Volume 12 - Fractography Part 8 doc

49 Fatigue fracture surfaces of pearlitic and ferritic ductile irons.. 50 Comparisons of fatigue and monotonic tension, bending, impact fracture surfaces for various ferritic ductile ir

ASM

Handbook Materials Characterization

Introduction: Atlas of Fractographs

Table 2 Causes of fractures illustrated in the Atlas of Fractographs for various ferrous and nonferrous alloys

Material

Parts Test specimens

Total Dimple rupture

Cleavage Fatigue (a) Decohesive

rupture (b)

4

5

4

8

4

7

ASM Handbook Metals Handbook Failure Analysis and Prevention

Table 3 Causes of fractures or failures illustrated in the Atlas of Fractographs for various engineered materials

Material

Parts Test specimens

Total Ductile Brittle Fatigue Mixed mode

1

1

8

3

8

Pure Irons: Atlas of Fractographs

Pure Irons

Fig 1 Grain-boundary cavitation in iron This is the mechanism by which metals typically fail when subjected to

elevated temperatures and low strain rates Composition, in parts per million: 70 C, 60 S, 54 O, 11 N, 40 P Rod, 13 mm (0.5 in.) in diameter, was made by vacuum induction melting, chill casting, and swaging Heat treatment: recrystallize for 30 min at 850 °C (1560 °F), austenitize for 1 h at 1100 °C (2010 °F), air cool Sample was tensile tested at 700 °C (1290 °F) and an initial strain rate of 1.1× 10 -6 /s Test was interrupted other 12 h, and the sample was then broken by impact at -100 °C (-150 °F) Fracture surface reveals cavitated grain boundaries The intergranular cavities nucleated on second-phase (iron sulfide) particles, examples of which are shown at A, B, and C SEM, 4500× (E.P George and D.P Pope, University of Pennsylvania)

Fig 2 Slip lines in iron Composition, in parts per million: 160 C, 40 S, 13 O, 6 N, 30 P Rod, 13 mm (0.5 in.) in

diameter, was made by vacuum induction melting, chill casting, and swaging Heat treatment: recrystallize for

30 min at 850 °C (1560 °F), austenitize for 1 h at 1100 °C (2010 °F), air cool Sample was tensile tested to failure at 700 °C (1290 °F) and an initial strain rate of 4.4 × 10 -5 /s Slip lines, smoothened by diffusional flow, are visible on grain boundaries of the elevated-temperature fracture surface Failure was intergranular and resulted from the nucleation, growth, and eventual coalescence of grain-boundary cavities (see Fig 1) In this case, however, cavity outlines were masked by the slip steps created on grain boundaries due to severe plastic deformation within the grains The wavy nature of the slip lines is a characteristic of body-centered cubic iron

SEM, 2180× (E.P George and D.P Pope, University of Pennsylvania)

Fig 3 Stereo pair of scanning electron microscope views of the fracture surface of a Charpy impact test bar of

high-purity iron The specimen was broken after being cooled to equilibrium in liquid nitrogen (-196 °C, or -321

°F) Flat cleavage has taken place on a variety of sharply divergent crystal planes There are fine river patterns evident on nearly all of the facets The cleavage steps that delineate the "river" systems are, however, very minute in height, providing only very small departures from a single crystallographic plane of crack growth Characteristic of iron when fractured at low temperature is the formation of "tongues," one of which is located

at the lower end of the very bright facet There even appears to be a river pattern on the left-hand portion of the "tongue." A second, smaller tongue projects from the facet shown obliquely at top left 1100×

Fig 4 Surface of a room-temperature tensile-test fracture in a specimen taken from an ingot prepared by

adding Fe2O3 to pure iron in a vacuum melt equilibrated at 1550 °C (2820 °F) in a silica crucible The ingot contained 0.07% O in the form of FeO The fracture surface contains dimples that initiated at globular FeO inclusions averaging 5.2 μm in diameter SEM, 1800×

