This chapter surveys both basic quality and basic reliability concepts as an introduction to the failure analysis professional. It begins with a section describing the distinction between quality and reliability and moves on to provide an overview of the concept of experiment design along with an example. The chapter then discusses the purposes of reliability engineering and introduces four basic statistical distribution functions useful in reliability engineering, namely normal, lognormal, exponential, and Weibull. It also provides information on three fundamental acceleration models used by reliability engineers: Arrhenius, Eyring, and power law models. The chapter concludes with information on failure rates and mechanisms and the two techniques for uncovering reliability issues, namely burn-in and outlier screening.

Inspections and microstructural analysis revealed intergranular corrosion of 6061-T6 aluminum alloy aircraft fuel line beneath ferrules. The cause of the corrosion was traced to the fuel line marking process, which involved electrolytic labeling. Although subsequent rinsing of the fuel lines washed off most of the electrolyte, some was trapped between the 6061-T6 tubing and the ferrule. This condition made intergranular corrosion of the fuel lines inevitable. The attack caused grains to become dislodged, giving the appearance of pitting. Corrosion penetrated approximately 0.13 mm (0.005 in.) into the tubing. Experiments indicated that the corrosion products were inactive. It was recommended that another marking process be used that does not involve corrosive materials. The prevention of electrolyte from being trapped between the tubing and ferrules by using a MIL-S-8802 sealant was recommended.

Two silica phenolic nozzle liners cracked during proof testing. The test consisted of pressuring the nozzles to 14.1 MPa (2050 psia) for 5 to 20 s. It was concluded that the failure was due to longitudinal cracking in the convergent exhaust-nozzle insulators, stemming from the use of silica phenolic tape produced from flawed materials that went undetected by the quality control tests, which at the time, assessed tape strength properties in the warp rather than the bias direction. Once the nozzle manufacturer and its suppliers identified the problem, they changed their quality control procedures and resumed production of nozzle liners with more tightly controlled fiber/fabric materials.

Fretting and/or fretting corrosion fatigue have been observed on such parts as main rotor counterweight tie rods, fixed-pitch propeller blades, propeller blade clamps, pressure regulator lines, and landing gear support brackets. Microcracks started from severe corrosion pits in a failed control rotor spar tube assembly made of cadmium-plated AISI 4130 Cr-Mo alloy steel. Inadequate design was responsible for the failure. A lower tine of the main rotor blade cuff failed in fatigue. The rotor blade cuff was forged of 2014-T6 aluminum alloy. Initial stages of crack growth displayed features typical of low stress intensity fatigue of aluminum alloys. The fatigue resulted from abnormal fretting owing to inadequate torquing of the main retention bolts. Aircraft maintenance engineers and owners were advised to adhere to specifications when torquing this joint.

A pinion gear made of AMS 6470 steel, nitrided all over, lost internal splined teeth due to wear. Spline failure of the power turbine gear caused an engine overspeed and disintegration. Excessive spline wear resulted from a new coupling being mated during overhaul with a worn gear spline. Wear on the spline teeth flanks of the coupling was attributed to severe wear on the mating gear (internal) spline teeth. The assigned cause was an inadequate maintenance procedure which resulted in a wear-damaged component being retained in the power train during engine overhaul. To prevent reoccurrence, specific inspection criteria were issued defining maximum limits for spline wear. A procedure and requirements were specified for installing the coupling and pinion gear at the next overhaul.

A farm-silo hoist used as the power source for a homemade barn elevator failed catastrophically from destructive wear of the worm. The hoist mechanism consisted of a pulley attached by a shaft to a worm that, in turn, engaged and drove a worm gear mounted directly on the hoist drum shaft. The worm and the worm gear were made of leaded cold-drawn 1113 steel and class 35-40 gray iron (nitrided in an aerated salt bath) respectively. The gearbox was found to contain fragments of the worm teeth and shavings that resembled steel wool. More than half of the worm teeth were revealed to be sheared off to almost half the depth. It was revealed on investigation that the drive pulley had been replaced with a larger pulley that generated more power than the gearbox could handle, causing failure by adhesive wear of the steel worm.

