Thermomechanical Analysis of Micromechanical Formation of Residual Stresses and Initial Matrix Failure in CFRP

Abstract

Process induced thermal residual stresses and matrix failure of unidirectional CFRP has been investigated by finite element methods. Partial discrete model composites consisting of a microscopic area of fibers and matrix surrounded by a homogeneous area were chosen. Four cases have been investigated concerning the formation of residual stresses and initial matrix failure: A free UD-laminate, a constrained UD-laminate, a cross ply laminate and a thick laminate which is subjected to a temperature gradient during cooling down. On the basis of experimental results from thermo-mechanical tests of the neat resin, the temperature dependent matrix stress-strain behavior as well as the parabolic failure criterion were formulated and introduced into the finite element program. The actual stress state on the microscopic level depending on different boundary condition could be described. The authors showed that the approach of a partial discrete model is suitable to determine the initial matrix failure of different macroscopic specimens under consideration of micro-mechanical effects. The results showed that high tri-axial stresses occur in the constrained laminate and the cross ply laminate, which lead to initial matrix failure in the 90• -ply. The consideration of a temperature gradient affects the stress distribution in the matrix though the influence on the maximum residual stress values is small. In this case, initial matrix failure can be excluded.

Finite Element Modeling to Predict Cure-Induced Microcracking in Three-Dimensional Woven Composites

“…In the post-processing, we utilize the so-called parabolic failure criterion suggested, for example, by Hobbiebrunken et al (2004):…”

“…In Hobbiebrunken et al (2004), these constants were determined for numerical modeling of unidirectional carbon/epoxy composites from uniaxial tensile and torsion tests. Note that more comprehensive studies are needed to identify these constants for triaxial tensile stress states relevant for curing of 3D woven composites.…”

“…Some significant experimental and modeling results related to the processinduced residual stresses are given in Hobbiebrunken et al (2004Hobbiebrunken et al ( , 2005 but those papers are devoted to unidirectional laminates and do not cover the three-dimensional composite architectures. The goal of this research is to develop experimentally validated realistic finite element models to predict cure-induced stresses in 3D woven composites.…”

Failure of Fiber-Reinforced Composites

“…Due to production process at elevated temperatures, most fiber-reinforced polymers are subject to a residual thermal stress between fibers and matrix. Here, the matrix material between two adjacent fibers is exposed to a geometrically induced stress concentration and therefore acts as weakest link when loading the material transverse to the fiber axis [5,[13][14][15].…”

Micromechanics analysis of progressive failure in cross-ply carbon fiber/epoxy composite under uniaxial loading

“…Several authors have used micro-scale finite element models to study failure in variety of cross-ply composites [10][11][12]. Unfortunately, no attempt has been made to obtain SAF for cross-ply polymer composites.…”