Fig 5 Stereo pair of scanning electron microscope fractographs of the surface of a tensile-test fracture

obtained at room temperature The alloy was a low-carbon iron to which an appreciable amount of Fe 2 O 3 had been added to form an aggregate of FeO inclusions The dimples that are characteristics of ductile rupture are evident here, and many of these dimples contain one or more globular oxide inclusions that are readily apparent There appear to be two sizes of these oxide inclusions some being about 6 μm in diameter and other about 3 μm in diameter Unlike many inclusions displayed in other fractographs, which have relatively smooth, unbroken contours several of the particles shown here possess sizable surface defects Some of these defects may be exposed internal shrinkage cavities The surfaces of the dimples show contours that vaguely resemble fatigue-striation marks The differences in topographic contours of the dimples displayed in this stereo pair of fractographs can be appreciated by viewing the fractographs stereographically, which provides a three- dimensional effect It then becomes apparent that the dimples are chimneylike cavities with nearly vertical walls in many instances and with bottoms at great depth that appear black and without detail The FeO inclusions appear to cling to the cavity walls, many at a point part way to the bottom of the "chimney." Most of the separating walls between adjacent chimneys are extremely thin, which makes it surprising that these walls did not rupture at a point closer to the bottom of the chimney 1200×

Sequence of SEM fractographs, at increasing magnifications (80×, 950×, and 5000×, respectively), that show a fracture in an iron alloy containing 0.14% S and 0.04% O The fracture was obtained by bending at room temperature Several spheroidal oxide inclusions are visible, most of them having diameters in the range of 1 to 3 μm The rectangle in Fig 6 indicates the area that is shown at higher magnification in Fig 7, and the rectangle in Fig 7 indicates the area that is shown at still higher magnification in Fig 8 The 6-μm-diam oxysulfide particle in Fig 8 shows a shrinkage cavity plus a white spot from an electron beam impingement in fluorescent x-ray analysis It is quite evident that, during the process of microvoid coalescence, the iron matrix has become detached from the globular inclusions at the metal-to-oxide and metal-to-sulfide interfaces, leaving these inclusions unaffected by

the applied stresses and severe deformation taking place around them

Fig 9 Fracture produced at room temperature by bending iron containing 0.02% C, 0.14% S, and 0.04% O

cast in a 7 × 7 × 20 cm (2.75 × 2.75 × 8 in.) ingot mold A carbon-FeO reaction caused blowholes such as shown at top Note the fine lamellar structure at bottom See also Fig 10 SEM, 75×

Fig 10 Higher-magnification view of the blowhole shown at the top in Fig 9, showing the interior of the

blowhole that resulted from the carbon-FeO reaction The pendants are droplets of a liquid oxysulfide that spread over the surface of the blowhole during freezing of the ingot SEM, 1400×

Fig 11 Fracture by room-temperature bending in casting of similar composition to that of casting in Fig 9, but

containing 1.1% Mn The numerous inclusions contained within the dimples are particles of manganese oxide and manganese sulfide trapped between growing dendrite branches SEM, 1500×

Fig 12 Higher-magnification view of the same fractured specimen shown in Fig 11 X-ray fluorescent analysis

of the inclusions indicates that some of them are Mn(Fe)O and that others are Mn(Fe)S, but the exact amount

of contained iron was not determined SEM, 4000×

Fig 13 Low-carbon iron containing a high percentage of oxygen, fractured in fatigue at room temperature A

large oxide inclusion has been nearly completely disengaged from its original pocket Fatigue striations detour around, or extend into, the pocket Crack propagation was from bottom to top SEM, 2400×

Fig 14 Intergranular fracture that was generated in a specimen of oxygen-embrittled Armco iron by a Charpy

impact test at room temperature The grain facets appear sharp and clean Note the secondary cracks, which follow grain boundaries See also Fig 15 and 16 for views of other regions of this fracture SEM, 55×

Fig 15 View of another region of the surface of the impact fracture shown in Fig 14, showing facets that

resulted from a combination of intergranular rupture and transcrystalline cleavage Note the array of small river patterns at the bottom edge of the large facet at center See also Fig 16 SEM, 655×

Fig 16 View of a third region of the surface of the impact fracture shown in Fig 14 and 15 Note the almost

perfect grain-boundary surfaces and the sharp edges and points at which the separated-grain facets meet The secondary cracks are equally clean separations SEM, 670×

Fig 17 Surface of tensile-test fracture in specimen of low-carbon, high-oxygen iron that was broken at room

temperature Many of the equiaxed dimples contain spheroidal particles of FeO The rectangle marks the area shown at higher magnification in Fig 18 and 19 SEM, 500×

