Volume 17 - Nondestructive Evaluation and Quality Control Part 18 pps

12, Dec 1972, p 249-253 Nondestructive Inspection of Boilers and Pressure Vessels Ultrasonic Inspection of Pressure Vessels Volumetric inspections are used to inspect the volume of ma

Fig 7 Nuclear pressure vessel, of 65 mm (2 in.) thick carbon steel, in which longitudinal, girth, and nozzle

welds were radiographically inspected using 50-Ci cobalt-60 as the radiation source, Eastman Kodak type AA radiographic film, and the inspection setups shown Dimensions given in inches

For radiographic inspection, cobalt-60 was selected as the radiation inspection because of the thickness of the material, and Eastman Kodak type AA film was selected for its relative speed to give the shortest exposure time The combination

of Co-60 and type AA film can sometimes result in marginal sensitivity that occasionally requires the reshooting of a specific region

Each longitudinal seam was radiographically inspected with eight shots spaced 350 mm (14 in.) apart using a 180 × 430

mm (7 × 17 in.) film The penetrameters were steel ASTM No 45 with a 1.5 mm ( in.) thick shim The Co-60 radiation source was placed 690 mm (27 in.) from the film The cassette was loaded with a 0.25 mm (0.010 in.) thick lead screen, two pieces of film, a 0.25 mm (0.010 in.) thick lead screen, and a 3 mm ( in.) thick lead back shield Section A-

A in Fig 7 shows the setup for radiographic inspection of the longitudinal seam

The girth weld was inspected using the panoramic technique, in which the Co-60, 50-Ci radiation source was placed at the center of the vessel and 28 film cassettes were placed around the outer surface (section B-B, Fig 7) The technique involves only one exposure of 25 min to radiograph the entire seam, but its success depends on the placement of the radiation source exactly in the center of the vessel The disadvantages of this technique include a lengthy setup time, a fairly complicated setup, and additional film costs if an error is made

An alternative technique is to place the radiation source outside the vessel and inspect the seam with 28 separate exposures This method would result in higher-quality radiographs, but a much longer total exposure time would be required

Three equally spaced steel ASTM No 45 penetrameters with 1.5 mm ( in.) thick shims were used for the 28 radiographs The film size and the types and thicknesses of screens and filters were the same as those used for the longitudinal seam welds

The nozzle seam weld was radiographed using eight 13-min exposures As shown in section C-C in Fig 7, the radiation source was placed outside the vessel and was offset at an angle of 7 to 10° away from the nozzle The source-to-film distance was 900 mm (36 in.) The variation in weld thickness at the nozzle required that two penetrameters be used One was flush to the sidewall, and the other had a 9 mm ( in.) thick shim, which compensated for the difference in weld thickness This two-penetrameter setup qualified the sensitivity of the radiograph because of the differential thickness of the weld being inspected A sensitivity of 2-2T and a density of 2.0 were specified for the radiographs of all three types of welds

Example 2: Radiographic Inspection for Creep Fissures in Reformer-Furnace Tubes

(Ref 14) About 1 year after start-up, two steam-methane reformer furnaces were subjected to short-time heat excursions because of a power outage, which resulted in creep bulging in the Incoloy 800 outlet pigtails, requiring complete replacement It was thought that during this heat excursion some of the reformer tubes experienced slight bulging, about 1%, by plastic deformation However, this type of bulging does not necessarily shorten the life of a tube in terms of creep

Each furnace had three cells, consisting of 112 vertical tubes per cell, each filled with a nickel catalyst The tubes were centrifugally cast from ASTM A297, grade HK-40 (Fe-25Cr-20Ni-0.40C), heat-resistant alloy The tubes had an outside diameter of 150 mm (5.85 in.) and a minimum wall thickness as-cast of 20 mm (0.802 in.), and they were more than 13.7

m (45 ft) in length The design limits for the tubes were 2200 kPa (320 psi), with a surface temperature of 958 °C (1757

°F) Operating limits were 2140 kPa (310 psi) at a surface temperature of 943 °C (1730 °F) The furnace cell is illustrated

in Fig 8(a)

Fig 8 Reformer-furnace cell from which cast tubes of ASTM A 297, HK-40, heat-resistant alloy were

radiographically inspected for the detection of creep fissuring (a) Schematic of furnace cell showing positions of radiographic sources and films Dimensions given in inches (b) Radiograph of a section removed from a failed tube that contained no catalyst showing fissures near the ruptured area (c) Same section as (b) but containing

a catalyst Fissures are visible but less apparent (d) Macrograph showing fissures in a tube that were detected

by radiography 6× (e) Macrograph showing fine fissuring that was not indicated by radiography 6×

The first tube failure occurred after 33,000 h of operation The unit was shut down, and the failed tube was removed for metallurgical inspection The results indicated that the tube failed from creep rupture (stress rupture) The tube failure instigated a project for detecting midwall creep fissuring Radiography had been reported to be limited to detecting only severe third-stage creep and not the early stages

Laboratory Radiography. Preliminary studies were made to determine type of radiation source and strength, radiation source-to-film distance, exposure time, and effect of catalyst in tube Figure 8(b) shows a laboratory radiograph of a section removed from the failed tube without catalyst Fissures were clearly visible near the ruptured area and diminished

to nondetectable fissures at a point 400 mm (16 in.) below the rupture The same tube section radiographed under similar conditions but with catalyst in the tube is shown in Fig 8(c) Fissures are visible, but it is apparent that the catalyst reduced the sensitivity by masking the smaller fissures

Liquid penetrant inspection and macroexamination of the specimen tube revealed the gross fissuring that was easily detectable and those fissures that were undetectable by radiography The largest fissures undetectable by radiography were approximately 3 mm (0.125 in.) long and 0.13 mm (0.005 in.) wide (Fissures have been detected that were between

5 and 10 mm, or 0.200 and 0.400 in., long.)

Based on the radiography and macroexamination results, it was decided that future tube replacements would be inspected

by radiography, preferably with the catalyst removed If the fissure were large enough to show on a radiograph, either with or without the catalyst, the tube could be expected to fail within 1 year

In-Service Radiographic Inspection. During shutdown of one furnace, the first full test of the radiographic inspection method was made All the tubes were measured for evidence of outside diameter growth, and the suspect areas

were strapped to identify the hottest zones Growth to failure in HK-40 material having a rough exterior surface is difficult to measure, because total creep of only about 1% has occurred by the time of failure Gaging did not show much, but strapping on some tubes clearly identified bulging in the hottest areas about 1.4 m (54 in.) above the burner terrace This was used as the basis for radiographing all other tubes

The catalyst was removed from the tubes in preparation for radiography Two IR-192 radiation sources having a strength

of 3300 GBq (90 Ci) and one 3700 GBq (100 Ci) source were used, mounted in jigs at distances of 750 and 900 mm (30 and 36 in.), respectively, from the film A tungsten collimator was also used to limit the emission to a single tube and permit the technicians to remain in the firebox during exposure Exposure time was 7 to 10 min per shot, and approximately 700 shots were taken

Shortly after the first furnace was back on stream, the second furnace was shut down and the tubes radiographed Using the same techniques but with a 3900 GBq (105 Ci) source, time to radiograph the second furnace was reduced about 25%

Twenty-four tubes in the first furnace and 53 in the second furnace showed significant fissuring One of these fissured tubes was left in the first furnace and 15 in the second; when these tubes eventually failed, they would provide an indication of remaining life after a known radiographic examination

One of the tubes that had been removed from the furnace was sectioned at two places through a radiographic indication

of a fissure and through an area containing no indications Macrographs of these sections are shown in Fig 8(d) and 8(e) Figure 8(d) shows fissuring that was detected by radiography Fine fissuring that was not seen on the radiograph is shown

in Fig 8(e)

Conclusion. Radiography was a practical, economical method of detecting the creep fissuring, and it provided advance information for purchase of replacement tubes However, because radiography was limited to detecting fissures caused by third-stage creep in tubes from which the nickel catalyst had been removed and because of the cost of removing the catalyst, ultrasonic techniques were developed for inspecting the tubes These techniques are described in Example 4 in this article

Reference cited in this section

14 R.R Dalton, Radiographic Inspection of Cast HK-40 Tubes for Creep Fissures, Mater Eval., Vol 30 (No

12), Dec 1972, p 249-253

Nondestructive Inspection of Boilers and Pressure Vessels

Ultrasonic Inspection of Pressure Vessels

Volumetric inspections are used to inspect the volume of material bounded by the surfaces of components and of piping Theoretically, any source of energy that penetrates the volume of a material can be used In practice, however, only x-rays, -rays, and ultrasonic waves are used In nuclear vessels, which are housed in a containment building, radiographic inspection methods for in-service inspection currently have limited use because of the need for access to both surfaces and because of the high -ray background in most areas of the containment building When radiography is used, other inspection methods such as acoustic emission can also be used for additional monitoring However, a large number of inspections are performed using various ultrasonic techniques

As described in the article "Ultrasonic Inspection" in this Volume, ultrasonic waves are generated by piezoelectric transducers that convert high-frequency electrical signals into mechanical vibrations These mechanical vibrations form a wave front, which is coupled to the vessel being inspected through the use of a suitable medium Several wave modes can

be used for inspection, depending on the orientation and location of the discontinuities that exist Longitudinal, shear, and surface waves are used separately in different techniques to reveal discontinuities that are respectively parallel to, at an angle to, and on or near the surface from which the inspection is performed These inspections are made with longitudinal wave transducers, pulse-echo, through transmission, pitch-catch, or delta techniques Most of the inspections are performed using pulse-echo straight longitudinal wave beams and angled shear wave beams from a single transducer

Manual ultrasonic inspection is performed with single-transducer pulse-echo techniques The ultrasonic wave is coupled to the component being inspected through a couplant (usually a glycerin or light oil) that transports the ultrasound between the face of the transducer and the surface of the component In longitudinal wave straight-beam inspection, ultrasonic waves enter normal to the surface and will detect discontinuities perpendicular to the direction of wave propagation In angle-beam inspection, the longitudinal wave is coupled through a plastic wedge at an angle to the surface The wave is again coupled to the component (still a longitudinal wave) and undergoes a mode conversion to a shear wave The angle of the resulting shear wave is dependent on the ratio of the combination of ultrasonic velocities in the plastic wedge and the metal component with the angle of the incident longitudinal wave in the plastic wedge Shear wave angles from 40 to 75° are used for inspection Indications are recorded and plotted from reference locations on the weld metal

Automated Ultrasonic Inspection. For accurate inspections, immersion ultrasonic inspection provides the highest level of inspection speed, accuracy, and repeatability The coupling of ultrasonic waves is done with much less variability

in couplant thickness and transducer pressure than occurs in manual techniques The immersion method permits automated scanning and digital recording of data with a high degree of precision and repeatability Additional advanced automated methods are described below in the section "In-Service Quantitative Evaluation" in this article

