Shammo Dutta

Hierarchically designed mechanical metamaterials involve nested levels of structural organization, mimicking natural structures (such as bones, wood, and bird feathers) to create advanced functional materials. Compositional hierarchy, a specific type of hierarchical strategy that involves the methodical assembly of discrete building blocks, offers unique advantages in engineering design due to its modular nature. This involves proper selection and spatial arrangements of distinct microstructures, as a result of which the desired macro-scale mechanical behavior can be achieved. Towards the design of such compositional hierarchical metamaterials, this paper presents a two-step design framework. First, material optimization of the design domain is performed using a parameterized elasticity matrix to obtain optimal conceptual designs. Second, building-block microstructure geometries are selected from a qualitative library and subjected to shape-size refinement to satisfy the desired kinematic or stiffness requirements. To construct the qualitative library, a novel parametrization scheme is initially introduced, which categorizes the planar orthotropic elasticity matrix into four distinct classes. Utilizing a kinetostatic load flow visualization technique, the candidate microstructure geometries are then populated within these four classes. The framework is validated for the design of a cantilever beam with a specified lateral stiffness requirement and the design of planar sheets that exhibit specified target deformation patterns. Thus, the present work provides a systematic and physically intuitive methodology applicable to arbitrary kinematic deformation and stiffness requirements.

Tendon-driven soft robotic systems are extensively recognized for their intrinsic flexibility, exceptional dexterity, potential for safe human interaction, and adaptability to unstructured environments. Modeling such systems is challenging because of their highly nonlinear continuum structure, coupled with complex tendon interactions and friction. Traditionally, modeling has been dominated by continuum Cosserat rods. While mathematically robust, the underlying assumptions of the Cosserat model restrict its use to simple, slender geometries. Moreover, the Cosserat rod models do not fully consider the complex tendon interactions in the system. Herein, we propose an alternative high-fidelity finite element model for tendon-driven soft robotics systems, incorporating a hyperelastic material model, contact definitions within and between the bodies, and frictional forces, to accurately capture the deformation of tendon-driven locomotion and manipulation systems, achieving a mean tip position error of less than 7.91%. In addition, the finite element modeling (FEM) is verified by comparing the complete shape of the actuators using curvature and torsion metrics. The efficacy of the proposed FEM is demonstrated through a parametric analysis of tendon-driven soft actuators and by modeling nonslender actuator geometries subjected to significant contact interactions. This research paves the way for scalable and generalizable computational models to enhance the practicality of tendon-driven soft robotics.

Mechanical metamaterials have garnered considerable interest for their enhanced properties, such as high strength-to-weight ratios, remarkable resilience, and superior energy absorption capabilities. Despite their advantages, localized stress concentrations in additively manufactured metamaterial geometries remain a challenge. In this article, we propose a bioinspired design optimization framework to achieve metamaterials with uniformly distributed stresses. The framework uses the maximum material utilization (MMU) metric to quantify and uniformly distribute stresses in metamaterial geometries. Optimization begins with the selection of an initial conceptual design from a qualitative library of planar metamaterials previously developed by the authors. Once we have a conceptual design, we optimize it using the MMU metric for both size and shape. We assessed our optimization methods on two planar auxetic metamaterials: negative Poisson’s ratio microstructures with low shear (NPLS) and negative Poisson’s ratio microstructures with high shear (NPHS). The optimized designs achieved a uniform stress distribution across the entire topology, at both the microstructural and material levels. We highlight the efficacy of our design methodology by using numerical simulations and experiments. In addition, we demonstrate the utility of stress-optimized metamaterials by conducting numerical dynamic impact tests on optimized and unoptimized NPLS lattices. The optimized lattice absorbed more energy than its unoptimized counterpart. This study paves the way for computationally inexpensive, insightful, and stress-based design optimization of metamaterials.

Auxetic materials have gained popularity in engineering applications owing to their unique deformation response mechanism. However, they have not been exploited to their full potential in engineering load bearing applications. The current paper, therefore, is focused on exploring and improving the deflection behaviour of auxetic beam structures. Initially, a single re-entrant unit cell and an array of auxetic clusters are modelled using Finite Element Method (FEM). These numerical models are then verified for its Poisson’s ratio and deflection behaviour using theoretical formulations. Subsequently, in the next phase, a comparison of the deflection characteristics of the in-use common beams with that of the conventional auxetic beam design is carried out. Much overlooked factors such as orientation and placements of auxetic clusters are introduced in beam designs and are exploited to improve the deflection characteristics of conventional auxetic beams. Through this assessment, the paper proposes two novel design concepts of Oriented Re-entrant Structures (ORS) and Assorted Re-entrant Structures (ARS) for improved load bearing response. Novel designs of ORS and ARS beams are observed to perform significantly better than the conventional auxetic and honeycomb beams. The newly proposed beam designs exhibit a 64% reduction in mass in comparison to the homogeneous beam. The usefulness of these designs are brought out by introducing the ARS auxetic beams into a real-world lightweight foot bridge design. The bridge designs with ARS cross beams demonstrates a better behaviour in comparison to the bridge designs with conventional cross beams in terms of both deformation and material usage. This work highlights the potential use of unconventional mechanical metamaterial structures in engineering load bearing problems to address the demands of green engineering and sustainability without compromising on its structural integrity.