Compression testing provides information about the compressive properties of the test specimen. As both rubber and plastic materials are frequently used in compressive applications, compression test procedures are common for these materials. These properties include compressive strength, compressive stress, compressive strain, deformation, compressive yield point and yield strength, Modulus of Elasticity, Secant Modulus, and slenderness ratio.
During a compression test, the test specimen is placed on compression platens or a specialized compression fixture and is compressed by applying load to the specimen. Compression testing for plastics and rubber materials often require a uniform, controlled speed rate or constant-rate-of-crosshead movement. Compression testing may be set up to determine the force required to cause a specified deflection, which is the change in thickness of the test specimen, or to measure the deflection when a specified compressive force is applied to the specimen. The sample may be placed in a compressometer with displacement transducers that record the amount of deformation on the specimen. Depending on the specimen geometry and dimensions, an extensometer or a deflectometer may also be used to measure the deformation.
Compressive strength is the maximum stress a test specimen can withstand during a compression test. Compressive stress, which may or may not be the same value as the compressive strength, is the compressive load per unit area of the cross sectional area prior to loading the specimen in compression. Compressive strength at failure corresponds to the compressive stress measurement obtained at the moment of failure of the specimen. Compressive strain is the ratio of compressive deformation, or the decrease in length produce in the gauge section of the specimen after it is loaded in compression, to the original gauge length of the specimen.
Compressive yield point is the first point on the compressive stress-strain diagram at which an increase in strain occurs without an increase in stress and the compressive yield strength is the stress value at the same point. If the test specimen fails in compression by a shattering fracture, the compressive strength has a very definite value. If the material does not shatter or does not exhibit a compressive yield point, the compressive strength is arbitrary depending upon the degree of distortion that is regarded as indicating complete failure.
Modulus of Elasticity is calculated by drawing a tangent to the initial linear portion of the stress-strain curve, selecting any point on this straight line portion, and dividing the compressive stress represented by this point by the corresponding strain.
Secant Modulus is the ratio of the compressive stress to the corresponding value of compressive strain. Slenderness ratio is the ratio of the length of a column of uniform cross section to its least radius of gyration.
Compression Test Setup
A typical compression test setup includes a testing machine capable of running tests at constant rates of crosshead motion and compression platens. The image on the left (from MTM Corporation) shows a compression test setup with two compression platens.
The deformation may be measured and calculated utilizing the crosshead position, a displacement transducer, an extensometer, or a deflection gage (shown on the left). The width, thickness, and length of the specimen are measured with a micrometer prior to placing it in between the compression platens or in the specialized compression tool. Depending on the instrument utilized to measure deformation, or the strain, the instrument may be directly attached to the specimen or it may be a part of the compression tool that the specimen sits on.
Compression Test Standards
Common compression test standard methods for plastics include ASTM D695 Standard Test Method for Compressive Properties of Rigid Plastics and ISO 604 Plastics – Determination of compressive properties.
Common compression test standards for rubber materials include ASTM D575 Standard Test Methods for Rubber Properties in Compression and ISO 7743- Rubber, vulcanized or thermoplastic — Determination of compression stress-strain properties.