Necking

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

Mechanics of polycarbonate during high-rate tension

Abstract: Polymeric materials often undergo large inhomogeneous deformations at high rates during their use in various impact-resistant energy-absorbing applications. For better design of such structures, a comprehensive understanding of high-rate deformation under various loading modes is essential. In this study, the behavior of polycarbonate was studied during tensile loading at high strain rates, using a splitcollar type split Hopkinson tension bar (SHTB). The effects of varying strain rate, overall imposed strain magnitude and specimen geometry on the mechanical response were examined. The chronological progression of deformation was captured with a high-speed rotating mirror CCD camera. The deformation mechanics were further studied via finite element simulations using the ABAQUS/Explicit code together with a recently developed constitutive model for high-rate behavior of glassy polymers. The mechanisms governing the phenomena of large inhomogeneous elongation, single and double necking, and the effects of material constitutive behavior on the characteristics of tensile deformation are presented.

In-situ investigation of the anisotropic mechanical properties of laser direct metal deposition Ti6Al4V alloy

Abstract: This study compares the microstructure and tensile properties of Ti6Al4V components fabricated by laser direct metal deposition (LDMD) additive manufacturing (AM) in the transverse and longitudinal directions. The results show anisotropic tensile properties with the transverse direction having high tensile and fracture strengths and the longitudinal direction having a high elongation and reduction of cross section. The anisotropic mechanical properties are attributed to the anisotropic microstructural distribution. The transverse tensile specimen is composed of short columnar prior-β grains which grow perpendicular to the tensile direction, and have a lamellar structure. Along the β grain boundary, αGB and large α colonies were identified. However, the longitudinal specimen shows that the long β structure is parallel to the tensile axis and that the microstructure is composed of basket-woven α phases with shorter α plates and smaller colony sizes compared with those in the transverse specimen. The fracture mechanism induced by the anisotropic microstructure along the transverse and longitudinal directions was compared by examining the fracture process in real-time using uniaxial in-situ scanning electron microscopy (SEM) tensile testing. The results show that shear fracture, which is caused by the vertical β grain boundaries and large α colonies with long α plates, occurs in the transverse specimen. The shear mode is the main reason behind the enhanced tensile strength and fracture strength due to the high resistance to microcrack propagation. However, in the longitudinal specimens, symmetric necking behavior due to the fine α grains resulted in uniform deformation of the grains on both sides of the grain boundaries, inducing greater elongation.

Lithospheric necking and regional isostasy at extensional basins 2. Stress‐induced vertical motions and relative sea level changes

Abstract: We investigate long-wavelength differential vertical motions at extensional basins which are the result of temporal changes in the intraplate stress field in the lithosphere. These stress-induced vertical motions are intimately coupled to the state of flexure (flexural curvature) of the lithosphere. We compare a previous model of flexure under the sediment load only (FSL model) and a recent more consistent model, which incorporates regional compensation of other vertical loads induced by lithospheric extension as well. These additional loads are controlled by the depth at which the necking of the lithosphere occurs (DON model). Tlie FSL model and models assuming a shallow level of necking (≤15 km) are very similar, they predict tilting of the basin with the hinge point landward of the shelf edge. Compression results in uplift of the landward pan of the basin, which generally corresponds to the coastal plain or inner to middle shelf, and downwarping of the outer shelf and slope. The differential vertical motions correspond to relative sea level cycles which are out of phase in different parts of the basin. Internal architecture of systems tracts can be different from predictions by eustasy. By contrast, deeper levels of necking (>20 km) predict a uniform sense of motion landward of the shelf edge. Type I sequence boundaries can be very easily induced for these conditions, and sequence stratigraphie features are probably hard to distinguish from predictions by eustasy. These results are for a pure elastic plate rheology. Stress-induced vertical motions for a more realistic depth-dependent rheology are more complicated because of the additional effect of stress-induced reduction of flexural rigidity. For these conditions, gradual buildup of tension causes a complete relative sea level cycle. For high tensional stress levels, total vertical motions become rather similar to vertical motions for compression. This may be of importance for the issue of global synchroneity of relative sea level changes.

A finite amplitude necking model of rifting in brittle lithosphere

Abstract: We formulate a mechanical model describing the formation of rifts as finite amplitude necking of an elastic-plastic layer overlying a fluid substrate. A perfectly plastic rheology is a continuum description of faulting in rift zones. Two important aspects of rift evolution are illustrated by this model: the evolution of the rift width as extension proceeds and the finite strain that occurs. A region at yield initially develops with a width determined by the thickness of the brittle layer, and the internal deformation within this yield zone is proportional to the topographic slope. As extension proceeds, the surface within the rift subsides, and the width of the subsiding yield zone decreases. At any stage of rifting, material in regions just outside the yield zone is deformed but no longer deforming. The width of these deformed regions increases with increasing extension. Vertical forces due to the mass deficit of the rift depression will flex the elastic layer outside the yield zone, creating flanking uplifts. The external force required to maintain active rifting increases with the amount of lithospheric stretching, indicating that rifting is a quasi-static, stable process. Because the yield zone will revert to elastic behavior if the external force causing extension is removed, the model predicts that the rift depression and flanking uplifts will be preserved after extension stops. Our simple mechanical model demonstrates the inherent relationship among graben formation, lithospheric thinning, and rift shoulder uplift in rift zones.

Tensile deformation and fracture of Ti–5Al–5V–5Mo–3Cr–1.5Zr–0.5Fe alloy at room temperature

Abstract: A combination of transmission electron microscopy, scanning electron microscopy and x-ray diffractometer is used to analyse the deformation mechanism and the fracture behavior of bi-model structure Ti–5Al–5V–5Mo–3Cr–1.5Zr–0.5Fe alloy during uniaxial tensile test. In the uniform deformation region, the strain of the primary α particles is dominated by the planar slipping and the dislocation tangling, and the deformation of the β matrix is accompanied by the dislocation tangling. In the necking region, the additional deformation twinning is found in the primary α particle, and the β phase is still strained by the dislocation tangling. The local shearing occurs in the necking region and results in the micro-twinning dominated deformation in secondary α precipitation and the 〈110〉β fiber texture. The room temperature tensile fracture of Ti-55531 alloy is extremely sensitive to the formation of the voids, and the critical dimension of the crack is the diameter of the primary α particles.