Thickness Measurements. Inspection should properly begin with thickness measurements of the shell, heads, nozzles, and piping Well-documented inspection records are necessary to complete a satisfactory inspection Records can indicate where to expect metal loss or corrosion on the component and therefore enable the suspect areas to be thoroughly inspected A comparison of measurements obtained during previous inspections will determine the amount of loss and corrosion rates Original or nominal thicknesses taken from specifications usually have tolerances too great to make them reliable in the determination of corrosion rates Vessels having a history of minimal corrosion require only a moderate amount of inspection, while those having a high corrosion rate or history of attack must be more thoroughly inspected Inspection experience and records provide the key to how much coverage should be given If the vessel is open, the thickness measurements can be made from the internal surfaces at any location the inspector desires an approach much more flexible than dependence on fixed corrosion-gaging points

The distance between thickness measurements can vary depending on the coverage desired, but a sufficient number of readings should be obtained to ensure a correct determination of vessel condition (distances generally range from to 1 m) In determining repair areas or the size and location of a patch, a grid pattern laid out on the vessel can be useful

Rough or badly pitted surfaces should have a small area ground smooth to permit the transducer to make good contact Transducers having a rubber membrane as a protective facing contact better on mildly pitted surfaces because the rubber will tend to conform to the surface Removal of loose scale or dirt by scraping or filing is often sufficient surface preparation

It is not necessary to drill gage holes or use other destructive methods to determine wall thickness Insulation, if present, must be removed at gage locations for external measurements, but the removal of insulation is not required if the measurements are to be taken from the inside

Special couplants enable many external measurements to be taken while the vessel is operating at temperatures of 370 °C (700 °F) or higher Corrections should be made for errors introduced by the high temperature Table 1 lists several correction factors for ultrasonic thickness measurements in carbon steels at elevated temperatures Additional information can be found in Ref 15

Table 1 Correction factors for ultrasonic thickness measurements in carbon steels at elevated temperatures

Measurement Temperature

Ultrasonic Micrometer

°C °F mm in mm in

Correction factor

For flaw detection, a continuous-scan method is often used, in which the transducer is carried on a rigid carriage and the ultrasound waves are coupled into the plate by a stream of water In some applications, the plate can be immersed, which simplifies the operation To eliminate some of the human errors, a flaw-alarm system is set up so that a flaw of predetermined size will trigger a warning of some kind

For the inspection of a weld, the weld shape may be such that the use of longitudinal waves is impossible, and a flaw such

as a crack at the fusion zone probably will not present enough reflective area to be detected from directly above it Nozzles located in hemispherical heads present special problems, and special techniques must be used to ensure a thorough inspection Using shear waves, the sound can be directed into a suspect area from almost any angle Large-scale drawings of the suspect area are needed to lay out correct angles and distances for the transducers (Fig 9) For components having a complicated design, a plastic model can be an aid to better visualization of the sound path and angles of reflection Thought given to the best approach and techniques can save hours in inspection and can ensure proper coverage Angle transducers of 45 and 60° will detect almost any weld discontinuity, particularly if both internal and external surfaces are available for inspection If the internal surface is not available, the nozzle can be filled with water and a search unit lowered to the correct depth to inspect the weld The search unit can be adjusted to any angle and turned from side to side while in use A plastic wedge on the search unit can be used to direct the sound beam through the nozzle body Correctly angled, the beam will be intercepted by any radial flaw in the nozzle body and can also reach the attachment weld on the far side The wedge can be shaped on a test block duplicating a nozzle section, and the sound path can be determined by through transmission (one transducer transmitting the signal and another, placed opposite, receiving the signal)

Fig 9 Typical layout used on large-scale drawings to determine transducer angles and distances for inspection

of difficult-to-reach weld areas

Shear wave search units intended for inspection at high temperatures may have the plastic wedge made of a special resistant material Such materials usually increase attenuation, and calibration must be made at temperatures close to that

heat-of the surface heat-of the part being inspected if detection heat-of a flaw heat-of given size is to be ensured

Examples of Application. The use of ultrasonics in the detection of discontinuities in a pressure vessel is discussed in the following examples Additional information on in-service inspection is presented in the following section of this article

Example 3: Use of Ultrasonic Inspection to Detect Creep Rupture in Stainless Steel Headers of an Ammonia-Plant Reformer Furnace

(Ref 16) A leak was detected in one of the coils in the radiant section of a primary reformer furnace used in an ammonia plant Furnace temperatures were reduced immediately, but it was necessary to continue firing of the furnace at about 540

to 595 °C (1000 to 1100 °F) for 16 h to reduce the catalyst Subsequent shutdown inspection revealed that the bottom of one of seven outlet headers had ruptured, causing a section about 100 mm (4 in.) wide by 460 mm (18 in.) long to fall to the furnace floor The outlet headers were 100 mm (4 in.) nominal diameter schedule 120 (11 mm, or 0.438 in., wall) pipe about 10 m (34 ft) long and were made of ASME SA-452, grade TP316H, stainless steel In the absence of hot spots, the surface of the outer tube ranged from 845 to 855 °C (1550 to 1575 °F) The unit had been on stream about 29,000 h prior

to failure

To get the unit back in service quickly, a section about 1.4 m (4 ft) in length was replaced The amount of header metal replaced was based on results from both visual and liquid penetrant inspections of the outer surface of the header The inner surface, where accessible, was also checked visually and by liquid penetrant inspection The six other headers were inspected at selected locations, and no surface cracks were detected

Investigation of the Removed Section. Metallographic examination of the 1.4 m (4 ft) section of the header that had been removed revealed that it had failed as a result of intergranular fissuring and oxidation, commonly termed creep rupture Severe intergranular fissuring was found throughout the cross section of the header sample Primary cracking occurred intergranularly through the grain boundaries, typical of high-temperature fissuring Close observation of the crack paths linking some of the larger fissures, however, did reveal local areas having grains that were much smaller than

those of the header metal The presence of these very small grains could only be the result of cold deformation and subsequent recrystallization at elevated temperatures Therefore, some plastic strain and cracking had occurred at temperatures well below the operating temperature, apparently as a result of thermal expansion and contraction stresses during startups and shutdowns, and then the cold-strained areas had recrystallized subsequently when they were reheated

to the service temperature Therefore, it follows that all of the fissuring did not take place during any one operating period Also, the microstructure in the ruptured area gave the following evidence that the header metal had not been subjected to gross overheating in service:

• The header metal still had a general grain structure that would be classified as being of fine size, while severe grain coarsening would have occurred if the header metal had been heated at temperatures between 1095 and 1315 °C (2000 and 2400 °F)

• The microconstituents present in the grains would have been dissolved at grossly elevated temperatures and would have been of much finer size if they had subsequently precipitated out of solid solution during the final 16 h of firing at 595 °C (1100 °F) to reduce catalyst

It was concluded from this investigation of the ruptured area that the header had failed by conventional long-time creep rupture as a result of exposure to operating temperatures probably between 900 and 955 °C (1650 and 1750 °F), rather than by short-time exposure at grossly elevated temperatures It was suspected that the furnace headers might not be as sound as the visual and liquid penetrant inspections of the surface metal had indicated; therefore, three ring sections from the removed 1.4 in (4 ft) long header section were selected for further study The sections were taken from the ruptured area (sample A), from a slightly bulged but nonruptured area (sample B), and from visually sound metal about 0.6 m (2 ft) from the rupture (sample C)

Inspection of the three samples revealed the presence of pinhead-size intergranular fissures throughout the cross sections

of samples B and C (Fig 10) This indicated that the header metal at these locations had been in an advanced stage of creep rupture even though the fissures or voids were of small size Sample C had fewer pinhead voids than sample B With continued service, the pinhead voids would have increased in number and grown in size and eventually would have linked up and caused failure Generally, the microstructure of the inner surface of the header in the area of rupture (sample A, Fig 10a) contained intergranular fissures (black areas), a relatively fine grain structure, precipitated microconstituents in the grains, and considerable amounts of carbides in the nonfissured grain boundaries The presence

of the carbides was evidence of carbon pickup in service

Fig 10 Unetched microscopic appearance of three samples of failed pipe from a reformer-furnace header of

TP316H stainless steel pipe and corresponding ultrasound responses of the three samples compared to the response of unused pipe Micrographs of (a) sample A, (b) sample B, and (c) sample C All 100× Black voids are intergranular fissures Ultrasound responses for (d) unused pipe, (e) sample C, (f) sample B, and (g) sample A Strong reflections were obtained from the unused pipe, and attenuation increased as the number of fissures increased in damaged pipes

Sample C had been removed from a visually undamaged header area located about 0.6 m (2 ft) from the rupture and about

250 mm (10 in.) from the sound end of the header section It was considered possible that the header metal that was in service and only 250 mm (10 in.) away from the area of sample C was also in an advanced stage of creep rupture It appeared probable that localized areas of the other six headers were in critical condition or that the problem was of a general nature Therefore, it was decided that the remaining headers in service should be inspected as soon as possible because obtaining replacement parts might require several months of lead time An ultrasonic attenuation method was considered to be the only possible nondestructive method for this evaluation, and development of a suitable system for use in the field was undertaken

Ultrasonic Inspection Equipment and Standards. It was determined that optimum results that permitted the determination of changes in attenuation as a function of the number of cracks present required the use of a 22-MHz search unit in conjunction with a pulse-echo instrument To minimize surface effects and curvature, a short water column was used to couple the sonic energy to the headers This water column was enclosed in a Lucite cylinder 32 mm (1.25 in.) long with a 6 mm (0.25 in.) thick polyurethane-foam gasket at the bottom The pliable gasket conformed to the curved

surfaces of the header The search unit was held in position by a setscrew Water flowed through the unit by gravity and escaped through a small hole in the Lucite cylinder at a point just above the front surface of the transducer

A section of unused type 316H stainless steel pipe and the three sections of header metal from the rupture area were used for reference Strong reflections were obtained from the undamaged pipe (Fig 10); increased attenuation was obtained from damaged piping as the number of cracks increased, as indicated by the ultrasonic responses in Fig 10(e) to 10(g)

The results of the test correlated well with the characterization of the number of cracks observed by optical microscopy (Fig 10a to 10c) Approximately 1 year later, with 37,336 h on stream, the plant was shut down, and in-service inspection was conducted using the ultrasonic technique developed