The AISI 1080 steel crankshaft of a large-capacity double-action stamping press broke in service and was repair welded. Shortly after the crankshaft was returned to service, the repair weld fractured. The repair-weld fracture was examined ultrasonically which revealed many internal reflectors, indicating the presence of slag inclusions and porosity. A low-carbon steel flux-cored filler metal was used in repair welding the crankshaft, without any preweld or postweld heating. This resulted in the formation of martensite in the HAZ. The repair weld failed by brittle fracture, which was attributed to the combination of weld porosity, many slag inclusions and the formation of brittle martensite in the HAZ. A new repair weld was made using an E312 stainless steel electrode, which provides a weld deposit that contains considerable ferrite to prevent hot cracking. Before welding, the crankshaft was preheated to a temperature above which martensite would form. After completion, the weld was covered with an asbestos blanket, and heating was continued for 24 h. During the next 24 h, the temperature was slowly lowered. The result was a crack-free weld.

Extensive slipper/wobbler failures occurred in the integrated drive generators that incorporated TiN coated wobblers, during the production acceptance test. Similar coated wobblers had passed the application tests. The nature of the failure was extensive gouging of the wobbler surface with discoloration and coating removal. The substrate material was E52100 which was through-hardened to HRC 55-60. The slippers that were in contact with the coated wobbler surface were made of AISI 06 material. A synthetic oil was used as the hydraulic fluid in the application. The failure in the wobblers was caused by lack of temperature control during application which resulted in localized surface rehardening. It was established that there was a significant difference in the grade of the hydraulic fluid that was used in the two test programs. Use of superior grade of hydraulic fluid was recommended in this case for the production acceptance tests.

An integral coupling and gear (Cr-Mo steel), used on a turbine-driven main boiler-feed pump, was removed from service after one year of operation because of excessive vibration. Spectrographic analysis and metallographic examination revealed the fact that gritty material in the gear teeth (found at visual inspection) was composed of the same material as the metal in the coupling. Beach marks and evidence of cold work, typical of fatigue failure, were found on the fracture surface. Chips remaining in the analysis cut were difficult to remove, indicating a strong magnetic field in the part. Evidence found supports the conclusions that failure of the coupling was by fatigue and that incomplete demagnetization of the coupling following magnetic-particle inspection caused retention of metal chips in the roots of the teeth. Improper lubrication caused gear teeth to overheat and spall, producing chips that eventually overstressed the gear, causing failure. Because the oil circulation system was not operating properly, metal chips were not removed from the coupling. Recommendations included checking the replacement coupling for residual magnetism and changing or filtering the pump oil to remove any debris.

Three spur gears made from 8622 Ni-Cr-Mo alloy steel formed a straight-line train in a speed reducer on a rail-mounted overslung lumber carrier. The gears were submitted for nondestructive examination and evaluation, with no accompanying information or report. Two teeth on one of the gears were found to be pitted, one low on profile and the adjacent tooth high on profile. The mating gear had a similar characteristic, two adjacent teeth with evidence of pitting and the same difference in profile. It was correctly deduced that the pitting occurred because the gears were in a static position under a reverberating load for an extended period of time.

A jack cylinder split open during simulated service testing. The intended internal test pressurization was reportedly analogous to typical service. The material and mechanical properties of the cylinder pipe were unknown, although subsequent testing showed that the pipe satisfied the requirements for a grade 1045 medium-carbon, plain carbon steel. Investigation (visual inspection, chemical analysis, 2% nital etched 119x images, and tension testing) supported the conclusion that the cylinder pipe burst in a mixed brittle-ductile manner due to overpressurization. It is likely that the bearing strength of the pipe was slightly compromised by a low-strength layer of decarburization. Recommendations included evaluating the testing procedure for the possibility of inadvertent overpressurization and analyzing successfully tested cylinders to identify changes in material, and perhaps heat treatment, that may have contributed to this failure.

An intermediate shaft (3 in. diam), part of a camshaft drive on a large diesel engine, broke after two weeks of service. Failure occurred at the end of the taper portion adjacent to the screwed thread. The irregular saw-tooth form of fracture was characteristic of failure from torsional fatigue. A second shaft carried as spare gear was fitted and failure took place in a similar manner in about the same period of time. Examination revealed that the tapered portion of the Fe-0.6C carbon steel shaft had been built up by welding prior to final machining. A detailed check by the engine-builder established that the manufacture of these two shafts had been subcontracted. It was ascertained that the taper portions had been machined to an incorrect angle and then subsequently built-up and remachined to the correct taper. The reduction in fatigue endurance following welding was due to heat-affected zone cracking, residual stresses, the lower fatigue strength of the weld deposited metal, and weld defects.

