Search Results for constant extension test

This chapter discusses the methodology of the tensile test and the effect of some of the variables on the tensile properties. The methodology and variables discussed are shape of the item being tested, method of gripping the item, method of applying the force, determination of strength properties other than the maximum force required to fracture the test item, ductility properties to be determined, speed of force application or speed of elongation, and test temperature. The chapter presents the definitions of the basic terms and their units, along with discussions of basic stress-strain behavior and the differences between related terms, such as stress and force and strain and elongation. It considers the parts of a tensile test, namely, test-piece preparation, geometry, and material condition; test setup and equipment; and test procedures. The chapter provides information on post-test measurements and describes the effect of strain concentrations and strain rate on tensile properties.

This chapter focuses on mechanical behavior under conditions of uniaxial tension during tensile testing. It begins with a discussion of properties determined from the stress-strain curve of a metal, namely, tensile strength, yield strength, measures of ductility, modulus of elasticity, and resilience. This is followed by a section describing the parameters determined from the true stress-true strain curve. The chapter then presents the mathematical expressions for the flow curve. The chapter reviews the effect of strain rate and temperature on the stress-strain curve and describes the instability in tensile deformation and stress distribution at the neck in the tensile specimen. It discusses the processes involved in ductility measurement and notch tensile test in tensile specimens. The parameter that is commonly used to characterize the anisotropy of sheet metal is covered. Finally, the chapter covers the characterization of fractures in tensile test specimens.

This appendix is a compilation of terms, definitions, abbreviations, and symbols associated with the mechanics and mechanisms of fracture.

This chapter reviews the concepts of fracture mechanics and their application to materials evaluation and the design of cryogenic structures. Emphasis is placed on an explanation of technology, a review of fracture mechanics testing methods, and a discussion on the many factors contributing to the fracture behavior of materials at cryogenic temperatures. Three approaches of elastic-plastic fracture mechanics are covered, namely the crack opening displacement, the J-integral, and the R-curve methods. The chapter also discusses the influence of thermal and metallurgical effects on toughness at low temperatures.

Fracture mechanics is the science of predicting the load-carrying capabilities of cracked structures based on a mathematical description of the stress field surrounding the crack. The fundamental ideas stem from the work of Griffith, who demonstrated that the strain energy released upon crack extension is the driving force for fracture in a cracked material under load. This chapter provides a summary of Griffith’s work and the subsequent development of linear elastic and elastic-plastic fracture mechanics. It includes detailed illustrations and examples, familiarizing readers with the steps involved in determining strain energy release rates, stress intensity factors, J-integrals, R-curves, and crack tip opening displacement parameters. It also covers fracture toughness testing methods and the effect of measurement variables.

This chapter provides a detailed overview of the creep behavior of metals and how to account for it when determining the remaining service life of components. It begins with a review of creep curves, explaining how they are plotted and what they reveal about the operating history, damage mechanisms, and structural integrity of the test sample. In the sections that follow, it discusses the effects of stress and temperature on creep rate, the difference between diffusional and dislocation creep, and the use of time-temperature-stress parameters for data extrapolation. It explains how to deal with time dependent deformation in design, how to estimate cumulative damage under changing conditions, and how to assess the effect of multiaxial stress based on uniaxial test data. It also includes information on rupture ductility, creep fracture, and creep-crack growth and their effect on component life and performance.

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.

This chapter is a compilation of terms and definitions related to tensile testing.

This chapter reviews the current technology and examines force application systems, force measurement, strain measurement, important instrument considerations, gripping of test specimens, test diagnostics, and the use of computers for gathering and reducing data. The influence of the machine stiffness on the test results is also described, along with a general assessment of test accuracy, precision, and repeatability of modern equipment. The chapter discusses various types of testing machines and their operations. Emphasis is placed on strain-sensing equipment. The chapter briefly describes load condition factors, such as strain rate, machine rigidity, and various testing modes by load control, speed control, strain control, and strain-rate control. It provides a description of environmental chambers for testing and discusses the processes involved in the force verification of universal testing machines. Specimen geometries and standard tensile tests are also described.

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 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 chapter addresses the challenge of selecting an appropriate stress-corrosion cracking (SCC) test to evaluate the serviceability of a material for a given application. It begins by establishing a generic model in which SCC is depicted in two stages, initiation and propagation, that further subdivide into several zones plus a transition region. It then discusses SCC test standards before describing basic test objectives and selection criteria. The chapter explains how to achieve the required loading conditions for different tests and how to prepare test specimens to determine elastic strain, plastic strain, and residual stress responses. It also describes the difference between smooth and precracked specimens and how they are used, provides information on slow-strain-rate testing and how to assess the results, and discusses various test environments and procedures, including tests for weldments. The chapter concludes with a section on how to interpret time to failure, threshold stress, percent survival, stress intensity, and propagation rate data, and assess the precision of the associated tests.

This chapter focuses on creep-rupture failure, or more precisely, the time required for such a failure to occur at a given stress and temperature. It begins with a review of creep-rupture phenomena and the various ways creep-rupture data are presented and analyzed. It then examines a large collection of creep-rupture data corresponding to different alloy designations and heat treatments, identifying key relationships, similarities, and differences. It also presents a test method developed by the authors in which twelve materials are tested over a range of temperature, stress, and time in order to determine multiheat constants that are then used to fit multiheat data from other materials and thus estimate rupture times.

This chapter discusses the concept of mean stress and explains how it is used in fatigue analysis and design. It begins by examining the stress-strain response of test samples subjected to cyclic forces and strains, noting important features and what they reveal about materials and their fatigue behaviors. It then discusses the challenge of developing hysteresis loops for complex loading patterns and accounting for effects such as ratcheting and stress relaxation. The sections that follow provide a summary of the various ways mean stress is described in the literature and the methods used to calculate or predict its effect on the fatigue life of machine components. The discussion also sheds light on why tensile mean stress is detrimental to both fatigue life and ductility, while compressive mean stress is highly beneficial.

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.