Stochastic Viscoelastic Delamination Onset Failure Analysis of Composites

The previously formulated deterministic viscoelastic quadratic time de pendent delamination onset criterion is generalized to complete three-dimensional stochastic environments. The analysis includes stochastic processes due to combined ran dom loads and random delamination failure stresses as well as random anisotropic vis coelastic material properties including the influence of stochastic temperature fields, mois ture contents and boundary conditions. It is shown that times for delamination onset occurrences in composites can be predicted probabilistically depending on any one or all of the above conditions. Illustrative examples are presented showing the relationship in terms of parametric variations between times to delamination and corresponding probabil ities that such events will occur. Since uniaxial tension, compression and shear vis coelastic delamination failure stresses decrease in time, the loading history is of significant importance. For cases where deterministic criteria predict no delamination failures, the present stochastic failure theory indicates high probabilities of failure at either early or long times depending on the load-time relations. The early time high probabilities of delamination onset predict short lifetimes and occur in conditions where the composite in ternal stresses relax at faster rates than the failure stresses are degrading. The effects of fiber orientation and of number of plies on delamination probabilities are also examined.