Failure analysis is an investigative process that uses visual observations of features present on a failed component fracture surface combined with component and environmental conditions to determine the root cause of a failure. The primary means of recording the conditions and features observed during a failure analysis investigation is photography. Failure analysis photographic imaging is a combination of both science and art; experience and proper imaging techniques are required to produce an accurate and meaningful fracture surface photograph. This article reviews photographic principles and techniques as applied to failure analysis, both in the field and in the laboratory. The discussion covers the processes involved in field and laboratory photographic documentations, provides a description of professional digital cameras, and gives information on photographic lighting and microscopic photography. Special techniques can be employed to deal with highly reflective conditions and are also described in this article.

This article outlines the basic steps to be followed and the range of techniques available for failure analysis, namely, background data assembling, visual examination, microfractography, chemical analysis, metallographic examination, electron microscopy, electron microprobe analysis, X-ray techniques, and simulations. It also describes the steps for analyzing the data, preparing the report, preservation of evidence, and follow-up on recommendations.

Examination of a damaged component involves a chain of activities that, first and foremost, requires good observation and documentation. Following receipt and documentation, the features of damage can be recorded and their cause(s) investigated, as this article briefly describes, for typical types of damage experienced for metallic components. This article discusses the processes involved in visual or macroscopic examination of damaged material; the interpretation of fracture features, corrosion, and wear damage features; and the analysis of base material composition. It covers the processes involved in the selection of metallurgical samples, the preparation and examination of metallographic specimens in failure analysis, and the analysis and interpretation of microstructures. Examination and evaluation of polymers and ceramic materials in failure analysis are also briefly discussed.

This article focuses on the visual or macroscopic examination of damaged materials and interpretation of damage and fracture features. Analytical tools available for evaluations of corrosion and wear damage features include energy dispersive spectroscopy, electron probe microanalysis, Auger electron spectroscopy, secondary ion mass spectroscopy, and X-ray powder diffraction. The article discusses the analysis and interpretation of base material composition and microstructures. Preparation and examination of metallographic specimens in failure analysis are also discussed. The article concludes with a review of the evaluation of polymers and ceramic materials in failure analysis.

Cracks were found on the wing leading edge of a test aircraft made from AZ31B magnesium alloy. Crack lengths were approximately 230 mm (9 in.) long on the left side and approximately 130 mm (5 in.) long on the right side. The cracks ran parallel to the leading edge. The 230-mm (9-in.) crack was received for examination. Visual examination of the submitted panel revealed two cracks. One crack ran through six adjacent fastener holes. Sections of the beveled edges of the holes were missing and corrosion was evident. Visual examination of the fastener holes after separation of the crack showed that the fracture faces were corroded. Optical examination of either side of the middle group of fastener holes showed that the area of suspected crack initiation had suffered excessive corrosion. Examination of the holes on the end of the crack showed fracture characteristics typical of fatigue and/or corrosion fatigue. It was concluded that crack propagation of the fracture in the wing panel occurred by a combination of corrosion and high-cycle fatigue in the end fastener holes. It was recommended that future panels be manufactured of 2024 aluminum.

A welded natural gas line of 400 mm OD and 9 mm wall thickness made of unalloyed steel with 0.22C had to be removed from service after four months because of a pipe burst. Metallographic examination showed the pipe section located next to the gas entrance was permeated by cracks or blisters almost over its entire perimeter in agreement with the ultrasonic test results. Only the weld seam and a strip on each side of it were crack-free. Based on this investigation, the pipeline was taken out of service and reconstructed. To avoid such failures in the future, two preventative measures may be considered. One is to desulfurize the gas. Based on tests, however, the desulfurization would have to be carried very far to be successful. The second possibility is to dry the gas to such an extent as to prevent condensate, and this corrosion, from forming no matter how low winter temperatures may drop. This measure was ultimately recommended, deemed more effective and cheaper.

Four nose wheels fabricated from 2014-T6 aluminum alloy and cold worked at the flange were examined. Visual examination showed that the failure started in the tube well area on the wheel with serial number 31. The failure initiated in the flange fillet on wheels with serial numbers 67, 217, and 250. Scanning electron microscopy (SEM) examination of the fractures showed that failure initiated by SCC or a corrosion pit on all failures examined. The failures then progressed by fatigue. Dye penetrant testing revealed no additional flaws on the wheels that had failed in the flange area. There was, however, one flaw area in the flange of the wheel that failed in the tube well. This flaw resembled a corrosion pit. It was concluded that failure of nose wheels 67, 217, and 250 was caused by cracking due to SCC or pitting. The failures progressed by fatigue. Because failure occurred in the same general area on all three wheels, these locations are suspect as being underdesigned. It was recommended that the nose wheel be redesigned and additional service data be accumulated to understand the contributing factors that resulted in wheel failure.

