The fracturing behavior of fiber-reinforced composites is often modeled using continuum damage mechanics-based approaches which commonly assume a linear softening damage law, i.e. a linearly decreasing stress with increasing strain. The objective of this paper is to assess the suitability of this assumption for composites and to systematically analyze the effect of the assumed shape of the softening damage law at material point, on the predicted fracturing behavior of the structure. The material considered is an epoxy/carbon-fiber twill woven composite. Numerical evaluations are conducted by predicting the strength size effect and progressive accumulation of damage in a structure and comparing with experimental measurements. Damage models with a wide range of shapes of softening laws are considered. These include linear, bilinear, trilinear, exponential, power law and sigmoidal. The effect of introducing a sudden drop in the damage law is also evaluated. All modeling is conducted via the crack band model (CBM), to ensure the mesh objectivity of results. It is found that despite using the right strength and fracture energy as input at the material point level, not all softening law shapes can accurately predict the strength size effect and post-peak softening. Especially the sudden drop feature leads to severe underprediction of strength. Bilinear softening shows the best combination of simplicity and accuracy, especially if formulated via the R-curve approach. Further insights are obtained by assessing the predicted fracture process zone size and fracture energy via the size effect method. Finally structure level predictions are made by considering plug initiated axial crushing of a cylindrical crushcan. The close ties between the shape of the material softening law and predicted process zone, and structural fracturing are revealed, and shown to be an important consideration in formulating damage models.

纤维增强复合材料的断裂行为通常使用基于连续损伤力学的方法进行建模，该方法通常采用线性软化破坏定律，即应力随应变线性降低。本文的目的是评估该假设对复合材料的适用性，并系统地分析假定的软化损伤定律形状在材料点上对结构的预测断裂行为的影响。所考虑的材料是环氧/碳纤维斜纹编织复合材料。通过预测强度大小效应和结构中损伤的逐步积累并与实验测量值进行比较来进行数值评估。考虑具有各种软化定律形状的损伤模型。其中包括线性，双线性，三线性，指数，幂律和S形。还评估了造成破坏定律突然下降的效果。所有建模都是通过裂缝带模型（CBM）进行的，以确保结果的网格客观性。结果发现，尽管在材料点级别使用正确的强度和断裂能作为输入，但并非所有的软化律形状都能准确预测强度大小效应和峰后软化。特别是突然掉落的特征导致强度的严重低估。双线性软化显示了简单性和准确性的最佳组合，尤其是通过R曲线方法进行配方时。通过使用尺寸效应方法评估预测的压裂过程带大小和压裂能量，可以获得更多的见解。最后，通过考虑圆柱状的破碎罐的塞子启动轴向破碎来进行结构水平预测。揭示了材料软化规律的形状与预测的过程区域以及结构破裂之间的紧密联系，并被证明是制定损伤模型时的重要考虑因素。