Metallographic examination is one of the most important procedures used by metallurgists in failure analysis. Typically, the light microscope (LM) is used to assess the nature of the material microstructure and its influence on the failure mechanism. Microstructural examination can be performed with the scanning electron microscope (SEM) over the same magnification range as the LM, but examination with the latter is more efficient. This article describes the major operations in the preparation of metallographic specimens, namely sectioning, mounting, grinding, polishing, and etching. The influence of microstructures on the failure of a material is discussed and examples of such work are given to illustrate the value of light microscopy. In addition, information on heat-treatment-related failures, fabrication-/machining-related failures, and service failures is provided, with examples created using light microscopy.

Following the crash of a Mirage III-0 aircraft (apparently caused by engine failure), a small crack was detected in a bolt hole in the wing main spar (AU4SG aluminum alloy). Because this area was considered to be critical to aircraft safety and similar cracking was found in other spars in service, the Royal Australian Air Force requested that the crack growth rate during service be determined. The loading history of the aircraft was made available in the form of flight by-flight records of the counts from the vertical accelerometer sensors fitted to the airframe and a series of “overstress” events recorded during the life of the aircraft. The bolt hole was examined by eddy current testing, visual examination, high-powered light microscope, and scanning electron microscope. Simulation tests were also conducted. The use of simulation specimens permitted actual crack growth rate data to be determined for the configuration of interest.

This article provides a discussion on the metallographic techniques used for failure analysis, and on fracture examination in materials, with illustrations. It discusses various metallographic specimen preparation techniques, namely, sectioning, mounting, grinding, polishing, and electrolytic polishing. The article also describes the microstructure examination of various materials, with emphasis on failure analysis, and concludes with information on the examination of replicas with light microscopy.

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.

A stamped coin exhibited visible discolored areas, seen as a tan haze on the surface. The discoloration was considered merely cosmetic. The nonstained and stained regions were studied using SEM/EDS. Greater amounts of aluminum and magnesium were found in the stained area as compared with the nonstained region. Some carbon and oxygen were detected in both areas, which may be suggestive of organic substances. Fourier transform infrared spectroscopy (FTIR) revealed traces of hydrocarbons and ether/alcohol materials in the stained area, suggesting that the stain was associated with a cellulose or carbohydrates (sugars). These findings, along with the appearance, suggest that a sugar-containing substance, such as coffee or a soft drink, dried onto the surface of this coin and caused the observed discoloration.

A flat spring for the main landing gear of a light aircraft failed after safe execution of a hard landing. The spring material was identified by chemical analysis to be 6150 steel. The fracture was found to have occurred near the end of the spring that was inserted through a support member about 25 mm thick and attached to the fuselage by a single bolt. Brinelling (plastic flow and indentation due to excessive localized contact pressure) was observed on the upper surface of the spring where the forward and rear edges of the spring contacted the support member. It was indicated by chevron marks that brittle fracture had started beneath the brinelled area at the forward edge of the upper surface of the spring. The origin of the brittle fracture was found to be a small fatigue crack that had been present for a considerable period of time before final fracture occurred. Fracture of the landing-gear spring was concluded to have been caused by a fatigue crack that resulted from excessive brinelling at the support point. Regular visual examinations to detect evidence of brinelling and wear at the support in aircraft with this configuration of landing-gear spring were recommended.

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.

A type 304 stainless steel tube that failed in a boiler stack economizer was analyzed to determine the cause. The investigation consisted of visual, SEM/EDS, and metallographic analysis. Several degradation mechanisms appeared to be at work, including pitting corrosion, chloride stress corrosion cracking, and fatigue fracture. Investigators concluded that the primary failure mechanism was fatigue fracture, although either of the other mechanisms may have eventually caused the tube to fail in the absence of fatigue.

This article reviews photographic principles, namely, visual examination, field photographic documentation, and laboratory photographic documentation, as applied to failure analysis and the specific techniques employed in both the field and laboratory. It provides information on the photographic equipment used in failure analysis and on film and digital photography. The article describes the basics of photography and the uses of different types of lighting in photography of a fractured surface. The article also addresses the techniques involved in macrophotography and microscopic photography as well as other special techniques.

