Compression Properties ASTM D695
ASTM D695: The Standard for Measuring Compressive Properties of Plastics
ASTM D695 is a widely recognized test method used to determine the compressive properties of plastics, both reinforced and unreinforced. This standard is critical in understanding the fundamental material properties of polymers, which are found in every industry and household worldwide. Along with its tensile test equivalent, ASTM D638, these two standards provide valuable insights into the behaviour of plastics when subjected to different stresses and loads. We have included an overview of the method towards the end of this page to help you.
What Does This Standard Measure?
ASTM D695 measures the compressive strength, compressive yield point, and modulus of a material. These properties describe how the material behaves under compression, which is important information for engineers and designers working with plastic components. The test specimen used for strength determination is typically a right cylinder or prism with a length that is twice its principal width or diameter. If the specimen is too thin, an anti-buckling support jig should be used to prevent buckling.
Scope of Test for ASTM D695
Objective: This testing methodology is established to determine the mechanical behavior of both unreinforced and reinforced rigid plastics, inclusive of high-modulus composites. The materials are subjected to compression at specified, low, uniform strain or load rates using standardized test specimens. The procedure is relevant for a composite modulus as high as 41,370 MPa (6,000,000 psi).
Units of Measurement: All measurements and values should be recognized in SI units as the primary standard. Any values provided within parentheses serve informational purposes only.
Relevant Test Methods: For assessment of compressive attributes in resin-matrix composites with oriented continuous, discontinuous, or cross-ply reinforcements, reference can be made to Test Methods D3410/D3410M or D6641/D6641M.
Safety Precautions: This standard doesn't encompass every potential safety concern related to its application. It's incumbent upon the users of this standard to adopt suitable safety protocols and to validate the relevance of regulatory constraints before its application. A dedicated precautionary directive can be found in Section 13.1.
Note: This test standard aligns with ISO 604.
Test Procedure for ASTM D695
Specimen Measurement:
Measure the specimen's width and thickness (or diameter) at multiple points along its length to an accuracy of 0.025 mm (0.001 in.).
Calculate the minimum value of the cross-sectional area and document.
Measure and record the length of the specimen.
Specimen Placement in Compression Tool:
Properly position the test specimen between the compression tool surfaces.
Ensure the center line of the specimen's long axis aligns with the center line of the plunger.
Confirm the ends of the specimen are parallel with the compression tool's surface.
Adjust the testing machine's crosshead until it slightly touches the top of the compression tool plunger.
Note: Compression tool might not be essential for testing lower modulus material (e.g., 700 MPa to 3500 MPa or 100,000 psi to 500,000 psi) if loading surfaces remain smooth, flat, and parallel.
Thin Specimen Placement in Jig:
Place thin specimens flush with the base and centered in the specified jig
Ensure nuts or screws on the jig are finger tight.
Insert the assembled unit into the compression tool.
Positioning the specimen flush with the base of the jig is recommended for convenience.
Ensure the jig's tightness is maintained as indicated for reproducible data.
For Compressive Strength/Yield Strength Analysis:
Set machine speed to 1.3 mm/min (0.050 in./min) and initiate.
Document the peak load experienced by the specimen during the assessment, typically the load at rupture.
For Stress-Strain Data Collection:
Equip the compressive strain indicator to directly measure specimen strain.
Set machine speed to 1.3 mm/min (0.050 in./min) and start.
Record loads and associated compressive strain at suitable strain intervals. If the test machine has an auto-recording feature, note the entire load-deformation curve.
Post the yield point, you can increase speed to 5-6 mm/min (0.20 to 0.25 in./min) until specimen failure. This adjustment is permitted only for relatively ductile materials and on machines with quick-response weighing systems.
Specimen Size
The specimen can be either a block or cylinder. For ASTM, the typical blocks are 12.7 x 12.7 x 25.4mm (½ by ½ by 1 in). and the cylinders are 12.7mm (½ in) in diameter and 25.4mm (1 in) long. For ISO, the preferred specimens are 50 x 10 x 4mm for modulus and 10 x 10 x 4mm for strength.
Data Analysis
Compressive strength and modulus are two useful calculations derived from the test. Compressive strength is calculated as the maximum compressive load divided by the minimum cross-sectional area. Compressive modulus is calculated as the change in stress divided by the change in strain.
Advice
Compression platens with integral spherical seats ensure platen to platen parallelism, a requirement of the standard.
To prevent buckling, use an anti-buckling support jig for thin specimens.
When reporting modulus calculation, verify both the extensometer and load cell within the calculation range.
ASTM D695 is part of the Plastics test method suite in Bluehill Universal.
If direct strain measurement and ASTM E83 Class B-2 compliance are not needed, use compliance correction to eliminate system compliance from crosshead displacement data.
Refer to the compliance correction whitepaper for more information.
Conclusion
ASTM D695 is a valuable test method for evaluating the compressive properties of plastics. Engineers and designers can use the results of this test to understand how different types of plastics behave under compression, which is critical information for selecting the right materials for a given application. By reviewing the full standard, professionals can gain a comprehensive understanding of the test setup, procedure, and result requirements.
Key Terms:
Compressive properties, compressive strength, modulus of elasticity, plastics.