Stress relaxation

About: Stress relaxation is a research topic. Over the lifetime, 12959 publications have been published within this topic receiving 270815 citations.

Network disentanglement and time‐dependent flow behaviour of polymer melts

Abstract: For small strains Lodge's rubberlike-liquid theory is a valid description of the rheological behaviour of polymer melts, but at higher strains the theory fails: The phenomenon of shear thinning which is characteristic for the shear flow of nearly all polymer liquids is not explained, and in elongation the deviation of the experimental data from the predictions of Lodge's theory reflect also a flow thinning, and not a strain hardening in spite of the pronounced S-shape of the stress-strain diagrams. Comparing the measured stress growth and stress relaxation data with the predictions of the theory, it must be concluded that the temporary physical network structure of the polymer melt is destroyed increasingly with the magnitude of the deformation. Hence, the number of entanglements decreases with increasing strain. For simple shear and uniaxial extension of a well-defined polyethylene melt the strain dependence of the relative entanglement density is discussed. The irreversibility of the disentanglement process of the network is considered.

Viscoelastic properties of linear associating poly(n-butyl acrylate) chains

Abstract: The linear viscoelastic properties of entangled hydrolysed poly(n-butyl acrylate) PnBA copolymers, with varying density of functional moieties, i.e., acrylic acid (AA) groups, have been investigated both experimentally and theoretically using small amplitude oscillatory shear and a modified version of the time marching algorithm (TMA). A second, low frequency, plateau, the level of which depends on temperature and AA groups content, is evident in the storage modulus of these copolymers. It indicates that terminal flow, i.e., chain motion at large length scales, is highly suppressed within the experimental timescale. This slow relaxation process is attributed to long lifetime aggregates (sticky junctions) of AA groups. At intermediate frequencies, the dynamic moduli decay with a slope of 0.5 indicative of a Rouse-like relaxation process. This behavior is well captured by the model when constraint release Rouse motion of the strands that are trapped between successive AA junctions or/and trapped entanglements is considered. One new parameter, pst, the probability of a monomer to act as a sticky junction, is introduced in the TMA to account for the influence of the functional AA groups on the chain dynamics. The validation of this simple model is then achieved by comparing experimental and predicted viscoelastic data.

Creep and stress relaxation in alpha-brass at low temperatures

Abstract: Creep of vacuum annealed polycrystalline 65/36 alpha-brass was studied at constant stress in the range 77–30°k, close to the yield point, at elongations not exceeding 3%. The tensile strain σ increased logarithmically with time, and the characteristic strain 8cr ≡ de/d log10 t, given by the slope of the creep curves in semi-logarithmic representation, decreased approximately linearly with increasing temperature, from 29 × 10−4 at 77°k to 11 × 10−4 at 300°k. The creep was correlated with logarithmic stress relaxation, and the relation 8Cr = - 8rel/χ was established. where χ ≡ d[sgrave]/d σ in the coefficient of work-hardening, and 8 rel≡ d[sgrave]/d log10 t the slope of the corresponding curve obtained with stress relaxation at constant elongation. Both the creep and the relaxation are ascribed to a process of residual slip propagation in which the migration of edge disclocations impeded by the drag of conservatively moving jogs is rate determining. The driving force on the jog is the component of...