Sudden and unexplained bearing cap bolt fractures were experienced with reduced-shank design bolts fabricated from 42 CrMo 4 steel, quenched and tempered to a nominal hardness of 38 to 40 HRC. Fractographic analysis provided evidence favoring stress-corrosion cracking as the operating transgranular fracture failure mechanism. Water containing H7S was subsequently identified as the aggressive environment that precipitated the fractures in the presence of high tensile stress. This environment was generated by the chemical breakdown of the engine oil additive and moisture ingress into the normally sealed bearing cap chamber surrounding the bolt shank. A complete absence of fractures in bolts from one of the two vendors was attributed primarily to surface residual compressive stresses produced on the bolt shank by a finish machining operation after heat treatment. Shot cleaning, with fine cast shot, produced a surface residual compressive stress, which eliminated stress-corrosion fractures under severe laboratory conditions.

To samples of helical compression springs were returned to the manufacturer after failing in service well short of the component design life. Spring design specifications required conformance to SAE J157, “Oil Tempered Chromium Silicon Alloy Steel Wire and Springs.” Each spring was installed in a separate heavy truck engine in an application in which spring failure can cause total engine destruction. The springs were composed of chromium-silicon steel, with a hardness ranging from 50 to 54 HRC. Chemical composition and hardness were substantially within specification. Failure initiated from the spring inside coil surface. Examination of the fracture surface using scanning electron microscopy showed no evidence of fatigue. Final fracture occurred in torsion. X-ray diffraction analysis revealed high inner-diameter residual stresses, indicating inadequate stress relief from spring winding. It was concluded that failure initiation was caused by residual stress-driven stress-corrosion cracking, and it was recommended that the vendor provide more effective stress relief.

This report covers case histories of failures in fixed-wing light aeroplane and helicopter components. A crankshaft of AISI 4340 Ni-Cr-Mo alloy steel, heat treated and nitrided all over, failed in bending fatigue. The nitrided layer was ground too rapidly causing excessive heat generation which induced grinding cracks and grinding burn. Tensional stresses resulting from grinding developed in a thin surface layer. On another crankshaft, chromium plating introduced undesirable residual tensile stresses. Such plating is an unsatisfactory finish for crankshafts of aircraft engines. Aircraft engine manufacturers and aeronautical standards require magnetic particle inspection to detect grinding cracks after reconditioning. Renitriding after any grinding is needed also, regardless of the amount of undersize as it introduces beneficial residual compressive stresses.

Plunger shafts machined from 4150 steel bar stock were involved in a series of fatigue failures. The fractures consistently occurred at two locations on the shafts: the shaft fillet and either side of a machined notch. The material specification for the shafts required 41xx series steel with a carbon content of 0.38 to 0.53%, a hardness of 35 to 40 HRC for the shaft, and a hardness of 50 to 55 HRC for the notch (which was case hardened). Analysis (visual inspection, chemical analysis, hardness testing, and magnetic particle inspection) supported the conclusions that all the fractures were fatigue-induced failures due to sharp radii in the fillets. The stress-concentrating effects of the fillets caused fatigue cracks to initiate and grow under cyclic loading until the crack depth was critical, causing the shaft to fail and rendering the assembly inoperative. Recommendations included increasing the radii of the notch and shaft fillets. If fatigue cracking had continued to be a problem with this component, shot peening of the subject radii would be appropriate. This process produces residual compressive stresses in the surface of the part, thereby retarding initiation of fatigue cracks.

During autofrettage of a thick-wall steel pressure vessel, a crack developed through the wall of the component. Certain forged pressure vessels are subjected to autofrettage during their manufacture to induce residual compressive stresses at locations where fatigue cracks may initiate. The results of the autofrettage process, which creates a state of plastic strain in the material, is an increase in the fatigue life of the component. Analysis (visual inspection, 50x/500x unetched micrographs, and electron microprobe analysis) supports the conclusion that the fracture toughness of the steel was exceeded, and failure through the wall occurred because of the following reason: the high level of iron oxide found is highly abnormal in vacuum-degassed steels. Included matter of this nature (exogenous) most likely resulted from scale worked into the surface during forging. Therefore, it is understandable that failure occurred during autofrettage when the section containing these defects was subjected to plastic strains. Because the inclusions were sizable, hard, and extremely irregular, this region would effect substantial stress concentration. No recommendations were made.

