A die-cast zinc adapter used in a snowthrower failed catastrophically in a brittle overload manner. The component had a chemical composition similar to standard zinc alloy ZA-27 (UNS Z35840), although the iron content was much higher and the copper slightly lower. The mechanical properties and alloy designation were not specified. Investigation (visual inspection, 187x SEM images, unetched 30x images, hardness testing, and chemical analysis) of both the failed adapter and an exemplar casting from known-good lot supported the conclusion that the casting failed as a result of brittle overload fracture due to excessive iron-zinc phase and gross porosity. These conditions acted synergistically to reduce the strength of the material. The composition was nonstandard, and the inherent brittleness suggested that it was unlikely that this material was an intentional proprietary alloy. No recommendations were made.

Electrical contact-finger retainers blanked and formed from annealed copper alloy C65500 (high-silicon bronze A) failed prematurely by cracking while in service in switchgear aboard seagoing vessels. In this service they were sheltered from the weather but subject to indirect exposure to the sea air. About 50% of the contact-finger retainers failed after five to eight months of service aboard ship. Investigation (visual inspection, 250x images etched with equal parts NH4OH and H2O2, emission spectrographic analysis, and stereoscopic views) supported the conclusion that the cracking was produced by stress corrosion as the combined result of: residual forming and service stresses; the concentration of tensile stress at outer square corners of the pierced slots; and preferential corrosive attack along the grain boundaries as a result of high humidity and occasional condensation of moisture containing a fairly high concentration of chlorides (seawater typically contains about 19,000 ppm of dissolved chlorides) and traces of ammonia. Recommendations included redesign of the slots, shot-blasting the formed retainers, and changing the material to a different type of silicon bronze-copper alloy C64700.

Failure analysis results were employed to identify a better alloy. Chipper knives used in the field to chip logs failed frequently. The knives were made of alloys with a composition of Fe-0.48C-0.30Mn-0.90Si-8.50Cr-1.35Mo-1.20W-0.30V. The development of tougher alloy steel with superior properties was initiated. The nominal composition of Fe-0.50C-0.30Mn-0.40Si-5.00Cr-2.00Mo was developed which achieved the goals of edge retention, resistance to softening under frictional heating, wear resistance, ease of heat treatment, dimensional stability in heat treatment, grindability, and low alloy cost. A chip harvester made from this composition was tested in field with older composition knives. It was found that the new knives outperformed the older knives. The key to the development was interpreted to be careful study of a number of failed knives with different problems used in different types of operations.

Failures of four different 300-series austenitic stainless steel biomedical fixation implants were examined. The device fractures were observed optically, and their surfaces were examined by scanning electron microscopy. Fractography identified fatigue to be the failure mode for all four of the implants. In every instance, the fatigue cracks initiated from the attachment screw holes at the reduced cross sections of the implants. Two fixation implant designs were analyzed using finite-element modeling. This analysis confirmed the presence of severe stress concentrations adjacent to the attachment screw holes, the fatigue crack initiation sites. Conclusions were reached regarding the design of these types of implant fixation devices, particularly the location of the attachment screw holes. The use of austenitic stainless steel for these biomedical implant devices is also addressed. Recommendations to improve the fixation implant design are suggested, and the potential benefits of the substitution of titanium or a titanium alloy for the stainless steel are discussed.

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.

Two tubular AISI 1025 steel posts (improved design) in a carrier vehicle failed by cracking at the radius of the flange after five weeks of service. The posts were two of four that supported the chassis of the vehicle high above the wheels. The original design involved a flat flange of low-carbon low-alloy steel that was welded to an AISI 1025 steel tube, and the improved design included placing the welded joint of the flange farther away from the flange fillet. Investigation (visual inspection and chemical analysis) supported the conclusion that the failures in the flanges of improved design were attributed to fatigue cracks initiating at the aluminum oxide inclusions in the flange fillet. Recommendations included retaining the improved design of the flange with the weld approximately 50 mm (2 in.) from the fillet, but changing the metal to a forging of AISI 4140 steel, oil quenched and tempered to a hardness of 241 to 285 HRB. Preheating to 370 deg C (700 deg F) before and during welding with AISI 4130 steel wire was specified. It was also recommended that the weld be subjected to magnetic-particle inspection and then stress relieved at 595 deg C (1100 deg F), followed by final machining.

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.

