Ridged plastics such as unreinforced, reinforced and high-modulus composites can be tested for quality using a variety of tests, including compression tests. One of the more common methods of compression testing is ASTM D695. This test is useful for determining the modulus of elasticity, yield stress, deformation beyond yield point, and compressive strength.


When testing your ridged plastic with ASTM D695, make sure you choose specimens that are made to the sizes specified in the standard and are free of visible surface flaws. In the case of the plastic specimen shattering during compression testing, the compressive strength has a very definite value. When the material does not fail in compression by a shattering fracture, the compressive strength can be arbitrary depending upon the degree of distortion that is regarded as indicating complete failure of the material. Many plastic materials will continue to deform in compression until a flat disk is produced, the compressive stress (nominal) rising steadily in the process, without any well-defined fracture occurring. Compressive strength can have no real meaning in such cases.

Testing Procedure

First, measure the width and thickness of the specimen at several points along its length. Calculate and record the minimum value of the cross-sectional area. Next, measure the length of the specimen and record the value. Place the test specimen between the surfaces of the compression tool, taking care to align the center line of its long axis with the center line of the plunger and to ensure that the ends of the specimen are parallel with the surface of the compression tool. Now adjust the crosshead until it just contacts the top of the compression tool plunger. If you only seek strength measurements, then set speed control at 1.3 mm/min (0.050 in./min) and start the machine. Record the maximum load carried by the specimen during the test. If stress-strain or design data is desired, then use an extensometer. Set the speed control at 1.3 mm/min (0.050 in./min) and start the machine. Record loads and corresponding compressive strain at appropriate intervals of strain or record the complete load-deformation curve.

A universal testing machine can be used for ASTM D695. Other fixtures may be helpful during testing such as an anti-bucking compression fixture and an axial extensometer. By means of our plastic testing expertise and modular product design, we will help find the testing solution that is right for you. Give one of our application engineers a call today for help with creating the best budget and testing plan according to ASTM D695

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