| This work focuses on multi-scale stress and damage initiation analyses of unidirectional and woven graphite/polyimide composites subjected to mechanical, thermal and aging loads. The macro, meso and micro scales representing respectively a specimen, woven fabric and fiber/matrix levels were investigated using both analytical and numerical methods. The Eshelby/Mori-Tanaka and finite element models based on the concept of a unit-cell were utilized to model visco-elastic and elasto-plastic effects in the unidirectional and woven systems both at room and elevated temperatures. To determine the properties of graphite fibers the analyses were performed from macro to micro scale, whereas in order to predict the composites response a reversed micro to macro approach was employed. Both mechanical and resonance testing results were used as input macro data for the fiber properties analysis. It was shown that using the proposed technique the elastic and thermal properties of T650, M40J and M60J graphite fibers can be determined from the macro data for the unidirectional and woven composites. Next, based on the fiber and neat resin data, the effective macro behavior of the T650/PMR-15 unidirectional and woven composites was predicted as a function of temperature. The predictions were subsequently compared with the available experimental results. The computations of the meso and micro stresses in the woven systems were conducted and the most critical combination of in-plane mechanical loads was determined. The analyses also revealed the difference between the micro stress states in the composite subjected to the Iosipescu and +/-45° shear tests. Finally, the long term relaxation modulus for PMR-15 resin was measured and simulation of aging at 315° in nitrogen was performed for the woven 8HS T650/PMR-15 composite. |
