7. F. McClintock and A. Argon, Mechanical Behavior of Material, Chapter 12, Residual Stress, Addison-Wesley 1966,p420-434 Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A Moore, Packer Engineering and william Dobson, Binary Engineering Associates, Inc. Analyzing Distortion Failures Distortion failures are often considered to be instances of relatively simple phenomena that are easy to analyze because deformation can occur only when the applied stress exceeds the flow strength of the material. On the contrary, distortion is not always the result of simple overload or use of an improperly processed part. Analysis of a distortion failure often must be exceptionally thorough and rigorous to determine the root cause of failure and, more importantly, to specify proper corrective action. The analyst must consider factors that may not have been anticipated in design of the part, such as material substitutions or process changes during manufacture and the misuse, abuse, or occurrence of complex stress fields in service A seemingly innocent substitution of material resulted in distortion of small volute springs made of cold-worked spring- tempered Inconel. Normally, the material was purchased as cold-flattened wire, but one lot of springs was formed from cold-rolled and slit strip because flattened wire could not be obtained in time to meet the delivery schedule. After a presetting operation, the strip springs were consistently out of tolerance; they had taken an excessive set. The Inconel strip had a hardness of 360 HV, compared to 390 HV for the wire that normally was used This represents a difference of about 10% in strength and accounts for the observed distortion. Had this problem gone undetected during manufacture, it might rell have resulted in distortion failures in service Analytical Procedure. The general process of materials failure analysis, adapted from the typical procedure for analysis of a distortion failure, is briefly summarized by the following ten steps Define the effect of the failure on the structure or assembly, and define the desired results of corrective action 2. Obtain all available design and service information 3. Examine the distorted part visually, making a record of observations, including a sketch or a photograph of the distorted part along with an undistorted part for comparison. Note all pertinent measurements of dimensions. It usually helpful to enter these measurements, which should be made with at least the same precision as in a quality-control inspection, alongside the design dimensions on a blueprint of the part 4. Perform laboratory tests as necessary to confirm the composition, structure, and other chemical or metallurgical characteristics of the distorted part 5. Trace the failed part through all manufacturing processes to discover whether process deviations occurred during 6. Compare the actual conditions of service with design assumptions 7. Compare the actual material properties with design specifications 8. Determine whether any differences found in the preceding two steps fully account for the distortion observed in the failed structure. If the differences do not fully account for the observed distortion, the information obtained in the second or fourth step is incorrect or incomplete 9. Prepare alternative courses of action to correct the variant factors that caused the observed distortion, and select the course that seems most likely to produce the desired result, which was defined in the first step 10. Test the selected course of corrective action to verify its effectiveness. Evaluate side effects of the corrective action, such as its effects on cost or on ease of implementatio xample 6: Deformation of a Gas-Nitrided Drive-Gear Assembly. Slipping of components in the left-side final drive train of a tracked military vehicle was detected after the vehicle had been driven 13, 700 km(8500 miles)in combined highway and rough-terrain service. No abnormal service conditions were reported in the history of the vehicle. The slipping was traced to the mating surfaces of the final drive gear and the adjacent splined coupling sleeve(Fig. 13). Failure analysis was conducted to determine the cause of the malfunction and to recommend corrective measures that would prevent similar failures in other vehicles
