Mechanical Testing of Materials

Abstract

Of the many properties of materials the mechanical ones are of great significance, since they deal with the principal phenomena regarding stability under force. Deformation under applied forces and the fracture of materials depend on their structure. The macroscopic responses of materials to the acting forces may result in their changing shape or even disintegrating, if these forces are sufficiently large. Interatomic forces must be overcome by external forces in order to cause shape changes in a material, which may eventually lead to its separation into two or more parts, depending on the atomic forces which resist any structural change, either in shape or dimension. The overall macroscopic behavior and the changes occurring in materials are inspected, tested experimentally and described in terms of the acting force per unit area, namely stress and the displacement per unit distance or strain. In a perfectly ideal material, free of lattice defects (which, in reality, does not exist, except in the form of whiskers), tremendous forces are required to cause the above changes. Real crystals contain various defects; lattice defects, such as dislocations, are responsible for the ease of deformation, which may often be observed in functioning machine elements. Mechanical engineers are expected to prevent this from occurring. To this end, laboratory tests must be performed in order to realize the practical potential of a given material of interest. In this chapter, a discussion of the mechanical behavior of materials on a macroscopic scale is described as observed during laboratory experiments intended to forecast the actual performance in real service. (An understanding of their real behavior will be the subject of Chap. 2.) The observed behavior, as revealed by the various laboratory tests, will be considered in terms of dislocations, providing a basic conceptual framework for the mechanical properties of materials.