CREEP AND REVERSE CREEP OF NATURAL AND PLASTICIZED BIRCH WOOD WHEN PRESSING ACROSS THE FIBERS.

Pressed (modified) wood is a technological process of drying, impregnation and pressing (R 54577-2011 State Standard). The density of pressed wood, depending on the degree of pressing, ranges from 750 to 850 kg/m3. Strength, hardness toughness of this wood is several times greater than of natural wood. Coal oil, TCL (thermocatalytic cracking liquid), shale oil are most often used as wood antiseptics. They are plasticizers and change the nature of deformations during pressing.Experiments with natural and were conducted to obtain comparative data. Pressed wood contained 10% of TCL oily antiseptic by weight of absolute dry wood. Birch wood was used as the starting material. In this case, an antiseptic agent in an amount of 8-10% by weight of dry wood is evenly distributed over the entire cross section of the specimen. Direct and reverse creep was studied to assess time changes in deformation composition of wood, pressed across fibers. Measurements of direct and reverse creep of wood were carried out on a lever installation. The obtained modes can be used to optimize the technology of obtaining plasticized wood, because plasticization of wood with TCL oil reduces toughness 3-5 times (in the first phase of deformation), in the second phase - 1.1-1.5 times, in the third phase - 2.5-3 times. Transverse strain coefficient increases 1.2-1.3 times. Using creep curves of plasticized wood, it was found that oil impregnation gives 4-5 times greater reduction in the proportion of elastic deformations and, accordingly, increases the residual ones.

Research on Genetic Characteristics of Slab Bulge in Continuous Casting Process

Bulge deformation is an important factor that causes inner crack and declines the quality for slab. This has a significant genetic phenomenon in continuous casting process. In order to explain this feature, a two-dimensional FEM model for slab that includes viscoelastic and dynamic characteristic has been built. Positive creep and reverse creep were introduced to explain this mechanism. Roll pitch, shell characteristic, slab withdrawing speed and other process parameters that influence bulge’s genetic level are discussed. A nonlinear equation was suggested to describe the relationships between these factors and genetic level of bulge.

Nano Scale Creep and the Role of Defects

The modulating force method in nanoindentation gives a direct measure of contact stiffness, and being insensitive to drift, allows the accurate observation of creep in small indents to be carried out over long time periods. We present results for a range of metals at room temperature. Strain rate indices similar to those for macroscopic creep are found. Reverse creep occurs for step unloading greater than about half the starting load. For electropolished tungsten, we find quite different behaviour before and after the sudden pop-in. Afterwards, creep is as in other metals, but beforehand, it is essentially zero. The slight changes of stiffness observed at the very smallest loads are due to diffusion of adsorbed surface films into the contact zone. Our results show that the dislocations nucleated and multiplied at pop-in provide the mechanism of creep.

The flow of polycrystalline metals under simple shear. II

The flow of lead of different purities under conditions of simple shear has been investigated over a wide range of strain, at different stresses, and mostly at a temperature in the neighbourhood of 27 °C. The phenomena have been studied both with unchanged direction of stress and with the direction of stress reversed during flow. At the beginning of forward creep the t 1/3 law is strictly obeyed, within a range of strain which is determined by the purity and grain size of the lead. The region within which the t 1/3 law holds, here called stage F I, is succeeded by stage F II, during which the strain increases according to a logarithmic law, the creep rate being proportional to the increase in strain. In stage F III, which begins at a strain of about 0.3, the creep is strictly linear until the strain approaches 2.0, when rupture starts. If the stress be reversed while stage F I is in progress a creep linear with time takes place, at a rate which is twice that of the forward creep at the moment of reversal, and this constant rate continues for a time equal to that of the forward creep. This initial stage R 0 is succeeded by a stage R I which is also governed by a t 1/3 law, the constant multiplying t 1/3 being proportional to that for stage F I at the same stress. This is followed by a stage R II in which the creep rate increases according to the same law which prevailed for stage F II, and this in turn is followed by a linear stage R III characterized by the same constant creep rate as F III. If the stress be reversed at a strain so large that the t 1/3 law has ceased to be valid, the reverse creep stages are markedly affected, which emphasizes the physical significance of the t 1/3 law. A law of corresponding times is enunciated, which connects the flow in stages R 0 and R I with that in stage F I. Photomicrographs and back-reflexion X-ray photographs have shown that the t 1/3 flow is accompanied by progressive slip in the grains and local rotation of the lattice, and that in the R 0 stage slip takes place on the same slip bands as were active in the F I stage. Recrystallization and grain growth occur during stages F II and R II. In stages F III and R III there is a balance between grain break-up and grain growth, and the slip direction shows a preference for the directions of principal stresses. By considering the variation of strain rate with stress and temperature in stage III, constants have been derived which indicate that the flow in this quasi- viscous stage resembles that of a single crystal far more than that of a polycrystalline metal. The general implications of the experimental findings are discussed.