Necking

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

Effect of loading history in necking and fracture

Abstract: The effect of Lode angle parameter, or the third deviatoric stress invariant, on plasticity and fracture is studied using flat-grooved transverse plane strain specimens. A generalized asymmetric plasticity model for isotropic materials with both pressure and Lode angle dependence is developed. Calibration method of the plasticity model is discussed in detail. Test results on 2024-T351 aluminum alloy confirmed the proposed plasticity model. Similarly, a generalized asymmetric 3D empirical fracture locus with six free parameters is proposed. The proposed fracture locus, which depends on both stress triaxiality (or pressure) and the Lode angle parameter, is calibrated using two types of methods: classical specimens under uniaxial testing, and the newly designed butterfly specimens under biaxial testing. Experimental results on 2024-T351 aluminum alloy, 1045 steel, and A710 steel validated the proposed 3D fracture locus. A concept of forming severity is introduced to study the loading history effect on metal forming limit diagram (FLD). Given the necking locus under proportional loading conditions, and using a non-linear accumulation rule of forming severity index, the proposed model well predicts the FLDs under different pre-loading conditions. As an extension of the ductile fracture locus defined and calibrated under proportional loading conditions, a new damage accumulation rule considering the loading history effect is proposed. The new model uses the accumulated difference between directions of the back stress tensor and the current stress tensor to describe the non-proportionality of a load path. Several types of tests with complex loading histories were designed and performed to study the loading history effect on ductile fracture. Extensive experimental studies on 1045 steel confirmed the proposed ductile fracture model. The proposed model is successfully applied to predict fracture of crushed prismatic tubes undergoing strain reversal. Thesis Supervisor: Tomasz Wierzbicki Title: Professor of Applied Mechanics

Yielding Behaviour of Martensite in Steel

Abstract: Although martensite is recognised as a very strong phase in carbon steels, its initial yielding commences at low stresses and the tensile stress-strain curve shows a smooth, rounded form. Evidence is presented from x-ray diffraction to show that this behaviour is due to the presence of intra-granular stresses that are residues after the shear transformation from austenite to martensite. These internal stresses are reduced in magnitude by plastic deformation and also by tempering. Reduction of internal stress due to plasticity is shown by a decrease in XRD line broadening after deformation. A simple model is presented in which the stress-strain behaviour is controlled by relaxation of the internal stresses almost up to the point of the ultimate tensile strength. It demonstrates that only a very small fraction of the material remaining in a purely elastic state provides a large stabilising effect resisting necking. A corollary of this is that the uniform elongation of martensitic steel actually increases with increase in the strength level. Effects of heat treatment are also reproduced in the model, including the increase in conventional yield stress (Rp 0.2 ) that occurs after low temperature tempering.

Resistance Spot Weld Failure Loads and Modes in Overload Conditions

Abstract: The failure modes of two single-weld specimens, the coach-peel and the tensile-shear specimens, were studied in detail. Weld overload experiments, along with optical and scanning electron microscopy, revealed that the coach-peel specimen failed by microvoid coalescence (ductile fracture) near the weld nugget/heat affected zone (HAZ) boundary and that the tensile-shear specimen failed predominately by localized necking (shear localization) near the HAZ/base metal boundary. Empirical data extracted from measurements performed on metallurgical cross sections of interrupted coach-peel and tensile-shear specimens established the deformed characteristic material distance (coach-peel) and the existence of a critical thickness strain for localized necking (tensile-shear). These quantities were used to predict weld failure via finite element analysis as described in Ref I. The work presented here is the first step in a larger project that is focused on developing a methodology for predicting spot weld overload failure in detailed finite element simulations of spot-welded joints. This methodology is based upon the failure phenomena (as reported here) and detailed characterization of the HAZ (as reported in Ref I). The main requirement of this predictive methodology is that it be adaptable to any combination of joint configuration and loading direction. This predictive methodology will serve as the basis for the final step of developing a model of resistance spot weld failure based upon a simpler representation of the spot weld that can be used in car crash simulation models.