Fatigue fractures are generally considered the most serious type of fracture in machinery parts simply because fatigue fractures can and do occur in normal service, without excessive overloads, and under normal operating conditions. This chapter first discusses the three stages (initiation, propagation, and final rupture) of fatigue fracture followed by a discussion of its microscopic and macroscopic characteristics. The relationship between stress and strength in fatigue is explained. The next section provides information that may help the uninitiated to appreciate some of the problems of laboratory fatigue testing and of the fatigue process itself. Finally, information on types and statistical aspects of fatigue is provided along with examples.

This chapter explains how metallography and hardness testing are used to evaluate the quality and condition of metal products. It also discusses the use of tensile testing, fracture toughness and impact testing, fatigue testing, and nondestructive test methods including ultrasonic, x-ray, and eddy current testing.

This chapter addresses the question of how to deal with multiaxial stresses and strains when using the strain-range partitioning method to analyze the effects of creep fatigue. It is divided into three sections: a general discussion on the rationale used in formulating rules for treating multiaxiality, a concise listing of the rules, and an example problem in which axial creep-fatigue data is used to predict the torsional creep-fatigue life of type 304 and 316 stainless steel. The chapter also includes a brief introduction in which the authors outline the challenges presented by multiaxial loading and set practical limits on the problem they intend to treat.

This chapter provides an overview of the possible mechanisms of failure for heat treated steel components and discusses the techniques for examining fractures, ductile and brittle failures, intergranular failure mechanisms, and fatigue. It begins with a description of the general sources of component failure. This is followed by a section on the stages of a failure analysis, which can proceed one after the other or occur at the same time. These stages of analysis are collection of background data, preliminary visual examination, nondestructive testing, selection and preservation of specimens, mechanical testing, macroexamination, microexamination, metallographic examination, determination of the fracture mechanism, chemical analysis, exemplar testing, and analysis and writing the report. The chapter ends with a discussion on various processes involved in the determination of the fracture mechanism.

This chapter presents various case histories that illustrate a variety of failure mechanisms experienced by the high-strength steel components in aerospace applications. The components covered are catapult holdback bar, AISI 420 stainless steel roll pin, main landing gear (MLG) lever, inboard flap hinge bolt, nose landing gear piston axle, multiple-leg aircraft-handling sling, aircraft hoist sling, internal spur gear, and MLG axle. In addition, the chapter provides information on full-scale fatigue testing, nondestructive testing, and failure analysis of fin attach bolts.

This chapter reviews the theories that have emerged from the widespread study of multiaxial fatigue and assesses their validity using data from different sources. It begins by providing background on the studies that the chapter draws on, pointing out differences in methodology and explaining how they influence test results and data. It then discusses the concept of critical planes and how they are used to correlate the effects of uniaxial loading with multiaxial fatigue behaviors. The section that follows covers the various methods used to analyze multiaxial fatigue and identifies one that best treats the general case. The chapter also defines two important factors, the triaxiality factor and the multiaxiality factor, and presents the results of an extensive study to determine how the two factors are related. One of the more interesting findings is that the atomic structure of a material has a significant effect on which theory best describes its fatigue behavior.

This chapter describes how notches affect the load-carrying capacity and fatigue life of materials under cyclic loads. It explains that stresses and strains can be three to four times higher in the vicinity of a notch, greatly accelerating fatigue damage. It discusses the use of stress concentration factors and how they are determined for the general case and for specific geometries, materials, and surface conditions. The chapter covers both elastic and plastic fatigue behaviors as well as a wide range of methods. It also explains how small nuances in loading can introduce tensile or compressive stress in the hysteresis loops causing variations in fatigue life as large as 50:1 depending on where the transition in fatigue behavior occurs.

This chapter provides a quantitative treatment of the cracking mechanisms associated with fatigue, drawing on the principles of fracture mechanics. It explains that although fracture mechanics originated with the aim of understanding sudden and catastrophic crack extension, the main premise of a stress field in the vicinity of the crack also applies to the study of cycle-by-cycle stable crack growth. A detailed review is given of the many developments and discoveries that helped shape the theory and methods collectively defined as crack mechanics, which the authors then employ to analyze the crack growth behavior of various materials, including steels and nonferrous alloys, under constant-amplitude loading. The authors then deal with the effects of complex loading using crack retardation and crack closure models to show how load fluctuations can slow crack growth rates and even cause total crack arrest. They also present the results of a study on crack initiation, propagation, and fracture in circular (rather than rectangular) specimens and a fatigue study on ductile and quasi-brittle materials.

This chapter discusses the effect of fatigue on polymers, ceramics, composites, and bone. It begins with a general comparison of polymers and metals, noting important differences in microstructure and cyclic loading response. It then presents the results of several studies that shed light on the fatigue behavior and crack growth mechanisms of common structural polymers and moves on from there to discuss the fatigue behavior of bone and how it compares to stable and cyclically softening metals. It also discusses the fatigue characteristics of engineered and composited ceramics and ceramic fiber-reinforced metal-matrix composites.

This appendix provides supplemental information on the metallurgical aspects of atomic structure, the use of dislocation theory, heat treatment processes and procedures, important engineering materials and strengthening mechanisms, and the nature of elastic, plastic, and creep strain components. It also provides information on mechanical property and fatigue testing, the use of hysteresis energy to analyze fatigue, a procedure for inverting equations to solve for dependent variables, and a method for dealing with the statistical nature of failure.

A design modification intended to reduce dowel bolt failures in an aircraft engine proved ineffective, prompting an investigation to determine what was causing the bolts to break. As the chapter explains, failure specimens were examined under various levels of magnification and subjected to chemical analysis and low-cycle fatigue tests. Based on their findings, investigators concluded that the bolts failed due to fatigue compounded by excessive clearances and poor surface finishes. The chapter provides a number of recommendations addressing these issues and related concerns.

Mechanical tests are performed to evaluate the durability of gears under load. The chapter first discusses the processes involved in the computations of stress for test parameters of gear. Next, the chapter reviews the four areas of specimen characterization of a test program, namely dimensional, surface finish texture, metallurgical, and residual stress. The following section presents the tests that simulate gear action, namely the rolling contact fatigue test, the single-tooth fatigue test, the single-tooth single-overload test, and the single-tooth impact test. Finally, the chapter describes the test procedures for surface durability (pitting), root strength (bending), and scoring (or scuffing) testing.

This appendix contains abbreviations and symbols related to aluminum alloy castings.

This data set contains the results of rotating-beam reversed-bending fatigue tests for a wide range of aluminum casting alloys. These fatigue curves are the results of tests on individual lots of material considered representative of the respective alloys and tempers.

This article reviews fatigue test methodologies, provides an overview of general fatigue behavior (crack initiation and propagation) in engineering plastics, and discusses some of the factors affecting the fatigue performance of polymers. In addition, it provides information on fractography that provides useful insight into the nature of fracture processes.