7. F. McClintock and A. Argon, Mechanical Behavior of Material, Chapter 12, Residual Stress, Addison-Wesley, 1966, p 420–434 Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A. Moore, Packer Engineering and William Dobson, Binary Engineering Associates, Inc. Analyzing Distortion Failures Distortion failures are often considered to be instances of relatively simple phenomena that are easy to analyze because deformation can occur only when the applied stress exceeds the flow strength of the material. On the contrary, distortion is not always the result of simple overload or use of an improperly processed part. Analysis of a distortion failure often must be exceptionally thorough and rigorous to determine the root cause of failure and, more importantly, to specify proper corrective action. The analyst must consider factors that may not have been anticipated in design of the part, such as material substitutions or process changes during manufacture and the misuse, abuse, or occurrence of complex stress fields in service. A seemingly innocent substitution of material resulted in distortion of small volute springs made of cold-worked springtempered Inconel. Normally, the material was purchased as cold-flattened wire, but one lot of springs was formed from cold-rolled and slit strip because flattened wire could not be obtained in time to meet the delivery schedule. After a presetting operation, the strip springs were consistently out of tolerance; they had taken an excessive set. The Inconel strip had a hardness of 360 HV, compared to 390 HV for the wire that normally was used. This represents a difference of about 10% in strength and accounts for the observed distortion. Had this problem gone undetected during manufacture, it might well have resulted in distortion failures in service. Analytical Procedure. The general process of materials failure analysis, adapted from the typical procedure for analysis of a distortion failure, is briefly summarized by the following ten steps: 1. Define the effect of the failure on the structure or assembly, and define the desired results of corrective action. 2. Obtain all available design and service information. 3. Examine the distorted part visually, making a record of observations, including a sketch or a photograph of the distorted part along with an undistorted part for comparison. Note all pertinent measurements of dimensions. It is usually helpful to enter these measurements, which should be made with at least the same precision as in a quality-control inspection, alongside the design dimensions on a blueprint of the part. 4. Perform laboratory tests as necessary to confirm the composition, structure, and other chemical or metallurgical characteristics of the distorted part. 5. Trace the failed part through all manufacturing processes to discover whether process deviations occurred during production. 6. Compare the actual conditions of service with design assumptions. 7. Compare the actual material properties with design specifications. 8. Determine whether any differences found in the preceding two steps fully account for the distortion observed in the failed structure. If the differences do not fully account for the observed distortion, the information obtained in the second or fourth step is incorrect or incomplete. 9. Prepare alternative courses of action to correct the variant factors that caused the observed distortion, and select the course that seems most likely to produce the desired result, which was defined in the first step. 10. Test the selected course of corrective action to verify its effectiveness. Evaluate side effects of the corrective action, such as its effects on cost or on ease of implementation. Example 6: Deformation of a Gas-Nitrided Drive-Gear Assembly. Slipping of components in the left-side final drive train of a tracked military vehicle was detected after the vehicle had been driven 13,700 km (8500 miles) in combined highway and rough-terrain service. No abnormal service conditions were reported in the history of the vehicle. The slipping was traced to the mating surfaces of the final drive gear and the adjacent splined coupling sleeve (Fig. 13). Failure analysis was conducted to determine the cause of the malfunction and to recommend corrective measures that would prevent similar failures in other vehicles
8 diam Splined coupling leeve Drive gea areo Deformed area Detail A B Fig. 13 Gas-nitrided 4140 steel (27-31 hrC) drive-gear assembly in which gear teeth deformed because of faulty design and low core hardness. Details a and b show deformed areas on drive-gear teeth and mating internal splines Dimensions given in inches Material and Fabrication. Specifications required that the gear and coupling be made from 4140 steel bar oil quenched and tempered to a hardness of 265 to 290 HB(equivalent to 27 to 31 HRC) and that the finish-machined parts be single stage gas nitrided to produce a total case depth of 0.5 mm(0.020 in )and a minimum surface hardness equivalent to 58 HRC Visual Examination. Low-magnification examination of the drive gear and coupling showed that the teeth of the gear and the mating contact surfaces of the internal splines on the coupling were almost completely worn away. The spline surfaces had been damaged mainly by severe indentation and plastic deformation. No spalling, cracking, or other indications of fatigue damage were visible on the splines nor was there any indication that abrasive wear had been the mechanism of failure Metal on the drive side of the gear teeth had been plastically deformed and subsequently removed due to wear(details a and B, Fig. 13). The damaged areas on the splines were wider(axial dimension) than the gear teeth, indicating that there had been excessive lateral play between the components Examination of the surfaces of the internal splines that contacted a gear on the opposite end of the coupling(not illustrated)revealed a much smaller amount of deterioration than on the failed end. For example, the machined flats on the crests of the splines showed no damage, the follower sides of the splines had very shallow, almost imperceptible wear areas that were only about 0. 