Figure 5 shows the change of resistance to the applied strain as a function of time in the cyclic loading tests for CFGFRP and CPGFRP.. Changes in electrical resistance solid line and ap
Trang 2892 SELF-DIAGNOSING OF DAMAGE IN CERAMICS AND LARGE-SCALE STRUCTURES
Glass fiber
Carbon-fiber - glass-fiber-reinforced plastics
Vinyl ester resin Carbon fiber
Figure 1 Schematics of the structural design for CFGFRP (a)
and CPGFRP (b) in the shape of rod.
The CFGFRP and CPGFRP consisting of unidirectional
re-inforced fiber have a diameter of 3 mm
The schematic structural designs for the CMC are
shown in Fig 2 The composites were fabricated by the
filament winding method using Si3N4 particles (Ube
In-dustries Co., Ltd SN-COA) as the matrix and SiC fiber
(Nippon Carbon Co., Ltd NL-401) as the reinforcement for
strengthening or toughening the composite A portion of
the fibers was replaced with tungsten wire (Nippon
tung-sten Co., Ltd.φ 30 µm) The conductive particles of TiN
(Japan New Metals Co., Ltd.) were dispersed in part of the
Si3N4matrix The volume fraction of the conductive phase,
which includes 40 vol% of TiN particles, was 0.13% These
conductive phases were formed near the surface (500µm in
Si3N4-SiC fiber-W fiber Si3N4-SiC fiber-(Si3N4+TiN)
Figure 2 Structural design for CMC containing tungsten wire (a) and TiN particles (b).
depth) which was the tensile surface in the bending tests.These composites were hot-pressed under 40 MPa at 1773
K in N2atmosphere for one hour The sintered specimenswere cut into 3× 4 × 45 mm bars for bending test pieces
Self-Diagnosis Function of FRP
Figure 3 presents two scanning electron micrographs of apolished transverse section and of a longitudinal section ofCPGFRP (2) The circles in Fig 3(a) and the white lines inFig 3(b) denote glass fibers The bright gray flakes are thedispersed carbon particles Note that the carbon particlesare sufficiently dispersed in the matrix and that the matrix
is well impregnated between glass fibers This means that
a percolation structure consisting of conductive particleshas been successfully achieved
The self-diagnosis functions of these materials wereevaluated through simultaneous measurements of stressand electrical resistance change as a function of appliedstrain in tensile loading tests The resistance change was
defined as relative change in resistance (R − R0)/R0, cated byR/R0in which R0denotes initial resistance be-fore loading The two types of loading selected were (1)
indi-a normindi-al tensile test until specimen frindi-acture indi-and (2) indi-acyclic loading–unloading test below the maximum stresslevel Figure 4 shows the electrical resistance changes andthe applied stress for CFGFRP and CPGFRP as a func-tion of the applied strain in the tensile tests The stresses
in both specimens were increased linearly in proportion tothe strains until fracture occurred of the carbon fiber or theglass fiber The CFGFRP indicates a slight change in resis-tance below a 0.6% strain due to the elongation of carbonfiber and shows a tremendous change around 0.7% strainowing to the fracture of the conductive fiber; namely the re-sistance of CFGFRP exhibits a nonlinear response to theapplied strain as shown in Fig 4(a) The initial resistance
R0for CPGFRP was higher than that for CFGFRP because
of a slight electrical contact between carbon particles inthe percolation structure As can be seen from Fig 4(b),the CPGFRP indicates a linear increase in resistance withincreasing tensile strain The response of the resistance toapplied strain appears at 0.01% strain (100µ strain) or
lower The linear increase in the resistance continues til the fracture of the composite Comparing Fig 4(a) with(b) illustrates CPGFRP’s higher sensitivity at the smallstrain level and the wider detectable strain range com-pared to CFGFRP These results mean that the percolation
Trang 3un-SELF-DIAGNOSING OF DAMAGE IN CERAMICS AND LARGE-SCALE STRUCTURES 893
Figure 3 SEM photographs of polished transverse section (a) and longitudinal section (b) of
CPGFRP with unidirectional glass fiber.
structure formed with the carbon particle enables more
sensitive and adaptable diagnosis of damage than the
structure consisting of carbon fiber The strong response of
resistance for CPGFRP was attributed to a local break in
electrical contact between carbon particles because of the
micro crack formation in the matrix or in the
rearrange-ment in the percolation structure under tensile stress It
should be noted that the dispersion of the carbon
parti-cles had no effect on the strength of the composite, since
the fracture stress and mode for CPGFRP were similar to
those of GFRP without carbon particles
Figure 5 shows the change of resistance to the applied
strain as a function of time in the cyclic loading tests for
CFGFRP and CPGFRP These FRP were loaded and
un-loaded cyclically under a gradual increase in stress The
resistance of CFGFRP showed poor response below 0.6%
strain and a drastic increase above 0.7% strain as shown
in Fig 5(a) From Fig 5(b), it can be seen that the change in
resistance of CPGFRP corresponded well with strain
fluc-tuation (3) It is noteworthy that the resistance decreased
Figure 4 Changes in electrical resistance (solid line) and applied stress (dashed line) as a function
of applied strain in tensile tests for CFGFRP (a) and CPGFRP (b).
but did not completely return to zero at the unloadingstate The residual resistance in CPGFRP appeared afterthe application of 0.2% strain, and then increased withthe increase to the maximum applied strain The maxi-mum resistance during loading, indicated byRmax, andthe residual resistance change after unloading, denoted by
Rres, were arranged according to the maximum strain plied in the past as shown in Fig 6 The residual resistance
ap-of CFGFRP appeared around the 0.4% strain and increaseddiscontinuously above 0.6% The appearance of residual re-sistance for CFGFRP owing to fracture of the carbon fiberwas limited in a narrow strain range The change in resid-ual resistance of CPGFRP correlated closely with previousmaximum strain over the wide strain range as shown inFig 6(b), suggesting that the CPGFRP has the ability todiagnose the maximum strain based on measurements ofpast residual resistance at an unloading state (3) A com-parison of Fig 6(a) and (b) shows that the CPGFRP per-forms a more useful diagnostic function of damage historyover the wide strain range than does the CFGFRP
Trang 4894 SELF-DIAGNOSING OF DAMAGE IN CERAMICS AND LARGE-SCALE STRUCTURES
Time / min
(b)
Figure 5 Change in resistance (solid line) and applied strain
(dashed line) as a function of time in cyclic loading test for the
CFGFRP (a) and CPGFRP (b).
The microstructure of CPGFRP after the loading–
unloading cycle induced 0.6% strain and 2.1% strain was
observed by scanning electron microscopy (SEM) as shown
in Fig 7 (2) Clearly, the number of micro cracks in the
ma-trix increased with the increase in applied strain Although
the elongation of CPGFRP affected the elasticity after
un-loading, the percolation structure did not return reversibly
2015
2015
Figure 6 Maximum resistance change at loading state and
resid-ual resistance change at unloading state as a function of applied strain in cyclic loading tests for the CFGFRP (a) and CPGFRP (b).
to the initial state because of the micro crack formation
in the matrix The irreversible change in the percolationstructure in the conductive phase was partly responsiblefor the appearance of obvious residual resistance over awide strain range
Trang 5SELF-DIAGNOSING OF DAMAGE IN CERAMICS AND LARGE-SCALE STRUCTURES 895
500 µm
Figure 8 SEM cross sections of polished CMC specimens The arrows point to the tungsten wire
(a) or to the area containing TiN particles (b).
Self-Diagnosis Function of CMC
The conductive phases in the CMC observed by SEM
are shown in Fig 8 (4) Three tungsten wires were
em-bedded near the tensile surface The conductive phase
containing dispersed TiN particles and SiC fibers was
observed as the bright area Some voids (white bareas)
appeared in the conductive phase; however, these defects
were thought to be insignificant for the damage
diagno-sis function because the amount was negligible The
inter-face between these conductive phases and the Si3N4matrix
did not show a remarkable reaction and exhibited good
adhesiveness
The self-diagnosis functions of the CMC were evaluated
by simultaneous measurements of stress and electrical
re-sistance changeR as a function of applied strain in
four-point bending tests The loading was performed two ways:
(1) a normal bending test until specimen fracture and (2)
cyclic loading–unloading tests below the maximum stress
level The dependence of the applied load and change in
re-sistance on displacement for the CMC is shown in Fig 9 (4)
Figure 9 Change in load and resistance as a function of displacement in the four-point bending
tests for the CMC containing tungsten wire (a) or TiN particles (b).
Similar fracture behavior peculiar to CMCs was observed
in both composites in which a part of the ultimate loadwas kept after fracture at a displacement of about 0.1 mm.The peculiar load–displacement curve explained from theextraction of SiC fibers from the Si3N4 matrix is shown
in Fig 10 The difference in the ultimate load and in theload-displacement curve for both composites was thought
to be due to the uneven quality of SiC–Si3N4 phase, andnot to the difference in conductive phase The nonlinear re-sponse of resistance changes to displacement was exhibited
in both composites The CMC with tungsten wire showed
a slight change in resistance in a small deformation, andthen a drastic change was accompanied by their own frac-ture as shown in Fig 9(a) The CMC containing TiN parti-cles exhibited a distinct change in resistance from a smalldisplacement to the fracture in the composite as shown
in Fig 9(b) These results suggest that the monitoring ofresistance for CMCs with percolation structures is advan-tageous for diagnosing damages to the composites.Figure 11 shows the hysteresis of resistance change
in loading–unloading bending tests under the ultimate
Trang 6896 SELF-DIAGNOSING OF DAMAGE IN CERAMICS AND LARGE-SCALE STRUCTURES
(a)
100 µm
(b)
200 µm
Figure 10 SEM images of fractured surface for the CMC specimens containing tungsten wire
(a) or TiN particles (b).
load (4) The resistance of CMCs containing tungsten wire
showed no change at the loading and unloading state,
which was expected from Fig 9(a) The applied load of some
50% of the ultimate load induced the increase in the
resis-tance for the CMCs containing TiN particles, and then the
increased resistance remained at about 80% of the
maxi-mum resistance after unloading It should be noted that
the loading–unloading cycle induced elastic deformation
for the CMCs without residual strain Hence, the residual
resistance was thought to be due to irreversible local
frac-ture in the conductive phase The residual phenomenon in
resistance change for the CMCs was more remarkable than
that for FRP shown in Fig 5(b), which was attributed to
the brittleness of the ceramic in the matrix
Figure 12 presents an attempt at repeatedly varying
the resistance for the CMC with tungsten wire or TiN
Figure 11 Change in load and resistance as a function of displacement in the loading–unloading
tests for the CMC containing tungsten wire (a) or TiN particles (b) The applied maximum load was 150 kN.
particles in cyclic bending test The applied load was,however, kept constant at 150 kN The residual resis-tance for the CMCs with tungsten wire indicated nochange, while that for the composites containing TiN par-ticles after unloading rapidly increased up to 10 cycles
It should be noted that the residual resistance tionally increased with an increasing number of repeti-tions after 20 cycles The linear response of residual re-sistance was thought to be attributed to the propagation ofmicro cracking in the conductive Si3N4–TiN phase Thisresult further confirms that the CMCs containing TiNparticles have the ability to diagnose cumulative damage
propor-to the composite through measurements of the residualresistance
The electrical conductive FRP and CMC were signed and produced by adding a conductive fiber or
Trang 7de-SELF-DIAGNOSING OF DAMAGE IN CERAMICS AND LARGE-SCALE STRUCTURES 897
Figure 12 Change in resistance as a function of number of
repe-titions in the cyclic bending tests for the CMC containing tungsten
wire and TiN particles.
