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 focuses on the life assessment methods for elevated-temperature failure mechanisms and metallurgical instabilities that reduce life or cause loss of function or operating time of high-temperature components, namely, gas turbine blade, and power plant piping and tubing. The article discusses metallurgical instabilities of steel-based alloys and nickel-base superalloys. It provides information on several life assessment methods, namely, the life fraction rule, parameter-based assessments, the thermal-mechanical fatigue, coating evaluations, hardness testing, microstructural evaluations, the creep cavitation damage assessment, the oxide-scale-based life prediction, and high-temperature crack growth methods.

This article discusses the methods for assessing creep-rupture properties, particularly, nonclassical creep behavior. The determination of creep-rupture behavior under the conditions of intended service requires extrapolation and/or interpolation of raw data. The article describes the various techniques employed for data handling of most materials and applications of engineering interest. These techniques include graphical methods, methods using time-temperature parameters, and methods used for estimations when data are sparse or hard to obtain. The article reviews the estimation of required creep-rupture properties based on insufficient data. Methods for evaluation of remaining creep-rupture life, including parametric modeling, isostress testing, accelerated creep testing, evaluation by the Monkman-Grant coordinates, and the Materials Properties Council (MPC) Omega method, are also reviewed.

The approaches to spectrum life prediction in components can be classified into two types, namely, history-based methods, using the life-fraction rule or other damage rules, and postservice evaluation methods. This article discusses the variables affecting the material crack growth rate behavior and those essential elements in making spectrum crack growth life prediction. It provides information on life assessment for bulk creep damage.

Predicting the service life of structural components involves creep-fatigue crack growth (CFCG) testing under pure creep conditions. This article provides a discussion on the loading condition and the type of ductile and brittle material showing creep behavior. It focuses on a description of the experimental method that should be followed in conducting tests of CFCG with various hold times. The article describes the testing conditions, definitions, and the necessary calculations of various crack-tip parameters considered during static and cyclic loading in time-dependent fracture mechanics. The parameters considered for static loading are C*, C(t), C*(t), C*h, Ct, and Cst(t). For cyclic loading, the parameters are delta Jc and (Ct)avg. An overview of life-prediction models is also provided.

This article focuses on the subject of proactive or predictive maintenance with particular emphasis on the control and prediction of corrosion damage for life extension and failure prevention. It discusses creep life assessment from the perspective of creep-rupture properties and creepcrack growth. Practical methods based on replication and parametric approaches are also discussed.

The lifetime assessment of polymeric products is complicated, and if the methodology utilized leads to inaccurate predictions, the mistakes could lead to financial loss as well as potential loss of life, depending on the service application of the product. This article provides information on the common aging mechanisms of polymeric materials and the common accelerated testing methods used to obtain relevant data that are used with the prediction models that enable service life assessment. Beginning with a discussion of what constitutes a product failure, this article then reviews four of the eight major aging mechanisms, namely environmental stress cracking, chemical degradation, creep, and fatigue, as well as the methods used in product service lifetime assessment for them. Later, several methods of service lifetime prediction that have gained industry-wide acceptance, namely the hydrostatic design basis approach, Miner's rule, the Arrhenius model, and the Paris Law for fatigue crack propagation, are discussed.

Fitness-for-service assessment procedures can be used to assess the integrity, or remaining life, of components in service. Depending on the operating environment and the nature of the applied loading, a structure can fail by a number of different modes: brittle fracture, ductile fracture, plastic collapse, fatigue, creep, corrosion, and buckling. This article focuses on the broad categories of these failure modes: fracture, fatigue, environmental cracking, and high-temperature creep. It also discusses the benefits of a fitness-for-service approach.

This article reviews the basic mechanisms of elevated-temperature behavior and associated design considerations, with an emphasis on metals. It discusses the key concepts of elevated-temperature design. These include plastic instability at elevated temperatures; deformation mechanisms and strain components associated with creep processes; stress and temperature dependence; fracture at elevated temperatures; and environmental effects. The article describes the basic presentation and analysis methods for creep rupture. It provides information on the application of these methods to materials selection and the setting of basic design rules. The article examines the limitations of high-temperature components as well as the alternative design approaches and tests for most high-temperature components.

This article presents typical problems encountered in the analysis of experimental creep and creep-rupture data and the possible solutions to these drawbacks. It provides information on planning the test and creep strain/time relationships. The exponential creep equation and the rational polynomial creep equation are discussed. The article also describes the dependence of stress and temperature on equation parameters and explains the lot-centered regression analysis.

This article presents effective stress equations that are based on the von Mises criterion, the Tresca criterion, and the Huddleston criterion. It describes the calculation of effective stresses for different cases: elastic stresses, steady-state creep stresses, stresses in a fully plastic case, and thermal stresses in a tube. The article illustrates the comparison of life predictions by the stress criteria and presents a simple mean diameter hoop stress equation, which is used for designing components. It also provides information on the multiaxial creep ductility of tubular components and multiaxial testing methods.

