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.

The spindle of a helicopter-rotor blade fractured after 7383 h of flight service. At every overhaul (the spindle that failed was overhauled six times and reworked twice), any spindle that showed wear was reworked by grinding the shank to 0.1 mm (0.004 in.) under the finished diam. The spindle was then shot peened with S170 shot to an Almen intensity of 0.010 to 0.012 A. Following shot peening, the shank was nickel sulfamate plated to 0.05 mm (0.002 in.) over the finished diam, ground to finished size, and cadmium plated. Visual and stereomicroscopic exam showed faint grinding marks and circumferential grooves on the surface near the fillet at the junction of the shank and fork, which should have been peened over and covered with peening dimples. Evidence found supports the conclusions that the spindle failed in fatigue that originated near the junction of the shank and fork. The nonuniformity of the shot-peened effect on the shank and fillet portions of the spindle resulted from incomplete peeing. The fracture was of the low-stress high-cycle type, initiated by stresses well below the gross yield strength and propagated by thousands of load cycles. No recommendations were made.

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 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.

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.

Helicopter rotor blade components that included the horizontal hinge pin, the associated nut, and the locking washer were examined. Visual examination of the submitted parts revealed that the hinge pin, fabricated from 4340 steel, was broken and that the fracture face showed a flat beach mark pattern indicative of a preexisting crack. The threaded area of the pin had an embedded thread that did not appear to come from the pin. A chemical analysis was conducted on the embedded thread and on an associated attachment to determine the origin of the thread. Analysis showed that the thread and nut were 4140 steel. Scanning electron fractographic examination of the fracture initiation site strongly suggested that the fracture progressed by fatigue. It was concluded that the failure of the horizontal hinge pin initiated at areas of localized corrosion pits. The pits in turn initiated fatigue cracks, resulting in a failure mode of corrosion fatigue. It was recommended that all of the horizontal hinge pins be inspected. Those pins determined to be satisfactory for further use should be stripped of cadmium, shot peened, and coated with cadmium to a minimum thickness of 0.0127 mm (0.0005 in.).

A T-bolt was part of the coupling for a bleed air duct of a jet engine on a transport plane. Specifications required that the 4.8 mm diam component be made of AISI type 431 stainless steel and heat treated to 44 HRC. The operating temperature of the duct is 425 to 540 deg C (800 to 1000 deg F), but that of the bolt is lower. The T-bolt broke after three years of service. The expected service life was equal to that of the aircraft. It was found that the bolt broke as a result of SCC. Thermal stresses were induced into the bolt by intermittent operation of the jet engine. Mechanical stresses were induced by tightening of the clamp around the duct, which in effect acted to straighten the bolt. The action of these stresses on the carbides that precipitated in the grain boundaries resulted in fracture of the bolt. Due to the operating temperatures of the duct near the bolt, the material was changed to A-286, which is less susceptible to carbide precipitation. The bolt is strengthened by shot peening and rolling the threads after heat treatment. Avoiding temperatures in the sensitizing range is desirable, but difficult to ensure because of the application.

A 4340 steel shaft, the driving member of a large rotor subject to cyclic loading and frequent overloads, broke after three weeks of operation. The driving shaft contained a shear groove at which the shaft should break if a sudden high overload occurred, thus preventing damage to an expensive gear mechanism. The rotor was subjected to severe chatter, which was an abnormal condition resulting from a series of continuous small overloads occurring at a frequency of around three per second. Investigation (visual inspection, hardness testing, and hot acid etch images) supported the conclusion that the basic failure mechanism was fracture by torsional fatigue, which started at numerous surface shear cracks, both longitudinal and transverse, that developed in the periphery of the root of the shear groove. These shear cracks resulted from high peak loads caused by chatter. The shear groove in the shaft had performed its function, but at a lower overload level than intended. Recommendations included increasing the fatigue strength of the shaft by shot peening the shear groove to minimize chatter.

