ASTM D1708 is a standardized test method to determine the tensile properties of plastic microtensile specimens. The tensile properties reported are yield stress, yield strain, tensile strength, break stress, and break strain. Modulus of elasticity shall not be determined when using D1708’s microtensile specimen.
ASTM D1708 is recommended for labs that have a historical material property data that was obtained through D1708 testing. If no such historical data exists, it is recommended to refer to ASTM D638 or ASTM D882. If material is limited for testing purposes, ASTM D638’s Type V specimen is similar in size to the microtensile specimen defined in D1708.
Plastic manufacturers must design plastic to meet the requirements of the end application. Whether they are supplying plastic to produce biomedical devices, bicycle helmets, or children toys, each application has specific strength requirements in addition to safety, weight, environmental resistance, and other requirements. Determining the tensile properties of a polymer provides useful strength and deformation data for the plastics engineering process. It is worth noting that the properties of plastics have a high sensitivity to the rate which rate they are pulled (tension) or pushed (compression). It is important to recognize this sensitivity as properties experienced in the end application can differ from those observed during a controlled test. For this reason, ASTM D1708 and other plastics testing standards are generally used for quality control in a production setting or for comparative purposes in a R&D setting.
Materials Testing System
As typical forces will not exceed 1 kN (225 lbf) and displacements will not exceed 2 inches (50 mm), we recommend a low force single column test frame such as those found in Instron's 3400 and 6800 series.
Gripping can be a challenge when testing subsized specimens. ASTM D1708 allows a minimum specimen shoulder length (the region of the specimen that is gripped) of 8 mm, which can cause difficulties due to insufficient specimen insertion into the grip’s jaw face. When insufficient specimen insertion exists, it is recommended to use grips that do not allow jaw face pivot as it can cause unwanted stress concentrations along the specimen. With sufficient specimen insertion, Instron recommends its 250 N capacity pneumatic side-action grips (2712-052) with serrated jaw faces. With insufficient jaw face insertion, Instron’s 1 kN capacity, manual wedge action grips (2716-016) are recommended.
Due to their small size, microtensile specimens can increase the chances of a pinch hazard when operators clamp the specimen with pneumatic grips. Instron’s 3400 and 6800 frames have an optional Smart-Close Air Kit that is fully integrated into the frame and software. The Smart-Close Air Kit minimizes pinch hazards by closing the jaw faces at a reduced speed and pressure during installation. When the test is ready to begin full pressure is applied, and the test routine starts.