This article presents the benefits of selecting plastics for products to be manufactured. It discusses the four key considerations for plastic part design: material, process, tooling, and design. The article provides a detailed discussion of the development sequence for plastic parts. The basis for the development sequence is twofold: first, to create the best solution for the application, and second, to minimize potential project risks through careful and thoughtful work habits.

Materials selection is closely related to the objectives of failure analysis and prevention. This article briefly reviews the general aspects of materials selection as a concern in both proactive failure prevention during design and as a possible root cause of failed parts. Coverage is more conceptual, with general discussions on the following topics: design and failure prevention, materials selection in design, materials selection for failure prevention, and materials selection and failure analysis. Because materials selection is just one part of the design process, the overall concept of design is discussed. The article also describes the role of the materials engineer in the design and materials selection process. It provides information on the significance of materials selection in both the prevention and analysis of failures.

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.

When a large LPG low-carbon steel storage tank was put into service for the first time and filled beyond the proof testing level, a brittle fracture crack initiated at a fillet weld between a stiffener ring and the wall. The crack propagated to a length of 5.5 m and arrested. Analysis showed that the plates satisfied the criteria of BS 4741. It was concluded that the cause of crack initiation was the lack of a mouse hole at the junction between the stiffening ring and the wall of the tank. The tank was repaired and put back in service. When it was filled beyond the proof test level, again a brittle crack was initiated at a horizontal weld defect and propagated vertically, destroying the tank and the liquefaction plant. The initiation site was a thumbnail elliptical crack in a horizontal weld, having a depth of 1.5 mm, and a length of 4.5 mm. This showed that as late the mid-1970s, misunderstanding of brittle fracture led to the wrong design and construction of an LPG storage tank. The best design specification is to use a correlation between LAST, the Lowest Anticipated Service Temperature, and the DBTT measured by either Charpy tests or DTT.

There was a fracture of an anchor link that rides on a post with a spherical surface. No bevel or radius was called for, leaving an extremely sharp edge. A fatigue crack grew from the top sharp edge. The casting had good toughness and strength (1448 MPa, or 210 ksi, tensile strength). This fatigue failure was the result of poor design, which incorporated a notch that acted as a stress raiser, leading to fatigue-crack growth. A generous radius was recommended to avoid similar failures.

Five cylinders out of a group of nine in a drying machine developed leaks after a few months service in a textile mill. Leakage was reported from locations between the hoop and body and from the circumferential welds. The materials in the affected area were 18/8 Ti and 18/10/3/Mo austenitic stainless steels. Examination of the cracks at high magnification revealed them to be of the stress-corrosion type. The welds were of satisfactory quality. Cracking was also visible at these locations, this again being of the stress corrosion type. The method of cylinder construction introduced a crevice between the outer hoop and the cylinder at the inboard edge so that during washing of the rolls, water could penetrate the crevice and subsequent heating would lead to the concentration of chlorides within the crevice. Redesign of the cylinder to eliminate the crevice was recommended.

After the mixing machines were introduced into service, excessive assembly stresses and inappropriate detail design caused the premature failures of ice cream drink mixer blades shortly. The mixer blade is slightly deformed by the contact between the wavy washer at the bottom of the assembly and the bends at the bottom shoulders of the two mixer arms. Multiple fatigue crack origins on the inside radii of the bends at the bottom shoulders was revealed by analysis of the failed mixer blades. It was revealed by metallographic examination that the shoulders on the arms were cold bent, introducing tensile residual stresses on the inside radii of the shoulders and creating a localized area of fatigue susceptibility due to the inherent notch sensitivity of cold-formed 300 series stainless steel. It was established that the physical root cause was the design of the mixer blade. The addition of a stand-off washer between the wavy washer and the bottom shoulders of the blade or modification of the shape of the wavy washer to prevent contact with the blade shoulders was recommended.

