Effect of fibre volume fraction on the fatigue behaviour of grp pultruded rod composites

The aim of this paper is to study the fatigue behaviour of GRP rods manufactured by a modified pultrusion technique. Constant-deflection flexural fatigue tests were performed at zero mean stress, i.e. a cyclic stress ratio R = -1. The frequency of the testing machine was 25 Hz. Failure was defined as a 30\% drop in the specimen stiffness (EI). The relationship between the endurance limit and the static flexural strength and fibre volume fraction, V-f, was determined, and the influence of V-f on the induced surface temperature, S-N relationships and failure modes was investigated. The results show that, in the range of V-f values investigated, as V-f increased fatigue strength increased. The relationship between the residual stiffness ratio (EI/(EI)(0)) and the normalized fatigue life (N/N-f) was independent of stress amplitude (S-a) and V-f. This result was supported by using the residual stiffness ratio as a failure criterion. The predicted values of endurance limit by using the proposed equation gave excellent agreement with the experimental data.