Numerical analysis of the process-induced residual stress in fiber-reinforced composite materials

Abstract

Composite materials are increasingly being used in aerospace structures due to their better mechanical properties compared to metals: high specific strength, specific modulus and fatigue endurance limit, low corrosion potential and coefficient of thermal expansion (CTE), etc. However, they can display some disadvantages which are inherent to their inhomogeneous nature. A convincing example is the residual stress induced during the manufacturing process. It results mainly from the following parameters: resin shrinkage, geometrical constraints, and thermal property differences between the fiber and matrix. Thus, for safety grounds, it is necessary to study the development of such residual stress during the fabrication process. However, in this work, we investigated the residual stress development through a numerical approach rather than experiments, which require tremendous time and/or money. Unidirectional and cross-ply micromechanical models were used with finite element method. They consisted of unit cells which represent the periodic microstructures of the carbon/epoxy and glass/epoxy composites. For symmetry grounds, only 1/4 of unidirectional unit cell and 1/8 of the cross-ply representative volume element were examined. The constitutive behavior of the epoxy matrix was described by a nonlinear cure and temperature-dependent viscoelastic model. It was implemented in a UMAT code written in FORTRAN and appended to commercial software ABAQUS. The residual stress distribution was obtained as follows: (i) the temperature distribution was simulated first, (ii) and then used as a predefined field for the mechanical analysis. In the unidirectional model, a maximum tensile stress was found within the resin at θM = 45o of the fiber-matrix interface. The fibers displayed compressive values mainly accumulated during the thermal cooling. The residual stress of the carbon/epoxy was lower than that of the glass/epoxy because of its higher transverse CTE. In the cross-...