Fracture of Lego Assemblies With Nacre-Like Archictectures

From concept car design to bio-inspired engineering solutions, the use and influence of architectured materials has undoubtedly grown in recent years. The possibility to optimize material properties indicates large opportunities in the field. One of the possible objectives in this specific class of materials is the exploitation of optimization of the material architecture to achieve overall enhancements of structural integrity. We drew inspiration from nacre (Mother of Pearl), a naturally-observed architectured material made up of ceramic tablets arranged in a brick like pattern and bonded by natural polymers. This biological architecture material, while made primarily of brittle ceramic, exhibits remarkable fracture toughness. Our project proposes using Legos as a macroscale model to study nacre-inspired architectured materials and to evaluate their fracture properties. Many variables are involved in the assembly of a Lego structure. We initially hypothesized the structure will perform best in standard pattern walls with the bricks stacked vertically, and that the fracture toughness of the wall can be determined using linear-elastic fracture mechanics.   Using standard 4 by 2, 2 by 2, and 1 by 2 Lego bricks, strength and fracture toughness values were experimentally determined using 3-point bend experiments. Several variables relating to strength were considered. Lego brick assemblies were tested with their studs oriented in horizontally and vertically stacked walls. The layout of Lego bricks assemblies was tested in a standard overlap and random overlap patterns. Uncracked and cracked walls were tested with varied crack sizes. Experiments were validated using different wall sizes while adhering to established aspect ratio standards for fracture tests. Three point bending tests were performed 10 times on each unique wall by applying incrementally increasing weight to the center of the wall. Video analysis was performed to evaluate any gradual deformations, deflections, and crack growth in the wall during testing.     As expected, walls with vertically-oriented studs outperformed other layouts. When crack growth was observed in this configuration, it was found to be hindered by the presence of the many weak bonding faces in the assembly. The type of pattern, standard overlap or random, had no distinguishable impact on the strength of the structure. However, on occasion, the random variation would leave brick separation overlapping with applied cracks, unintentionally lengthening the cracks. Surprisingly, though the crack length was varied from 0 to 37.5% of the overall wall height, crack length was shown to have little impact on the force required to cause the system to fail. In these cases, failure did, in general not emerge from the crack but rather from defects away from the crack. The Lego assembly appears as insensitive to cracks. Only as the crack length was extended to 50% of the wall height did we observe failure to emerge from the crack, and strength to decline. We find linear elastic fracture mechanics only to apply to systems in which the crack is large to the Lego block dimensions.     From these experiments, we demonstrate that design of Nacre-inspired architectured materials employ layouts and building block sizes at the microscale components in a manner that would hinder crack propagation in order to best realize the enhanced mechanical properties enabled in architectured materials. Though the nature of applying Lego bricks did not appear to accurately represent a clear crack propagation model in fracture mechanics, certain essential data trends still hold as vertically-oriented studded walls and standard brick patterns would more often lead to tougher layouts.    Understanding fracture mechanics in Lego-built systems will allow for further research and application in the proper manufacturing and loading of architectured materials. Additionally, Lego blocks are an accessible and fun way to engage with K-12 students about engineering. Experiments from this research can be adapted to form an activity to reach out to students in the community to teach about problem solving, engineering, and building strong structures.    This work was supported by NSF Award 1662177.