The Fatigue Performance of Notched Composite Material Systems

Abstract

In this work results are reported from an experimental fatigue programme. The role of the constituent materials upon the fatigue performance of a given composite material system was studied. This involved comparisons of the residual strength, damage development, and in some cases fatigue life. A new method of approach, based upon a unique characteristic fatigue parameter, termed the damage transition stress, St, was also introduced. It has been shown that the employment of the characteristic parameter can simplify the comparison of different material systems considerably. Four graphite reinforced material systems were evaluated: (a) standard epoxy system, (b) toughened epoxy system, (c) thermoplastic system and, (d) high temperature system. It was found that three of the four material systems examined (a, b, and d) were more sensitive to hygrothermal environments, so far as the development of fatigue damage is concerned. In general, the tensile residual strength, in all material systems, showed a marked improvement, whereas the effect of environment and fatigue damage combination upon the compressive strength was more complex. Presentation of the fatigue data in the form of linear S-N plots allowed the determination of St, and its dependence upon environment and material systems. A material dynamic toughness was also identified.