FATIGUE LIFE PREDICTION ON COMPOSITE MATERIALS

Fatigue life prediction on composite materials is studied analytically using degradation and damage models,resultant strain and fatigue modulus.Definition of fatigue modulus,new damage models using fatigue modulus and resultant strain,and prediction of fatiguelife on composite materials using degradation and damage models are discussed.This approach can accurately predict the multi-stress level fatigue life as well as single-stress level fatigue life of composite materials.Defining fatigue damage on composite materials is not a simple problem.Common variables used to define damage of composite materials are secant modulus,residual strength,strain,compliance,amount of acoustic emission,crack densitv,crack length,and number of debonded or failed fibers.These damage models are tabulated in Table 1.However,one variable can not be used universally to represent the degree of damage in various composite materials.Fatigue modulus is defined using secant modulus and resultant strain during fatigue cycle.Fatigue modulus represents the physical damage in composites with different orientation and materials.,specially metal-matrix and ceramic-matrix composites for high temperature applications.Damage model Ⅰ (D=[F₀-F(n)]/[F₀-Ff]),model Ⅱ (D=ε(n)-εf) and model Ⅲ (D=[ε(n)-ε₀]/[εf-ε₀]) are defined as functions of fatigue modulus and resultant strain.The structure of damage model,boundary conditions and physical meanings are inserted in order to interpret the proposed damage models.Also the effect of applied stress on the accumulated damage is studied analytically using proposed damage models.Fatigue life is predicted by the following procedures:(1)establish the fatigue modulus degradation,(2) find fatigue life equation as a function of fatigue modulus,(3)calculate the fatigue life using strain failure criterion.Degradation models for composite damage are generalized and found that three-pare meter degradation model (dF/dn=ACn^C-1/BF^B-1) and two-parameter power function model(dF/dn=-ACn^C-1) are most suitable to predicting fatigue life of composites.Also the predicted two-stress.level fatigue life using the proposed damage models are reasonably close to the experimental data.