Compression Testing of Rigid Plastics
ASTM D695 describes a test method for rigid plastics in compression. There are a few variations in the test, depending on the size of the sample being used.
The test can be performed very basically between two compression platens on a universtal testing machine. It can also be performed at a more advanced level using strain gauges and a special subpress compression fixture. For reduced size samples, a special compression jig is used to hold the specimen
The most popular specimen size is a 12.7mm diameter cylinder which is at least 2 times as tall as the diameter. Rod samples are different and must have a slenderness ratio of 1 to 11-16. Tube samples can be tested under this specification as well as reduced size samples with diameters less than 3.2mm.
The special ASTM D695 jig uses rectangular flat samples which look similar to a tensile test specimen. The height is usually 79.4mm and the thickness from 1mm to 12.7mm.
Basic Mode: The test can be performed quickly and easily between two compression platens as shown in the video. Performing the test in this fashion may not meet the standards for axial parellelism between the two compression surfaces.
Subpress: The subpress has two guide rods on the side to help with the parallelism requirement. The guided press translates a much more uniform, controlled, and even compression to the plastic material.
Compression Jig for Sub-Size samples: A special ASTM D695 compression fixture is needed for sub-sized samples with diameter of less than 3.2mm. The specimen stands straight up and down in the fixture, with the dogboned ends of the sample sticking out of the top and bottom. The bottom of the sample rests on the base of the square block at the bottom of the fixture. The top of the sample sticks out and is compressed with the top compression platen. The jig helps to prevent the sample from buckling while it is compressed. The fixture should be assembled and tightened by hand. Tightening the jig together with wrenches can compress the sample from the sides and skew data results.
Dual-Column Universal Testing Machine: The Galdabini line of testing systems can arrive pre-programmed and ready to perform this test out of the box. These testing machines are very easy to use, affordable, and can arrive as a plug & play solution.
Compressive Yield Strength is taken at the yield point and divided by the area of thickness of the sample. The units are given in megapascals or pounds per square inch. Offset yield and Modulus of Elasticity (MOE) are also taken.
Slight variations of this fixture can cover ASTM D3846, prEN 3850, ISO 14126, GOST 9550-81, SACMA-SRM1-Type 1 and SACMA-SRM1-Type 2.