particles, and the self-diagnosis functions for these
con-ductive composites were investigated Compared with
the composites that include conductive fiber or wire the
composites with the percolation structure consisting of
conductive particles were found to be capable of
diag-nosing deformation or damage in the composites The
composites containing carbon particles appeared
capa-ble of diagnozing damage at the sensitivity level of a
small strain and in a detectable strain range
Concern-ing the detectable strain level, the FRP showed an
ex-cellent response to the resistance change and to the
applied strain This is a suitable range for the health
monitoring of structural materials such as concrete
con-struction It was also found through measurements of the
residual resistance that the FRP composites are capable
of memorizing the maximum applied strain or stress The
CMCs with percolation structures consisting of TiN
par-ticles exhibited superior resistance to small deformation
changes It should finally be noted that the CMC materials
proved capable of diagnosing cumulative damage for the
composites by evaluating the residual resistance, and that
these self-diagnosis functions are easily obtained by simple
measurements of electrical resistance
APPLICATION OF THE SELF-DIAGNOSIS COMPOSITE
TO CONCRETE STRUCTURES
A new type composite was developed that had a
self-diagnosis function for health monitoring and damage
detection in materials (1–7) The composite, which has
elec-trical conductivity as well as reinforced fibers, provides
a signal of electrical resistance change corresponding to
the degree of damage in the material This self-diagnosis
composite offers also some advantages in properties, cost,
and simplicity, compared with other materials or systems
such the an optical fiber and the strain gauge A concrete
structure is the best application for the self-diagnosiscomposite because the composite has a good sensitivity tomicro cracking in concrete materials, shows high strength
in reinforcing concrete material, and provides ease both inits attachment and in the measurement of electrical con-ductivity The study was aimed at determining whetherthe composite was useful for measuring damage and frac-ture in concrete blocks and piles Particularly, the appli-cation into concrete piles was treated as a typical exam-ple of concrete construction limiting the direct observation
of damage or fracture after a serious load has been plied in its utilization Also investigated, by bending testsand electrical resistance measurements were the functionand performance of the composites when embedded in mor-tar/concrete blocks and concrete piles
ap-Specimen and Experiment
Two kinds of glass-fiber reinforced plastics compositeswere fabricated in this study The first composite includedcarbon fibers substituted for some of the glass fibers; itselectrical conductivity was called CF The second compositeinvolved carbon powders dispersed in a part of the plasticthat formed the percolation structures as a conductive path(CP) The CF and CP composites were embedded into mor-tar specimens and concrete specimens reinforced by steelbars or rods by the following procedures Figure 13(a–c)shows the structure and arrangement of the composites
in the three concrete specimens types The first type is arectangular mortar block specimen with the CP compos-ites The second type is a rectangular concrete block speci-men with the CP and CF composites and two steel bars.The third type is a concrete pile specimen having the CPcomposites and 16 steel bars The pile type specimens havebeen pre-stressed at 14.3 MPa applied by the tension stress
of the steel bars, while the block type was free from stress
pre-Figure 14 illustrates the methods used for bending testsfor the block and the pile type specimens with differentlengths and distances The electrical resistance change(R/R0, where R is an increase of resistance and R0
is an initial resistance) of the composites was measuredsimultaneously in the loading tests The strain gaugemeasurement attached on the tension-side surface of thespecimen was also used Photographs the actual bendingtests for the block and the pile specimens are shown inFig 15
Mortar Block Tests
The CP composite was embedded in the tensile side ofthe mortar specimens in order to demonstrate the self-diagnosis function Figure 16 shows the applied load andresistance change of the CP composite as a function ofdisplacement in a bending test The embedded CP com-posite was located 8 mm apart from the tensile sur-face of the mortar The load–displacement curve indicatesdiscontinuous changes at points A and B, which corre-spond to the crack formation and propagation in the mortarspecimen, respectively The crack formation and propaga-tion are shown in photographs of the mortar specimen.The resistance of the CP composite begins to increase
Trang 8898 SELF-DIAGNOSING OF DAMAGE IN CERAMICS AND LARGE-SCALE STRUCTURES
Figure 13 Structure and arrangement of the composites in the three types of concrete specimens.
(a) Type-1, a rectangular mortar block specimen with the CP composite (b) Type-2, a rectangular concrete block specimen with the CP and CF composites (c) Type-3, a concrete pile specimen with
Figure 14 Different bending tests for the block-and-pile type
specimens with different length and distances corresponding to
type-1, type-2, and type-3.
slightly before crack formation Note that the increase inresistance appears simultaneously with the micro crackformation and that a discontinuous resistance change
is generated in response to the crack propagation Theresidual resistance was observed in the FRP material af-
ter unloading at point D The resistance change of
em-bedded CP composite corresponds well to the propagation
of damage inflicted on the mortar specimen Once again,the results demonstrate that the embedded CP compositehas the ability to diagnose micro crack formation/propa-gation and loading history in cement-based structuralmaterials
The behavior of residual resistance for the CP compositeembedded in a mortar specimen was investigated in detail
by cyclic bending tests Figure 17 presents the sis of resistance changes by cyclic loading–unloading testsunder 40% of ultimate load The application of load causedmicro crack formation, and then the crack was closed at anunloading state as shown in Fig 17 It should be notedthat the crack was eliminated, but the behavior of themicro crack induced residual resistance after unloading.The application of higher load (60% of ultimate load) madehigher residual resistance after unloading These resultssuggested that the CP composite embedded in the mortarspecimen has the ability to diagnose the closed micro crack,namely the hysteresis of micro crack formation by evalu-ation of the residual resistance even after the crack hasclosed
Trang 9(b)
(c)
Figure 15 Bending tests in progress for the mortar block specimens (a), the concrete block
speci-mens (b), and the pile specispeci-mens (c).
0 1 2 3 4 5 6 7
0 5 10 15 20 25 30 35
Figure 16 Changes in resistance (solid line) and applied load (dashed line) in a bending test for
CPGFRP rod embedded in mortar specimen These points (A–D) on the graph correspond to the photographs of the mortar specimen.
899
Trang 10900 SELF-DIAGNOSING OF DAMAGE IN CERAMICS AND LARGE-SCALE STRUCTURES
3
Load
∆R/R02.5
12
3
45
6
Loaded
Unloaded
Figure 17 Changes in resistance (dashed line) and applied load (solid line) in the cyclic loading–
unloading tests, under 40% of the ultimate load.
Concrete Block Tests
Figure 18 shows the results of load, strain, andR/R0of
CP and CF composites as a function of time in the
bend-ing test for the concrete block (6) The stain change, which
followed closely the loading curve, indicates that a
mi-cro crack formed at about 200µ strain and the steel bars
yielded at about 1000µ strain The strain gauge was broken
in the loading test owing to the crack propagation in the
surface of the concrete specimen TheR/R0 of the CP
composite is initiated at about 300 s, which corresponds to
the stage of crack formation TheR/R0of the CP
compos-ite increased with an increased load up to the maximum
load at about 1000 s TheR/R0of CF is scarcely detected
until the high load level when it increases suddenly near
the maximum load Both the CP and CF composites do not
break in the test because of their high strength and
flexi-bility It should be noted that the CP composite shows good
sensitivity in the small stain range as well as a continuous
response in the wide strain range up to the final fracture
of the specimen
Figure 19 provides the results of a cyclic loading test for
the block-type specimen (6) In all, eight cycles of loading
and unloading with an increased load level were carried
out in this test The strain change and theR/R0 of CP
composite responded well to the load curve from a lower
load level, while theR/R0of CF did not act until a higher
load was applied It was also found that the CP composite’s
residual resistance appeared only after the cycles of the
medium load level
The block specimen is shown in Fig 20 (a–c) as it
ap-peared in the cyclic bending test (6) The cracks are clearly
initiated from the tension-side surface at a low load, and
they grow with an increased load level until the specimen
finally breaks owing to steel bar fracture
Concrete Pile Tests
Figure 21 gives the results of the cyclic bending test for the
type-3 concrete pile specimen (6) The specimen included
only the CP composite because of the sensitivity it showedunder a small load, which was higher than that for the CFcomposite as confirmed in Figs 18 and 19 This test aimed
to increase the sensitivity of the CP composite, which isarranged near the tension-side surface of the pile speci-men Figure 21(d) is the result from the enlargedR/R0axis of the CP composite in Fig 21(c) TheR/R0 of the
CP composite in the pile responds well in a wide range
of loading as shown in Fig 21(c) The CP composite cated near the tension-side surface of the pile specimenindicates good sensitivity in the lower load levels as shown
lo-in Fig 21(d) TheR/R0of the CP composite in the lowerload range is very similar to the strain change in Fig.21(b), which means that the CP composite can signal asmaller strain before the crack forms in the pile surface
In these pile tests there is no clear indication of ual resistance phenomena as detected in the block tests,probably because of the effect of pre-stress in the pilespecimens
resid-The appearances of the pile specimen in the cyclic ing test are shown in Fig 22(a–c) (6) The crack forms at
bend-a low lobend-ad, its growth occurs with bend-an increbend-ased lobend-ad, bend-andfinally the pile fractures after the test has ended
Performance of the Self-diagnosis Composites
In the bending tests of the concrete block, the CP ite produced good results compared to the CF composite.Remarkably, the electrical resistance of the CP compos-ite increased under a small strain to detect a micro crackformation at about 200µ, and it responded well to small de-formations before the crack formation The CP compositeshowed continuous resistance change up to a large strainlevel near the final fracture of the concrete structures re-inforced by steel bars It was also found that the CP com-posites embedded in mortar/cement block specimens havethe ability to diagnose the hysteresis of micro crack forma-tion by the evaluation of the residual resistance even afterunloading
Trang 11compos-SELF-DIAGNOSING OF DAMAGE IN CERAMICS AND LARGE-SCALE STRUCTURES 901
100
(a)
806040
200
10000
1050
5000
500 1000 1500
Time / s
2000 2500
Figure 18 Load (a), strain (b), andR/R0 (c, d) of CP and CF
composites as a function of time in the bending test for the
block-type specimen.
100
(a)
806040
200
400030002000
10000
400030002000
1050
400030002000
5000
0 1000 2000 3000 4000 5000 6000
Time / s
7000 8000
Figure 19 Load (a), strain (b), and R/R0 (c, d) of CP and
CF composites in the cyclic loading test for of the block type specimen.
Trang 12902 SELF-DIAGNOSING OF DAMAGE IN CERAMICS AND LARGE-SCALE STRUCTURES
(a)
(b)
(c)
Figure 20 Appearances of the block specimen during the cyclic
bending test (a) A low load level; (b) a high load level; (c) after the
test.
Such excellent properties can be attributed to the
perco-lation structure of the carbon particles dispersed within a
section of the plastic matrix phase The conductive path in
the percolation structure of carbon particles, which is very
different from the conductive path in carbon continuous
fibers, can react to small strains that are lower than 200
µ This may be due to its flexible structure which is filled
with faint gaps and cracks as seen in the microstructures of
the carbon particles mixed with plastics The phenomenon
of the residual electrical resistance at the unloading state
suggests that the distorted structure at the loading state
does not completely return to its original shape at the
unloading state The residual resistance phenomenon has
a possibility for the hysteresis function of an applied load
250
(a)
200150100
500
0 2000 4000 6000 8000 110 1.2 10
Time / s
30002500
(b)
200015001000
5000
800060004000
20
−2
800060004000
0.20
0 2000 4000 6000 8000 110 1.2 10
Time / s
Figure 21 Load (a), strain (b) andR/R0 (c, d) of CP composite in.
Trang 13SENSOR ARRAY TECHNOLOGY, ARMY 903
(a)
(b)
(c)
Figure 22 Appearances of the pile specimen in the cyclic bending
test of type-3 (a) A low load level; (b) a high load level; (c) after the
test.
The continuous change of resistance in the CP composite
contributes to the damage detection of concrete structures
The percolation structure in the fiber-reinforced structure
can keep its flexible structure up to the final fracture
It is necessary to arrange the CP composite in concrete
specimens to optimize the function The location near a
tension-side surface and far from steel bars is effective
in order to obtain a quick response to applied stress and
crack formation The existence of prestress (compression)
in concrete structures can dull the sensitivity of the
com-posite For the CP composite near the tension-side surface
in the pile specimen, its clear sensitivity proves that the
performance of the composite overcomes the influence of
prestress
Two kinds of glass-fiber reinforced plastic composites
with carbon powder (CP) or carbon-fiber (CP) were
in-troduced into the mortar/concrete specimens, with block
and concrete pile types and electrical resistance change
(R/R0) of the composites being measured in the bending
tests TheR/R0of the CP composite in the block specimen
showed a good sensitivity in a small strain range to detect
crack formation in the mortar/concrete and a continuous
change in a large strain range up to the final fracture of the
specimen, while theR/R of the CF composite increased
suddenly at a certain strain of the specimen The CP posite had the good response to cyclic load patterns in thebending test of the block specimen and indicated the resi-dual resistance at an unloading state TheR/R0of the CPcomposite in the pile specimen with prestress showed goodresults to the loading patterns before and after micro crackformation in the bending test The arrangement of the CPcomposite near the tension-side’s surface, and far from thesteel bars in the pile, effectively improved the sensitivity ofthe composite The excellent self-diagnosis function of the
com-CP composite in the concrete structures was considered to
be mainly caused by the flexibility in the percolation ture of carbon particles
struc-BIBLIOGRAPHY
1 N Muto, H Yanagida, T Nakatsuji, M Sugita, and Y.
Ohtsuka J Am Ceram Soc 76 (4): 875–879 (1993).
2 M Takada, S.-G Shin, H Matsubara, and H Yanagida J Jpn.
Soc Compos Mater 25: 225–230 (1999).
3 Y Okuhara, S.-G Shin, H Matsubara, and H Yanagida.
Trans MRS-J 25 (2): 581–584 (2000).
4 M Takada, H Matsubara, S.-G Shin, T Mitsuoka, and
H Yanagida J Ceram Soc Jpn 108 (4): 397–401 (2000).
5 Y Okuhara, S.-G Shin, H Matsubara, H Yanagida, and
N Takeda Proc SPIE (2000), in press.