The right landing gear on a twin-turboprop transport aircraft collapsed during landing. Preliminary examination indicated that the failure occurred at a steel-to-aluminum (7014) pinned drag-strut connection due to fracture of the lower set of drag-strut attachment lugs at the lower end of the oleo cylinder housing. Two lug fractures that were determined to be the primary fractures were analyzed. Results of various examinations indicated that stress-corrosion cracking associated with the origins of the principal fractures in the connection was the cause of failure. It was recommended that the design be modified to avoid dissimilar metal combinations of high corrosion potential.

Several pressurized air containers (i.e., diving tanks) made of non-heat-treatable Al-5Mg aluminum alloy failed catastrophically. Catastrophic failure occurred when a subcritical stress corrosion crack reached a critical size. Critical crack size for unstable propagation was reached prior to wall penetration, which could have led to subsequent loss of pressure, resulting in explosion of the cylinder. It was recommended that more stress corrosion resistant alloys be used for sea diving applications. Furthermore, cylinders should have a reduced wall thickness that can be determined employing the “leak-before-break” design philosophy, developed using fracture mechanics, to eliminate the possibility of catastrophic ruptures.

This article focuses on the general root causes of failure attributed to the casting process, casting material, and design with examples. The casting processes discussed include gravity die casting, pressure die casting, semisolid casting, squeeze casting, and centrifugal casting. Cast iron, gray cast iron, malleable irons, ductile iron, low-alloy steel castings, austenitic steels, corrosion-resistant castings, and cast aluminum alloys are the materials discussed. The article describes the general types of discontinuities or imperfections for traditional casting with sand molds. It presents the international classification of common casting defects in a tabular form.

High failure rates in the drive shafts of 40 newly acquired articulated buses was investigated. The drive shafts were fabricated from a low-carbon (0.45%) steel similar to AISI 5046. Investigators examined all 40 buses, discovering six different drive shaft designs across the fleet. All of the failures, a total of 14, were of the same type of design, which according to finite-element analysis, produces a significantly higher level of stress. SEM examination of the fracture surface of one of the failed drive shafts revealed fatigue striations near the OD and ductile dimpling near the ID, evidence of high-cycle fatigue. Based on the failure rate and fatigue life predictions, it was recommended to discontinue the use of drive shafts with the inferior design.

Proper stress analysis during component design is imperative for accurate life and performance prediction. The total stress on a part is comprised of the applied design stress and any residual stress that may exist due to forming or machining operations. Stress-corrosion cracking may be defined as the spontaneous failure of a metal resulting from the combined effects of a corrosive environment and the effective component of tensile stress acting on the structure. However, because of the orientation dependence in aluminum, it is the residual stress occurring in the most susceptible direction that must be considered of primary importance in material selection for design configuration. A Navy UH-1N helicopter main rotor blade grip manufactured from a 2014-T6 aluminum alloy forging failed because of a design flaw that left a high residual tensile stress along the short transverse plane; this in turn provided the necessary condition for stress corrosion to initiate. A complete failure investigation to ascertain the exact cause of the failure was conducted utilizing stereomicroscopic examination, scanning electron microscopy, metallographic inspection and interpretation, energy-dispersive chemical analysis, physical and mechanical evaluation. Stereomicroscopic examination of the opened crack fracture surface revealed one large fan-shaped region that had propagated radially through the thickness of the material from two distinct origin areas on the internal diam of the grip. Higher magnification inspection near the origin area revealed a flat, wood-like appearance. Scanning electron microscopy divulged the presence of substantial mud cracking and intergranular separation on the fracture surface. Metallographic examination revealed intergranular cracking and substantial leaf separation along the elongated grains parallel to the fracture surface. Chemical composition and hardness requirements were found to be as specified. The blade grip failed due to a stress corrosion crack which initiated on the inner diam and propagated in the short transverse direction through the thickness of the component. The high residual tensile stress in the part resulting from the forging and exposed after machining of the inner diam, combined with the presence of moisture, provided the necessary conditions to facilitate crack initiation and propagation.

The drive shaft in a marine propulsion system broke, stranding a large vessel along the Canadian seacoast. The shaft was made from quenched and tempered low-alloy steel. Fractographic investigation revealed that the shaft failed under low rotating-bending variable stress. Fatigue propagation occurred on about 95% of the total cross section of the shaft, under both low-cycle and high-cycle fatigue mechanisms. It was found that the fillet radius at the fracture’s origin was smaller than the one provisioned by design. As a result, the stresses at this location exceeded the values used in the design calculations, thus causing the initiation of the cracking. Moreover, although the shaft had been quenched and tempered, its actual hardness did not have the optimal value for long-term fatigue strength.

