Mechanical Testing of Materials

Tensile or tension testing is a fundamental and most commonly used test for the characterization of the mechanical behavior of materials. The test consists of pulling a sample of material and measuring the load and the corresponding elongation. Main properties measured from the test include Young’s modulus, yield, ultimate and breaking strength, ductility, resilience, tensile toughness and Poisson’s ratio. The tensile properties are used in selecting materials for engineering applications and in research and development of new materials or processes. In this chapter we will present the basic characteristics of the time-independent tensile test. More specifically, we will discuss the testing procedure, the specimen characteristics, the engineering and true stress–strain curves, the yield and strain hardening behavior, the hysteresis loop in plastic deformation, the Bauschinger effect, the state of stress in the neck area, the approximate McGregor method, idealized and analytical expressions of stress–strain curves, the definitions of ductility, resilience and tensile toughness, the Poisson’s ratio, the bulk modulus, and standards for the tensile test. Finally, we will present values of material properties obtained from the tension test for some engineering metals and polymers.

In this chapter we consider the compression, bending, torsion and multiaxial testing of materials. Compression is a fundamental test as it is tension. The initial portions of stress–strain curves in compression for most materials have the same characteristics as in tension. Various material properties such as modulus of elasticity, elastic and proportional limit, yield stress may be defined from the initial portion of the stress–strain curve in compression. Many materials, even though they have the same elastic behavior in tension and in compression, their post-yield and failure characteristics in compression are quite different than in tension. We present the specimen types and the stress–strain curves in compression, and we compare the material behavior in compression to that in tension. Also, we present the bending and torsion tests and a series of tests for the multiaxial characterization of materials. They include biaxial tension tests, biaxial tension/compression strip tests, tube tests, spherical vessel tests, combined tension/compression–torsion ring tests, combined tension/compression–torsion tube tests, combined tension/compression–torsion–internal pressure tube tests. Finally, we present failure envelopes under multiaxial loading for isotropic, anisotropic and uneven materials.

In this chapter we present indentation methods for measuring the modulus of elasticity and the hardness of materials at macro, micro and nanoscale levels. For macroindentation testing we present the Brinell, Meyer, Vickers and Rockwell tests; for microindentation the Vickers and Knoop tests; for nanoindentation the elastic contact method and nanoindentation tests for measuring the fracture toughness of brittle materials at small volumes and the interfacial fracture toughness of thin films on substrates using conical and wedge indenters.

Fracture mechanics constitutes a powerful method for the determination of the load-carrying capacity of structures and machine components in the presence of cracks. This approach is in contrast to traditional failure criteria (maximum stress/strain, Tresca, von Mises, Coulomb–Mohr, etc.) which ignore the presence of defects. Since structures and machine components cannot be constructed without defects, on the grounds of practicality, fracture mechanics is used to determine either the safe operating load for a prescribed crack size or the safe crack size for a prescribed operating load. Design by fracture mechanics necessitates a parameter known as fracture toughness which characterizes the resistance of a material to crack extension. Fracture toughness is a material property and it should be size independent. It expresses the ability of a material to resist fracture in the presence of cracks. Fracture toughness is analogous to the yield or ultimate stress used in design by the conventional failure criteria. In this chapter we consider the following fracture mechanics failure criteria: the stress intensity factor criterion, the J-integral criterion, the crack opening displacement criterion and the strain energy density criterion, and present the experimental procedure for the determination of fracture toughness for each criterion. Furthermore, we discuss the stress intensity factor failure criterion for dynamic fracture.

