Stress relaxation

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

Dynamics of sorbitol and maltitol over a wide time-temperature range

Abstract: The relaxation behaviour of two molecular glass-forming systems, namely sorbitol and maltitol, are investigated in the large temperature range relevant to the glass-transition. These data are obtained by combining three techniques, i.e. low-frequency mechanical spectroscopy, medium and high frequency dielectric spectroscopy, and viscosity measurements. This procedure allows to determine the relaxation map of these polyols on a wide time range [10-9-107 s]. Two different relaxation processes can be observed. The principal α-relaxation process exhibits a complex behaviour, comprising a non-Arrhenius temperature dependence above T g (supercooled liquid state), and an Arrhenius behaviour below T g (glassy state). A secondary β-relaxation is observed at higher frequencies with an Arrhenius temperature dependence. The secondary process appears in the same time-temperature range in both polyols. Consequently the molecular root of this relaxation is most likely the same in these complementary chemical systems. On the other hand, the time scale on which the α and β processes cross is very different for these two polyols. We relate this feature to the differences in the relative contributions of intra and inter-molecular interactions due to the different chemical architecture of these polyols.

Fatigue of solder joints in surface mount devices

Abstract: Lifetime studies of a 16 I/O surface-mounted solder joint array undergoing isothermal cyclic fatigue in torsion shear under fixed plastic strain range show a strong correlation with creep fatigue and a creep-cracking mechanism. Experimental lifetime data follow an inverse dependence on matrix creep. Experimental measurement of the steady-state shear creep rate versus shear stress defines the creep characteristic that is sensitive to changes in metallurgical structure. The amounts of grain boundary and matrix creep taking place during a fatigue cycle are derived from experimental creep data combined with stress-strain hysteresis data obtained in steady-state cycling. Initially, thicker solder joints have a larger grain size than thinner solder joints, giving more matrix creep during fatigue and a faster failure rate. Fatigue increases the mean grain size of the solder joint as determined by the creep-rate-versus-stress characteristic and microstructure. Effects of grain size and joint thickness on lifetime are discussed. A maximum in the creep fatigue rate occurs at 333 K (60°C).

Study of Shear Stress Relaxation in Well‐Characterized Polymer Liquids

Abstract: Shear stress relaxation data were measured for a range of shear strains on several entangled polymer liquids in order to test time‐strain factorability and the Doi‐Edwards predictions about strain dependence. Undiluted samples and concentrated solutions of well‐characterized linear and star polybutadienes with narrow molecular weight distributions were used. Some departures from time‐strain factorability were found, but these appear to be nonsystematic, i.e., they do not correlate in any obvious way with the polymer structure. The strain dependence, however, follows the pattern observed by Osaki and Kurata for polystyrene solutions. The Doi‐Edwards prediction gives a good account of the results at intermediate entanglement densities (35,000≲cM≲150,000 for linear and star polybutadienes), but systematic departures are found at higher values. The causes of the latter are unknown.