A review on mechanical metamaterials: Exploring negative Poisson's ratio, energy absorption, failure mechanisms, and current research frontiers

Mechanical metamaterials have become increasingly attractive due to their numerous attributes, including low weight, high strength, negative Poisson's ratio, and high energy absorption. Traditionally, metamaterials have been utilised in wave manipulation such as in optics, acoustics and electromagnetism. Due to the mentioned attributes, there has been a notable rise in the adoption/research of these metamaterials for mechanical and structural applications. This article aims to present a review of the properties and state-of-the-art progress in mechanical metamaterials. A comprehensive review of the literature is presented, highlighting the state-of-the-art in negative Poisson's ratio (NPR), high energy absorption, failure mechanisms, design strategies, and 4D metamaterials. It is observed that the research is currently focusing on the creation/design of novel metamaterials and the enhancement of the properties of existing structures. Some of the common unit cells adopted for mechanical metamaterials include lattice-based, origami/Kirigami-inspired, cellular, or hierarchical unit cells. While great achievements can be reported on NPR and high-energy absorption metamaterials, there is a lag in the failure studies of mechanical metamaterials, especially on fracture toughness. Furthermore, there are concerted efforts to apply topology optimisation, numerical simulations, and machine learning to design higher-performing metamaterials. It is observed that machine learning and data-based design strategies are promising areas of research for the development of high-performance materials. The review article is motivated by the absence of a comprehensive review on mechanical metamaterials despite many research outputs. Several gaps are presented, and this article will be a useful resource for the development of novel mechanical metamaterials.