Two fuel injection pump gears that were nitrided in a cyanide bath were submitted by the engine manufacturer for examination of hardness distribution and failure analysis. The gears showed signs of wear after only comparatively brief operation. They were made of normalized unalloyed steel C 45 (Material No. 1.0503) according to DIN 17200 and were normalized. Gear 1 with 1905 h of operation showed at one side pittings on both flanks of the teeth as well as incipient fractures. Gear 2 with 1713 h of operation also showed at one side incipient fractures of the nitride layers at the outer part of the teeth. The nitride layer did not stand up to the high and one-sided compressive stress applied in this case and could not prevent pitting. It could even have accelerated the wear by the incipient break down. Gas nitriding at greater depth under application of a suitable special steel or case hardening would have been better under these circumstances.

Presence of transverse marks which were remnant of grinding was indicated in a failed valve spring made from ground rod. The shot-peening pattern was light at this location. A transverse crack was found to grow from one such mark under the influence of local stress fields until it was reoriented to the plane normal to the major tensile axis by sufficient loading. The shot-peening procedure was altered to create adequate surface compression at all stressed points on the springs.

A failure analysis case study on railroad rails is presented. The work, performed under the sponsorship of the Department of Transportation, addresses the problem of shell and detail fracture formation in standard rails. Fractographic and metallographic results coupled with hardness and residual stress measurements are presented. These results suggest that the shell fractures form on the plane of maximum residual tensile stresses. The formation of the shells is aided by the presence of defects in the material in these planes of maximum residual stress. The detail fracture forms as a perturbation from the shell crack under cyclic loading and is constrained to develop as an embedded flaw in the early stages of growth because the crack is impeded at the gage side and surface of the rail head by compressive longitudinal stresses.

This paper summarizes the results of a failure analysis investigation of a fractured main support bridge made of 7075 aluminum alloy from an army helicopter. The part, manufactured by “Contractor IT,” failed component fatigue testing while those of the original equipment manufacturer (OEM) passed. Metallurgical data collected during this investigation indicated that the difference in fatigue life between the components fabricated by IT and by OEM may be attributable to a difference in dimensions at the web where fatigue crack initiation occurred. The webs of the two OEM parts examined had cross-sectional thicknesses significantly larger than the web cross-sectional thicknesses of the IT components. Recommendations included changing the web reference dimension of 0.38 in. to include a tolerance range based upon a fracture mechanics model. Also, the shot peening process should be controlled especially at the critical areas of the web, to assure complete coverage and proper compressive residual stresses.

A steel spring used in an automotive application suddenly began to fail in the field, although “nothing had changed” in the fabrication process. Fatigue tests using springs fabricated prior to field failures lasted 500,000 cycles to failure, whereas fatigue tests performed on springs fabricated after field failures lasted only 50,000 cycles to failure. It was discovered that the percent coverage of shot peening prior and subsequent to the increase in failure incidence was much less than 100%, with a shot peening time of 12 min. The residual-stress state of “as fabricated” springs in three conditions were evaluated using XRD: springs manufactured prior to failure incidence increase, 12 min peen; springs manufactured following failure incidence increase, 12 min peen; and 60 min peen. The conclusion was that the failure occurred because low peening time significantly decreased the compressive residual-stress levels in the springs. Recommendation was made to increase the time the spring was shot peened from 12 to 60 min.