6 H Nishimura, T Sugiyama, Y Okuhara, S.-G Shin, H.
Matsubara, and H Yanagida, Proc SPIE 3985, 335 (2000).
SENSOR ARRAY TECHNOLOGY, ARMY
signi-needs for characterizing in situ structural integrity
char-acteristics of corrosion and barely visible impact damage(BVID) to determine “damage susceptibility” must be ad-dressed This article presents a new concept for onboardreal-time monitoring using conductive polymer sensor ar-ray technology
BACKGROUND
Both commercial and military service personnel currentlyemploy “walk-around” structural inspection as a corner-stone of condition-based maintenance This means that
a hierarchy of inspections is required to ensure thatfleet readiness and availability requirements are met.Structural inspection includes daily inspection, phasedmaintenance based on aircraft operating time, conditional
Trang 14904 SENSOR ARRAY TECHNOLOGY, ARMY
Figure 1 Key sensing locations on aircraft.
Enging inlet 5A
Landing geat(nose and main)
6A1C
5B4A
1A
3C
2B
Load bearingantenna
Fuel tank andweapons pulon
External skin(upper andlower)Wing fold
Horizontal/verticalstabilizer7A,B
Engine AftexhaustWing
tanksLeading and trailing edges
Gun bay area
3A
2A
Cockpit canopy
Redomebulkhead
inspection based on the mission and location of the aircraft,
and calendar-based inspection
Although condition-based maintenance inspection is
mature and is reliable in most cases, its application in
future military and commercial systems has significant
drawbacks notably high cost and intensive effort
Cur-rently, the cost to maintain a Navy aircraft is up to
$200,000 per year A 1996 Naval Center for Cost Analysis
AMOSC report indicates that the direct cost of
maintain-ing Navy aircraft and ships is at least $15.0 B per year As
much as 25 to 30% of operating revenue is spent on
main-tenance for commercial air carriers According to a 1995
study by the office of the Under Secretary of Defense, 47%
of the Navy’s active duty enlisted force (173,000 sailors)
and 24% of the Marine Corps (37,600 marines) are assigned
to maintenance functions The mandate to reduce
man-power while performing duties faster, cheaper, better, and
more reliably is a reality in both military and commercial
transportation
In addition to these issues, problem areas exist
specifi-cally for maintaining structural integrity, including BVID
and hidden and inaccessible corrosion The increased use
of composite materials in aircraft structures introduces the
potential for BVID, a maintenance-induced damage effect
At least 30% of all maintenance is related to structural
repair due to tool dropping and in-service damage A
sig-nificant amount of the loss of structural integrity is due to
hidden corrosion as well as corrosion located in inaccessible
areas (wheel wells, landing gear areas, and fuel tanks) The
practice of applying surface treatments of various types to
provide adequate protection, in some cases overcoating the
surface with several layers, causes considerable weight
in-crease This increase results in loss of fuel savings and
proved aircraft performance
TECHNICAL APPROACH
A trade study was performed to identify and assess
po-tential aircraft inspection areas that could benefit from
conductive polymer sensor array technology The trade
study involved the identification of seven key areas of a
generic fighter aircraft (F-18 or equivalent) The areas dressed in the study were external wing structure, inter-nal wing and fuselage structure, including landing gearand cockpit canopy, communications, external stores, andempennage structure The study addressed specific parts
ad-of these identified areas and included a problem tion, a proposed sensing layout approach, and a sensingconfiguration Figure 1 is a drawing of the F-18 aircraftthat shows the functional layout of the seven aircraft sens-ing areas for possible future technology insertion Thesensing areas are mapped to the aircraft geometry, labeled
defini-by area, and keyed to the full-scale trade study chart shown
in Table 1 The chart highlights the details of the tradestudy effort and contains specifics on subassemblies, in-cluding a general problem description It maps the prob-lems using three different types of sensing: “M/C” refers tomoisture/corrosion sensing, “ID” refers to impact detection,and LBA refers to “load-bearing antenna.” For each sens-ing approach, three packaging options exist: (1) a confor-mal sensor array, which would cover a larger surface areasuch as an external wing area of more than several squarefeet; (2) a conformal sensor applique to provide sensingcoverage in a smaller area (a few square inches, possiblywith significant contour shapes); and (3) a conformal bootassembly The conformal boot design would involve fabri-cating a preformed structure—a sensory boot that fits thespatial constraints of the aircraft contour An example ofthis configuration would be a preformed boot fit over theleading edge or radome bulkhead assembly
Sensor Development
A conductive polymer sensor array design provides thecapability for multifunction conformal sensing Honeywellhas developed polymer sensors to sense moisture (i.e.,electrolyte) conditions and the presence of moisture/fluidsacross an extended surface area A primary maintenanceconcern is the need to sense and quantify moisture trappedbetween the protectant system layer and the aircraft sur-face that could cause corrosion Typically, the moisture is
an electrolyte, an electrically conducting fluid that has ions
in solution The polymer sensor array has been designed to
Trang 15SENSOR ARRAY TECHNOLOGY, ARMY 905
Table 1 Aircraft Trade Study Chart
Sensing Sensing Aircraft Area Part/Assembly Problem Definition Approacha Configuration
1 Wing external rLeading edges rFlap and drive assembly rM/C rConformal array
rTrailing edges rImpact (BVID) rID rConformal boot
rCorrosion—wing attach fitting
rErosion
rExternal skin (upper rImpact (BVID due to maintenance/ rM/C rConformal array
rCorrosion (fastener area)
rWing fold rCorrosion in hinge area rM/C rConformal tape
rWing attachment fatigue
2 Communications rRadome bulkhead rCorrosion (dissimilar± F-galvanic) rM/C rConformal bootsupport rWing antenna rPhased-array antenna rLBA rConformal applique
3 Fuselage rCockpit canopy rCorrosion—dissimilar interface rM/C rConformal applique
(galvanic)
rLanding gear rCorrosion in wheel well area, main rM/C rConformal applique
landing gear assembly
rGun bay area rCorrosion—dissimilar interface rM/C rConformal applique
4 Wing internal rWing tank rFuel leakage in web area (wet bay) rM/C rConformal applique
rElectrical connector/ground straps
5 Engine rEngine inlet rImpact (BVID) from debris/bird strike rID rConformal applique
rAft engine exhaust area rCorrosion—moisture rM/C
6 External stores rFuel tank pylon rCorrosion—dissimilar interface rM/C rConformal applique
rWeapons pylon rErosion
7 Empennage rHorizontal stabilizer rPivot shaft corrosion rM/C rConformal applique
rVertical stabilizer box rCorrosion
aM/C = moisture/corrosion; ID = impact detection; LBA = load-bearing antenna.
detect the “presence” of an electrolyte, which can be
seawa-ter, acid rain, lavatory fluids, fuel, hydraulic fluid,
chemi-cals, or cargo by-products
The basic design is implemented by printing a
spe-cific pattern design on a flexible substrate material,
cur-ing it, and layercur-ing it uscur-ing a pressure-sensitive adhesive
A typical pattern developed for electrolyte sensing is a
transducer design that has alternating electrode pairs
Figure 2 illustrates the pattern layout for a polymer sensor
array The figure shows a set of dedicated electrode pairs,
each of which operates as a sensory element The sensor
is designed to function as a linear 2-D array that
mea-sures the “location” where the electrolyte is sensed and
the “amount” of electrolyte based on exposure across the
sensor array
Electrode linewidth ~ 1/32 in.
1/2 in.
IDT (interdigitated tranducer) electrode # 1
6 in.
To scanning electronics
C0024 1-11
Figure 2 Pattern layout of polymer sensor array.
Detection of Corrosivity Four conditions must exist
be-fore corrosion can occur: (1) the presence of a metal that willcorrode an anode; (2) the presence of a dissimilar conduc-tive material (i.e., cathode) that has less tendency to cor-rode; (3) the presence of a conductive liquid (electrolyte);and (4) an electrical path between anode and cathode Acorrosion cell is formed because of the electrochemical ef-fect, if these four conditions exist, as shown in Fig 3 In
a typical aircraft coating application, paint applied to thesurface of the metal acts as a moisture barrier to protect thebare metal from exposure to an electrolyte The paint filmprevents the corrosion cell from functioning by separatingthe electrolyte from the anodic and cathodic sites on themetal surface If this paint layer is damaged by erosion,heat exposure, or aging, the cell is activated, and corrosionoccurs
Figure 3 also highlights the concept of using a polymersensor array to detect corrosivity when a corrosion cell isformed in an aircraft lap joint As shown, the linear sen-sor array senses the “conductivity” of the trapped fluid byconducting a current through the fluid that is between IDTelectrode pairs The fluid’s conductive property is, by defi-nition, “the ability to act like an electrolyte and conduct acurrent, or a measure of its corrosivity.”
The concept of performing corrosive environmental
“exposure susceptibility” index monitoring to minimizescheduled inspections and provide direct cost savings isshown in Fig 4 The basic idea is continuous monitor-ing of the actual exposure of each aircraft to corrosive
Trang 16906 SENSOR ARRAY TECHNOLOGY, ARMY
Moisture migration ID
Polymersensor
agents enter
at unsealedskin edges
Aircraft fastener
Paint layer
Electronflow
Anodic area Cathodicarea
Metal
Paintlayer
Paint erosioneffect Electrolyte(i.e.fuel,
water)
C0 024 1-01
Figure 3 Simplified corrosion cell and lap joint application.
environmental factors (moisture ingress into protective
coating, type of corrosive agent, etc.) and then scheduling
corrosion inspections based on these measurements, rather
than on preset rules that are only loosely related to
corro-sion Typical preset rules that an exposure susceptibility
index would replace are calendar-based (i.e., inspection
ev-ery 30 days) or usage-based (i.e., inspection evev-ery 10 h of
operation) inspections One can think of the system as a
“corrosion odometer” whose a readout steadily increases
according to the corrosiveness of the environment to which
the aircraft is exposed Maintenance personnel can
inter-mittently check the odometer and inspect as needed The
exposure susceptibility index provides a reliable method
for scheduling corrosion inspections that (1) is based on
the true exposure of the aircraft, which leads to a higher
degree of susceptibility to corrosion; (2) appropriately
re-flect variations in exposure due to short-term weather
pat-terns; and (3) can be consistently applied to aircraft of a
given type at any location in the world
The sensor array approach can sense and calculate an
exposure index to ingress of an electrolyte (i.e., water) and
the “wetness” effect of the electrolyte The wet/dry cycle of
exposure is a strong indicator of how susceptible an aircraft
is to corrosion; wetness is a basic requirement for corrosion
Figure 4 Exposure susceptibility index.
to occur The wetness exposure index is defined as the
in-tegral over time of the function FW(W) Here W is the
time-varying output of a “wetness” sensor (1= wet, 0 = dry)
that quantifies the total corrosive effect of wetness FWis asimple function that gives the exposure index on a conve-nient scale, so an abbreviated inspection is called for eachtime the index passes through a multiple of 100, for exam-ple Thus, for severe environments such as Puerto Rico, anincrease by 100 every 15 days could occur, compared to anincrease by 100 every 90 days in Denver
Further improvement of the exposure susceptibility dex can be obtained by adding other environmental factorsthat influence corrosion, including the concentration of theelectrolyte, the temperature, and the conductivity (corro-sivity factor)
in-Figure 5 illustrates the index calculation concept andshows the maintenance cost saving concept in detail Thedesign approach is set up to collect and analyze the en-vironmental factors related to structural health (mois-ture ingress, impact forces, etc.) that could lead to loss ofstructural integrity These factors are collected and inte-grated as a “cumulative index” to determine (1) the level of
“susceptibility” to failure and (2) whether maintenance isrequired at a given location in the aircraft The cumulativeindex value, it is envisioned, will be represented as a simple
index
Figure 5 Maintenance cost saving tutorial.
Trang 17SENSOR ARRAY TECHNOLOGY, ARMY 907
Force appliedvia structuralimpact
Semiconductorpolymer layer
Applique filmF
Polymersensorpattern
Figure 6 Force-sensing resistor (FSR).
whole number from 0 to 100 (which indicates the level of
susceptibility; a higher number indicates that more
poten-tial for damage may exist) that could be read out by
mainte-nance personnel from the aircraft maintemainte-nance debriefing
interface at scheduled inspection intervals The crew could
then decide to perform scheduled maintenance or bypass
the action This would directly reduce the cost of
main-taining the aircraft by eliminating or reducing the number
of inspections In addition, reducing the time for a
main-tenance procedure based on the polymer sensor system’s
ability to identify the general structural location where the
repairs may be needed and the type of repair required (i.e.,
impact damage vs corrosion) will result in additional
op-erational cost savings
Impact Detection The polymer sensor for
mois-ture/corrosion sensing can also sense impact forces caused
by maintenance-induced damage or operational servicing
To provide sensing for impact forces, the polymer sensor
array is configured with an additional semiconductor
poly-mer layer, as shown in Fig 6 The design approach is set
up to operate as a force-sensing resistor (FSR) An FSR
operates on the principle of converting force applied via
Figure 8 Example of off-the-shelf FSR product.
a structural impact to an equivalent voltage output
As pressure is applied, individual electrode pairs areshunted, causing a decrease in electrical resistance Themeasurement of impact force magnitude, impact directionvector along the sensor array, and impact surface area can
be quantified, depending on polymer composition, shuntpattern and shunt shape, and the method for applyingpressure (hemispherical vs flat) Figure 7 shows a typi-cal curve of sensor response The figure is a plot of electri-cal resistivity versus applied force and has an active sens-ing region of two to three orders of magnitude from lowimpedance (kilohms) to high impedance (megohms) Thesensor response is approximately a linear function of forceacross a wide range of applied pressure The first abrupttransition that occurs is at low pressure This point is calledthe “breakover point” where the slope changes Above this
region, the force is approximately proportional to 1/R until
Localized region of particles (Higher density)
External force F
Electrically insulation polymer matrix
Active sensing region
Figure 9 Polymer matrix sensor.