1 mm(0.004 in )wide, and the drive side of each spline had a gently radiused, concave area of damage approximately 6.4 X 25 mm(0. 25x I in. )and 0. 13 mm(0.005 in. )deep at its center. However, the amount of damage on the drive side of these splines was considered to be excessive for only 13, 700 km(8500 miles )of service Conformance to Material Specifications. Spectrographic analysis of the two components confirmed that composition was ithin the range specified for 4140 steel. Core hardness was in the required range of 27 to 31 hRC (264 to 294 HB) Total case depth, as determined by the microscopic method on polished specimens etched in 2% nital, was satisfactory Surface hardness, as measured using a Knoop indenter, was equivalent to 50 HRC, substantially lower than the required value of 58 HRC Microstructure of the cases and cores of the two components was examined at 500x on polished sections etched in 2% nital. There was a white layer(nitrogen-rich iron nitride, Fe2N)about 0.025 mm(0.001 in )thick on the surfaces of the gear teeth and the splines, and grain-boundary networks of iron nitride were present to a slight degree near the surface The microstructure of the core consisted mainly of tempered martensite, but contained large amounts of blocky ferrite Thefileisdownloadedfromwww.bzfxw.com
Fig. 13 Gas-nitrided 4140 steel (27–31 HRC) drive-gear assembly in which gear teeth deformed because of faulty design and low core hardness. Details A and B show deformed areas on drive-gear teeth and mating internal splines. Dimensions given in inches Material and Fabrication. Specifications required that the gear and coupling be made from 4140 steel bar oil quenched and tempered to a hardness of 265 to 290 HB (equivalent to 27 to 31 HRC) and that the finish-machined parts be singlestage gas nitrided to produce a total case depth of 0.5 mm (0.020 in.) and a minimum surface hardness equivalent to 58 HRC. Visual Examination. Low-magnification examination of the drive gear and coupling showed that the teeth of the gear and the mating contact surfaces of the internal splines on the coupling were almost completely worn away. The spline surfaces had been damaged mainly by severe indentation and plastic deformation. No spalling, cracking, or other indications of fatigue damage were visible on the splines nor was there any indication that abrasive wear had been the mechanism of failure. Metal on the drive side of the gear teeth had been plastically deformed and subsequently removed due to wear (details A and B, Fig. 13). The damaged areas on the splines were wider (axial dimension) than the gear teeth, indicating that there had been excessive lateral play between the components. Examination of the surfaces of the internal splines that contacted a gear on the opposite end of the coupling (not illustrated) revealed a much smaller amount of deterioration than on the failed end. For example, the machined flats on the crests of the splines showed no damage, the follower sides of the splines had very shallow, almost imperceptible wear areas that were only about 0.1 mm (0.004 in.) wide, and the drive side of each spline had a gently radiused, concave area of damage approximately 6.4 × 25 mm (0.25 × 1 in.) and 0.13 mm (0.005 in.) deep at its center. However, the amount of damage on the drive side of these splines was considered to be excessive for only 13,700 km (8500 miles) of service. Conformance to Material Specifications. Spectrographic analysis of the two components confirmed that composition was within the range specified for 4140 steel. Core hardness was in the required range of 27 to 31 HRC (264 to 294 HB). Total case depth, as determined by the microscopic method on polished specimens etched in 2% nital, was satisfactory. Surface hardness, as measured using a Knoop indenter, was equivalent to 50 HRC, substantially lower than the required value of 58 HRC. Microstructure of the cases and cores of the two components was examined at 500× on polished sections etched in 2% nital. There was a white layer (nitrogen-rich iron nitride, Fe2N) about 0.025 mm (0.001 in.) thick on the surfaces of the gear teeth and the splines, and grain-boundary networks of iron nitride were present to a slight degree near the surface. The microstructure of the core consisted mainly of tempered martensite, but contained large amounts of blocky ferrite. The file is downloaded from www.bzfxw.com