Several crankshaft failures occurred in equipment that was being used in logging operations in subzero temperatures. Failure usually initiated at a cracked pin oil hole, and the failure origin was approximately 7.6 mm (0.3 in.) from the shaft surface. The holes were produced by gun drilling, giving rise to surface defects. The fracture surface was characteristic of fatigue in that it was flat, relatively shiny, and exhibited beach marks. The crack surface was at a 45 deg angle to the axis of the shaft, indicating dominant tensile stresses. The material was the French designation AFNOR 38CD4 (similar to AISI type 4140H) and was in the quenched-and-tempered condition, with a yield strength of about 760 MPa (110 ksi). It was treated to have compressive surface stresses, and the prior-austenite grain size was ASTM 8. Analysis (visual inspection, stress analyses, and macrographs) supported the conclusion that failure was caused by fatigue stress caused by surface defects in the oil holes. Recommendation includes drilling the oil holes by a technique that essentially eliminates surface defects.

A rail section that failed due to fatigue showed a smooth surface with well-developed conchoidal markings. This indicated successive stages of crack propagation, characteristic of fatigue failure. The crack was one of several which developed in the sections of curved rail which formed the lower roller path on which the superstructure of a walking drag-line excavator slewed. The cracking, which ran horizontally, developed at the junction of the underside of the rail head with the web and originated at surface defects in the form of grooves present on the castings. It was concluded that the cracking was caused by lateral deflection of the rails under in-service loads. The web of a rail would normally be loaded in compression but, should lateral movements occur, then it would be subjected to bending stresses and fatigue cracks could break out in regions where excessive tensile components predominated.

A wire hoisting rope on a drilling rig failed during a lift, after a few cycles of operation, causing extensive damage to support structures. The failure investigation that followed included mechanical property testing and chemical, metallurgical, and finite element analysis. The rope was made from multiple strands of 1095 steel wire. Its chemical composition, ferrite-pearlite structure, and high hardness indicate that the wire is a type of extra improved plow steel (EEIPS grade). The morphologies of the fracture surfaces suggest that the wires were subjected to tensile overloading. This was confirmed by finite element analysis, which also revealed compressive contact stresses between the wires and between the rope and sheave surface. Based on the results, it was concluded that a tensile overload, due to the combined effect of a sudden load and undersized sheave, is what ultimately caused the rope to fail.

The secondary cooling water system pressure boundary of Savannah River Site reactors includes expansion joints utilizing a thin-wall bellows. While successfully used for over thirty years, an occasional replacement has been required because of the development of small, circumferential fatigue cracks in a bellows convolute. One such crack was recently shown to have initiated from a weld heat-affected zone liquation microcrack. The crack, initially open to the outer surface of the rolled and seam welded cylindrical bellows section, was closed when cold forming of the convolutes placed the outer surface in residual compression. However, the bellows was placed in tension when installed, and the tensile stresses reopened the microcrack. This five to eight grain diameter microcrack was extended by ductile fatigue processes. Initial extension was by relatively rapid propagation through the large-grained weld metal, followed by slower extension through the fine-grained base metal. A significant through-wall crack was not developed until the crack extended into the base metal on both sides of the weld. Leakage of cooling water was subsequently detected and the bellows removed and a replacement installed.

Several mercury diffusion pump stages in the Tritium Purification process at the Savannah River Site (SRS) have been removed from service for scheduled preventive maintenance. These stages have been examined to determine if failure has occurred. Evidence of fatigue around the flange portion of the pump has been seen. In addition, erosion and cavitation inside the throat of the venturi tube and corrosion on the other surface of the venturi tube has been observed. Several measures are being examined in an attempt to improve the performance of these pumps. These measures, as well as the noted observations, are described. Six stages [two machined (MP) and four electron beam (EB) welded] from the mercury diffusion pumps operating in the Tritium Purification process at SRS have been analyzed to determine their condition after nine months of usage. Several cracks were found around the necked region of the two MP stages. The EB welded stages, however, seemed to perform better in service; only two of four stages showed cracking. The cracking is caused by fatigue that has been enhanced by high stresses and tritium in the flange area. The EB welded stage appears to be a step in the right direction. Since the EB weld is a shrink fit, the surface is in compression, thereby eliminating crack propagation. In addition, shot peening has been employed to produce a compressive material surface since fatigue usually originates at the surface. Pitting was observed down the throat of the venturi. This pitting was caused by cavitation and erosion along the length of the venturi tube. Corrosion and pitting was seen on the exterior walls of the diffuser tubes. Stress-corrosion cracks were observed emanating from these corrosion pits. The corrosion likely occurred from the chloride ions present in the process cooling water. Shot peening is now being used in an attempt to place the outside of the diffuser tube in compression to eliminate the stress-corrosion cracking.

