Inspired by the vast amounts of investigations carried out on three-dimensional (3D) printed structures and their recent accelerated developments, the present review paper comprehensively describes the current trends as well as promising findings of 3D printed energy absorbing structures (EAS) for crashworthiness application. Particular attention is paid to the mechanical behaviour and crushing performance of 3D printed EAS. The main 3D printing technological processes, their material feedstocks choices and unique structural designs, investigated recently, are discussed in detail. Deformation modes obtained by 3D printed EAS under different loading conditions are identified. Additionally, salient suggestions with future realisation of complex 3D printed EAS are provided. This review will serve as a springboard to propel the technological advancement of additively manufactured EAS incorporated into moving vehicles and utilised as protective devices. Hence, setting the goals to encourage novel research that guarantees the efficient protection of lives and valuables during mild and catastrophic impacts.

The increasing motivation behind the recently wide industrial applications of sandwich and composite double panel structures stems from their ability to absorb sounds more effectively. Meticulous selection of the geometrical and material constituents of both the core and panels of these structures can produce highly desirable properties. A good understanding of their vibro-acoustic response and emission index such as the sound transmission loss (STL) is, therefore, a requisite to producing optimal design. In this study, an overview of recent advances in STL of sandwich and composites double panels is presented. At first, some salient explanation of the various frequency and controlled regions are given. It then critically examines a number of parameter effects on the STL of sandwich and composite structures. Literatures on the numerical, analytical and experimental solutions of STL are systematically presented. Efficient and more reliable optimization problems that maximize the STL and minimize the objective functions capable of degrading the effectiveness of the structure to absorb sounds are also provided.

The emergence of 4D printing from additive manufacturing has opened new frontiers in crashworthiness application. Energy-absorbing structures with fixed geometrical shapes and irreversible deformation stages can be programmed such that after mild or extreme deformation, their initial shapes, properties and functionalities can be recovered with time when actuated by external stimuli. This survey delves into the recently-accelerated progress of shape memory/recovery energy-absorbing metamaterials (EAMM) and energy-absorbing smart/intelligent structures (EASS). First, the introduction gives some fundamental concepts of metamaterials and their application to energy-absorbing structures. Next, some common 3D printing technologies that have led to 4D printed EAMM and EASS are succinctly described. Shape memory materials, their functional properties and recovery process, are then discussed. Finally, various recoverable/reversible energy absorbers with their future challenges and perspectives, are presented. With well-tailored 4D printed EAMM and EASS, reusability with minimal maintenance and higher energy absorption capacity can be retained.