Trang 18908 SENSOR ARRAY TECHNOLOGY, ARMY
Table 2 PTF Resistor Versus Other Resistor Technologya
a Source: G Harsanyi, ed., Polymer Films in Sensor Applications—Technology, Materials, Devices and Their Characteristics Technomic, 1995.
a saturation region is reached When the force reaches this
magnitude, applying additional force does not decrease the
resistance substantially
Figure 8 is a photo illustration of a commercially
avail-able off-the-shelf FSR product called Uniforce, which has
an operating range of 0–1000 psi
Another type of conductive polymer sensor is a
poly-mer matrix sensor that consists of electrically conducting
and nonconducting particles suspended in a matrix binder
Figure 9 shows a cross-sectional view of a polymer
matrix sensor Typical design construction includes a
ma-trix binder and filler Mama-trix binders include polyimides,
polyesters, polyethylene, silicone, and other
nonconduct-ing materials Some typical fillers include carbon black,
copper, silver, gold, and silica Particle sizes typically are of
the order of fractions of microns in diameter and are
formu-lated to reduce temperature dependence, improve
mechan-ical properties, and increase surface durability Applying
an external force to the surface of a sensing film causes
A/D Mux
1-in.
space FSRelements
Aircraft composite access panel (approx 24 in 2 )
V sense
− Equivalent circuit (voltage divider)
to µC
f applied
Structural BVD
Access panel
(after f x exceeded)
Damage ID
Damage threshold (f x )
Force applied (psi) (f x )
Figure 10 Structural impact damage tutorial.
particles to touch each other and decreases the overall trical resistance
elec-Table 2 illustrates the typical performance of polymerthick-film (PTF) resistor technology and other resistortechnologies The table includes a summary for thin films,semiconductor, and continuous metal films The significantadvantage of PTF resistor technology over all other resistorsensing is the cost to fabricate devices The PTF cost factor
is achieved by the ability to print resistive material viastencil, screen printing, and ink-jet printing techniques
A prime example of using FSR technology for aerospacesensing is structural integrity monitoring Today’s com-mercial and aerospace structures incorporate a largeamount of composite materials to reduce structural weightand increase load-bearing properties Composites aresusceptible to damage from impact forces experienced
in operation, including debris picked up from runwaysand maintenance-induced damage caused by droppedtools Figure 10 illustrates the system-level concept of
Trang 19SENSOR ARRAY TECHNOLOGY, ARMY 909
Patchantenna(conductive film)
Nonconductivefilm layer
Ground plane layer
50-100 mmsquare
Stripfeed
Detailed view
of antenna
Figure 11 Example of conformal antenna.
impact-damage-detection-based applied force versus
dam-age for a composite aircraft panel A matrix array of FSR
elements integrated into the aircraft panel is shown Panel
construction involves printing FSR elements directly on
the panel surface or on a film layer, which is then bonded
to the panel using a pressure-sensitive adhesive layer The
polymer patterns incorporated on the panel include a
com-bination of sensor elements and electrical interconnects
implemented with conductive polymer materials
To measure and record impact forces in real time, the
output of each FSR element is converted to an equivalent
voltage via a simple voltage divider circuit and is provided
as input for a dedicated data acquisition system Each
FSR element output is routed to an analog multiplexer
10 5'
6'Conformal
antenna array
Antenna
Antenna
EMIgenerator
EMIgenerator(broadband)
Figure 12 Conformal antenna
threshold value fxindicates that barely visible structuraldamage has occurred The effects of detected damage can
be read out by maintenance personnel periodically to termine if structural repair is needed it or is marked assuspect, and the vehicle is returned to active service Aset of damage identification threshold values could be re-tained for each major structural component of the air-craft in a 3-D map database to perform maintenance ondemand
de-Conformal Antennas A significant feature of polymer
sensor array technology is the arrays’ ability to operate
as a low observable (LO) conformal antenna The polymersensor has been tested in laboratory conditions to detectbroadband frequencies of several megahertz without anyoptimization of the polymer circuit pattern The confor-mal antenna capability offers a significant benefit of in-creasing detection of “bad guy” signature threats Testsperformed by aircraft primes have indicated that confor-mal load-bearing antennas improve detection by a fac-tor of 6 to 14 times In addition, the conformal polymerconstruction makes it suitable for phased-array antenna
Trang 20910 SENSORS, SURFACE ACOUSTIC WAVE SENSORS
Figure 13 Wireless transceiver module for self-contained
Controlantenna
Sensorarray
Controlantenna
Sensorarray
Controlantenna
Aircraft access panel Aircraft access panel Aircraft access panel
C00241-08
Figure 14 Wireless structural panel sensor web.
design for munitions and guided projectiles Figure 11
illustrates the feasibility of using the polymer design
for antenna functions Figure 12 highlights the use of a
broadband EMI source and detection of electromagnetic
wave pickup at increasing distances up to 10 ft from the
antenna
Communications Debriefing
A significant system-level issue is how to obtain data on
environmental factors during aircraft inspections without
increasing the workloads of maintenance personnel This
can be achieved by providing a wireless link for data access
Figure 13 illustrates a photo of a 2× 3 × 0.125 in wireless
transceiver module for field maintenance communications
The module consists of a low-frequency (128 kHz) receiver
interface, a dedicated high-frequency (315 MHz) ter interface, dedicated control logic, and internal RAMmemory The radio-frequency (RF) system can be read atranges of 6–30 ft and operates at 2µa in standby mode The
transmit-RF module is powered by a high-energy-density lithiumbutton-cell battery Future applications will include an
RF module that features a very low profile height of 4mils and capability for RF power scavenging This uniquecapability implies that no batteries will be required to com-municate and debrief the sensor suite Up to 100 RF mod-ules can be read simultaneously by a dedicated wireless
RF reader
Figure 14 illustrates a concept for wireless sensor munications to debrief a suite of aircraft structural compo-nents A field maintenance technician is shown holding apersonal data assistant that has a wireless interface Thestatus of the structural integrity of each component could
com-be assessed by issuing a polling command to search andidentify the health status of a designated structural panel.The wireless interface within each structural componentwould read the poll message, determine if the messagerequest is intended for that component, and the designatedpanel will then return the health status to the maintenancetechnician
SENSORS, SURFACE ACOUSTIC WAVE SENSORS
As a result of these advantages and the strong demand,the development of SAW microsensors has grown rapidlyduring the last twenty years
Trang 21SENSORS, SURFACE ACOUSTIC WAVE SENSORS 911
term acoustic wave refers to the class of waves that
dis-places particles of the solid, liquid, or gas medium in which
they propagate Therefore, acoustic waves are considered
mechanical waves compared to electromagnetic waves,
which can propagate in a vacuum because they do not
re-quire a medium or have related particle displacements
The term surface acoustic wave (SAW) usually refers to
the class of acoustic waves that propagates at a solid
sur-face, versus bulk waves, which propagate within a solid
The first type of SAW was discovered by Rayleigh in 1885
It has longitudinal particle displacements (in the
propaga-tive direction) and transverse particle displacements
(per-pendicular to the propagative direction) that are normal to
the substrate surface This type of SAW is called a Rayleigh
wave There are several other types of SAWs that are
dis-tinguished primarily by their wave particle displacements
and are usually allowed only for certain crystallographic
orientations For example, a wave that has transverse
par-ticle displacements in the plane of the substrate and
propa-gates just below the surface is called a surface skimming
bulk wave (SSBW) It occurs on ST-cut quartz Bulk waves
are also classified as longitudinal or transverse (shear)
based on particle displacements Classic reviews of
acou-stic waves in solids are provided by Auld (4) and Kino (5)
A SAW microsensor normally consists of two metal
in-terdigital transducers (IDTs) fabricated on a
piezoelec-tric substrate The IDTs are patterned from a thin metal
film (usually aluminum) that has been deposited on the
substrate The patterning is done by using standard
pho-tolithographic techniques Figure 1 illustrates a single
de-lay line (channel) SAW sensor The term dede-lay line is used
for this design because it can be used for this application in
signal processing The operation of a SAW microsensor is
as follows SAWs are launched onto the delay path (Fig 2a)
via the reverse piezoelectric effect when an RF signal at the
microsensor’s operating frequency is applied to the input
IDT These SAWs travel across the delay path (Fig 2b) to
the output IDT where they are converted back into
electri-cal signals via the piezoelectric effect (Fig 2c) The velocity
and amplitude of the SAWs are the sensor outputs
The acoustic velocity V of any material is a function of
the elastic constant c and density ρ of the material For the
simplest (isotropic) case, the velocity is given by V = (c/ρ)1/2
(5) The relationship between the SAW velocity, frequency
f , and wavelength λ, is given by V = f λ The
aftenua-tion of the waves is primarily a funcaftenua-tion of the viscosity
Delaypath
InterdigitalTransducer
Piezoelectricsubstrate
Figure 1 Diagram of a single channel (delay line) SAW
microsensor.