In the EMD-2 Joint Directed Attack Munition (JDAM), the A357 aluminum alloy housing had been redesigned and cast via permanent mold casting, but did not meet the design strength requirements of the previous design. Mechanical tests on thick and thin sections of the forward housing assembly revealed tensile properties well below the allowable design values. Radiology and CT evaluations revealed no casting defects. Optical microscopy revealed porosity uniformly distributed throughout the casting on the order of 0.1 mm pore diam. Scanning electron microscopy revealed elongated pores, which indicated turbulent filling of the mold. Spherical pores would have indicated the melt had been improperly degassed. Based on these findings, it was recommended that the manufacturer analyze and redesign the gating system to eliminate the turbulent flow problem during the permanent mold casting process.

A fan support casting failed unexpectedly while running at 1800 rpm in pulp at 65 deg C (150 deg F). The leading edge of the blade exhibited deep spongy holes leading to reduced section and finally to fracture of the part when the remaining section size was insufficient to support the load. Analysis showed the support casting to be a standard 8620 type composition with a hardness of 311 HRB. The design of the casting was not streamlined. There were several square corners present where great pressure differences could be generated. This was a case of erosion-corrosion with the classic spongy appearance of cavitation. Two changes were proposed: streamlining the part to avoid abrupt changes in fluid flow; and a change in alloy to a more corrosion-resistant material (304 or preferably 316) to increase the tenacity of protective films.

The information provided in this article is intended for those individuals who want to determine why a casting component failed to perform its intended purpose. It is also intended to provide insights for potential casting applications so that the likelihood of failure to perform the intended function is decreased. The article addresses factors that may cause failures in castings for each metal type, starting with gray iron and progressing to ductile iron, steel, aluminum, and copper-base alloys. It describes the general root causes of failure attributed to the casting material, production method, and/or design. The article also addresses conditions related to the casting process but not specific to any metal group, including misruns, pour shorts, broken cores, and foundry expertise. The discussion in each casting metal group includes factors concerning defects that can occur specific to the metal group and progress from melting to solidification, casting processing, and finally how the removal of the mold material can affect performance.

Several 6063-T6 aluminum alloy extension ladders of the same size and type collapsed in service in the same manner; the extruded aluminum alloy 6063-T6 side rails buckled, but the rungs and hardware remained firmly in place. The ladders had a maximum extended length of 6.4 m (21 ft) with a recommended maximum angle of inclination of 75 deg (15 deg from vertical). Investigation (visual inspection, hardness testing, metallographic examination, stress analysis, and tensile tests) supported the conclusion that the side rails of the ladders buckled when subjected to loads that produced stresses beyond the yield strength of the alloy. Recommendations included increasing the thickness of the flange and web of the side-rail extrusion.

The torque-arm assembly (aluminum alloy 7075-T73) for an aircraft nose landing gear failed after 22,779 simulated flights. The part, made from an aluminum alloy 7075-T73 forging, had an expected life of 100,000 simulated flights. Initial study of the fracture surfaces indicated that the primary fracture initiated from multiple origins on both sides of a lubrication hole that extended from the outer surface to the bore of a lug in two cadmium-plated flanged bushings made of copper alloy C63000 (aluminum bronze) that were press-fitted into each bored hole in the lug. Sectioning and 2x metallographic analysis showed small fatigue-type cracks in the hole adjacent to the origin of primary fracture. Hardness and electrical conductivity were typical for aluminum alloy 7075. This evidence supported the conclusion that the arm failed in fatigue cracking that initiated on each side of the lubrication hole since no material defects were found at the failure origin. Recommendations included redesign of the lubrication hole, shot peeing of the faces of the lug for added resistance to fatigue failure, and changing of the forging material to aluminum alloy 7175-T736 for its higher mechanical properties.

A coil made of a nickel-chromium alloy (Material No. 2.4869) with approx. 80Ni and 20Cr had burned through after a brief period of operation as a heating element in a brazing furnace. The protective atmosphere consisted of an incompletely combusted coal gas. Furnace temperature reached 1150 deg C. This type of selective oxidation at which the easily oxidized chromium burns, while the nickel is not attacked, is caused by mildly oxidizing gases and is sometimes designated as green rot. Under these conditions, chromium-containing steels and alloys whose oxidation resistance is based upon formation of tight oxide layers are not stable.

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.