Metallic Stretching and Creep Testing Methods
Metals used in structural components and for industrial and engineering applications must be tested for their mechanical properties. Stretching and creep testing is one such form of testing which a range of metals must go through in order to determine their resistance to strain and deformation under different loading conditions.
Stretching tests assess the force needed to elongate a sample, also known as tensile strength. Creep tests evaluate how a sample responds to extended time and a sustained amount of force. This type of testing is especially important for safety-critical applications such as electronics cooling components, helicopter rotors, or large bridges.
Stretching Tests
To begin testing, a sample is selected and cut according to the shape and size given in the applicable ASTM standard. A cross section is then measured and given in the report. This test is conducted using a standard tensile test machine. Specimens are strained until they achieve the specified amount of deformation, and the maximum force needed to achieve that deformation is measured as the yield or ultimate strength.
The longer the sample, the easier it is to deform without fracturing. For this reason, a clearly defined, uniform sample is preconditioned by stretching it to the first point of yield (i.e., the point of maximum force) and then releasing it. This is done over several cycles so that at the time of testing, the sample is “pre-stretched” and has a much greater resistance to deformation.
Creep Tests
Creep testing is done by loading a specimen to the specified load, maintaining that load for a definite period of time, and then measuring the amount of strain or relaxation during and after that time, respectively. Generally, creep tests require substantial amounts of time, so the magnitude of strain (or relaxation) is calculated by measuring its rate over time.
A typical creep test includes heating the sample in an oven to a temperature of about 100ºC and then stretching it to the desired percentage of elongation. The load is then maintained for a specified duration (typically 1000 hours) and the amount of strain is monitored over that period of time. The strain is then converted to a creep strain rate which is then used to predict the creep-rupture strength of the material.
Conclusion
Metallic stretching and creep testing methods provide a safe and reliable way to measure the mechanical properties of metals. By understanding the physical and chemical nature of a metal, as well as its performance in different loading conditions, it is possible to find and use the optimal material for any application.