Several diesel-engine rocker levers (malleable iron similar to ASTM A 602, grade M7002) failed at low hours in overspeed, over-fuel, highly loaded developmental engine tests. Identical rocker levers had performed acceptably in normal engine tests. The rocker levers were failing through the radius of an adjusting screw arm. The typical fracture face exhibited two distinct modes of crack propagation: the upper portion indicated overload at final fracture, whereas the majority of the fracture suggested a fatigue fracture. Investigation (visual inspection, 1.5x/30x/60x magnification, and nital etched 300x magnification) supported the conclusion that the rocker levers failed in fatigue, with casting defects, or spiking, acting as stress raisers to initiate failures in highly loaded engine tests. Recommendations included shot peening of the levers as an interim measure to reduce the possibility of failure and redesign to increase the cross-sectional area of the levers.

New aircraft wing panels extruded from 7075-T6 aluminum exhibited an unusual pattern of circular black interrupted lines, which could not be removed by scouring or light sanding. The panels, subsequent to profiling and machining, were required to be penetrated inspected, shot peened, H2SO4 anodized, and coated with MIL-C-27725 integral fuel tank coating on the rib side. Scanning electron microscopy and microprobe analysis (both conventional energy-dispersive and Auger analyzers) showed that the anodic coating was applied over an improperly cleaned and contaminated surface. The expanding corrosion product had cracked and, in some places, had flaked away the anodized coating. The corrodent had penetrated the base aluminum in the form of subsurface intergranular attack to a depth of 0.035 mm (0.0014 in.). It was recommended that a vapor degreaser be used during cleaning prior to anodizing. A hot inhibited alkaline cleaner was also recommended during cleaning prior to anodizing. The panels should be dichromate sealed after anodizing. The use of deionized water was also recommended during the dichromate sealing operation. In addition, the use of an epoxy primer prior to shipment of the panels was endorsed. Most importantly, surveillance of the anodizing process itself was emphasized.

The jackscrew drive pins on a landing-gear bogie failed when the other bogie on the same side of the airplane was kneeled for tire change. The pins, made of 300M steel, were shot peened and chromium plated on the outside surface and were cadmium plated and painted with polyurethane on the inside surface. The top of the jackscrew was 6150 steel. Both ends of the pins were revealed to be dented where the jackscrew had pressed into them and were observed to have been resulted due to overdriving the jackscrew at the end of an unkneeling cycle. These dented areas were found to be heavily corroded with chromium plating missing. A heavily corroded intergranular fracture mode was revealed by chromium-carbon replicas of the areas of fracture origin. Deep corrosion pits adjacent to the fracture origins and directly beneath cracks in the chromium plate were revealed by metallographic examination. It was concluded that stress-corrosion cracks grew out from the rust pits. The pin material was changed from 300M steel to PH 13-8 Mo stainless steel, which is highly resistant to rusting and SCC and the jacking control system was modified to prevent overdriving.

A connecting rod from a failed engine ruptured in fatigue without evidence of excessive stresses, detonation, overheating, or oil starvation. The origin of the fatigue failure was completely mutilated but decarburization was observed. Significant amounts of decarburization (0.010 to 0.015 in.) were found also in other forgings, such as exhaust rocker arms, main rotor drag brace clevises, bolts of carriage diagonal struts, and spring legs of main landing gears. The failure mode was low-stress, high-cycle fatigue involving tension and bending loads. The main cause was a manufacturing deficiency. The usual way to eliminate decarburization is to machine off the soft skin or employ better quality control when making them. Many aircraft manufacturers employ forged parts with machined surfaces or with shot-peened as-forged surfaces without excessive decarburization.

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.