In-Service Ultrasonic Inspection. The standards and equipment used to develop the ultrasonic technique were used for the field inspection of the reformer furnace Ultrasonic readings were taken on the horizontal header metal between each vertical tube and on both sides of any header welds Over 350 readings were made In addition, at some locations, readings were taken on the top, bottom, and sides It was found that the headers all contained internal voids throughout their lengths, of varying numbers between those of samples B and C The inspection results indicated that all headers were in an advanced stage of creep rupture (stress rupture) but that no areas had fissured to a degree that they needed immediate replacement The replacement piping that had been installed during the previous shutdown 13 months earlier gave a "good" signal similar to unused material The technique developed for the field survey was not adaptable for determining the conditions of the welded joints, so none of the readings represents welds

On the basis of results of the in-service inspection, two conclusions were reached First, the furnace was deemed serviceable, and second, in the absence of local hot spots, the headers would survive for a reasonable period of time

Example 4: Ultrasonic Inspection for Creep Fissures in Reformer-Furnace Tubes

(Ref 17) Preliminary studies were made to establish whether sound would transmit through cast heat-resistant alloy

HK-40 tubes from a reformer furnace Several tests were conducted, and the most likely method was found to be the measurement of attenuation losses in a dual sensor using the through transmission method in an immersion tank The tests showed that sound would transmit in HK-40 tubes with the use of a low-frequency transducer Also, the rough outer surface of a centrifugal casting was the significant factor, rather than poor transmission in the large-grain cast microstructure Immersion methods minimized the surface roughness condition and provided sufficient acoustic energy in the tubes A dual search unit, with a sending transducer and a receiving transducer to produce a refracted-angle sound beam, was the best means for passing sound through the tube and across the plane of fissure formation The attenuation of this beam, measured by the receiver, was a measure of fissure density

Figure 11 shows the results of ultrasonic C-scan amplitude recordings on three sample tubes in an immersion tank By moving the dual search unit at a constant speed longitudinally along the tube and indexing at the end of each traverse, and continuously recording the received ultrasonic signal, attenuation patterns in fissured tubes were recorded Sample tubes with known fissures showed that some or all of the sound transmission was interrupted, depending on the density of the fissures (compare Fig 11a and 11b) Sample tubes with no fissures showed full sound transmission (Fig 11c)

Fig 11 Ultrasonic C-scan recording of three sample alloy HK-40 tubes from a steam-methane reformer

furnace (a) Severely fissured tube (b) Tube with small fissures (c) Tube with no fissures Light areas represent sound attenuation (no through signal); black areas represent no sound attenuation (through signal)

Strip chart recordings of sound attenuation were made on sections of sample tubes By evaluation of the strip charts, the tubes were categorized as to sound attenuation and a grading scale of 1 to 5 was arbitrarily established A rating of 1 represented good sound transmission through unfissured tubes, and a rating of 5 represented little or no sound transmission through severely fissured tubes Intermediate values represented various degrees of fissuring

Field Tests. On the basis of the preliminary tests, a field unit was built for subsequent furnace inspections that could be clamped on a tube and mechanically operated The basic principle of the field unit is shown in Fig 12

Fig 12 Creep fissures in a centrifugally cast HK-40 reformer-furnace tube that are detectable by ultrasonic

inspection and by radiography with nickel catalyst in tube (a) Tube cross section 0.45× (b)Tube wall 2.5× (c) Enlargement of inside diameter portion of wall shown at bottom in (b) 7.5×

The first full plant test with the unit was conducted with catalyst in the tubes At that time, the furnace had operated over 50,000 h The unit was calibrated on sample tubes with known conditions Two men were in the furnace with voice communications to a third man outside the furnace operating the supporting ultrasonic equipment With the unit clamped

to the tube, a single scan was made at the critical hot area at each level Ultrasonic scanning time at each location was less than 30 s, during which the attenuation information was recorded on a strip chart for later interpretation About 32 h was required to inspect the entire furnace

The tubes were graded on the scale of 1 to 5 established during testing of the sample tubes Of the 295 tubes tested, about 15% had ratings of 5 About 30% of these tubes were replaced; the remainder were left in the furnace for a time-to-failure test Of the 15 known fissured tubes left in the furnace at the previous shutdown (see Example 2), only 3 showed severe creep fissures by ultrasonic inspection and were replaced The remaining 12 tubes had ratings of 4

Metallographic examination was made of specimens taken from the removed tubes to determine the reliability of the ultrasonic unit Results indicated that the unit was so sensitive that it could detect mild third-stage creep (not detectable by radiography) as well as severe fissures (detectable by radiography) Because of the sensitivity of the unit, there was some difficulty in differentiating between mild and severe fissuring Radiography with catalyst in the tubes can detect only the most severe creep fissures, but it was used as the basis on which tubes were replaced

The tubes with indications of severe creep fissures (ratings of 5) were radiographed Seven tubes showed definite fissures

on the radiographs, even with catalyst in place These seven tubes and seven additional tubes that showed questionable fissures were replaced One of the known fissured tubes left in the furnace during the previous shutdown showed questionable fissures on the radiograph taken with catalyst and had an ultrasonic rating of 5 This tube was again allowed

to remain in the furnace for a time-to-failure test Metallographic features of a tube with severe creep fissures that had a

rating of 5 with the ultrasonic unit, subsequently confirmed with radiography (with catalyst in the tube), are shown in Fig

12

Conclusions. The ultrasonic unit is a good field inspection tool for centrifugally cast alloy HK-40 tubes Further refinement of the device is necessary to discriminate between mild and severe fissures and thus eliminate the need for radiography to determine the most severe fissures

An ultrasonic inspection system provides the following advantages:

• Higher speed

• Increased coverage at lower cost

• Increased sensitivity

• Need for fewer radiographs

• Elimination of the need for removal of catalyst to effect inspection

In addition, maintenance work need not be interrupted while inspection is in progress, as it must be for radiography

References cited in this section

15 D.J Evans, Field Application of Nondestructive Testing in the Petroleum and Petrochemical Industries, in

Materials Engineering and Sciences Division Biennial Conference, American Institute of Chemical

Engineers, 1970, p 484-487

16 B Ostrofsky and N.B Heckler, Detection of Creep Rupture in Ammonia Plant Reformer Headers, in

Materials Engineering and Sciences Division Biennial Conference, American Institute of Chemical

Engineers, 1970, p 472-476

17 R.R Dalton, Ultrasonic Inspection of Cast HK-40 Tubes for Creep Fissures, Mater Eval., Vol 32 (No 12),

Dec 1974, p 264-268

Nondestructive Inspection of Boilers and Pressure Vessels

In-Service Quantitative Evaluation (Ref 18, 19, 20)

The structural integrity of the reactor pressure vessel receives considerable attention because the vessel is the primary containment for the reactor coolant In the United States, periodic in-service examination of the vessel is performed according to section XI of the ASME Boiler and Pressure Vessel Code Ultrasonic methods of quantitative nondestructive evaluation (NDE) are those most commonly used to accomplish in-service examinations Nearly all of the examinations are performed with remote-controlled equipment Many innovative devices and specialized ultrasonic techniques have been developed to examine components with complex geometry, which are often extremely difficult to access Some of the more difficult areas to examine are the under-clad region nozzle inner radii, nozzle-to-shell welds, dissimilar-metal welds in the safe-ends, and seam welds in areas of complex shape (Ref 18)

Fracture mechanics is used to evaluate indications detected during in-service examinations Accurate measurements of the sizes and locations of all defects are required Furthermore, the probabilistic failure prediction methodology now being used requires additional information on the probability of detecting flaws with each NDE technique These requirements are the driving force in the current trend toward additional regulatory requirements for the quantitative demonstration of NDE performance (Ref 18)

The probability of detection as a function of flaw size and the accuracy of flaw size measurement are the important characteristics of each NDE technique that are to be measured in performance demonstration (Ref 18) Much of the work

to date in the area of performance demonstrations and quantitative NDE of pressure vessels has been carried out at the Electric Power Research Institute (EPRI) Some of the published EPRI work is contained in the extensive list of Selected

References found at the end of this article Additional information on the principles of quantitative analysis can be found

in the Section "Quantitative Nondestructive Evaluation" in this Volume

In designing NDE performance demonstrations, both the examination sensitivity and the mechanical precision

of the scanning device must be addressed to determine if they are adequate to detect and size the defects of concern (Ref 18) Accordingly, the scope of a demonstration can be broken into three conveniently separate parts:

• The mechanical handling system

• The ultrasonic system

• The data-recording system

Mechanical Handling (Ref 18) The intent of testing the mechanical system is to measure and document the accuracy, backlash, and repeatability of the complete remote positioning system over its full range of operation The tolerances of the mechanical system, when combined with those of the ultrasonic system, must give adequate flaw sizing and location capability for any required fracture mechanics analysis

Tests of Ultrasonic and Data-Recording Systems (Ref 18) The intent of these tests is to demonstrate that the intended inspection procedures are capable of detecting and sizing all flaws of potential concern to the safety of the reactor pressure vessel The vessel regions to be addressed can be categorized as follows:

• The region of the vessel in the vicinity of the cladding/base metal interface

• The nozzle regions, including nozzle inner radius, nozzle-to-vessel welds, and safe-end welds (up to and including the pipe-to-safe-end weld)

• The remaining welds that can be further categorized as circumferential or longitudinal welds These are separately identified because of the different relative directions of weld, cladding, and curvature

Full-sized specimens representing the appropriate component are required Intentional flaws are introduced of the size and type of concern For example, these may be thermal fatigue cracks in the nozzle inner radius or intergranular stress-corrosion cracks in the nozzle-to-safe-end weld The number of defects required in the demonstration is dependent on the purpose of that demonstration Tests can be classified as either performance demonstration or validation Additional information can be found in Ref 18 and in the Selected References that follow this article

NDE of Clad Vessels (Ref 19) In the early 1980s, there was much concern in the United States about pressurized thermal shock, a series of events that started with a considerable primary fluid loss, the addition of cold make-up water, and the subsequent repressurization of the system Under these conditions, small cracks (6 mm, or 0.25 in., in depth), if present immediately under the stainless steel vessel clad, could act as initiation sites for crack growth This concern caused many reactor pressure vessel inspection development efforts to be focused on the detection of small cracks in the innermost region of the vessel inner surface The following sections discuss several advanced systems developed at EPRI for the detection and sizing of flaws, the automatic discrimination of flaw signals, and computer-aided sizing from signals using crack tip diffraction methods

The ultrasonic data recording and processing system (UDRPS) is a high-speed, general-purpose device that consists of a large minicomputer, high-speed data channel processor, color video display, and disk/tape storage devices The UDRPS uses a detection criterion that is based on the signal-to-local-noise ratio threshold and the apparent motion of the target within the field of view of the moving transducer Resulting patterns of indications are color coded for signal-to-noise ratio and are viewed by an analyst for the presence of formations suggestive of defects Crack length and depth are also estimated from several image display modes that are available to the operator Figure 13 shows the UDRPS results on a test block Crack length and depth measurement capabilities are shown in Fig 14

