Fracture and Failure Properties of Hierarchically Architectured Materials

在我们之前的项目“分层材料的纤维束和纤维网络失效模型”中，我们使用理想化模型研究了具有分层微结构的(准)脆性材料与具有非分层微结构的(准)脆性材料的破坏机制的不同之处。在我们的模拟中，我们发现非分级结构由于损伤(微裂纹)的扩散积累而失效，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，没有观察到相干的裂纹扩展，而是通过微裂纹的持续积累和最终聚合来破坏。基于这一观察结果，我们发现，由统计上不可靠的材料组成的承重结构的裂纹容限可以显著提高，从而避免了裂纹尖端应力集中驱动的裂纹扩展。同时，所研究的层次结构显示出足够的结构冗余度，以避免当大型结构的强度由其最薄弱环节控制时发生的统计尺寸效应。通过对有缺口和无缺口的纸样进行单轴拉伸试验，验证了模拟结果的正确性。结果表明，采用这种分层结构，缺口试样的断裂应力可提高5倍以上。在原始纸样和具有随机非分级切割图案的样本上的参考测试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。虽然最初的项目专注于二维模型和结构，但现在我们将模拟和实验性地生产和测试三维样本。样品制造将通过加法制造完成，模拟将使用逼真的梁网络模型来表示这种3D加法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作和测试，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从脆性破坏扩展到其他失效模式，如蠕变和疲劳破坏。这将使我们能够调查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.