Splined rotor shafts (constructed from 1151 steel) used on small electric motors were found to miss one spline each from several shafts before the motors were put into service. Apparent peeling of splines on the induction-hardened end of each rotor shaft was revealed by visual and stereo-microscopic examination. One tooth on each shaft was found to be broken off. It was revealed by metallographic examination of an unetched section through the fractured tooth that the fracture surface was concave and had an appearance characteristic of a seam. Partial decarburization of the surface was revealed after etching with 1% nital. The presence of a crack, with typical oxides found in seams at its root, was disclosed by an unetched section through the shaft in an area unaffected by induction heating. The etched samples revealed similar decarburization as was noted on the fracture surface of the tooth. It was concluded that the seam had been present before the shaft was heat treated and these seams acted as stress raisers during induction hardening to cause the shaft failure. It was recommended that the specifications should specify that the shaft material should be free of seams and other surface imperfections.

A forged aluminum alloy 2014-T6 catapult-hook attachment fitting (anodized by the chromic acid process to protect it from corrosion) from a naval aircraft broke in service. Spectrographic analysis, visual examination, microscopic examination, and tensile analysis showed minute cracks on the inside surface of a bearing hole, and small areas of pitting corrosion were visible on the exterior surface of the fitting. The analysis also revealed a small number of rosettes, suggestive of eutectic melting, in an otherwise normal structure. These examinations and analyses support the conclusion that the presence of chromic acid stain on the fracture surface proved that the forging had cracked before anodizing. This suggest that the crack initiated during straightening, either after machining or after heat treatment. The structure and composition of the alloy appear to have been acceptable. Ductility was acceptable so rosettes found in the microstructure are believed to have been nondamaging. Had they contributed to the failure, the ductility would have been very low. The recommendations included inspection for cracks and revising the manufacturing process to include a fluorescent liquid-penetrant inspection before anodizing, because chromic acid destroys the penetrant. This inspection would reduce the possibility of cracked parts being used in service.

The brine-heater shell in a seawater-conversion plant failed by bursting along a welded joint connecting the hot well (C70600 per ASTM B 466) to the heater shell (ASTM A285, grade C steel). Three cracks in the welded joints between the heater shell and the hot well were revealed by visual inspection. It was observed that crack 1 and 2 were covered with high-temperature oxidation products which revealed that the surfaces had been separated for quite some time. A very high discontinuity stress which existed at the longitudinal welds between the hot well and the heater shell was revealed by stress analysis. It was interpreted that the cracks had originated shortly after the heater was put into operation and propagated slowly initially. The rate of propagation was interpreted to have increased due to discontinuity stresses greater than yield strength of the material. It was concluded that the brine heater cracked and fractured because it was overstressed in normal operation. The heater design was modified to make the heater shell and the hot well two separate units. A relief valve was recommended in the heater or in the steam line near the heater.

A series of poppet-valve stems fabricated from 17-4 PH (AISI type 630) stainless steel failed prematurely in service during the development of a large combustion assembly. The poppet valves were part of a scavenging system that evacuated the assembly after each combustion cycle. The function of the valve is to open and close a port; thus, the valve is subjected to both impact and tensile loading. Analysis (visual inspection, hardness testing, and stress analysis) supported the conclusions that the valve stems were impact loaded to stresses in excess of their yield strength. That they failed in the threaded portion also suggests a stress-concentration effect. Recommendations included changing the material spec to a higher-strength material with greater impact strength. In this case, it was recommended that the stems, despite any possible design changes, be manufactured from an alloy such as PH 13-8Mo, which can be processed to a yield strength of 1379 MPa (200 ksi), with impact energies of the order of 81 J (60 ft·lbf) at room temperature.

To permit bolting of a 90 lb/yd. flat-bottomed rail to a steel structure, rectangular slots 2 in. wide x 1 in. deep were flame-cut in the base of the rail at 2 ft intervals to suit existing bolt holes. During subsequent handling, one of the rails (which were about 25 ft long) was dropped from a height of approximately 6 ft on to a concrete floor and it fractured into 11 pieces, each break occurring at a slot. The sample piece submitted for examination showed a wholly brittle fracture at each end, the fractures having originated at the sharp corners of the slots. During flame-cutting, a narrow band of material on each side of the cut was raised above the hardening temperature. When the torch had passed the rate of abstraction of heat from this zone by conduction into the cold mass of the rail was sufficiently rapid to amount to a quench and thus cause local hardening. The steel in the regions of the slots possessed little capacity for deformation, and fracturing of the martensitic layer, under cooling or impact stresses, would be likely to occur. The slots should have been cut mechanically.

