Leakage from the top of a fire-extinguisher case, made of 1541 steel tubing and closed by spinning was observed during testing. Three small folds were observed on the surface by visual examination and one was sectioned. A very fine transverse fissure through the section was revealed. Streaks of ferrite were observed by metallographic examination. It was concluded that cracking of the top of the fire-extinguisher case was the result of ferrite streaks formed due to metal overheating. The temperature of the metal was recommended to be controlled so that the spinning operation is done at a lower temperature to avoid formation of ferrite streaks.

An air bottle, machined from a solid block of aluminum alloy 2219-T852, displayed liquid-penetrant crack indications after assembly welding. The air bottle was machined to rough shape, a 3.8 mm (0.15 in.) wall thickness cylindrical cup with a 19 mm (3/4 in.) wall thickness integral boss on one side. After annealing, hot spinning, annealing a second time, and tack welding a port fitting, the assembly was torch preheated to 120 to 150 deg C (250 to 300 deg F). The port fitting was then welded in place. Final full heat treatment to the T62 temper was followed by machining, testing, and inspection. The crack indications were found only on one side of the boss and on the lower portion of the hot-spun dome region. The metallographic specimens revealed triangular voids and severe intergranular cracks. The cracks displayed the glossy surfaces typical of melted and resolidified material. The localized cracks in the air bottle were from grain-boundary eutectic melting caused by local torch overheating used in preparation for assembly welding of a port fitting. A change in design was scheduled to semiautomatic welding without the use of preheating for the joining of the port fitting for the dome opening.

A bolt breaks along a change in cross section well below its rated capacity. An anchoring screw spins freely in place, having snapped at its first supporting thread. A motor unexpectedly disengages its load, its driveshaft having fractured near a keyway. Such failures – involving axles, leaf springs, engine rods, wing struts, bearings, gears, and more – can occur, seemingly without cause, due to vibrational fracture. Vibrational fractures begin as cracks that form under cyclic loading at nominal stresses which may be considerably lower than the yield point of the material. The fracture is proceeded by local gliding and the development of cracks along lattice planes favorably orientated with respect to the principal stress. This non-reversible process is often misleadingly called “fatigue” and presents significant challenges to engineering teams that ill-advisedly take to searching for material faults. Several examples of notch-induced vibrational fractures are presented along with guidelines for investigating their cause.

A hydroextractor installed new for the drying of sugar massecuite consisted of a metal basket fixed to a vertical spindle. Disruption occurred just after the machine had been run up to speed and was not preceded by any abnormal behavior. The basket assembly consisted of a Ni-Cr-Mo steel shell and two end plates. It was designed to spin at 2200 rpm, using centrifugal force to expel liquids through nearly 3000 drilled holes in the shell wall. Investigators found that the shell separated completely from the bottom plate. The top plate, though it cracked radially, remained attached over most of its circumference. The basket also contained a 22-gauge Monel metal liner that had been perforated by stabbing, raising pronounced burrs that faced each hole. Apart from the local spots of corrosion due to the lining, the inner surface of the basket showed little evidence of general corrosion. What caused the basket to fail was the presence of corrosion-fatigue cracks or fissures radiating from the holes. A secondary cause was that the scantlings of the basket were too light.

Several compressor disks in military fighter and trainer aircraft gas turbine engines cracked prematurely in the bolt hole regions. The disks were made of precipitation-hardened AM355 martensitic stainless steel. Experimental and analytical work was performed on specimens from the fifth-stage compressor disk (judged to be the most crack-prone disk in the compressor) to determine the cause of the failures. Failure was attributed to high-strain low-cycle fatigue during service. It was also determined that the cyclic engine usage assumed in the original life calculations had been under estimated, which led to low-cycle fatigue cracking earlier than expected. Fracture mechanics analysis of the disks was carried out to assess their damage tolerance and to predict safe inspection intervals.

Two AISI type 316 stainless steel dished ends failed through the formation of intergranular stress-corrosion cracks (IGSCC) within a few months of service. The dished ends failed in the straight portions near the circumferential welds that joined the ends to the cylindrical portions of the vessel. Both dished ends were manufactured from the same batch and were supplied by the same manufacturer One of the dished ends had been exposed to sodium at 550 deg C (1020 deg F) for 500 h before failure due to sodium leakage was detected. The other dished end was used to fabricate a second vessel that was kept in storage for 1 year Clear evidence of sensitization was found in areas where IGSCC occurred. Sensitization was extensive in the dished end that had been exposed to sodium at high temperature, and it occurred in a narrow band similar to that typical of weld decay in the dished end that had been kept in storage. Solution annealing was recommended to relieve residual stress, thereby reducing the probability of failure. It was also recommended that the carbon content of the steel be lowered, i.e., that a 316L grade be used.

