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.
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.
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 StrengthThe 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.
Compression tests are performed on universal testing machines. These machines are available in a variety of different sizes and force capacities ranging from 0.02 N to 2,000 kN. Most low force testing is performed on a tabletop machine such as Instron's 6800 Series, while higher force applications require floor model frames such as those found in Instron's Industrial Series.
A universal testing machine must be equipped with testing software and application-specific platens and accessories, such as extensometers. The type of material being tested will determine the type of accessories needed, and a single machine can be adapted to test any material within its force range simply by changing the fixturing.
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