Experiments on fracture of materials are made for various purposes. Of primary importance are those through which criteria predicting material failure by deformation and/or fracture are investigated. Since the demands of engineering application always precede the development of theories, there is another kind of experiment where conditions under which a particular material can fail are simulated as closely as possible to the operational situation but in a simplified and standardized form. In this way, many of the parameters corresponding to fracture such as toughness, Charpy values, crack opening distance (COD), etc. are measured. Obviously, a sound knowledge of the physical theories governing material failure is necessary as the quantity of interest can seldom be evaluated in a direct manner. Critical stress intensity factors and critical energy release rates are examples. Standard test of materials should be distinguished from basic experi ments. They are performed to provide routine information on materials responding to certain conditions of loading or environment. The tension test with or without a crack is among one of the most widely used tests. Because they affect the results, with size and shape of the specimen, the rate of loading, temperature and crack configuration are standardized to enable comparison and reproducibility of results. The American Society for Testing Materials (ASTM) provides a great deal of information on recommended procedures and methods of testing. The objective is to standardize specifications for materials and definition of technical terms.
