What is Compression Testing?

Compression testing is one of the most fundamental types of mechanical testing, alongside tensile and flexion tests. Compression tests are used to determine a material’s behavior under applied crushing loads, and are typically conducted by applying compressive pressure to a test specimen (usually of either a cuboid or cylindrical geometry) using platens or specialized fixtures on a universal testing machine. During the test, various properties of the material are calculated and plotted as a stress-strain diagram which is used to determine qualities such as elastic limit, proportional limit, yield point, yield strength, and, for some materials, compressive strength.

 

Compression testing on an Instron

 

Why Perform a Compression Test?

Compression testing allows manufacturers to assess the integrity and safety of materials, components, and products during several phases of the manufacturing process. The potential applications can vary from strength testing of a car windshield to endurance testing of concrete beams used in construction. Materials that exhibit high tensile strength tend to (but do not always!) exhibit low compressive strength. Likewise, materials high in compressive strength tend to exhibit low tensile strength. Therefore, compression testing is often used on brittle materials such as concrete, metals, plastics, ceramics, composites, and corrugated materials like cardboard. These materials are often used in a load-bearing capacity where their integrity under compressive forces is critical.

compression test stress/strain curve

 

Unlike tensile tests, which are usually conducted to determine the tensile properties of a specific material, compression tests are often performed on finished products. Common items such as tennis balls, golf balls, water bottles, protective cases, plastic pipes, and furniture are all examples of products that need to be evaluated for their compressive strength. For example, an engineer may want to conserve plastic by creating water bottles with thinner walls, but the bottles must still be strong enough to be packed in pallets and stacked on top of each other for transport. Compression testing can help the engineer fine tune the balance between product strength and material conservation.

Ultimate Compressive Strength

The ultimate compressive strength of a material is the value of compressive stress reached when the material fails completely. When brittle materials reach their ultimate compressive strength they are crushed, and the load drops drastically. Materials with higher ductility, (most plastics) do not rupture, but instead continue deforming  until the load is no longer being applied to the specimen, but rather between the two compression platens. In these cases, compressive strength can be reported as specific deformations such as 1%, 5%, or 10% of the test specimen's original height.

Industry-Specific Testing Standards

There are many ASTM and ISO standards related to compression testing a variety of materials. For example, the furniture, automotive, and mattress industries follow ASTM D3574, which measures the indention force deflection of polyurethane foam. This test measures the initial softness of the foam by measuring the force when the foam is compressed to 25% of its original thickness. The test then measures how supportive it is by measuring the force when it is compressed to 65% of its original thickness. Automotive seating engineers specify the indentation force deflection value of the foam they want in their final product, and the manufacturing location will perform the test several times per shift to guarantee that each and every seat being manufactured has the same feel in regards to softness and supportiveness.


Common Compression Test Standards

ASTM D3574-17 - Standard Test Methods for Flexible Cellular Materials

ASTM D695-15 - Compressive Properties of Rigid Plastics

AITM 0010 - Airbus Test Method for Determination of Compression Strength After Impact

ASTM C109 - Compression Testing 2-Inch Concrete Cubes

ASTM D2844 - Moisture Exudation Test Compacted Soils

ASTM D3846 - In-Plane Shear Strength of Reinforced Plastics

ASTM D575-91 - (2012) Method A Rubber Properties in Compression

ASTM F1306 - Slow Rate Penetration Resistance of Flexible Barrier Films and Laminates

ASTM F2267 - Evaluating Spinal Invertebral Body Fusion Devices under Axial Compression

EN12430 - Point Compression of Thermal Insulation

ISO 7886-1 - Testing Sterile Hypodermic Syringes

ISO 844 - Compression Properties of Rigid Cellular Plastics