Compression test

The compression test determines the mechanical properties of materials under crushing loads. There are many aircraft structures that carry compression loads, such as the undercarriage during take-off and landing or the upper wing surface during flight, and therefore the mechanical behaviour of their materials must be determined by compression testing. It is often assumed that the tension and compression properties of materials are the same, yet this is only true for the elastic modulus. Other important mechanical properties, such as yield strength, may be different under tension and compression. It is essential that the mechanical properties of materials used in compression-loaded aerospace components are measured under compression loading, rather than assuming they are identical to the tensile properties.

The compression test involves squashing a small material specimen under increasing load until the point of mechanical instability. Compression specimens are usually short, stubby rods with a short length L to wide diameter D ratio (usually L/D < 2) to prevent buckling and shearing modes of deformation. The reaction of the specimen against an increasing compression load is measured during the test and, from this, the compression stress–strain curve is determined. The compression curve is simply the reverse of the tension curve at small strains within the elastic regime because the elastic modulus is the same. As the strain increases the difference between the compression and tension curves often becomes significant. As the specimen is squashed, it becomes shorter and fatter, and the load needed to keep it deforming rises. The specimen does not experience necking and so the stress increases until an instability occurs, such as cracking. The main mechanical properties which are determined from the compression test are the compression modulus and compression yield strength.