A maintenance worker was injured when his 3/4 in. (19 mm) open-ended wrench failed, fracturing in overload fashion along the jaw. The failed wrench was unavailable for testing, but an identical one that failed in the same manner was acquired and subjected to hardness, chemistry, SEM, and metallurgical analyses. SEM imaging revealed microvoid coalescence within the fracture zone. The microvoids were flat and smooth edged indicating insufficient bonding. In addition, a cross sectional sample, mounted and etched using alkaline chromate, revealed an oxygen-rich zone in the jaw. It was concluded that the failures stemmed from forging laps in the jaw that broaching failed to remove.

A bridge wheel from a 272,160 kg stripper crane fractured in the web near the rim after one year of service. The wheel was forged from 1055 steel, and the tread, hub faces, and hub bore were machined. Beach marks indicative of fatigue at ten locations were revealed by macroscopic examination of the fracture surfaces. The surface of the web was heavily scaled and decarburized. A gross forging defect extending about 1.8 mm along the fracture surface was disclosed by examination of a micrograph of a section through one of the fatigue origins. Shallower forging defects were visible along the web surface. Fatigue cracking of the wheel was initiated at forging defects in the web. Replacement wheels were machined all over and were magnetic particle inspected to detect any cracks that could act as stress raisers.

A felt guide roll fractured in-service on a paper manufacturing machine, damaging the belt as well as multiple dryer rolls, nearby felt guide rolls, and the frame of the machine. The investigation included visual and stereoscopic examination, chemical and microstructural analysis, microhardness and tensile testing, stress calculations, and vibration measurements. Based on the results, the roll fracture was attributed to high-cycle fatigue associated with a plug weld over one of the five threaded fasteners added to secure a balance weight inside the roll. The balance weight was installed to compensate for variations in wall thickness (i.e., weight distribution) of the pipe product used to make the roll. According to the investigation, resonance and vibration, which were initially considered, did not cause the failure.

Two investment-cast A356 aluminum alloy actuators used for handles on passenger doors of commercial aircraft fractured during torquing at less than the design load. Visual examination showed that cracking had occurred through a machined side hole. Fractography revealed that the cracks originated in hot tear locations in the castings. Microprobe analysis of fracture surfaces in the hot tear region indicated a much higher silicon-to-aluminum ratio compared with the overload fracture area. No microstructural anomalies related to the failure were found during metallographic examination. It was concluded that the strength of the castings had been compromised by the presence of the casting defects. Modification of the gating system for casting was recommended to eliminate the hot tear zone. It was also suggested that the balance of the castings from the same manufacturing lot be radiographically inspected.

Following three similar failures of load chains on manually operated geared pulley-blocks of 1-ton capacity, a portion of one of the chains was obtained for examination. The chain was made of mild steel and the links had been electrically butt-welded at one side. In the case of the sample obtained, the failure in service had resulted from fracture of one of the links in the plane of the weld. Six of the other links in the vicinity showed cracks in the welds in various stages of development. Microscope examination showed a crack in an early stage of development and also from an apparently sound link, the prepared surfaces lying in the planes of the links. This examination revealed that the welds were initially defective. Discontinuities were present in both cases adjacent to the insides of the links, of a type indicative of either inadequate fusion or incomplete expulsion of oxide, etc., at the time of the upset, i.e. the pressing together of the ends of the links to complete the welding. It was evident from the examination that the service failures were due to the use of chain that was initially defective.

A detailed failure analysis was conducted on an ammonia refrigerant condenser tube component that failed catastrophically during its initial hours of operation. Evidence collected clearly demonstrated that the weld between a pipe and a dished end contained a sharp unfused region at its root (lack of penetration). Component failure had started from this weld defect. The hydrogen absorbed during welding facilitated crack initiation from this weld defect during storage of the component after welding. Poor weld toughness at the low operating temperature facilitated crack growth during startup, culminating in catastrophic failure as soon as the crack exceeded critical length.

A mild steel hook that was part of the auxiliary hoist of an electric overhead crane used in a foundry was of the shank type and the rated safe working load was 15 tons. Failure took place in a wholly brittle manner, and occurred transversely through the back of the hook. From the direction in which the fracture developed, as indicated by the radial lines on its surface, it was evident that a preexisting defect served to initiate the brittle fracture. Material adjacent to the fracture was decarburized and contained numerous globules of oxide and slag. It was evident, therefore that a fissure was formed during the manufacture of the hook and had not developed in service. The failure was associated with a surface defect, and it was recommended that the other similar hooks at the establishment be crack detected and any similar discontinuities eliminated.

