Behaviors of Composite Materials to Withstand Impact and Structural Compressive Design Allowableness

In this paper the relationship between CAI obtained by NASA test standard and compressive design allowableness of composite structure are discussed based on the concept of structural design allowableness and the compressive-after-impact behaviors of composites, and it is pointed out that the traditional CAI values could not reflect the behaviors of composite materials to withstand impact adequately and no relationship between CAI values and structural compressive design allowableness existed. Based on the integrity requirements of composite structures, experimental study by authors and the conclusions from available references, it is pointed out that the behaviors of composite materials to withstand impact should include both the damage resistance and the damage tolerance. Large numbers of test data verify that the knee point phenomenon of composite material to withstand impact existed. Based on failure mechanisms, it is proposed that the behavior evaluation system of composite system to withstand impact is based on the properties at the knee point, that is, the maximum capability to keep integrity of the front plies (the maximum allowable contact force) characterizing the damage resistance and the threshold of dent depth(or impact energy)-compressive strength(or failure strain) curves characterizing the damage tolerance.