Transverse and longitudinal CTE measurements of carbon fibers and their impact on interfacial residual stresses in composites

In situ transmission electron microscopy (TEM) is utilized to evaluate the coefficients of thermal expansion (CTE) of two polyacrylonitrile (PAN) based (T1000 and IM7) and one pitch-based (P55) carbon fiber in the temperature range 20-1100 degrees C. The transverse morphology of the fibers is studied using field emission scanning electron microscope and selected area diffraction (SAD) patterns in a TEM and is co-related to the fiber transverse CTE. The PAN-based IM7 and T1000 fibers revealed a transversely isotropic structure, which was consistent with their transverse CTE measurements. Even though the pitch-based P55 fiber revealed cylindrical orthotropy, it did not translate to orthotropy in the transverse CTE. Finite element models of unidirectional IM7/977 composite, utilizing the present IM7 CTE values, are compared to experimental lamina scale CTE results where good agreement was obtained for cool-down regime. The models also indicate that the Tresca stresses in the epoxy matrix at the fiber-matrix interface exceed the allowable strength at cryogenic regimes suggesting possible fiber-matrix debonding and transverse matrix cracking.