Compression Testing Explained: Methods, Standards, and Applications
Representative Infinita Engineering Visual explaining the four-step workflow for Compression Testing.What Is ASTM D695 Compression Testing?
ASTM D695 is the Standard Test Method for Compressive Properties of Rigid Plastics. It determines how unreinforced and reinforced rigid plastics — including high-modulus composites up to a compressive modulus of 41,370 MPa — behave under uniform compressive loading, generating data on compressive modulus, yield stress, and compressive strength.
Specimen Preparation
- Block specimens: 12.7 x 12.7 x 25.4 mm (0.5 x 0.5 x 1 in)
- Cylindrical specimens: 12.7 mm diameter x 25.4 mm length (a slenderness ratio requirement applies for rod samples, typically 11-16:1)
- Minimum specimen count: five for isotropic materials, ten for anisotropic materials
- For thin laminates (under 3.2 mm) or high-modulus composites, an anti-buckling support jig is used to prevent premature buckling failure
Test Procedure
Specimen dimensions are measured and the specimen is centered in a compression fixture with platens aligned parallel to the loading axis — platen-to-platen parallelism is critical to valid results. The testing machine applies a compressive load at a constant crosshead speed until yield or fracture. Compressive stress-strain data is captured throughout, with a compressometer or extensometer used to measure strain when compressive modulus is required. The maximum load sustained is divided by the original cross-sectional area to calculate compressive strength; for materials that flatten without a distinct fracture, compressive strength is defined at an arbitrary specified deformation.
Also Read – ASTM D2850: Triaxial Compression Testing of Soils — Method & Applications
Reported Properties
- Compressive modulus (elastic region slope of the stress-strain curve)
- Compressive strength (maximum stress sustained)
- Yield stress and strain, where a yield point exists
- Engineering and true stress-strain curves
Industry Specifications Referencing ASTM D695
- Related composite methods: ASTM D3410/D3410M and D6641/D6641M for oriented fiber-reinforced composites
- EN 2850 Method B, SACMA SRM-1R-94, and Boeing BSS 7260 Type III & IV: variants developed for high-modulus/reinforced laminates using compression support fixtures
Conclusion
ASTM D695 is a foundational compressive property test for rigid plastics used across automotive and aerospace component design. Because thin or high-modulus specimens are prone to buckling rather than true compressive failure, proper fixture selection — supported vs. unsupported — is essential to getting a valid, comparable result.
How is the compression test performed? The specimen is placed between two parallel compression plates in a universal testing machine. A controlled load is applied until the specimen reaches a specified deformation or fails.
Why is specimen alignment important? The specimen must be centred and aligned between the compression plates. Incorrect alignment can create bending or uneven stress, resulting in inaccurate and inconsistent measurements.
What factors can affect compression test results? Results may be influenced by specimen dimensions, loading speed, temperature, moisture, surface condition, manufacturing process, and material orientation. Consistent preparation and conditioning are essential.
Which rigid plastics can be tested? Common materials include ABS, nylon, polycarbonate, PVC, acrylic, thermosetting plastics, filled polymers, and fiber-reinforced composites. The method can be adapted to many rigid polymer systems.
Why is compression testing important? It helps confirm whether a plastic component can withstand expected service loads without excessive deformation, buckling, or failure. The results support material selection, product design, quality control, and safety validation.
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