Compression Testing
Compression testing is used to determine compression strength, usually applied to materials of high compression but low tensile strength, in which the specimen is subjected to increasing compressive forces until failure occurs. Compression tests are used to determine the material behavior under a load. The maximum stress a material can maintain during a defined time period under a load (constant or progressive) is determined.
A compression test determines behavior of materials under crushing loads. The specimen is compressed and deformation at various loads is recorded. Compressive stress and strain are calculated and plotted as a stress-strain diagram which is used to determine elastic limit, proportional limit, yield point, yield strength and, for some materials, compressive strength. When the limit of compressive strength is reached, brittle materials are crushed.
A compression test is simply the opposite of the tension test with respect to the direction of loading. In compression testing the sample is squeezed while the load and the displacement are recorded. Compression tests result in mechanical properties that include the compressive yield stress, compressive ultimate stress, and compressive modulus of elasticity.
Compression testing is frequently done to a specific limit or to a break (rupture). Depending on the type of material being tested, break detection can be defined when the test is performed to a break. If the test is performed to a specific limit, either a load limit or deflection limit is used.
Ultimate compressive strength is the stress required to rupture a specimen. This value is much harder to determine for a compression test than it is for a tensile test since many materials do not exhibit rapid fracture in compression. Materials such as most plastics that do not rupture can have their results reported as the compressive strength at a specific deformation such as 1%, 5%, or 10% of the sample's original height.
For materials such as concrete, the compressive strength is the most important material property that engineers use when designing and building a structure. Compressive strength is used to determine whether a concrete mixture meets the requirements of the job specifications.
Common compression testing results are:
• Load at Rupture
• Deflection at Rupture
• Work at Rupture
• Maximum Load
• Deflection at Maximum Load
• Work at Maximum Load
• Stiffness
• Chord Slope
• Offset Yield
• Stress
• Strain
TEST STANDARDS:
• ASTM E9 - 09 Standard Test Methods of Compression Testing of Metallic Materials at Room Temperature
• ASTM C864 - 05(2011) Standard Specification for Dense Elastomeric Compression Seal Gaskets, Setting Blocks, and Spacers
• ASTM D575 - 91(2012) Standard Test Methods for Rubber Properties in Compression
• ASTM D905 - 08e1 Standard Test Method for Strength Properties of Adhesive Bonds in Shear by Compression Loading
• ASTM D1710 - 08 Standard Specification for Extruded Polytetrafluoroethylene (PTFE) Rod, Heavy Walled Tubing and Basic Shapes