Thermomechanical fatigue (TMF) refers to the process of fatigue damage under simultaneous changes in temperature and mechanical strain. This article reviews the process of TMF with a practical example of life assessment. It describes TMF damages caused due to two possible types of loading: in-phase and out-of-phase cycling. The article illustrates the ways in which damage can interact at high and low temperatures and the development of microstructurally based models in parametric form. It presents a case study of the prediction of residual life in a turbine casing of a ship through stress analysis and fracture mechanics analyses of the casing.

The overarching goal of life-prediction research is to develop models for the various types of time dependencies in the crack-tip damage accumulation that occur in materials subjected to elevated temperatures. This article focuses on describing the models based on creep, oxidation kinetics, evolution of crack-tip stress fields due to creep, oxygen ingress, and change in the microstructure. It also provides a summary of creep-fatigue modeling approaches.

This article describes the general aspects of creep, stress relaxation, and yielding for homogeneous polymers. It then presents creep failure mechanisms in polymers. The article discusses extrapolative methods for the prediction of long-term creep failure in polymer materials. Then, the widely used models to simulate the service life of polymers are highlighted. These include the Burgers power-law model, the Findley power-law model, the time-temperature superposition (or equivalence) principle (TTSP), and the time-stress superposition principle (TSSP). The Larson-Miller parametric method, one of the most common to describe the material deformation and rupture time, is also discussed.

This article reviews the applied aspects of creep and stress-rupture failures. It discusses the microstructural changes and bulk mechanical behavior of classical and nonclassical creep behavior. The article provides a description of microstructural changes and damage from creep deformation, including stress-rupture fractures. It also describes metallurgical instabilities, such as aging and carbide reactions, and evaluates the complex effects of creep-fatigue interaction. The article concludes with a discussion on thermal fatigue and creep fatigue failures.

Thermomechanical fatigue (TMF) is the general term given to the material damage accumulation process that occurs with simultaneous changes in temperature and mechanical loading. TMF may couple cyclic inelastic deformation accumulation, temperature-assisted diffusion within the material, temperature-assisted grain-boundary evolution, and temperature-driven surface oxidation, among other things. This article discusses some of the major aspects and challenges of dealing with TMF life prediction. It describes the damage mechanisms of TMF and covers various experimental techniques to promote TMF damage mechanisms and elucidate mechanism coupling interactions. In addition, life modeling in TMF conditions and a practical application of TMF life prediction are presented.

The principal types of elevated-temperature mechanical failure are creep and stress rupture, stress relaxation, low- and high-cycle fatigue, thermal fatigue, tension overload, and combinations of these, as modified by environment. This article briefly reviews the applied aspects of creep-related failures, where the mechanical strength of a material becomes limited by creep rather than by its elastic limit. The majority of information provided is applicable to metallic materials, and only general information regarding creep-related failures of polymeric materials is given. The article also reviews various factors related to creep behavior and associated failures of materials used in high-temperature applications. The complex effects of creep-fatigue interaction, microstructural changes during classical creep, and nondestructive creep damage assessment of metallic materials are also discussed. The article describes the fracture characteristics of stress rupture. Information on various metallurgical instabilities is also provided. The article presents a description of thermal-fatigue cracks, as distinguished from creep-rupture cracks.

This article provides an overview of the structural design process and discusses the life-limiting factors, including material defects, fabrication practices, and stress. It details the role of a failure investigator in performing nondestructive inspection. The article provides information on fatigue life assessment, elevated-temperature life assessment, and fitness-for-service life assessment.

Life assessment of structural components is used to avoid catastrophic failures and to maintain safe and reliable functioning of equipment. The failure investigator's input is essential for the meaningful life assessment of structural components. This article provides an overview of the structural design process, the failure analysis process, the failure investigator's role, and how failure analysis of structural components integrates into the determination of remaining life, fitness-for-service, and other life assessment concerns. The topics discussed include industry perspectives on failure and life assessment of components, structural design philosophies, the role of the failure analyst in life assessment, and the role of nondestructive inspection. They also cover fatigue life assessment, elevated-temperature life assessment, fitness-for-service life assessment, brittle fracture assessments, corrosion assessments, and blast, fire, and heat damage assessments.

Structural alloys are commonly subjected to a variety of thermal and thermomechanical loads. This article provides an overview of the experimental methods in thermal fatigue (TF) and thermomechanical fatigue (TMF) and presents experimental results on the structural materials that have been considered in TF and TMF research. Life prediction models and constitutive equations suited for TF and TMF are covered. The structural materials discussed include carbon steels, low-alloy steels, stainless steels, aluminum alloys, and nickel-base high-temperature alloys. The article explains crack initiation and crack propagation in TF and TMF. It describes thermal ratcheting and thermal shock behavior of structural metallic materials. The article concludes with information on life prediction of structural materials under TF and TMF.