This chapter compares and contrasts the high-temperature behaviors of metals and composites. It describes the use of creep curves and stress-rupture testing along with the underlying mechanisms in creep deformation and elevated-temperature fracture. It also discusses creep-life prediction and related design methods and some of the factors involved in high-temperature fatigue, including creep-fatigue interaction and thermomechanical damage.

Creep occurs in any metal or alloy at a temperature where atoms become sufficiently mobile to allow the time-dependent rearrangement of structure. This chapter begins with a section on creep curves, covering the three distinct stages: primary, secondary, and tertiary. It then provides information on the stress-rupture test used to measure the time it takes for a metal to fail at a given stress at elevated temperature. The major classes of creep mechanism, namely Nabarro-Herring creep and Coble creep, are then covered. The chapter also provides information on three primary modes of elevated fracture, namely, rupture, transgranular fracture, and intergranular fracture. The next section focuses on some of the metallurgical instabilities caused by overaging, intermetallic phase precipitation, and carbide reactions. Subsequent sections address creep life prediction and creep-fatigue interaction and the approaches to design against creep.

Boiler tubes subjected to cyclic or fluctuating loads over extended periods of time are prone to fatigue failure. Fatigue can occur at relatively low stresses and is implicated in almost 80% of the tube failures in firetube boilers. This chapter covers the most common forms of boiler tube fatigue, including mechanical or vibrational fatigue, corrosion fatigue, thermal fatigue, and creep-fatigue interaction. It discusses the causes, characteristics, and impacts of each type and provides several case studies.

This chapter provides a detailed review of creep-fatigue analysis techniques, including the 10% rule, strain-range partitioning, several variants of the frequency-modified life equation, damage assessment based on tensile hysteresis energy, the OCTF (oxidation, creep, and thermomechanical fatigue) damage model, and numerous methods that make use of creep-rupture, crack-growth, and void-growth data. It also discusses the use of continuum damage mechanics and includes examples demonstrating the accuracy of each method as well as the procedures involved.

This chapter describes the phenomenological aspects of fatigue and how to assess its effect on the life of components operating in high-temperature environments. It explains how fatigue is measured and expressed and how it is affected by loading conditions (stress cycles, amplitude, and frequency) and factors such as temperature, material defects, component geometry, and processing history. It provides a detailed overview of the damage mechanisms associated with high-cycle and low-cycle fatigue as well as thermal fatigue, creep-fatigue, and fatigue-crack growth. It also demonstrates the use of tools and techniques that have been developed to quantify fatigue-related damage and its effect on the remaining life of components.

Strain-range partitioning is a method for assessing the effects of creep fatigue based on inelastic strain paths or strain reversals. The first part of the chapter defines four distinct strain paths that can be used to model any cyclic loading pattern and describes the microstructural damages associated with each of the four basic loading cycles. The discussion then turns to fatigue life prediction for different types of materials and more realistic loading conditions, particularly those in which hysteresis loops have more than one strain-range component. To that end, the chapter considers two cases. In one, the relationship between strain range and cyclic life is established from test data. In the other, a rule is required to determine the damage of each concurrent strain and the total damage of the cycle is used to predict creep-fatigue life. The chapter presents several such damage rules and discusses their applicability in different situations.

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 outline of the failure modes and mechanisms associated with most boiler tube failures in coal-fired power plants. Primary categories include stress rupture failures, water-side corrosion, fire-side corrosion, fire-side erosion, fatigue, operation failures, and insufficient quality control.

This chapter provides an overview of factors that must be considered in the design of structural components for satisfactory service performance in terms of mechanical behavior of steel castings. The chapter discusses designing against yielding, excessive deflection, and creep and stress rupture. The chapter describes the three main approaches to evaluating and designing structures relative to fatigue resistance: the S-N curve approach for high cycle fatigue, the strain range approach for low cycle fatigue, and the fracture mechanics approach. Two approaches to design against brittle fracture are described, the ductile to brittle transition concept and the fracture mechanics approach. The chapter also discusses several types of corrosion behavior and emphasizes the need to interact with corrosion specialists in the design process. It illustrates the unique advantages that designers may gain by designing components as castings to achieve low stress concentrations economically.

