Effect of mean stress on fatigue behaviour of GFRP pultruded rod composites

In weight-sensitive applications such as aircraft and space vehicles, high-performance composites (FRP) were used because of their high fatigue strength to weight ratio. To clarify the effect of mean stress on the behaviour of the S-N diagram of GFRP composites, constant deflection flexural fatigue tests were conducted on standard unidirectional glass fibre (ER 1150 F-183)-reinforced polyester (QL 8520 A) pultruded rods. The frequency of the testing machine was 25 Hz. The failure criterion was defined as when the residual stiffness of the test specimen reached 70\% of the specimen stiffness at the start of the test. The results show that mean stress has a slight effect on the material constant (A; slope of the S-N power function). The relationship between the specific stiffness ratio EI/(EI)(o) and the cycle ratio N/N-f was independent of the values of initial stress amplitude and mean stress. A master diagram for GFRP composite rods has been constructed for various cyclic lives. The area under the curves of the master diagram represents the safe design range, based on average values, of initial stress amplitude (plotted on the ordinate) and mean stress (plotted on the abscissa). Copyright (C) 1996 Published by Elsevier Science Limited.