Determination of tensile fatigue life of unidirectional CFRP specimens by strand testing

This paper is concerned with the determination of time and temperature depe ndent uniaxial static-strength and fatigue life of unidirectional carbon fi ber reinforced plastics (CFRPs). A novel test system has been designed with fan-shaped grip ends to secure the CFRP strands. Constant temperature is m aintained in a chamber surrounding the central portion of the CFRP strand o nly. Tensile strengths for carbon fiber reinforced by epoxy resin (CF/Ep) s trands under constant strain-rate (CSR) and fatigue loading were measured a t various loading-rates and temperatures. Corresponding experiments were co nducted also by the conventional split-disk method using the CF/Ep, ring as the specimen. Experimental results from these two methods are compared and discussed. The strand-testing method described in this study provided excellent result s of tensile fatigue life in the longitudinal direction of unidirectional C FRPs. Advantages of this method are the small size of the apparatus and its uniaxial state of stress in the test portion of the specimen. Fatigue live s in tension of CF/Ep strands were measured at a single frequency and vario us temperatures. Assuming the validity of the time-temperature superpositio n principle, a scheme is presented to draw two sets of master curves for te nsile failure stress in fatigue from data measured as previously described. In addition, evidence is presented to support the concept that tensile fai lure stress in fatigue follows the principle; the shift factor is the same with the matrix resin studied in this work. Further, evidence is presented to support that the tensile failure stress in fatigue obeys the principle w ith the shift factor for the matrix resin.