Superelastic nitinol wires that fractured under various conditions were examined under a scanning electron microscope in order to characterize the fracture surfaces, produce reference data, and compare the findings with prior published work. The study revealed that nitinol fracture modes and morphologies are generally consistent with those of ductile metals, such as austenitic stainless steel, with one exception: Nitinol exhibits a unique damage mechanism under high bending strain, where damage occurs at the compression side of tight bends or kinks while the tensile side is unaffected. The damage begins as slip line formation due to plastic deformation, which progresses to cracking at high strain levels. The cracks appear to initiate from slip lines and extend in shear (mode II) manner.

A stick-shift from an automobile transmission failed in service. Failure occurred by stable fatigue crack propagation at locations R and L. Plausibly, the crack initiated at location R due to maximum tensile stress, which was perhaps further concentrated by a surface imperfection. As the crack progressed, the service load shifted to the section marked L. This overload on location L appears to have caused another fatigue crack that propagated first over the smooth-banded region. Catastrophic failure occurred when the component became unstable under the applied load.

A crumpled piece of sheet metal had two cracks in a T-junction shape. The relative locations of shear lips in the cracks allowed deduction of which crack happened first, and which direction the cracks propagated.

On 13 Dec 1994, two massive detonations leveled portions of an ammonium nitrate plant near Sioux City, IA. The primary explosion allegedly occurred in defectively-designed titanium sparger piping inside the neutralizer vessel. Investigation however, revealed the explosion occurred because of unsafe plant operations and poor maintenance procedures. Specifically, the ammonium nitrate within the 18,000 gal capacity neutralizer vessel had become contaminated and made highly acidic. The operators then injected superheated steam directly into the ammonium nitrate in the neutralizer vessel.

Microstructural examinations on transverse cross sections of a steam reformer tube, showed the presence of large macrovoids elongated in the radial direction and emanating from the internal surface of the tube. The macrovoids were located at the interdendritic regions, and were partially filled by a Mn-Fe bearing chromium oxide film. The areas adjacent to the oxide film were chemically depleted in C, Cr and Mn and rich in Fe and Ni. Associated with this depletion were a large concentration of microvoids. It was suggested that the dissolution of carbides in areas surrounding the macrovoids and the concentration of stresses at their tips, caused extensive localized plastic deformation which led to the formation of microvoids and subsequently to the spalling of the oxide film. The non-protective character of the film induced a progressive deterioration of the grain boundaries properties. Grain boundary sliding and dislocation motion were enhanced, causing a local increase in the steady state strain rate and the premature failure of the tube.

While an attempt was being made to start the diesel engine of a dragline type excavator, a severe explosion took place, the operator unfortunately being killed. The engine was normally started by compressed air from an air bottle or receiver charged to a pressure of 350 psi by a small auxiliary engine driven compressor fitted on the excavators. On an occasion when this had broken down, the air bottle was replenished by the engineer-in-charge from a cylinder of compressed air. The compressor engine remained unserviced and when the starting bottle again became depleted, the excavator operator re-charged it himself from another cylinder without the knowledge of the engineer. The engine was started satisfactorily, and the bottle charged again to 350 psi in readiness for the start on the following day. It was while an attempt was being made to start the engine on this occasion that the explosion took place. The cylinder used by the operator for re-charging the receiver contained not air but oxygen.

The cover of a feed cock fitted to an economic boiler suddenly blew off and the plug lifted sufficiently to permit the escape of water. It was found that all four of the studs securing the cover had fractured. In each case fracture had occurred at the end of one of the screwed portions adjacent to the shank. The fractures were of a short nature, with no evidence of progressive cracking by fatigue, nor was there any sign of stretching prior to failure. The fractured faces and the shanks of the studs were of rusted appearance. Microscopic examination of the material showed it to be an austenitic nickel-chromium stainless steel, stabilized by titanium and of the free machining type. Multiple transgranular cracking characteristic of failure from stress-corrosion cracking, was present to an extensive degree. It was considered probable that there had been slight leakage of water from the valve over a period and evaporation resulted in a solution which favored failure from stress-corrosion cracking. If corrosion resistant studs were desired, those of bronze or Monel metal are to be preferred.

Following a freight train derailment, part of a fractured side frame was retained for study because a portion of its fracture surface exhibited a rock candy appearance and black scale. It was suspected of having failed, thereby precipitating the derailment. Metallography, scanning electron microscopy, EDXA, and x-ray mapping were used to study the steel in the vicinity of this part of the fracture surface. It was found to be contaminated with copper. Debye-Scherrer x-ray diffraction patterns obtained from the scale showed that it consisted of magnetite and hematite. It was concluded that some copper was accidentally left in the mold when the casting was poured. Liquid copper, carrying with it oxygen in solution, penetrated the austenite grain boundaries as the steel cooled. The oxygen reacted with the steel producing a network of scale outlining the austenite grain structure. When the casting fractured as a result of the derailment, the fracture followed the scale in the contaminated region thus creating the “rock candy” fracture.

A few Cr-Mo steel piston rods from different production batches were found identically cracked in the eye end near the radius after chrome plating and baking treatment. Two of them cracked in the plating stage itself instantly broke on slight tapping. Cracking initiated from the outer base surface of the forked eye end. The 40 mm diam forged piston rods were subjected to plating after heavy machining on the part without any stress-relieving treatment. Also, time lapses between plating and baking were varied from 3 to 11 h. The brittle cracking along forked eye-end radius portion was attributed to hydrogen embrittlement that occurred during chrome plating.

