Cellular Metals and Ceramics for Defence Applications

Abstract

INTRODUCTIONA cellular material or foam is a dispersion of a gas in a solid matrix. The presence of a large proportion of gas phase in the foam helps in significant weight reduction, and imparts special mechanical properties vastly different from its bulk material. The use of cellular materials allows the simultaneous optimisation of stiffness, strength, energy absorption, overall weight, thermal conductivity, surface area, and gas permeability (the last two for open-cell foams). As such, these materials are highly desirable for a wide range of engineering applications, and can be found commonly in the natural world (e.g., wood and bone). Until recently, however, only wood was used to a great extent. Only during the twentieth century have manmade polymer foams been available for insulation, cushioning, padding, and packaging, but the high structural efficiency of cellular materials has been little used. Techniques now exist for fabricating foams not only of polymers, but of metals, ceramics, and glasses as well. These newer foams are increasingly being used structurally, for insulation, and in systems for absorbing the kinetic energy of impacts. The purpose of the review is to introduce the cellular materials which are having a unique combination of properties and their possible applications in defence and other sectors. The useful properties are described 1 in Fig. 1. These properties can be exploited to have novel engineering designs pertaining to defence, aerospace, and automobile applications.Foams are commonly classified into two types: closedpore foams and open-pore (or reticulated) foams. The closedpore foams have structures resembling a network of soap bubbles and have higher compressive strength due to their structures. DMRL-developed aluminium foam, e.g., belongs to this category. The open-pore foams are identical to the closed-cell ones except the membranes have been removed which produce large channels of interconnected cells, the main advantage of which is its 'flow-through' capability and access to their internal surface. Liquids and gases can flow through the structure with minimal resistance. If the non-solid phases of the structure are non-random and in the form of closelyspaced channels, it is usually called honeycomb structure. This honeycomb structures have one-directional channel with anisotropic mechanical properties and permeability. The channel diameter can vary from several microns to several milimeters based on a variety of metals and alloys 2 . There is a special class of closed-cell foams called syntactic foams. Syntactic foams are composite materials synthesised by filling a metal, polymer or a ceramic matrix with hollow particles called microballoons. The presence of hollow particles results in lower density, higher strength, a lower thermal expansion coefficient, and, in some cases, radar or sonar transparency. Tailorability is one of the biggest advantages of these materials. The matrix material can be selected from almost any metal, polymer or ceramic. A wide variety of microball...

Developments on auxetic closed cell foam pressure vessel fabrications

“…Auxetic closed cell foam studies have only focussed on fabrication [39][40][41]48], and high compressive strain rate testing [60]. With conventional closed cell foam being common for impact protection and energy absorption devices [20,21,24], indentation and impact studies-targeting potential benefits of auxetic behaviour [1,7,8]-could be focusses of further work.…”

Effect of the manufacturing parameters on the pore size and porosity of closed-cell hybrid aluminum foams

“…Aluminum foams are the most frequently produced metallic foams due to their unusual combination of mechanical properties (good energy and sound absorption, low density), see in [18]. The foaming agents and the manufacturing parameters used in the production of aluminum foams are presented in [19] which also offers a review of their advantages and the issues relevant to their use.…”

Influence of the Manufacturing Parameters on the Compressive Properties of Closed Cell Aluminum Foams

“…Among metallic foams, aluminum foams are the most widely manufactured material because they provide a rare combination of properties such as low density, blast amelioration, high energy absorption during compressive static and dynamic loading, flame resistance and sound absorption [10]. A review of the process parameters and foaming agents used in the manufacturing methods of aluminum foam has been presented in [11].…”