Search Results for high-temperature fatigue

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 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.

Fatigue failures occur due to the application of fluctuating stresses that are much lower than the stress required to cause failure during a single application of stress. This chapter describes three basic factors that cause fatigue: a maximum tensile stress of sufficiently high value, a large enough variation or fluctuation in the applied stress, and a sufficiently large number of cycles of the applied stress. The discussion covers high-cycle fatigue, low-cycle fatigue, and fatigue crack propagation. The chapter then discusses the stages where fatigue crack nucleation and growth occurs. It describes the most effective methods of improving fatigue life. The chapter also explains the effect of geometrical stress concentrations on fatigue. In addition, it explores the environmental effects of corrosion fatigue, low-temperature fatigue, high-temperature fatigue, and thermal fatigue. Finally, the chapter discusses a number of design philosophies or methodologies to deal with design against fatigue failures.

This chapter gives a brief overview of the role of fatigue in component failures. It presents examples of fatigue failures along with statistics on the causes and costs of fatigue damage in various industries. It also includes a chapter-by-chapter summary of the content in the book, noting that the book deals primarily with fatigue at temperatures below the creep range with high-temperature fatigue being treated in a companion publication.

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 explains how the authors assessed the potential risks of creep-fatigue in several aerospace applications using the tools and techniques presented in earlier chapters. It begins by identifying the fatigue regimes encountered in the main engines of the Space Shuttle. It then describes the types of damage observed in engine components and the methods used to mitigate problems. It also discusses the results of analyses that led to changes in design or approach and examines fatigue-related issues in turbine engines used in commercial aircraft.

This chapter compares and contrasts empirical approaches for partitioning hysteresis loops and predicting creep-fatigue life. The first part of the chapter presents experimental partitioning methods, explaining how they can be used to partition any loading cycle into its basic strain-range components. The methods covered include rapid cycling between peak stress extremes, half-cycle rapid loading and unloading, and variations of the incremental step-stress approach. The methods are then compared based on their ability to predict creep-fatigue life. The chapter goes on from there to describe how fatigue life can be estimated from ductility measurements when cyclic data are unavailable or are likely to change. It also explains how cyclic life is influenced by the time-dependent nature of creep-plasticity and the physical and metallurgical effects of environmental exposure.

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.

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 is largely a compendium of best practices and procedures for minimizing the effects of fatigue. It explains how to make products more resistant to fatigue by choosing the right materials and manufacturing processes, avoiding geometries and features that concentrate strains, preventing or removing surface damage, and by inducing compressive mean stresses that prolong fatigue life. It also discusses the use of property conditioning and restoration treatments, the benefits of interference fits and processes such as coaxing, the effects of assembly damage and operating overload, the importance of surface cleanliness and finish, and the role of inspection, testing, replacement, and repair in safe-life and fail-safe designs. Examples highlighting the benefits and potential pitfalls of proof loading tests are included as well.

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 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.

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.

A low-pressure turbine rotor blade failed in service, causing extensive engine damage. A section of the blade broke off around 25 mm from the root platform, producing a flat fracture surface that appeared smooth on one end and grainy elsewhere. Based on their examination, investigators concluded that the nickel-base superalloy blade was exposed to high temperatures and stresses, initiating a crack that propagated under cyclic loading. This chapter provides a summary of the investigation and the insights acquired using scanning electron fractography, metallography, and hardness measurements.