Effects of Free Surface and Heterogeneous Residual Internal Stress on Stress-Driven Grain Growth in Nanocrystalline Metals

By reevaluating the experimental study of Zhang et al. (2005), here we demonstrate that the extent of grain growth, previously proposed to be solely driven by external stress, may have been significantly overestimated. A new physical mechanism, termed as free surface assisted stress-driven grain growth (or self-mechanical annealing), is proposed and discussed in detail. Representing the cooperative effect of free surface and heterogeneous residual internal stress, the proposed mechanism is considered more favorable than the traditional pure stress-driven mechanism for interpreting the abnormal grain growth widely observed in deforming nanocrystalline metals at room temperature.

Stress Field Simulation of Compound Cast Rollers during Centrifugal Casting Process

The stress field of compound cast rollers during centrifugal casting process was investigated in this work. A finite element (FE) model was built to simulate the internal stress fields during cooling process, in which phase transformation was considered. The mechanical parameters employed in stress numerical simulation were measured by using thermal/mechanical simulator. The residual internal stress of the compound cast roller by centrifugal casting method was measured by using X-ray stress analyzer. The simulated results are in agreement with the measured ones very well. According to this model, the temperature and stress fields of the compound cast roller during centrifugal casting process were simulated.

Temperature Effects and Calculation Method of Closure Temperatures for Concrete-filled Steel Tube Arch Rib of Dumbbell-shape Section

This paper has done continuous on-site experimental researches on temperature field and temperature effects that are under the influence of hydration heat of concrete during the molding process of dumb-bell CFST arch bridge. Analysis on time-history law of temperature field and temperature effects is made. Results reveal that temperature variation of hydration heat of the concrete within the steel tube is showed as follows: temperature rising-continuous high temperature—temperature dropping—equalizing. The structural temperature field, generated under the effect of hydration- heat in the process of molding arch rib, is nonlinear temperature field. Temperature field of concrete hydration heat has an obvious effect on crown section. Finally, practical methods to calculate the closure temperature of CFST arch rib and the effective measures to reduce the residual internal stress of temperature of arch rib are proposed.

Gun Liner Emplacement Using Elastomeric Materials

The development of a process to emplace a refractory metal liner inside a gun tube is described. The process consists of filling the liner with an elastomeric material and then slipping this arrangement into the gun tube whose inner diameter is close to the outer diameter of the liner. The ends of the liner are plugged with plastic disks and pressure is applied to the elastomeric material by a load frame. This pressure can produce a residual internal stress within the steel gun tube that produces a frictional bond between the liner and gun tube. Initial efforts have resulted in bond strengths over 3 ksi (21 MPa). In addition, by tailoring the degree of lubrication between the elastomeric material and the liner, a graded autofrettage can be produced in the steel gun tube.

Determination of Residual Stresses in Medium Density Fibreboard

Summary Due to the moisture and temperature gradients developed during hot pressing of medium density fibre-board (MDF), residual stresses occur within the board as it equilibrates to room conditions. It would be extremely useful to measure these residual stresses and to determine their effects on board properties such as moduli of elasticity and rupture in bending, internal bond strength and dimensional stability. In this article two methods, namely dissection and hole drilling, have been adapted to measure residual internal stress distributions in six different samples of industry produced MDF. The dissection method involves cutting several pieces of MDF perpendicular to the thickness direction at different depths. The residual stresses released by the dissection can be determined by measuring the curvatures of cut pieces and knowing their elastic moduli. The hole drilling method, on the other hand, involves mounting three strain gauges on the surface of a piece of MDF and drilling a hole to release residual stresses in close proximity. The released stresses are manifested as strains in the forms of which can be measured in three directions on the surface of the board. A theoretical model for predicting residual stresses involving various parameters has been developed and an excellent agreement with the experimental results from both the dissection and hole drilling methods has been achieved. Linear moisture expansion coefficient appears to have the greatest influence on residual stress. When compared against each other, the residual stresses measured by the hole drilling method show some shortcomings towards the centre of the board. While all six of the MDF boards exhibited similar trends in their residual stress distributions, significant differences were identified in the magnitudes of residual stress measured. Finally, some preliminary results linking the residual stress with the thickness swell of the samples and their surface densities have been presented.

Influence of Silicon Incorporation on the Properties of Hard a-C:H Films

ABSTRACT- Results on the properties of silicon incorporated hard a-C:H films deposited on the cathode of a glow discharge decomposition system using methane and silane gaseous mixtures are reported. Obtained samples showed an increased deposition rate, high hardness comparable to that of a-C:H, and a large decrease of the internal stress upon silicon incorporation. The structure of the samples seems to be similar to diamond-like a-C:H, as revealed by the total hydrogen content, infrared absorption and hydrogen effusion experiments. Our results indicate that the observed reduction of residual internal stress may be attributed to a less compact material and/or to a smaller density of voids containing hydrogen in comparison to pure a-C:H.