THE FATIGUE SENSITIVITY OF FIBERGLASS GLIDERS

Abstract

A preliminary analysis of the fatigue strength of fiberglass gliders has been carried out in the course of a current Australian investigation to substantiate an optimum economic life and, if warranted, carry out a life extension program for fiberglass gliders in Australia. This has led to some significant conclusions relevant to the fatigue performance of fiberglass gliders, and a brief account of the project is presented here and problems it has brought out are discussed with reference to the main investigation. The main conclusions of the paper are that the conventional view of the satisfactory fatigue performance of fiberglass structures contains some potential dangers. The flat S-N curves of this material which underlie its high fatigue performance also contribute to a very large variability in life. They also cause a large reduction in fatigue life for any increase in operating stress and with the current lack of knowledge of the effective stress concentration factor (or fatigue strength reduction factor) of complete fiberglass structures, as distinct from notched specimens, this is a major problem. Similarly, the high residual strength maintained by fiberglass during the fatigue life until final failure is approached, also carries a counteracting disadvantage in that it reduces the advance warning available to a safety-by-inspection or fail safe approach to fatigue safety. It is suggested that the Australian Joint Program on fatigue of fiberglass gliders will help to overcome these uncertainties and provide a better understanding of the fatigue behaviour of a complete fiberglass structure.