Gear and Spline Configuration. Measurements of the gear teeth and splines in areas showing little damage established that the parts had been manufactured in conformance with the engineering drawings. The splines were straight axially and convex radially. However, the gear teeth that failed were convex in both directions in the vicinity of the pitch line. This design, which was intended to facilitate alignment and adjustment, provided extremely small contact areas between tooth and spline surfaces and hence very heavy localized loading. The gear that engaged the other end of the internally splined coupling(not shown in Fig. 13)was designed to provide larger contact areas with the splines in the vicinity of the pitch line, providing lighter and more uniform local loading Conclusions. The premature failure occurred by crushing, or cracking, of the case as a result of several factors, which are listed below in order of importance Design that produced excessively high localized stresses at the pitch line of the mating components Specification of a core hardness(27 to 31 HRC) too low to provide adequate support for a 0.5 mm(0.020 in.) thick case or to permit attainment of the specified surface hardness of 58 HRC after nitriding, actual surface hardness was 50 HRC The presence of large amounts of blocky ferrite in the core(a microstructure conducive to case crushing) as a result of faulty heat treatment before nitriding he presence of a white nitride layer about 0.025 mm(0.001 in )thick at the surface and of nitride networks in the case Recommendations. Measures to correct the first two deficiencies listed above were recommended as the most important steps in obtaining adequate drive-train performance. First, the excessively high local stresses at the pitch line should be reduced to an acceptable level by modifying the gear-tooth contour in the vicinity of the pitch line to provide a wider and longer initial contact area than in the original design. Second, a core hardness of 35 to 40 HRC Should be specified to provide adequate support for the case and to permit attainment of the specified surface hardness of 58 HRC. Closer control of heat treating, which would be necessary to produce the recommended higher core hardness consistently, would also eliminate the presence of blocky ferrite in the core For maximum service life, consideration should also be given to controlling single-stage gas nitriding to minimize the thickness of the white layer and the extent of nitride networks in the case or to use double-stage gas nitriding to provide a diffused nitride layer and specifying final lapping or honing to remove the white layer Analysis of Distortion and Deformation Revised by Roch J. shipley and David A Moore, Packer Engineering and william Dobson, Binary Engineering Associates, Inc. Special Types of Distortion Failure Analysis of distortion failures can be particularly difficult when there is no apparent permanent deformation of the part or when complex stress fields are involved. In this section, three types of distortion failure are discussed, which may provide useful insights into the problems of analyzing unusual mechanisms of distortion Elastic Distortion. A distortion failure does not necessarily involve yielding under a single application of load Unacceptable distortion can occur within the elastic regime. All parts will deflect elastically under load. But if a part ordinarily made of a high-modulus alloy is made of a low-modulus alloy, it will deflect more under a given load than if it were made of the high-modulus alloy. If this greater amount of deflection places the part in the path of another part in an assembly, it could be said to have failed by elastic distortion. As mentioned earlier, a change in the modulus of a material because of a change in temperature can cause an elastic-distortion failure. Elastic buckling of a long, slender column is another type of distortion failure in which the yield strength of the material is not exceeded(unless the structure collapses Ratcheting. Cyclic strain accumulation, or ratcheting, requires that a part be stressed by steady-state loading, either uniaxial or multiaxial, and that a cyclically varying strain in a direction other than the direction of principal stress be superimposed on the part. In ratcheting, an oscillating load or a cyclic variation of temperature strains the material beyond the yield point on alternate sides of a single member, or on alternate members of a structure, during each half-cycle. With succeeding cycles, plastic strain accumulates, with the result that one or more of the overall dimensions of the member or the structure change relatively uniformly along the direction of steady-state stress. Deformation produced by a cyclic variation in load is known as isothermal ratcheting(even though a temperature change may occur simultaneously with the
