Many composite laminates exhibit the Resistance-curve (R-curve) effect where energy absorption increases with incremental delamination growth. The energy absorbing mechanisms can be related to interlaminar non-linear fracture behaviors. Cohesive zone model (CZM) is useful for capturing the non-linear fracture process zone (FPZ). Researchers have proposed various traction softening laws because crack opening displacement (COD) corresponding to complete fracture (δmax) will vary for different materials and this will affect the shape of the traction softening law for that material. There is still no consensus on a general function form of traction softening law so far. Thus, in this work, six tested cases (different materials) from literatures are studied to compare the effectiveness of different softening laws, including linear, exponential, modified linear, modified exponential, multi-linear and four-parametric exponential functions. Especially, a unified data reduction method is used for calculating the damage onset parameters K0 and σ0, and J-integral-crack opening displacement (J-COD) curves where only experimental load–displacement data are needed. All softening laws are determined using calculated J-COD curves. Numerical studies shown that the effectiveness of different softening laws can be ranked as follows: multi-linear = four-parametric exponential > modified linear = modified exponential > exponential = linear > no softening (without considering FPZ). Meanwhile, it is found that the dimensionless parameter δmax/δ0 (COD corresponding to complete failure and damage onset, respectively) is effective for selecting an appropriate softening law. This work can provide references for simulating mode I quasi-static delamination growth with significant R-curve effect.
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