Fig 13 UDRPS flaw detection result on a heavy-section test block Source: Ref 19

Fig 14 UDRPS estimates of the flaw depths (a) and flaw lengths (b) in a heavy-section test block Inspection

technique: 45° shear and 60° shear Source: Ref 19

Flaw discriminators provide the ability to distinguish among ultrasonic signal types Feasibility studies have been conducted to demonstrate the use of integrated ultrasonic inspection and pattern recognition systems for distinguishing among slag inclusions, cracks, and spurious clad-noise signals

Pressure Vessel Imaging Systems. The present inspection method for weld zones of nuclear reactor pressure vessels uses a pulse-echo ultrasonic technique for both preservice and in-service inspections Pulse-echo inspection data are not sufficiently accurate to satisfy the demands of structural analysis by fracture mechanics methods This has resulted

in the development of imaging systems that combine conventional ultrasonic inspection techniques (B-scan, C-scan, pulse echo) with acoustic holography, which provides real-time three-dimensional estimations of flaw size and depth and more accurate information for fracture mechanics analysis

Computer-aided sizing through crack tip diffraction involves the use of advanced digital processing methodologies that utilize spectral features of signals diffracted from the under-clad crack tips to distinguish more accurate depth estimates from less reliable ones This technology is based on the development of sizing algorithms that are available as computer codes

References cited in this section

18 A.J Willets, F.V Ammirato, and J.A Jones, Objectives and Techniques for Performance of In-Service

Examination of Reactor Pressure Vessels, in Performance and Evaluation of Light Water Reactor Pressure Vessels, American Society of Mechanical Engineers, 1987, p 79-86

19 G.J Dav and M.M Behravesh, U.S Developments in the Ultrasonic Examination of Pressure Vessels, Int

J Pressure Vessels Piping, Vol 28, 1987, p 3-17

20 P.C Riccardella, J.F Copeland, and J Gilman, Evaluation of Flaws or Service Induced Cracks in Pressure

Vessels, in Performance and Evaluation at Light Water Reactor Pressure Vessels, American Society of

Mechanical Engineers, 1987, p 87-94

Nondestructive Inspection of Boilers and Pressure Vessels

Acoustic Emission Monitoring of Pressure Vessels

When discontinuities exist in a metal, a stress concentration occurs at the tips of discontinuities; under increasing applied stress, deformation occurs first at these discontinuities This deformation, which may be plastic flow, microcracking, or even large-scale cracking, produces signals that, by means of suitable amplification, can be recorded as acoustic emission

The emission is converted to an electrical signal by means of a piezoelectric transducer, which is mounted on the pressure vessel The transducer is contained in a simple housing, which can be bonded to the vessel with glue or with a film of grease The signal is then amplified (often using a preamplifier close to the transducer), filtered to remove low-frequency extraneous mechanical and electrical noise, and then recorded Different types of analysis are used, and the signals can be shaped as pulses to aid quantitative interpretation of data

Stressing of a discontinuity can produce a continuous emission and bursts of high amplitude Analysis of these signals can

be made in any of several ways; for example, each ring of the transducer above a set threshold can be counted or, alternatively, each high-amplitude burst can be counted as a discrete pulse The number of counts per second or the integrated count is then compared with vessel pressure for strain

Application. A series of search units is used for monitoring the emission of sound energy from stressed pressure vessels All the outputs can be digitally stored for subsequent analysis Additionally, some of the circuits can be continuously monitored to give an immediate indication of high acoustic activity and therefore the existence of a severe discontinuity that could cause fracture Any source of acoustic emission can be located by measurement of the time taken for the signal

to reach different search units at known locations On-line or subsequent location techniques can be used, preferably in conjunction with a small computer The number of search units necessary depends on the degree of accuracy required in defining the sources of the emissions

Most of the information in any signal generated from a source within a metal occurs in the 1 to 2 MHz frequency range; however, at these frequencies, the signal attenuation is high, so that the higher frequencies in the stress wave are soon attenuated to near the background noise over relatively short distances (a few meters) The lower-frequency components

of the wave in the 50 to 500 kHz frequency range can transmit over larger distances, because of less attenuation The lower-frequency signals in the 50-kHz range can be detected over relatively large distances on an uncoated surface This means that some general source location can be performed using the lower-frequency components of the acoustic emission signal for large transducer separations, provided the signal attenuation (which increases when a vessel is coated and/or buried) does not reduce the surface wave to the extent that the true signal is lost in the background noise (Ref 9)

Source identification, however, requires the use of wide-band transducers operating at high frequencies and located as close as possible to the source of the emission Only then is it possible to obtain some indication of the original waveform, and even then it will have been modified by its passage through the material to the surface so that care must be used in interpretation

Other factors influencing the interpretation of results include:

• Defect type, size, shape, orientation, and location

• Chemical composition of the vessel material

• Vessel geometry and its associated pipework

• Epoxy-type coatings

Details on these factors can be found in Ref 9 and in the article "Acoustic Emission Inspection" in this Volume

Proof Testing. Because the acoustic emission technique depends on a changing state of stress, especially around a discontinuity, the most convenient time for application to most pressure vessels is at the first proof test With a sufficient number of search units, it is possible to monitor the entire vessel and locate the areas where discontinuities exist Subsequent ultrasonic inspection can confirm the existence of very small acceptable discontinuities in the position indicated by the acoustic emission technique

Test to Failure (Ref 9) In an attempt to improve the interpretation of acoustic emission data, tests have been conducted

on operating vessels, static vessels, and vessels deliberately tested to failure Data on wave propagation and failure mechanisms have been recorded and used to develop a reliable acoustic emission integrity evaluation technique The following example illustrates the use of these tests

Example 5: Test to Failure of a 10-Year-Old Pressure Vessel

(Ref 9) A vessel that had been in service for over 10 years and was operated at a normal pressure of 9.6 MPa (1400 psi)

at 550 °C (1020 °F) was tested by acoustic emission and analyzed by subsequent fractographic examination The defects causing its removal from service were extensive cracks in the nozzle reinforcement These cracks ran circumferentially around the nozzle penetration, and those on the inside surface of the vessel were up to 25 mm (1 in.) deep The test program was designed to pressurize the vessel to failure after many pressure cycles designed to accelerate the growth of flaws in the nozzle by cyclic fatigue Numerous pressurizations were carried out at progressively increasing pressures, including a group of 995 cycles in the range of 7 to 24 MPa (1000 to 3500 psi)

The last four cycles were monitored with acoustic emission The last three cycles were up to 58 MPa (8400 psi), and in the final pressurization, the failure of the vessel occurred at 62 MPa (9000 psi) There was considerable acoustic emission activity detected in the early stages of the test program, but this is thought to have resulted from movement of the vessel supported by observations using television monitors After the pressure had exceeded 56 MPa (8100 psi), this acoustic emission activity diminished concurrently with the vessel becoming more settled on its supports Two active source areas were localized at the bottom of adjoining nozzles It was noted in this test that some of the location patterns were distorted due to some propagation paths being around nozzle holes in the vessel During the final stages of the test, the acoustic emission event rate further diminished, which was partly due to the reduced pumping rate caused by the vessel expansion

More acoustic emission activity was detected from the base of the central nozzle (No 3, Fig 15) than most of the other locations, and this primary initiation point for failure was later confirmed by independent fractograhy examination

Fig 15 Schematic of pressure vessel tested to failure and monitored by acoustic emission Dimensions given in

millimeters Source: Ref 9

The failure was almost entirely brittle in character and appeared to be from localized exhaustion of ductility, with no apparent involvement of any significant preexisting defect It appeared to propagate axially along the vessel from the initiation point and then to branch circumferentially between nozzle Nos 2, 3, and 4, as shown in Fig 15 Half of the vessel then lifted and separated, causing an axial fracture diametrically opposite the underside of these nozzles This fracture acted as a hinge, which allowed this section to lift The only evidence of ductile fracture was in a narrow shear lip adjacent to the external surface of the vessel

The absence of a flaw at the point of initiation was somewhat surprising; however, the highly localized acoustic emission activity and subsequent failure may have resulted from a steep strain gradient adjacent to the reinforced and highly restrained nozzle penetration The failure of the preexisting cracks to propagate confirms predictions of stress analysis that they were not located in highly stressed regions during simple pressurization Their formation during service resulted from loadings imposed by external supports and pipework, rather than from internal pressure

The conditions encountered in this test are clearly different from those that would exist in a nondestructive proof test The vessel suffered extensive plastic deformation at pressure considerably higher than any conceivable proof test pressure Also, the failure occurred by the local exhaustion of ductility in a highly strained region, rather than by the initiation of yielding in a highly stressed region However, the ability of the equipment to identify localized acoustic emission sources was clearly demonstrated

Inspection During Fabrication. During fabrication, acoustic monitoring can be used to detect cracking during or after the welding process Stress-relief cracking can be identified as it occurs, although provision must be made for keeping the transducers at a low temperature This is normally done by the use of waveguides, the extremities of which hold the transducers outside the stress-relieving furnace

In-service inspection may consist of monitoring during periodic proof testing, during normal pressure cycles, or

continuously during normal operation When the vessel is pressurized to a level less than that to which it has been previously subjected (during, for example, the proof tests), little or no acoustic emission occurs Therefore, on subsequent pressurizations a quiet vessel will be obtained unless a crack has extended in service because of corrosion or fatigue On pressurizing after crack growth, the stress system at the enlarged crack will be changed from that in the proof test, and further emission will be obtained

Flaw Location. In many cases, especially on large pressure vessels, it becomes necessary not only to detect acoustic emissions but also to locate the source of the signals This can be accomplished by uniformly spacing multiple search units over the surface area of a pressure vessel and monitoring the time of arrival of the signals to the various search-unit locations Because of the high velocity of sound and the relatively close spacing of search units on a steel vessel, time resolutions must be made in microseconds to locate the source to within a centimeter In most cases, inspection requirements are such that data must be available in a short period of time Therefore, most systems of this type utilize a computer for handling and displaying the data

Reference cited in this section

9 B.R.A Wood, Acoustic Emission Applied to Pressure Vessels, J Acoust Emiss., Vol 6 (No 2), 1989, p

125-132

Nondestructive Inspection of Boilers and Pressure Vessels

References

1 Outlook on Nondestructive Examination, Nucleonics Week, 30 June 1988

2 ASME Boiler and Pressure Vessel Code: Section II Material Specifications, Part A Ferrous Materials; Section III, Division 1 Nuclear Power Plant Components; Section V Nondestructive Examination; Section VIII Division 1 Pressure Vessels, Division 2 Alternative Rules for Pressure Vessels; Section IX Welding and Brazing Qualifications; Section XI Rules for Inservice Inspection of Nuclear Power Plant Components, American Society of Mechanical Engineers

