Papers by Keyword: Microvoid

Abstract: Al0.5CoCrFeNi high-entropy alloy (HEAp) reinforced AA2218 metal matrix composites (MMCs) by stir casting and successive rolling. Mechanical characteristics of the AA2218 HEAp MMCs are analysed. The stir-casted AA2218 HEAp MMCs' ultimate tensile strength rose by 74.3 percent when HEAp was added at a weight percentage of 4 wt percent. When the MMCs were made by rolling, they had greater mechanical qualities than those made by RTR. Higher rolling deformation and lower HEAp mass fraction led to greater mechanical characteristics discrepancies between the AA2218 HEAp MMCs formed by CR and RTR. In the AA2218 HEAp MMCs after RTR, there were voids that were not present in the CR MMCs. Micro holes and the mechanical properties of metal matrix composites were also discussed in detail.

Abstract: This paper has studied heat shrinkable properties of COC/PETG blending film with microvoids inside. The shrink curve, shrink force, film structure and light barrier effect were studied by use of electron stretcher, SEM, shrink force tester, ultraviolet spectrophotometer and so on. The results indicated that COC/PETG blending film with many microvoids inside can receive more than 70% shrinkage and lower shrink force by comparing transparent PETG shrink film. At the same time, COC/PETG shrink film has excellent UV-light barrier effect from 310-400nm which also has good visual light barrier effect from 400-800nm because of it 85% opacity and 25% transparency.

Abstract: For hydrogenated amorphous silicon (a-Si:H) films deposited at temperatures between 423 K and 623 K (a-Si:H_423K and so on), the light-induced changes in the internal friction between 80 K and 400 K were studied. The internal friction is associated with H₂ motion in microvoid networks, and shows the mild temperature dependence between about 80 K and 300 K (Q^-1_80-300K) and the almost linear increase above 300 K (Q^-1_>300K). Both Q^-1_80-300K and Q^-1_>300K decrease with increasing the deposition temperature, and show the mild temperature dependence in a-Si:H_623K. The white light soaking with 100 mW/cm² (WLS₁₀₀ and so on) below 300 K caused a change in Q^-1_80-300K and no changes in Q^-1_>300K, respectively, and the light-induced changes in Q^-1_80-300K recovered after annealing at 423 K. The wide distribution of activation energies for H₂ motions between microvoids indicate that most of neighboring microvoids are connected through windows, i.e., the microvoid networks are existing in a-Si:H, and the spatially loose or solid structures are responsible for the low or high activation energies for the H₂ motion between microvoids, respectively. Furthermore, the light-induced hydrogen evolution (LIHE) was observed for WLS₂₀₀ to WLS₄₀₀ in a vacuum between 400 and 500 K, resulting in the disappearance of the internal friction due to the H₂ motion in the microvoid network.

Abstract: Hydrogen in the steel wall can cause hydrogen embrittlement of the wall material and thereby change the carrying capacity of the vessel. A theoretical model of hydrogen diffusion in the steel wall of a high pressure vessel was established and the formula of hydrogen content in the steel wall was deduced. Based on the hydrogen content formula, the formula of hydrogen pressure within microvoids which naturally exist in the steel wall of a spherical pressure vessel was deduced. At last, as an example to demonstrate the meaning of solving hydrogen pressure in microvoids, by using a representative volume element (RVE) model to carry out FE numerical simulation, the effects of hydrogen pressure on equivalent mechanical properties of the wall material were studied. It is found that the higher the gas pressures are, the lower the ultimate strength, specific elongation and percentage contraction of area are, which is in good accordance with the phenomena of metal hydrogen embrittlement.

Abstract: Little work has been published concerning the transferability of Gurson’s ductile damage model parameters in specimens tested at different strain rates and in the rolling direction of a Grade A ship plate steel. In order to investigate the transferability of the damage model parameters of Gurson’s model, tensile specimens with different constraint level and impact Charpy specimens were simulated to investigate the effect of the strain rate on the damage model parameters of Gurson model. The simulations were performed with the finite element program ABAQUS Explicit [1]. ABAQUS Explicit is ideally suited for the solution of complex nonlinear dynamic and quasi–static problems [2], especially those involving impact and other highly discontinuous events. ABAQUS Explicit supports not only stress–displacement analyses but also fully coupled transient dynamic temperature, displacement, acoustic and coupled acoustic–structural analyses. This makes the program very suitable for modelling fracture initiation and propagation. In ABAQUS Explicit, the element deletion technique is provided, so the damaged or dead elements are removed from the analysis once the failure criterion is locally reached. This simulates crack growth through the microstructure. It was found that the variation of the strain rate affects slightly the value of the damage model parameters of Gurson model.

Abstract: This study pays attention to reveal the material properties that control resistance curve for ductile crack growth (CTOD-R curve) on the basis of the mechanism for ductile crack growth, so that the R-curve could be numerically predicted only from those properties. The crack growth tests using 3-point bend specimens with fatigue pre-crack were conducted for two steels that have different ductile crack growth resistance with almost the same CTOD level for crack initiation, whereas both steels have the same “Mechanical properties” in terms of strength and work hardenability. The observation of crack growth behaviors provided that different mechanisms between ductile crack initiations from fatigue pre-crack and subsequent growth process could be applied. It was found that two “Mechanical properties” associated with ductile damage of steel could mainly influence CTOD-R curve; one is a resistance of ductile crack initiation estimated with critical local strain for ductile cracking from the surface of notched specimen, and the other one is a dependence of stress triaxiality on ductility obtained with circumferentially notched round-bar specimens. The damage model for numerically simulating the R-curve was proposed taking the two “ductile properties” into account, where ductile crack initiation from crack-tip was in accordance with critical local strain based criterion, and subsequent crack growth GTN (Gurson-Tvergaard-Needleman) based triaxiality dependent damage criterion. The proposed model accurately predicted the measured R-curve for the two steels used with the same “strength properties” through ductile crack initiation to growth.

Abstract: Generally, as the hold time of static load increases, the degradation of material becomes more rapid and the creep-fatigue life decreases. Under the creep-fatigue interaction, the cause of life decrease was mainly the initiation and growth of microvoid with increasing hold time. In this study, using the newly developed alloy of P92, the artificial creep-fatigue degradation tests and its ultrasonic inspection were carried out to evaluate creep-fatigue life. From the relations between the creep-fatigue life and the nondestructive degradation assessment by backscattered ultrasound inspection, the new nondestructive life evaluation parameter, SDA (Slope of Decaying Amplitude) was proposed. Also, to verify the nondestructive life evaluation ability, the life evaluation results by SDA were compared with those of the destructive life evaluation obtained through the fraction of cavity area. From the test result of backscattering Rayleigh surface wave, the linear relationship between SDA and experimental life could be obtained. The good agreement between the experimental life and the predicted life by SDA was also obtained. Finally, it can be stated that the new life evaluation/degradation parameter, SDA (Slope of Decaying Amplitude) could be utilized for the evaluation of the material degradation under creep-fatigue interaction.

Abstract: This paper investigates the influence of various hold times on creep-fatigue life at 600oC. The relationship between the crack growth behavior and hold time was studied, and a metallurigical investigation to examine the effect of creep was performed. To examine the relationship between creep-fatigue life and microvoids, the fraction of micro-voids/cavity area was analyzed at the crack tip. The crack growth rate of the HAZ was found to be faster than that of base metal while creep-fatigue life was found to be shorter. Finally, it can be stated that the fraction of cavity area, Fca could be utilized for the life prediction under creep-fatigue interaction. As the hold time increased, the creep damage was observed along the prior austenite grain boundaries and inside and boundaries of delta-ferrite.