Variation of the elastic characteristics of carbon/epoxy fiber laminate as a result of fatigue damage

Abstract

Carbon/epoxy fiber laminates are increasingly used in critical structures of aircrafts, ships, etc. Fatigue resistance is an important requirement for such structures. Material degradation due to the gradual accumulation of fatigue damage is accompanied by a decrease in stiffness. A decrease in stiffness during fatigue damage may result in a change in the natural frequencies of the structures. The aim of this study is to obtain experimental data on the variation of the elastic characteristics of a carbon/epoxy fiber laminate as a result of progressive fatigue damage. As the studied characteristics of the laminate, four elasticity parameters of the monolayer are considered (two Young's moduli, the shear modulus and Poisson's ratio). These characteristics are commonly used in laminate strength and durability models. The research technique is based on an experimental study of natural frequencies and eigenmodes of samples under fatigue testing. Fatigue tests of the samples are carried out under cyclic tension while maintaining constant amplitude and the average load value. The fatigue test of the sample was stopped several times to perform modal tests. The modal tests were carried out using scanning laser vibrometry. The determination of the four above-mentioned elastic parameters of the monolayer is considered as the inverse identification problem using the results of modal tests. The dependences of the mentioned elastic characteristics on the relative fatigue life are obtained. These dependencies can be used to develop methods for predicting the fatigue life of carbon fiber parts.