3 "Recommended Practice for Nondestructive Testing Personnel Certification," SNT-TC-1A, American Society for Nondestructive Testing, 1988

4 "Standard Recommended Practice for Magnetic Particle Examination," E 709, Annual Book of ASTM Standards, American Society for Testing and Materials

5 "Standard Practice for Liquid Penetrant Inspection Method," E 165, Annual Book of ASTM Standards,

American Society for Testing and Materials

6 "Standard Specification for Ultrasonic Angle-Beam Examination of Steel Plates," A 577, Annual Book of ASTM Standards, American Society for Testing and Materials

7 "Standard Specification for Straight-Beam Ultrasonic Examination of Plain and Clad Steel Plates for

Special Applications," A 578, Annual Book of ASTM Standards, American Society for Testing and

12 R.H Ferris, A.S Birks, and P.G Doctor, Qualification of Eddy Current Steam Generator Tube

Examination, in NDE in the Nuclear Industry, ASM INTERNATIONAL, 1987, p 71-73

13 V.S Cecco and F.L Sharp, Special Eddy Current Probes for Heat Exchanger Tubing, in NDE in the Nuclear Industry, ASM INTERNATIONAL, 1987, p 109-174

14 R.R Dalton, Radiographic Inspection of Cast HK-40 Tubes for Creep Fissures, Mater Eval., Vol 30 (No

12), Dec 1972, p 249-253

15 D.J Evans, Field Application of Nondestructive Testing in the Petroleum and Petrochemical Industries, in

Materials Engineering and Sciences Division Biennial Conference, American Institute of Chemical

Engineers, 1970, p 484-487

16 B Ostrofsky and N.B Heckler, Detection of Creep Rupture in Ammonia Plant Reformer Headers, in

Materials Engineering and Sciences Division Biennial Conference, American Institute of Chemical

Engineers, 1970, p 472-476

17 R.R Dalton, Ultrasonic Inspection of Cast HK-40 Tubes for Creep Fissures, Mater Eval., Vol 32 (No

12), Dec 1974, p 264-268

18 A.J Willets, F.V Ammirato, and J.A Jones, Objectives and Techniques for Performance of In-Service

Examination of Reactor Pressure Vessels, in Performance and Evaluation of Light Water Reactor Pressure Vessels, American Society of Mechanical Engineers, 1987, p 79-86

19 G.J Dav and M.M Behravesh, U.S Developments in the Ultrasonic Examination of Pressure Vessels, Int

J Pressure Vessels Piping, Vol 28, 1987, p 3-17

20 P.C Riccardella, J.F Copeland, and J Gilman, Evaluation of Flaws or Service Induced Cracks in Pressure

Vessels, in Performance and Evaluation at Light Water Reactor Pressure Vessels, American Society of

Mechanical Engineers, 1987, p 87-94

Nondestructive Inspection of Boilers and Pressure Vessels

Selected References

Replication Microscopy

• J.J Balaschak and B.M Strauss, Field Metallography in Assessment of Steam Piping in Older Fossil

Power Plants, in Microstructural Science, Vol 15, ASM INTERNATIONAL, 1987, p 27-36

• C.J Bolton, B.F Dyson, and K.R Williams, Metallographic Methods of Determining Residual Creep

Life, Mater Sci Eng., Vol 46, 1980, p 231-239

• P.B Ludwigsen, The Replica Method for Inspection of Material Structures and Crack Detection,

Structure, No 15, Sept 1987, p 3-5

• B Neubauer and U Wedel, NDT: Replication Avoids Unnecessary Replacement of Power Plant

Components, Power Eng., Vol 88 (No 5), May 1984, p 5

• N Nilsvang and G Eggeler, A Quantitative Metallographic Study of Creep Cavitation in a 12%

Chromium Ferritic Steel (X20 CrMoV 12 1), Pract Metallogr., Vol 24, 1987, p 323-335

• E.V Sullivan, Field Metallography Equipment and Techniques, in Microstructural Science, Vol 15,

ASM INTERNATIONAL, 1987, p 3-11

Quantitative NDE (1980)

• A.J Boland et al., "Development of Ultrasonic Tomography for Residual Stress Mapping," EPRI

NP-1407, Electric Power Research Institute, May 1980

• A.E Holt, "Defect Characterization by Acoustic Holography Volume 1: Imaging in Field Environments," EPRI NP-1534, Electric Power Research Institute, Sept 1980

• P.H Hutton, "Development of an Acoustic Emission Zone Monitor and Recorder for BWR Cracking Detection," EPRI NP-1408, Electric Power Research Institute, June 1980

Pipe-• W Lord, "Magnetic Flux Leakage for Measurement of Crevice Gap Clearance and Tube Support Plate Inspection," EPRI NP-1427, Electric Power Research Institute, June 1980

• W Lord and R Palanisamy, Magnetic Probe Inspection of Steam Generator Tubing, Mater Eval., Vol 38

(No 5), May 1980

• V.I Neeley et al., "Technology Transfer Phase of Advanced Ultrasonic Nuclear Reactor Pressure Vessel

Inspection System," EPRI NP-1535, Electric Power Research Institute, Sept 1980

• G.P Singh and J.L Rose, "Ultrasonic Field Analysis Program for Transducer Design in the Nuclear Industry," EPRI NP-1335, Electric Power Research Institute, Feb 1980

• S Wenk et al., "NDE Characteristics of Pipe Weld Defects," EPRI NP-1590-SR, Electric Power

Research Institute, Sept 1980

Quantitative NDE (1981)

• D.G Eitzen et al., "Fundamental Development for Quantitative Acoustic Emission Measurements," EPRI

NP-2089, Electric Power Research Institute, Oct 1981

• D.G Eitzen et al., "Summary of Fundamental Development for Quantitative Acoustic Emission

Measurements," EPRI NP-1877, Electric Power Research Institute, June 1981

• T.D Jamison et al., "Studies of Section XI Ultrasonic Repeatability," EPRI NP-1858, Electric Power

Research Institute, May 1981

• W Lord, "Development of a Finite Element Model for Eddy-Current NDT Phenomena," EPRI NP-2026, Electric Power Research Institute, Sept 1981

• W.R McDearman et al., "Steam Generator Support Plate Radiography," EPRI NP-2042, Electric Power

Research Institute, Sept 1981

• E.J Parent et al., "Profilometry for Steam Generator Tube Dents," EPRI NP-2141, Electric Power

Research Institute, Nov 1981

• C.O Ruud, "Review and Evaluation of Nondestructive Methods for Residual Stress Measurements," EPRI NP-1971, Electric Power Research Institute, Sept 1981

Quantitative NDE (1982)

• S Brown, "Field Experiences With Multifrequency-Multiparameter Eddy Current Technology," EPRI NP-2299, Electric Power Research Institute, March 1982

• W.E Cramer et al., "Application of an Eddy Current Technique to Steam Generator U-Bend

Characterization," EPRI NP-2339, Electric Power Research Institute, April 1982

• G.J Dau et al., Automatic Analysis of Eddy Current Signals, in Proceedings of the 5th International Conference on Inspection of Pressurized Components, Institute of Mechanical Engineers, 1982

• E.S Furgason and V.L Newhouse, "Evaluation of Pulse-Echo Ultrasound for Steam Generator Support Plate Gap Measurement," EPRI NP-2285, Electric Power Research Institute, June 1982

Tube-to-• M.E Lapides, Factors Influencing Detection, Location, and Sizing of Flaws: Intergranular Stress

Corrosion Cracking (IGSCC), in Proceedings of the 5th International Conference on Inspection of Pressurized Components, Institute of Mechanical Engineers, 1982

• M.E Lapides and T.U Marston, In-Service Inspection of Heavy Section Castings: Techniques, Results,

Implication, in Proceedings of the Seminar on Improvements in Power Plant Casting Quality (St Charles,

IL), American Society for Metals, 1982

• J.L Rose et al., "A Physically Modelled Feature Based Ultrasonic System for IGSCC Classification,"

Paper presented at the ASNT Spring Conference (Boston), American Society for Nondestructive Testing, March 1982

Quantitative NDE (1983)

• C Bradshaw, Benefits of Automatic Steam Generator Tube Inspection Data Acquisition, in Proceedings

of the SMIRT-7 Post Conference (Monterey, CA), 1983

• S.D Brown, "Eddy Current NDE for Intergranular Attack," EPRI NP-2862, Electric Power Research Institute, Feb 1983

• S.D Brown, "Steam Generator U-Bend Eddy Current NDE," EPRI NP-3010, Electric Power Research Institute, April 1983

• M.E Lapides, "Radiographic Detection of Intergranular Stress Corrosion Cracking: Analysis, Qualification, and Field Testing," EPRI NP-3164-SR, Electric Power Research Institute, Oct 1983

• W Lord, "Magnetic Flux Leakage for Measurement of Crevice Gap Clearance and Tube Support Plate Inspection," EPRI NP-2857, Electric Power Research Institute, Feb 1983

• W.R McDearman et al., "Steam Generator Support Plate Radiographic Evaluation System," EPRI

NP-3253, Electric Power Research Institute, Jan 1983

• V.I Neeley, "Development of a Production Prototype Pressure Vessel Imaging System," EPRI NP-3253, Electric Power Research Institute, Oct 1983

• R.B Thompson et al., "A Prototype EMAT System for Inspection of Steam Generator Tubes," EPRI

NP-2836, Electric Power Research Institute, Jan 1983

• M.E Lapides, MINAC: Portable, High Energy Radiographic Source; Experience and Applications, in

Non-Destructive Examination for Pressurised Components, Elsevier, 1984

• M.E Lapides, Radiographic Detection of Crack-Like Defects in Thick Sections, Mater Eval., Vol 42 (No

6), May 1984

• C.R Mikesell and S.N Liu et al., Detection of Intergranular Stress Corrosion Cracking Using Automated Ultrasonic Techniques, in Proceedings of the Nondestructive Testing and Electrochemical Methods of Monitoring Corrosion in Industrial Plants, ATME E-7/G-1 Symposium, Montreal, Canada, May 1984

Quantitative NDE (1985)

• M.L Fleming, Inspection of Pipe, Tubing and Plate, in Proceedings of the 1985 ASNT Spring Conference

(Washington, D.C.), American Society for Nondestructive Testing, 1985

• M.L Fleming, Field Experience in Automated Ultrasonic Inspection of Stainless Steel, in Proceedings of the 1985 Pressure Vessels and Piping Conference (New Orleans, LA), American Society of Mechanical