Fig 18 Enlargement of the area within the rectangle in Fig 17, showing the surface contours of the dimple

cavities of the very ductile fracture Dimly visible in the central dimple is a globular particle of FeO; the particle

is shown more clearly in Fig 19 SEM, 2400×

Fig 19 Same fracture-surface area as that shown in Fig 18, but processed with a different exposure to bring

out the shape and size of the globular particles of FeO in the central dimple The small dark spot at the lower right on the FeO particle is a shrinkage area SEM, 2400×

Fig 20 Fe, 0.01% C, 0.24% Mn, and 0.02% Si, heat treated at 950 °C (1740 °F) h, air cooled The structure

is ferrite Hardness, 62 HV The fracture was generated by impact at -196 °C (-321 °F) Cleavage steps beginning at the twin at top form a sharply defined river pattern Crack propagation was in direction of arrow SEM, 300×

Fig 21 Surface of an impact fracture in a notched specimen of wrought iron The longitudinal stringers of slag

in the material are parallel to the direction of fracture, which gives the surface this typically "woody" appearance Compare with Fig 22 6×

Fig 22 A companion notch-impact fracture to the one shown in Fig 21, but here the longitudinal stringers of

slag are normal to the fracture surface Individual particles of slag are not readily visible at this magnification, but they cause the speckled appearance 6×

Fig 23 External surface of a specimen of Armco iron that had been etch-pitted and then subjected to tension

during one cycle of a 2.5° bend test Note the slip steps See also Fig 24 TEM p-c replica, 5000×

Fig 24 Same specimen as in Fig 23 after 5.5 cycles of 2.5° bending At center is the same area as in Fig 23

(reversed left to right) showing slip-band cracks that have grown at slip steps TEM p-c replica, 2000×

Fig 25 Cleavage fracture in Armco iron broken at dry-ice temperature (-78.5 °C, or -109.3 °F) The light band

shows where cleavage followed a twin-matrix interface The black meandering line is a shear step through the thickness of the twin TEM p-c replica, 3000×

Fig 26 Cleavage fracture in Armco iron broken at dry-ice temperature (-78.5 °C, or -109.3 °F), showing facets

of which most have the same orientation Facets that depart from the general orientation appear lighter or darker than the majority TEM direct carbon replica, 3000×

Fig 27 Cleavage fracture in Armco iron broken at -45 °C (-49 °F) Instead of cleavage steps, tear ridges

(occasionally forming river patterns) were produced here by microscopic plastic flow TEM p-c replica, 2000×

Fig 28 Cleavage fracture in Armco iron broken at -196 °C (-321 °F), showing river patterns, tongues, and

(from bottom right to top left) a grain boundary TEM p-c replica, 3000×

Gray Irons: Atlas of Fractographs

Gray Irons

Fig 29 Fatigue fracture of sand-cast gray iron bread-crumb grinder The ASTM A159 part was machined and

hot dip galvanized after casting Also, a lockpin hole (not visible) was drilled through the casting in a region containing type C, size 3 graphite flakes Cause of failure was traced to a crack that initiated at one of these

flakes Inset: broken grinder in as-received condition 1.5× (C.-A Baer, California Polytechnic State University)

30 and 31: Fracture surface of nil-ductility sample that cracked during casting or machining Fracture initiated at and propagated from graphite-ferrite interfaces SEM, 190× and 1900× Fig 32 and 33: Solidification structure and graphite morphology of permanent mold cast, gray iron sample with nil ductility The crack propagated along the preferred orientation of solidification Dendrite spacing was

a narrow 50 m Graphite was fine tipped, roughly cylindrical, and isolated from ferrite cells a morphology that apparently has an adverse effect on ductility and is also a major contributor to graphite-ferrite interface cracking 2% nital, 82× and 330× Fig 34 and 35: Fracture surface of sample having normal ductility Fracture was artificially generated by impact Note how material resisted graphite-ferrite interface cracking Microstructure (not shown) was more isotropic than that of the nil- ductility casting, with a wider dendrite spacing (85 m) Graphite was medium sized, interconnected,

and penetrated ferrite cells SEM, 200× and 1000× (D.C Wei, Kelsey-Hayes Company)

