MECHANICAL BEHAVIOUR OF WOVENGRAPHITE/POLYIMIDE COMPOSITES WITH MEDIUM ANDHIGH MODULUS GRAPHITE FIBERS SUBJECTED TOBIAXIAL SHEAR DOMINATED LOADS

摘要

A major limitation of woven fiber/polymer matrix composite systems is the inability ofthese materials to resist intralaminar and interlaminar damage initiation and propagation under sheardominatedbiaxial loading conditions. There are numerous shear test methods for woven fabriccomposites, each with its own advantages and disadvantages. Two techniques, which show muchpotential, are the Iosipescu shear and ±45° tensile tests. In this paper, the application of these two tests forthe room and high temperature failure analyses of woven graphite/polyimide composites is brieflyevaluated. In particular, visco-elastic micro-, meso- and macro-stress distributions in a woven eightharness satin (8HS) T650/PMR-15 composite subjected to these two tests are presented and their effect onthe failure process of the composite is evaluated. Subsequently, the application of the Iosipescu tests tothe failure analysis of woven composites with medium (T650) and high (M40J and M60J) modulusgraphite fibers and PMR-15 and PMR-II-50 polyimide resins is discussed. The composites were tested assuppliedand after thermal conditioning. The effect of temperature and thermal conditioning on theinitiation of intralaminar damage and the shear strength of the composites was established. It has beenshown in this work that the onset of intralaminar damage and the shear strength of the T-650/PMR-15composite are very strongly dependent on the type of test. The biaxial tension/shear stress fields in thegage sections of the ±45o specimens resulted in much lower estimates of the critical loads for the initiationof intralaminar damage and the composite strength in comparison with the Iosipescu test data at bothroom temperature and at 316oC. This agreed very well with the conclusions from visco-elastic micro- andmeso-stress analyses. It has also been shown in this research that the room and high temperature (at316°C) ultimate shear strengths of the T-650/PMR-15, M40J/PMR-II-50 and M60J/PMR-II-50 compositesystems tested using the Iosipescu shear test increase linearly with an increase in the strains at failure ofthe T-650, M40J and M60J fibers. The room and high temperature shear strengths of the M60J fibersystem are the lowest followed by the strengths of the M40J and subsequently the T-650 fiber compositesystems. No noticeable effect of the thermal conditioning and moisture on the shear strengths of the M40Jand M60J fiber composites was found. However, the critical shear stresses for the initiation of towcracking in the M40J and M60J fiber Iosipescu specimens increased substantially after pre-conditioning.