TAILIEUCHUNG - Mechanical properties of polymers and composites-Nielsen Episode 6

Tham khảo tài liệu 'mechanical properties of polymers and composites-nielsen episode 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Elastic Moduli 43 SANDWICH SHEAR ANNULAR POCCHETINO OR CONE AND PLATE TORSIONAL DEFLECTiOn Figure 4 Some test modes for measuring shear response. Arrows indicate direction of the force or displacement applied to that surface. be denoted as D for tensile compliance J for shear and B for bulk. The compliance-modulus relationships for elastic materials have been given in equations to those for viscoelastic materials are given in subsequent chapters. If a disklikc specimen is sheared between two end plates by rotation of one over the other to obtain the shear modulus then at any moderate twist angle the strain and strain rate vary along the radius so only an effective shear modulus is obtained. For better results the upper plate is replaced with a cone of very small angle. Figure 4 shows fche cone-and-plate and two other possible test geometries for making shear measurements. At high frequencies of 104 to 107 Hz Young s modulus of fibers and film strips can be measured by wave propagation techniques 16 49-53 . An appropriate equation when the damping is low is E pv2 14 where p is the density of the material and v is the velocity of the ultrasonic wave in it. III. RELATIONS OF MODUL TO MOLECULAR STRUCTURE The modulus-time or modulus-frequency relationship or graphically the corresponding curve at a fixed Temperature is basic to an understanding of the mechanical properties of polymers. Either can be converted directly to the other. By combining these relations curves with a second major response curve or description which gives the temperature dependence of these time-dependent curves one can cither predict much of the response of a given polymer under widely varying conditions or make rather 44 Chapter 2 detailed comparisons of the responses of a set of polymers 7 16 54-56 . These frequency time response curves serve to delineate the effects of changes in backbone structure i. e. the type of polymer or copolymer molecular weight and molecular weight .

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