Mechanical behaviour

Mechanical properties can probably be regarded as the most signifieant aspeet of materials, as adequate mechanical properties are required for any application. This is not to say that other physical properties, such as density and thermal conductivity, can be neglected as these are also vital in materials selection. In understanding the mechanical properties of agro-polymers it is useful to compare their behaviour to that of synthetic polymers. 

Mechanical properties are essentially a description of a material s response to an external load and its ability to reversibly or irreversibly deform as a result of it. These properties are generally evaluated by standardized methods. Mechanical properties of natural polymers (or agro-polymers) are not characterized any differently than conventional polymers and important properties include strength. Young s modulus, stress at yield, elongation and toughness (energy to break), etc. 

Mechanical properties of biomaterials compared to currently available synthetic polymers. Adapted with permission from B. Lagrain, B. Goderis, K. Brijs and J. A. Delcour, Biomacromolecules, 2010, 11, 533-541. Copyright 2010 American Chemical Society. The shadowed triangle shows systems of soy, sunflower isolate, starch/zein, soy/corn starch, whey, zein, and wheat protein.  

The mechanical properties of agro-polymers are generally a lot weaker than those of commodity engineering plastics. Their properties are also strongly dependent on moisture content as well as other plasticizers because of their characteristic hydrophilicity. Apart from these, the most prevalent factors alfecting the mechanical properties include processing conditions and composition. 

Water is required in the manufacture of both thermoplastic starch and thermoplastic protein. For starch, it ensures gelatinization and the formation of a continuous thermoplastic phase. In proteins, it lowers the glass transition and denaturing temperature, allowing processing. However, dry resin ensures dimensional stability directly after injection moulding and over time. 

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Polymers have found widespread applications because of their mechanical behaviour. They combine the mechanical properties of elastic solids and viscous fluids. Therefore, they are regarded as viscoelastic materials. Viscoelastic... 

Before discussing tire complex mechanical behaviour of polymers, consider a simple system whose mechanical response is characterized by a single relaxation time x, due to tire transition between two states. For such a system, tire dynamical shear compliance is [42]... 

The mechanical behaviour of a polymer as a function of temperature is summarized in figure C2.1.15. The... 

In the last section we considered tire mechanical behaviour of polymers in tire linear regime where tire response is proportional to tire applied stress or strain. This section deals witli tire nonlinear behaviour of polymers under large defonnation. Microscopically, tire transition into tire nonlinear regime is associated with a change of tire polymer stmcture under mechanical loading. 

Just as the electrical behaviour of a real diatomic molecule is not accurately harmonic, neither is its mechanical behaviour. The potential function, vibrational energy levels and wave functions shown in Figure f.i3 were derived by assuming that vibrational motion obeys Hooke s law, as expressed by Equation (1.63), but this assumption is reasonable only... 

H. LI. D. Pugh, ed.. Mechanical Behaviour of Materials Under Pressure, Applied Science Publishers Ltd., London, 1971. 

R. W. Davidge, Mechanical Behaviour of Ceramics, Cambridge University Press, 1979. 

The beginnings of the enormous field of solid-state physics were concisely set out in a fascinating series of recollections by some of the pioneers at a Royal Society Symposium (Mott 1980), with the participation of a number of professional historians of science, and in much greater detail in a large, impressive book by a number of historians (Hoddeson et al. 1992), dealing in depth with such histories as the roots of solid-state physics in the years before quantum mechanics, the quantum theory of metals and band theory, point defects and colour centres, magnetism, mechanical behaviour of solids, semiconductor physics and critical statistical theory. 

Dislocations are involved in various important aspects of materials apart from mechanical behaviour, such as semiconducting behaviour and crystal growth. I turn next to a brief examination of crystal growth. 

In 1964, two competing series of slender volumes appeared one, the Macmillan Series in Materials Science , came from Northwestern Morris Fine wrote a fine account of Phase Transformations in Comlen.ted Systems, accompanied by Marvin Wayman s Introduction to the Crystallography of Martensite Transformations and by Elementary Dislocation Theory, written by Johannes and Julia Weertman. The second series, edited at MIT by John Wulff, was entitled The Structure and Properties of Materials , and included slim volumes on Structure, Thermodynamics of Structure, Mechanical Behaviour and Electronic Properties. 

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