title: Damage Tolerance and Failure Analysis of Hierarchically Patterned Materials
Creators: name: Pournajar, Mahshid
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024

description: Hierarchically (micro)structured materials contain elements that repeatedly ap- pear on multiple scales in a self-similar manner. Architectures of this type are sup- posed to provide enhanced toughness in materials made from brittle constituents. Network models have been developed to explore the failure behavior of hierar- chical structures. While modeling approaches have confirmed the mechanical advantages of these microstructural arrangements, experimental validation of the viability and applicability of these models remains an open question. In this thesis we apply both simulation and experimental approaches to explore how the hierarchical architecture of materials improves toughness and enhances damage tolerance in different loading modes. Conducting experiments on quasi- two-dimensional materials, we examine the failure behavior of paper sheets as well as polystyrene (PS) sheets as simple prototypes of a disordered quasi-brittle material. Different microstructures are imposed in the form of laser cut hier- archical and non-hierarchical reference slice patterns. Fault tolerance is investi- gated considering both intact samples and samples with pre-existing macroscopic cracks. To simulate fracture in different loading conditions, failure is reached under displacement-controlled tensile loading and time dependant creep constant sub-critical load. We record acoustic emission signals to monitor damage, and we use digital image correlation technique (DIC) to reconstruct surface strain pat- terns. We perform similar tests in two-dimensional (2D) beam network model (BNM) simulations, which allow us to average results over multiple realizations of structural disorder and local strength fluctuations. We demonstrate that hierarchical patterning effectively suppresses strain local- ization and crack propagation. As a result, hierarchical patterned samples show higher failure load, apparent fracture toughness, and work of fracture. Under creep conditions, the hierarchically patterned samples sustain larger creep strains at higher stress levels. Creep curves exhibit peculiar behavior characterized by multiple minima in creep rate, attributed to the repeated arrest of emerging lo- calization bands. Our results indicate that hierarchical patterning enhances fault tolerance by dramatically altering failure mechanisms. In order to further explore the relation- ship between microstructure and fracture behavior, and to investigate how the network structure responds to damage, we implement methods of graph theory to quantify statistical correlation between network measures and failure propensity. We study the possibility of predicting failure locations by quantifying the edge betweenness centrality (EBC) of the load carrying beams in our quasi-brittle sim- ulation models. We evaluate the effectiveness of the EBC metric as a structural predictor of future failure events for varying degrees of local-strength disorder highlighting, as above, the differences between hierarchical and reference systems.