Finite element modeling of the sensitivity of carbon/epoxy laminates to void content

Abstract

This thesis will review mechanical property degradation that is available and then utilize what test data is published to provide the remaining aspects. Recognizing that tensile and compressive moduli degrade differently with respect to void content, an iterative solution which queries directional stress in the finite element model (FEM) will be employed. This technique will be implemented through custom ANSYS APDL source code that compares mechanical property (tension/compression) assignment and element stress state. The code monitors mechanical property modifications made for each solution iteration and terminates upon convergence (zero modifications required). This modeling approach is superior to typical orthotropic material modeling with differences in deflection and stress noted for a simple rectangular plate model. A maximum difference of 2.6% for plate displacement, 9.5% for tensile stress, and 39.5% for compressive stress has been predicted from the sensitivity study conducted. Clearly, the effect on compressive stress is the most significant and should be considered when evaluating porous laminates. The methodology developed to simulate the effect of void content on mechanical properties of FRC materials is simple to implement in FEMs and adaptable to other fiber/matrix material combinations thus satisfying the intent of this thesis.

The purpose of this thesis is to develop a methodology for simulating the structural behavior of porous carbon/epoxy composite materials. Research and testing has been performed to quantify the impact on some of the mechanical properties however no solution for collecting and utilizing all of the data has been published. Laboratory tests confirm that void content impacts the in plane tensile strength (IPT) and interlaminar shear (ILS) strength. Largely absent from this testing is the effect on in plane compression (IPC) strength. Generally, FRC are modeled with orthotropic material properties, thus assuming that tensile and compressive effects are similar. On the contrary, it is noted from testing that flexural strength is impacted by void content and therefore the relationship between tensile and compressive moduli is affected.