Fatigue is the process of damage and failure of materials and structures due to cycling loading. It was demonstrated that under repeated loading materials fail at stresses well below the ultimate strength, and microscopic damage accumulates until cracks or other forms of macroscopic damage develop. Failure due to fatigue loading is called “fatigue failure”. The number of loading cycles leading to failure of structural or machine components is called fatigue life. The main objective of fatigue analysis is to determine the fatigue life for a repeated fluctuating load of constant or variable amplitude. In this chapter we will consider two major approaches for the study of fatigue failure. The stress-based approach which is based on nominal stresses and does not account on the mechanisms of fatigue failure, and the fracture mechanics approach which considers the micro and macromechanisms of fatigue failure and uses the principles of fracture mechanics for initiation and propagation of cracks. This approach provides a better understanding of fatigue failure by analyzing the initiation, propagation and instability processes of fatigue cracks. For both approaches we present the mechanical tests performed on specimens subjected to cyclic loading. The objective of the tests is to generate fatigue life and crack growth data and demonstrate the safety of a material or structure. The chapter concludes with a study of the environment-assisted fracture.

Creep is the slow, continuous, inelastic, permanent deformation of materials that increases with time under the action of constant stress. Generally occurs at high temperature, but can also happen at room or very low temperatures, albeit much slower. Creep results in large mechanical stress when loads are applied for long time. The rate of creep deformation is a function of the material’s properties, exposure time, temperature and applied load. In this chapter we first present rheological models for the understanding of the three major types of material deformation including elastic, plastic and creep deformation. Based on these models we give constitutive equations for steady-state and transient creep. Also we study relaxation behavior, which is the decrease of stress when a material is held at constant strain. Linear and nonlinear creep and relaxation are analyzed. Finally, we present mechanical tests for the creep, recovery and relaxation characterization of materials.

In this chapter we present methods of material testing at high strain rates. Mechanical behavior of materials at high strain rates differs considerably from that at quasi-static rates. Special testing equipment, methods and experimental arrangements are needed. We will present the following tests: high-speed load frames for low rates, drop weight impact and pendulum impact including the Charpy and Izod tests for intermediate rates, the split-Hopkinson (Kolsky) bar impact for compression, tension and torsion, the Taylor impact and the expanding ring for high rates, and the plate impact for very high rates. In all tests we will provide the testing equipment, experimental procedure and instrumentation, data analysis and discussion. The chapter concludes with the ASTM standards for testing at high strain rates.

Nondestructive testing (NDT) methods include a wide group of analysis techniques to evaluate the properties of a material, component or system without causing damage. They are important for the assessment of fabrication quality and for the detection of early damage through alteration in microstructures leading to premature failure. They allow the investigator to carry out examinations without invading the integrity of the engineering component under investigation. In this chapter we will present the following NDT methods: dye penetrant, magnetic particles inspection, eddy currents, radiography, ultrasonics and acoustic emission.

Concrete is a heterogeneous composite material full of cracks, which play a vital role in its mechanical behavior and failure. A distinct characteristic of concrete is that it presents a softening behavior, as opposed to steels which present a hardening behavior. In concrete the crack-tip region is accompanied by a large fracture process zone in which microfailure mechanisms including matrix microcracking, debonding of cement-matrix interface, crack deviation and branching take place. All these mechanisms contribute to the energy of fracture. Concrete structures exhibit the so-called size effect according to which the failure load of a structure depends on its size. The size effect can only be explained with fracture mechanics concepts, while classical theories, such as elastic or plastic limit analysis cannot take into consideration this effect. In this chapter we briefly present the basic tests for the characterization of the mechanical behavior of concrete. They include compression, tension and bending tests. Furthermore, we present the basic principles of fracture mechanics applied to concrete, test methods for the determination of the critical strain energy release rate and an analysis of the size effect of concrete structures.

Composites are produced from two or more constituent materials with dissimilar chemical or physical properties that are emerged to create a material with properties unlike the individual elements. Of the various types of composites the most important category is the fiber composites, in which a matrix is reinforced with continuous fibers. Fiber composites are anisotropic materials that need special methods for the study of their mechanical behavior, much more complicated than those of isotropic materials. In this chapter we present test methods for the mechanical characterization of a unidirectional lamina of fiber reinforced composite materials under tension, compression and shear. We also present methods for the determination of the interlaminar fracture toughness of laminates under mode-I, II, III and mixed-mode loading. Finally, we introduce the sandwich materials, and study their failure mechanisms.