The effect of minimum stress and stress amplitude on the fatigue life of non strain crystallising elastomers

Fatigue tests on ethylene propylene (EPDM) and styrene-butadiene (SBR) rubber revealed physical behaviour that is not seen in conventional linear elastic solids. Uniaxial cyclical tests, using cylindrical dumbbell specimens, having a minimum stress of zero (σmin=0) and varying stress amplitude (σa), predictably gave decreased fatigue life with increased stress amplitude and hence maximum stress (σmax). However, tensile uniaxial cyclic tests where σmin was increased in successive tests whilst the alternating stress (σa) remained constant, produced longer fatigue lives for higher values of σmax. Ethylene propylene and styrene-butadiene compounds were chosen for the tests because they do not strain crystallise during deformation and consequently this phenomenon has no influence. The results show that σmax cannot be used as a criterion to predict fatigue life of elastomers. Preliminary evaluation of recorded data of stress versus strain gave evidence that energy controls fatigue life rather than stress and strain. However, the role played by dissipated energy plays remains open to discussion. Experimental results on filled and unfilled rubber materials are evaluated and discussed.