The 4340 steel main rotor yoke of a helicopter failed during a hovering exercise. Visual examination of the yoke revealed no evidence of gross external damage. Visual fracture surface examination, macrofractography, scanning electron micrography, and metallography of a section cut from the yoke in the region of the cracking indicated that the failure was caused by fatigue-crack initiation and growth from severe corrosion damage to a pillow-block bolt hole. Corrosion occurred because of failure of the protection scheme. An upgraded corrosion protection scheme for the bolt holes was recommended, along with nondestructive inspection of the region at intervals determined by fractographic analysis of the fatigue crack growth.

This article describes the characteristics of tubing of heat exchangers with respect to general corrosion, stress-corrosion cracking, selective leaching, and oxygen-cell attack, with examples. It illustrates the examination of failed parts of heat exchangers by using sample selection, visual examination, microscopic examination, chemical analysis, and mechanical tests. The article explains corrosion fatigue of tubing of heat exchangers caused by aggressive environment and cyclic stress. It also discusses the effects of design, welding practices, and elevated temperatures on the failures of heat exchangers.

A routine examination on a seat ejection system found that the catapult attachment swivel fabricated from 7075-T651 aluminum alloy plate contained cracks on opposite sides of the part. This swivel, or bath tub, does not experience extreme loads prior to activation of the catapult system. Some loads could be absorbed however, when the aircraft is subjected to G loads. Visual examination of the part revealed that cracks through the wall thickness initiated on the inner walls of the fixture. Scanning electron microscopy (SEM) and electron optical examination revealed that the cracking pattern initiated and progressed by an intergranular failure mechanism. It was concluded that failure of the catapult attachment swivel fixture occurred by SCC. It was recommended that the 7075 aluminum ejection seat fixture be supplied in the T-73 temper to minimize susceptibility to SCC.

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 forged aluminum alloy 2014-T6 catapult-hook attachment fitting (anodized by the chromic acid process to protect it from corrosion) from a naval aircraft broke in service. Spectrographic analysis, visual examination, microscopic examination, and tensile analysis showed minute cracks on the inside surface of a bearing hole, and small areas of pitting corrosion were visible on the exterior surface of the fitting. The analysis also revealed a small number of rosettes, suggestive of eutectic melting, in an otherwise normal structure. These examinations and analyses support the conclusion that the presence of chromic acid stain on the fracture surface proved that the forging had cracked before anodizing. This suggest that the crack initiated during straightening, either after machining or after heat treatment. The structure and composition of the alloy appear to have been acceptable. Ductility was acceptable so rosettes found in the microstructure are believed to have been nondamaging. Had they contributed to the failure, the ductility would have been very low. The recommendations included inspection for cracks and revising the manufacturing process to include a fluorescent liquid-penetrant inspection before anodizing, because chromic acid destroys the penetrant. This inspection would reduce the possibility of cracked parts being used in service.

A crack was found in an aircraft main wing spar flange fabricated from 7079-T6 aluminum alloy during a routine nondestructive x-ray inspection after the craft had logged 300 h. Scanning electron microscopy (SEM) revealed an intergranular fracture pattern indicative of stress-corrosion cracking (SCC) and fatigue striations near the crack origin. Visual examination of the crack edge revealed that the installation of the fasteners produced a fit up stress. Further inspection of the opened fracture showed that the crack had been present for some time because a heavy buildup of corrosion products was seen on the fractured surface. Metallographic examination of the flange in the area of fracture initiation showed the presence of end grain exposure, which would promote SCC. Electron optical examination of the fracture clearly showed the flange was cracking by a mixed mode of stress corrosion and fatigue. The cracking was accelerated because of an inadvertent fit up stress during installation. The age of the crack could not be established. However, a reevaluation of prior x-ray inspections in this area would result in some close estimate of the age of the crack. End grain exposure further promoted SCC.

The master connecting rod of a reciprocating aircraft engine revealed cracks during routine inspection. The rods were forged from 4337 (AMS 6412) steel and heat treated to a specified hardness of 36 to 40 HRC. H-shaped cracks in the wall between the knuckle-pin flanges were revealed by visual examination. The cracks were originated as circumferential cracks and then propagated transversely into the bearing-bore wall. No inclusions in the master rod were detected by magnetic-particle and x-ray inspection. Three large inclusions lying approximately parallel to the grain direction and fatigue beach marks around two of the inclusions were revealed by macroscopic examination of the fracture surface. Large nonmetallic inclusions that consisted of heavy concentrations of aluminum oxide (Al2O3) were revealed by microscopic examination of a section through the fracture origin. The forging vendors were notified about the excess size of the nonmetallic inclusions in the master connecting rods and a nondestructive-testing procedure for detection of large nonmetallic inclusions was established.

