High temperature degradation of graphite/epoxy composites

Abstract

The problem of determining the response of a laminated composite plate exposed to a high temperature environment while mechanically loaded is approached by identifying the underlying mechanisms and addressing them separately. The approach is general, but the work focuses on the response of AS4/3501-6 graphite/epoxy composites. The mechanisms studied and modeled in this work are thermal response, degradation chemistry, and changes in mechanical material properties. The thermal response of an orthotropic plate exposed to convective heating is modeled using generalized heat transfer theory. The key parameters identified as controlling the thermal response include well-known parameters from heat transfer literature and a new parameter called the geometry-orthotropy parameter. From these parameters, the accuracy with which a multi-dimensional temperature distribution may be approximated using a onedimensional thermal model is quantified. The degradation chemistry of 3501-6 epoxy is studied through thermogravimetric analysis (TGA) experiments conducted in an inert atmosphere. A model of degradation based on a single Arrhenius rate equation is developed. Reaction constants for the degradation model are determined empirically and the validity of the model is verified through separate TGA experiments. A novel method for assessing the degradation state of a sample with an unknown thermal history is proposed. Analyses employing the method achieve estimates of the degradation state within 0.3 to 28% of the actual values. Changes in mechanical material properties are quantified by measuring the modulus and tensile strength of unidirectional [0]4 and [90]12 coupons exposed to temperatures as high as 400°C in a furnace. Some coupons are loaded to failure while exposed to the test temperature, others are first cooled to room temperature, allowing at-temperature and residual properties to be directly compared. Transverse properties are very sensitive to temperature around the glass transition temperature, but may recover when the coupon cools. Transverse properties are also very sensitive to small values (-0.03) of degradation state. Longitudinal properties are less sensitive to these variables. Temperature and degradation state are identified as appropriate metrics for quantifying changes in material properties. Models of the measured properties as functions of these variables are developed. A methodology for integrating models of the various mechanisms underlying structural response is presented. The thermal response model, degradation chemistry model, and material property models developed in this work are integrated with a thermomechanical response model based on classical laminated plate theory and implemented in a one-dimensional predictive code. This work establishes a foundation upon which a complete mechanism-based integrated model of the response of mechanically-loaded composites exposed to high temperatures may be developed. Specific recommendations for further work are provided.