X-ray diffraction (XRD) residual-stress analysis is an essential tool for failure analysis. This article focuses primarily on what the analyst should know about applying XRD residual-stress measurement techniques to failure analysis. Discussions are extended to the description of ways in which XRD can be applied to the characterization of residual stresses in a component or assembly and to the subsequent evaluation of corrective actions that alter the residual-stress state of a component for the purposes of preventing, minimizing, or eradicating the contribution of residual stress to premature failures. The article presents a practical approach to sample selection and specimen preparation, measurement location selection, and measurement depth selection; measurement validation is outlined as well. A number of case studies and examples are cited. The article also briefly summarizes the theory of XRD analysis and describes advances in equipment capability.

An ASTM A 504 carbon steel railway car wheel that was used on a train in a metropolitan railway system failed during service, causing derailment. The wheel was completely fractured from rim to hub. Macrofractography of the fracture surface showed road grime, indicating that the crack had existed for a considerable time prior to derailment and initiated in the flange. Failure propagated from the flange across the rim and down the plate to the bore of the hub. Two zones that exhibited definite signs of heating were observed. The fracture initiation site was typical of fatigue fracture. No defects were found that could have contributed to failure. The wheel conformed to the chemical, microstructural, and hardness requirements for class A wheels. Failure was attributed to repeated severe heating and cooling of the rim and flange due to brake locking or misapplication of the hand brake. It was recommended that the brake system on the car be examined and replaced if necessary.

Two clamps that support overhead power lines in an electrified rail system fractured within six months of being installed. The clamps are made of CuNiSi alloy, a type of precipitation-strengthening nickel-silicon bronze. To identify the root cause of failure, the rail operator led an investigation that included fractographic and microstructural analysis, hardness testing, inductively coupled plasma spectroscopy, and finite-element analysis. The fracture was shown to be brittle in nature and covered with oxide flakes, but no other flaws relevant to the failure were observed. The investigation results suggest that the root cause of failure was a forging lap that occurred during manufacturing. Precracks induced by the forging defect and the influence of preload stress (due to bolt torque) caused the premature failure.

Failed ferrous components were analyzed from a crane that operated on an offshore platform. The crane failed during operation and fell into the sea. The brake spring on the boom hoist was found to have fractured in four places. The spring contained a line defect (seam) that was the source of each crack. The fracture of the oil quenched and tempered (HRC 50 ASTM A229) spring was by stress-corrosion cracking after the crane fell into the sea because fatigue cannot account for the fractures observed. The crane failure was caused by an overload created by the operator catching a free-falling load.

A valve spring made of 4.1 mm diam wire, designed to withstand 10,000,000 stress cycles, fractured after only 2,000,000 cycles. The surface displayed impressions which indicated it had been treated by shot blasting. The spring has broken in two places. Fracture 1 was a torsional fatigue fracture which has started from a lobe-like surface defect and not, as is usual, from a point on the most highly stressed inner surface. Fracture 2, on the other hand, was a bending fatigue fracture with a starting point on both the inner and the outer surface of the spiral. The objective of the shot blasting, to put the surface into a state of even compressive internal stress, which must first be overcome during subsequent bending and torsional loading before the boundary zone comes under tensile stress, was therefore not realized in this case. On the contrary, the shot blasting led to a state of internal stress which favored fracture of the spring.