A steel pot used as crucible in a magnesium alloy foundry developed a leak that resulted in a fire and caused extensive damage. Hypotheses as to the cause of the leak included a defect in the pot, overuse, overheating, and poor foundry practices. Scanning electron microscopy, transmission electron microscopy, optical microscopy, and x-ray microanalysis in conjunction with dimensional analysis, phase diagrams and thermodynamics considerations were employed to evaluate the various hypotheses. All evidence pointed to an oxide mass in the area where the hole developed, likely introduced during the steelmaking process.

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 hot rolled, low-carbon steel pot used to melt magnesium alloys leaked, releasing about 35 kg (80 lb) of molten magnesium onto the foundry floor and causing an extensive fire. Due to the fire, the original leakage hole could not be investigated. Samples of the failed pot were polished and etched and were found to be composed of ferrite and pearlite mixtures, as would be expected. However, the sample taken from a location about 75 mm (3 in.) from the hole contained a cluster of unusually large inclusions. By removing the beryllium window from in front of the detector, EPMA spectra were obtained from the inclusions and from the steel matrix. The inclusion spectrum contained primarily iron and oxygen, whereas the matrix spectrum contained primarily iron. X-ray maps were made to show the distribution of iron and oxygen. These results indicated that the inclusions were iron oxide. A similar inclusion at the failure site in the melting pot may have reacted violently with the molten magnesium, causing the leak.

This article commences with a discussion on the characteristics of stress-corrosion cracking (SCC) and describes crack initiation and propagation during SCC. It reviews the various mechanisms of SCC and addresses electrochemical and stress-sorption theories. The article explains the SCC, which occurs due to welding, metalworking process, and stress concentration, including options for investigation and corrective measures. It describes the sources of stresses in service and the effect of composition and metal structure on the susceptibility of SCC. The article provides information on specific ions and substances, service environments, and preservice environments responsible for SCC. It details the analysis of SCC failures, which include on-site examination, sampling, observation of fracture surface characteristics, macroscopic examination, microscopic examination, chemical analysis, metallographic analysis, and simulated-service tests. It provides case studies for the analysis of SCC service failures and their occurrence in steels, stainless steels, and commercial alloys of aluminum, copper, magnesium, and titanium.

Stress-corrosion cracking (SCC) is a form of corrosion and produces wastage in that the stress-corrosion cracks penetrate the cross-sectional thickness of a component over time and deteriorate its mechanical strength. Although there are factors common among the different forms of environmentally induced cracking, this article deals only with SCC of metallic components. It begins by presenting terminology and background of SCC. Then, the general characteristics of SCC and the development of conditions for SCC as well as the stages of SCC are covered. The article provides a brief overview of proposed SCC propagation mechanisms. It discusses the processes involved in diagnosing SCC and the prevention and mitigation of SCC. Several engineering alloys are discussed with respect to their susceptibility to SCC. This includes a description of some of the environmental and metallurgical conditions commonly associated with the development of SCC, although not all, and numerous case studies.

Extensive cracking was found in a batch of die-cast ZAMAK 3 solenoid valve seats during commissioning of the system in which they were installed. Scanning electron microscopic and chemical analyses conducted on one of the failed valve seats showed that the composition of the alloy was different from that specified. The presence of excess aluminum and lead impurities that had segregated to the grain boundaries, coupled with an inadequate amount of magnesium, resulted in intergranular corrosion and subsequent intergranular failure. Corrosion was accelerated by storage in a humid environment in a coastal area. It was recommended that proper chemical analysis of the zinc-aluminum alloy be carried out as a quality control procedure.

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.

A failure analysis investigation was conducted on a fractured aluminum tailwheel fork which failed moments after the landing of a privately owned, 1955 twin-engine airplane. Nondestructive evaluation via dye-penetrant inspection revealed no discernible surface cracks. The chemical composition of the sand-cast component was identified via optical emission spectroscopy and is comparable to an aluminum sand-cast alloy, AA 712.0. Metallographic evaluation via optical microscopy and scanning electron microscopy revealed a high degree of porosity in the microstructure as well as the presence of deleterious intermetallic compounds within interdendritic regions. Macrohardness testing produced hardness values which are noticeably higher than standard hardness values for 712.0. The primary fracture surfaces indicate evidence of mixed-mode fracture, via intergranular cracking, cleaved intermetallic particles, and dimpled cellular regions in the matrix. The secondary fracture surface demonstrates similar features of intergranular fracture.

