Damage retardation in composite laminates by fiber prestress

The present work is concerned with retardation of damage in composite materials and structures by application of fiber prestress. The main topics considered are improvement of composite strength by fiber prestress, distribution of optimized fiber prestress for reduction and cancellation of free edge stresses, loss of prestress due to viscoelastic matrix deformation, and enhancement of dynamic strength of ceramic armor plates.; A distinguishing property of prestressed composite laminates is the presence of residual stresses induced by release of fiber prestress. These residual stresses change total stress distribution in the fibers, matrix and plies of the laminate subjected to mechanical loads. To establish conditions for damage initiation in prestressed composite laminates, we have selected a maximum stress failure criterion. Enhancement of composite strength derived from fiber prestress is best visualized in terms of translation of damage envelopes plotted in the laminate stress space.; An analysis of free edge stresses in prestressed composite laminates is presented. A novelty of the approach is that fiber prestress release is modeled through application of equivalent compressive tractions in the fiber direction of each prestressed ply. We have found an exact magnitude of fiber prestress that cancels thermal residual stresses caused by cooling from processing temperature. An optimized fiber prestress distribution is evaluated which minimizes free edge stresses without violating constraints imposed on local stresses by stress failure criterion.; A constitutive model for nonlinear viscoelastic EPON 828 matrix material suggested by Ellyin [31] is used for viscoelastic analysis of prestressed composite laminates. The effect of inelastic and thermal deformations is modeled through application of equivalent eigenstrains to the elastic composite. Transformation field analysis method is used to derive governing equations for the time-dependent local stresses in the matrix and fibers of each ply. The amount of stress relaxation is estimated by plotting damage envelopes for the viscoelastic laminate after sufficiently large periods of time. It is found that composite strength is not much reduced by creep deformation of the matrix and the loss of prestress with time can be counterbalanced by application of fiber prestress of higher magnitudes.; Further benefits of fiber prestress are obtained in designing ceramic armor plate encapsulated into prestressed composite laminate. Compressive stresses generated in the armor plate by fiber prestress can significantly improve its dynamic strength and penetration resistance. Optimization procedure is developed to find fiber prestress distribution that create maximum compression in the ceramic. Effect of matrix viscoelastic deformation and applied temperature gradients on relaxation of initial compressive stresses in the ceramic is found negligible.; The subject of transverse matrix cracking has received a considerable amount of attention. A model for analyzing matrix crack initiation and progression is developed. This model appears to be quite versatile and can be applied to the laminates made of thin plies with arbitrary layups. Two criteria—stress and energy criteria, are used to describe damage progression in composite laminates. Expressions for energy release rates and stress distributions are obtained for plies with arbitrary crack densities. Initial ply strength is found as a function of ply thickness. Damage evolution laws are obtained for several layup configurations and ply thicknesses; unknown parameters of the present model, such as initial flaw density are obtained by comparison of numerical results with experimental data.