The relationships between a resistance curve ( R - curve ) , the corresponding fracture process zone length , the shape of the traction / displacement softening law , and the propagation of fracture are examined in the context of the through - the - thickness fracture of composite laminates . A procedure that accounts for R - curve toughening mechanisms by superposing bilinear cohesive elements is proposed . Simple equations are developed for determining the separation of the critical energy release rates and the strengths that define the independent contributions of each bilinear softening law in the superposition . It is shown that the R - curve measured with a Compact Tension specimen test can be reproduced by superposing two bilinear softening laws . It is also shown that an accurate representation of the R - curve is essential for predicting the initiation and propagation of fracture in composite laminates .