On 15 March 2000, a National Railroad Passenger Corporation (Amtrak) train traveling from Chicago to Los Angeles derailed in Carbondale, KS. After the initial on-scene investigation, 12 pieces of rail were sent to the materials laboratory for examination. Ten of them were from the point of derailment (POD). A vertical crack was observed in the head of the rail (vertical split head). The crack was at least 233 in. (591 cm) long, continuing through the entire lengths of most pieces recovered from the POD. The vertical fracture surface had features consistent with overstress fracture with short-term exposure to an oxygen-rich environment. Fracture features emanated from longitudinally-aligned inclusions rich in aluminum.

A newly installed pipeline leaked during cleaning prior to hydrotest at a pressure of approximately 400 psig. The intended hydrotest pressure was 750 psig. The pipeline was constructed from spiral-welded API 5L-X65 HSLA steel and was intended for seawater injection. Analysis included nondestructive testing, metallography, and scanning electron microscopy. Based on the results, the failure was attributed to transit fatigue, caused during highway transportation. Cracks along the toes of the weld from both the outside and inside surfaces, the transgranular nature of cracking, and the presence of fatigue striations all supported transit fatigue as the damage mechanism.

A piece of wheel flange separated from the main landing gear wheel of a C130 aircraft as it taxied on a runway. The wheel was a 2014-T61 aluminum alloy forging and had been in service nearly 20 years. Fractographic evidence indicated that the initial crack growth was caused by high-cycle fatigue. The crack grew to approximately 8 in. in length before final catastrophic fracture. Fatigue analyses accurately predicted the cyclic life demonstrated by the failed wheel since its last inspection, assuming an initial crack length of 13 to 25 mm (0.5 to 1.0 in.). It was recommended that the inspection interval be reduced to one-third of its original duration for the current level of inspection reliability, or that inspection procedures be improved in order that cracks substantially smaller than 13 mm (0.5 in.) can be reliably detected.

On 9 March 2000, a gasoline pipeline failed near Greenville, TX releasing approximately 12,000 barrels of fuel. After the on-scene portion of the investigation was completed, an 8.5 ft. (2.6 m) section of the 28 in. (71 cm) diam pipe was sent to the materials laboratory for examination. Examination included optical and scanning electron microscopy of the fracture surfaces and metallographic examination of cross sections through the fracture surface. From the outer to inner edge of the fracture surface, three different areas were observed. Fracture features in area 1 were obliterated by corrosion. The fracture features in region 2 were relatively smooth, and striations were observed, typical of fatigue. In region 3, dimple features were observed, typical of ductile overstress. Also, corrosion pits were observed on the outer surface of the pipe section in locations where the protective black tar-like coating was cracked.

One main undercarriage axle made of high strength alloy steel was subjected to simulated fatigue test for 6000 h of service. After only 300 h it broke in two along the sharp radius. The fracture revealed a coarse, irregular, and brittle surface before final fracture by thick angular shear lip zone. The presence of micropores in the cleavage facets as well as at the grain boundaries and hairline type crack indications under SEM examination were all suggestive of hydrogen embrittlement. On the basis of investigation results and observations, it was concluded that the transverse breakage of the axle had occurred intergranularly in a brittle manner, possibly, initiated by a shallow zone of fatigue along the sharp radius acting as stress riser.

A number of machined end frame steel forgings made of Cr-Si-Mn alloy showed tiny cracks during magnetic particle inspection after heat treatment. The cracks were mostly confined to base edges and fillet radius. No significant abnormality was observed in chemical composition and microstructure. SEM, optical microscopy, and gas analysis revealed that the subsurface discontinuous cracks at the bore edges and in the fillet radius of the heat-treated end frame component had occurred due to hydrogen embrittlement, and not because of faulty heat treatment. This conclusion was supported by the presence of cracklike indications in machined bore surface of the annealed part.

Lakes in zinc die castings are areas encompassed by irregular lines or waves on flat or slightly contoured surfaces which are intended to look uniform. The laked areas have to be removed by polishing before the castings can be plated. This adds considerably to the overall cost of production. Castings examined were of an automobile name-plate holder with two flat sides of approximately 113 sq cm. All castings produced during a trial showed laking defects, the number and position varying from casting to casting. It was found that formation of metal waves and lakes depended primarily on the design of the gate and runner system and operating conditions. High flow efficiencies, with adequate feeding to all sections of the die, and short cavity fill times are desirable.

A tube in a radiant superheater, the boiler of which is coal fired, failed by creep after 17 years of service. The failed tube was specified to be made of ASME SA-213, grade T-22. Measurable swelling of the tube diameter by about 2.4 mm and tube wastage caused by corrosion or erosion were observed. Log stress versus Larson-Miller Parameter (LMP) plots were produced to assess the remaining life of the superheater. It was revealed that the estimated operating temperature of 1060 deg F was higher than the estimated design temperature of 1000 deg F and that the tube wastage had increased the actual operating stress. Tube wastage and high operating temperatures hastened the failure. A better understanding of the material condition of this superheater was recommended to verify all the suspect hot tubes.

Several waterwall tubes in a power station boiler failed after ten years of service. The boiler is a suspension type equipped with 30 IK boxes where retractable soot blowers are inserted to clean the inside of the boiler using high-pressure steam. The tubes, which operate at 693 deg F (367 deg C) and 2935 psi (20.5 MPa), failed near the IK boxes as a result of thermal fatigue. Thermal fatigue damage was accelerated by repetitive exposure to water droplets from the soot blower and the associated rapid cooling.