A study of process-induced stress and deformation in thick-section thermosetting composite laminates is presented. A methodology is proposed for predict ing the evolution of residual stress development during the curing process. A one- dimensional cure simulation analysis is coupled to an incremental laminated plate theory model to study the relationships between complex gradients in temperature and degree of cure, and process-induced residual stress and deformation. Material models are proposed to describe the mechanical properties, thermal and chemical strains of the thermoset resin during cure. These material models are incorporated into a micromechanics model to pre dict the effective mechanical properties and process-induced strains of the composite dur ing cure. Thermal expansion and cure shrinkage contribute to changes in material specific volume and represent important sources of internal loading included in the analysis. Tem perature and degree of cure gradients that develop during the curing process represent fun damental mechanisms that contribute to stress development not considered in traditional residual stress analyses of laminated composites. Model predictions of cure dependent epoxy modulus and curvature in unsymmetric graphite/epoxy laminates are correlated with experimental data. The effects of processing history (autoclave temperature cure cy cle), laminate thickness, resin cure shrinkage and laminate stacking sequence on the evo lution of process-induced stress and deformation in thick-section glass/polyester and graphite/epoxy laminates during cure are studied. The magnitude of process-induced residual stress is sufficient to mitiate transverse cracks and delammations. The results clearly indicate that the mechanics and performance of thick-section thermoset laminates are strongly dependent on processing history.

No abstract

An investigation into the two-dimensional cure simulation of thick thermosetting composites is presented. Temperature and degree of cure distributions within arbitrary cross-sectional geometries are predicted as a function of the autoclave temperature history. The heat conduction equation for two-dimensional, transient anisotropic heat transfer is coupled to the cure kinetics of the thermosetting composite material. A heat generation term, expressed as a function of cure rate and the total heat of reaction, is introduced to account for the heat liberated during the curing process. A generalized boundary condition formulation is employed, enabling arbitrary temperature boundary conditions to be enforced straightforwardly. An incremental, transient finite difference solution scheme is implemented to solve the pertinent governing equations and boundary conditions. The boundary-fitted coordinate system (BFCS) transformation technique is combined with the Alternating Direction Explicit (ADE) finite difference method in the solution strategy. Complex gradients in temperature and degree of cure are predicted and the influence of the tool on the curing process is demonstrated. Correlation between experimentally measured and predicted through-the-thickness temperature profiles in glass/polyester laminates are presented for various arbitrary temperature cure cycle histories. Several typical glass/polyester and graphite/epoxy structural elements of arbitrary cross-section (ply-drop and angle bend) are analyzed to provide insight into the non-uniform curing process unique to thick-sections. Spatial gradients in degree of cure are shown to be strongly dependent on part geometry, thermal anisotropy, cure kinetics and the temperature cure cycle. These spatial gradients directly influence the quality and in-service performance of the finished component by inducing warpage and residual stress during the curing process.

No abstract

No abstract