Delaypath
λ
(a)
Piezoelectric substrate
an electrical component on a piezoelectric substrate ondary physical parameters that affect the previously men-tioned (primary) parameters also affect the SAWs’ charac-teristics They include temperature, stress, pressure, andelectric and magnetic fields A change in the SAW velo-city due to mass on the surface is commonly referred to as
Sec-“mass loading.” Mass loading and amplitude attenuationare the most commonly used sensing mechanisms for SAWsensors and are the primary focus of this review article.The change in SAW velocity VR has been related tothe mass of a thin nonviscous (lossless) film on the sensorsurface by Wohltjen (6)
VR
VR = (k1+ k2) f h ρ , (1)
where VRis the SAW velocity, k1and k2are substrate
ma-terial constants, f is the SAW frequency, and h and ρ arethe height and density of the thin film layer, respectively.Therefore, the change in SAW velocity due to a layer de-
pends on h ρ of the layer Because the units of h ρ are kg/m2,this is also the layer surface densityρs The change in SAW
velocity is determined experimentally by measuring thephase shift,φ or the frequency shift f of the SAWs that
are related to the change in SAW velocity by (6)
α = ω2 n
Trang 22912 SENSORS, SURFACE ACOUSTIC WAVE SENSORS
whereα is the attenuation change, ω2is the angular
fre-quency (2πf), C is a SAW-film coupling constant, and G is
the loss modulus (complex part of the shear modulus) of
the film which is directly related to its viscosity The term
acoustically thin denotes a film that does not resonate at
the SAW microsensor’s operating frequency As film
thick-ness increases, it can resonate at the operating frequencies
A macroscopic analogy for a resonant film would be a plate
of Jello which, if shaken at a certain frequency, will also
resonate when the Jello is high enough
The earliest uses of SAW devices as microsensors were
reported by Das in 1978 (8) for measuring pressure
(phys-ical) and by Wohltjen in 1979 (9) for measuring thin film
properties (chemical) These sensor applications resulted
from the observed high sensitivity of SAW signal
process-ing devices to “external” physical parameters such as
tem-perature changes and package stress, as well as
“inter-nal” properties of the films deposited on the SAW
sub-strate A major application of SAW sensors has been highly
sensitive mass detectors (microbalances) Wohltjen stated
that SAW sensors have a potential mass sensitivity 200
times greater than the better known quartz crystal
mi-crobalance due to their higher operating frequency (6)
The effect of frequency on SAW velocity is illustrated by
Eq (1) SAW devices can operate at frequencies higher
than 1 GHz, compared to about 10 to 50 MHz for the quartz
microbalance which operates by using bulk (shear)
acous-tic waves However, because noise in measurement
elec-tronics increases as frequency increases, the practical
fre-quency limit for SAW sensors may be closer to 500 MHz
SAW and bulk wave devices have significantly different
geometries and fabrication techniques SAW devices are
fabricated by standard microelectronic fabrication
meth-ods, whereas bulk wave devices are manufactured
individ-ually as small disks that have thin metal film electrodes on
each side Thus, SAW devices are typically less expensive
and have a much wider range of designs than bulk devices
The following are additional advantages of SAW sensors
(1) They can be configured in “smart” designs by using two
sensing channels on the same substrate where one is a
reference This allows the sensor to be self-compensating
for interfering environmental parameters such as
tem-perature (10) (2) Because SAW microsensors are sensitive
to several parameters, they can provide an amplified
sen-sor response via multiple detection mechanisms (3) They
are easy to use in wireless sensing applications in both the
active mode, as the frequency control element in a
trans-mitter (8), and the passive mode, as an energy reflector
(11) The passive mode is particularly interesting because
the sensor does not need a power source but is read using
a special FM radar type system SAW devices are also
con-sidered one of the earliest types of microelectromechanical
systems, or MEMS devices MEMS devices are usually
de-fined by having both mechanical and electrical components
or functions in a single unit and are fabricated using
mi-croelectronic fabrication techniques SAW devices fit this
description because they have acoustic waves (mechanical)
that are launched and detected electrically
Commercial SAW microsensor based systems are
currently available for gas and biological sensing,
gas-chromatography vapor sensing, and chilled-surface,
dew-point hygrometry All of these systems capitalize onthe high sensitivity of the SAW microsensor to smallmass changes systems for chemical and biological sens-ing have been developed by Microsensor Systems (now
a subsidiary of Sawtek) (12) These include “Vaporlab,”which uses an array of SAW microsensors coated withproprietary films and pattern recognition to identify thevapor, and the SAW “Minicad,” which uses the sametechniques to detect chemical warfare agents The SAWgas-chromatography system was developed by ElectronicSensor Technology (13) This system uses a single bareSAW microsensor and can be used as an electronic nose inseveral gas sensing applications that have been validated
by the EPA The SAW dew-point hygrometer was developed
by Microconversion Technologies Co (14) This ter, the “Ultra DP5,” also uses a bare SAW microsensor, inthis case to provide precision measurements of water vaporconcentration
hygrome-This article presents a fairly wide range of SAWmicrosensor applications that are based on the personalresearch and development experience of the author Theseapplications include detection of water vapor and othergases; thin polymer film characterization, including adhe-sion, surface properties, and curing; chilled-surface dew-point measurements; measurement of surface energy andcleanliness; and temperature measurement A review ofacoustic wave biosensors has been provided by Andle andVetelino (15) Additional SAW microsensor applicationsare reviewed in books by Ballantine et al (16) and Thomsonand Stone (17) These books also provide more comprehen-sive descriptions of SAW microsensor theory, design, andapplications SAW device design procedures can also befound in the literature (2,3)
EXPERIMENTAL PROCEDURES FOR SAW SENSING
The two most common methods for measuring SAW
velo-city are the phase and frequency techniques (9,16) The
experimental setup for the phase technique requires plying an RF signal (from a signal generator) to the inputIDT of the SAW sensor A vector voltmeter is then used
ap-to moniap-tor both the phase and amplitude of the SAWs, asshown in Fig 3 The experimental setup for the frequencytechnique requires using the SAW sensor as the frequencycontrol element in an oscillator circuit A frequency counter
is then used to monitor the oscillatory frequency, as shown
in Fig 4 The advantages of the phase technique are ease ofuse, stability, and easily obtainable amplitude information
RF signalgenerator
SAWmicrosensor
Vectorvoltmeter
Figure 3 The experimental setup for the phase (vector
volt-meter) technique.
Trang 23SENSORS, SURFACE ACOUSTIC WAVE SENSORS 913
Frequencycounter
SAWmicrosensor
Amplifier
Figure 4 The experimental setup for the frequency (oscillator)
technique.
However, this technique typically involves relatively
ex-pensive laboratory equipment The advantages of the
fre-quency technique are high sensitivity and less expensive
equipment; however, oscillatory stability can be a problem,
and additional circuitry is required to obtain the amplitude
information
DISCRIPTION OF APPLICATIONS
AND EXPERIMENTAL RESULTS
Gas Detection: Water Vapor and Hydrocarbons
One of the most widely studied applications of SAW
mi-crosensors has been measuring gas concentration The
most commonly used configuration consists of a very thin
film (<1 µm) applied to the SAW microsensor These films
are carefully selected or designed to provide both high
sen-sitivity and selectivity to the gas of interest and also
long-term reliability To study the response of the SAW sensor
to gas concentration, the sensors are placed in a chamber
in which the atmosphere is controlled by a gas delivery
system
The measurement of water vapor concentration and
rel-ative humidity have been of high interest for many years
because of their effect on human comfort and health More
recently, the measurement of water vapor has become very
important in several other fields, including meteorology,
agriculture, and manufacturing due to the effects of
wa-ter vapor on weather forecasting, product quality, and
the large energy costs of drying processes Polymer-coated
SAW devices have been studied as an improved means to
measure relative humidity Polymers are good candidates
for sensing films due to their ease of processing, widely
customizable properties, and relatively low cost Polyimide
is a readily available polymer that is widely used in
mi-croelectronic applications It has the advantages of
dura-bility at high temperatures, low dielectric constants, and
ease of application Therefore, it was chosen for this work
Table 1 Maximum SAW Phase Shift of Polyimide-Coated SAW Microsensors for Various Gasesa
Figure 5 Water vapor response of photosensitive and
nonphoto-sensitive polyimide films (15).
In addition, photosensitive polyimides have recently beendeveloped that reduce the number of processing steps re-quired for patterning The major drawback of polyimide
in microelectronic applications is that it typically absorbsmore than 2% by weight of water vapor when placed inhigh humidity However, this property allows its use as ahumidity sensing film
Figure 5 (18) shows the response to water vapor of twoSAW microsensors coated by different polyimide films, onethat was photosensitive and one that was nonphotosen-sitive The photosensitive polyimide had about twice thesensitive to water vapor as the nonphotosensitive poly-imide, as indicated by the maximum phase shifts (at 100%relative humidity) of 35◦ and 18◦, respectively The dif-ference in the responses was attributed to the more openmolecular structure of the photosensitive polyimide (whichcomes in precured form) The higher sensitivity suggeststhat the photosensitive polyimide would be preferred for arelative humidity sensor However the long-term stability
of the film needs to be studied A comparison of a SAW midity sensor with other types of low cost humidity sensorsindicated that SAW sensors have the potential for the high-est sensitivity at low relative humidities but would prob-ably be more expensive than capacitive or resistive typeswhen signal conditioning circuitry is considered (19).Polyimide has also been used to measure hydrocarbonand alcohol vapors Selective hydrocarbon measurement is
hu-of high interest to the petroleum refining industry Table 1summarizes the results of studies that used SAW microsen-sors coated by nonphotosensitive and photosensitive poly-imides to detect three different hydrocarbons, methyl ethylketone (MEK) vapor, and water vapor The maximum SAW
Trang 24914 SENSORS, SURFACE ACOUSTIC WAVE SENSORS
microsensor phase responses are shown for polyimide films
before and after 4 months of aging (18) The responses were
similar in shape to those for water vapor (Fig 5) but
dif-fer in magnitude Neither polyimide was selective among
isooctane, n-octane, and n-heptane, but the
nonphotosensi-tive polyimide had good selectivity between water vapor
and MEK (large responses) versus n-octane, n-heptane,
and isooctane (small responses) Aging had a significant
effect on water and MEK responses for both polyimide
types However, only aging significantly affected the
hep-tane and ochep-tane responses of the photosensitive polyimide
These results suggest that the structure of the
photosen-sitive film may become more open or that its viscoelastic
properties changed due to additional curing, as the
poly-imide film aged Other investigators have used a similar
polyimide as a light guide (20) and have shown that
poly-imide film can select between n-heptane and isooctane gas
molecules This selectivity was attributed to the different
cross-sectional areas of these molecules The poor
selec-tivity of the SAW microsensor between n-heptane and
isooctane was attributed to a different molecular
struc-ture of the film caused by the differences in film
process-ing or thickness from that of the light guide work These
results illustrate some of the key difficulties in
develop-ing appropriate films for chemical sensors, namely, poor
selectivity and long-term stability The most promising
ap-proach to the selectivity problem for many gases appears
to be the use of a sensor array using pattern recognition
such as that used by Microsensor Systems or a
chromatog-raphy system that uses pattern recognition such as that of
Electronic Sensor Systems There is also room for SAW
sensor-based systems designed for specific gases or
ap-plications such as the Microconversion Technologies Co
hygrometer
Polymer Film Characterization: Surface Treatments
and Adhesion
The SAW microsensor has been used to characterize the
ef-fects of surface treatments on thin polyimide films and as
a nondestructive indicator of film adhesion Surface
treat-ments are of high interest because they are commonly used
to modify film properties, particularly surface energy The
surface energy is important because it is directly related
to the adhesion of additional layers to the film and to the
film’s ability to absorb vapors This is particularly
impor-tant in the microelectronics industry The characterization
method consisted of measuring changes in the water vapor
response of the films as a function of the film parameter of
interest
The effects of plasma and chemical surface treatments
on the water uptake of polyimide films are illustrated in
Table 2 (21) which shows the maximum water vapor
re-sponse (100% relative humidity) for polyimide films that
were untreated, sputtered, exposed to KOH, and coated
by Teflon-AF The maximum phase shift for untreated film
was about 40◦ This compares to the smallest response of
about 5◦for Teflon-AF treated film, to about 12◦for
sput-tered film, to a maximum response of about 80◦ for KOH
treated film These results indicate that the surface
treat-ments significantly affect the water uptake of polyimide
Table 2 Maximum Water Vapor Response for Polyimide Film Subjected to Various Surface Treatments
Surface Treatment Phase Change (degrees)
Adhesion of thin films is directly related to film ability Therefore, a method that can measure the adhesion
reli-of thin films nondestructively would be extremely useful.The water uptake response of thin polyimide films wasexamined as a possible nondestructive indicator of film–substrate interfacial characteristics and adhesion The wa-ter uptake response was measured for two polyimide filmswhich were identical except for the surface treatment used
to prepare the substrates before film application For thiswork, a dual channel SAW microsensor was used because
it can directly measure the response difference between
two films The experimental setup used for this study is
a slightly modified version of the vector voltmeter (phase)setup previously described (Fig 3) The modifications in-clude applying the signal generator output to both SAWmicrosensor channels by using a splitter and putting one ofthe vector voltmeter probes at each of the two output IDTs,
as shown in Fig 6 The difference in the water uptake
responses of two polyimide films, one applied over silaneadhesion promoter and one applied without promoter isshown in Fig 7 (22) The positive phase shift indicatesthat less water was absorbed in the film that used pro-moter Because the two films were identical except for theinterfacial region, these results suggest that the adhesionpromoter prevented water from entering the interface andthat a significant amount of water was present at the in-terfacial region of the film/substrate without promoter.This agrees with neutron scattering studies of wateradsorption at similarly treated polyimide/silicon interfaces(23) and suggests that the SAW technique may provide
a simple and nondestructive indication of adhesion thatcould be used in process control
RF signalgenerator
Polyimidefilm
Polyimidefilm
Vectorvoltmeter
Figure 6 A dual delay line SAW microsensor that has two
poly-imide film samples and the experimental setup for the tive phase technique.
Trang 25compara-SENSORS, SURFACE ACOUSTIC WAVE SENSORS 915
0
10
2468
Figure 7 The difference in the water uptake of polyimide films
applied with and without silane adhesion promoter (22).
