Necking

About: Necking is a research topic. Over the lifetime, 5280 publications have been published within this topic receiving 113945 citations.

Mechanical properties and deformation mechanism of Mg-Al-Zn alloy with gradient microstructure in grain size and orientation

Abstract: The surface mechanical attrition treatment was taken to fabricate the gradient structure in AZ31 magnesium alloy sheet. Microstructural investigations demonstrate the formation of dual gradients with respect to grain size and orientation, where the microstructural sizes decreased from several microns to about 200 nm from center area to treated surface, while the c -axis gradually inclined from being vertical to treated plane towards parallel with it. According to tensile results, the gradient structured sample has yield strength of 305 MPa in average, which is increased by about 4 times when compared with its coarse-grained counterpart. Meanwhile, contrary to quickly failure after necking in most traditional magnesium alloys, the failure process of gradient structure appears more gently, which makes it has 6.5% uniform elongation but 11.5% total elongation. The further comparative tensile tests for separated gradient layers and corresponding cores demonstrate that the gradient structured sample has higher elongation either in uniform or in post-uniform stages. In order to elucidate the relationship between mechanical properties and deformation mechanisms for this dual gradient structure, the repeated stress relaxation tests and pole figure examinations via X-ray diffraction were conducted in constituent gradient layer and corresponding core, as well as gradient structured sample. The results show that the pyramidal dislocations in dual gradient structure are activated through the whole thickness of sample. Together with the contribution of grain-size gradient, more dislocations are activated in dual gradient structure under tensile loading, which results in stronger strain hardening and hence higher tensile ductility.

On Some Issues in Triaxial Extension Tests

Abstract: Results of triaxial extension tests on dry sand are presented. Special attention is paid to the factors influencing the results in triaxial extension tests. It is found that factors such as accuracy of the axial force measurement, axial force carried by the rubber membrane surrounding the specimen, gravity, and inhomogeneous deformation have a much larger influence on the results in triaxial extension tests than in triaxial compression tests. Accurate measurement of the axial force is achieved by placing the load cell inside the pressure chamber. The influence of the confining pressure on the axial force measurement is avoided through a proper construction. The force carried by the rubber membrane is corrected with reference to the result of a tension test on a strip of rubber membrane. The effect of gravity is accounted for by adding the axial stress due to gravity to the applied axial stress. The inhomogeneous deformation is traced by three lateral strain collars placed along the specimen height. Results of triaxial extension tests cannot be appreciated if these influential factors are not taken into account. The failure mode of the specimen is found to be influenced largely by the initial density. Inhomogeneous deformation in the form of necking develops in the upper part of dense specimens and in the middle of loose specimens. For dense specimens, the necked region becomes wider with advanced deformation and develops into one single or two intersecting shear bands. For loose specimens, however, no shear bands can be observed.

Semi-automated image analysis of the true tensile drawing behaviour of polymers to large strains

Abstract: An image analysis system has been developed using commercially available hardware with custom software to investigate the deformation behaviour of solid polymers in uniaxial tension. This technique provides a rapid, semi-automated non-contacting method for determining true process stress-strain-strain-rate behaviour for both homogeneous and inhomogeneous deformation. The relative displacements of printed transverse grid lines are determined from images captured during a standard monotonic tensile test, providing local measures of strain. The examination of a time series of images allows the generation of true strain-rate data, and concurrent monitoring of the total draw force from the load cell allows the generation of true stress data at those times when the images are captured. Therefore, it is possible to produce a series of process uniaxial true stress-strain curves for individual “elements” of material within the gauge length of the specimen. Synthetic elastomers drawn at ambient temperature have been found to display relatively homogeneous deformation, resulting in a simple process axial stress-strain curve for the single-speed test, whereas in the case of inhomogeneous deformation (“necking”) exhibited by polypropylene, it is verified that each element of material experiences a slightly different deformation process. This spatially variant deformation is related to the original location of the particular element with respect to the point of neck initiation.