Compound bow handle risers that had failed in service and during assembly along with an unassembled riser were submitted for analysis. The risers were die cast from magnesium-base alloy AM60A. Inspection of the failed risers and metallurgical investigations conducted on the stock riser revealed the presence of cold shuts at the same site in all specimens. It was recommended that all risers be thoroughly inspected and that the bow company work with their die casting shop to design a mold with acceptable filling characteristics.

Two nuts were used to secure the water-supply pipes to the threaded connections on hot-water and cold-water taps. The nut used on the cold-water tap fractured about one week after installation. Examination of the fracture surfaces of the coldwater nut did not reveal any obvious defects to account for the fracture, but there were indications of excessive porosity in the nut. The fracture had occurred through the root of the first thread that was adjacent to the flange of the tap. It was found that the nut from the cold-water tap failed by SCC. Apparently, sufficient stress was developed in the nut to promote this type of failure by normal installation because there was no evidence of excessive tightening of the nut. Corrosion testing of the nuts indicated that the fractured nut was highly susceptible to intergranular corrosion because of either a deficiency in magnesium content or excessive impurities, such as lead, tin, or cadmium. This composition problem with zinc alloys was recognized many years ago, and particular attention has been directed toward ensuring that high-purity zinc is used. This corrective measure reportedly resulted in virtual elimination of this type of defect.

Despite extensive aircraft landing gear design analyses and tests performed by designers and manufacturers, and the large number of trouble-free landings, aircraft users have experienced problems with and failures of landing gear components. Different data banks and over 200 failure analysis reports were surveyed to provide an overview of structural landing gear component failures as experienced by the Canadian Forces over the last 20 years on more than 20 aircraft types, and to assess trends in failure mechanisms and causes. Case histories were selected to illustrate typical problems, troublesome failure mechanisms, the role of high strength aluminum alloys and steels, and situations where fracture mechanics analyses provided insight into the failures. The two main failure mechanisms were: fatigue occurring mainly in steel components, and corrosion related problems with aluminum alloys. Very few overload failures were noted. A number of causes were identified: design deficiencies and manufacturing defects leading mainly to fatigue failures, and poor materials selection and improper maintenance as the principal causes of corrosion-related failures. The survey showed that a proper understanding of the failure mechanisms and causes, by thorough failure analysis, provides valuable feedback information to designers, operators and maintenance personnel for appropriate corrective actions to be taken.

One of five underground drain lines intended to carry a highly acidic effluent from a chemical-processing plant to distant holding tanks failed in just a few months. Each line was made of 304L stainless steel pipe 73 mm (2 in.) in diam with a 5 mm (0.203 in.) wall thickness. Lengths of pipe were joined by shielded metal arc welding. Soundness of the welded joints was determined by water back-pressure testing after several lengths of pipe had been installed and joined. Before completion of the pipeline, a pressure drop was observed during back-pressure testing. An extreme depression in the backfill revealed the site of failure. Analysis (visual inspection, electrical conductivity, and soil analysis) supported the conclusions that the failure had resulted from galvanic corrosion at a point where the corrosivity of the soil was substantially greater than the average, resulting in a voltage decrease near the point of failure of about 1.3 to 1.7 V. Recommendations included that the pipelines be asphalt coated and enclosed in a concrete trough with a concrete cover. Also, magnesium anodes, connected electrically to each line, should be installed at periodic intervals along their entire length to provide cathodic protection.