Note that this dual channel design has been widely
stu-died as a method of adjusting SAW microsensors for
unde-sirable effects and can be considered a “smart design.” For
example, the most common application of the dual
chan-nel design has been for temperature compensation which
is necessary because the SAW velocity for many SAW
sub-strates is sensitive to temperature When the dual
chan-nel design, is used in conjunction with the vector voltmeter
setup in Fig 6, it results in canceling the temperature
re-sponse of the substrate because it is the same for both
chan-nels However, because only one channel can be coated by
a sensing film, the temperature response of the film itself
is not compensated for
Polymer Film Characterization: Curing
and Glass Transition Temperature
Polymer films are widely used in microelectronics as
re-placements for more traditional materials such as
in-organic coatings on integrated circuits and ceramic printed
circuit boards (PCBs) This is due to their low cost, ease
of fabrication, and the ability to modify their properties
easily to ensure compatibility with fabrication processes
The increased use of thin polymer films in microelectronic
applications has resulted in the need for new
characteri-zation methods because these films are much smaller and
thinner than polymer films used previously and are
there-fore not always compatible with existing characterization
techniques For example, the curing processes of some
new high-temperature polymer films are not fully
under-stood Of particular interest are changes in mass and
vis-coelasticity during curing Thermogravimetry, a common
method used to study curing, is the measurement of mass
changes caused by outgassing of solvents and other
chemi-cal changes in polymers during curing It involves heating
the sample while simultaneously weighing it on a precision
balance The balances currently used can measure mass
changes of the order of micrograms The mass changes in
thin polymer films are in the parts per million range, so,
a relatively large amount of the polymer must be tested to
obtain mass changes that are measurable by these
bal-ances This results in measuring the bulk properties of
the polymer which can be significantly different from the
50 100 150 200 250 300 350 400
3002001000
Film resonance
First harmonic
Figure 8 The temperature-compensated phase and amplitude
response of a polyimide film during cure (24).
thin film properties Therefore, a highly sensitive nique is needed to monitor mass and viscoelastic changes inthin polymer films during curing A surface acoustic wave(SAW) system was developed that can measure the masslost due to water outgassing during the cure of thin poly-mer films in a temperature range of 20 to 400◦C It canalso measure the apparent glass transition temperature ofacoustically thin films and film resonance of acousticallythick films The principle limitations of the system werethe limited accuracy of temperature compensation and thelimited ability to separate mass loss effects from viscoelas-tic effects
tech-The SAW sensor used was similar to that in Fig 1, andthe polymer film to be tested was applied to the delay path
A sensor test chamber contained the SAW sensor and aheater and allowed dry nitrogen gas purging to preventwater sorption by the polymers The temperature compen-sation was done by curve fitting the temperature responsedata of an uncoated sensor because this provided muchbetter compensation than the dual delay line technique atthis high temperature range
Figure 8 (24) shows the temperature-compensated plitude and phase responses for a 1.2-µm thick polyimidefilm Both the phase and amplitude initially decreased withincreasing temperature, indicating that the polymer wassoftening, until a minimum in amplitude was reached atabout 135◦C Because the phase continued to decrease atthis temperature, this corresponds to the apparent glasstransition temperature (a function of the sensor operat-ing frequency) described by Martin et al (7) The firstfilm resonance point is indicated by the second amplitudeminimum at 255◦C because it corresponds to a sharp in-crease in phase There was also a phase increase of 43◦between 175 and 210◦C which was attributed to water out-gassing caused by the reaction of the polyamic acid to formpolyimide monomers This agrees reasonably with the pre-dicted 65◦phase change based on the expected mass lostdue to water outgassing It also agrees with work done
am-by others (25) which showed that the water outgassing ofpolyimide during cure occurs between 175 and 225◦C Thedifference in the measured and theoretical phase changemay be accounted for by partial imidization during the softbake of the polymer application process or by further soft-ening of the polymer A second resonance point was also
Trang 26916 SENSORS, SURFACE ACOUSTIC WAVE SENSORS
observed at the third amplitude minimum (and a
concur-rent phase increase) at 335◦C This is the first harmonic
of film resonance This was predicted by others (17), but
it was not previously observed because their studies did
not exceed 150◦C The further large increase in phase at
higher temperatures may indicate the curing reaction of
the polyimide in which polyimide monomers combine to
form polymer chains This would cause hardening of the
polymer and therefore, an increase in phase
In summary, this system can measure the mass lost due
to water outgassing during cure of thin polymer films to
2% of total polymer mass in a temperature range of 20 to
400◦C It can also measure the apparent glass transition
temperature of acoustically thin films, and film resonance
including the first harmonic of acoustically thick films
The principle limitations of the system are the accuracy
of temperature compensation, how well mass losses can
be separated from viscoelastic effects, and how well the
glass transition temperature can be separated from
acous-tic resonance When water outgasses, the apparent glass
transition temperature and film resonance occur at
differ-ent temperatures as they did in this polyimide study, and
the responses are distinguishable However, should a
poly-mer be tested in which the mass loss occurs at the same
temperature as film resonance, the mass loss is most likely
to be masked by the larger film resonance response
There-fore, this system can provide a powerful technique for thin
polymer film analysis, but the user must have some idea
what to expect and the system’s limitations Improvements
to this system could include identifying SAW substrate
ma-terials by linear temperature responses using lithium
nio-bate or lithium tantalate
Dew-Point Measurement
The SAW microsensor has proven to be a very useful
tool for studying water vapor condensation and
measur-ing the dew point Dew-point measurement provides one
of the most accurate and widely used methods for
mea-suring the absolute water vapor content of a gas This
has traditionally been done by using an optical
chilled-mirror, dew-point hygrometer (25) These instruments
de-tect condensation and dew-point by measuring changes
in the reflectivity of a condensing surface (26) Although
dew-point hygrometers perform better than polymer-film
based and other types of resistive and capacitive humidity
sensors, they have some drawbacks, including instability
due to mirror contamination, inability to detect the
frost-point transition, limited resolution, and high cost A SAW
microsensor-based, dew-point hygrometer offers a
chilled-surface technique for dew-point measurements that has
improved performance at lower cost The SAW
microsen-sor’s small size also suggests its application for dew-point
measurements inside small structures such as
microelec-tronic packages where water vapor can affect yield and
device reliability A description of the SAW dew-point
grometer and some examples of the SAW dewpoint
hy-grometer’s advantages follow
To study condensation using the SAW microsensor, two
changes were made to the previous test setups First, the
frequency technique was used for some of the studies due
to its expectedly higher sensitivity The principal design
difference between a frequency device and the phase vice shown in Fig 1 is a much shorter delay path This al-lows only one mode of oscillation Sensitivity comparisons,based on theory, indicate that a surface density of 1µg/cm2results in a 10◦SAW phase shift for a 80 MHz phase de-vice versus a 1.43 kHz frequency shift for a 50 MHz fre-quency device Assuming a 0.1◦phase resolution and 1 Hzfrequency resolution for the phase and frequency systems,respectively, the frequency system would provide about
de-10 times better resolution The second change was adding
a temperature control system which was required to lowerthe temperature of the SAW sensor until water (dew) con-densed on its surface The temperature at which this oc-curs is defined as the dew point (26) A thermoelectriccooler and a PC-based data acquisition and control systemwere included that detected the amount of condensation onthe SAW sensor and then maintained the desired amount
of condensation so that an accurate reading of the perature could be made The SAW velocity change, whichcorresponds directly to the condensation density on thesensor, was used as the feedback parameter to maintainthe predetermined condensation density that was specifiedvia the control software The desired condensation densitywas determined by balancing the need for fast responsetime (less condensation) and minimizing dew-point mea-surement error (more condensation) Dew-point error isdiscussed in the section on surface energy and cleaning.The temperature of the sensor was measured by using
tem-a resistive tempertem-ature device (RTD) A ditem-agrtem-am of thissystem is shown in Fig 9, and includes an optical micro-scope that was used to image the condensation and a LED-phototransistor setup that is discussed later
The ability of the SAW microsensor to measure densation density accurately was examined by correlat-ing SAW microsensor measurements of condensation withoptical microscope images (27) The condensation density(g/cm2) was determined from the optical images by esti-mating the total mass of all water drops in a specified area
con-of the image using drop diameter and contact angle formation The contact angle of the water drops was used
in-to obtain drop height The SAW frequency was linearlyrelated to the condensation density, the sensitivity wasabout 1.5 Hz/ng/cm2, and the minimum mass resolutionwas 18.5 ng/cm2 This mass resolution is more than anorder of magnitude smaller than optical techniques
RF signalgenerator
Vectorvoltmeter
Personalcomputer
LEDGasinlet
Front view
RTD
PhototransistorSAW sensorThermoelectricdeviceHeat sinkMicroscope
Figure 9 Diagram of the hybrid SAW/optical dew-point
mea-surement system.
Trang 27SENSORS, SURFACE ACOUSTIC WAVE SENSORS 917
A novel hybrid SAW-optical system was developed to
obtain meaningful comparisons between the performance
of the SAW and optical chilled-mirror, dew-point
mea-surement techniques This system permitted simultaneous
measurement of condensation using both the SAW and
op-tical methods The design of this system involved adding a
light emitting diode (LED), a phototransistor light
detec-tor, and an aluminum-mirror film to the SAW sensor delay
path Condensation on the aluminum mirror changed the
phototransistor output voltage which was used in a
feed-back control system, similar to that used for SAW
veloc-ity, to maintain the mirror at the dew point This setup
is also shown is Fig 9 The hybrid SAW/optical system
allowed direct comparisons between the SAW and
opti-cal chilled-mirror techniques of sensitivity and the effects
of surface contamination and the frost-point transition on
dew-point measurements One of the key findings was that
the SAW system could maintain a constant condensation
density on the sensor without the dew coalescing (small
dew drops combining into fewer large drops) This was
ob-served by microscope while controlling the condensation
density by using the SAW velocity Conversely, the
opti-cal technique could not maintain a constant condensation
density, and coalescence of the dew deposit resulted along
with an increase in the SAW phase Figure 10 (27) shows
the reflections voltage and condensation density plotted as
a function of time as the dew point was lowered from−10
to−20◦C These data indicate that the condensation
den-sity increased by approximately 100% while the reflection
voltage was held constant (to within 0.1%) Microscopic
imaging indicated that the condensate was dew
(super-cooled) and not frost The condensation density change for a
constant reflection voltage indicated that reflection voltage
was not proportional to condensation density This lack of
a direct relationship can adversely affect control system
stability and result in coalescence of the dew which, in
turn, can further adversely affect control system
stabil-ity These effects typically result in the need for a
dry-off cycle when using optical techniques Therefore, an
advantage of the SAW velocity technique is direct
mea-surement of condensation surface density which results
0246
8Reflection voltage
Condensation density
Figure 10 Condensation density variation when controlled by
optical detector voltage (27).
in a more stable system than the indirect ments provided by the optical and SAW attenuationtechniques
measure-The effect of surface contamination on dew-point surements was also examined The amount of condensa-tion which caused the same (60 mV) change in the opti-cal detector voltage for clean and contaminated surfaces,respectively, was studied by using SAW phase and micro-scopic images (27) The clean surface had 5.5µg/cm2 ofcondensation versus more than 30µg/cm2for the contami-nated surface By comparison, when the SAW sensor wasused to measure the dew point, the condensation density(measured with microscopic images) did not change as thesurface became contaminated This indicates that a SAWdew-point sensor can provide more stable and accuratedew-point measurements in dirty environments
mea-The dew to frost-point transition and its effected onSAW velocity and amplitude and optical detector voltagewere also examined (27) The frost-point transition ob-served by a microscope began at about −23.5◦C, as indi-cated by a mixture of dew and ice crystals on the sensorsurface Therefore, supercooled dew was present beforefrost formed The dew became completely frozen at
−24.2◦C This transition occurred during a period of aboutsix minutes Detection of the frost-point transition around
−23◦C is in agreement with that of other investigatorswho reported frost-point transition temperatures rangingfrom−18 to 28◦C, depending on the condensation densityand gas flow rate (28,29) As the dew deposit froze, it alsocoalesced, whereas the SAW velocity simultaneously re-turned (increased) to approximately the same value as thatwhen no condensation was present This increase was at-tributed to the different acoustic velocities of ice versuswater and the observed coalesced state of the frost de-posit, which resulted in a much smaller contact area withthe sensor surface This resulted in significant instability
of dew-point measurements during the frost-point sition Similar instability occurred when controlling thecondensation density by using the optical reflection volt-age However, it was found that SAW amplitude could beused to maintain constant condensation density during thefrost-point transition, thus providing continuous dew-pointmeasurements, as well as an indication of the frost-pointtransition
tran-The resolution of the SAW dew-point hygrometer (MCT)was compared with those of EG&G (now Edgetech) Model
2000 and General Eastern Model Hygro-M3 optical point hygrometer Figure 11 (13) shows a constant dewpoint measured by the three hygrometers The resolution
dew-of the SAW microsensor was about±0.02◦C verus±0.2◦Cfor the EG&G and±0.5◦C for the General Eastern The su-perior performance to the SAW hygrometer was attributed
to its direct and more precise measurement of tion density The accuracy differences were expected to beresolved by recalibration
condensa-The Measurement of Surface Energy and Effects
of Surface Cleaning
A SAW microsensor technique based on the dew-point tem was also examined as a novel method for measuringsurface energy and the effects of surface cleaning The
Trang 28sys-918 SENSORS, SURFACE ACOUSTIC WAVE SENSORS
Figure 11 The dew point measured by the
SAW-based hygrometer and two commercial
op-tical dew-point hygrometers (EG&G Model 2000
and General Eastern Model Hygro-M3) (14).