A two-section extension ladder, made from 6061-T6 aluminum alloy extrusions and stampings that were riveted together at each rung location and at the ends of side rails, broke in service after having been used at the sites of several fires by the fire department of a large city. The fracture surfaces were examined visually and by optical (light) stereomicroscopy. Material testing showed a sample to be within the specified material limits for aluminum alloy 6061. Microscopic examination showed no significant differences in microstructure or grain size among the four T-sections, and thickness measurements at various locations indicated that thicknesses were well within standard industry tolerances for aluminum extrusions in this size range. However, hardness testing of the four T-sections showed that in two, hardness was considerably lower than the acceptable hardness for the T6 temper and were within the range for 6061-T4 (acceptable hardness, 19 to 45 HRB). This indicated they had been naturally aged at room temperature after solution heat treatment instead of artificially aged as per specs. Edge cracking in two of the T-sections was the result of improper conditions during extrusion of the T-sections; however, this condition was not a primary cause of failure.

This paper describes an investigation on the failure of a large leaded bronze bearing that supports a nine-ton roller of a plastic calendering machine. At the end of the normal service life of a good bearing, which lasted for seven years, a new bearing was installed. However the new one failed catastrophically within a few days, generating a huge amount of metallic wear debris and causing pitting on the surface of the cast iron roller. Following the failure, samples were collected from both good and failed bearings. The samples were analyzed chemically and their microstructures examined. Both samples were subjected to accelerated wear tests in a laboratory type pin-on-disk apparatus. During the tests, the bearing materials acted as pins, which were pressed against a rotating cast iron disk. The wear behaviors of both bearing materials were studied using weight loss measurement. The worn surfaces of samples and the wear debris were examined by light optical microscope, SEM, and energy-dispersive x-ray microanalyzer. It was found that the laboratory pin-on-disk wear data correlated well with the plant experience. It is suggested that the higher lead content ~18%) of the good bearing compared with 7% lead of the failed bearing helped to establish a protective transfer layer on the worn surface. This transfer layer reduced metal-to-metal contact between the bearing and the roller and resulted in a lower wear rate. The lower lead content of the failed bearing does not allow the establishment of a well-protected transfer layer and leads to rapid wear.

Rollover accidents in light trucks and cars involving an axle failure frequently raise the question of whether the axle broke causing the rollover or did the axle break as a result of the rollover. Axles in these vehicles are induction hardened medium carbon steel. Bearings ride directly on the axles. This article provides a fractography/fracture mechanic approach to making the determination of when the axle failed. Full scale tests on axle assemblies and suspensions provided data for fracture toughness in the induction hardened outer case on the axle. These tests also demonstrated that roller bearing indentions on the axle journal, cross pin indentation on the end of the axle, and axle bending can be accounted for by spring energy release following axle failure. Pre-existing cracks in the induction hardened axle are small and are often difficult to see without a microscope. The pre-existing crack morphology was intergranular fracture in the axles studied. An estimate of the force required to cause the axle fracture can be made using the measured crack size, fracture toughness determined from these tests, and linear elastic fracture mechanics. The axle can be reliably said to have failed prior to rollover if the estimated force for failure is equal to or less than forces imposed on the axle during events leading to the rollover.

Four cadmium-plated ASTM A193 grade B studs from a steam line connector associated with a power turbine failed unexpectedly in a nil-ductility manner. Fracture surfaces were covered with a light-colored, lustrous deposit. Optical microscope, SEM, and EDS analyses were conducted on sections from one of the studs and revealed that the coating on the fracture surface was cadmium. The fracture had multiple origins, and secondary cracks also contained cadmium. The fracture topography was intergranular. The failures were attributed to liquid metal embrittlement caused by the presence of a cadmium plating and operating temperatures at approximately the melting point of cadmium. It was recommended that components exposed to the cadmium be replaced.

Following an accident in which a light pickup truck left the road and overturned, one of the rear axles, made of approximately 0.30C steel, was found to be fractured adjacent to the bearing lock nut. A keyway was present in the failed area, as were threads for the lock nut. Fracture surfaces of the failed axle and exemplar fractures obtained from simulation tests were studied using scanning electron microscope. The examination showed that the outer perimeter fracture in the axle was very flat and composed of cleavage and that the interior portion was composed of both cleavage and dimples. No evidence of prior cracking was found. The exemplar specimens from the simulation impact testing failed in a manner consistent with that observed in the axle. The examination confirmed that the failure was a one-time impact overload fracture and not the result of any prior crack in the material, indicating that the axle failure did not initiate the accident.