Initiation and axial propagation of kink bands in fiber composites

The failure of fiber-reinforced composites in compression is sudden, uncontrolled and results in the dynamic formation of narrow zones of inclined kinked fibers and in a drastic reduction in load bearing capacity. Experimental results are presented which demonstrate that such materials do maintain significant post-failure strength and ''ductility.'' Following the onset of failure. kink bands which were initiated dynamically could be made to propagate (broaden) in the fiber direction. Under displacement controlled loading, the propagation of the bands takes place in a steady-state manner. In the case of the APC-2/AS4 composite analyzed, quasi-static propagation of this failure look place at a stress level which was 40% of the strength of the intact material. The propagation of the band involved progressive addition of narrow zones of fibers from the adjacent intact material by rotation and breaking. This new characteristic stress of such materials has been called propagation stress. Several requirements and guidelines for establishing the true value of this material property, free of influence from experimental conditions and specimen geometry, are outlined. (C) 1997 Acta Metallurgica Inc.