Components of a latch assembly used in a consumer safety restraint exhibited a relatively high failure rate. The failures were occurring after installation but prior to actual field use when failure could result in severe injury. Cracking occurred within retaining tabs used to secure a metal slide on an older design, whereas newer components showed no signs of failure. The latch assembly components were injection molded from an unfilled commercial grade of a polyacetal copolymer. Investigation of failed parts (including visual inspection, a specially designed proof load test, 59x SEM images, micro-FTIR in the ATR mode, and DSC/TGA/MFR analysis) showed no evidence of contamination or degradation from the molding process. The conclusion was that the parts failed via brittle fracture associated with stress overload. The stress overload was accompanied by severe apparent embrittlement resulting from a relatively high strain rate event and/or significant stress concentration. A relatively sharp corner formed by a retaining tab on the older design was shown to be a primary cause of the failures.

Field fatigue failures occurred in a hand-operated gear shift lever mechanism made of 1049 medium carbon steel hardened to 269 to 285 HB. The failures occurred in the 3.18 mm (0.127 in.) radius. Redesign increased the shift lever's diameter to 25 mm (1 in.) and the radius to 4.75 mm (0.187 in.). Also, instead of the as-forged surface, it was expedient to machine the radius. The as-forged surface at 360 MPa (52 ksi) maximum working stress would not ensure satisfactory life because the recalculated maximum stress was 390 MPa (57 ksi). However, the machined surface with a maximum working stress of 475 MPa (69 ksi) gives a safe margin above the 390 MPa (57 ksi) requirement for design stress. Interpreting these values, the forged surface should have a life expectancy of 1,000,000 cycles of stress. However, because the load cycle was somewhat uncertain, the machined radius was chosen to obtain a greater margin of safety. Redesigning eliminated the failures.

A weld that attached the head to the shell of a preheater containing steam at 1.4 MPa (200 psi) and was used in the manufacture of paper cracked in service. The original joint contained a 6.4 by 50 mm backing ring that had been tack welded to the inside surface of the shell in a position to project beyond the fully beveled top edge of the shell. The projecting edge of the ring fitted against a deep undercut on the inner corner of the rim of the head. The internal 90-deg angle in this undercut was sharp, with almost no fillet. A bevel from the lower edge of the undercut to the outside of the head completed the groove for the circumferential attachment weld. Investigation (visual inspection and actual size views etched in hot 50% hydrochloric acid) supported the conclusion that cracking occurred in the HAZ in the head of the original design, originating in the sharp corner of the undercut, which was an inherent stress raiser. Recommendations included revised joint design to ensure full root penetration.

This article discusses the three legal theories on which a products liability lawsuit is based and the issues of hazard, risk, and danger in the context of liability. It describes manufacturing and design defects of various products. The article explains a design that is analyzed from the human factors viewpoint and details the preventive measures of the defects, with examples. It presents four paramount questions relating to the probability of injury which are asked even when one executes all possible preventive measures carefully and thoroughly.

This article describes the six fundamental factors that decide a tool's performance. These are mechanical design, grade of tool steel, machining procedure, heat treatment, grinding, and handling. A deficiency in any one of the factors can lead to a tool and die failure. The article presents a seven-step procedure to be followed when looking for the reason for a failure. A review of the results of the seven-point investigation may lead directly to the source of failure or narrow the field of investigation to permit the use of special tests.

This article focuses on the general root causes of failure attributed to the casting process, casting material, and design with examples. The casting processes discussed include gravity die casting, pressure die casting, semisolid casting, squeeze casting, and centrifugal casting. Cast iron, gray cast iron, malleable irons, ductile iron, low-alloy steel castings, austenitic steels, corrosion-resistant castings, and cast aluminum alloys are the materials discussed. The article describes the general types of discontinuities or imperfections for traditional casting with sand molds. It presents the international classification of common casting defects in a tabular form.

This article provides a description of the microscale models and mechanisms for deformation and fracture. Macroscale and microscale appearances of ductile and brittle fracture are discussed for various specimen geometries and loading conditions. The article reviews the general geometric factors and materials aspects that influence the stress-strain behavior and fracture of ductile metals. It highlights fractures arising from manufacturing imperfections and stress raisers. The article presents a root cause failure analysis case history to illustrate some of the fractography concepts.