Engineers, 1985

• V.I Neeley, "Application of Medical Ultrasonic Testing Technology to the Utility Industry," EPRI

NP-4034, Electric Power Research Institute, May 1985

• R.B Thompson et al., "Ultrasonic Scattering From Intergranular Stress Corrosion Cracks: Derivation and

Application of Theory," EPRI NP-3822, Electric Power Research Institute, Jan 1985

Quantitative NDE (1986)

• F.V Ammirato et al., "Development of Improved Procedure for Examination of Dissimilar-Metal Welds in

BWR Nozzle-to-Safe-End Welds," EPRI NP-4606-LD, Electric Power Research Institute, May 1986

• M.J Avioli, Jr., Modeling Ultrasonic Flaw Detection, EPRI J., Sept 1986

• F.L Becker et al., NDT of Steam Piping and Headers, in Proceedings of the Fossil Plant Inspections Workshop, Electric Power Research Institute, 1986

• E.S Furgason et al., "Digital Techniques to Improve Flaw Detection by Ultrasound Systems," EPRI

NP-4878, Electric Power Research Institute, Oct 1986

• B.P Hildebrand, "Investigation of Advanced Acoustic and Optical Nondestructive Evaluation Techniques," EPRI NP-4897, Electric Power Research Institute, Oct 1986

• K Krzywosz, Trends and Recent Developments in NDE of Steam Generator Tubes, in Proceedings of the Fifth Annual Steam Generator NDE Workshop (Myrtle Beach, SC), Steam Generator Owners Group II,

1986

• S.M Walker, Ultrasonic Examination of Corrosion-Resistant Clad Weldments, in Proceedings of the Southwest Research Institute 14th Nuclear Power Educational Seminar, Southwest Research Institute,

1986

• A.J Willetts et al., "Evaluation of the Ultrasonic Data Recording and Processing System (UDRPS)," EPRI

NP-4397, Electric Power Research Institute, Jan 1986

• D MacDonald and E.K Kietzman, Comparative Evaluation of Acoustic Holography Systems, in

Proceedings of the 8th International Conference on NDE in the Nuclear Industry (Orlando, FL), ASM

• R.B Thompson et al., "Modeling Ultrasonic Inspection of Nuclear Components Beam Models and

Applications," EPRI NP-5330, Electric Power Research Institute, Aug 1987

• A.J Willetts and E.K Kietzman, UT Techniques for Detection and Sizing of Under-Clad Cracks in Reactor

Pressure Vessels, in Proceedings of the 8th International Conference on NDE in the Nuclear Industry

(Orlando, FL), ASM INTERNATIONAL, 1987

Quantitative NDE (1988)

• F Ammirato, "NDE of Cast Piping in the Nuclear Industry," Paper presented at the IAEA Specialists Meeting on the Inspection of Austenitic and Dissimilar Metal Welds, Espoo, Finland, International Atomic Energy Agency, June 1988

• F Ammirato et al., Ultrasonic Examination of Dissimilar-Metal Welds in PWR and BWR Plants, in Destructive Examination in Relation to Structural Integrity, Proceedings of the Post-SMiRT Seminar #3,

• B.P Newberry and R.B Thompson, Prediction of Surface Induced Ultrasonic Beam Distortions, in Review

of Progress in Quantitative NDE-8, Plenum Press, 1988

• J.L Rose, "Wave-Propagation Studies for Improved Ultrasonic Testing of Centrifugally Cast Stainless Steel," EPRI NP-5979, Electric Power Research Institute, Aug 1988

• J Saniee, T Wong, and N.M Bilgutay, Optimal Ultrasonic Flaw Detection Using a Frequency Diversity

Technique, in Review of Progress in Quantitative NDE-8, Plenum Press, 1988

• T Sasahara and F Ammirato, "Automated Ultrasonic Pipe Examination and Interpretation," EPRI

NP-5760, Electric Power Research Institute, April 1988

• R Shankar, Field Application of Integrated Ultrasonic Feature-Based and Imaging-Based Analysis, in

Proceedings of the 9th International Conference on Non-Destructive Evaluation in the Nuclear Industry,

ASM INTERNATIONAL, 1988

• R Shankar et al., Feature-Enhanced Ultrasonic Imaging Application of Signal Processing and Analysis, in Non-Destructive Examination in Relation to Structural Integrity, Proceedings of the Post-SMiRT Seminar

#3, Elsevier, 1988

Introduction to Quantitative Nondestructive Evaluation

Vicki E Panhuise, Allied-Signal Aerospace Company, Garrett Engine Division

Introduction

THE RELIABILITY of a nondestructive inspection (NDI) procedure was defined in the article "Reliability of Flaw

Detection by Nondestructive Inspection" in Volume 11 of the 8th Edition of Metals Handbook as a quantitative measure

of the efficiency of that procedure in finding flaws of specific type and size During the years since that article was published, many inspection reliability programs have been conducted, and various quantitative measurements have been cited to express the procedure capabilities To establish the basis for NDI method reliability, it is necessary to review the history of NDI and its relation to reliability methods

Historical Development of Quantitative Measurement Techniques

Nondestructive inspection methods are specified for material and/or component inspection requirements to maintain the necessary quality for the final service life of the material/component In most industries, the inspection requirements are defined in a specification that describes the sensitivity level of the inspection method as well as the rejectable flaw size

An example of a specification requirement is given in Table 1; the specification is based on longitudinal wave inspection using flat-bottom holes (FBH) It defines the ultrasonic inspection requirements for product over 13 mm (0.5 in.) thick Table 1 indicates the defect detection/rejection limits for each quality class material For example, quality class AA requires that single discontinuities be detected at a sensitivity level equivalent to a No 3 (1.2 mm, or in., diam) FBH

In addition, multiple discontinuities* shall be detected at a sensitivity level equivalent to a No 1 (0.4 mm, or in., diam) FBH

Table 1 Typical NDI acceptance criteria

Five classes of ultrasonic quality are established for longitudinal wave inspection These classes are defined for inspection involving flat-bottom reflectors in ultrasonic references

Quality class Single discontinuity (a) ,

FBH No

Multiple discontinuities (a) , FBH No

Maximum linear discontinuity, in

Maximum loss of back reflection, %

C As established by purchaser and vendor for specific part

Source: Ref 1

(a)

FBH numbers indicate diameter in multiples of 0.4 mm ( in.) of FBH in ultrasonic reference

(b) 11% of a No 3 FBH is equivalent to a No 1 FBH and can be used in place of the response from the No 1 FBH

(c) 44% of a No 3 FBH is equivalent to a No 2 FBH and can be used in place of the response from the No 2 FBH

Each specification also defines a procedure for demonstrating the capability to detect defects at the required sensitivity levels In AMS 2630A, this defined procedure is a calibration technique used to set up the ultrasonic instrumentation Ultrasonic standards have been designed to establish the performance of the inspection system Procedures have been defined by the American Society for Testing and Materials for the manufacture of these ultrasonic reference blocks for longitudinal wave testing (Ref 2, 3) Finally, the specifications require specific training for the inspectors For conformance with AMS 2630A, personnel must be certified to and function within the limits of their levels of certification as specified by the American Society for Nondestructive Testing (Ref 4) Further details of ultrasonic inspection procedures are described elsewhere in this Volume

These specification requirements were designed to control the inspection processes and the quality of the inspection results However, several catastrophic failures of major engineering systems (such as the F-111, space shuttle, nuclear reactors, and the Alaskan pipeline) and the development of advanced materials such as composites were major forces in the development and application of NDI technology (Ref 5) In concert with these events, a new design method using linear elastic fracture mechanics (LEFM) required inspection methodology for detecting defects in production and in

service (for a description of LEFM, see Mechanical Testing, Volume 8 of ASM Handbook, formerly 9th Edition Metals Handbook) Linear elastic fracture mechanics design assumes the presence of structural defects and then allows the

designer to answer the following questions:

• What is the critical flaw size that will cause failure for a given component subject to service stress and temperature conditions?

• How long can a precracked structure be safely operated in service?

• How can a structure be designed to prevent catastrophic failure from preexisting cracks?

• What inspections must be performed to prevent catastrophic failure?

The ability to answer these questions forms the basis of nondestructive evaluation (NDE), which involves

damage-tolerant design approaches and is centered on the philosophy of ensuring safe operation in the presence of flaws The U.S Air Force has used damage tolerance analysis, as shown in Fig 1 The initial component in the as-manufactured condition

is assumed to have a flaw of length a0 This flaw length is based on the manufacturing inspection capability or material

flaw size distribution The growth of the flaw is predicted for service usage and will reach a critical flaw size, af, after t0

flight hours The current Air Force philosophy requires an inspection at half the time required for the potential crack to

grow to critical size This inspection is assumed to detect and remove any flaw of size larger than aNDE The assumption creates the requirement to determine NDE probability of detection (POD) for this current design practice, as described in the articles that follow in this Section

Fig 1 Crack growth life curve for damage-tolerant design

The driving functions described above caused the NDE/NDI community to evaluate the inspection capabilities The first evaluation in the aerospace industry was conducted by Lockheed in the 1970s and was called the "Have Cracks, Will Travel" program (Ref 6) This program was established to determine if airframe inspection would repeatedly detect cracks

so that engineers could use LEFM philosophy for design The study concluded that the overall reliability of NDI performed by the Air Force and evaluated in the program falls below that which has been previously assumed by established guidelines (Ref 6) The most significant result of the program was that the 90/95 reliability criteria could not

be attained for any flaw size with typical inspection techniques applied by the average technician Many advancements have been made in NDE/NDI technology to improve the inspection capability since this study These technologies are described throughout this Volume

References cited in this section

1 "Ultrasonic Inspection of Product over 0.5 in (13 mm) Thick," Aerospace Material Specification 2630A, Society of Automotive Engineers, 1980 (original release 1960)

2 "Standard Practice for Fabricating and Checking Aluminum Alloy Ultrasonic Standard Reference Blocks," E

127, Annual Book of ASTM Standards, American Society for Testing and Materials

3 "Standard Recommended Practice for Fabrication and Control of Steel Reference Blocks Used in Ultrasonic

Inspection," E 428, Annual Book of ASTM Standards, American Society for Testing and Materials

4 "Recommended Practice, Personnel Qualification and Certification in NDT," SNT-TC-IA, American Society for Nondestructive Testing

5 W Rummel, Recommended Practice for a Demonstration of Nondestructive Evaluation (NDE) Reliability on

Aircraft Production Parts, Mater Eval.,Vol 40 (No 9), Aug 1982, p 922

6 W.H Lewis, et al., "Reliability of Nondestructive Inspections Final Report," Report SA-ALC/MME

76-6-38-1, United States Air Force, Air Logistics Center, Kelly Air Force Base, 1978

