Density‐Graded Cellular Solids: Mechanics, Fabrication, and Applications

Abstract

Cellular solids have gained extensive popularity in different areas of engineering due to their unique physical and mechanical properties. Recent advancements in manufacturing technologies have led to the development of cellular solids with highly controllable microstructures and properties modulated for multiple functionalities at low structural weights. The concept of density gradation in cellular solids has recently gained attention due to its potentials in opening new doors to the development of lightweight structures that offer optimal physical and mechanical properties without compromising their favorable characteristics. Herein, a comprehensive insight into the fundamental concepts, fabrication, and current and potential applications of density‐graded cellular solids in various areas of science and engineering is provided. Cellular solids are broadly classified into two main categories: foams and lattice structures. An overview of the fundamental concepts in each category is presented, followed by details on the characterization approaches and some of the most novel processing techniques utilized in fabricating the structures. The uses of density‐graded structures in load‐bearing, acoustic, and biomedical applications are highlighted. The state of the art in each category and the current trends in application‐specific optimization of density‐graded structures are discussed. The review concludes with an outlook of the future directions in this exciting field.

A Classification of Aperiodic Architected Cellular Materials

“…Instead, this high-level classification of all aperiodic cellular materials is arrived at empirically, by observation of structures in nature, as well as a review of the engineering literature. In addition to these observations, engineering applications leveraging aperiodic cellular materials have traditionally involved metal foams, but there is a growing body of research that explores ideas such as the programmable insertion of defects into periodic cellular materials [9,10], gradation in cell size or thickness [11], and the use of multiple-unit cell shapes and managing transitions between them [12,13]. Several approaches have also been proposed for the design of these aperiodic cellular materials [14], and a selection of these methods, most commonly the Voronoi tessellation, have also been implemented in commercial design software [15].…”

“…Instead, this high-level classification of all aperiodic cellular materials is arrived at empirically, by observation of structures in nature, as well as a review of the engineering literature. Gradation is the most commonly studied form of aperiodicity in cellular materials and is defined as a prescribed spatial variation in a geometric feature of a unit cell [11]. In Figure 3, the feature being graded is the width of the rectangular unit cell, which increases from left to right.…”

“…One such example, using the commercially available nTopology software [15], is shown in Figure 4b. Gradation is the most commonly studied form of aperiodicity in cellular materials and is defined as a prescribed spatial variation in a geometric feature of a unit cell [11]. In Figure 3, the feature being graded is the width of the rectangular unit cell, which increases from left to right.…”

Tuning the Mechanical Behavior of Density-Graded Elastomeric Foam Structures via Interlayer Properties

“…Polymeric foams are widely used in various applications due to their superior mechanical and thermal properties, including excellent energy absorption performance, specific strength, insulating properties, low structural weight, and low production cost. Cell architecture design and the use of flexible polymers as the base materials have facilitated the development of flexible polymeric foams with tunable mechanical properties and controlled degrees of deformation recovery, leading to the widespread use of this class of materials in applications where repeated loading events are common. − However, one of the major drawbacks in the use of polymeric foams as protective structures stems from the dichotomy between specific mechanical strength and strain energy dissipation . It is understood that the mechanical load-bearing capacity of polymeric foams is directly correlated with their density: i.e., the presence of thinner cell walls in the structure results in lower strengths.…”

Mechanical and Thermal Properties of Functionally Graded Polyolefin Elastomer Foams

“…The generation of a cell size gradient inside a foam can be associated with variation in the foaming agent concentration or temperature across the foam thickness under production [ 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ]. This asymmetric spatial feature can lead to superior results in terms of mechanical properties and thermal insulation, which can make them useful in a variety of applications, including thermal or sound insulation, high strength at low weight and impact resistance [ 24 , 32 , 33 , 34 ].…”