This article examines the concept of fractography as applied to elastomeric rubbery materials. It considers four general categories of physical root failure causes: design defects, material defects, manufacturing defects, and service life anomalies. Examples of real-world failures of rubber articles, with numerous accompanying figures, are representative of the four root failure categories.

Drive cables from a rubber processing machine were failing in less than 8 h of operation, the expected service life being much greater than 100 h. Comparison cables were tested to failure under known stress conditions, including tensile overload, torsional loading, reversed bending alternating stress, and buckling (compressive) cyclic loading. The mode of failure was found to be reversed bending fatigue caused by drive cables moving over guide pulleys of small radii. Modifications of the machinery and drive cable system were suggested.

A forging of 7075-T6 aluminum alloy, which formed a support for the cylinder of a cargo door, cracked at an attachment hole. Fluorescent penetrant inspection showed the crack ran above and below the hole out onto the machined flat surface of the flange. A 6500x electron fractograph proved the crack to be a forging defect called a cold shut. Because defects of this type are usually detected when the raw forging is inspected, this occurrence was considered to be an isolated instance.

Examination of a cracked nose landing gear cylinder made of AISI 4340 Cr-Mo-Ni alloy steel proved that the part started to fail on the inside diam. When the nucleus of the stress-corrosion crack was studied in detail, iron oxide was found on the fracture surface. A 6500x micrograph revealed this area also displayed an intergranular texture. One of a group of small grinding cracks on the ID of the cylinder nucleated the failure. Other evidence indicated the cracks developed when the cylinder was ground during overhaul.

Ground maintenance personnel discovered hydraulic fluid leaking from two small cracks in a main landing gear cylinder made from AISI 4340 Cr-Mo-Ni alloy steel. Failure of the part had initiated on the ID of the cylinder. Numerous cracks were found under the chromium plate. A 6500x electron fractograph showed cracking was predominantly intergranular with hairline indications. Leaking had occurred only 43 h after overhaul of the part. Total service time on the part was 9488 h. It was concluded that cracking on the ID was caused by hydrogen embrittlement which occurred during or after overhaul. The specific source of hydrogen which produced failure was not ascertainable.

A nose landing gear cylinder made from AISI 4340 Ni-Cr-Mo alloy steel was found cracked and leaking, causing partial depressurization. Investigation revealed the crack to be a stress-corrosion type, judging by the 6500x electron fractograph. It had started in a region of concentrated, large non-metallic inclusions near the chromium-plated ID of the cylinder. Also, there were breaks in the chromium plate and pits in the underlying base metal. The cylinder had been in service for 18,017 h, and 5948 h had passed since the first and only overhaul. Substandard plating of the ID at this time ultimately resulted in pitting of the metal. The combination of surface pitting and stress concentration at the nearby inclusions resulted in stress-corrosion cracking.

After a quick-release fitting of an ejection seat broke, an investigation was performed to determine the manner and cause of crack propagation. Most fractography-based investigations aim to characterize only qualitative characteristics, such as the fracture orientation and origin position, topology, and details of interactions with microstructural features. The aim of this investigation was to use quantitative fractography as a tool to extract information, including striation spacing and size of the stretched zone, in order to make a direct correlation with fracture mechanic concepts. As the crack propagated, striations were created on the fracture surface as a result of service-induced load changes. The size of the striations were measured to estimate crack propagation rate. Remaining lifetime estimates were also made. The dimensions of plastically stretched zones found at the tips of the cracks were evaluated using electron micrograph stereo image pairs to characterize local fracture toughness. To complete the failure analysis, nondestructive evaluation, metallographic examination, and chemical investigations were carried out. No secondary cracks could be found. Most of the broken parts showed that the microstructure, the hardness, and the chemical composition of the Al-alloy were within the specification, but some of the cracked parts were manufactured using a different material than that specified.

