Search Results for Compressive stress

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.

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.

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.

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.

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.

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 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.

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.

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.