Prediction Of Fatigue Life Of Composite Laminates By Empirical Formula Method

With the rapid development of China's aerospace industry in the 21 st century,composite materials have been applied more and more in the aerospace field.At the same time,the requirements for the properties of composite materials in the aerospace field have been continuously improved.Fatigue damage has gradually appeared in composite structures and is becoming an important factor restricting its further application.Therefore,the fatigue problem of composite materials is becoming a concern of users and engineers,but the related research in the field of fatigue damage of composite materials is insufficient.Based on the fatigue test of composite laminates and the fatigue theory of composite materials,the fatigue life of various laminates is deduced by using S-N curves of a few unidirectional plates,and a set of empirical formulas for predicting the fatigue life of composite laminates is established.Firstly,the extended Tsai-Hill fatigue failure criterion is deduced by using the classical theory of laminates,and the fatigue life prediction model of composite unidirectional plates is established.The S-N curves of unidirectional plates with different angles are obtained by this method.The accuracy of the model is verified by comparing with the experimental values,and then the feasibility of predicting the fatigue life of laminates by empirical formula method is proved..Then the theoretical basis of the empirical formula for fatigue life of laminates is introduced,and the ideas and basic assumptions for solving the stress of each single layer in laminates are given.Secondly,the existing fatigue test data of unidirectional and laminates are processed and analyzed,and the empirical formulas of fatigue life are established when the strength of laminates is known.Then the fatigue tests of other composite materials with 0 degree unidirectional and laminates are carried out.The comparative analysis of the fatigue life obtained from the tests and the empirical formulas verifies the accuracy and reliability of the empirical formulas.Finally,the formulas for calculating the stresses of 0 degree unidirectional laminates in symmetrical laminates with 0,90 and± 45 degrees are derived.Based on the empirical formulas established above,the empirical formulas for fatigue life of 0 degree laminates are assumed to be the fatigue life of laminates,and the empirical formulas for fatigue life of laminates with unknown strength are established.Based on Miners linear damage criterion and stiffness degradation theory,the formula is modified by layer-by-layer failure method,and the correction coefficient is introduced to obtain an empirical formula which can accurately predict the fatigue life of laminates.