tailieunhanh - Mechanical properties of polymers and composites-Nielsen Episode 10

Tham khảo tài liệu 'mechanical properties of polymers and composites-nielsen episode 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Creep and Stress Relaxation 83 and Halpin 58 have similarly reported that the creep rate is independent of the stress level lor four different types of elastomers for strains up to about 2 X . Landel and Stedry 59 appear to have been the first to publish explicit stress-relaxation data showing the independence of strain and time for SBR up to very large strains and a polyurethanc rubber llavsky and Prins 60 and co-workers have presented similar explicit results for a series of polyurethanc rubbers. Thus the response can be separated or factored into functions of time and of stress. Factorizability when it holds offers a powerful simplification to any attempt to develop theories or descriptions of polymer response whether phenomenologically OMnolecularly based as well as an often stringent test of their validity. The factorizability is easily tested since plots of log r or log e 0 versus log will all be linear with the same slope. In the more general case the curves will not be linear but they will still be parallel. A cross plot of the strains at any given time against the stress will give the resulting isochronal constant time e -- Ơ or r - e relationship. Using this factorizability of response into a time-dependent and a straindependent function. Landel et ai. 61 62 have proposed a theory that would express tensile stress relaxation in the nonlinear regime as the product of a time-dependent modulus and a function of the strain Xư U r FK X G r F X Ị 3Ó Here is the usual small-strain tensile stress-relaxation modulus as described and observed in linear viscoelastic response . the same E 0 as that discussed up to this point in the chapter . The nonlinearity function describes the shape of the isochronal stress-strain curve. It is a simple function of X which however depends on the type of deformation. Thus for uniaxial extension F X X - X zSw X- V3 31 The underlying nonlinearity function m X which is independent of the type of deformation is very similar for