Papers by Keyword: Plastic Instability

Abstract: In this paper, the physical and chemical analysis method was used to analyze the leakage of a regeneration gas heater device accident. The result shows that this device was heated seriously at local area, and oxidized severely. The carbon content around the leakage is obviously lower than the limit of standard, and the grain deformation at crack tip of the leakage is obvious.It shows that reason of device failure is caused by typical plastic Instability, and low carbon content leads to insufficient mechanical properties at high temperature. According to data from manufacturer, the unreasonable arrangement of thermo-couples may be the root cause of this leakage.It can be considered that replace the higher grade materials conservatively, while optimizing the arrangement of thermo-couples.

Abstract: The aim of this work is to study the plastic instabilities occurring on the stamped sheets during deep drawing process. The analysis of the plastic deformation of the material showed that the deformation occurs in bi-axial extension at the bottom of the punch due to thinning of the sheet, in local necking together at the vertical wall level of the sheet and below the blank holder due to thickening of the sheet. As a first step, an experimental characterization of the material is undertaken, whose experimental tests made it possible to determine the fundamental characteristics of the material. In the second step, a study of the material behaviour during forming process by numerical simulation using Abaqus finite element code is proposed. The various simulations undertaken showed the variation of the two parameters; the blank holder force and the friction effect. The blank holder force and friction, applied respectively to the blank flange region and between the tool-blank surfaces, make it possible to optimize the deformation limits and to repel any instability which may appear on the material in deep drawing. The simulations carried out on Abaqus code allow to visualize the material behaviour during deformation, by locating the thinning and necking zones on the sheet and from there, in order to locate areas at risk of failure. An optimization of the process is proposed by varying the considered parameters in a validated numerical model. Satisfactory results have been obtained which clearly show the failure and the safe zones.

Abstract: In this work, we focus on a new generation of polymer named Polybutylene terephthalate (PBT). In order to analyse and determine true behaviour of this polymer, a special experimental method was used. Hence, the true stress/strain responses are investigated under a large plastic deformation in different stress triaxiality frameworks with a particular attention on the volumetric strain evolution, with their decomposition to an elastic volumetric strain, plastic volumetric strain and the pure shear. Moreover, the effect of stress triaxiality on the plastic instability and the fracture strain is also examined. With the plastic instability analysis, it was found that plastic strain hardening increases gradually with the triaxiality. Finally, in order to evaluate the damage of this polymer, a theoretical damage formula is proposed.

Abstract: In this study, we present an experimental/numerical methodology which aims to improve 3D thin sheet hydroforming considering coupled constitutive equations formulated in the framework of irreversible processes accounting for isotropic hardening as well as isotropic ductile damage. The experimental study is dedicated to the identification of stress-strain flow from the global measure of pole displacement, thickness evolution and internal pressure expansion. Indeed, Hill48 yield surface anisotropy parameters and coefficients of the Swift law coupled to ductile damage allowing to locate plastic instability zones of hydroformed sheets are identified with three dies cavities shapes. Or during the hydroforming processes severe mesh distortion of element occur after a few incremental steps. Hence an automatic mesh generation with remeshing capabilities is essential to carry out the FEA. The proposed technique based on geometrical criteria includes adaptive refinement and coarsening procedure is integrated in a computational environment.

Abstract: In this study, we present an experimental/numerical methodology which aims to improve 3D thin sheet hydroforming considering coupled constitutive equations formulated in the framework of of irreversible processes accounting for isotropic hardening as well as isotropic ductile damage. The experimental study is dedicated to the identification of stress-strain flow from the global measure of pole displacement, thickness evolution and internal pressure expansion. Or during the hydroforming processes severe mesh distortion of element occur after a few incremental steps. Hence an automatic mesh generation with remeshing capabilities is essential to carry out the FEA. The proposed technique based on geometrical criteria includes adaptive refinement and coarsening procedure is integrated in a computational environment.

Abstract: Miniature specimen techniques viz. small-punch tests (SPT) have been carried out at room temperature in order to correlate the microstructural degradation of 2.25Cr-1Mo steel with that of SPT parameters. Microstructural degradation of this steel has been introduced as a result of thermal ageing corresponding to Larson-Miller parameters (LMP) values of 33,012, 35,402, 37,846 and 38,374. SPT parameters viz. total area and area under the region of plastic instability of the load-displacement curve have been found to decrease with an increase in LMP values. A strength parameter viz. UTS obtained using uniaxial tensile tests has also been found to decrease with an increase in LMP values. The results indicated that miniature specimen techniques viz. small-punch test could be successfully used to assess the degradation of microstructures in 2.25Cr-1Mo steel generated due to their exposure to high temperatures.

Abstract: The superplastic bulging test of AZ31B magnesium alloy sheet of 0.6mm thick was carried out on Alliance RT/50 tensile machine at 573K and 3.3×10^-4S^-1. It is found that either in tensile-compressive deformation or in bi-axis tensile deformation, the judgment criterion for local necking of superplastic deformation is dε₂=0. The superplastic forming limit diagram(FLD) at 573K and 3.3×10^-4S^-1 was established for the first time.

Abstract: The use of Finite Element Simulation allows accurate predictions of stress and strain distributions in complex stamped parts. The onset of necking is strongly dependent on the strain paths imposed to the parts and therefore the prediction of localized necking can be a difficult task. Numerical models of plastic instability have been used to predict such behavior and recent and more accurate constitutive models have been applied in these calculations. In many manufacturing areas such as automotive, aerospace, building, packaging and electronic industries, the optimization of sheet metal processes, through the use of numerical simulations, has become a key factor to a continuously increasing requirement for time and cost efficiency, for quality improvement and materials saving. This paper makes an analysis of the evolution of strain gradients in stamped parts. The combination of Finite Element Analysis with a Plastic Instability Model, developed to predict localized necking under complex strain paths, shows that it is possible to predict failure with precision. Several constitutive laws are used and comparisons are made with experiments in stamped benchmark parts. Considering non linear strain paths, as detected in stamped parts, more accurate failure predictions are achieved. The work described in this paper shows the need to include a post processor analysis of failure, capable of predicting the behavior of the material under non linear strain paths. Taking this phenomenon into account, it is shown that it is possible to increase the accuracy of the onset of localized necking prediction.

Abstract: The superplastic bulging test of AZ31B magnesium alloy sheet of 0.6mm thick was carried out on Alliance RT/50 tensile machine at 573K and 3.3×10 −4 −1 s . It is found that either in tensile-compressive deformation or in bi-axis tensile deformation, the judgment criterion for local necking of superplastic deformation is 0 2 dε = . The superplastic forming limit diagram
FLDat 573K and 3.3×10 −4 −1 s was established for the first time.

Abstract: Generally rupture of steam generator tubes occurs accompanying significant plastic deformation. In this study, the burst pressure of a damaged steam generator tube is calculated from the plastic instability analysis using the finite element method. Two wear types, flat and circumferential types are considered. An equation for the burst pressure is proposed by using the concept of strength reduction factor and the Svensson equation. The analysis results are also compared with the experiment data from published references and they show a good agreement with the experiment data.