Overhaul mechanics discovered a crack in an AISI 4340 Cr-Mo-Ni alloy steel pivot bolt when grinding off the chromium plating. The bolt had served for an estimated 10,000 h and was replated when last overhauled. On checking the bolt, several fine cracks were found on the surface. A 6500x micrograph revealed the intergranular nature of a crack. By trying different grinding procedures, investigators were able to reproduce this type of failure in the laboratory. It was concluded that grinding cracks initiated the failure. It should be noted that governing specifications prohibit grinding on high-strength steel; chromium should be stripped by electrochemical methods.

After completing a fatigue test of an aluminum alloy component machined from a 7079-T6 forging, technicians noted a 5 in. crack which ran longitudinally above and through the flange. When the fracture face was examined by light microscopy, observers could not ascertain the exact mode of fracture. Electron fractography revealed that five different modes of crack growth were operative as the part failed. Region 1 was a shallow zone (about 0.002 in. at its deepest) of dimpled structure typical of an overload failure. Region 2 was a zone that grew by a stress corrosion mechanism. Through a fatigue mechanism was operative in Region 3, it was not the cause of the large crack. Region 4, which covered 50% of the fracture area, developed mainly by stress corrosion. This zone gradually changed into the combination of intergranular and transgranular overload in Region 5, which covered approximately the remaining 50% of the fracture. Apparently, after stress corrosion moved halfway through, the part failed by overload. This failure analysis proved that a crack, originally thought to be a fatigue failure, was actually a stress corrosion crack.

Important clues about the probable cause of a gun tube explosion were obtained from a fractographic and metallographic examination of the fragments. The size, distribution, and surface markings of fragments may be used to localize the explosion and deduce its intensity. Microstructural features such as voids, adiabatic shear, and structural surface alterations also indicate the explosion intensity and further allow a comparison of the tube structure near and away from the explosion zone. These, and other metallurgical characteristics, are illustrated and discussed for cases of accidental and deliberately caused explosions of large caliber gun tubes.

The quantitative characterization of fracture surface geometry, that is, quantitative fractography, can provide useful information regarding the microstructural features and failure mechanisms that govern material fracture. This article is devoted to the fractographic techniques that are based on fracture profilometry. This is followed by a section describing the methods based on scanning electron microscope fractography. The article also addresses procedures for three-dimensional fracture surface reconstruction. In each case, sufficient methodological details, governing equations, and practical examples are provided.

A ductile iron T-hook hook was reported to have fractured in service. It was further reported that the hook had been subjected to a load that did not exceed 5900 kg (13,000 lb) at the time of fracture. No information was provided regarding the type of metal used to manufacture the hook. A failure analysis was requested to determine the cause of fracture. Two hooks were submitted for examination. Analysis (visual inspection, 2.7x light fractography, chemical analysis, 110x SEM fractography, 27x/110x/215x nital-etched micrographs) supported the conclusions that this component fractured in service as a consequence of ductile tensile overload. Evidence indicates that the fractured region was subjected to a load exceeding the capacity of the material. Because the information available from the service application indicated that the component had not been subjected to a stress that exceeded 5900 kg (13,000 lb), the observations made in this investigation suggested that either the load was underestimated or that the indicated load was applied at a more rapid rate (perhaps with a jerk), which would tend to increase the effective force of the load.

The fatigue failure of a wire rope used on a skip hoist in an underground mine has been studied as part of the ongoing research by the Bureau of Mines into haulage and materials handling hazards in mines. Macroscopic correlation of individual wire failures with wear patterns, fractography, and microhardness testing were used to gain an understanding of the failure mechanism. Wire failures occurred predominantly at characteristic wear sites between strands. These wear sites are identifiable by a large reduction in diameter; however, reduction in area was not responsible for the location of failure. Fractography revealed multiple crack initiation sites to be located at other less noticeable wear sites or opposite the characteristic wear site. Microhardness testing revealed hardening, and some softening, at wear sites.