A tube in a radiant superheater, the boiler of which is coal fired, failed by creep after 17 years of service. The failed tube was specified to be made of ASME SA-213, grade T-22. Measurable swelling of the tube diameter by about 2.4 mm and tube wastage caused by corrosion or erosion were observed. Log stress versus Larson-Miller Parameter (LMP) plots were produced to assess the remaining life of the superheater. It was revealed that the estimated operating temperature of 1060 deg F was higher than the estimated design temperature of 1000 deg F and that the tube wastage had increased the actual operating stress. Tube wastage and high operating temperatures hastened the failure. A better understanding of the material condition of this superheater was recommended to verify all the suspect hot tubes.

Several large chromium-plated 4340 steel cylinders were removed from service because of deep longitudinal score marks in the plating. One of the damaged cylinders and a mating cast aluminum alloy B850-T5 bearing adapter that also exhibited deep longitudinal score marks were submitted for examination. Analysis (visual inspection, manual testing of the hardness and adherence of the chromium plating, 100x microscopic examination, and hardness testing) supported the conclusions that high localized loads on the cylinder had resulted in chipping of the chromium plating, particles of which became embedded in the aluminum alloy adapter. The sliding action of the adapter with embedded hard particles resulted in scoring of both the cylinder and the adapter. If the cylinder alone had been available for examination, it might have been concluded that the scoring had been caused by entrapped sand or debris from an external source. No recommendations were made.

A crankshaft was overloaded on a test stand and suffered an incipient crack in the crank pin. The crack run generally parallel to the longitudinal axis and branched off at the entrance into the two fillets at the transition to the crank arm. It consisted of many small cracks, all of which propagated at an angle of approximately 45 deg to the longitudinal axis, and therefore were caused by torsion stresses. Neither macroscopic nor microscopic examination determined any material or processing faults. Experience has shown that torsion vibration fractures of this kind usually appear in comparatively short journal pins at high stresses. This crankshaft fracture was an example of the damage that is caused or promoted neither by material nor heat treatment mistakes nor by defects of design or machining, but solely by overstressing.

The shaft of an exciter that was used with a diesel-driven electric generator broke at a fillet after ten hours of service following resurfacing of the shaft by welding. The fracture surface contained a dull off-center region of final ductile fracture surrounded by regions of fatigue that had been subjected to appreciable rubbing. The fracture appeared to be typical of rotary bending fatigue under conditions of a low nominal stress with a severe stress concentration. It appeared that the fatigue cracks initiated in the surface-weld layer. The weld deposit in the original keyway displays a lack of fusion at the bottom corner. Fatigue fracture of the shaft resulted from stresses that were created by vibration acting on a crack or cracks formed in the weld deposit because of the lack of preheating and postheating. Rebuilding of exciter shafts should be discontinued, and the support plate of the exciter should be braced to reduce the amount of transmitted vibration. Also, the fillet in the exciter shaft should be carefully machined to provide an adequate radius.

Field failures, traced to internal cracks that were initiated from gross nonmetallics, were encountered in the upset portion of forged 4118 steel shafts. Ultrasonic inspection was thought to be the best method for detection from the location of these cracks, their orientation, and the size of the shaft. A longitudinal beam was sent in from the end of the shaft. The shaft was observed to have a radially drilled oil hole 9 mm in diam. Since there was a variation in flaw orientation, testing of the shaft was desired from both the long and short end. The rejection level was set at 20% of full screen and was based on the size of flaws observed when the shafts were cut up. The inclusions were considered to be rejectable if the size was larger than 20 mm diam. Similar flaws were observed in larger shafts, but no flaws were observed once the shafts were sectioned. It was interpreted that the flaw signals were false and had happened when a portion of the beam struck the oily surface of the longitudinal oil hole. The problem was solved by removing the oil film from the longitudinal oil hole.

An alloy IN-690 (N06690) incinerator liner approximately 0.8 mm (0.031 in.) thick failed after only 250 h of service burning solid waste. Investigation supported the conclusion that the root cause of the failure was overfiring during startup and sulfidation of the nickel-base alloy. No recommendations were made.