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This glossary contains the definitions of terms related to the characterization and failure analysis of plastics.

An electric transport vehicle, similar to an electric trolley or subway rail car, experienced frequent breakdowns due to in-service fractures of torsion springs that support the weight of an overhead electric pickup assembly. Scanning electron microscopy and metallographic examinations determined that the fractures stemmed from electric arc damage. Intergranular quench cracks in the transformed untempered martensite on the surface of the spring provided crack initiations that propagated during operation causing fatigue fracture.

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.

A 10-cm (4-in.) chain link used in operating a large dragline bucket failed after several weeks in service. The link was made of cast low-alloy steel (similar to ASTM A487, class 10Q) that had been normalized, hardened, and tempered to give a yield strength of approximately 1034 MPa (150 ksi). A hydrogen flake approximately 5 cm (2 in.) in diam was observed at the center of the fracture surface. Beach marks indicative of fatigue encircled the hydrogen flake and covered nearly all of the remaining fracture surface. The failure of this linkways caused by an excessive hydrogen content. Two steps were taken to combat this type of failure. First, when service conditions did not require high hardness to combat wear, the links were produced of a steel having a yield strength of about 690 MPa (100 ksi) rather than 1034 M

A CrN coated restrike punch was made of WR-95 (similar to H-11), which was fluidized bed nitrided. The coated punch was used on hot Inconel at about 1040 deg C (1900 deg F). However, a water-soluble graphite coolant was used to maintain the punch temperature at 230 deg C (450 deg F). Visual and binocular inspection at 64+ revealed presence of cracks and complete washout of coating in the working area of the failed punch. Comparison of metallographic cross sections of used and unused punches revealed a significant microstructural transformation in case of the used punch. Presence of a yellow porous layer was clearly evident between the nitrided layer and the coating, in case of the used punch. Cracks were observed to propagate from the outer surface into the bulk. Oxidation was evident along the cracks. The microstructural transformation observed in the case of the used punch was a clear indication of high temperature exposure (due to insufficient cooling) during application. The most probable cause of failure was thermal fatigue.

Examination of a 7075-T6 aluminum alloy pylon strut revealed cracks in two locations on the ears of the strut. Because the part was still intact, the cracks had to be forced open so that the fractures could be examined. Scanning electron microscopy (SEM) of the opened cracks showed that the crack surfaces were covered with a mud crack pattern suggestive of stress-corrosion cracking (SCC). The T6 temper is susceptible to SCC. It was concluded that cracking of the strut could have been aggravated by the hard landing experienced by the aircraft. The strut, however, contained stress-corrosion cracks which were present before the landing. It was recommended that an inspection for SCC be made of all pylon struts with a similar service life.

A coupling in a line-shaft vertical turbine pump installed in a dam foundation fractured after a very short time. The coupling material was ASTM A582 416 martensitic stainless steel. Visual, macrofractographic, and scanning electron microscopic examination of the coupling showed that the fracture was brittle and was initiated by an intergranular cracking mechanism. The mode of fracture outside the crack initiation zone was transgranular cleavage. No indication of fatigue was found. The failure was attributed to improper heat treatment during manufacture, which resulted in a brittle microstructure susceptible to corrosion. The crack initiated either by stress-corrosion or hydrogen cracking. It was recommended that the couplings in the system be examined for surface cracking and, if present, corrective measures be taken.

A crane hook of 200T rated capacity failed suddenly at an indicated load of 143T, while the crane was undergoing a load test. Fracture took place through the intrados of the hook at the region of maximum stress. The jib and other portions suffered subsequent damage following the sudden release of the load. Fracture was wholly of the brittle cleavage type except for a small crescent shaped lip at the top right-hand side. In this zone, fracture occurred at an angle of 45 deg to the general plane of fracture, indicative of failure in shear. Failure of the hook had taken place where a deposit of weld metal had been made, probably to eliminate a surface defect but apparently, without complete removal of the defect down to sound metal prior to welding. On many occasions it is preferable to blend out surface defects by local dressing. The effect of the resulting loss of strength is insignificant compared with the increased chance of failure associated with a weld repair.

