Fracture and Failure Properties of Hierarchically Architectured Materials

在我们之前的项目“纤维束和纤维网络模型用于分层材料的破坏”中，我们使用理想化模型来研究具有分层微观结构的（准）脆性材料与具有非分层微观结构的材料的破坏机制有何不同。在我们的模拟中，我们发现非分层结构的失效是由于损伤（微裂纹）的弥漫性积累导致临界裂纹的形核和扩展。另一方面，在模拟的分层结构中，没有观察到相干裂纹扩展，而是通过微裂纹的持续累积和最终合并而发生破坏。基于这一观察，我们发现由统计上不可靠的材料组成的承载结构的裂纹容限可以通过分层结构来显著提高，以避免裂纹尖端应力集中驱动的裂纹扩展。同时，所研究的分层结构显示出足够程度的结构冗余，以避免当大型结构的强度由其最薄弱的环节控制时发生的统计尺寸效应。通过对有缺口和无缺口纸样品进行单轴拉伸试验，验证了仿真结果。纸张的结构是通过使用激光切割机来创建负载平行切割的分层模式。结果表明，这种分层结构可使缺口试样的断裂应力提高5倍。对原始纸张样本的参考测试，以及随机的非分层切割图案的样本，没有显示出这种效果。在目前的后续项目中，我们希望通过系统的模拟和实验来深化这些结果。虽然最初的项目侧重于二维模型和结构，但我们现在将模拟和实验生产并测试三维样品。样品制造将通过增材制造完成，模拟将使用逼真的梁网络模型来表示这种3D增材制造结构的力学行为。除纸张外，我们将使用其他材料进行样品制造和测试，以确保结果的通用性。在建模和仿真领域，我们将把以前的研究重点放在脆性破坏上，扩展到其他破坏模式，如蠕变和疲劳破坏。这将使我们能够研究在脆性破坏中观察到的分层结构的积极影响是否会延续到其他破坏模式。

In our previous project 'Fiber Bundle and Fiber Network Models for Failure of Hierarchical Materials' we used idealized models to investigate how failure mechanisms of (quasi)brittle materials with hierarchical microstructures differ from those with non-hierachical microstructure. In our simulations we found that non-hierarchical structures failed by diffuse accumulation of damage (microcracks) leading to the nucleation and propagation of a critical crack. In simulated hierarchical structures, on the other hand, no coherent crack propagation was observed and failure instead occurred by sustained accumulation and ultimate coalescence of microcracks.Based on this observation we found that crack tolerance of load-carrying structures consisting of statistically unreliable materials can be significantly improved by hierarchical structuring to avoid crack propagation driven by crack tip stress concentrations. At the same time, the investigated hierarchical structures showed a sufficient degree of structural redundancy to avoid statistical size effects that occur when the strength of large structures is controlled by their weakest links. The simulation results were validated by experiments on notched and un-notched paper samples tested in uni-axial tension.Paper sheets were structured by using a laser cutter to create hierarchical patterns of load-parallel cuts. It was found that the rupture stress of notched samples could be increased by a factor up to 5 by such hierarchical structuring. Reference tests on pristine paper samples, and on samples with random non-hierarchical cut patterns, did not show this effect. In the present follow-up project we want to deepen these results by systematic simulations and experiments. While the initial project focused on two-dimensional models and structures, we will now simulate and experimentally produce and test also three-dimensional samples. Sample manufacturing will be done by additive manufacturing, and simulations will use realistic beam network models to represent the mechanical behavior of such 3D additively manufactured structures. Beyond paper, we will use other materials for sample manufacturing and testing in order to ensure generality of the results. In the field of modelling and simulation, we will extend the previous investigation, which has focused on brittle failure, to other failure modes such as creep and fatigue failure. This will allow us to investigate whether the positive effects of hierarchical structuring observed in brittle failure carry over to those other failure modes.