TAILIEUCHUNG - Mechanical properties of polymers, composites-Nielsen 2010 Part 5

Tham khảo tài liệu 'mechanical properties of polymers, composites-nielsen 2010 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 2 Elastic Moduli I. ISOTROPIC AND ANISOTROPIC MATERIALS A. Isotropic Materials Elastic moduli measure the resistance to deformation of materials when external forces are applied. Explicitly moduli M arc the ratio of applied stress cr to the resulting strain e M - I In general there are three kinds of moduli Young s moduli E shear moduli G and bulk moduli K. The simplest of all materials are isotropic and homogeneous. The distinguishing feature about isotropic elastic materials is that their properties are the same in all directions. Unoriented amorphous polymers and annealed glasses are examples of such materials. They have only one of each of the three kinds of moduli and since the moduli are interrelated only two moduli are enough to describe the elastic behavior of isotropic substances. For isotropic materials 9GK 3K G 2 33 34 Chapter 2 It is not necessary to know the bulk modulus to convert E to G. If the transverse strain of a specimen is determined during a uniaxial tensile test in addition to the extensional or longitudinal strain e their ratio called F oisson s ratio V can be used 3 E 20 1 p - 3K 1 - 2v 4 For soft elastic solids V 1 for glassy polymers V is in the range to often about . B. Anisotropic Materials Anisotropic materials have different properties in different directions 17 . l-Aamples include fibers wood oriented amorphous polymers injection-molded specimens fiber-filled composites single crystals and crystalline polymers in which the crystalline phase is not randomly oriented. Thus anisotropic materials are really much more common than isotropic ones. But if the anisotropy is small it is often neglected with possible serious consequences. Anisoiropic materials have far more than two independent clastic moduli generally a minimum of five or six. The exact number of independent moduli depends on the symmetry in the system 1-7 . Anisotropic materials will also have different contractions in different directions and hence a set of .

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