By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Sep 23 2024A recent article published in Advanced Materials demonstrated fabricating cement-based materials inspired by cortical bone's toughening mechanism using a hybrid (three-dimensional (3D) printing/casting) process. Experimental and theoretical methods were employed to illustrate the non-brittle fracture mechanism in the fabricated materials.
Study: Tough Cortical Bone‐Inspired Tubular Architected Cement‐Based Material with Disorder. Image Credit: Remberto Nieves/Shutterstock.com
Background
Natural materials exceptionally possess fracture toughness and strength, the two often competing mechanical properties, using modest constituents assembled into complex arrangements and hierarchical architectures.
However, attaining fracture toughness and strength simultaneously is difficult in engineering materials. For instance, concrete, the most commonly used man-made material globally, suffers from intrinsically low fracture toughness owing to the limited toughening mechanisms. Thus, cement-based materials, including concrete, mortar, and cement paste, exhibit low fracture toughness under tension.
The human cortical bone has high fracture toughness attributed to its hierarchical structure based on a tubular architecture resisting the initiation and propagation of cracks through various intrinsic and extrinsic multi-scale toughening mechanisms. Thus, this study harnessed the geometry of tubes in such a tubular architecture to engineer a new toughening mechanism and enhance the fracture behavior of cementitious materials.
Methods
A cement paste was prepared in a vacuum mixer using Type I cement, deionized water, range-reducing admixture (HRWRA), and viscosity-modifying admixtures (VMA). The mixing was performed in two steps: pre-mixing for 25 s followed by mixing at 400 rpm for 90 s at 70% vacuum and subsequent mixing at 400 rpm for 90 s at 100% vacuum.
Tubular specimens were fabricated using hybrid 3D printing/casting techniques from the prepared cement paste. The first step involved 3D-printing of positive mold using polyvinyl alcohol (PVA). This PVA mold was used to prepare a negative mold of the tubular specimen from urethane rubber. Finally, the fresh cement paste was poured into the urethane rubber mold to cast tubular architected materials, which were cured for seven days in a controlled environment.
The mechanical properties of prepared specimens were investigated experimentally through a three-point bending (3PB) test, which revealed their modulus of rupture (MOR). Alternatively, their fracture toughness was determined using the single-edge notch bend (SENB) test. Both mechanical tests were performed on prismatic specimens of 40×40×130 mm3 dimensions using an electromechanical frame.
At least three repetitions were performed for each experimental test, and the obtained data was analyzed statistically using the Data Analysis Toolbox in Microsoft Excel. One-tailed F-test and T-test with a confidence interval of 95% were used to determine the significant difference between the variance and mean of the data, respectively.
Results and Discussion
Compared to the general single brittle failure in the reference cast specimens, the specific-load versus displacement curves of tubular architected specimens obtained from the SENB test highlighted a distinct globally non-brittle and non-abrupt fracture response. Unlike the catastrophic failure at the first peak load in brittle specimens, the tubular architected designs postponed the abrupt failure and exhibited multiple load drops and increased steps. This allowed for overall hardening and softening behaviors beyond the first peak.
The first load drop in the behavior of circular and elliptical specimens corresponded to crack instigation from the sharp notch to the tube in the bottommost row. The subsequent increase in the load reinitiated the crack from the first tube before the initial notch, propagating toward the successive row of tubes.
Further load drops led to additional crack interaction with the tubes before the crack tip in both designs. This phenomenon was considered responsible for the overall non-brittle and non-abrupt response of the circular and elliptical tubular architected cementitious specimens compared to the cast counterparts.
The specific MOR of the circular and elliptical specimens was similar to that of the solid reference. Thus, engineering tubular architecture to enhance the fracture toughness of the brittle cementitious material did not compromise its strength. These results highlighted the capability of tubular architecture to overcome the mutually exclusive nature of these two characteristics in brittle materials by developing a stable crack propagation.
Conclusion and Future Prospects
Overall, the researchers successfully leveraged intentional tubular defects in cement-based materials to enhance their specific fracture toughness. The underlying competition between tube size and shape enabled engineering stepwise cracking, which significantly enhanced fracture toughness through rising R-curves (specific fracture toughness versus crack growth). Thus, crack-resistant characteristics could be enabled in brittle cement-based materials.
The researchers suggest extending the proposed methodology to quasi-brittle mortar and concrete, enhancing their fracture toughness through deliberate design and harnessing defects. Notably, additive manufacturing techniques and robotic fabrication can be implemented to scale such designs for mortar and concrete.
Furthermore, a statistical mechanics approach to quantify disorders in architected materials can enable novel design methods for the material and the disorders it contains. It may help gain insights into the mechanics-geometry relationships across various architected materials (both brittle and ductile).
Journal Reference
Gupta, S. & Moini, R. (2024). Tough Cortical Bone‐Inspired Tubular Architected Cement‐Based Material with Disorder. Advanced Materials. DOI: 10.1002/adma.202313904, https://onlinelibrary.wiley.com/doi/10.1002/adma.202313904
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