Design Approach Using Damage Tolerant Composites Materials with Rising Fracture Resistance (R-Curve Behaviour)

Composite structures can fail by delamination. For some composite materials, denoted damage tolerant materials, the fracture resistance increases with crack length (R-curve behavior). We consider three design approaches: Level 1 is a traditional linear elastic fracture mechanics design approach where the applied J integral value, J, should be kept below J₀, so that no crack growth occurs, and cyclic crack growth is assessed by the Paris-Erdogan relation of an unbridged crack. Level 2 is restricted to steady-state problems for which the J integral is independent of crack length, so that under monotonic loading unstable cracking will not occur unless J reaches J_ss . The design can be made using analytical models. Level 3 is design using cohesive laws. Level 3 is more general, but also more demanding; the cohesive laws must be known, and a numerical model must be used. We illustrate the Level 1 and Level 2 approaches by analyzing delamination crack growth from a ply-drop under static and cyclic loading. The Level 2 design stresses for both static and cyclic crack growth were significantly higher than that for Level 1. Finally, we discuss the implications regarding safety factors and the limitations of the proposed approaches.