Gear and Spline Configuration. Measurements of the gear teeth and splines in areas showing little damage established that the parts had been manufactured in conformance with the engineering drawings. The splines were straight axially and convex radially. However, the gear teeth that failed were convex in both directions in the vicinity of the pitch line. This design, which was intended to facilitate alignment and adjustment, provided extremely small contact areas between tooth and spline surfaces and hence very heavy localized loading. The gear that engaged the other end of the internally splined coupling (not shown in Fig. 13) was designed to provide larger contact areas with the splines in the vicinity of the pitch line, providing lighter and more uniform local loading. Conclusions. The premature failure occurred by crushing, or cracking, of the case as a result of several factors, which are listed below in order of importance: · Design that produced excessively high localized stresses at the pitch line of the mating components · Specification of a core hardness (27 to 31 HRC) too low to provide adequate support for a 0.5 mm (0.020 in.) thick case or to permit attainment of the specified surface hardness of 58 HRC after nitriding; actual surface hardness was 50 HRC · The presence of large amounts of blocky ferrite in the core (a microstructure conducive to case crushing) as a result of faulty heat treatment before nitriding · The presence of a white nitride layer about 0.025 mm (0.001 in.) thick at the surface and of nitride networks in the case Recommendations. Measures to correct the first two deficiencies listed above were recommended as the most important steps in obtaining adequate drive-train performance. First, the excessively high local stresses at the pitch line should be reduced to an acceptable level by modifying the gear-tooth contour in the vicinity of the pitch line to provide a wider and longer initial contact area than in the original design. Second, a core hardness of 35 to 40 HRC should be specified to provide adequate support for the case and to permit attainment of the specified surface hardness of 58 HRC. Closer control of heat treating, which would be necessary to produce the recommended higher core hardness consistently, would also eliminate the presence of blocky ferrite in the core. For maximum service life, consideration should also be given to controlling single-stage gas nitriding to minimize the thickness of the white layer and the extent of nitride networks in the case or to use double-stage gas nitriding to provide a diffused nitride layer and specifying final lapping or honing to remove the white layer. Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A. Moore, Packer Engineering and William Dobson, Binary Engineering Associates, Inc. Special Types of Distortion Failure Analysis of distortion failures can be particularly difficult when there is no apparent permanent deformation of the part or when complex stress fields are involved. In this section, three types of distortion failure are discussed, which may provide useful insights into the problems of analyzing unusual mechanisms of distortion. Elastic Distortion. A distortion failure does not necessarily involve yielding under a single application of load. Unacceptable distortion can occur within the elastic regime. All parts will deflect elastically under load. But if a part ordinarily made of a high-modulus alloy is made of a low-modulus alloy, it will deflect more under a given load than if it were made of the high-modulus alloy. If this greater amount of deflection places the part in the path of another part in an assembly, it could be said to have failed by elastic distortion. As mentioned earlier, a change in the modulus of a material because of a change in temperature can cause an elastic-distortion failure. Elastic buckling of a long, slender column is another type of distortion failure in which the yield strength of the material is not exceeded (unless the structure collapses). Ratcheting. Cyclic strain accumulation, or ratcheting, requires that a part be stressed by steady-state loading, either uniaxial or multiaxial, and that a cyclically varying strain in a direction other than the direction of principal stress be superimposed on the part. In ratcheting, an oscillating load or a cyclic variation of temperature strains the material beyond the yield point on alternate sides of a single member, or on alternate members of a structure, during each half-cycle. With succeeding cycles, plastic strain accumulates, with the result that one or more of the overall dimensions of the member or the structure change relatively uniformly along the direction of steady-state stress. Deformation produced by a cyclic variation in load is known as isothermal ratcheting (even though a temperature change may occur simultaneously with the
