An evaluation of the elastic properties and thermal expansion coefficients of medium and high modulus graphite fibers

In this work, the elastic properties and coefficients of thermal expansion of T650-35, M40J and M60J graphite fibers were determined from the macroscopic properties of either unidirectional and/or woven composites of these fibers embedded in polyimide resins. The T650-35 fibers were embedded in a PMR-15 matrix, whereas the M40J and M60J fibers were embedded in a PMR-II-50 polyimide. The three-component oscillator resonance method was employed to determine the elastic properties of the unidirectional and woven composites and their neat resins. The macroscopic coefficients of thermal expansion of the composites and the neat resins were measured by length dilatometry. Subsequently, the fiber properties were calculated from the unidirectional composite macro-data using the Eshelby/Mori-Tanaka approach. For the woven composites, a finite element approach based on the concept of a representative volume element was employed to determine the elastic and thermal properties of the fibers. In the case of the T650-35 fibers, both the longitudinal and transverse elastic and thermal properties of the fibers determined from the unidirectional and woven composites agreed very well with each other. However, for the M40J fibers, noticeable differences were observed between the fiber properties determined from the unidirectional and woven system, which was attributed to the lack of transverse isotropy of the unidirectional system. Since the properties of the M60J fibers were evaluated only from the woven system no direct comparison could be made between the properties obtained from the unidirectional and woven composite architectures. Overall, the methodology was shown to be highly applicable for the accurate determination of fiber properties from both unidirectional and woven systems.