Plastics tensile standards, such as ASTM D638 and ISO 527-2, cover a range of plastics including thermosets, thermoplastics, and fiber-reinforced plastics. Test specimens may be rigid, semi-rigid, molded or extruded plastics, and are commonly in the shape of a dumbbell or dogbone.
The mechanical properties of plastics vary depending on the type of plastic, as well as the additives that may be incorporated into the formulation. Properties, such as strength, ductility and toughness, are influenced by different types of additives. To determine the appropriate testing equipment, it is important to understand how material behavior changes as a result of the various additives. Additives such as talc or glass will stiffen a material while other additives such as rubber will increase the ductility of a plastic. As these material characteristics change, equipment requirements will often change.
If a material becomes more ductile as a result of adding rubber to the formulation, we suggest using a long travel extensometer to measure strain to failure. The opposite is true regarding additives that stiffen materials. Stiffer materials generally strain less and have smaller elastic regions. Since modulus is measured within the elastic region, high-resolution extensometers are often required to accurately measure modulus.
Selecting the appropriate extensometer can be a difficult task when interested in measuring both modulus and strain to failure. For some types of extensometers, such as a static clip-on extensometer, accuracy is a function of the amount of extensometer travel. The more travel an extensometer is capable of performing, the lower the accuracy. This makes it difficult to measure both modulus and strain to failure for plastics that have relatively high strain. Since the standard requires direct strain measurement only up until the yield point, static clip-on extensometers are suitable for the determination of elastic modulus modulus..
Automatic extensometers, such as the MFX 500 and the Advanced Video Extensometer (AVE),
have high accuracy throughout the full range of travel. This makes them ideal for applications that require high accuracy earlier on in the test, but also the ability to measure strain to failure. These extensometers can be used on manual testing systems, as well as fully-automated testing systems that utilize robotic specimen handling.
A suitable system for testing these types of samples would be our 3300, 5500 or 5900 electromechanical testing machine with mechanical wedge grips. Pneumatic side acting grips are also suitable as long as the appropriate capacity is used. The choice of jaw faces may vary depending on sample type, but serrated faces are most common. Both the MFX 500 and Advanced Video Extensometer (AVE) are recommended.