4mechanicsofmaterials.pptVIP

Chapter 4 Mechanics of Materials We might take the obvious definition that a material has failed when it has broken into two or more parts. However, it has already been pointed out that in most applications a member would be unserviceable due to excessive distortion long before it actually ruptured . Consequently, we will relate failure to yielding and consider that a material has failed when it will no longer return to its original shape upon release of the loads. In a simple tensile test we would then say that a ductile([′d?ktail] adj.（金属等）延性的，可延展的；（粘土等）可塑的，柔软的) material has failed when the material begins to yield. Then for uniaxial stress, failure occurs when the stress reaches the yield stress, in either tension or compression. The maximum stress, based on the original area, developed in a material before rupture is called the ultimate strength of the material, and the term may be modified as the ultimate tensile, compressive, or shearing strength of the material. Ductile materials undergo considerable plastic tensile or shearing deformation before rupture. True strain, like true stress, is computed on the basis of the actual length of the test specimen during the test and is used primarily to study the fundamental properties of materials. Strains are usually relatively small in materials used in engineering structures, often less than 0.1%, and their accurate determination require special measuring equipment. 真正的应变，和实际应力一样，在实验当中试样的实际长度的基础上计算出来的，而且主要用来研究材料的基本性能。 用于工程 结构中的材料，应变相对较小，通常低于0.1 ％ ，它的准确得出需要特殊的测量设备。 The difference between nominal stress and strain, computed from initial dimensions of the specimen, and true stress and strain is negligible([′neɡlid??bl] adj.可不计的，可忽视的)for stresses usually encountered in engineering structures