Results of failure analyses of two aircraft crankshafts are described. These crankshafts were forged from AMS 6414 (similar composition to AISI 4340) vacuum arc remelted steels with sulfur contents of 0.003% (low sulfur) and 0.0005% (ultra-low sulfur). A grain boundary sulfide precipitate was caused by overheat of the low sulfur steel, and an incipient melting of grain boundary junctions was caused by overheat of the ultra-low sulfur steel. The precipitates and incipient melting in these two failed crankshafts were observed during the examination. As expected, impact fractures from the low sulfur steel crankshaft contained planar dimpled facets along separated grain boundaries with a small spherical manganese sulfide precipitates within each dimple. In contrast, planar dimpled facets along separated grain boundaries of impact fractures from the ultra-low sulfur crankshaft steel contained a majority of small spherical particles consisting of nitrogen, boron, iron, carbon, and a small amount of oxygen. Some other dimples contained manganese sulfide precipitates. Fatigue samples machined from the ultra-low sulfur steel crankshaft failed internally at planar grain boundary facets. Some of the facets were covered with nitrogen, boron, iron, and carbon film, while other facets were relatively free of such coverage. Results of experimental forging studies defined the times and temperatures required to produce incipient melting overheat and facets at grain boundary junctions of ultra-low sulfur AMS 6414 steels.

This article provides a discussion on the metallographic techniques used for failure analysis, and on fracture examination in materials, with illustrations. It discusses various metallographic specimen preparation techniques, namely, sectioning, mounting, grinding, polishing, and electrolytic polishing. The article also describes the microstructure examination of various materials, with emphasis on failure analysis, and concludes with information on the examination of replicas with light microscopy.

Metallographic examination is one of the most important procedures used by metallurgists in failure analysis. Typically, the light microscope (LM) is used to assess the nature of the material microstructure and its influence on the failure mechanism. Microstructural examination can be performed with the scanning electron microscope (SEM) over the same magnification range as the LM, but examination with the latter is more efficient. This article describes the major operations in the preparation of metallographic specimens, namely sectioning, mounting, grinding, polishing, and etching. The influence of microstructures on the failure of a material is discussed and examples of such work are given to illustrate the value of light microscopy. In addition, information on heat-treatment-related failures, fabrication-/machining-related failures, and service failures is provided, with examples created using light microscopy.

This article commences with a summary of fatigue processes and mechanisms. It focuses on fractography of fatigue. Characteristic fatigue fracture features that can be discerned visually or under low magnification are described. Typical microscopic features observed on structural metals are presented subsequently, followed by a brief discussion of fatigue in nonmetals. The article reviews the various macroscopic and microscopic features to characterize the history and growth rate of fatigue in metals. It concludes with a description of fatigue of polymers and composites.

Fatigue failure of engineering components and structures results from progressive fracture caused by cyclic or fluctuating loads. Fatigue is an important potential cause of mechanical failure, because most engineering components or structures are or can be subjected to cyclic loads during their lifetime. This article focuses on fractography of fatigue. It provides an abbreviated summary of fatigue processes and mechanisms: fatigue crack initiation, fatigue crack propagation, and final fracture,. Characteristic fatigue fracture features that can be discerned visually or under low magnification are then described. Typical microscopic features observed on structural metals are presented subsequently, followed by a brief discussion on fatigue in polymers and polymer-matrix composites.

This article provides some new developments in elevated-temperature and life assessments. It is aimed at providing an overview of the damage mechanisms of concern, with a focus on creep, and the methodologies for design and in-service assessment of components operating at elevated temperatures. The article describes the stages of the creep curve, discusses processes involved in the extrapolation of creep data, and summarizes notable creep constitutive models and continuum damage mechanics models. It demonstrates the effects of stress relaxation and redistribution on the remaining life and discusses the Monkman-Grant relationship and multiaxiality. The article further provides information on high-temperature metallurgical changes and high-temperature hydrogen attack and the steps involved in the remaining-life prediction of high-temperature components. It presents case studies on heater tube creep testing and remaining-life assessment, and pressure vessel time-dependent stress analysis showing the effect of stress relaxation at hot spots.

This article discusses pressure vessels, piping, and associated pressure-boundary items of the types used in nuclear and conventional power plants, refineries, and chemical-processing plants. It begins by explaining the necessity of conducting a failure analysis, followed by the objectives of a failure analysis. Then, the article discusses the processes involved in failure analysis, including codes and standards. Next, fabrication flaws that can develop into failures of in-service pressure vessels and piping are covered. This is followed by sections discussing in-service mechanical and metallurgical failures, environment-assisted cracking failures, and other damage mechanisms that induce cracking failures. Finally, the article provides information on inspection practices.

This article describes some of the welding discontinuities and flaws characterized by nondestructive examinations. It focuses on nondestructive inspection methods used in the welding industry. The sources of weld discontinuities and defects as they relate to service failures or rejection in new construction inspection are also discussed. The article discusses the types of base metal cracks and metallurgical weld cracking. The article discusses the processes involved in the analysis of in-service weld failures. It briefly reviews the general types of process-related discontinuities of arc welds. Mechanical and environmental failure origins related to other types of welding processes are also described. The article explains the cause and effects of process-related discontinuities including weld porosity, inclusions, incomplete fusion, and incomplete penetration. Different fitness-for-service assessment methodologies for calculating allowable or critical flaw sizes are also discussed.