This article focuses on manufacturing-related failures of injection-molded plastic parts, although the concepts apply to all plastic manufacturing processes It provides detailed examples of failures due to improper material handling, drying, mixing of additives, and molecular packing and orientation. It also presents examples of failures stemming from material degradation improper use of metal inserts, weak weld lines, insufficient curing of thermosets, and inadequate mixing and impregnation in the case of thermoset composites.

Sheet forming failures divert resources from normal business activities and have significant bottom-line impact. This article focuses on the formation, causes, and limitations of four primary categories of sheet forming failures, namely necks, fractures/splits/cracks, wrinkles/loose metal, and springback/dimensional. It discusses the processes involved in analytical tools that aid in characterizing the state of a formed part. In addition, information on draw panel analysis and troubleshooting of sheet forming failures is also provided.

The primary purpose of this article is to describe general root causes of failure that are associated with wrought metals and metalworking. This includes a brief review of the discontinuities or imperfections that may be common sources of failure-inducing defects in the bulk working of wrought products. The article addresses the types of flaws or defects that can be introduced during the steel forging process itself, including defects originating in the ingot-casting process. Defects found in nonferrous forgings—titanium, aluminum, and copper and copper alloys—also are covered.

Distortion often is observed in the analysis of other types of failures, and consideration of the distortion can be an important part of the analysis. This article first considers that true distortion occurs when it was unexpected and in which the distortion is associated with a functional failure. Then, a more general consideration of distortion in failure analysis is introduced. Several common aspects of failure by distortion are discussed and suitable examples of distortion failures are presented for illustration. The article provides information on methods to compute load limits, errors in the specification of the material, and faulty process and their corrective measures to meet specifications. It discusses the general process of material failure analysis and special types of distortion and deformation failure.

A deformed steel tube was received for failure analysis after buckling during a heat-treat operation. The tube was subjected to various metallurgical tests as well as nondestructive testing to confirm the presence of residual stresses. The microstructure of the tube was found to be homogenous and had no banded structure. However, x-ray diffraction analysis confirmed the presence of up to 6% retained austenite which likely caused the tube to buckle during the 910 °C heat treating procedure.

Two clamps that support overhead power lines in an electrified rail system fractured within six months of being installed. The clamps are made of CuNiSi alloy, a type of precipitation-strengthening nickel-silicon bronze. To identify the root cause of failure, the rail operator led an investigation that included fractographic and microstructural analysis, hardness testing, inductively coupled plasma spectroscopy, and finite-element analysis. The fracture was shown to be brittle in nature and covered with oxide flakes, but no other flaws relevant to the failure were observed. The investigation results suggest that the root cause of failure was a forging lap that occurred during manufacturing. Precracks induced by the forging defect and the influence of preload stress (due to bolt torque) caused the premature failure.

The failure of a high speed steel twist drill which caused injury to the user was investigated thoroughly to settle a legal suit. The drill was being used to remove a stud that broke in the vertical wall of a metalworking machine (upsetter) after drilling a pilot hole. The drill had shattered suddenly with a bang which caused a chip to be dislodged and cause the injury. A large nonmetallic inclusion parallel to the axis near the center of the drill was revealed in an unetched longitudinal section. Carbide bands in a martensitic matrix were indicated in an etched sample. It was concluded by the plaintiff's metallurgist that the failed drill was defective as the steel contained nonmetallic inclusions and carbide segregation which made it brittle. It was revealed by the defendant that the twist drill met all specifications of M1 high-speed steel and investigated several other drills without failure to prove that the failure was caused by use in excessive conditions. It was revealed by examination that the point of the broken drill was not the original point put on at manufacture but came from regrinding. Both technical and legal details have been discussed.

