Numerical/Experimental Correlation of Three-Dimensional Thermal Stress Distributions in Graphite/Epoxy Laminates

Abstract

Three-dimensional finite element analysis was used to predict thermal stress distribu tions in thick graphite/epoxy laminates. The laminates analyzed were [O_n/90_n]_s and [+45_n/-45_n]_s. For the [O_n/90 _n]_s laminates, high thermal in-plane tensile normal stresses were predicted in the transverse (2-) directions in both the 0° and 90 ° plies. High inter laminar normal (z- or 3-direction) tensile stresses were predicted to exist in the 90° plies near the center of two opposite free edges, with large compressive interlaminar normal stresses along the other two edges. Stresses were predicted to be low in all corners. In the [+45_n/-45_n]_s laminates, high in-plane transverse (2-) direction normal tensile stresses were also predicted. High tensile interlaminar normal stresses were predicted to exist at two diagonally opposite corners, with large compressive interlaminar normal stresses at the other corners. Subsequent experiments using thick laminates with similar stacking sequences slowly cooled to -150°C confirmed the predicted stress distribu tions and the existence of a "free corner" effect in angle-ply laminates, an effect which could not have been predicted without performing a fully three-dimensional analysis. Absence of inplane symmetry in mid-plane symmetric angle-ply laminates was predicted analytically and verified experimentally. Failure mode and location were shown, analytically (based on a maximum stress failure criterion) and experimentally, to depend on stacking sequence.