Ductile Irons: Atlas of Fractographs

Ductile Irons

Brittle cleavage fracture of ductile iron spur gear (ASTM A536, grade 100-70-03) due to improper heat treatment Tensile strength was 544 MPa (78.9 ksi), much less than the 690 MPa (100 ksi) required by the specification Elongation was nil; the specification called for 3% min The induction- hardened case on the teeth was shallower and harder than specified (50 + HRC versus 46 HRC), and the martensitic microstructure had not been tempered as specified Direct cause of fracture was the presence of inverse chill (carbides in thick sections of the casting) associated with microporosity This carbidic material, which formed at thermal centers due to segregation of carbide-forming elements, increased hardness, decreased tensile strength, and promoted brittle fracture Proper heat treatments would have corrected the deficiencies in case structure and properties and would also have prevented the occurrence of inverse chill Fig 36: Fracture surface at core of gear directly below origin Note transcrystalline (cleavage) mode of fracture SEM, 200× Fig 37: Photomicrograph of core Matrix is 100% pearlite Note presence of inverse chill (carbides) and associated porosity 2% nital, 200× Fig 38: Fracture face at casting surface and in the induction-hardened case SEM, 20× Fig 39: Boxed area

in Fig 38 Note cleavage fracture appearance and nodule surrounded by cracked material believed to

be carbidic SEM, 200× Fig 40: Microstructure directly below fracture surface and in case There is a carbide-appearing envelope around each nodule of temper carbon that formed as the inverse chill

decomposed The two nodules at upper left also have retained austenite around them Note the secondary crack extending between the casting surface and the carbide envelope of a temper nodule 2% nital, 200× (G.M Goodrich, Taussig Associates Inc.)

Fig 41 Surface of a fatigue-test fracture in an experimental crankshaft of induction-hardened 80-60-03 ductile

iron with a hardness of 197 to 225 HB Fatigue-crack origin is at arrow A Porosity at arrow B was unrelated to fracture initiation 2.5×

Fig 42 Surface of a fatigue-test fracture in an experimental crankshaft of ductile-iron with a hardness of 241

to 255 HB Note the multiple fatigue-crack origins at the journal edge (at right) Fatigue beach marks are evident, which is unusual in cast iron Actual size

Fig 43 Surface of a fatigue fracture in an experimental crankshaft broken in a fatigue test The material is

ductile iron with a hardness of 241 HB The origin of the fatigue crack is at the edge of the journal, at arrow Actual size

Fig 44

Fig 45

Fig 46 How fatigue cracks propagate through ductile irons Fig 44 and 45: In an as-cast, commercial pearlitic ductile iron, the crack changes direction from grain to grain, usually following nodule-matrix interfaces Fig 45 etched in 2% nital, both at 80× Fig 46: Crack propagation through the tempered martensite of a heat-treated ductile iron is more matrix controlled than in either a pearlitic (Fig 44 and 45) or ferritic microstructure 100× (F.J Worzala, University of Wisconsin)

Fig 47 Fig 48 Fig 49

Fatigue fracture surfaces of pearlitic and ferritic ductile irons Compositions of pearlitic irons: 3.63 to 3.80% C, 0.34% Mn, 2.02 to 2.66% Si Compositions of ferritic iron: 3.75 to 2.82% C, 0.34% Mn, 2.30

to 2.66% Si Striations noted in all cases Fig 47 and 48: Mixture of striations and fractured pearlite lamellae on fracture surfaces of commercial pearlitic ductile irons Striations are the fine steplike features, not the macroscopic waviness or undulations SEM, 207× and 198× Fig 49: A high load fatigue fracture surface of a ferritic ductile iron SEM, 375× (F.J Worzala, University of Wisconsin)

Fig 51 Fig 50

Comparisons of fatigue and monotonic (tension, bending, impact) fracture surfaces for various ferritic ductile iron microstructures Fatigue fractures are characterized by striations (see Fig 47, 48, and 49),

by relatively little opening up or stretching of matrix material around nodules, and by nodules that appear to have had large pieces of graphite broken off of them Mode of crack propagation in monotonic fractures is very ductile with considerable stretching of nodule-bearing cavities The demarcation line between fatigue (FAT) and monotonic (FRA) fracture is noted in each of the three fractographs Fig 50: Commercial ferritic iron (2.30 to 2.66% Si) tested at room temperature SEM, 90× Fig 51: Same material as in Fig 50, but during tensile portion of test at -40 °C (-40 °F) SEM, 90× Fig 52: High-silicon ferritic ductile iron (3.5% Si) SEM, 90× (F.J Worzala, University of

