Modeling of localization and fracture phenomena in strain and stress space for sheet metal forming

Abstract

In this study, a model based on a strain localization level to overcome the shortcomings of the wellestablished Forming Limit Diagram (FLD) in predicting the physical phenomenon of necking is introduced. An optical measurement system was used to capture the strain history of the Nakazima experiment until rupture occurred. In order to measure the fracture strain more accurately, a further method is introduced, which is based on the microscopic measurements of ruptured regions. This model is validated using a 3-point bending test. The results show the ability of the method to predict failure under bending conditions as well. Additionally, failure is investigated based on the pressure sensitivity and the Lode dependency. The results show that the triaxiality at the failure point is independent of the loading path.

Modelling of localized ductile fracture with volumetric locking‐free tetrahedral elements

“…Focussing on ductile fracture, the fracture process has been observed to initiate from diffused void nucleation and growth, followed by void coalescence, and eventually lead to an experimentally localized crack growth and fracture. [47][48][49] Since the nonlocal term at the macro-level characterizes the interactions between active micro-processes, the length scale parameter in a gradient enhancement should decrease with damage. A more intuitive formulation with a decreasing length scale (c) was also considered in Geers et al, 45 where in Equation (32) decreases with increasing damage.…”

Investigation on Fracture of a 6014-T4 Aluminum Alloy Sheet in the Flanging and Hemming Process Based on Numerical and Experimental Methods

“…This difference indicates that the Max failure criteria derived from FLC needs further correction. A localization level forming limit diagram (LL-FLD) based on the fracture region micro-measurement method (thinning method) has been shown to be more suitable criteria for failure prediction on forming processes with small bending radii [16,17]. In addition, because shell element was used, the Max failure values in the simulation were calculated based on the strain state of the neutral layer of sheet metal.…”

“…Considering the complexity of precipitation, only the mechanical properties of 6014-T4 were researched in this study, while the metallographic study of the structure is one of our future research directions. Moreover, in order to obtain a better correspondence between simulation and experiment, more accurate evaluation criteria need to be adopted, such as the LL-FLD criteria proposed by Gorji et al [16,17], and the grain gradient over the thickness must be considered.…”

Applications of Finite Element Simulation in the Development of Advanced Sheet Metal Forming Processes

“…The curve represents the maximum values of principle strains ε1 and ε2 determined by measuring the strains at material failure. The different methods for determining the forming limit curve have been proposed by the Nakazima test, hydraulic bulge test, Marciniak test, and others [9]. At LUT, a Zwick BUP 200 system is taken to measure FLDs in combination with an optical device, a GOM Aramis system, to measure the strain field on the test sample and to determine the major strains at the begin of necking.…”