The A2 tool steel mandrel, part of a rolling tool used for mechanically joining two tubes was fractured after making five rolled joints. A 6.4 mm diam hole was drilled by EDM through the square end of the hardened mandrel due to difficulty was experienced in withdrawing the tool. The fracture progressed into the threaded section and formed a pyramid-shape fragment after it was initiated at approximately 45 deg through the hole in the square end. An irregular zone of untempered martensite with cracks radiating from the surface of the hole (result of melting around hole) was revealed by metallographic examination. A microstructure of fine tempered martensite containing some carbide particles was exhibited by the core material away from the hole. Brittle fracture characteristics with beach marks were exhibited by the fracture surfaces which is characteristic of a torsional fatigue fracture. As a corrective measure, the hole through the square end of the mandrel was incorporated into the design of the tool and was drilled and reamed before heat treatment and specified hardness of the threaded portion and square end of the mandrel was reduced.

A cross-recessed die of D5 tool steel fractured in service. The die face was found to be subjected to shear and tensile stresses as a result of the forging pressures from the material being worked. The presence of numerous slag stringers was revealed by microscopic examination of an unetched longitudinal section taken through the die. The pattern was microscopically revealed after etching with 5 % nital to be due to severe chemical segregation or banding. Considerable variation in the hardness, corresponding to the banded and non-banded regions across the face of the specimen was observed. The fracture was found to have originated near the high-stress region of the die face examination of the fracture surface. Failure of the die was concluded to have originated in an area of abnormally high hardness which is prone to microcracking during heat treatment for this grade of tool steel

The head of a socket spanner made of heat-treated 0.40C-0.34Cr steel cracked in service. The pronounced fibrous structure of the component became evident as soon as it was etched with 2% nital. Folds in the material originating from the shaping process were visible, and the micrograph showed that cracks ran along these folds oriented according to the fiber. The fissures, with the exception of the hardening crack, were partly filled with oxide and showed signs of decarburization at the edges. From this it could be assumed that parts of the external skin had been forced into the folds during forging. This evidence supported the conclusion that even through there was some indication of chemical segregation, the folds made during forging initiated the main crack. Furthermore, even if the steel had been more homogeneous, hardening cracks would probably have been promoted by the coarse fissures at the fold zones.

In a continuously cast aluminum press stud, two small foreign metal slivers were found that had caused difficulties with the cable sheathing press. Spectroscopic examination revealed the slivers consisted of a chromium-molybdenum-vanadium steel with minor (unintentional) additions of copper, nickel, and cobalt. A steel of similar composition, X38Cr-MoV5 1 (W-No. 2343) was used for hot working tools. The sliver originated from a damaged press tool.

The pointed ends of several stainless steel forceps split or completely fractured where split portions broke off. All the forceps were delivered in the same lot. The pointed ends of the forceps are used for probing and gripping very small objects and must be true, sound, and sharp. Analysis supported the conclusion that the failures to be the result of seams in the steel that were not joined during hot working. Recommendations included that closer inspection of the product take place at all stages of manufacturing. Inspection at the mill will minimize discrepancies at the source, and the inspection of the finished product will help detect obscure seams.

Several newly developed liquid propane gas (LPG) cylinders made from Fe-0.13C-0.42Mn steel failed, each fracturing in the longitudinal direction. One of the cylinders was thoroughly analyzed to determine the cause. Deep-drawing flaws were observed on the inner wall of the cylinder, oriented in the direction of the fracture and roughly equal in length. Flaws about 1.3 mm deep, steps, and a chevron pattern were observed on the fractured surface as were cleavage facets, revealed by SEM. Hardness was relatively high and the microstructure near the fracture surface appeared elongated. In addition, the stress intensity factor KI calculated from the value of the internal pressure was lower than that estimated by the fracture toughness test. All of this suggests that the tanks were not sufficiently annealed and prone to brittle fracture. The analysis thus proves that cracks initiated by deep-drawing flaws were the primary cause of failure.

