Several of the aluminum alloy 6061-T6 drawn seamless tubes (ASTM B 234, 2.5 cm (1.0 in.) OD with wall thickness of 1.7 mm (0.065 in.)) connecting an array of headers to a system of water-cooling pipes failed. The tubes were supplied in the O temper. They were bent to the desired curvature, preheated, then solution treated, water quenched, and then aged for 8 to 10 h. Analysis (visual inspection, slow-bend testing, 65x macrographic analysis, macroetching, spectrographic analysis, hardness tests, microhardness tests, tension tests, and microscopic examination) supported the conclusions that bending of the connector tubes in the annealed condition induced critical strain near the neutral axis of the tube, which resulted in excessive growth of individual grains during the subsequent solution treatment. Recommendations included bending the connector tubes in the T4 temper as early as possible after being quenched from the solution temperature. The tubes should be stored in dry ice after the quench until bending can be done. The tubes should be aged immediately after being formed. Flattening and slow-bend tests should be specified to ensure that the connector tubes had satisfactory ductility.

A blade from the engine cooling fan of a pickup truck fractured unexpectedly. The blade was made from type 301 stainless steel in the extra full hard tempered condition with a hardness of 47 HRC. Failure analysis indicated that the blade fractured in three modes: crack initiation, fatigue crack propagation, and final rapid fracture in a ductile manner The fatigue crack originated near a rivet hole.

This article briefly introduces some commonly used methods for mechanical testing. It describes the test methods and provides comparative data for the mechanical property tests. In addition, creep testing and dynamic mechanical analyses of viscoelastic plastics are also briefly described. The article discusses the processes involved in the short-term and long-term tensile testing of plastics. Information on the strength/modulus and deflection tests, impact toughness, hardness testing, and fatigue testing of plastics is also provided. The article describes tension testing of elastomers and fibers. It covers two basic methods to test the mechanical properties of fibers, namely the single-filament tension test and the tensile test of a yarn or a group of fibers.

Engineering plastics, as a general class of materials, are prone to the development of internal stresses which arise during processing or during servicing when parts are exposed to environments that impose deformation and/or temperature extremes. Thermal stresses are largely a consequence of high coefficients of thermal expansion and low thermal diffusivities. Although time-consuming techniques can be used to analyze thermal stresses, several useful qualitative tests are described in this article. The classification of internal stresses in plastic parts is covered. The article describes the effects of low thermal diffusivity and high thermal expansion properties, and the variation of mechanical properties with temperature. It discusses the combined effects of thermal stresses and orientation that result from processing conditions. The article also describes the effect of aging on properties of plastics. It explains the use of high-modulus graphite fibers in amorphous polymers.

During a refit of a twenty-year-old Naval destroyer, two cracks were found on the inside of the killed carbon-manganese steel hull plate at the forward end of the boiler room. The cracks coincided with the location of the top and bottom plates of the bilge keel. Metallurgical examination of sections cut from the cracked area identified lamellar tearing as the principle cause of the cracking. This was surprising in 6 mm thick hull plates. Corrosion fatigue and general corrosion also contributed to hull plate perforation. Although it is probable that more lamellar tears exist near the bilge keel in other ships and may be a nuisance in the future, the hull integrity of the ships is not threatened and major repairs are not needed.

Engineering component and structure failures manifest through many mechanisms but are most often associated with fracture in one or more forms. This article introduces the subject of fractography and aspects of how it is used in failure analysis. The basic types of fracture processes (ductile, brittle, fatigue, and creep) are described briefly, principally in terms of fracture appearances. A description of the surface, structure, and behavior of each fracture process is also included. The article provides a framework from which a prospective analyst can begin to study the fracture of a component of interest in a failure investigation. Details on the mechanisms of deformation, brittle transgranular fracture, intergranular fracture, fatigue fracture, and environmentally affected fracture are also provided.

This article provides an overview of fractography and explains how it is used in failure analysis. It reviews the basic types of fracture processes, namely, ductile, brittle, fatigue, and creep, principally in terms of fracture appearances, such as microstructure. The article also describes the general features of fatigue fractures in terms of crack initiation and fatigue crack propagation.

This article reviews the impact response of plastic components and the various methods used to evaluate it.. It describes the effects of loading rate on polymer deformation and the influence of temperature and strain rate on failure mode. It discusses the advantages and limitations of standard impact tests, the use of puncture tests for assessing material behavior under extreme strain, and the application of fracture mechanics for analyzing impact failures. It also develops and demonstrates the theory involved in the design and analysis of thin-walled, injection-molded plastic components.

Following an accident in which a light pickup truck left the road and overturned, one of the rear axles, made of approximately 0.30C steel, was found to be fractured adjacent to the bearing lock nut. A keyway was present in the failed area, as were threads for the lock nut. Fracture surfaces of the failed axle and exemplar fractures obtained from simulation tests were studied using scanning electron microscope. The examination showed that the outer perimeter fracture in the axle was very flat and composed of cleavage and that the interior portion was composed of both cleavage and dimples. No evidence of prior cracking was found. The exemplar specimens from the simulation impact testing failed in a manner consistent with that observed in the axle. The examination confirmed that the failure was a one-time impact overload fracture and not the result of any prior crack in the material, indicating that the axle failure did not initiate the accident.