A 4340 steel piston engine crankshaft in a transport aircraft failed catastrophically during flight. The fracture occurred in the pin radius zone. Fractographic studies established the mode of failure as fatigue under a complex combination of bending and torsional stresses. SEM examination revealed that the fracture origin was a subsurface defect-a hard refractory (Al2O3) inclusion—in the zone close to the pin radius. Chemical analysis showed the crankshaft material to be of inferior quality. It was recommended that magnetic particle inspection using the dc method be used to cheek for cracks during periodic maintenance overhauls.

The results of a failure analysis of a series of Cr-Mo-V steel turbine studs which had experienced a service lifetime of some 50,000 h are described. It was observed that certain studs suffered complete fracture while others showed significant defects located at the first stress bearing thread. Crack extension was the result of marked creep embrittlement and reverse temper embrittlement (RTE). Selected approaches were examined to assess the effects of RTE on the material toughness of selected studs. It was observed that Auger electron microscopy results which indicated the extent of grain boundary phosphorus segregation exhibited a good relationship with ambient temperature Charpy data. The electrochemical polarization kinetic reactivation, EPR, approach, however, proved disappointing in that the overlapping scatter in the minimum current density, Ir, for an embrittled and a non-embrittled material was such that no clear decision of the toughness properties was possible by this approach. The initial results obtained from small punch testing showed good agreement with other reported data and could be related to the FATT. Indeed, this small punch test, combined with a miniature sample sampling method, represents an attractive approach to the toughness assessment of critical power plant components.

An aftercooler was of conventional design and fitted with brass tubes through which cooling-water circulated. Air at 100 psi pressure was passed over the outsides of the tubes, entering the vessel near to the upper tubeplate on one side and leaving it by a branch adjacent to the lower tubeplate on the opposite side. After a mishap, the paint had been burned off the upper half of the shell. Internally, most of the tubes were found to be twisted or bent. The casing of the pump used to circulate the cooling water was also found to be cracked after the mishap. All the evidence pointed to the probability that a fire had occurred within the vessel. Some months before the failure, one of the tubes situated towards the center of the nest developed a leak. Owing to the difficulty of inserting a replacement tube, the defective one was scaled by means of a length of screwed rod fitted with nuts and washers at each end. This assembly became loose, thereby allowing air under pressure to enter the waterside of the cooler and expel the water, leading to overheating and ultimately to the damage described.

Pinhole defects were found in a main combustion chamber made from NARloy-Z after an unexpectedly short time in service. Analysis indicated that the throat section of the liner had been exposed to very severe environmental conditions of high temperature and high oxygen content, which caused ductility loss and grain-boundary separation. The excessive oxygen content in the liner was attributed to diffusion from an oxygen-rich environment that had resulted from nonuniform mixing of propellants. The internal oxygen embrittled the alloy and reduced its thermal conductivity, which resulted in a higher hot-gas wall temperature and associated degradation of mechanical properties.

An ASTM A 504 carbon steel railway car wheel that was used on a train in a metropolitan railway system failed during service, causing derailment. The wheel was completely fractured from rim to hub. Macrofractography of the fracture surface showed road grime, indicating that the crack had existed for a considerable time prior to derailment and initiated in the flange. Failure propagated from the flange across the rim and down the plate to the bore of the hub. Two zones that exhibited definite signs of heating were observed. The fracture initiation site was typical of fatigue fracture. No defects were found that could have contributed to failure. The wheel conformed to the chemical, microstructural, and hardness requirements for class A wheels. Failure was attributed to repeated severe heating and cooling of the rim and flange due to brake locking or misapplication of the hand brake. It was recommended that the brake system on the car be examined and replaced if necessary.

Gears in a strip mining dragline failed in service. The material was identified as a low-alloy (NiCrMoV) steel. SEM analysis indicated that the initial fracture and subsequent fractures resulted from impact or a suddenly applied load. Mechanical testing indicated that the gears had low impact strength. Failure was attributed to low toughness caused by the absence of, or improper, heat treatment. Casting defects identified during metallographic examination were determined to be the fracture initiation site, but were considered less significant than the low as-received impact strength of the material. It was recommended that the equipment manufacturer implement an appropriate heat treatment to meet the impact requirements of the application.