Hydraulic cylinder housings were being fabricated from 4140 grade seamless steel tubing. During production, magnetic-particle inspection indicated the presence of circumferential and longitudinal cracks in a large number of cylinders. Analysis (visual inspection, dye penetrant inspection, 50x/90x/400x SEM micrographs, and metallographic analysis) supports the conclusion that the cracking problem in these components was identified as quench cracks due to their brittle, intergranular nature and the characteristic temper oxide on the fracture surfaces. Although the steel met the compositional requirements of SAE 4140, the sulfur level was 0.022% and would account for the formation of the sulfide stringers observed. Apparently, the combination of the clustered, stringer-type inclusions and the quenching conditions were too severe for this component geometry. The result was a high incidence of quench cracks that rendered the parts useless. Recommendations included changing the specification, requiring the steel to have lower sulfur concentrations. Magnetic-particle cleanliness standards should be imposed that will exclude material with harmful clusters of sulfide stringers, for example, modified AMS 2301.

A motorboat engine connecting rod forged from carbon steel fractured in two places and cracked at the small end during service. The analysis (visual inspection, 50x micrographs of sections etched with 2% nital, magnetic-particle inspection, and metallographic examination) supported the conclusion that the connecting rods were rendered susceptible to fatigue-crack initiation and propagation by the notch effect of coarse folds formed during the forging operation. One fracture was caused by fatigue resulting from operating stresses, and the other was a secondary tensile fracture. No recommendations were made.

A connecting rod from a motor boat was broken in two places at the small end. At position I there was a fatigue fracture brought about by operational stress, whereas the fibrous fracture surface II was a secondary tensile fracture. Furthermore the transition on the other side of the rod was cracked symmetrically to the fatigue fracture (position III). Magnetic inspection showed indications of cracking at the transition between the rod and small end in six other connecting rods from the same batch. Metallographic investigation showed the connecting rods were rendered susceptible to fatigue by the notch effect of coarse scale-filled folds formed during forging.

An explosion occurred in a portion of a horizontal, U-shaped expansion loop in a steam main approximately 10-in. diam which had been operating at 400 psi for six years. Steam conditions varied from 538 deg C (450 deg F) saturated to 343 deg C (650 deg F) superheated. Fracture occurred longitudinally through the upper wall over a length of approximately 68 in. The sample received for examination was ultrasonically tested, which indicated a band of internal defects extending 1 in. in from the edge. Subsequently, the portion of the pipe embodying the other side of the rupture was obtained for examination. Transverse sections through this and the mating portion already received, followed by magnetic crack detection, revealed the presence of defective zones. Subsequent ultrasonic examination of other sections of the steam main indicated suspect areas in a number of lengths of pipe. These defects were basically laminations of a similar form to those which resulted in the failure of the portion of pipe.

In TAKR 300 (Bob Hope) Class transport ships, the builder observed cracking of steel cloverleaf vehicle tie-down deck sockets following installation. Sockets were made from AH36 steel plate by flame cutting and cold coining, then submerged-arc welded to the shop deck. Cracks initiated from the tip of the cloverleaf pattern in >300 cases aboard several cargo vessels in various stages of construction. Consultants who analyzed the situation concluded that the problem may have been corrosion and hydrogen embrittlement. Three possible mechanisms of failure were considered: overload failure; fatigue fracture; and, environmentally-assisted cracking. Testing indicated overload failure was the cause. Remedial actions were taken to improve the fracture properties of the deck socket. A modified manufacturing process was developed involving milling and cutting instead of coining to round the comers of the flame-cut cloverleaf lobe. This new manufacturing process solved the problem.

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.

Countersunk riveted joints in aluminum sheet are widely employed in the aircraft industry. The preparation of the sheet for the riveting process consists either of countersinking where the sheet is sufficiently thick or of dimpling. Metallographic assessment of dimple defects is described in specimens made of clad aluminum sheet of alloy type AlZnMgCu1.5. Addressed are a dimple with partially missing stamped surface (bell-mouth), a cylindrical prominence because the dimpling force was too great and the stamping cylinder force too low, and a dimple with flashes at the top surfaces of the sheet as a result of play between the stamping cylinder and the anvil head (ringed dimple). Frequently, overlapping of several defects occurs, especially with steel or titanium sheet, with the result that it is difficult to identify the defects.