Characterisation of Diffuse and Local Necking of Aluminium Alloy Sheets using DIC Technique

This paper introduces a new method for the characterisation of the boundary of diffuse and local necking based on DIC measurements during tensile tests. A series of images illustrate the extension of diffuse necking and show the occurrence of local necking as well. The evaluation of strain distribution gives the exact description of processes using both time dependent and non-dependent methods.

BIFURCATION ANALYSIS VERSUS MAXIMUM FORCE CRITERIA IN FORMABILITY LIMIT ASSESSMENT OF STRETCHED METAL SHEETS

The present contribution deals with the prediction of diffuse necking in the context of forming and stretching of metal sheets. For this purpose, two approaches are investigated, namely bifurcation and the maximum force principle, with a systematic comparison of their respective ability to predict necking. While the bifurcation approach is of quite general applicability, some restrictions are shown for the application of maximum force conditions. Although the predictions of the two approaches are identical for particular loading paths and constitutive models, they are generally different, which is even the case for elasticity, confirming the distinct nature of the two concepts. Closed-form expressions of the critical stress and strain states are derived for both criteria in elasto-plasticity and rigid-plasticity for a variety of hardening models. The resulting useful formulas in rigid-plasticity are shown to also accurately represent the elasto-plastic critical states for small ratios of the hardening modulus with respect to Young's modulus. Finally, the well-known expression of Swift's diffuse necking criterion, whose foundations are attributed in the literature to the maximum force principle, is shown here to originate from the bifurcation approach instead, providing a sound justification for it.

Experimental Investigation on the Instability of Al Sheet in Hydraulic Bulging Based on ESPI

Electronic speckle pattern interferometry (ESPI) was applied to hydraulic bulging to analyze the entire deformation process, especially the instability evolution of Al 1060 sheet. Speckle patterns of the specimens were captured continuously to record the deformation fields. The development of full-field strain rate during bulging was represented by the fringe patterns real-timely. The emergence of the defect and its subsequent transformation into groove and crack were revealed clearly by the aberration region in the fringe patterns. The onset of diffuse necking and localized necking were determined by the strain rate distribution curves calculated based on the fringe patterns. Results indicate that ESPI is a satisfying method to analyze the instability and fracture of sheet metals even in three dimension deformation. The growth of micro-crack caused a banded weak region. Diffuse necking occurred in the weak region due to strain localization. A groove generated in the banded weak region and then extended along the length direction. The remarkable strain localization in the groove indicated the onset of localized necking. The thickness of the groove decreased quickly and finally caused fracture. The accurately and quickly determined necking strains would lead to a safer FLD.

A Consistent Criterion for Diffuse Necking in Sheet Metals Using Hill’s 1979 Yield Surface

A review of some existing criteria for diffuse necking in sheet metals is given and their limitations are discussed. The introduction into production of new sheet materials whose plastic deformation is impossible to be modeled using Hill’s 1948 anisotropic yield function necessitates improvements of these existing criteria to accurately describe their necking behavior. In this paper, a generalization of the existing diffuse necking criteria for materials describable by Case IV of Hill’s 1979 anisotropic yield function is presented. The proposed criterion is consistent with the previous criteria. It predicts a significant effect of Hill’s 1979 yield surface shape factor on the critical principal strain in the range of negative minor strains while in the range of biaxial tension this influence is small.