A shrunk-fit 18 Mn-5Cr steel retaining ring failed without warning during normal unit operation of a 380 MW electrical generator. The cause of the ring failure was determined to be intergranular stress-corrosion cracking (IGSCC) because of the high strength of the ring material and the presence of moist hydrogen used to cool the ring. Factors which promoted the failure were higher than normal strength levels in the ring material, lower than normal ring operating temperatures, possible moisture in the lubrication oil system, periodic poor performance of the hydrogen dryers, and a ring design which allowed water to become trapped in a relief groove. These factors caused pitting in the ring in an estimated 100 hours of operation. The ring had been inspected previously 18 months prior to the failure and no defects or pitting were found. Calculations showed that a 0.127-cm (0.050-in.) deep pit could grow to a critical size in 3000 to 4000 hours of operation. To prevent further failures, it was recommended that the ring be replaced with an 18 Mn-18Cr alloy with superior resistance to IGSCC. A program of periodic inspection and replacement of other retaining rings in the system was also recommended.

To forged AISI 4140 steel trailer kingpins fractured after 4 to 6 months of service. Fractographic and metallographic examination revealed that cracks were present in the spool-flange shoulder region of the defective kingpins prior to installation on the trailers. The cracks grew and coalesced during service. Consideration of the manufacturing process suggested that the cracks were the result of overheating of the kingpin blanks prior to forging, which was exacerbated during forging by deformation heating in the highly-strained region. This view was supported by results of two types of tensile tests conducted near the incipient melting temperature at the grain boundaries. All kingpins made by the supplier of the fractured ones were ultrasonically inspected and six more anticipated to fail were found. It was recommended that the heating of forging blanks be more carefully controlled, especially with respect to the accuracy of the optical pyrometer temperature readout. Also, procedures must be developed such that forging blanks that trigger the over-temperature alarm are reliably and permanently removed from the production line.

Metallurgical SEM analysis provides many insights into the failure of biomedical materials and devices. The results of several such investigations are reported here, including findings and conclusions from the examination a total hip prosthesis, stainless steel and titanium compression plates, and hollow spinal rods. Some of the failure mechanisms that were identified include corrosive attack, corrosion plus erosion-corrosion, inclusions and stress gaps, production impurities, design flaws, and manufacturing defects. Failure prevention and mitigation strategies are also discussed.

Several stainless steel coils cracked during a routine unwinding procedure, prompting an investigation to determine the cause. The analysis included optical and scanning electron microscopy, energy-dispersive x-ray spectrometry, and tensile testing. An examination of the fracture surfaces revealed a brittle intercrystalline mode of fracture with typical manifestations of clear grain facets. Branched and discrete stepwise microcracks were also found along with unusually high levels of residual hydrogen. Mechanical tests revealed a marked loss of tensile ductility in the defective steel with elongations barely approaching 8%, compared to 50% at the time of delivery weeks earlier. Based on the timing interval and the fact that failure occurred at operating stresses well below the yield point of the material, the failure is being attributed to hydrogen-induced damage. Potential sources of hydrogen are considered as are remedial measures for controlling hydrogen content in steels.

An exhaust diffuser assembly failed prematurely in service. The failure occurred near the intake end of the assembly and involved fracture in the diffuser cone (Corten), diffuser in take flange (type 310 stainless steel), diffuser exit flange (type 405 stainless steel), expansion bellows (Inconel 600), and bellows intake flange (Corten). Individual segments of the failed subassemblies were examined using various methods. The analysis indicated that the weld joint in the diffuser intake flange (type 310 stainless steel to Corten steel) contained diffusion-zone solidification cracks. The joints had been produced using the mechanized gas-metal arc welding process. Cracking was attributed to improper control of welding parameters, and failure was attributed to weld defects.

A commercial hybrid-iron golf club fractured during normal use. The club fractured through its cast aluminum alloy hosel. Optical analysis revealed casting pores through 20% of the hosel thickness. Mechanical properties were determined from characterization results, then used to construct a finite element model to analyze material performance under failure conditions. In addition, a full scale structural test was conducted to determine failure strength. It was concluded that the club failed not from ground impact but from a force reversal at the bottom of the downswing. Large moments generated during the downswing aggravated by manufacturing defects and stress concentration combined to create an overload condition.

This article aims to identify and illustrate the types of overload failures, which are categorized as failures due to insufficient material strength and underdesign, failures due to stress concentration and material defects, and failures due to material alteration. It describes the general aspects of fracture modes and mechanisms. The article briefly reviews some mechanistic aspects of ductile and brittle crack propagation, including discussion on mixed-mode cracking. Factors associated with overload failures are discussed, and, where appropriate, preventive steps for reducing the likelihood of overload fractures are included. The article focuses primarily on the contribution of embrittlement to overload failure. The embrittling phenomena are described and differentiated by their causes, effects, and remedial methods, so that failure characteristics can be directly compared during practical failure investigation. The article describes the effects of mechanical loading on a part in service and provides information on laboratory fracture examination.