The outlet-piping system of a steam-reformer unit failed by extensive cracking at four weld locations. The welded system consisted of Incoloy 800 (Fe-32Ni-21Cr-0.05C) pipe and fittings. The exterior surfaces of the system were insulated with rock wool that did not contain weatherproofing. On-site visual examination and magnetic testing indicated severe external corrosion of most of the piping. The system showed extensive cracking in weld HAZ. One specimen indicated that corrosion extended to a depth of 3.2 mm and cracks were seen at the edge of the cover bead and in the HAZ of the weld. Metallographic examination showed that cracking was intergranular and that adjacent grain boundaries had undergone deep intergranular attack. Examination at higher magnification revealed heavy carbide precipitation, primarily at grain boundaries, indicating that the alloy had been sensitized, which resulted from heating during welding. Electron probe x-ray microanalysis showed the outside surface of the tube did not have the protective chromium oxide scale normally found on Incoloy 800. The inside surface of the tube had a thin chromium oxide protective scale. This evidence supported the conclusions that the deep oxidation greatly decreased the strength of the weld HAZ and cracking followed.

Visual examination, optical and scanning electron microscopy were used to determine the cause of failure in the connector groove of a marine riser coupling. The specified steel was AISI 4142 (0.40 to 0.45% C; 0.75 to 1.00% Mn; 0.20 to 0.35% Si; 0.80 to 1.10% Cr; 0.15 to 0.25% Mo) normalized from 9000C. Microscopic examination revealed the crack's initiation point and subsequent propagation. SEM examination of chemically stripped corrosion showed that corrosion fatigue and stress corrosion might have contributed to the initial slow crack growth. Impact tests revealed a fracture transition temperature in excess of 1000C. The sequence of events leading to failure was detailed. The main recommendation was to quench and temper existing couplings and to use a lower carbon quenched and tempered steel for new couplings.

Two 38 mm (1.5 in.) diam threaded stud bolts that were part of a steel mold die assembly from a plastics molding operation were examined to determine their serviceability. Chemical analysis showed the material to be a plain carbon steel that approximated 1045. Visual examination revealed evidence of severe hammer blows to the clevis and boss areas and a gap between the die and the underside of the boss. Magnetic particle inspection showed cracks at the thread roots that, when examined metallographically, were found to contain MnS stringers. The cracking of the threads was attributed to a poor stud bolt design, which allowed a high stress concentration to occur at the base of the threads upon application of a lateral load. It was recommended that bolts of a new design that incorporated a stress-relieving groove be used. Threading of the bolt to eliminate the gap between the lower face of the boss and the die and an improved method of inserting or removing the bolt to avoid hammering (use of a wrench on a square or hexagonal boss) were also recommended.

The rear wall tube section of a boiler that had been in service for approximately 38 years was removed and examined. Visual examination of the tube revealed a small bulge with a through-wall crack. Metallography showed that the microstructure of the bulged area consisted of a few partially decarburized pearlite colonies in a ferrite matrix. The microstructure remote from the bulged area consisted of pearlite in a ferrite matrix. EDS analysis of internal deposits on the tube detected a major amount of iron, plus trace amounts of other elements. The evidence indicated that the bulge and crack in the tube resulted from hydrogen damage. Examination of the remaining water circuit boiler tubing using nondestructive techniques and elimination of any heavy deposit buildup was recommended.

A gas-fired, ASTM A-106 Grade B carbon steel vaporizer failed on three different occasions during attempts to bring the vaporizer on line. Dye penetrant examination indicated the presence of multiple packets of ductile cracks on the inside of the coil radius at the bottom of the horizontal axis coils. Visual examination of the inside of the tubing indicated the presence of a carbonaceous deposit resulting from decomposition of the heat-exchanging fluid. Subsequent metallographic examination and microhardness testing indicated that the steel was heated to a temperature above the allowable operating temperature for the fluid. The probable cause for failure is thermal fatigue due to the localized overheating. Flow conditions inside the tubing should be reexamined to ensure suitable conditions for annular fluid flow.

The T-section cross member of the lifting sling failed in service while lifting a 966 kg (2130 lb) load. The L-section sling body and the cross member were made of aluminum alloy 5083 or 5086 and were joined by welding using aluminum alloy 4043 filler metal. The fracture was found by visual examination to have occurred at the weld joining the sling body and the cross member. Inadequate joint penetration and porosity was revealed by macrographic examination of the weld. Lower silicon content and a higher magnesium and manganese content than the normal for alloy 4043 filler metal were found during chemical analysis. It was revealed by examination of the ends of the failed cross member that a rotational force that had been applied on the cross member caused it to fracture near the sling body. It was concluded that brittle fracture at the weld was caused by overloading which was attributed to the misalignment of the sling during loading. Aluminum alloy 5183 or 5356 filler metal was recommended to be used to avoid brittle welds.