Fig 56

Fracture modes in slow monotonic loading (tension or bending) of pearlitic ductile irons These cast irons consist of a pearlitic matrix with ferritic rings of varying thickness surrounding graphite nodules They exhibit predominantly brittle fracture with river patterns in pearlitic areas However, ductile fracture can occur in areas where several graphite nodules are closely spaced Fig 53: Typical slow monotonic fracture surface of pearlitic ductile iron Note river patterns and both constrained and opened ferritic rings (areas of brittle and ductile fracture, respectively) SEM, 230× Fig 54: Fracture surface with isolated graphite nodules Brittle fracture results because the ferritic rings are under severe mechanical constraint due to the surrounding stronger and nondeformable matrix SEM, 260× Fig 55: Fracture surface at cluster of graphite nodules Note the considerable deformation of the unrestrained ferrite Fracture occurs here by ductile tearing and microvoid coalescence SEM, 385× Fig 56: River patterns indicative of brittle failure on fracture surface of a pearlitic ductile iron SEM, 385× (F.J Worzala, University of Wisconsin)

Fig 60

Fracture modes in slow monotonic loading (tension or bending) of ferritic ductile irons The only

modes operative at ambient temperature are ductile tearing and microvoid coalescence The propagating crack will cut through any "lumps" of nonspheroidal graphite that may be present Fig 60: Fracture surface of ferritic ductile iron subjected to slow monotonic loading is characterized by ductile tearing and microvoid coalescence Internodular ferrite undergoes a substantial amount of deformation SEM, 300× Fig 61: Fracture surface of ferritic ductile iron with nonspheroidal graphite shows how crack cuts through the irregularly shaped graphite lumps while leaving graphite nodules intact SEM, 534× (F.J Worzala, University of Wisconsin)

As test temperature falls below ambient, the slow monotonic fracture mode for ferritic ductile irons undergoes a gradual ductile-to-brittle transition Fig 62: Fracture surface of ferritic ductile iron tested

at -40 °C (-40 °F) Note river patterns and plateaus characteristic of brittle fractures SEM, 150× Fig

63 and 64: High-magnification views compare internodular areas of ferritic ductile irons tested at room temperature and at -40 °C (-40 °F), respectively SEM, ~1900× (F.J Worzala, University of

Slow monotonic (tension or bending) fracture characteristics of heat-treated ductile irons Fig 65: Normalized high-nodularity ductile iron tested at room temperature Note cleavage facets extending to the nodule, secondary cracking, and evidence of nodule cavity elongation This material is not as tough

as ferritic or pearlitic ductile iron SEM, 550× Fig 66: Fracture surface of normalized low-nodularity iron Presence of nonspheroidal graphite fosters the creation of long ridges and valleys These graphitic "lumps" apparently upset and redirect the crack plane SEM, 550× Fig 67: Austempered ductile iron, with a microstructure of bainite and tempered martensite, is stronger but less ductile than ferritic or pearlitic ductile iron Note fine cleavage features When viewed macroscopically, the fracture surface is very flat and reflective SEM, 270× (F.J Worzala, University of Wisconsin)

Fig 69 Fig 68

Fracture modes in impact (fast monotonic) loading of a ferritic ductile iron tested at -45 °C (-50 °F) Fig 68: Fracture surface reveals little deformation with no stretching around graphite nodules Several nodules appear to have been broken Also, the matrix failed in an entirely brittle mode, as evidenced by the numerous river patterns and cleavage and quasi-cleavage steps Compare with the slow monotonic (tension or bending) fracture in Fig 62 Although they generally resemble each other, the fast fracture exhibits no graphite cavity stretching or microvoids, and nodules remain intact in the slow fracture SEM, 220× Fig 69: Internodular area of impact-loaded specimen in Fig 68 Fracture in this region was entirely brittle Microtongues lie on the iron {112} plane and intersect the cleavage

Fig 71 Fig 70

Presence of nonspheroidal graphite affects impact fracture of ductile irons by producing a rough fracture surface with continuous change in crack path direction even at low test temperatures Fig 70: Fracture surface of ferritic ductile iron tested at -18 °C (0 °F) SEM, 240× Fig 71: Fracture surface of pearlitic ductile iron impact tested at -75 °C (-100 °F) SEM, 100× (F.J Worzala, University of Wisconsin)

Fig 73 Fig 72

Impact fracture of ductile irons at test temperatures above the nil-ductility transition (NDT) temperature is accompanied by gross plastic deformation (ductile behavior) Fig 72: Fracture surface

of low-nodularity ferritic ductile iron tested at 65 °C (150 °F) SEM, 80× Fig 73: Pearlitic ductile iron

tested at 205 °C (400 °F) Note microvoid coalescence and ductile tearing SEM, ~400× (F.J Worzala, University of Wisconsin)

