Abstract The large strain fracture of non-linear complex solids concerns a wide range of applications, such as material forming, food oral processing, surgical instrumental penetration as well as more recently, the design of biodegradable composites for packaging and bio-medical use. Although simulations are a powerful tool towards understanding and designing such processes, modelling ductile fracture in materials such as soft natural composites imposes a new challenge, particularly when the fracture patterns cannot be pre-defined. Here we bring to light new information on these aspects of benefit to the multidisciplinary community, by characterising and modelling the deformation and fracture of short cellulose fibre starch extruded composites. Hyperviscoelastic-Mullins damage laws show merits in modelling such complex systems. Yet they are inferior to a viscoplastic-damage law able to capture exactly their highly non-linear, rate dependent and pressure dependent pseudo-plastic stress-strain response. The viscoplastic-damage law also predicts fracture based on experimental toughness values without pre-specifying the crack path in a Finite Element (FE) model, displaying superiority over the conventional cohesive zone approach. Yet, despite using a toughness parameter to drive crack propagation, spurious mesh dependency is still observed while other previously unreported sources of error imposed by the finite element aspect ratio are also highlighted. The latter is rectified by developing a novel numerical strategy for calculating the characteristic element length used in the damage computations. Inherent mesh dependency suggests that non-local damage models may be essential to model this newly investigated class of natural composites.

中文翻译：

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关于高度非线性生物复合材料的本构和损伤行为建模——粘塑性损伤定律计算的网格敏感性

摘要 非线性复杂固体的大应变断裂涉及广泛的应用，例如材料成型、食品口腔加工、外科器械穿透以及最近用于包装和生物医学用途的可生物降解复合材料的设计。尽管模拟是理解和设计此类过程的强大工具，但在软质天然复合材料等材料中模拟延展性断裂带来了新的挑战，尤其是在无法预先定义断裂模式时。在这里，我们通过对短纤维素纤维淀粉挤出复合材料的变形和断裂进行表征和建模，为多学科社区带来了这些方面的新信息。Hyperviscoelastic-Mullins 损伤定律在对此类复杂系统进行建模时显示出优点。然而，它们不如能够准确捕捉其高度非线性、速率相关和压力相关的假塑性应力应变响应的粘塑性损伤定律。粘塑性损伤定律还基于实验韧性值预测断裂，而无需在有限元 (FE) 模型中预先指定裂纹路径，显示出优于传统内聚区方法的优势。然而，尽管使用韧性参数来驱动裂纹扩展，但仍然观察到虚假网格依赖性，同时还突出显示了其他先前未报告的有限元纵横比造成的误差来源。后者通过开发一种用于计算损伤计算中使用的特征单元长度的新型数值策略来纠正。