Delamination behavior of a carbon-fiber-reinforced thermoplastic polymer at high temperatures

Abstract

This study focuses on the behavior of delamination crack propagation of a carbon-fiber-reinforced polymer at high temperatures. At first, the elastic properties of unidirectionally reinforced CF/PEEK at high temperatures were examined in order to evaluate the stress intensity factor, K, as a leading parameter of fracture mechanics. Fracture toughness and slow crack propagation in creep and fatigue were then investigated by the use of double-cantilever-beam specimens. The results obtained are summarized as follows. (1) The CF/PEEK shows excellent fracture toughness at temperatures up to 373 K, but the toughness decreases at higher temperatures because of creep. (2) In static creep, the creep deformation of matrix is confined near the crack tip as a consequence of constraint by the fibers, and the crack propagation rate is correlated well with the stress intensity factor, K. (3) Crack propagation in high-temperature fatigue is classified into two types, time-dependent and cycle-dependent. In the former, the crack propagation rate per unit time, da dt is governed by the stress intensity factor, and the relationship agrees well with that in static creep. In the latter, the propagation behavior resembles that in room-temperature fatigue, and the crack propagation rate per unit cycle, da dN is controlled by the stress intensity factor range, ΔK. The transition condition between the two types is given by the characteristic equation represented as a function of K for time-dependent crack propagation and ΔK for cycle-dependent propagation.