Think Tank : How important is materials testing?


Engineered structures and machines, or their associated components, tend to fail in service because of fracture or excessive deformation. To prevent such failures, the materials or component designer estimates how much stress, or load per unit area, needs to be tolerated during in-service conditions, and specifies materials that can withstand the anticipated stresses. Mechanical property tests, play an important role, indicating which materials may safely be employed in various service environments.

Non-destructive and destructive materials testing examines the mechanical loading of a material up to break or up to a specific deformation. The tests can take place under different environmental conditions to replicate in-service influences.

Through material characteristic values, materials testing delivers a clear definition of the material properties, which in turn allows for the comparison between different materials.

Materials testing is not only performed at research institutes, but it also helps companies obtain valuable knowledge for the development of new products, and the ongoing improvement of existing products.

A variety of test methods can be applied in materials testing. In quasi-static or monotonic materials testing, loading on the specimen is relatively slow and constant. In static materials testing the strength and deformation behaviour of specimens and components, predominantly subjected to tension, compression, and flexure, as well as shearing or torsion, is determined. Static materials testing, relative to dynamic materials testing is performed with lower test speeds.

Static tension tests require test specimens, usually cylindrical or flat, a test machine with appropriate force capacity that applies, measures, and records various loads and specimen extensions, and an appropriate set of grips to firmly hold the test piece under load. In the static tension test, the test machine uniformly elongates the test sample. A portion of the test section, referred to as the gauge length, is measured at different loads using a device called an extensometer and these measurements are used to calculate strain.

Testing machine specimen grips are designed to transfer the load smoothly into the test piece without producing localised stress concentrations. The ends of the test piece are usually enlarged so that if slight concentrations of stress are present these will be directed to the specimen gauge section, and specimen failures will occur only where the measurements are being taken.

For dynamic testing the specimen is subjected to an impact load, or the load periodically influences the specimen over a longer period. Dynamic materials testing refers to the (destructive) test on materials or components, which is performed with quick or dynamic movement. Examples include pendulum impact testers, drop weight testers, or high-speed tensile testing systems.

In cyclic materials testing, loading on the specimen takes place in continuously recurring load cycles. Depending on the machine, these load cycles can be in the form of tensile/compression, pulsating or alternating load in sinusoidal shape, triangle shape, etc.

Materials that comfortably 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 values.

The stresses acting on a material during in-service conditions, are usually random rather than cyclic. Several cumulative fatigue damage theories have been generated to enable engineers to extrapolate from cyclic test data, predictions of material behaviour under random stresses. These theories are not always applicable to many materials, and a technique, which involves mechanical application of pseudo-random fatigue stresses, statistically matched to in-service conditions, is now employed in the majority materials testing laboratories.

In destructive testing, specimens are extracted from a parent material or component and tested for mechanical loads. The specimen is usually destroyed or significantly altered and after the conclusion of the test, the tested component, or material specimen can no longer be used.

Destructive materials testing particularly, plays an important role in the automotive industry and aerospace engineering, since here material fatigue presents a very high-risk factor. However, materials and component testing has also become indispensable in many sectors including medical engineering.

In most test methods the specimen is destroyed and rendered unusable. These include the tension test, compression test, flexure test, fatigue test, fracture mechanics, impact test, sheet metal forming, shear test, biaxial test, and creep test.

In non-destructive testing (NDT), the quality of a specimen is tested without causing damage. In this way it can be ensured that the material quality is high enough for further processing and that it can reliably withstand loads for the long-term. Non-destructive test methods include hardness testing, friction test, rebound test, component test and functionality testing.

The quality of a material going into a manufactured product is as important as the reliability of the production process. Materials testing helps us to understand and quantify whether a specific material or treatment is suitable for a particular application or environment and complements considerations related to product safety and long-term durability.

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