Transverse compressive stress relaxation in unidirectional CFRP composites: Thermal cycling effects and relaxation relief treatments

UD-CFRP exhibits pronounced viscoelasticity under transverse compressive stress. In plate-type anchorage systems, the material is subjected to sustained transverse clamping stresses over extended periods and is therefore prone to creep deformation and stress relaxation governed primarily by the resin matrix and the fiber–matrix interface, which in turn result in a reduction in clamping stress and a deterioration of the anchorage load-carrying capacity. Meanwhile, in practical engineering environments, UD-CFRP is often subjected to temperature variations, which further complicate the evolution of clamping stress. To this end, compressive stress-relaxation tests were first performed on UD-CFRP under different temperature ranges and transverse compressive stress levels to characterize its stress relaxation behavior under coupled thermal-cycling and stress conditions. Subsequently, the residual compressive mechanical properties after stress relaxation were evaluated. Finally, two methods for compensating for compressive stress relaxation, namely over-preloading and re-preloading, were proposed. The results indicate that, under thermal cycling, the evolution of transverse compressive stress relaxation with time and temperature exhibits a distinct four-stage pattern; After long-term compressive stress relaxation, the transverse compressive strength of UD-CFRP remains essentially unchanged, whereas the elastic modulus increases; Under the same upper temperature limit and stress level, the stress relaxation rate, post relaxation static mechanical properties, and microscopic failure characteristics under thermal cycling were comparable to those under the constant temperature condition at elevated temperature. In addition, the proposed stress relaxation compensation methods were effective in controlling the stress relaxation rate.