Microcrack initiation and growth in an annealed ferritic ductile iron (3.6% C, 2.2% Si, 0.3% Mn, 0.7% Ni, 0.2% Mo) Sample polished, etched in 2% nital, and then plastically deformed by amount indicated Tensile load applied horizontally Fig 76 and 77: After 610 and 670 strain units, respectively SEM, both at 1200× Fig 78 and 79: After 700 and 760 strain units, respectively SEM, both at 600× (R.C Voigt and L.M Eldoky, University of Kansas)

Fig 80 Fig 81

Effects of plastic deformation on internodule bridges in ferritic ductile iron Material and sample preparation same as in Fig 76, 77, 78, and 79 Fig 80: Surface plastic deformation and microcracking ahead of the primary crack front Note strain concentration at narrow internodule bridge Tensile load applied horizontally SEM, 950× Fig 81: Severe plastic deformation associated with tearing of internodule bridge Note separation of secondary graphite ring from both the primary graphite and

Fig 82 Microvoids and microtearing common to fractures of wide internodule bridges in ferritic ductile iron

Material same as in Fig 76, 77, 78, and 79 Surface perpendicular to fracture surface and polished and etched

in 2% nital SEM, 300× (R.C Voigt and L.M Eldoky, University of Kansas)

Fig 84 Fig 83

Typical fracture surface morphologies for an annealed ferritic ductile iron Composition: 3.6% C, 2.2% Si, 0.3% Mn, 0.7% Ni, 0.2% Mo (same as in Fig 76, 77, 78, 79, 80, 81, and 82) Fig 83: Dimpled rupture (ductile fracture) at room temperature SEM, 800× Fig 84: Quasi-cleavage (brittle fracture)

at low temperature SEM, 1600× (R.C Voigt and L.M Eldoky, University of Kansas)

Fig 87 Fracture surface of ferritic-pearlite ductile iron in Fig 85 and 86 The low-temperature fracture

occurred via a brittle, quasi-cleavage mode SEM, 715× (R.C Voigt and L.M Eldoky, University of Kansas)

Fig 88 Ductile-to-brittle transition in an annealed ferritic ductile iron (same alloy as in Fig 83 and 84) Above

demarcation line is region of dimpled rupture (the ductile fracture surface of the test sample after partial fracture at room temperature) Below line is region of quasi-cleavage (the brittle fracture surface of the test sample after final fracture at low temperature) SEM, 30× (R.C Voigt and L.M Eldoky, University of Kansas)

Fracture transition zone at center of the ferritic ductile iron sample in Fig 88 Fig 89: Subsurface fracture transition zone SEM, 140× Fig 90: Higher magnification view of Fig 89 Note how far microcracks formed at room temperature had propagated by time of final low-temperature fracture SEM, 275× Fig 91: High magnification view of transition zone Visible features include nodule decohesion and microplastic deformation and tearing ahead of the primary crack front SEM, 1,100×

Fig 92 Fracture surface of a ferritic-pearlitic ductile iron Note ductile fracture of ferrite in matrix around

nodules and cleavage (brittle) fracture of pearlite in matrix SEM, 50× (W.L Bradley, Texas A&M University)

Fig 93 Regions of fatigue precracking (at right) and crack extension or fracture (at left) in the fracture surface

of a ferritic ductile iron compact tension specimen Note how crack ignores nodules in fatigue and grows almost exclusively through nodule-nucleated voids during ductile fracture ~80× (W.L Bradley, Texas A&M University)

Fig 94

Ductile fracture in ferritic ductile iron Fig 94: SEM, 160× Fig 95: Example of the finely dimpled ductile fracture that occurs between the much larger voids nucleated at graphite nodules SEM, 2000×

Fig 96 Fatigue precracked region on the fracture surface of a ferritic ductile iron compact tension specimen

Morphology is typical of fatigue fractures in this material SEM, 500× (W.L Bradley, Texas A&M University)

Fig 97 Brittle cleavage fracture in ferritic ductile iron SEM, 1000× (W.L Bradley, Texas A&M University)

Malleable Irons/White Irons: Atlas of Fractographs

Malleable Irons/White Irons

Fracture sequence (increasing strain) illustrating localized plastic deformation and microcrack initiation and propagation ahead of the primary crack in a ferritic malleable iron (ASTM A47, grade