Several wires in aluminum conductor cables fractured within 5 to 8 years of, service in Alaskan tundra. The cables were comprised of 19-wire strands; the wires were aluminum alloy 6201-T81. Visual and metallographic examinations of the cold-upset pressure weld joints in the wires established that the fractures were caused by fatigue loading attributable to wind/thermal factors at the joints. The grain flow at the joints was transverse to the wire axis, rendering the notches of the joints sensitive to fatigue loading. An additional contributory factor was intergranular corrosion, which assisted fatigue crack initiation/propagation. The failure was attributed to the departure of conductor quality from the requirements of ASTM B 398 and B 399, which specify that “no joints shall be made during final drawing or in the finished wire” and that the joints should not be closer than 15 m (50 ft). The failed cable did not meet either criterion. It was recommended that the replacement cable be inspected for strict compliance to ASTM requirements.

A commercial aircraft wheel half, machined from an aluminum alloy 2014 forging that had been heat treated to the T6 temper, was removed from service because a crack was discovered in the area of the grease-dam radius during a routine inspection. Neither the total number of landings nor the roll mileage was reported, but about 300 days had elapsed between the date of manufacture and the date the wheel was removed from service. The analysis (visual inspection, macrographs, micrographs, electron microprobe) supported the conclusions that the wheel half failed by fatigue. The fatigue crack originated at a material imperfection and progressed in more than one plane because changes in the direction of wheel rotation altered the direction of the applied stresses. Recommendations included rewriting the inspection specifications to require sound forgings.

An AMS 4126 (7075-T6) aluminum alloy impeller from a radial inflow turbine fractured during commissioning. Initial examination showed that two adjacent vanes had fractured through airfoils in the vicinity of the vane leading edges, and one vane fractured through an airfoil near the hub in the vicinity of the vane trailing edge. Some remaining vanes exhibited radial and transverse cracks in similar locations. Binocular and scanning electron microscope examinations showed that the cracks had been caused by high-cycle fatigue and had progressed from multiple origins on the vane surface. Structural analysis indicated that the fatigue loading probably had been caused by forced excitation, resulting in the impeller vibrating at its resonant frequency. It was recommended that the impeller design, control systems, and material of construction be changed.

This article addresses the forms of corrosion that contribute directly to the failure of metal parts or that render them susceptible to failure by some other mechanism. It describes the mechanisms of corrosive attack for specific forms of corrosion such as galvanic corrosion, uniform corrosion, pitting and crevice corrosion, intergranular corrosion, and velocity-affected corrosion. The article contains a table that lists combinations of alloys and environments subjected to selective leaching and the elements removed by leaching.

During the preproduction stages of forging, an initial batch of 50 mm (2 in.) diam Al-4Cu alloy (L77) extruded bar stock material was found to be cracking randomly. Failure analysis was conducted to determine the metallurgical factors underlying the phenomenon. Microexamination of sections across the defects revealed intergranular cracks tracing a path of round, segregated particles and oxide film discontinuities. The segregated particles were rich in copper It was concluded that the cracking was the result of segregations occurring in poor-quality raw material. The source of segregation was suspected to be the use of improperly made master alloys. Use of improved melting techniques and proper master alloys was recommended.

Corrosion is the electrochemical reaction of a material and its environment. This article addresses those forms of corrosion that contribute directly to the failure of metal parts or that render them susceptible to failure by some other mechanism. Various forms of corrosion covered are galvanic corrosion, uniform corrosion, pitting, crevice corrosion, intergranular corrosion, selective leaching, and velocity-affected corrosion. In particular, mechanisms of corrosive attack for specific forms of corrosion, as well as evaluation and factors contributing to these forms, are described. These reviews of corrosion forms and mechanisms are intended to assist the reader in developing an understanding of the underlying principles of corrosion; acquiring such an understanding is the first step in recognizing and analyzing corrosion-related failures and in formulating preventive measures.

This article briefly reviews the various metallurgical or environmental factors that cause a weakening of the grain boundaries and, in turn, influence the occurrence of intergranular (IG) fractures. It discusses the mechanisms of IG fractures, including the dimpled IG fracture, the IG brittle fracture, and the IG fatigue fracture. The article describes some typical embrittlement mechanisms that cause the IG fracture of steels.