The side supporting flange of the bottom platen of an 800 ton hydraulic press fractured after 9 x 10's cycles under a maximum load of 530 tons. The platen material specified in the design was cast steel 52. Metallographic examination of the fracture surface indicated that the platen had failed in fatigue as a result of a high stress concentration in a sharp 0.6 mm (0.02 in.) radius fillet. Stress analysis and fracture mechanics predictions revealed that there was also danger of fatigue failure for platens with the design radius of 10 mm (0. 4 in.) if the press operates at 800 tons. It was recommended that the remaining life of similar presses be assessed periodically controlling the cracks, their dimensions, and their propagation rates. An increase in the radius of the fillet was also recommended.

Nineteen out of 26 bolts in a multistage water pump corroded and cracked after a short time in a severe working environment containing saline water, CO 2 , and H 2 S. The failed bolts and intact nuts were to be made from a special type of stainless steel as per ASTM A 193 B8S and A 194. However, the investigation (which included visual, macroscopic, metallographic, SEM, and chemical analysis) showed that austenitic stainless steel and a nickel-base alloy were used instead. The unspecified materials are more prone to corrosion, particularly galvanic corrosion, which proved to be the primary failure mechanism in the areas of the bolts directly exposed to the working environment. Corrosion damage on surfaces facing away from the work environment was caused primarily by chloride stress-corrosion cracking, aided by loose fitting threads. Thread gaps constitute a crevice where an aggressive chemistry is allowed to develop and attack local surfaces.

An elbow of 70 mm OD and 10 mm wall thickness made from St 35.29, and exposed to 315 atmospheres internal pressure in an oil hydraulic shear installation, cracked lengthwise after a short operating period. Because the stress was not sufficient to explain the fracture of this elbow under this pressure, an investigation was conducted to establish whether material or processing errors had occurred. Microscopic examination showed that a ferritic-pearlitic structure in select locations was very fine-grained. Other signs of fast cooling as compared to normally formed structure of the core zone were noted. It was also possible that the pipe was resting on a cold plate during bending or that it came in touch with a cold tool. This apparently caused the strains at the transition to the cross-sectional part that had been cooled more slowly. The location of the crack at just this point gave rise to the conclusion that it was formed either by the sole or contributive effect of these stresses.

A ceiling in a concrete structure was hung on flat bars with a cross section of 30 x 80 mm. The bars were borne by a slit steel plate and supported by tabs that were welded onto the flat sides. One of the bars fractured during mounting when it was dropped from a height of about 1 m onto the opposite support. The fracture was a grainy forced rupture that propagated from one of the fillet welds. Investigation showed a steel was selected for this important construction that was prone to aging and that in fact had aged through cold deformation during straightening and then was welded yet. The bar could withstand mounting and subsequent static loading as long as it was treated with care, as could be expected from the good deformation characteristics of the static tensile test. The question is, however, whether occasional impacts or shocks can be assuredly avoided. This risk could have been eliminated if a killed steel of quality groups 2 or 3 according to DIN 17 100 had been used.

A 75 cm OD x 33 mm thick pipe in a horizontal section of a hot steam reheat line ruptured after 15 years in service. The failed section was manufactured from rolled plate of material specification SA387, grade C. The longitudinal seam weld was a double butt-weld that was V-welded from both sides and failure was found to propagate along the longitudinal seam and its HAZ. The fracture surface near the inner wall of the pipe was found to have a bluish gray appearance, while the fracture surface near the outer wall was rust colored (oxides). The transverse-to-the-weld specimen from the longitudinal seam weld was revealed to have lower elongation and a shear type failure rather than the cup-cone failures. It was concluded that the welded longitudinal seam exhibited embrittlement. A low-ductility intergranular fracture that progressed through the weld metal was revealed by scanning electron microscopy. The cracks were revealed to be in existence for some time before the final failure which was indicated by the extent and amount of corrosion products. It was concluded that low ductility was responsible for the original initiation of cracks in the pipe.

Overload failures refer to the ductile or brittle fracture of a material when stresses exceed the load-bearing capacity of a material. This article reviews some mechanistic aspects of ductile and brittle crack propagation, including a discussion on mixed-mode cracking, which may also occur when an overload failure is caused by a combination of ductile and brittle cracking mechanisms. It describes the general aspects of fracture modes and mechanisms. The article discusses some of the material, mechanical, and environmental factors that may be involved in determining the root cause of an overload failure. It also presents examples of thermally and environmentally induced embrittlement effects that can alter the overload fracture behavior of metals.

