Tensile Testing, also known as tension testing, is a destructive engineering and materials science test whereby controlled tension is applied to a sample either as a load for proof testing or until it fully fails.
This is one of the most common mechanical testing techniques. It is used to find out how strong a material is and also how much it can be stretched before it breaks. This test method is used to determine yield strength, ultimate tensile strength, ductility, strain hardening characteristics, Young's modulus and Poisson's ratio.
Ultimate Tensile Strength (UTS)
The UTS is the maximum stress that a specimen is exposed to during testing. This may differ from the specimen's strength when breaking depending on if it is brittle, ductile or has properties of both. These material properties can change depending on environment, for example in extreme hot or cold conditions.
Hooke's Law defines the relationship between the applied force and the elongation shown in the test specimen where the ratio of stress to strain is constant.
Modulus of Elasticity
The modulus of elasticity measures the stiffness of a specimen whereby the material will return to its original condition once the load has been removed. Once the material has been stretched to the point where it no longer returns to its original length and permanent deformation is shown, Hooke's Law no longer applies. This is known as the elastic or proportonal limit.
The yield strength is the point at which plastic deformation occurs under stress. This is determined during testing over a measured gauge length via the use of devices known as extensometers. The devices may be either be mechanical clip on or video where non-contact is a limitation, e.g. elevated temperature testing.
The Offset Method is used on materials where the tensile properties make the yield strength difficult to determine, such as with metals or plastics.
In materials that do not have a well-defined linear region, alternative methods are used to determine the modulus, including secant modulus and tangent modulus.
The strain of a specimen is the measurement of the change in length that the item undergoes during testing divided by the original gauge length. Known as Engineering strain or nominal strain.
Tensile Specimen Preparation
Tensile test specimens are prepared in a variety of ways depending on the test specifications. The most commonly used specifications are BS EN ISO 6892-1 and ASTM E8M. Most specimens use either a round or square standard cross section with two shoulders and a reduced section gauge length in between. The shoulders allow the specimen to be gripped while the gauge length shows the deformation and failure in the elastic region as it is stretched under load. The reduced cross section gauge length of specific dimensions assists with accurate calculation of engineering stress via load over area calculation.