A tension test is a mechanical test that measures how much force a material can tolerate, before it tears or breaks when it is pulled. Materials such as metal, rubber, plastic, or fiber can be tested using a universal mechanical testing instrument or other kinds of tensile test machines. Testing equipment can be adjusted to provide the desired amount of force and according to how long the material is subjected to it. Graphs are used to identify at what level of force a material fails, and what its ultimate tensile strength is, which the strongest load put on the material during the test.
Principles that help scientists interpret the results of a tension test include "Hooke’s Law," which describes the correlation of force to how a material elongates during a test. The area on a graph where the two variables correspond is referred to as linear. Material stiffness, called the modulus of elasticity, is identifiable while the material still retains its integrity after being subjected to the load. The material is permanently altered when it reaches its elastic limit, even when the load is taken away. At this point a graphical line will change direction from the straight, consistent path found at the start of the test.
Yield strength is when a material starts to show strain under load during a tension test. Strain can be measured as a ratio of how the length changes during the test to its length beforehand, or as the length at any given moment during the test. Tensile testing systems are operated by securing the material into the machine, and triggering the cross heads in the machine to apply force. The rate of how force is applied can be adjusted. Sometimes the temperature is adjusted as well to determine how the material responds to loads when it is hot or cold.
When a material is subject to a tension test, it is prepared in samples that take the form of cylinders or flat plates. Wires and fibers that are in the form of a tube are tested in their regular form using the appropriate grips in tensile test machines. Materials testing is conducted by engineers who need to choose a material for an application, in manufacturing to monitor the quality of products, and to analyze the strength of materials used in construction. A tension test also helps researchers see how long a product, under specific conditions, can last before it is no longer usable.
By Andrew Kirmayer