This article focuses primarily on what an analyst should know about applying X-ray diffraction (XRD) residual stress measurement techniques to failure analysis. Discussions are extended to the description of ways in which XRD can be applied to the characterization of residual stresses in a component or assembly. The article describes the steps required to calibrate instrumentation and to validate stress measurement results. It presents a practical approach to sample selection and specimen preparation, measurement location selection, and measurement depth selection, as well as an outline on measurement validation. The article also provides information on stress-corrosion cracking and corrosion fatigue. The importance of residual stress in fatigue is described with examples. The article explains the effects of heat treatment and manufacturing processes on residual stress. It concludes with a section on the XRD stress measurements in multiphase materials and composites and in locations of stress concentration.

X-ray diffraction (XRD) residual-stress analysis is an essential tool for failure analysis. This article focuses primarily on what the analyst should know about applying XRD residual-stress measurement techniques to failure analysis. Discussions are extended to the description of ways in which XRD can be applied to the characterization of residual stresses in a component or assembly and to the subsequent evaluation of corrective actions that alter the residual-stress state of a component for the purposes of preventing, minimizing, or eradicating the contribution of residual stress to premature failures. The article presents a practical approach to sample selection and specimen preparation, measurement location selection, and measurement depth selection; measurement validation is outlined as well. A number of case studies and examples are cited. The article also briefly summarizes the theory of XRD analysis and describes advances in equipment capability.

One of the coils in the radiant section of a primary reformer furnace used in an ammonia plant was found leaking. The bottom of one of seven outlet headers (made of ASME SA-452, grade TP316H, stainless steel) was revealed during examination to be ruptured. It was revealed by metallurgical examination that it had failed as a result of intergranular fissuring and oxidation (creep rupture). The ruptured area revealed that the header had failed by conventional long-time creep rupture as a result of exposure to operating temperatures probably between 900 and 955 deg C. Three samples from different sections (ruptured area, slightly bulged but nonruptured area and visually sound metal) were inspected. The presence of pinhead-size intergranular fissures throughout the cross sections of the latter two samples was observed. An ultrasonic attenuation method was employed to investigate the remaining headers. All headers were revealed by ultrasonic readings to be in an advanced stage of creep rupture and no areas were found to be fissured to a degree that they needed immediate replacement. As a conclusion, the furnace was deemed serviceable and it was established that in the absence of local hot spots, the headers would survive for a reasonable period of time.

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.

This article focuses on characterizing the fracture-surface appearance at the microscale and contains some discussion on both crack nucleation and propagation mechanisms that cause the fracture appearance. It begins with a discussion on microscale models and mechanisms for deformation and fracture. Next, the mechanisms of void nucleation and void coalescence are briefly described. Macroscale and microscale appearances of ductile and brittle fracture are then discussed for various specimen geometries (smooth cylindrical and prismatic) and loading conditions (e.g., tension compression, bending, torsion). Finally, the factors influencing the appearance of a fracture surface and various imperfections or stress raisers are described, followed by a root-cause failure analysis case history to illustrate some of these fractography concepts.

While there are many fracture mechanisms that can lead to the failure of a plastic component, environmental stress cracking (ESC) is recognized as one of the leading causes of plastic failure. This article focuses on unpacking the basic concepts of ESC to provide the engineer with a better understanding of how to evaluate and prevent it. It then presents factors that affect and contribute to the susceptibility of plastic to ESC: material factors, chemical factors, stress, and environmental factors. The article includes the collection of background information to understand the circumstances surrounding the failure, a fractographic evaluation to assess the cracking, and analytical testing to evaluate the material, design, manufacturing, and environmental factors.

On 13 Dec 1994, two massive detonations leveled portions of an ammonium nitrate plant near Sioux City, IA. The primary explosion allegedly occurred in defectively-designed titanium sparger piping inside the neutralizer vessel. Investigation however, revealed the explosion occurred because of unsafe plant operations and poor maintenance procedures. Specifically, the ammonium nitrate within the 18,000 gal capacity neutralizer vessel had become contaminated and made highly acidic. The operators then injected superheated steam directly into the ammonium nitrate in the neutralizer vessel.

Over a period of 2 or 3 years, 40 to 50 premature failures of drawn high-tensile, pearlitic high-carbon (0.8 wt% C) steel wires used as cables for towing targets behind aircraft occurred. Six service failures were examined in detail. Four types of failure characteristics were noted. A close examination of wire that had been flown several times without failure was also made, and dynamic tests were conducted to investigate the fracture characteristics of wire subjected to dynamic loading. It was concluded that dynamic shock loading transmitted by the target during unsteady flight conditions was the major cause of failure. Recommendations emphasized the need for a suitable shock absorber to be fitted at the constant-tensioning device of the winch system.

Optical testing of plastics includes the characterization of materials and the analysis of optical components. If a material is tested for transmission, haze, yellowness, and refractive index, the knowledge of its optical properties is nearly complete. For optical components, surface irregularity, birefringence, and internal contamination must also be considered. These characteristics are a function of the material and the fabrication method. Gloss and color also are affected by the base material and measured as optical properties.