Numerical Analysis of Micro-Residual Stresses in a Carbon/Epoxy Polymer Matrix Composite during Curing Process

Abstract

The manufacturing process in thermoset-based carbon fiber-reinforced polymers (CFRPs) usually requires a curing stage where the material is transformed from a gel state to a monolithic solid state. During the curing process, micro-residual stresses are developed in the material due to the different chemical–thermal–mechanical properties of the fiber and of the polymer, reducing the mechanical performance of the composite material compared to the nominal performance. In this study, computational micromechanics is used to analyze the micro-residual stresses development and to predict its influence on the mechanical performance of a pre-impregnated unidirectional CFRP made of T700-fibers and an aeronautical grade epoxy. The numerical model of a representative volume element (RVE) was developed in the commercial software Abaqus® and user-subroutines are used to simulate the thermo-curing process coupled with the mechanical constitutive model. Experimental characterization of the bulk resin properties and curing behavior was made to setup the models. The higher micro-residual stresses occur at the thinner fiber gaps, acting as triggers to failure propagation during mechanical loading. These micro-residual stresses achieve peak values above the yield stress of the resin 55 MPa, but without achieving damage. These micro-residual stresses reduce the transverse strength by at least 10%, while the elastic properties remain almost unaffected. The numerical results of the effective properties show a good agreement with the macro-scale experimentally measured properties at coupon level, including transverse tensile, longitudinal shear and transverse shear moduli and strengths, and minor in-plane and transverse Poisson’s ratios. A sensitivity analysis was performed on the thermal expansion coefficient, chemical shrinkage, resin elastic modulus and cure temperature. All these parameters change the micro-residual stress levels and reduce the strength properties.

Physical and Mathematical Modeling of the Distribution of Load Forces on the Feminal Component of an Endoprosthesis of the Hip Joint Under Real Conditions

“…As for integration by variable r, then here the integration is performed within the range of values that now depends on two parameters. From the dimensionless length of the prosthesis stem λ and the length of the contact area and are determined by condition (2). To find the limits of integration, we will substitute (1) in ( 2) and solve the obtained relation r for both parts of inequality (2).…”

“…From the dimensionless length of the prosthesis stem λ and the length of the contact area and are determined by condition (2). To find the limits of integration, we will substitute (1) in ( 2) and solve the obtained relation r for both parts of inequality (2). As a result, we will have a value R λ and R λ1 , respectively, for the upper and lower limits of integration: ; , (7) and the integral itself for determining the required area is reduced to the form: (8) In the last entry, it is emphasized that the dependence of the area S is investigated not only on the dimensionless length of the prosthesis stem:…”

A Numerical Model to Predict the Relaxation Phenomena in Thermoset Polymers and Their Effects on Residual Stress during Curing, Part II: Numerical Evaluation of Residual Stress

“…Generally, these stress entities are more consistent in a two-phase component than in epoxy only components due to the thermal and mechanical properties mismatching between the two phases [ 17 , 28 , 38 , 39 ]. In addition, in a polymer-based composite laminate, the orthotropy of ply causes an in-thickness mismatch of mechanical and thermal properties, such as angle-ply laminate, that produces distortions and residual stresses in the composite part.…”