291919.219.419.619.82020.220.420.620.821
measurement of the surface energy of thin metal and
polymer films is of high interest in the microelectronics
industry due to its relationship to film cleanliness and
adhesion and ultimately, to microelectronic device
reliabi-lity (31) Surface energy is also important for
determin-ing the biocompatibility of materials and for developdetermin-ing a
better understanding of gas sorption on sensing surfaces
Surface energy is defined as the energy spent to create a
surface and is closely related to the reactivity or
wettabil-ity of that surface Wettabilwettabil-ity indicates how well a liquid
spreads across a solid surface (32) The presence of organic
films or contamination on a surface can also be measured
because they usually change the surface energy of a
ma-terial Plasma cleaning is commonly used to remove
or-ganic and mobile ion contamination from integrated
cir-cuits and multichip modules (MCMs) before encapsulation,
as an alternative to solvents and detergents that typically
contain chlorofluorohydrocarbons (CFCs) and other
haz-ardous materials Plasma treatments have also been used
to improve the adhesion of conductors and insulators by
changing the surface structure and energy of the metal or
polymer layer (33) Improved methods for measuring the
results of plasma treatments and cleaning are important
for optimizing these processes
The traditional method for obtaining information about
contamination, surface energy, and wettability of solid
ma-terials and the effects of plasma treatments is contact angle
measurements The contact angle is the angle created at
the liquid–solid–vapor interface when a drop is placed on a
solid surface A surface that has a small contact angle wets
better and has a higher surface energy than a surface that
has a large contact angle Small and large contact angles
and the effects of surface treatments on them can easily be
observed by examining the interface of water drops on the
hood of a car before and after it is waxed, respectively
Con-tact angle is commonly measured by dispensing a fluid on
the surface in question and observing the drop by using a
goniometer The advancing angle is measured as the drop
volume is increased, and the receding angle is measured as
the drop volume is decreased The difference between theadvancing and receding angle is known as contact anglehysteresis Contact angle hysteresis is caused by surfacemicroroughness and heterogeneity and by chemical inter-actions between the surface and the fluid (32) The prin-cipal limitations of contact angle measurements are hys-teresis, difficulty in measuring angles less then 20◦, andpoor reproducibility resulting from variations in operatortechnique Detailed reviews of contact angle and surfaceenergy are provided in (32,34)
The purpose of these SAW microsensor studies was todetermine if a technique for measuring the surface en-ergy and the effects of surface treatments for common elec-tronic materials could be developed that did not suffer fromthe errors typical of contact angle measurements The ex-perimental technique used was to measure the dew pointwhile maintaining extremely small condensation densities
on the sensor surface This resulted in a measured dewpoint that was higher than the true dew point, dewpointerror, which can then be compared for various surfaces Al-ternatively, the condensation density on various surfaces
at a specific temperature above the dew point can also becompared The ability of the SAW microsensor to mea-sure a dewpoint error was first demonstrated by coating
a SAW sensor with known highly hydrophobic (low face energy and high contact angle) or hydrophilic (highsurface energy and low contact angle) materials Films
sur-of Teflon-AF and polyimide 1µ thick were used,
respec-tively Figure 12 shows the deviation from the dew point(dew-point error) as a function of condensation density forquartz, polyimide, and Teflon-AF surfaces (27) The dew-point error was greater for the hydrophilic material (poly-imide) than for the hydrophobic material (Teflon-AF).For example, at a condensation density of 0.2µg/cm2,the dew-point error for polyimide was approximately 2◦Cversus 0◦C for the Teflon-AF This inversely correlates withthe contact angles of 30◦and 90◦for polyimide and Teflon-
AF, respectively, as expected Noted that the temperature
of the SAW sensor was at the true dew point for all surfaces
Trang 29SENSORS, SURFACE ACOUSTIC WAVE SENSORS 919
0.0
−2
0246810
Figure 12 Deviation from the dew point (dew-point error) as a
function of condensation density for quartz, polyimide, and
Teflon-AF surfaces (27).
when the condensation density exceeded 0.8 µg/cm2 A
dew-point measurement error does not occur when large
dew densities are present because condensation forms on
existing drops of water
The ability of SAW dew-point error measurements to
measure surface energy and the effects of various surface
treatments on quartz is illustrated in Table 3 (35), which
shows contact angles and dew-point errors at 0.1 µg/cm2
for several sensor surface treatments listed in order of
de-creasing contact angle There appears to be reasonably
good correlation between decreasing contact angle and
in-creasing dew-point error, considering the previously
men-tioned limitations of contact angle measurements The
most significant deviation was HCI which had the largest
contact angle but not the smallest dew-point error
Con-tamination between experiments is a possible source of
error for all measurements These results suggest that
dew-point error may provide a measurement of surface
en-ergy which does not suffer from hysteresis, however, more
work is necessary in this area to establish a more definitive
relationship
Temperature Measurement
Acoustic temperature sensors (ATSs) use the temperature
dependence of the acoustic velocity in a piezoelectric
sub-strate to measure temperature Their principle advantages
Table 3 Contact Angles and Dew-Point Error for Several SAW Sensor Surface Treatmentsa
Advancing Contact Angle Receding Contact Angle Dew-Point Error
Figure 13 The frequency versus temperature response for an
SSBW ATS (36).
over other types of temperature sensors are ease of tegration with other acoustic sensors, low self-heating,robustness, wide temperature range, a digital form of out-put, low noise susceptibility when used in an oscillator,relatively low cost, high resolution, and short responsetimes Their main disadvantage is that they require her-metic packaging to prevent inaccuracies caused by sen-sor contamination This slows the response time and in-creases cost Surface skimming bulk wave (SSBW) de-vices have been found much less sensitive to surface con-tamination than other acoustic devices because the waveshave horizontal displacements and can propagate just be-neath the surface of the substrate Therefore, they maynot require hermetic packaging However, these deviceshave not been studied as temperature sensors The objec-tives of this work were to study the temperature charac-teristics and contamination sensitivity of an SSBW ATS.The introductory section provides a description of thiswave
in-Figure 13 shows the frequency versus temperature sponse for an ATS characterized in a Styrofoam cham-ber (36) A second-order curve fit yielded a first-ordertemperature coefficient of frequency (TCF) of approxi-mately 31.5 ppm/◦C The temperature deviation from thesecond-order curve fit indicated a resolution of ±0.22◦Cacross a 78◦C temperature range The TCF of 31.5 ppm/◦Ccould provide a theoretical temperature resolution of bet-ter than 0.0003◦C if a 1 Hz frequency resolution is avail-able Figure 14 shows the test setup used to compare the
Trang 30re-920 SENSORS, SURFACE ACOUSTIC WAVE SENSORS
Signalgenerator
Signalgenerator
Vectorvoltmeter
Vectorvoltmeter
Figure 14 The test setup used to compare the mass sensitivities
of SSBW and SAW microsensors (36,37).
mass sensitivities of SSBW and SAW acoustic delay lines
(36) A novel dual delay line design was used where a SAW
channel was aligned at 90◦to the SSBW channel (37) The
phase shift of the SSBW due to mass loading was
deter-mined by maintaining a fixed surface density of condensed
water by using the SAW device (as described in the
previ-ous section) while monitoring the phase shift of the SSBW
device using a vector voltmeter Figure 15 (36) shows the
phase shift due to mass loading for the SSBW and SAW
delay lines The SSBW phase shift was more than an
or-der of magnitude less than that of the SAW for the same
mass loading Assuming a TCD of 32 ppm/◦C, 5µg/cm2of
contamination would result in a temperature error of 16◦C
for SAW versus 0.4◦C for SSBW ATSs, respectively The
SSBW response was attributed more to the effect of the
water on IDT capacitance than mass loading, and
there-fore, it is most likely that it can be significantly reduced
by a protective coating These results suggest that SSBW
ATSs may have applications in acoustic chemical sensing
where temperature information is also desired Note that
ATSs do require calibration and that the procedures are
not simple because SAW microsensor calibration can be
affected by the electronic circuitry used
0
1009080706050403020100
2
4Mass loading (µg /cm2)
The ATS is the application most likely to be cialized next It will be most useful when used in con-junction with other SAW-based chemical sensors due tocost considerations Commercial applications for coatedSAW microsensors beyond those described in the intro-duction hinge on the other key chemical sensor require-ments of selectivity and long-term reliability Further ad-vances here are most likely to come from improved filmchemistry which has been a relatively slow process Thehigh sensitivity of the SAW microsensor to condensation
commer-at tempercommer-atures slightly above the dew point and the lack
of hysteresis of the dew-point measurement suggest that itmay have advantages over contact angle for measuring sur-face energy and for characterizing certain polymers, metalsurfaces, and surface treatments, if further developed Pos-sible applications of this SAW microsensor system include
in situ monitoring of surface and interfacial treatments for
process control in microelectronics manufacturing ilarly, the use of a SAW microsensor for characterizingthin polymer films is also promising but requires furtherdevelopment
Sim-BIBLIOGRAPHY
1 S.M Sze, Semiconductor Sensors Wiley, NY, 1994.
2 C Campbell, Surface Acoustic Wave Devices and Their Signal
Processing Applications Academic Press, Boston, 1989.
3 D.P Morgan, Surface-Wave Devices for Signal Processing.
Elsevier, Amsterdam, 1991.
4 B.A Auld, Acoustic Fields and Waves in Solids Krieger,
Malabar, FL, 1973, 1990.
5 G.S Kino, Acoustic Waves: Devices, Imaging, and Analog
Sig-nal Processing Prentice-Hall, NJ, 1987.
6 H Wohltjen, Sensors and Actuators 5: 307–325 (1984).
7 S.J Martin, G.C Frye, and S.D Senturia, Anal Chem 66:
2201–2219 (1994).
8 P Das, C Lanzl, and D Barone, IEEE Ultrasonics Symp., Sept.
1978, Piscataway, NJ, pp 458–463.
9 H Wohltjen, Anal Chem 51(9): 1458–1475 (1979).
10 A Bryant, D.L Lee, and J.F Vetelino, IEEE Ultrasonics
Symp., 1981, pp 171–174.
Trang 31SHAPE MEMORY ALLOYS, APPLICATIONS 921
11 X.Q Bao, W Burkhard, V.V Varadan, and V.K Varadan,
IEEE Ultrasonics Symp., Denver, CO, Oct 1987, pp 583–
585.
12 Sawtek, Orlando, FL 32860, www.microsensorsystems.com.
13 Electronic Sensor Technology, Newbury Park, CA
16 D.S Ballantine, R.M White, S.J Martin, A.J Ricco,
E.T Zellers, G.C Frye, and H Wohltjen, Acoustic Wave
Sen-sors Academic Press, San Diego, 1997.
17 M Thompson and D.C Stone, Surface-Launched Acoustic
Wave Sensors Wiley, NY, 1997.
18 P.R Story, D Nesthus, R.D Mileham, C Feger and D.W.
Galipeau, Soc Plast Eng Annu Tech Conf Indianapolis, IN,
24 R.D Mileham, J.D Sternhagen, and D.W Galipeau, IEEE
Trans Ultrasonics Ferroelectrics, and Frequency Control 45:
1273–1280 (1998).
25 J.J McAndrew and D Boucheron, Solid State Technol 55–60
(Feb 1992).
26 R.G Wylie, D.K Davies, and W.A Caw, in Humidity and
Mois-ture, Vol 1, A Wexler and R.E Ruskin, eds., Reinhold, NY,
1965, pp 125–134.
27 K.A Vetelino, P.R Story, R.D Mileham, and D.W Galipeau,
Sensors and Actuators B 35–36: 91–98 (1996).
28 L.C Paine and H.R Farrah, in Humidity and Moisture, Vol 1,
A Wexler and R.E Ruskin, eds., Reinhold, NY, 1965, pp 174–
188.
29 T.I Inamatsu, C Takahashi, and T Furuya, Proc 1985 Int.
Symp Moisture Humidity, Washington, DC, 1985, pp 379–
382.
30 EG&G Moisture and Humidity Systems, EG&G Dewpoint
Er-ror Analysis, EG&G Applications Data, EG&G Moisture and
Humidity Systems, Burlington, MA, 1976.
31 K.A Vetelino, P.R Story, B.L DeClercq, and D.W
Gali-peau, Int J Microcircuits Electron Packag 19: 212–218
(1996).
32 R.J Good, in Contact Angle, Wettability and Adhesion, K.L.
Mittal, ed., VSP, Utrecht, The Netherlands, 1993.
33 C.P Wong, Polymers for Electronic and Photonic Applications.
Academic Press, San Diego, 1993.
34 S Wu, Polymer Interface and Adhesion Marcel Dekker, NY,
1982.
35 L.D Eske and D.W Galipeau, Colloids Surf A, 154: 33–51
(1999).
36 C.E Wold, J.D Sternhagen, R.D Mileham, K.D Mitzner, and
D.W Galipeau, IEEE Ultrasonics Symp., 1999.