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.

Some examples of equipment failures involving high temperature operation are presented. They include some steam generator superheater components and a pump shaft that should not have been at high temperature. Metallographic analysis is used to determine the causes of failure in each case.

A 4340 steel shaft, the driving member of a large rotor subject to cyclic loading and frequent overloads, broke after three weeks of operation. The driving shaft contained a shear groove at which the shaft should break if a sudden high overload occurred, thus preventing damage to an expensive gear mechanism. The rotor was subjected to severe chatter, which was an abnormal condition resulting from a series of continuous small overloads occurring at a frequency of around three per second. Investigation (visual inspection, hardness testing, and hot acid etch images) supported the conclusion that the basic failure mechanism was fracture by torsional fatigue, which started at numerous surface shear cracks, both longitudinal and transverse, that developed in the periphery of the root of the shear groove. These shear cracks resulted from high peak loads caused by chatter. The shear groove in the shaft had performed its function, but at a lower overload level than intended. Recommendations included increasing the fatigue strength of the shaft by shot peening the shear groove to minimize chatter.

The contamination of lubrication with powdered stone resulted in progressive wear of the internal surfaces of a bearing. Because of the motion of rollers, the inner race exhibited an unusual cyclic washboard wear pattern. Because of a lack of bearing conformity, wear progressed into severe coarse-grain spalling.

A turbine vane made of cast cobalt-base alloy AMS 5382 (Stellite 31; composition: Co-25.5Cr-10.5Ni-7.5W) was returned from service after an undetermined number of service hours because of crack indications on the airfoil sections. This alloy is cast by the precision investment method. Analysis (visual inspection, 100x/500x metallographic examination of sections etched with a mixture of ferric chloride, hydrochloric acid, and methanol, and bend tests) supported the conclusions that cracking of the airfoil sections was caused by thermal fatigue and was contributed to by low ductility due to age hardening, subsurface oxidation related to intragranular carbides, and high residual tensile macrostresses. No further conclusions could be drawn because of the lack of detailed service history, and no recommendations were made.

An unusual type of defect was discovered during hydraulic testing of a water-tube boiler after repairs to the superheater tubes following erosion from soot-blowers. When the pressure reached 700 psi, slight leakage was found to be taking place from one of the superheater tubes in a region where there appeared to be a split, approximately 8 in. long. What was thought to be a split was actually a pronounced fold. Microscopic examination showed that a corrosion-fatigue fissure had developed from one of the inside corners of the fold, presumably as a result of the fluctuating bending stresses to which this portion of the tube would be subjected because of the discontinuity in the tube wall. It was from this fissure that the leakage occurred. It was evident that the defect developed during the manufacture of the tube, probably in the course of a drawing or rolling operation without an internal plug. The diam of this portion of the tube was reduced by local collapse and folding of the section rather than by longitudinal extension of the tube itself.

A shaft, which carried both a worm wheel and hoist barrel, fractured at a reduction in diameter adjacent to a mating gearbox. The appearance of the fracture was characteristic of a fatigue failure of a rotating shaft resulting from excessive bending stresses. Cracks of the fatigue type broke out all around the circumference at the change of section and progressed inwards. Microscopic examination of the material showed it to be an alloy steel in the hardened and tempered condition, with no abnormal features. It was considered that the bending stresses due to the deflection of the shaft arising from misalignment were responsible for the fatigue failure, which occurred in a region of stress concentration where insignificant fillet radius had been provided.

An antifriction bearing made from a nylon/ polyethylene blend failed. The bearing came into contact with a steel shaft. Investigation (visual inspection and 417X images) supported the conclusion that movement of the shaft against the bearing caused abrasion due to fine iron oxide particles. No recommendations were made.