Mesoscale lamina fatigue damage model for fiber-reinforced polymer composite laminates

Abstract

Carbon fiber-reinforced polymer (CFRP) composite laminates used as loadbearing structures, such as skin of aircraft wings and wind turbine blades, are likely to experience fatigue loading. The resulting complex stresses could cause fatigue damage in the laminas in the form of matrix cracking, interface delamination, and fiber fracture. The prediction of the reliability of these structures requires an accurate constitutive damage model of the composite material. However, the previous available damage models are based on stress control condition and limited to low cycle fatigue loading conditions. In this respect, the research proposes and examines a new universal damage-based material fatigue model of unidirectional lamina. The mesoscale model incorporates the observed degradation of the lamina strength and stiffness properties in defining the material damage under cyclic loading conditions. Hashin’s stress-based criteria for damage under monotonic loading are extended and used to describe the fatigue damage accumulation process. The normalized model is employed to describe the fatigue degradation of the strength and stiffness properties. The model acknowledges the effects of mean stress on the damage and fracture process. It is observed that the shear strength of the CFRP composite lamina is the first to degrade when compared to other properties. The predicted fatigue damage evolution characteristics are examined for a typical material point in the CFRP composite lamina throughout the fatigue loading. For this purpose, a finite element (FE) model of a CFRP composite laminate plate with a through central hole is subjected to tensiontension cyclic stressing ( k = 0.1) in transverse fiber direction. Since cycle-by-cycle life calculations are impractical given the large number of anticipated fatigue cycles involved, a load-cycle block sequence is introduced to address the computational efficiency of the fatigue life prediction routine. The number of load cycles represented by each block is dictated by the rate o f the property degradation. The size of a load cycle block is determined by the residual property curve that exhibits the shortest fatigue life, Nf under the operating fluctuating load cycles. Nonlinear characteristic evolution of the fatigue damage with the applied stress cycles is demonstrated. The critical level of the fatigue damage, determined based on the total dissipated energy of fracture, denotes the nucleation of the fatigue crack through the separation of the material point. From the case study, with the operating stresses of a 22max=18.85 MPa and T12max =1.30 MPa, the accumulated matrix tension fatigue damage at the critical point reaches g MT = 1.0 after n d = 1.93x103 cycles have elapsed. The collection of the separated material points throughout the applied fatigue cycles represents the propagated fatigue crack. The calculation routine is readily implemented in any standard FEA software for damage-based fatigue life prediction of fiber-reinforced polymer (FRP) composite laminate structures. The unified fatigue damage model developed in this thesis is significant for industrial sectors, dealing with FRP composite design, fabrication, reliability prediction, and failure analysis of loadbearing structures.