Estimating the fatigue strength characteristics of multilaminated composite materials

Abstract

Since calculations of the fatigue strength characteristics of composites and components from them require long-term and expensive experiments, it is important to develop a fatigue endurance criterion which would make it possible to predict the corresponding characteristics on laminated composites from the results of testing individual layers. Since it is complicated to describe the nucleation and development of structural damage in the case of composites it is more efficient to use phenomenological approaches of which the models based on the hypothesis of damage cumulation are used most extensively. Damage cumulation in composites under cyclic loading is described by means of hereditary relations [1-3] which make it possible to take into account in an integral manner the effect of loading history on degradation on the mechanical properties of materials. The existence of several defect cumulation mechanisms has made it possible to describe damage cumulation as a multidimensional process and introduce tensor [1] and vector measures of damage [3]. In this work we use the force criterion of the hereditary type described in [4] and linked with the nonlinear determining equation derived by Rabotnov [4], and on the basis of the linear variant and the theory of laminated plates, we propose a method of predicting the fatigue strength characteristics of multilaminated composites with a polymer matrix using the results of fatigue tests on unidirectional specimens loaded under different angles in relation to the reinforcement direction. The failure condition of unidirectional composite materials [4] was formulated assuming that two types of failure were possible in a plane stress state: the first type of failure -failure from rupture of reinforcement fibres, the second type of failure -failure at the interface between the fibre and the matrix. Damage cumulation is described by the stresses acting in the main axes of orthotropy of the material and fracture of fibres takes place under the effect of stresses in the reinforcement direction, whereas fracture at the interface takes place as a result of the combined effect of stresses in the direction normal to that of reinforcement and shear stresses. According to [4] failure as a result of rupture of the fibres takes place when characteristic stresses a h [[, acting along the fibres and linked with the level of cumulated damage, reach the critical value a* LL' which are regarded as the strength of a defect-free material in the reinforcement direction. Similar failure at the fibre--matrix interface takes place when the combination of the characteristic stresses %x in the direction perpendicular to the reinforcement, and of the shear stresses %, related to the corresponding values of the strength of the defect-free material in tensile loading across the reinforcement direction a*x and shear strength r* reaches the critical value equal to unity: ahll = ai~ ; CTh .1. T h ---7+ ~ = 1. (1) Gl