Durability is a generic term used to describe the performance of a material or a component made from that material in a given application. In order to be durable, a material must resist failure by wear, corrosion, fracture, fatigue, deformation, and exposure to a range of service temperatures. This chapter covers several types of component and material failure associated with wear, temperature effects, and crack growth. It examines temperature-induced, brittle, ductile, and fatigue failures as well as failures due to abrasive, erosive, adhesive, and fretting wear and cavitation fatigue. It also discusses preventative measures.

This chapter demonstrates the versatility of the strain-range partitioning method and its application to creep-fatigue problems involving complex loading histories. It begins with a derivation showing that it is possible to assess the damage of hysteresis loops combining two or more strain ranges using generic loops based on fundamental data. It then explains how to treat problems involving sequential loading with both healing and damage cycles and presents a general solution for combining two loops with arbitrary amounts of the four strain-range components. The chapter also derives closed-form equations that account for interactions among any number of adjacent loops and can be used, through successive application, to analyze any loading history.

Combustion turbines consist of a compressor, a combustor, and a turbine. As commonly configured, the compressor and turbine mount on a single shaft that connects directly to a generator. This chapter reviews the materials of construction, damage mechanisms, and life-assessment techniques for nozzles and buckets. It also presents key information from a detailed review of the literature and the results of a survey on combustion-turbine material problems.

This chapter covers the failure modes and mechanisms of concern in steam turbines and the methods used to assess remaining component life. It provides a detailed overview of the design considerations, material requirements, damage mechanisms, and remaining-life-assessment methods for the most-failure prone components beginning with rotors and continuing on to casings, blades, nozzles, and high-temperature bolts. The chapter makes extensive use of images, diagrams, data plots, and tables and includes step-by-step instructions where relevant.

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.

Increasing the efficiency of power plants by operating at higher temperatures and pressures and adding a double-reheat feature comes at the expense of shortened lifetimes for critical components. This chapter provides an overview of the material-related problems associated with advanced steam plants and their respective solutions. The discussion covers the selection of materials on a component-specific basis for boilers as well as steam turbines.

This chapter explains why it is sometimes necessary to separate inelastic from elastic strains and how to do it using one of two methods. It first discusses the direct calculation of strain-range components from experimental data associated with large strains. It then explains how the method can be extended to the treatment of very low inelastic strains by adjusting tensile and compressive hold periods and continuous cycling frequencies. The chapter then begins the presentation of the second approach, called the total strain-range method, so named because it combines elastic and inelastic strain into a total strain range. The discussion covers important features, procedures, and correlations as well as the use of models and the steps involved in predicting thermomechanical fatigue (TMF) life. It also includes information on isothermal fatigue, bithermal creep-fatigue testing, and the predictability of the method for TMF cycling.

This chapter briefly outlines some of the basic aspects of failure analysis, describing some of the basic steps and major concerns in conducting a failure analysis. A brief review of failure types from fracture, distortion, wear-assisted failure, and environmentally assisted failure (corrosion) is also provided.

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 addresses the cumulative effects of fatigue and how to determine its impact on component lifetime and performance. It begins by defining a loading history and its corresponding hysteresis loops that exposes the deficiencies of some of the theories discussed. It then proceeds to demonstrate the methods commonly used to analyze cumulative fatigue damage and its effect on component life starting with the classical linear damage rule. After pointing out the inherent limitations of the model, it presents a method that incorporates two linear damage rules, one applying prior to crack initiation and the other after the crack has started. Although the method accounts somewhat better for loading-order effects, the transition in behavior that the rules presume to model occurs prior to any signs of cracking. Two modified versions of the double linear damage rule method, neither of which are related to a physical crack initiation event, are subsequently presented along with several applications showing how the different methods compare. The examples provided include two-level and multilevel tests, a gas-turbine engine compressor disk, and the cumulative damage associated with the irreversible hardening of type 304 stainless steel.

This chapter familiarizes readers with the mechanisms involved in creep and how they are related to fatigue behavior. It explains that what we observe as creep deformation is the gradual displacement of atoms in the direction of an applied stress aided by diffusion, dislocation movement, and grain boundary sliding. It describes these mechanisms in qualitative terms, explaining how they are driven by thermal energy and how they can be analyzed using creep curves and deformation maps. In addition, it examines the types of damage associated with creep, presents a number of creep strain and strain rate equations, explains how to determine creep constants, and reviews the findings of several studies on cyclic loading. It also discusses the development of a novel test that measures the cyclic creep-rupture resistance of materials in tension and compression.