load variation). Progressive growth due to plastic strain incurred during a change in temperature is called thermal ratcheting Ratcheting may ultimately result in ductile fracture or in failure by low-cycle fatigue At elevated temperatures, ratcheting must be distinguished from creep or stress relaxation. Ratcheting is solely a strain- dependent phenomenon, whereas creep and stress relaxation are time-dependent phenomena. Exposure to elevated temperature for an extended length of time, typically hours to thousands of hours, is necessary for creep or stress relaxation to occur, but extensive deformation by ratcheting can occur in short periods of time-sometimes only minutes Ratcheting can appear to be time dependent when the cyclic strains are imposed at regular intervals. However, the factor that distinguishes ratcheting is the occurrence of plastic strain during both halves of the cyclic variation In general, the proper corrective action for failures by ratcheting involves changing the design of the part or the conditions of service to reduce the magnitude of the service stresses or specifying a material with a higher yield strength for the application Inelastic Cyclic Buckling Cyclic strain softening is a continuous decrease of elastic limit or tangent modulus that occurs Bsth imposition of alternating stresses below the net section yield strength. Columns made from materials that exhibit this avior can fail by lateral displacement at the midspan(buckling) under stresses much lower than those predicted by classical design Table 1 presents the results of a test in which cylindrical specimens of cold-worked 1020 steel, resembling tensile specimens with threaded ends, were stressed by alternating tensile and compressive loads of equal magnitude(Ref 8) Buckling occurred with stresses approximately 60 to 90% of the 0.2% offset yield strength, and the cycles ranged from at the highest stresses to 1900 at the lowest. When an aluminum alloy was tested in the same manner, buckling did not occur in a range of stresses below the 0. 2% offset yield strength. Aluminum alloys are among those that do not exhibit clic strain softening Table 1 Inelastic cycle buckling of cylindrical specimens of cold-worked 1020 steel Peak stress tress cycles MIPa At buckling At failure failure mode 60064887-94 552 0 130 143 380 421 61 1900 2377 400 4730 Tensile strength of the steel: 690 MPa(100 ksi); yield strength, 620 MPa(90 ksi) (a) Buckling, with or without fracture (b) Buckling followed by buckling-induced fracture (c)Low-cycle fatigue fracture Source: Ref 8 Reference cited in this section 8. C.R. Preschmann and R I Stephens, Inelastic Cyclic Buckling, Exp Mech., Vol 12(No 9), Sept 1972, p 426-428 Analysis of Distortion and deformation Revised by Roch ]. Shipley and David A Moore, Packer Engineering and william Dobson, Binary Engineering Associates, Inc. Deformation Related to other types of failure Previous discussion and examples have shown how distortion of a critical component can impede the functionality of the component itself or a larger assembly. In some cases, the root cause of the distortion is a problem elsewhere in a Thefileisdownloadedfromwww.bzfxw.com
load variation). Progressive growth due to plastic strain incurred during a change in temperature is called thermal ratcheting. Ratcheting may ultimately result in ductile fracture or in failure by low-cycle fatigue. At elevated temperatures, ratcheting must be distinguished from creep or stress relaxation. Ratcheting is solely a straindependent phenomenon, whereas creep and stress relaxation are time-dependent phenomena. Exposure to elevated temperature for an extended length of time, typically hours to thousands of hours, is necessary for creep or stress relaxation to occur, but extensive deformation by ratcheting can occur in short periods of time—sometimes only minutes. Ratcheting can appear to be time dependent when the cyclic strains are imposed at regular intervals. However, the factor that distinguishes ratcheting is the occurrence of plastic strain during both halves of the cyclic variation. In general, the proper corrective action for failures by ratcheting involves changing the design of the part or the conditions of service to reduce the magnitude of the service stresses or specifying a material with a higher yield strength for the application. Inelastic Cyclic Buckling. Cyclic strain softening is a continuous decrease of elastic limit or tangent modulus that occurs with imposition of alternating stresses below the net section yield strength. Columns made from materials that exhibit this behavior can fail by lateral displacement at the midspan (buckling) under stresses much lower than those predicted by classical design. Table 1 presents the results of a test in which cylindrical specimens of cold-worked 1020 steel, resembling tensile specimens with threaded ends, were stressed by alternating tensile and compressive loads of equal magnitude (Ref 8). Buckling occurred with stresses approximately 60 to 90% of the 0.2% offset yield strength, and the cycles ranged from 1 4 at the highest stresses to 1900 at the lowest. When an aluminum alloy was tested in the same manner, buckling did not occur in a range of stresses below the 0.2% offset yield strength. Aluminum alloys are among those that do not exhibit cyclic strain softening. Table 1 Inelastic cycle buckling of