Shaft misalignment and rotor unbalance contribute to the premature failure of many machine components. To understand how these failures occur and quantify the effects, investigators developed a model of a rotating assembly, including a motor, flexible coupling, driveshaft, and bearings. Equations of motion accounting for misalignment and unbalance were then derived using finite elements. A spectral method for resolving these equations was also developed, making it possible to obtain and analyze dynamic system response and identify misalignment and unbalance conditions.

Failure occurred in two TOW flight missile cases in unrelated launchings. After an extensive investigation, it was concluded that stress corrosion was the most likely cause of failure. Subsequent to this conclusion, inner wall softening was observed in an unfired TOW flight missile case. Questions arose as to how the softening occurred and whether or not it had contributed to the failures. This report contains the results of a study which resolved that inner wall softening could not have been present in the failed missile cases.

Leakage which developed from two storage vessels handling a mixture of trimethyl formate and chloroform took place from the dished head at the edge of the circumferential weld to the shell which incorporated a backing ring. Some shallow pitting had occurred under the backing ring on the shell side behind the tack welds securing the backing strip to the shell. Intermittent pitting had also occurred along the head side of the weld at the other end the vessel. There was no pitting along the main longitudinal weld of the shells in any vessel nor around any of the branches set into the shells. The material of the original vessels was specified as BS 970 - 1966. En 58J. Sections taken through pitted areas from both head welds showed preferential attack along the grain-boundaries, some grains becoming completely detached. The location of the pitting and preferential attack was at such a distance from the weld that the heat of welding could have raised the metal temperature to 550 to 700 deg C (1292 deg F). The corrosion of the shell material which occurred at the shell side of the weld under the backing ring is also an example of crevice corrosion.

A rocket-motor case made of consumable-electrode vacuum arc remelted D-6ac alloy steel failed during hydrostatic proof-pressure testing. Close visual examination, magnetic-particle inspection, and hardness tests showed cracks that appeared to have occurred after austenitizing but before tempering. Microscopic examinations of ethereal picral etched sections indicated that the cracks appeared before or during the final tempering phase of the heat treatment and that cracking had occurred while the steel was in the as-quenched condition, before its 315 deg C (600 deg F) snap temper. Chemical analysis of the cracked metal showed a slightly higher level of carbon than in the component that did not crack. X-ray diffraction studies of material from the fractured dome showed a very low level of retained austenite, and chemical analysis showed a slightly higher content of carbon in the metal of the three cracked components. Bend tests verified the conclusion that the most likely mechanism of delayed quench cracking was isothermal transformation of retained austenite to martensite under the influence of residual quenching stresses. Recommendations included modifying the quenching portion of the heat-treating cycle and tempering in the salt pot used for quenching, immediately after quenching.

Flow forming technology has emerged as a promising, economical metal forming technology due to its ability to provide high strength, high precision, thin walled tubes with excellent surface finish. This paper presents experimental observations of defects developed during flow forming of high strength SAE 4130 steel tubes. The major defects observed are fish scaling, premature burst, diametral growth, microcracks, and macrocracks. This paper analyzes the defects and arrives at the causative factors contributing to the various failure modes.

An amusement ride failed when a component in the ride parted, permitting it to fly apart. The ride consisted of a central shaft supporting a spider of three arms, each of which was equipped with an AISI 1040 steel secondary shaft about which a circular platform rotated. The main shaft rotated at about 12 rpm and the platforms at a speed of 20 rpm. The accident occurred when one of the secondary shafts on the amusement ride broke. The point of fracture was adjacent to a weld that attached the shaft to a 16 mm thick plate, which in turn bore the platform support arms. Investigation (visual inspection, 0.4x magnification, and stress analysis) supported the conclusion that a likely cause for the fatigue failure was the combination of residual stresses generated in welding and centrifugal service stresses from operation that were accentuated by areas of stress concentration at the undercut locations. Without the excessive residual stress, the shaft dimensions appeared ample for the service load. Recommendations included applying the fillet weld with more care to avoid undercutting. The residual stresses could be minimized by pre-weld and post-weld heat application.