8 FLEXURAL (BENDING) STRESS/STRAIN

Although rubbers are, by design or accident, deformed by bending in some practical applications, it is only very rarely that bending or flexural tests are carried out. This is in contrast to the situation with rigid plastics, including ebonite, where flexural tests are often used and are well standardised.

In most applications where bending apparently takes place, the rubber is also deformed in shear, tension or compression, for example in a shaped door seal, when the test for stiffness would be a compression test on the actual part. Generally, rubbers are not stiff enough in flexure to support appreciable loads so that there is not much need for flexural tests and, at the same time, the lack of stiffness makes such tests a little difficult to carry out with precision. There are, however, some cases where stiffness in bend is of interest, for example with thin sheet and coated fabrics as a measure of handle .

Flexural tests for plastics are usually of the three point loading type, as in ISO 178 ^[122], where the test piece in the form of a flat strip is supported near its ends and a load applied in the centre. On using such a test for soft rubbers, it is immediately apparent that very low forces are realised and that rubbers will deform vastly more than the small strains for which the test is valid. It would seem sensible that if the force/deformation characteristics of a rubber in flexure are...