Two sections of a galvanized cable 10.5 A 160 GR +NORM M 9533 (round stranded cable of normal type, h + 6, Langslay, right-handed) were examined. One had a 100 mm long blackish-brown tarnished zone obviously caused by localized heating at one end, inside which the hemp core was missing, and the other corresponded to the original condition of the cable. The cause of the damage was unknown. About a third of the wires had fractured and the rest had been cut. All were tensile fractures with a relatively high degree of necking. The cause of the localized heating was unknown. It can only be concluded from the investigation that the temperature did not exceed the Ac3 point of the wire material, which should be about 750 deg C, and that the heating lasted a fairly long time.

Three bearing bosses from the cover of scrap shears were sent in for examination. They had torn off the base plate to which they had been welded by fillet welds all around. Two of these were examined. They showed entirely the same symptoms. The bosses had broken away on three sides along the welds. The cleaved fractures in the burned notches propagated partially above and partially below several incipient cracks which may have been fatigue fractures. Metallographic sections showed that the fractures had occurred either at the burned notches near the transition from the weld to the sheet, or else they ran in the sheet material next to the weld. The quality of the welds could not be judged because the opposite fracture pieces to which they adhered had not been sent in. It was concluded that the breakaway of these bosses was at least favored by overheating and hardening.

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.

Several cases of failures in gray cast iron paper machine dryer rolls were evaluated. The rolls were found have ground outer cylindrical surfaces on which the paper web is dried. They were found to rotate about their longitudinal axes at speeds from 50 to 250 rpm while containing saturated steam from 35 to 380 kPa. Failures were found to occur in the shell body, in a head near a hand hole or a manhole opening, or in a head near the journal-to-head interface. A cleavage fracture was revealed by scanning electron microscopy regardless of the driving stress for failure. Fracture surface were found to exhibit chevron marks typical of fatigue or raised points or tears pointing in the direction of the probable origin of failure. The characteristics of the thinwall cast iron structures like the variation in composition due to pouring from multiple ladles, variation in solidification rates, and variation in tensile strength to be noted during inspection were described.

Cylinder fatigue can result from abnormal heating in service. Fatigue can be experienced also by piston heads, exhaust valves, and turbosupercharger housings (castings). Pistons from different engines series can sometimes fit, but because of slight design modifications, they may not function properly. Circumferential cracks and fractures near the head-to- barrel junctions have occurred on numerous cylinders of reciprocating piston engines. In most instances, cracks were caused by high cyclic pressures and high temperatures resulting most probably from detonation. At times, fractures or cracks (or both) were also caused by a combination of unfavorable temperature distribution (and possibly excessive pressures around the cylinder barrel), un-nitrided internal surfaces of cylinder barrels, and inadequate thread contours, which caused high stress concentrations at the thread roots. One example of the most common type of cylinder failure is illustrated.

A recuperator for blast heating of a cupola furnace became unserviceable because of the brittle fracture of several finned tubes made of heat resistant cast steel containing 1.4C, 2.3Si and 28Cr. The service temperature was reported as 850 deg C. This led to the suspicion that the fracturing had something to do with the precipitation of sigma phase. Metallographic examination showed that the multiaxial stresses caused by sigma phase formation and the related embrittlement was the cause for the fracture of the recuperator. A steel of lower chromium content with no or little tendency for sigma phase formation would have had adequate corrosion resistance at the relatively low service temperature.

Applying general techniques of failure analysis, the authors deduced that an in-flight explosion brought down a passenger plane. Other evidence pinpointed the location of the explosive, an important factor in establishing responsibility.

A 150 cm ID boiler drum made form ASTM A515, grade 70, steel failed during final hydrotesting at a pressure of approximately 26 MPa. Brittle fractures were revealed in between two SA-106C nozzles and remainder was found to involve tearing. Short, flat segments of fracture area, indicative of pre-existing cracks, were revealed by examination of the fracture surface at the drain grooves arc gouged at the nozzle sites. A thin layer of material with a dendritic structure was observed at the groove surface. The dendritic layer was revealed by qualitative microprobe analysis to contain over 1% C, higher than the carbon content of the base metal. The cracks in the drain groove surface could have occurred after arc gouging, during subsequent stress-relieving, or during the hydrostatic test. Flame cutting is not recommended for the type of steel used in the boiler drum because it can lead to local embrittlement and stress raisers, potentially initiating major failures.

Two 25 x 40 mm (1 x 1.5 in.) AISI 4150 hot-rolled steel bars that cracked during heat treatment were examined to determine whether the heat treating procedure had contributed to the failure. Metallographic examination of a cross section taken through the fracture revealed an oxide coating on both sides of the fracture surface. The oxide was also found on the top and bottom sides of the sample. Sawcut sides of the bar did not exhibit the oxide layer The presence of the oxide in the fracture, combined with its absence on all exterior surfaces, indicated that the fracture occurred as a result of an oxide seam in the original material rather than from oxide from heat treating. Nondestructive testing prior to machining and heat treatment was recommended.