Ultra-low-cycle tension of fabric CFRP along the warp

The paper presents the results of experimental studies of deformation, acoustic emission and accumulation of micro-damages in plain weave fabric carbon fiber reinforced plastic (CFRP) under cyclic tension along the warp threads. Strain measurements were carried out using an optical extensometer, which ensures accurate positioning of the specimen along the loading direction. The loading program included up to 100 stretching cycles with an asymmetry coefficient of R = 0.1 followed by failure. All specimens passed the pre-cyclic loading program. The maximum stress values in the cycles corresponded to 25...90 % of the static tensile strength. It is revealed that in the process of cyclic tension, a one-sided accumulation of strain (cyclic creep) occurs. The accumulated strain depends nonmonotonically on the maximum stress in the cycle, increasing to 600 MPa and decreasing at high stresses. Analysis of the strain kinetics in cycles revealed an increase in the tangential modulus, which is obviously due to the straightening of the fibers. During rupture, an increase of 5-10 % in residual strength was noted in comparison with the ultimate strength under single loading. Acoustic emission phenomena occur both in half-cycles of stretching and in half-cycles of unloading, the Kaiser effect is not observed. Cyclic tests at a temperature of 80 ° C showed a sharp decrease in the number of acoustic events and the disappearance of events in the medium frequency ranges. This is due to the fact that heating leads to a decrease in residual technological stresses in the mesostructure elements of the fabric composite, causing changes in the process of micro-damage across the fibers and to a shift in the places where the warp and weft threads intertwine. In all cases, the failure of the specimens occurs when the longitudinal strain reach a value of 1.50 ± 0.06%. This can serve as a basis for predicting strength in other cyclic loading programs.