FINITE ELEMENT-BASED EVALUATION OF RESIDUAL STRESSES IN CARBON FIBER-REINFORCED POLYMER (CFRP) AT MICROSCALE USING FIBER PUSH-OUT EXPERIMENT

Abstract

Residual Stresses that develop during manufacturing process of Carbon Fiberreinforced Polymer (CFRP) composites can have negative effects on material strength and fatigue properties. The ability to reliably represent residual stresses in the material microstructure may be essential for accurate prediction of composite material properties using numerical micromodels. In this work, a methodology that uses Finite Element Model Updating (FEMU) and experimental data from Fiber Push-Out experiments is presented for evaluation of microscale residual stress in CFRPs. Recently, Fiber Push-Out experiments with in-situ scanning electron microscope (SEM) imaging conducted by the authors on CFRP specimens revealed significant through-thickness matrix deformation/sink-in in the matrix-rich region after nearby fibers are pushed out, a phenomenon attributed to the release of residual stress after the fiber-interface is broken. To characterize and reproduce residual stress in the Push-Out test samples, a two-step approach is proposed. First, a microscale Finite Element Method (FEM) model is built based on the geometry of the Push-Out area. Second, an FEMU procedure determines effective matrix properties associated with stress build-up that will reproduce the matrix Sink-in value as measured experimentally. In this preliminary work, residual stresses are attributed to the development of thermal strains in the matrix during cool down, and an effective Coefficient of Thermal Expansion (CTE) property for the epoxy material is obtained from the FEMU methodology. FEMU results suggest that epoxy matrix’s common CTE (on the order of 50ppm/oC) is too small to explain the observed matrix Sink-in. Preliminary evaluations suggest high level of residual stress in matrix which might initiate and aggravate microstructure defects, and potentially interfere with fiber-matrix interface properties measurements using the Fiber Push-Out Experiment.