Search Results for Strain

This chapter discusses the fundamentals of plastic deformation and the role of strain and strain rate in sheet metal forming processes. It describes the conditions associated with uniform deformation, the significance of engineering and true strain, the effect of volume constancy on the tensile response of isotropic and anisotropic materials, and how infinitesimal strains or strain rates are used to express and analyze instantaneous deformation and local stain. It also discusses the concept of principal strain and strain paths and explains how to determine, and when to use, equivalent strain and strain rate.

This chapter discusses the role of plastic deformation in forging and the effect of strain and strain rate on metal flow. It demonstrates the use of stress tensors and shows how metal flow can be represented qualitatively by the displacement of volume elements and quantitatively by the distribution of velocity components and strain rates. It describes the conditions associated with homogeneous deformation in a frictionless upset forging and explains how they can also be obtained using engineering and true stress-strain curves.

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.

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 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.

High strain rate tensile testing is used to understand the response of materials to dynamic loading. The behavior of materials under high strain rate tensile loads may differ considerably from that observed in conventional tensile tests. This chapter discusses the processes involved in determining strain rate effects in tension by conventional tensile tests and covers expanding ring tests, flat plate impact tests, split-Hopkinson pressure bar tests, and rotating wheel tests.

This chapter provides a detailed analysis of the cyclic stress-strain behavior of materials under uniaxial stress and strain cycling. It first considers the case of a stable material under constant-amplitude strain cycling then broadens the discussion to materials that harden or soften with continued strain reversals. It compares and contrasts the response patterns of such materials, explaining how the movement of dispersed particles and dislocations influences their behavior. It then examines the behavior of materials under uniaxial strain reversals of varying amplitude and explains how to construct double-amplitude stress-strain curves that account for complex straining histories. For special cases, those involving complex materials such as gray cast iron or highly complex straining patterns, the chapter presents other methods of analysis, including the rainflow cycle counting method, mechanical modeling based on displacement-limited elements, Wetzel’s method, and deformation modeling. It also explains the difference between force cycling and stress cycling and presents alternate techniques for predicting whether a material will become harder or softer in response to strain cycling.

The stress-strain curves in this data set are representative examples of the behavior of several cast alloys under tensile or compressive loads. The curves are arranged by alloy designation. Each figure cites the original source of the curve and provides pertinent background information as available. Compressive tangent modulus curves are presented for certain alloys. The effects of cyclic loading are given on several curves.

This chapter discusses three measurements parameters: temperature, strain, and magnetic field strength. It stresses the measurement of temperature because it is the primary variable in nearly all low-temperature material properties. The chapter contains information on methods and auxiliary materials. Areas of frequent concern, such as thermal contact, heat leak, thermal anchoring, thermal conductivity of greases, insulators, lead wires, ground loops, and feedthroughs are also reviewed. The chapter provides an overview and historical development of temperature scales because the practical use of all thermometers is associated with some approximation of the thermodynamic temperature scale. A short section is devoted to types of temperature measuring devices. The characteristics of commercially available resistance-type strain gauges at low temperatures are stressed.

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 appendix lists etch compositions and procedures that reveal strain patterns in aluminum and nickel-base superalloys.