Decrease In Critical Shear Stress Research Articles

Experimental study and molecular dynamics simulation of wall slip in a micro-extrusion flowing process

Wall slip is one of the most important characteristics of polymer melts’ elasticity behaviours as well as the most significant factor which affects the flow of polymer melts. Based on the traditional Mooney method, through a double-barrel capillary rheometer, the relationship between velocities of wall slip, shear stress, shear rate, diameters of dies, and temperature of polypropylene (PP), high-density polyethylene (HDPE), polystyrene (PS), and polymethylmethacrylate (PMMA) is explored. The results indicate that the velocities of the wall slip of PP and HDPE increase apparently with shear stress and slightly with temperature. Meanwhile, the rise of temperature results in the decrease of critical shear stress. The wall-slip velocities of PS and PMMA are negative which means that the Mooney method based on the adsorption–desorption mechanism has determinate limitation to calculate the wall-slip velocity. Based on the entanglement–disentanglement mechanism, a new wall-slip model is built. With the new model, the calculation values of velocity of PP and HDPE correspond to the experimental values very well and the velocities of PS and PMMA are positive. The velocities of PS and PMMA increase obviously with the rise of shear stress. The rise of temperature results in the increase of velocity and decrease of critical shear stress. Then, the molecular dynamics simulation is used to investigate the combining energy between four polymer melts and the inside wall. The results show that at the given temperature and pressure, the molecules of PS and PMMA combine with atoms of the wall more tightly than those of PP and HDPE which means when wall slip occurs, the molecules of PS and PMMA near the wall will adsorb to the surface of the wall. However, those of PP and HDPE will be easy to slip. Therefore, the wall-slip mechanism of PP and HDPE is the adsorption–desorption mechanism, and that of PS and PMMA is the entanglement–disentanglement mechanism. According to the different wall-slip mechanisms of four polymers, an all-sided calculation method of wall-slip velocity is raised which consummates the theory of wall slip of polymer melts.

Degradation of surfactant solutions by age and by a flow singularity

Four different surfactants were investigated in a closed hydraulic loop in order to find the influence of age of the solution on drag reducing effectiveness and on decrease of critical shear stress. It shows that aging of a particular surfactant is dependent on its concentration. Different sensitivity of solutions to flow disturbances is also shown.

Influence of Fine Sand Mixture on Yield Value and Critical Shear Stress of Cohesive Sediment

Water content is the most important parameter for rheological property of cohesive sediment. Mud in estuary often contain 10-70% fine sand and mixture of sand considerably influences rheology of mud. Vane tests were conducted for measuring the yield stress of mud and it was found that the yield stress decrease by mixture of fine sand. In flume experiments for initial movement of mud bed, it was found that the increase of sand content causes the decrease of critical shear stress.

Anomalies in the curve of critical shear stress versus temperature for single crystals of certain iron-chromium-nickel alloys at low temperatures

1. In the case of single crystals of the metastable alloys 000Kh18N10 and 000Kh18N15, the curves of critical shear stress versus temperature show anomalies due to γ→ξ transformation. In 000Kh18N10 the γ−ξ transformation begins near 200°K as a result of cooling, whereas in 000Kh18N15, it begins near 77°K as a result of slight plastic deformation. 2. The decrease in critical shear stress in 000Kh18N10 begins at a higher temperature when the cooling rate is decreased.

Deformation hardening of copper at low temperatures

1. A decrease in critical shear stress at 1.4°K was detected for all three fundamentally oriented copper single crystals. A similar temperature dependence for the elastic limit in copper polycrystals was not detected. 2. The studied hardening curve irregularities were either caused by avalanche, slip effects or twinning, depending on the single crystal strain axis. 3. The calculated activation parameters confirmed that the process controlling the plastic deformation in copper was the thermally activated intersection of gliding dislocations with forest dislocations. 4. An anomalous increase in relaxation depth at 1.4°K in comparison to the values at higher temperatures was detected.

AlおよびMg中における酸化物粒子と転位との相互作用の電顕観察

Interaction between dislocations and oxide particles was observed with a transmission electron microscope. The specimens of aluminium and magnesium were prepared so that an array of oxide particles was introduced in the middle part of each specimen along the direction of the final rolling. After heat-treatment or heat-treatment and working, the specimens were thinned by electrolytic polishing and were set in an electron microscope. As the dislocations were moved by the emission of electron beam, microphotographs were taken intermittently. The results obtained are as follows: (1) Rapid cooling from annealing temperature resulted in the formation of small dislocation loops in aluminium matrix. There were, however, denuded zones of these loops in the vicinity of the array of particles as well as grain boundaries. There relatively long dislocations associated with particles were observed. In the case of magnesium specimens, the generation of dislocations from the interface of matrix and particles and consequently the formation of dislocation networks were observed. (2) Many dislocations in both specimens which were generated and moved by emission of electron beam, interacted with particles. In aluminium specimens, arrests of moving dislocations and cross slips were preferentially observed. In magnesium specimens, however, the formation of dislocation loops as well as arrest and bending of dislocations was observed. (3) According to the working degree, cell structures, especially in aluminium, were formed, an array of particles taking part in its formation. However, with increase in working degree there were no appreciable differences in the cell structures between the matrix and the vicinity of the array of particles. (4) Finally, some considerations were made on the decrease in critical shear stress required for dislocation to pass through particles dispersed non-uniformly.