cylindrical specimens of cold-worked 1020 steel Peak stress Stress cycles MPa ksi At buckling At failure Failure mode 600–648 87–94 (a) 552 80 14 20 (a) 490 71 130 143 (a) 441 64 380 691 (b) 421 61 1900 2377 (b) 400 58 … 4730 (c) Tensile strength of the steel: 690 MPa (100 ksi); yield strength, 620 MPa (90 ksi). (a) Buckling, with or without fracture. (b) Buckling followed by buckling-induced fracture. (c) Low-cycle fatigue fracture. Source: Ref 8 Reference cited in this section 8. C.R. Preschmann and R.I. Stephens, Inelastic Cyclic Buckling, Exp. Mech., Vol 12 (No. 9), Sept 1972, p 426–428 Analysis of Distortion and Deformation Revised by Roch J. Shipley and David A. Moore, Packer Engineering and William Dobson, Binary Engineering Associates, Inc. Deformation Related to Other Types of Failure Previous discussion and examples have shown how distortion of a critical component can impede the functionality of the component itself or a larger assembly. In some cases, the root cause of the distortion is a problem elsewhere in a The file is downloaded from www.bzfxw.com
mechanism. The distortion results from overload(stress too high), material deficiency(material too weak),or a manufacturing process variation(residual stress related ) Sometimes the root cause of the distortion is obvious, other times more subtle. Distortion is also often observed in connection with other types of failures. The characteristics of the distortion observed, or the absence of distortion, can provide important and sometimes critical insights into the sequence of events leading to an ultimate failure To restate the obvious, deformation occurs when stresses exceed the critical point for yield of the material. Fracture may or may not be the end result in the sequence, but if there is distortion, it most certainly occurred prior to the fracture Corrosion or wear may contribute by removing material and decreasing the load-carrying capability of a cross section While no two situations are exactly the same, a few composite examples illustrate the concept and the related thought processes of the failure analyst A motor vehicle runs off the road, and the driver claims there was a problem with control immed Inspection of the damaged vehicle after the accident cannot locate all the parts associated with the steering and suspension, so the first investigator concludes that these parts were loose, finally fell off, and led to the loss of control. Subsequent engineering failure analysis and more careful examination of the parts that are available show clear distortion on the parts that were adjacent to the missing items. This deformation could not have occurred if the missing parts had simply fallen off. The forces of the collision caused deformation followed by fracture and separation of some parts from the remainder of the vehicle. Figure 14 shows one instance of the deformation observed in this type of situation. The threads of a missing bolt left impressions on the inside surface of a hole as the members sheared with respect to each other In another motor vehicle accident, fractured parts are found after the accident. Again, the driver claims a problem with control. The fracture surfaces are so damaged that microscopic examination is inconclusive regarding the fracture mechanism. (In typical accidents, this damage occurs by post-accident smashing of the fracture,or exposure to heat, fire, or corrosive conditions. )One investigator concludes that a critical component failed in fatigue and led to the loss of control. However, careful examination of the component and comparison with an exemplar of the same exact part number shows the deformation that occurred prior to separation. Furthermore, the directionality of this deformation was such that it could not have occurred during vehicle operation, but it was consistent with the forces of the collision. Thus, the collision caused deformation followed by fracture Macroscopic suggestions of a fatigue crack could be explained by geometric stress concentrators A component of a piping system is erroneously fabricated from the wrong material and is in the process of failing prematurely by a combination of corrosion, erosion, and wear. The plant in which this component is installed experiences an explosion and fire, and the deteriorating component is found deformed and fractured afterward Careful analysis of the material properties and component dimensions, including deformation patterns, is undertaken to determine if the deformation and fracture is consistent with normal system operating pressures and temperatures acting on a deteriorated component, causing a leak, and subsequent explosion. Analysis is also undertaken to consider the alternative possibility that the forces of an explosion originating elsewhere may have caused the deformation and fracture of this component. Some gas appliance flexible connectors manufactured and sold during the 1960s and 1970s are known to be susceptible to separation of the end fitting through long-term and progressive deterioration. Unfortunately,a small number of such connectors still remain in service. a home is destroyed by a gas explosion and fire that occur under suspicious circumstances. However, investigators also find one of the suspect connectors and question whether it separated and leaked natural gas before the explosion or whether it separated as a result of the forces of the explosion. Careful inspection of the post-explosion appearance of the connector can provide information to answer this question