Note cited in this section

* Multiple discontinuities are defined in AMS 2630A as two or more indications above the level established for the class that occur within 16 cm3 (1.0 in.3) of the inspected surface

Introduction to Quantitative Nondestructive Evaluation

Vicki E Panhuise, Allied-Signal Aerospace Company, Garrett Engine Division

NDE Reliability

The following article in this Section "Fracture Control Philosophy" describes the U.S Air Force philosophy for current design practices Damage-tolerant design requires knowledge of the reliability of the inspection technique used to detect flaws or damage In the initial stages of using this design approach, a one-number characterization of NDE capability was

in use The one-number characterization was the minimum crack (flaw) length for which there is a fixed degree of confidence that at least a fixed probability of cracks will be detected Typically, the minimum crack length was chosen such that, at the 95% confidence level (CL), at least 90% of all cracks greater than this length will be detected The number is referred to as the 90/95 (POD/CL) crack length

Probability of detection functions for describing the reliability of an NDE technique have been the subject of many

studies The ideal inspection system POD function is shown schematically in Fig 2 All flaws larger than aNDE would be

detected all of the time, while all flaws smaller than aNDE would not Obviously, no ideal system exists, and the POD functions used produce a continuous curve Figure 3 shows a POD curve for an automated eddy current inspection of titanium bolt holes Two confidence level POD curves are shown, 50 and 95% Berens and Hovey completed a study that compared POD analysis techniques to determine the optimum calculation method (Ref 7) Additional information concerning the analysis of NDE data for the determination of reliability is available in the article "NDE Reliability Data Analysis" in this Section

Fig 2 Schematic of probability of detection curves

Fig 3 POD curve for the automated eddy current inspection of titanium bolt holes

The accuracy of the POD function is dependent on the demonstration program design The data acquired during the demonstration program testing can influence the resulting prediction of reliability For example, consider the cases presented in Fig 4, which graphically presents the specimen/crack depth histogram for two case studies If the design

requirement is such that aNDE (that is, 90/95 crack depth) must be 250 μm (10 mils) or less, only the specimens in case study No 2 could demonstrate this capability That is, to demonstrate 90/95 crack size, the specimens fabricated for the

demonstration program must have cracks less than aNDE and some larger If it is assumed that all the flaws were detected

in both cases, the 90/95 crack sizes predicted are as follows:

90/95 crack size

Case study

μm mils

1 417 16.40

Fig 4 Specimen crack depth distribution for two case studies (a) Case study No 1 (b) Case study No 2

This case study, which demonstrates the criticality of planning the reliability demonstration program, showed only the importance of specimen flaw size to the final outcome of the reliability study

In the following articles in this Section, fracture control philosophy is discussed, reliability demonstrations that have been completed are reviewed, and the analysis of NDE data is examined The final article in this Section, "Models for Predicting NDE Reliability," provides information on advanced modeling studies for predicting reliability for the inspection of a particular component The objective of this Section is to provide the necessary information and resources

to conduct an effective NDE reliability program

Reference cited in this section

7 A.P Berens and P.W Hovey, "Flaw Detection Reliability Criteria Volume I Methods and Results, Final Report," Report AFWAL-TR-84-4022 Volume I, United States Air Force Material Laboratory, Wright- Patterson Air Force Base, 1984

Introduction to Quantitative Nondestructive Evaluation

Vicki E Panhuise, Allied-Signal Aerospace Company, Garrett Engine Division

References

1 "Ultrasonic Inspection of Product over 0.5 in (13 mm) Thick," Aerospace Material Specification 2630A, Society of Automotive Engineers, 1980 (original release 1960)

2 "Standard Practice for Fabricating and Checking Aluminum Alloy Ultrasonic Standard Reference

Blocks," E 127, Annual Book of ASTM Standards, American Society for Testing and Materials

3 "Standard Recommended Practice for Fabrication and Control of Steel Reference Blocks Used in

Ultrasonic Inspection," E 428, Annual Book of ASTM Standards, American Society for Testing and

Materials

4 "Recommended Practice, Personnel Qualification and Certification in NDT," SNT-TC-IA, American Society for Nondestructive Testing

5 W Rummel, Recommended Practice for a Demonstration of Nondestructive Evaluation (NDE)

Reliability on Aircraft Production Parts, Mater Eval.,Vol 40 (No 9), Aug 1982, p 922

6 W.H Lewis, et al., "Reliability of Nondestructive Inspections Final Report," Report SA-ALC/MME

76-6-38-1, United States Air Force, Air Logistics Center, Kelly Air Force Base, 1978

7 A.P Berens and P.W Hovey, "Flaw Detection Reliability Criteria Volume I Methods and Results, Final Report," Report AFWAL-TR-84-4022 Volume I, United States Air Force Material Laboratory, Wright-Patterson Air Force Base, 1984

Fracture Control Philosophy

William D Cowie, United States Air Force, Aeronautical System Division, Propulsion Directorate

Introduction

FRACTURE CONTROL PHILOSOPHIES are being used in the design, development, and life management of United States Air Force (USAF) turbine engine and airframe components This article describes the fracture control program for turbine engine components The section "Applications (Case Studies)" of the article "Applications of NDE Reliability to Systems" in this Volume provides an overview of fracture control programs for both airframe and turbine engine components

The establishment of a fracture control philosophy and the implementation of a fracture control program have been integral components of the USAF turbine engine development process since 1978 They have been applied to new engine programs as part of the USAF Engine Structural Integrity Program (ENSIP) described in military standard MIL-STD-

1783 (issued formally in 1984) This military standard was reviewed and approved by the Aerospace Industries Association of America in 1982

Fracture control philosophy has also been applied to existing inventory USAF engines through structural durability and damage tolerance assessments In all, fracture control programs have been applied or are being applied to the F-100, TF-

34, F100-PW-220, F100-PW-229, F110-GE-100, F110-GE-129, F101-GE-102, F109-GA-100, F-119, F120, and T406 engines and have resulted in the implementation of enhanced nondestructive evaluation (NDE) methods (for example, eddy current inspection) at manufacturing and at field/depot These inspections have been successful in detecting early cracking and in accelerating corrective actions Several developmental efforts in the last 5 years have identified fluorescent penetrant inspection process improvements that must be implemented within industry and Air Force depots to improve flaw detection reliability The need to quantify detection reliability for imbedded defects is also identified

The engine development process has been evolutionary in terms of the application of upgraded requirements The new process of fracture control, sometimes referred to as damage tolerance, is contained in ENSIP, and it involves material selection as well as design and life management Recent experience clearly demonstrates that the damage tolerance requirement is cost effective when assessed on a life cycle basis

Fracture Control Philosophy

William D Cowie, United States Air Force, Aeronautical System Division, Propulsion Directorate

Overview of ENSIP

In the past 16 years, a large number of structural problems have occurred in USAF gas turbine engines Many of these were safety problems that resulted in loss of aircraft, and an even greater number affected durability, causing a high level

of maintenance and modification costs All of these problems have adversely affected fleet readiness The Engine Structural Integrity Program was intended to reduce these problems substantially and was developed based on the following specific lessons:

• It is unrealistic (and can be dangerous) to assume defect-free structure in safety-of-flight components

• Critical parts (and part details) and potential failure modes must be identified early and appropriate control measures implemented

• Internal thermal and vibratory environments must be identified early in the engine development

• Predicted analytical stresses must be verified by test for complex components

• Materials and processes must be adequately characterized (particularly, the fracture properties)

• Design stress spectra, component test spectra, and full-scale engine test spectra must be based on the anticipated service usage of the engine, that is, accelerated mission-related testing

• Potential engine/airframe structural interactions must be defined and accounted for

• Management procedures (such as individual engine tracking procedures and realistic inspection and maintenance requirements) must be defined and enforced

The Engine Structural Integrity Program was established by the Air Force to provide an organized and disciplined approach to the structural design, analysis, development, production, and life management of gas turbine engines, with the goal of ensuring engine structural safety, increased service readiness, and reduced life cycle costs The five major tasks associated with ENSIP are the development of design information; design analysis and component and material characterization; component and core engine testing; ground and flight engine testing; and production quality control and engine life management Each major task is subdivided into a number of subtasks (Table 1) that guide the development process

Table 1 Tasks of the engine structural integrity program

Development plans

ENSIP master Durability and damage control Material and process characterization Corrosion prevention and control Inspection and diagnostics

Operational requirements

Design service life and usage requirements Design criteria

Task II: Design analysis and material characterization and development tests

Design duty cycle

Material characterization

Design development tests

Structural/thermal analysis

Installed engine inspectability

Manufacturing and quality control

Task III: Component and core engine tests

Component tests

Strength Vibration Damage tolerance Durability

Core engine tests

Thermal survey Vibration strain and flutter boundary survey

Ground engine tests

Strength Damage tolerance Accelerated mission test Thermal survey Vibration strain and flutter boundary survey

Flight engine tests

Fan strain survey Thermal survey Installed vibration Deterioration

Updated analyses

Structural maintenance plan

Operational usage survey

Individual engine tracking

Durability and damage tolerance control actions (production)

The Engine Structural Maintenance Plan represents the output of the ENSIP program This plan identifies and defines individual part life limits, the necessary inspection periods for each fracture-critical part, and the inspection procedure The basic components of the Engine Structural Maintenance Plan are as follows:

• Structural safety is obtained in ENSIP by requiring a structure with a damage tolerance that is capable

of accommodating flaws induced either in manufacture or service

• Durability design requirements stipulate that the economic life of the engine must exceed the specified design service life of the aircraft when flown to the design usage spectra

• Maintainability criteria require that old parts fit and function with new parts, that repair life be defined,

and that inspectability and structural diagnostics be designed into the engine and its components

• A materials and process characterization plan controls materials development through key engine development points

• Environmental definition requirements specify the thermal, dynamic, and steady-state stress; the stress spectra; and the component sensitivities

• A comprehensive ground test policy is utilized to ensure compliance with safety, durability, and maintainability requirements

• A usage and tracking policy is used to form the basis of an engine life management program

Fracture Control Philosophy

William D Cowie, United States Air Force, Aeronautical System Division, Propulsion Directorate

• It is unrealistic (and can be dangerous) to assume defect-free structure in safety-of-flight components

• Critical parts (and part details) and potential failure modes must be identified early and appropriate control measures implemented

• Internal thermal and vibratory environments must be identified early in the engine development

• Predicted analytical stresses must be verified by test for complex components

• Materials and processes must be adequately characterized (particularly, the fracture properties)

