When subjected to a tension (pulling apart), a material elongates and eventually breaks. A simple static tension test determines the breaking point of the material and its elongation, designated as strain (change in length per unit length).
A static tensile strength test is commonly used to determine two things: whether a material can withstand a specified load, and, what is the ultimate breaking (tensile) strength of a material. These are not necessarily the same thing. These tests are often used to determine the acceptable working loads of component materials, or entire product assemblies.
It defines the point at which a material will no longer elastically deform (return to original state), plastically deform (does not return to original state), and fail. The ultimate tensile strength of a material will usually far exceed that of the working load of a material. This difference helps to establish a safety margin to offset any variance in the material during manufacture
The most common testing machine used in tensile testing is the universal testing machine. This type of machine has two crossheads; one is adjusted for the length of the specimen and the other is driven to apply tension to the test specimen. There are two types: hydraulic powered and electromagnetically powered machines.
Materials that survive a single application of stress frequently fail when stressed repeatedly. This phenomenon, known as fatigue, is measured by mechanical tests that involve repeated application of different stresses varying in a regular cycle from maximum to minimum value.
“Fatigue” testing gives data to predict the in-service life of materials. stresses acting upon a material in the real world are usually random in nature rather than cyclic.
It usually uses higher speeds and frequencies, and up to millions of cycles when compared to static testing.
Material fatigue involves a number of phenomena, among which are atomic slip (in which the upper plane of a metal crystal moves or slips in relation to the lower plane, in response to a shearing stress), crack initiation, and crack propagation. Thus, a fatigue test may measure the number of cycles required to initiate a crack, as well as the number of cycles to failure.
A cautious designer always bears the statistical nature of fatigue in mind, for the lives of material specimens tested at a common stress level always range above and below some average value. Statistical theory tells the designer how many samples of a material must be tested in order to provide adequate data; it is not uncommon to test several hundred specimens before drawing firm conclusions.
Many materials, sensitive to the presence of flaws, cracks, and notches, fail suddenly under impact. The most common impact tests employ a swinging pendulum to strike a notched bar; heights before and after impact are used to compute the energy required to fracture the bar and, consequently, the bar’s impact strength. In nonmetal tests, however, the striking hammer falls vertically in a guide column, and the test is repeated from increasing heights until failure occurs.
Some materials vary in impact strength at different temperatures, becoming very brittle when cold. Tests have shown that the decrease in material strength and elasticity is often quite abrupt at a certain temperature, which is called the transition temperature for that material. Designers always specify a material that possesses a transition temperature well below the range of heat and cold to which the structure or machine is exposed. Thus, even a building in the tropics, which will doubtless never be exposed to freezing weather, employs materials with transition temperatures slightly below freezing.