Necking

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

Deformation behaviors of an armchair boron-nitride nanotube under axial tensile strains

Abstract: Deformation behaviors of an (8,8) boron-nitride nanotube (BNNT) under axial tensile strains were investigated via molecular dynamics (MD) simulations. The Tersoff potential was employed in the simulations with potential parameters determined by fitting the MD simulations results to those obtained from density functional theory calculations for BNNTs with the aid of the force-matching method. Variations in the axial stress, bond lengths, bond angles, radial buckling, and slip vectors with tensile strain were all examined. The axial, the radial, and tangential components of the slip vector were employed to monitor the local elongation, the local necking, and the local twisting deformations, respectively, near the tensile failure of the BNNT. From this study, it was noted that the BNNT started to fail at the failure strain of 26.7%. The components of the slip vector grew abruptly and rapidly after the failure strain, especially for the axial component. This implies that the local elongation dominates the tensile failure of the BNNT. With further axial tensile strains, subsequent bond breaking was found in the BNNT and finally resulted in a chain-like failure mode before complete breaking of the BNNT. No apparent yielding was noticed before the tensile failure of the BNNT.

Effect of inertia on the necking behaviour of ring specimens under rapid radial expansion

Abstract: Dynamic necking is analysed numerically for plane strain specimens under rapid deformation. Inertia and finite strain effects are accounted for, with the material described by a simple isotropic hardening elastic-plastic constitutive model, so that thermal softening and material strain-rate sensitivity are neglected. Plane strain tensile test specimens are analysed to confirm a size dependence of final failure found previously for round bars under rapid extension. The imperfection-sensitivity of this size dependence is discussed. Main focus is on necking in thin'ring specimens, thus modelling experiments in which electromagnetic loading was employed to expand ring specimens very rapidly. For perfect rings under axisymmetric loading there is no wave propagation around the circumference; but small initial imperfections change that, and a large number of necks around the circumference are predicted for various imperfections

Effects of strain rate on dynamic mechanical behavior and microstructure evolution of 5A02-O aluminum alloy

Abstract: This paper studies the effects of strain rate on dynamic mechanical behavior and microstructure evolution of 5A02-O aluminum alloy at room temperature. Based on the results of the dynamic tensile tests and compressive tests at strain rates of 1000–5000 s −1 by split Hopkinson bar as well as the results of quasi-static tests at strain rate of 0.001 s −1 , it is shown that with increasing strain rate, the flow stress and tensile strength significantly increase and notable strain hardening and thermal softening behaviors are observed for 5A02-O with elongation of 63.00% and softening ratio of 73.23% at the strain rate of 4000 s −1 . The strain rate sensitivity for 5A02-O is enhanced in the range of 1000–3000 s −1 . Scanning electron microscopy (SEM) observations illustrate that the fracture surfaces are characterized by larger and deeper dimple-like structure with more precipitates at higher strain rates, which indicates the ductile failure mode. The enhancement of ductility is interpreted via the inertia effect which may contribute to diffuse necking, slow down the necking development and delay the onset of fracture. Furthermore, transmission electron microscopy (TEM) observations show that higher strain rate leads to higher dislocation density, smaller cell size with thinner cell wall and the appearance of dislocation wall with parallel dislocation lines. Dislocation cells are incomplete under dynamic deformation. In addition, the micro-hardness of 5A02-O increases with increasing strain rate.

Determination of the Forming Limit Diagrams Using Image Analysis by the Corelation Method

Abstract: This paper deals with a new method of strain measurement applied to the experimental determination of the forming limit diagrams of thin steel sheets. This method uses the correlation technique to determine the displacement field between two images taken on the same area of the sample at two different strain levels. The strains are calculated from this field. The range of strains determined between two images may be very wide. This allows to compare the initial image to one before the sample fracture or two successive images during the straining. This method shows that the strain localization may occur early during the forming process and so the onset of localized necking is not a sudden phenomenon. A method is proposed to draw the forming limit diagrams for necking.

A Unified Damage Approach for Predicting Forming Limit Diagrams

Abstract: Plastic deformation in sheet metal consists of four distinct phases, namely, uniform deformation, diffuse necking, localized necking, and final rupture. The last three phases are commonly known as nonuniform deformation. A proper forming limit diagram (FLD) should include all three phases of the nonuniform deformation. This paper presents the development of a unified approach to the prediction of FLD to include all three phases of nonuniform deformation. The conventional method for predicting FLD is based on localized necking and adopts two fundamentally different approaches. Under biaxial loading, the Hill's plasticity method is often chosen when α (= ∈ 2 /∈ 1 ) 0 or when the biaxial stretching of sheet metal is significant. The M-K method, however, suffers from the arbitrary selection of the imperfection size, thus resulting in inconsistent predictions. The unified approach takes into account the effects of micro-cracks/voids on the FLD. All real-life materials contain varying sizes and degrees of micro-cracks/voids which can be characterized by the theory of damage mechanics. The theory is extended to include orthotropic damage, which is often observed in extensive plastic deformation during sheet metal forming. The orthotropic FLD model is based on an anisotropic damage model proposed recently by Chow and Wang (1993). Coupling the incremental theory of plasticity with damage, the new model can be used to predict not only the forming limit diagram but also the fracture limit diagram under proportional or nonpropor-tional loading. In view of the two distinct physical phenomena governing the cases when α (=∈ 2 /∈ 1 ) 0, a set of instability criteria is proposed to characterize all three phases of nonuniform deformation. The orthotropic damage model has been employed to predict the FLD of VDIF steel (Chow et at., 1996) and excellent agreement between the predicted and measured results has been achieved as shown in Fig. 1. The damage model is extended in this paper to examine its applicability and validity for another important engineering material, namely aluminum alloy 6111-T4.