• Design stress spectra, component test spectra, and full-scale engine test spectra must be based on the anticipated service usage of the engine, that is, accelerated mission-related testing

• Potential engine/airframe structural interactions must be defined and accounted for

• Management procedures (such as individual engine tracking procedures and realistic inspection and maintenance requirements) must be defined and enforced

The Engine Structural Integrity Program was established by the Air Force to provide an organized and disciplined approach to the structural design, analysis, development, production, and life management of gas turbine engines, with the goal of ensuring engine structural safety, increased service readiness, and reduced life cycle costs The five major tasks associated with ENSIP are the development of design information; design analysis and component and material characterization; component and core engine testing; ground and flight engine testing; and production quality control and engine life management Each major task is subdivided into a number of subtasks (Table 1) that guide the development process

Table 1 Tasks of the engine structural integrity program

Development plans

ENSIP master Durability and damage control Material and process characterization Corrosion prevention and control

Inspection and diagnostics

Operational requirements

Design service life and usage requirements Design criteria

Task II: Design analysis and material characterization and development tests

Design duty cycle

Material characterization

Design development tests

Structural/thermal analysis

Installed engine inspectability

Manufacturing and quality control

Component tests

Strength Vibration Damage tolerance Durability

Core engine tests

Thermal survey Vibration strain and flutter boundary survey

Ground engine tests

Strength Damage tolerance Accelerated mission test Thermal survey Vibration strain and flutter boundary survey

Flight engine tests

Fan strain survey Thermal survey Installed vibration

Task V: Engine life management

Updated analyses

Structural maintenance plan

Operational usage survey

Individual engine tracking

Durability and damage tolerance control actions (production)

The Engine Structural Maintenance Plan represents the output of the ENSIP program This plan identifies and defines individual part life limits, the necessary inspection periods for each fracture-critical part, and the inspection procedure The basic components of the Engine Structural Maintenance Plan are as follows:

• Structural safety is obtained in ENSIP by requiring a structure with a damage tolerance that is capable

of accommodating flaws induced either in manufacture or service

• Durability design requirements stipulate that the economic life of the engine must exceed the specified design service life of the aircraft when flown to the design usage spectra

• Maintainability criteria require that old parts fit and function with new parts, that repair life be defined, and that inspectability and structural diagnostics be designed into the engine and its components

• A materials and process characterization plan controls materials development through key engine development points

• Environmental definition requirements specify the thermal, dynamic, and steady-state stress; the stress spectra; and the component sensitivities

• A comprehensive ground test policy is utilized to ensure compliance with safety, durability, and maintainability requirements

• A usage and tracking policy is used to form the basis of an engine life management program

Fracture Control Philosophy

William D Cowie, United States Air Force, Aeronautical System Division, Propulsion Directorate

ENSIP and Fracture Control Philosophy Policy

Damage tolerance is defined as the ability of the engine to resist failure due to the presence of flaws, cracks, or other damage for a specified period of usage The damage tolerance or fracture control philosophy used in ENSIP is shown in Fig 4 Components are designed for crack growth so that the safety limit exceeds two times the required inspection interval The safety limit or residual life is the time for assumed initial flaws to grow and cause failure Because the requirement is to inspect at one-half the safety limit, the design goal for the safety limit is two times the required design life (that is, no inspections) The minimum design requirement for the safety limit is two times the planned depot visit interval An important aspect of the damage tolerance requirement is that it applies only to fracture-critical components

Fig 4 Damage tolerance approach to life management of cyclic-limited engine components The safety limit or

residual life is the time for the initial flaw to grow and cause failure The size of the initial flaw, ai, is based on the inspection method or material defect distribution (for imbedded defects)

Fracture-critical components are defined as those components whose failure will result in probable loss of the aircraft due to noncontainment or, for single-engine aircraft, power loss that presents sustained flight because of direct part failure or by causing other progressive part failures Damage tolerance requirements are applied only to fracture-critical components (that is, components that must maintain their integrity during flight) and not, in general, to durability-critical components (that is, components that affect maintenance schedules) As expected, component classification is affected by aircraft engine configuration (single engine or multiengine) Component classification is established early and

is identified in the contract

Initial Flaw Size. Initial flaws are assumed to exist in fracture-critical components Experience has shown that premature cracking (that is, crack initiation prior to the LCF limit) occurs at high-stress areas and where components initially contained both material- and manufacturing-related quality variations (voids, inclusions, machining marks, scratches, sharp cracks, and so on) The fracture control or damage tolerance requirement assumes a sharp crack as the initial flaw when characterizing these abnormal initial conditions The assumed initial imbedded flaw sizes are based on the intrinsic material defect distribution or the NDE methods to be used during manufacture The assumed surface flaw size also depends on the NDE capability An inspection reliability of 90% probability of detection (POD) at the lower-bound 95% confidence level (CL) is required for the assumed initial flaw sizes

The assumed initial flaw size to account for intrinsic material defect distribution should encompass 99.99% of the defect population if a scatter factor of two is used to establish the inspection interval, or 99.9% if a scatter factor of one is used

If embedded defects cannot be inspected in service, the 99.99 percentile (or the 99.9 percentile) is used to satisfy the design life requirement

An initial flaw size not less than 0.75 mm (0.030 in.) in length (for surface flaws) or 0.4 × 0.4 mm (0.015 × 0.015 in.) in size (corner cracks) for nonconcentrated stress areas (bores, webs, and so on) is required Initial flaw sizes for other surface locations (holes, fillets, scallops, and so on) will be consistent with the demonstrated capability (90% POD/95% CL) of the inspection systems proposed for use It is recommended that the initial design and sizing of components be based on 0.75 mm (0.030 in.) long surface flaws or 0.4 × 0.4 mm (0.015 × 0.015 in.) corner cracks at all locations This design recommendation is based on the initial flaw size that can be detected with fluorescent penetrant inspection This includes fully automated fluorescent penetrant inspection systems that are being developed to meet the 0.75 mm (0.030 in.) and 0.4 × 0.4 mm (0.015 × 0.015 in.) inspection criteria

These flaw sizes are intended to represent the maximum size of the damage that can be present in a critical location after manufacture and/or inspection The specification of these flaw sizes is based on the demonstrated flaw detection capability of the nondestructive inspection (NDI) method During design of the components, the assumed initial flaw size that is appropriate for various NDI methods is:

• 0.75 mm (0.030 in.) surface length where the NDI method is fluorescent penetrant inspection

• 0.25 mm (0.010 in.) surface length where the NDI method is eddy current or ultrasonic inspection

• 1.3 mm2 (0.002 in.2) area for imbedded defects utilizing ultrasonic inspection

• 5 mm (0.200 in.) surface length and imbedded sphere = 0.2 × thickness for weldments

• When initial flaw sizes are based on material defect distribution, selected size shall encompass 99.99%

of the distribution

• Demonstration that assumed flaw sizes can be reliably detected with a 90% POD and a 95% CL

The capabilities of the NDI method must be demonstrated by the contractor The design of NDE reliability experiments is discussed in the article "NDE Reliability Data Analysis" in this Volume

Residual strength is defined as the load-carrying capability of a component at any time during the service exposure period, considering the presence of damage and accounting for the growth of damage as a function of exposure time The requirement is to provide limit load residual strength capability throughout the service life of the component In other words, the minimum residual strength for each component (and location) must be equal to the maximum stress that occurs within the applicable stress spectra based on the design duty cycle Normal or expected overspeed due to control system tolerance and engine deterioration is included in the residual strength requirement, but fail-safe conditions, such as burst margin, are excluded The residual strength requirement is illustrated in Fig 5

Inspection Intervals. It is highly desirable to have no damage tolerance inspections required during the design lifetime of the engine This in-service noninspectable classification requires that components be designed such that the residual life or safety limit be twice the design life Designing components as in-service noninspectable is a requirement for those components or locations that cannot

be inspected during the depot maintenance cycle

However, the weight penalty incurred to achieve a safety limit/residual life/damage growth interval twice the design life may be prohibitive on some components/locations Therefore, in-service inspections will be allowed on some components subject to justification The basis for the justification is characterization of the costs as a function of the requirements as established by trade studies Cost is usually expressed in terms of weight or life cycle cost, and the requirement in terms of safety limit/residual life/damage growth interval

The depot or base-level inspection interval for damage tolerance considerations should be compatible with the overall engine maintenance plan Once again, it is highly desirable that the inspection interval be equal to the design service life of the parts in the hot gas path (that is, the hot-part design service life, which is equal to one-half the design lifetime of the engine) because this is the expected minimum depot or maintenance interval for the engine or module It is required that the minimum damage tolerance inspection interval be contained in the contract specification

Flaw Growth. It is required that the assumed initial flaw sizes will not grow to critical size and cause failure of a component due to the application of the required residual strength load in two times the inspection interval The flaw growth interval is set equal to two times the inspection interval to provide a margin for a variability that exists in the total process (that is, inspection reliability, material properties, usage, stress predictions, and so on) Factors other than two should be used when individual assessments of the variables that affect crack growth can be made (for example, to account for observed scatter in crack growth during testing)

Fig 5 Diagram of the residual strength requirement

It is important that the effects of vibratory stress on unstable crack growth be accounted for in establishing the safety limit Experience shows that the threshold crack size can be significantly less than the critical crack size associated with the material fracture toughness, depending on the material, the major stress cycle, and the vibration stress As shown in Fig 6, the conventional Goodman diagram may not disclose the true sensitivity of initial defects to vibratory stresses The threshold crack size must be established at each individual sustained-power condition (idle, cruise, intermediate) using the appropriate values of steady stress and vibratory stress The smallest threshold crack size will be used as a limiting value

in calculating the safety limit if it is less than the critical crack size associated with the material fracture toughness

Fig 6 Interaction of vibratory stress and initial flaws (a) Large vibratory stress required to initiate crack (b)

Low vibratory stress will propagate cracks The crack growth threshold, At, represents the threshold of vibratory motion that will cause the growth of a given crack size

Fracture Control Philosophy

William D Cowie, United States Air Force, Aeronautical System Division, Propulsion Directorate

Fracture Control Verification

Verification that the fracture control policy is met is accomplished by the development and implementation of a Damage Tolerance Control Plan, by analysis and test, and by the implementation of reliable inspection methods during manufacture and field/depot maintenance

A Damage Tolerance Control Plan is prepared that identifies and schedules each of the tasks and interfaces in the functional areas of design, materials selection, tests, manufacturing control, and inspection Specific tasks that are addressed in the Damage Tolerance Control Plan are:

• Trade studies for design concepts/material/weight/performance/cost

• Analysis

• Development and qualification tests

• Fracture-critical parts list

• Zoning of drawings

• Basic materials fracture data

• Material properties controls

• Traceability

• NDI requirements