6 COMPRESSION STRESS/STRAIN

A compression stress/strain test is in many ways easier to carry out than a tensile test, and in view of the large number of applications of rubber in compression, should be more often used. Frequently, it would be logical for the test piece to be the complete product and a compressive force applied as it would be in service. Usually a constant rate of deformation would be appropriate and the force and corresponding deformation recorded without attempts at calculating the resultant stresses and strains.

Specially prepared test pieces for measuring material properties are usually in the form of a disc or short cylinder, the compressive force being applied to the circular faces. There are, in theory, two conditions under which the test pieces can be compressed: either with perfect slippage between the rubber and the compressing members or with complete absence of slip. Generally, perfect slippage is impossible to achieve and most applications involve either rubber bonded to metal or compressed between surfaces that virtually eliminate slip.

If there were perfect slippage, every element of the test piece would be subjected to the same stress and strain and a cylindrical test piece would remain a true cylinder without any barrelling. Under these conditions stress and strain are approximately related by (but see also Section 1):

where: F = compression force, A = initial cross-sectional area of test piece, E = Young's modulus G = shear modulus, ?. = ratio of compressed height to initial height.

The...