In a large class of engineering structures it is essential that the materials employed should be both strong and ductile, so that not only shall the structure be able to resist heavy loads, but that if by any chance it is overloaded it shall not collapse suddenly. In order to ascertain whether a metal is suitable for a particular structure, its strength and ductility are determined experimentally. The test most commonly in use consists in applying a gradually increasing pull to a bar of the metal until fracture takes place. The maximum load supported per unit of the original cross-sectional area of the bar is called the tensile strength or tenacity of the metal, and the elongation of an initial measured length, expressed as a proportion of that length, is called the extension, and is used as an index of the ductility of the metal.