Investigating the Secrets of Nature-Inspired Fracture Toughness: A Deep Dive into the Anisotropic Crack-Bridging Model

Bio-inspired materials, particularly those mimicking the brick-and-mortar architecture of nacre, represent a fascinating area of materials science.  Nacre is renowned for its remarkable strength, toughness, and resilience. The structure of nacre is akin to a brick-and-mortar arrangement. It consists of mineral ‘bricks’ (usually aragonite, a form of calcium carbonate) separated by thin layers of organic ‘mortar’. This layered, hierarchical structure is key to its mechanical properties. In fact, these materials often involve more sustainable production processes, reducing environmental impact. They provide a combination of strength, toughness, and lightness that is difficult to achieve with conventional materials. Scientists and engineers create materials that mimic this natural architecture. They use various synthetic and natural components to replicate the ‘bricks’ and ‘mortar’, aiming to achieve similar mechanical properties. These materials are used in aerospace and automotive industries for lightweight yet strong components. Their ability to absorb impact makes them ideal for these applications. Moreover, in the biomedical field, their biocompatibility and mechanical properties make them suitable for implants, prostheses, and bone repair materials. They can also be used in the design of protective gear, such as helmets and body armor, due to their high toughness and impact resistance.

Bio-inspired materials represents a significant step in material innovation, pushing the boundaries of what’s possible in material science. The development of bio-inspired materials with brick-and-mortar architecture similar to nacre involves a blend of biology, chemistry, materials science, and engineering, promoting interdisciplinary research. In a new study published in the peer-reviewed Journal Composite Structures by PhD candidate Yunqing Nie, Prof.  Dongxu Li, and Prof.  Qing Luo from the College of Aerospace Science and Engineering at the National University of Defense Technology, the researchers conducted a comprehensive study to understand the fracture toughness of bio-inspired materials, specifically focusing on brick and mortar architectures similar to nacre. They developed a microstructure-based anisotropic fracture model to accurately predict the fracture toughness of these materials, acknowledging the importance of material anisotropy in their analysis​​.

The team established a microstructure-based anisotropic fracture model, recognizing the inadequacy of previous models that largely ignored material anisotropy. They used a multi-scale approach, integrating the J-integral method with a novel crack-bridging model. This model accounted for the toughening effect of geometric sizes and material parameters, such as the aspect ratio of ‘bricks’ and the stiffness ratio between the brick and mortar phases. Significant attention was devoted to the crack-bridging mechanism, which was identified as a dominant contributor to crack resistance, especially in the initial stages of crack growth. The researchers explored the relationship between the anisotropic properties of materials and their fracture behaviors, a linkage that had not been thoroughly investigated before. The authors developed the Anisotropic Crack-Bridging Model. The model was based on a 2D composite representing brick and mortar structures, akin to nacre, which consisted of mineral platelets (stiff phase) and organic matrix (soft protein phase). They focused on a semi-infinite Mode-I crack perpendicular to the platelets’ longitudinal direction, crucial for understanding fracture failures in engineering materials​​. The model concentrated on the toughening mechanism of crack-bridging. They made assumptions about the initial crack length and analyzed the stress distribution near the crack tip in anisotropic materials, considering the remote force and the length of the bridging zone​​. They studied the crack opening displacement and its contributions, including the dissipated energy by crack extension in the matrix and the contribution of the crack-bridging mechanism​​.

The team demonstrated their model capability to capture the effects of anisotropic properties on toughening behavior. When considering anisotropy, the model showed that ignoring anisotropic properties could lead to a significant overestimation of the toughening ratio, suggesting that material anisotropy is essential for accurately predicting fracture toughness in these architectures​​. The study also showed that the aspect ratio of platelets and the thickness of platelets are critical parameters affecting the toughening mechanism. Increasing the bridging zone length enhances the closing stress on the crack face, leading to more efficient toughening mechanisms. The findings suggested that both the thickness and length of platelets significantly influence fracture properties​​. Moreover, the researchers examined the effects of various material properties such as the Young’s modulus of mineral platelets, shear modulus, and cohesive law parameters of the organic matrix. They observed that increasing the Young’s modulus of mineral platelets extends the critical bridging zone and increases the equivalent elastic modulus. The adjustments in the interfacial shear modulus and the stiffness ratio between mineral platelets and organic matrix also significantly affected the overall fracture toughness​​.

The authors performed dimensional analysis to establish a general law for estimating the fracture toughness of brick and mortar architectures. This involved formulating a toughening ratio as a function of various parameters, including platelet size and material characteristics. The analysis revealed that increasing certain dimensionless parameters greatly enhances the toughening ratio, indicating a need for optimal values to achieve a balance among strength, stiffness, and toughness​​.

The study by Dr. Yunqing Nie  and colleagues provides significant insights into the design of high-performance, bio-inspired materials. By acknowledging and incorporating anisotropy into the fracture model, the study offers a more accurate and realistic representation of how these materials behave under stress, a vital step forward from the traditionally used isotropic models. The authors successfully provided a robust framework for designing stronger and tougher materials by mimicking nature’s brick and mortar structure, potentially revolutionizing various fields, from aerospace to civil engineering. The study highlights the importance of geometric and material parameters in influencing fracture toughness, offering a path to optimize these properties for better performance.