37 US Pat Pending, 1999, D.W Galipeau and R.D Mileham.
SHAPE MEMORY ALLOYS, APPLICATIONS
CAROLYNRICE
Cordis-NDC Fremont, CA
INTRODUCTION
Shape-memory alloys have been engineered for tions and devices since the first discovery of the shape-memory effect in the 1930s The majority of this designactivity was initiated by the discovery of Nitinol (nickel–titanium alloy) in 1962, and since then more than 10,000patents have been issued for applications using shape-memory alloys (1) This article reviews a number of theseapplications, discusses aspects of design, and illustratesrepresentative examples
applica-TYPES OF SHAPE-MEMORY ALLOYS General Theory
Shape-memory alloys (SMAs) are known primarily for onefundamental and unique property—the ability to remem-ber and recover from large strains without permanentdeformation Unlike most conventional metals that re-cover less than 1% strain before plastic deformation, SMAsundergo a diffusionless, thermoelastic martensitic phasetransformation that enables the material to deform via atwinning process rather than the conventional dislocationslip mechanism and allows complete recovery of strains aslarge as 8% The metallurgical phenomena that explainthese martensitic transformations are detailed in manysources (2–5) The discussion to follow includes only a briefsummary of SMA behavior to review its basic properties
Shape-Memory Effect The most well-known form of
transformation behavior exploited in SMAs is thermallyinduced shape change, often labeled the shape-memoryeffect (SME) A material component may be deformed,
or strained, at low temperatures, and when heated, itreverses this strain and remembers its prestrained shape.The low-temperature, deformable martensite phase trans-forms to a more stable austenite phase at higher tem-peratures This transformation occurs across a tempera-ture range, known as a transformation temperature range(TTR) This range for Nitinol (Ni–Ti) is approximately 30 to
50◦C, and is also known as temperature hysteresis The As(austenitic start) temperature is the beginning of the trans-formation to austenite upon heating, the Af (austeniticfinish) is the finish of the transformation to austenite,
Ms(martensitic start) is the beginning of the martensitictransformation upon cooling, and Mf (martensitic finish)
is the finish of the transformation to martensite A cal SMA stress–strain curve, depicted in Fig 1a, demon-strates shape-memory behavior at temperatures below thematerial Mftemperature A schematic example of a shape-memory application is shown in Fig 1b
Trang 32typi-922 SHAPE MEMORY ALLOYS, APPLICATIONS
AppliedLoading
AppliedHeating
InitialPosition
InitialPosition
Superelastic Effect Shape Memory Effect
Figure 1 Typical SMA behavior in tensile tests and bending applications: (a) stress-strain curve
of shape memory (martensite) material, (b) schematic of a shape memory application, (c) stress–
strain curve of superelastic (austenite) in tension, (d) superelastic behavior in a bending application.
Shape-memory alloys may also be trained to exhibit
a two-way shape memory effect Similar to the thermal
shape-memory effect, two-way shape memory (TWSM)
requires special thermomechanical processing to impart
shape memory in both martensitic and austenitic phases
A trained shape in the austenitic phase reverts to a
sec-ond trained shape upon cooling, allowing the material to
cycle between two different shapes This TWSM is
theoret-ically ideal for many shape-memory applications; however,
practical uses are limited due to behavior instability and
complex processing requirements
Superelastic Effect This effect, known also as
pseudoe-lastic, describes material strains that are recovered
isothermally to yield mechanical shape-memory behavior
The phenomenon is essentially the same as the thermal
shape-memory effect, although the phase transformation
to austenite (Af) occurs at temperatures below the expected
operating temperature If the austenitic phase is strained
by an applied load, a martensitic phase is induced by
stress, and the twinning process occurs as if the material
had been cooled to its martensitic temperature When the
applied load is removed, the material inherently prefers
the austenitic phase at the operating temperature, and
its strain is instantly recovered A typical stress–strain
curve is depicted in Fig 1c, and a schematic example of asuperelastic application is shown in Fig 1d The stress–strain curve indicates a difference in stress levels dur-ing loading and unloading, that is known as superelasticstress–strain hysteresis
Alloys
Several alloys have been developed that display ing degrees and types of shape-memory behavior Themost commercially successful have been Ni–Ti, Ni–Ti-Xand Cu-based alloys, although Ni–Ti and ternary Ni–Ti–Xalloys are used in more than 90% of new SMA applications(6) Ni–Ti alloys are more expensive to melt and producethan copper alloys, but they are preferred for their duc-tility, stability in cyclic applications, corrosion resistance,biocompatibility, and higher electrical resistivity for resis-tive heating in actuator applications (6)
vary-The most common Cu-based alloys, Cu–Al–Ni and Cu–Zn–Al, are used for their narrow thermal hysteresis andadaptability to two-way memory training Ni–Ti ternaryalloys are used to enhance other parameters Examplesinclude Ni–Ti–Nb for wide thermal hysteresis, Ni–Ti–Fefor extremely low TTR, Ni–Ti–Cr for TTR stability duringthermomechanical processing, and Ni–Ti–Cu for narrowthermal hysteresis and cyclic stability (7)
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Material Forms
SMAs are manufactured in many of the conventional forms
expected of metal alloys: drawn round wire, flat wire,
tubing, rolled sheet, and sputtered thin films Additional
forms include shaped components, centerless ground
ta-pered wires and tubing, alternate core wire (Ni–Ti filled
with a conductive or radiopaque material), PTFE coated
wire, stranded wire, and embedded composites At present,
Ni–Ti–X alloys are the most readily available in all of these
forms
The processing of SMA material is critical for
optimiz-ing shape-memory behavior Many adjustments can be
made to optimize the properties of a material form for a
particular application; however, most efforts are made to
optimize a balance of strain recovery, ductility, and
ten-sile strength SMAs such as Ni–Ti are melted using
ex-treme purity and composition control, hot worked to bars
or plates, cold worked to their final form, and subjected to
specialized thermomechanical treatments to enhance their
shape-memory properties
DESIGNING WITH SHAPE MEMORY ALLOYS
Shape-memory alloys have intrigued engineers and
inven-tors for more than 30 years One might conclude from the
large number of SMA patents that have been issued and
the knowledge that relatively few of the ideas have been
commercially successful that the majority of these designs
have not fully accounted for the unique behaviors,
limita-tions, and constraints of SMAs The focus of this section is
to highlight the properties best used in SMA applications
and to discuss SMA design considerations
Functional Properties
SMA applications are often categorized in terms of the
spe-cific material property used The majority of these
pro-perties are either thermal shape memory or mechanical
shape memory (superelastic), but some unique properties
are only indirectly related to these shape-memory effects
General categories of applications are classified according
to these properties
Shape Memory The thermally activated ability of a
shape memory material to change shape yields several
types of applications that can be summarized in three
dis-tinct categories: applications that use the shape change to
display motion, those that actuate, and those that harness
stresses produced from constraining the recovery of the
shape-memory material
Displayed motion, also referred to as free recovery,
de-scribes applications that exploit the pure motion of thermal
shape memory (8) An example of this application, a moving
butterfly, is displayed in Fig 2 These butterflies, produced
by Dynalloy, Inc., use a specially processed form of Ni–Ti
wire to move wings back and forth for thousands of cycles
without significant signs of fatigue This processed wire,
known as FlexinolTM, changes shape via cyclic heating by
electric current The small mass of the butterfly body is
sufficient to extend the Ni–Ti wire when cooled, but the
Ni–Ti wire can contract and close the wings when heated
to its stronger austenitic shape
Actuation applications are designed to perform work
A simplified example is a mass suspended from a memory tension spring When cooled, the weaker marten-sitic phase deforms, and the spring is extended by the mass.When heated to austenite, the spring recovers its shapewith forces sufficient to lift the weight, resulting in actua-tion that performs work
shape-Constrained recovery applications use the change inmaterial strength from martensite to austenite to pro-duce a stress that can be harnessed as a clamping force
A popular example of a constrained recovery application
is a shape-memory coupling which is expanded at lowtemperatures, then heated to shrink and clamp to join twopipes
Superelasticity Unlike thermal shape-memory
applica-tions, which can be categorized into several types, cations that exploit this mechanical shape memory aredefined as those that require high strain recovery at
appli-Figure 2 Photograph of a FlexinolTM actuated butterfly tesy of Dynalloy, Inc.).
Trang 34(cour-924 SHAPE MEMORY ALLOYS, APPLICATIONS
Figure 3 Suture retrieval loops designed to recover their shape once deployed from a 6 fr cannula
(courtesy of Shape Memory Applications, Inc.).
operating temperatures Many examples of applications
that use superelasticity are found in the medical
indus-try (Fig 3), but one of the most well known is found in
consumer eyeglass frames marketed as Flexon® by
Mar-chon Eyewear, Inc (Fig 4)
Energy Absorption for Vibrational Damping An energy
absorbing ability found in both the martensitic and
austenitic phases of SMAs is indirectly related to their
shape-memory behavior The energy absorption of SMA
materials has demonstrated excellent vibrational
damp-ing characteristics, which can be harnessed for use in
various damping applications The types of devices that
exploit this property are classified in three categories of
damping : martensitic, martensitic transformation, and
superelastic
Martensitic damping devices operate by using only the
martensitic phase of SMAs Energy is absorbed by the
martensite during its twin reorientation process, and
acco-mmodates large strains for high-amplitude, low-frequency
loading They offer the best damping characteristics ofthe three categories, and although they cannot recoverlarge strains without subsequent heating, they provideexcellent damping properties across a broad temperaturerange
Martensitic transformation damping elements are signed to operate near martensitic transition temperaturesfor peak performance in vibrational attenuation Thispeak is due to a sharp increase in internal friction dur-ing the martensitic phase transformation These dampingelements offer ideal properties for low-amplitude, high-frequency vibrations within a small operating temperaturerange (9) This type of device could be used in ski materi-als to damp vibrations when the ski is in contact with snow(6)
de-Superelastic damping devices use the plateau sis portion of the stress–strain curve for properties similar
hystere-to those of a rubber band Superelastic SMA materials arepretensioned to reach this stress–strain plateau, and anyadditional strains are accommodated easily by changes in
Trang 35SHAPE MEMORY ALLOYS, APPLICATIONS 925
Figure 4 Deformation resistant eyeglass frames (courtesy of
Marchon Eyewear, Inc.).
the applied load This property optimizes a combination of
damping capacity, shape recovery, and temperature range
of operation (9) Unlike martensitic damping elements,
superelastic devices recover their original shape when
vi-brational loading is removed Suggested superelastic
ten-sioning devices are presented in a U.S patent regarding
hysteretic damping (10); one example is shown in Fig 5 (9)
These SMA tension elements cycle through a
superelas-tic stress–strain hysteresis to dissipate energy and serve
as a damping mechanism Vibrations due to
environmen-tal impacts such as violent winds and earthquakes deform
the tensioned elements, and when the vibrational impact
is lessened, the elements recover their shapes
Cavitation-Erosion Resistance Cavitation erosion is a
phenomenon that affects equipment and machinery in
many industries Small bubbles explode with large
Figure 5 Schematic of a superelastic damping device, using
loops of SMA wire in tension Reprinted with permission from D.E.
Hodgson and R.C Krumme, Damping in Structural Applications,
SMST Proceedings, 1994.
impacts, causing pitting and erosion in metallic surfacesand reducing the service life of expensive equipment Boththe martensitic and austenitic phases of SMAs have dis-played cavitation-erosion resistance; they recover from im-pact and minimize material loss when exposed to vibratorycavitation Studies that explored the performance of Ni–Ti
on stainless steels have indicated that both martensiticand austenitic Ni-Ti have significant potential for coveringand protecting equipment that suffers wear from cavita-tion erosion Ni–Ti cladding could be used in applicationssuch as machinery, hydraulics, large hydroelectric genera-tor turbines, and ship propellers (11)
Low Elastic Modulus The martensitic phase of SMA
ma-terials is soft and pliable, in contrast to the stiff, springycharacteristics of the austenitic phase This softness, orlow effective (nonlinear) elastic modulus, is often used
in applications that require deformability and excellentfatigue characteristics This property is exploited alone
or in conjunction with a shape-memory effect in resistant applications
fatigue-An example of a low elastic modulus application isshown in Fig 6: a martensitic tool developed by St JudeMedical, Inc., is used by surgeons during open heartsurgery to orient a tissue-restraining device During thisprocedure, surgeons must make adjustments to optimizethe tool geometry for each patient, and the use of SMAsallow surgeons to bend the martensitic handle to an ap-propriate angle Upon completing the operation, the tool issterilized in an autoclave where it is exposed to elevatedtemperatures and reverts to its trained, austenitic shape.Due to its ability to recover large strains repeatedly, thesetools are marketed for both fatigue resistance and shape-memory properties
Design Constraints and Considerations
When assessing a potential design challenge, designers areoften anxious to develop a solution that uses the uniqueand exciting properties of SMAs It is critical, however, fordesigners to understand the complexity of SMA behavior
As a general rule, if conventional materials and designscan be applied to yield an acceptable and desirable result,the use of SMAs to provide an alternative solution willincrease complexity and cost SMAs are best used whentheir unique properties are necessary for design success—when conventional materials cannot meet the demands ofthe application
The design of SMA applications requires more than ditional design techniques and textbook methods Due tothe many unique properties of SMA materials, several con-siderations specific to SMA design must be addressed andaccounted for This section discusses the majority of issuesthat should be addressed before designing an applicationusing SMAs
tra-General Guidelines
Recoverable Strain The expected recoverable strain of
SMA material must be within the limitations of the alloy