Transverse and Longitudinal CTE Measurements of Carbon Fibers and their Impact on Interfacial Residual Stresses in Composites

Abstract

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 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.

Fibre hybridisation in polymer composites: A review

“…Both fibres will have the tendency to change their length due to their coefficient of thermal expansion (CTE). The CTE of carbon fibre is typically between -1 and +1 K -1 [14,28,29], while the CTE of glass fibre is 5-10 K -1 [14,30]. This causes the glass fibres to increase their length upon heating, while carbon fibres After the resin is cured and the composite is cooled down, the glass fibres will shrink, while the carbon fibres will more or less maintain their length.…”

On the fatigue response of a bonded repaired aerospace composite using thermography

“…Carbon fibres are thermally anisotropic, as they possess a negative CTE in the longitudinal direction and a positive CTE in the transverse direction [42][43][44]. However, Wong reported positive CTE values at the two principal directions [32].…”

Real-Time Debonding Monitoring of Composite Repaired Materials via Electrical, Acoustic, and Thermographic Methods

“…where ρ is the mass density, C p the specific heat and constant pressure, α the Coefficient of Thermal Expansion (CTE), T the absolute temperature, ΔΤ the change in temperature in Kelvin degrees, K m the thermoelastic coefficient and Δσ the change in the sum of principal stresses. For a typical CFRP plate, CTE varies or even changes sign for the longitudinal and transverse direction [38], and as a result, the thermoelastic effect is different in the two principle directions [39,40].…”