Cellular Solids

5 Key excerpts on "Cellular Solids"

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  eBook - ePub

  Soft Materials

  Structure and Dynamics

  • John R. Dutcher, Alejandro G. Marangoni(Authors)
  • 2004(Publication Date)
  • CRC Press

    (Publisher)

  13 Biogenic Cellular Solids

  MARTIN G. SCANLON University of Manitoba, Winnipeg, Manitoba, Canada

  I. INTRODUCTION

  A. Classification

  Cellular Solids are one category of soft solids that are found in various forms throughout nature and the food chain.Much of the credit for rationalization of the diverse groups of materials within this category is due to Gibson and Ashby [1 ,2 ], whose approach of combining ‘‘simple mechanics with scaling ideas’’ has greatly simplified our understanding of their structure–property relationships. Their book, Cellular Solids, has been described as ‘‘a classic’’ [3 ] and has been well cited in the biological sciences as well as in the engineering literature.

  Although Cellular Solids are frequently indexed separately from foams, they are actually a subset of them, comprised of a solid network associated with a fluid phase. Due to the continuity of the solid phase, Cellular Solids are stable over reasonable periods of time, in contrast to the inherent instability of liquid foams [4 ]. However, because the primary means of fabricating Cellular Solids is by the setting (rigidification) of a liquid foam, issues of stability in liquid foam structures [3 ] have a bearing on the structure, and hence the properties, of Cellular Solids [5 ,6 ].

  Cellular Solids are also a subset of porous solids (if the fluid phase is air) or saturated porous solids (if the fluid phase is liquid). The distinguishing feature for Cellular Solids is their high porosity (volume fraction >70%), a porosity cutoff that ensures that cell wall thickness relative to cell wall length is small, permitting simplification of the analysis of the mechanical properties [2 ]. The utility of Cellular Solids’ residence within this subset of materials is that rigorous bounds on many physical properties of the Cellular Solids can be defined [7 ]. This is beneficial from a food processing perspective in that it may be desirable to know the extent to which a parameter such as the modulus, which often correlates well with sensory assessements of texture [8

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  Energy Absorption of Structures and Materials

  • G Lu, T X Yu(Authors)
  • 2003(Publication Date)
  • Woodhead Publishing

    (Publisher)

  10 Cellular materials Cellular materials have good energy-absorption characteristics. This chapter presents their stress–strain relations, the fundamental mechanics at the cell level and their impact response. Materials discussed include honeycombs, foams, woods and cellular textile materials. Gibson and Ashby (1997) give a comprehensive treatment of various prop-erties of Cellular Solids in their book. In this chapter, we will start our dis-cussion by presenting the basic properties and mechanics for honeycombs, foams and woods, which are relevant to energy absorption. 10.1 Honeycombs Honeycombs are a typical type of cellular material. They (and foams) are widely used as the core structure in sandwich panels, for example. Honey-combs can also be used alone as good energy-absorbing materials. Their structure is essentially two-dimensional and regular. Hence they are easier to analyse than foams, which have three-dimensional cell structures. In this section, we describe the crushing behaviour of honeycombs. Foams will be discussed in the next section. 10.1.1 Cell structure, relative density, stress–strain curves and densification strain Most honeycomb cells are hexagonal in section (Fig. 10.1), but other shapes are also possible such as triangular, square, rhombic or circular (Chung and Waas, 2002a, b). The materials from which the cells are made are man-made polymers, metals, ceramics and paper. In this section, we will restrict our discussion to honeycombs with hexagonal cells. As shown in Fig. 10.1, a typical honeycomb consists of series of hexago-nal cells whose dimensions are defined by cell wall lengths, l and c , the angle between two cell walls q and the cell wall thickness h . Deformation caused by loading in the global plane of the honeycomb, X 1 X 2 , is known as in-plane 268 Cellular materials 269 response, while that caused by loading in the X 3 direction is out-of-plane response.

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  Cellular Ceramics

  Structure, Manufacturing, Properties and Applications

  • Michael Scheffler, Paolo Colombo, Michael Scheffler, Paolo Colombo(Authors)
  • 2006(Publication Date)
  • Wiley-VCH

    (Publisher)

