Journal of Composite Materials

This paper deals with buckling analysis of rectangular composite laminates with circular holes under inplane static loadings. The first-order shear deformation theory and the variational energy method are employed in mathematical formulation, and the nine-node Lagrangian finite element method is used for finding critical loads. The effects on critical load by hole size, plate thickness ratio, material modulus ratio, ply lamination geometry, loading types, and boundary conditions are investigated. Numerical solutions are shown in graphical form where some comparisons are made with those of given litera tures.

The thermo-viscoelastic boundary value problem for anisotropic materials is formulated and a numerical procedure is developed for the efficient analysis of stress and deformation histories in composites. The procedure is based on the finite element method and therefore it is applicable to composite laminates containing geometric discon tinuities and complicated boundary conditions. Using the present formulation, the time- dependent stress and strain distributions in both notched and unnotched graphite/epoxy composites have been obtained. The effect of temperature and ply orientation on the creep and relaxation response is also studied.

This study addresses residual stress and its effect on the performance of thermoset filamentary composites. Both cure shrinkage and thermal shrinkage in the matrix generate various residual stresses. In this work, a molecular additive was used to nearly eliminate the cure shrinkage of the cast thermoset resin and thereby to significantly reduce the residual stress in the composite.

In spite of the reduction in residual stress, the damage accumulation behavior was un changed : transverse cracking studies of (0₂/90₂) _s laminates showed identical crack onset and crack accumulation in both control and experimental composites (with and without additive, respectively). The surprising lack of effect of reduced residual stress on damage accumulation is explained by considering the effect of the additive on elastic constants and fracture sensitivity.

A model for the ply-slip deformation behavior of a laminate composite is presented which allows prediction of the stresses, displacements, and strains developed during the forming process. The model takes into account the viscous nature of the matrix and the stress relaxation achievable via ply slippage. The stress response is inherently time dependent and is strongly affected by the forming rate. Results calculated for the isother mal three point bending of a unidirectional lay-up agree well with previous experimental data. An important parameter, the maximum system time constant, is identified and related to the material properties and part geometry. Comparisons with experiments and simulation results for nonisothermal forming are also discussed.

Nonlinear equations of motion of symmetrically laminated anisotropic plates are derived considering von Karman strains and transverse shear effects. Using a single-mode Galerkin procedure, a nonlinear modal equation is obtained and a direct equivalent linearization method is employed for solution of this equation. The response of acoustic excitation of moderately thick composite panels is then studied. Further, the effects of transverse shear on the large deflection vibration of laminates under random ex citation are studied. Mean-square deflections and mean-square inplane stresses are ob tained for symmetric graphite-epoxy laminates. Using equilibrium equations and con tinuity conditions, the mean-square transverse shear stresses are calculated with results obtained useful in the sonic fatigue design of composite aircraft panels. The analysis is presented in detail for simply supported plates with analogous equations for a clamped case also obtained.