Fracture and Failure Properties of Hierarchically Architectured Materials

• Applicant: Professor Dr. Michael Zaiser
• Subject Area: Computer-Aided Design of Materials and Simulation of Materials Behaviour from Atomic to Microscopic Scale ; Mechanical Properties of Metallic Materials and their Microstructural Origins ; Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: since 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 313904396

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.

• DFG Programme: Research Grants
• International Connection: Finland
• Cooperation Partner: Professor Dr. Mikko Juhani Alava
• Co-Investigator: Privatdozent Paolo Moretti, Ph.D.