Effect of Deformation Temperatures on High Temperature Tensile Instability of Austenitic Stainless Steel

The characteristics of instability have been investigated on two low nickel austenite stainless steel at the temperatures of 950–1200 °C through the uniaxial tensile test and tensile unload test. The results show geometrical instability is not concurrent with load instability in high temperature tensile. The deformation of specimens are uniform before the load instability, then appear non-uniform in some local area of specimen but they do not directly lead to the geometric instability. With the increase of deformation temperature, load instability strain and strain hardening exponent n are both no obvious variation, and load instability true strain is close to n and depends on n. The geometric instability strain and strain rate sensitivity coefficient m both increase with the increase of temperature, and the strain between load instability and geometric instability depends on m and is the main part of the geometric instability strain.

On thermal softening and adiabatic shear failure of dynamically compressed metallic specimens in the Kolsky bar system

The issues of thermal softening and adiabatic shear failure, in dynamically compressed metals, are revisited through experiments in the Kolsky bar system. Various materials were compressed by single- and multi-step loadings and the results were analysed through a new approach to the issue of instability strain, which is based on the temperatures existing in the specimens just prior to the onset of instability. These temperatures are compared with the threshold temperatures, which mark the steep decrease in the strength–temperature curves. This approach accounts for most of the materials we tested. However, the brittle behaviour of the titanium and magnesium alloys, which fail at a very low strain, should be treated by a different approach.

Analysis of Thermoviscoplastic Effects on Dynamic Necking of Pure Copper Bars during Impact Tension

In order to make effective measure for the stress, strain and strain rate in the specimen, a tensile split Hopkinson bar (TSHB) was optimized in the first loading duration and the thermoviscoplastic models for a pure copper were determined. The deformation of the pure copper bar during the multi-tensile loading including the necking process in the optimized TSHB testing was recorded by a high speed camera and was used in providing an extensional check of the determined thermoviscoplastic models of the pure copper. The thermoviscoplastic models determined in a certain range of strain were employed in numerically simulating the large deformation of the pure copper bar to adjust the thermoviscoplastic model to describing large strain range. The instability strain of thermoviscoplastic materials in simple tension given by Batra et al was compared with the experimental strain at diffuse necking of the pure copper bar and the computed instability strain based on the load-average strain of the pure copper bar. The local necking of the pure copper bar in the optimized TSHB testing was numerically simulated using the micro-mechanical data given by Ragab and was compared with the recorded necking.

Analysis of Thermoviscoplastic Effects on Dynamic Necking of Pure Copper Bars during Impact Tension

In order to make effective measure for the stress, strain and strain rate in the specimen, a tensile split Hopkinson bar (TSHB) was optimized in the first loading duration and the thermoviscoplastic models for a pure copper were determined. The deformation of the pure copper bar during the multi-tensile loading including the necking process in the optimized TSHB testing was recorded by a high speed camera and was used in providing an extensional check of the determined thermoviscoplastic models of the pure copper. The thermoviscoplastic models determined in a certain range of strain were employed in numerically simulating the large deformation of the pure copper bar to adjust the thermoviscoplastic model to describing large strain range. The instability strain of thermoviscoplastic materials in simple tension given by Batra et al was compared with the experimental strain at diffuse necking of the pure copper bar and the computed instability strain based on the load-average strain of the pure copper bar. The local necking of the pure copper bar in the optimized TSHB testing was numerically simulated using the micro-mechanical data given by Ragab and was compared with the recorded necking.

Analysis of Thermoviscoplastic Effects on Dynamic Necking of Pure Copper Bars during Impact Tension

In order to make effective measure for the stress, strain and strain rate in the specimen, a tensile split Hopkinson bar (TSHB) was optimized in the first loading duration and the thermoviscoplastic models for a pure copper were determined. The deformation of the pure copper bar during the multi-tensile loading including the necking process in the optimized TSHB testing was recorded by a high speed camera and was used in providing an extensional check of the determined thermoviscoplastic models of the pure copper. The thermoviscoplastic models determined in a certain range of strain were employed in numerically simulating the large deformation of the pure copper bar to adjust the thermoviscoplastic model to describing large strain range. The instability strain of thermoviscoplastic materials in simple tension given by Batra et al was compared with the experimental strain at diffuse necking of the pure copper bar and the computed instability strain based on the load-average strain of the pure copper bar. The local necking of the pure copper bar in the optimized TSHB testing was numerically simulated using the micro-mechanical data given by Ragab and was compared with the recorded necking.

A Study on Bursting Pressure of Thin-Walled Cylinders

In this paper, we investigated the bursting pressure of thin-walled cylinders. Considering the strain hardening behavior of materials and the geometry deformation of pressure vessels, we derived the instability strain of thin-walled cylinders with a Swift-type stress-strain relationship, and used it as a failure criterion. Consequently, the instability stress was obtained and used to determine the maximum load-bearing capacity of thin-walled cylinders, that is, bursting pressure. The analytical solutions were compared with finite element analysis and bursting experimental results on different size thin-walled cylindrical pressure vessels manufactured from three different materials. It was turned out that it is reasonable to adopt instability strain as a failure criterion and use instability pressure as burst pressure. In the finite element analysis, the material parameters used were from raw experimental data or fitted values of experimental data. For both cases, finite element predications on instability strain and bursting pressure gave around the same values, close to experimental results. Therefore, based on finite element analyses, the instability strain and bursting pressure can be calculated by using true stress-strain curves directly measured from experiments, without the need to assume any specific material type.