Papers by Keyword: Plastic Behaviour

Abstract: The formability of the magnesium alloy sheets is poor at room temperature even though the magnesium alloy sheets are attractive because of their excellent characteristics. Hydro-forming technology, especially the pulsating hydro-forming, may be a way to improve the formability. Finite element simulations have been conducted to investigate the formability of AZ31B magnesium alloy sheet in hydro-bugling by pulsating hydraulic pressure. The influence of linear pressure, pulsating pressure and pulsating frequency on the maximum bulging height and minimum wall thickness of the formed parts have been analyzed. The research results show that the formability of AZ31B magnesium alloy sheet can be improved dramatically by pulsating hydro-forming. And enough bugling height can be obtained by lesser forming hydraulic pressure. The AZ31B sheet has an excellent performance in formability when the pulsating frequency is properly selected.

Abstract: Forging of a rib-web shape in rolled AZ31B magnesium alloy was conducted in the transverse direction at speeds of 0.01-10 mm s^-1 in the temperature range 300-500 °C with the objective of validating the flow anisotropy. The finite element programme DEFORM was used to simulate the forging process to obtain the local values of strain and strain rate. Forgings done along the transverse direction at temperatures higher than 400 °C resulted in a symmetrical cup-shape while those done at lower temperatures exhibited an elliptical boat-shape with the major axis coinciding with the rolling direction and the minor axis aligning with the normal direction. This anisotropy of flow was due to the strong basal texture in the rolled plate and the dominance of prismatic slip at lower temperatures. At higher temperatures, pyramidal slip dominates along with cross- slip as the recovery mechanism, which reinstates the symmetry of flow by destroying the initial texture.

Abstract: The shot peening process is largely used for the surface treatment and forming. The residual stress distribution developed within material may induce distortion of the component. The residual stress formed during the shot peening process is simulated numerically. The elastic-plastic constitutive model is adopted to describe the plastic behavior of the target material. The influence of shot peen speed on residual stress and deformation distribution is discussed.

Abstract: Low stress repeated impact experiments and test were carried out on medium carbon steel and stainless steel samples which is clad with high-strength Co-based or Ni-based alloy. The results showed that under low stress repeated impact load, which is much lower than the yield strength of material, plastic deformation will be occurred on the coating and part of its below base material. The average plastic deformation degree caused by each impact is increased at first, and then declined with the increase of impact times. Accumulated impact will lead to macroscopic plastic deformation and material hardening. The degree of deformation and the size of deformation area are related to the impact stress value and the material’s strength. The degree of deformation and hardening decline from the exterior to the interior, and only occur on the impact surface and a certain depth below, forming a ‘deformable area’. Based on our analysis, we consider that low stress repeated impact deformation is a kind of accumulative fatigue damage. The energy absorbed by material under repeated impact load, is larger than that absorbed at the same peak value of stress under static load or static fatigue load. Moreover, the energy absorbability is inversely proportional to the impact distance. Repeated impact may increase the movability of atom, reduce the critical shearing stress, that make the dislocation be initiated and increased easily.

Abstract: Silica aerogels have been studied with the objective of understanding the mechanical behavior of these extremely porous (pore volume higher than 85%) glassy materials. Elastic and plastic behaviors are investigated using Hg porosimetry. Because of the peculiar structure of these materials, Hg liquid cannot enter their porous network and consequently induces an isostatic pressure. Due to the high compliance of the solid network, under isostatic pressure aerogels display an irreversible shrinkage caused by plastic deformation. The magnitude of the plastic shrinkage and the increase of the associated mechanical properties depend on the different parameters (porosity, elastic properties and structural features). The structural features are followed by X Rays scattering. The irreversible compaction can be explained by siloxane bond formation between clusters constituting the porous materials, retaining the strained structure. The pore collapse mechanism is favored by the large pores structure and loose cluster structure (low fractal dimension). This densification process could offer a new way to synthesize porous glasses at room temperature.

Abstract: Different sets of silica aerogels (classical aerogels, partially dense aerogels, composite aerogels) have been studied in the objective to understand the mechanical behaviour of these extremely porous solids. The mechanical behaviour of xerogels and aerogels is generally described in terms of brittle and elastic materials, like glasses or ceramics. The main difference compared to silica glass is the order of magnitude of the elastic and rupture modulus which are 104 times lower. However, if this analogy is pertinent when gels are under a tension stress (bending test) they exhibit a more complicated response when the structure is submitted to a compressive stress. The network is linearly elastic under small strains, then exhibits yield followed by densification and plastic hardening. As a consequence of the plastic shrinkage it is possible to compact and stiffen the gel at room temperature. These opposite behaviours (brittle and plastic) are surprisingly related to the same kinds of gel features: pore volume silanol content and the pore size. Both elastic modulus and plastic shrinkage depend strongly on the volume fraction of pores and on the condensation reaction between silanols. On the mechanical point of view (rupture modulus and toughness), it is shown that pores size plays likely an important role. Pores can be considered as flaws in the terms of fracture mechanics and the flaw size, calculated from rupture strength and toughness is related to the pore size distribution.

Abstract: The results of monotonic and cyclic uniaxial compression tests, in which the deviatoric component of the stress is predominant, carried out on green and recrystallized iron compacts with different levels of density are presented and discussed in order to analyse the macro and micromechanisms governing the mechanical behaviour of non-sintered PM materials. The plastic deformation of the particles, especially at the contact areas between neighbouring particles, produces an internal friction responsible for the main features observed in the behaviour of green metallic compacts. These experimental results show important discrepancies with the plasticity models, Cam-Clay and Drucker-Prager Cap, used to simulate the powder compaction stage. Possible causes for these discrepancies are pointed out.