Static and Fatigue Properties of UD Carbon Fibre Composites at 77 K

Abstract

The applicability of CFRP in cryogenics has been the subject of numerous publications^1–3. However, extensive uncertainties still exist as to how the properties of the matrix affect the strength and fatigue resistance of the composite under cyclic loading. In the course of cooling to low temperatures different thermal contraction of fibre and matrix give rise to thermal stresses and strains which affect the mechanical properties of the UD-composite. The carbon fibre is prestressed in compression while the matrix undergoes tensile prestresses and strainsl. These tensile strains are especially dangerous because the polymer gets brittle, and the cryogenic fracture strain decreases to a few percent. Hence, the formation of matrix cracks has been observed in composites due to thermal cycling^4–6, especially when using brittle duroplastic matrix polymers, such as epoxies. At the same time, shear stresses arise between the fibre and the matrix. In case of a low interfacial strength this might result in premature localized debonding.