The safety valve on a steam turbogenerator was set to open when the steam pressure reaches 2400 kPa (348 psi). The pressure had not exceeded 1790 kPa (260 psi) when the safety-valve spring shattered into 12 pieces. The steam temperature in the line varied from about 330 to 400 deg C (625 to 750 deg F). Because the spring was enclosed and mounted above the valve, its temperature was probably slightly lower. The 195 mm (7 in.) OD x 305 mm (12 in.) long spring was made from a 35 mm (1 in.) diam rod of H21 hot-work tool steel. It had been in service for about four years and had been subjected to mildly fluctuating stresses. Analysis (visual inspection, 0.3x photographs, 0.7x light fractographs, and metallographic examination) supported the conclusions that the spring failed by corrosion fatigue that resulted from application of a fluctuating load in the presence of a moisture-laden atmosphere. Recommendations included replacing all safety valves in the system with new open-top valves that had shot-peened and galvanized steel springs. Alternatively, the valve springs could be made from a corrosion-resistant metal-for example, a 300 series austenitic stainless steel or a nickel-base alloy, such as Hastelloy B or C.

Type 316L stainless steel pipes carrying brine at 120 deg C (250 deg F) and at a pH of about 7, failed by perforation at or near circumferential butt-weld seams. The failure was examined optically and radiographically in the field. Specimens were removed and examined metallographically and with a SEM in the laboratory. The examinations revealed a combination of failure mechanisms. The pitting failure of the welds was attributed to localized attack of an activated surface, in which anodic pits corroded rapidly. Additionally, SCC driven by residual welding stresses occurred in the base metal adjacent to the welds. Use of highly stressed austenitic stainless steels in high-chloride environments having a temperature above 65 deg C (150 deg F) should be discouraged. Solution annealing or shot peening to reduce residual stresses may be advisable. If heat treatment is not feasible after welding, the substitution of a more corrosion-resistant alloy, such as Incoloy 800 or 825, may be necessary.

Two large tension springs fractured during installation. The springs were manufactured from a grade 9254 chromium-silicon steel spring wire. The associated material specification allows wire in the cold-drawn or oil-tempered (quenched-and-tempered) condition. The specified wire tensile strength range was 1689 to 1793 MPa (245 to 260 ksi). The finished springs were to be shot peened for greater fatigue resistance. Investigation (visual inspection, 3x images, 2% nital etched 148x SEM images, chemical analysis, hardness testing, and EDS analysis) supported the conclusion that the springs failed during installation due to the presence of preexisting defects. Crack surfaces were found to be corroded and phosphate coated, indicating that the cracks occurred during manufacture. Installation, which presumably entailed some axial extension, resulted in ductile overload failure at the crack sites. Recommendations included evaluating the manufacturing steps to identify the process(es) wherein the cracking was likely occurring. It was further recommended that a suitable nondestructive method such as magnetic particle inspection be implemented.

A steel valve spring meeting Steel-Iron-Test 1570 fractured during the high-stress condition of the operation of its valve. Metallographic examination of a transverse section adjacent to the fracture and a longitudinal section through the crack showed the steel was free of major defects and was of high purity, although a number of minor surface defects such as rolling laps were found. The spring was heat treated and its surface strengthened by shot-peening, but the surface was also decarburized to a depth of approximately 0.03 mm which resulted in a lowering of the surface hardness. The fracture of this valve spring is therefore primarily due to surface defects, and secondly perhaps also to weak surface decarburization. No recommendation resulted from the investigation except to note that comparatively minor effects suffice to cause fractures in highly stressed springs.

A recurring piston shaft failure problem on the billet-loading tray of an extrusion press was investigated. Two shafts fractured within a period of 10 days. The shaft was machined from normalized EN3 (AISI C1022) steel stock without further treatment. Visual, microstructural, chemical, and mechanical (hardness and tensile properties) analyses of failed shaft specimens were conducted. The examinations showed that the shafts had failed by fatigue. It was recommended that a low-alloy steel (e.g., 3% Ni-Cr) in the hardened and tempered condition and subjected to shot-peening surface-hardening treatment be used. The provision of a stop to reduce bending stresses was also recommended.

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.