  But at another we must think of the lattice not only as a set of connected struts, but as a “material” in its own right, with its own set of effective properties, allowing direct comparison with those of monolithic materials. Historically, foams, a particular subset of lattice-structured materials, were studied long before attention focused on lattices of other types. Early studies assumed that foam properties depended linearly on relative density ~ &=& s (i.e., the volume fraction of solid in the material), but for most foams this is not so. A sound understanding of their mechanical properties began to emerge in the 1960s and 1970 with the work of Gent and Thomas [1] and Patel and Finnie [2]. Work since then has built a com- prehensive understanding of mechanical, thermal, and electrical properties of foams, summarized in the texts “Cellular Solids” [3], “Metal Foams, a Design Guide” [4], and a number of conference proceedings [5–9]. The ideas have been applied with success to ceramic foams, notably by Green et al. [10–13], Gibson et al. [14–17], and Vedula et al. [18, 19]. The central findings of this body of research are summarized briefly in Section 1.1.3. One key finding is that the deformation of most foams, whether open- or closed-cell, is bending-dominated – a term that is explained more fully below. A con- sequence of this is that their stiffnesses and strength (at a given relative density) fall far below the levels that would be expected of stretch-dominated structures, typified by a fully triangulated lattice. To give an idea of the difference: a low-connectivity 4 1.1 Cellular Solids – Scaling of Properties lattice, typified by a foam, with a relative density of 0.1 (meaning that the solid cell walls occupy 10 % of the volume) is less stiff by a factor of 10 than a stretch-domi- nated, triangulated lattice of the same relative density.

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  Handbook of Polymer Testing

  Physical Methods

  • Roger Brown(Author)
  • 1999(Publication Date)
  • CRC Press

    (Publisher)

  Although the physical structure is usually complex, which is to say that the solid and gaseous phases will comprise cells, struts, cell walls, pinholes, and other irregularities, it is generally true to say that the chemical makeup of both the solid and gaseous phases is homogeneous in the fully cured foam. A rigid foam then 375 376 Hillier produced as a thermal insulation medium relies on the thermal insulation properties of the enclosed gas in the cells. It is usually assumed that the nature of the gas phase is homo-geneous. There is an optimum size associated with the cell to give the minimum thermal conductivity. If the cells are too large or interconnecting and thermal transfer is allowed by convection, then the thermal conductivity will increase. A solid form of the polymer will of course have minimal thermal insulation properties. The nature of the polymer then is important in several ways. It must have properties that are appropriate for the final application while maintaining an appropriate degree of thermal insulation. Many different types of polymeric cellular thermal insulation are in use today. The properties required are very diverse, and the choice of polymer will depend on the application. As another example we may take flexible foam designed for domestic seating or cushioning. A solid rubber or plastic without any cellular form will have very little in the way of cushioning properties, at least as far as domestic upholstery is concerned. If cells are introduced without any consideration given to the interconnecting nature of those cells, then if those cells are completely closed and nonconnecting the cushioning charac-teristics will be poor and the feel will approximate to sitting on a balloon. A properly formulated foam with the correct degree of interconnecting cell networks will produce a foam with good cushioning and fatigue properties.

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  Porous Materials

  Processing and Applications

  • Peisheng Liu, Guo-Feng Chen(Authors)
  • 2014(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Chapter One

  General Introduction to Porous Materials

  Abstract

  Because porous materials are a new type of engineering materials, people are not generally aware of them. But they are worthy of note due to their valuable attributes of function and structure. This chapter gives a brief overview of the topic so that readers might have a rudimental understanding of it. Some primary concepts are introduced for porous materials, including porous metals, porous ceramics, and polymer foams, and their main groups and chief features.

  Keywords Porous materials Primary concepts Porous metals Porous ceramics Polymer foams

  Porous materials widely exist around us and play a role in many aspects of our daily lives; among the fields they can be found in are energy management, vibration suppression, heat insulation, sound absorption, and fluid filtration. Highly porous solids have relatively high structural rigidity and low density of mass, so porous solids often serve as structural bodies in nature, including in wood and bones [1 ,2 ]; but human beings use porous materials more functionally than structurally, and develop many structural and functional integrative applications that use these materials fully [3 ,4 ]. This chapter will introduce the elementary concepts and features of this kind of material.

  1.1 Elementary Concepts for Porous Materials

  Just as their name implies, porous materials contain many pores. Porous solids are made of a continuously solid phase that forms the basic porous frame and a fluid phase that forms the pores in the solid. The latter can consist of gas, when there is a gaseous medium in the pore, or of liquid, when there is a liquid medium in the pore.

  In that case, can all materials with pores be referred to as porous? Perhaps surprisingly, the answer is “no.” For instance, holes and crannies that are the result of defects will lower a material’s performance. This result is not what designers want, and so these materials cannot be termed porous. So-called porous materials must possess two essential characteristics: one is that the material contains a lot of pores, and the other is that the pores are designed specifically to achieve the expectant index of the material’s performance. Thus, the pore of porous materials may be thought as a functional phase what designers and users hope to come forth within the material, and it supplies an optimizing action for the performance of the material.

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