mechanism. The distortion results from overload (stress too high), material deficiency (material too weak), or a manufacturing process variation (residual stress related). Sometimes the root cause of the distortion is obvious, other times more subtle. Distortion is also often observed in connection with other types of failures. The characteristics of the distortion observed, or the absence of distortion, can provide important and sometimes critical insights into the sequence of events leading to an ultimate failure. To restate the obvious, deformation occurs when stresses exceed the critical point for yield of the material. Fracture may or may not be the end result in the sequence, but if there is distortion, it most certainly occurred prior to the fracture. Corrosion or wear may contribute by removing material and decreasing the load-carrying capability of a cross section. While no two situations are exactly the same, a few composite examples illustrate the concept and the related thought processes of the failure analyst: · A motor vehicle runs off the road, and the driver claims there was a problem with control immediately prior. Inspection of the damaged vehicle after the accident cannot locate all the parts associated with the steering and suspension, so the first investigator concludes that these parts were loose, finally fell off, and led to the loss of control. Subsequent engineering failure analysis and more careful examination of the parts that are available show clear distortion on the parts that were adjacent to the missing items. This deformation could not have occurred if the missing parts had simply fallen off. The forces of the collision caused deformation followed by fracture and separation of some parts from the remainder of the vehicle. Figure 14 shows one instance of the deformation observed in this type of situation. The threads of a missing bolt left impressions on the inside surface of a hole as the members sheared with respect to each other. · In another motor vehicle accident, fractured parts are found after the accident. Again, the driver claims a problem with control. The fracture surfaces are so damaged that microscopic examination is inconclusive regarding the fracture mechanism. (In typical accidents, this damage occurs by post-accident smashing of the fracture, or exposure to heat, fire, or corrosive conditions.) One investigator concludes that a critical component failed in fatigue and led to the loss of control. However, careful examination of the component and comparison with an exemplar of the same exact part number shows the deformation that occurred prior to separation. Furthermore, the directionality of this deformation was such that it could not have occurred during vehicle operation, but it was consistent with the forces of the collision. Thus, the collision caused deformation followed by fracture. Macroscopic suggestions of a fatigue crack could be explained by geometric stress concentrators. · A component of a piping system is erroneously fabricated from the wrong material and is in the process of failing prematurely by a combination of corrosion, erosion, and wear. The plant in which this component is installed experiences an explosion and fire, and the deteriorating component is found deformed and fractured afterward. Careful analysis of the material properties and component dimensions, including deformation patterns, is undertaken to determine if the deformation and fracture is consistent with normal system operating pressures and temperatures acting on a deteriorated component, causing a leak, and subsequent explosion. Analysis is also undertaken to consider the alternative possibility that the forces of an explosion originating elsewhere may have caused the deformation and fracture of this component. · Some gas appliance flexible connectors manufactured and sold during the 1960s and 1970s are known to be susceptible to separation of the end fitting through long-term and progressive deterioration. Unfortunately, a small number of such connectors still remain in service. A home is destroyed by a gas explosion and fire that occur under suspicious circumstances. However, investigators also find one of the suspect connectors and question whether it separated and leaked natural gas before the explosion or whether it separated as a result of the forces of the explosion. Careful inspection of the post-explosion appearance of the connector can provide information to answer this question