Engineering Twist

An emerging frontier in mechanical engineering is efficient analysis of multi-body systems made of materials with structures that span length scales. A new material modelling paradigm is forming, to treat such structures as uniform bodies (homogenisation). The experimental processes and uncertainty quantifications to define these material models are neither sufficiently understood nor developed for widespread uptake. An exciting example of multiscale structures, mechanical metamaterials (e.g., lattices), are engineered to have effective properties that cannot usually be achieved with the materials they are made from. Mechanical metamaterials can improve functionality, or sustainability by structuring sustainable materials to mimic unsustainable ones. As such, metamaterials are an EPSRC priority; having funded the UK MetaMaterials Network and after accepting “The MetaMaterials Revolution” into the big ideas pool. Their unusual properties are often used for applications with complex load cases, such as sporting impact protection, biomedical devices, or aerospace vibration isolation. With effective properties achieved by relatively large internal features (e.g., cells), and requirement for high-performance in complex load cases, they make ideal multiscale analysis demonstrators. Most engineers base understanding on classical elasticity theory, where a body deforms evenly, by moving in three axes when stretched or compressed. Classical theories become inaccurate when materials or structures have large internal features, like cells, because stretching or compressing causes sections to twist or deform unevenly. Mechanical metamaterials, particularly ones that twist in response to a uniaxial load (i.e., force-talk coupling), clearly show non-classical effects, as do many biological materials. The typical approach to undertake computer simulations of such materials is modelling geometries that replicate their complex structures. These are computationally expensive, so design of such materials is based on experiments, or simulations on super computers, that are often inefficient or unfeasible. Homogenisation methods are well developed for small deformations, but those for large deformations are not. Micromorphic theories allow internal features (micro) to change shape or orientation (morph). These provide the degrees of freedom required to simulate mechanical metamaterials or biological tissue under large deformations. The extra terms in micromorphic theories vary with applied deformation and deformation rate. An unsolved trade-off exists between rigorous definition of effective properties, and model complexity. For such methods to be widely adopted efficient characterisations for each extra variable, and the level of uncertainty caused by negating or fixing them, must be discovered. Understanding the limits of each variable allows exciting discovery of extreme properties toward such limits. The principal investigator (OD) recently showed that force-talk coupling, a micromorphic effect, makes mechanical metamaterials harder to indent. Impact and benefits: To transform simulation efficiency of lattices, and other materials with large internal features (e.g., cells), so they can be included in full-system simulations used from biomedical devices and protective equipment to transport and energy sectors. Aim: To develop and demonstrate micromorphic models, characterisation protocols, and uncertainty quantification, to drive a disruptive step change in mechanical metamaterial simulation and design. Obj 1: To create a testbed of mechanical metamaterials with force-torque coupling, microstructurally faithful models, and micromorphic models. Obj 2: To characterise and simulate the large strain and depth indentation response of materials with force-torque coupling. Obj 3: To explore simulation and characterisation at the high strain rates expected during sporting impacts. Obj 4: To undertake partner-set characterisation and simulation challenges, demonstrating levels of agreement and simulation costs for provided mechanical metamaterial products.