Predicting Sheet Forming Limit of Aluminum Alloys for Cold and Warm Forming by Developing a Ductile Failure Criterion

2017 ◽  
Vol 139 (11) ◽  
Author(s):  
Z. Q. Sheng ◽  
P. K. Mallick
Keyword(s):  
Failure Criterion ◽  
Sheet Thickness ◽  
Ductile Failure ◽  
Alloy Sheet ◽  
Warm Forming ◽  
Forming Limit ◽  
Hollomon Parameter ◽  
Effect Function ◽  
Sheet Materials ◽  
Ductile Failure Criterion

In this study, the forming limit of aluminum alloy sheet materials is predicted by developing a ductile failure criterion (DFC). In the DFC, the damage growth is defined by Mclintock formula, stretching failure is defined at localized necking (LN) or fracture without LN, while the critical damage is defined by a so-called effect function, which reflects the effect of strain path and initial sheet thickness. In the first part of this study, the DFC is used to predict forming limit curves (FLCs) of six different aluminum sheet materials at room temperature. Then, the DFC is further developed for elevated temperature conditions by introducing an improved Zener–Hollomon parameter (Z′), which is proposed to provide enhanced representation of the strain rate and temperature effect on limit strain. In warm forming condition, the improved DFC is used to predict the FLCs of Al5083-O and failure in a rectangular cup warm draw process on Al5182 + Mn. Comparison shows that all the predictions match quite well with the experimental measurements. Thanks to the proposal of effect function, the DFC needs calibration only in uniaxial tension, and thus, provides a promising potential to predict forming limit with reduced effort.

Predicting Sheet Forming Limit of Aluminum Alloys for Cold and Warm Forming by Developing a Ductile Failure Criterion

2017 ◽  
Author(s):  
Z. Q. Sheng ◽  
P. K. Mallick
Keyword(s):  
Failure Criterion ◽  
Sheet Thickness ◽  
Ductile Failure ◽  
Alloy Sheet ◽  
Warm Forming ◽  
Forming Limit ◽  
Hollomon Parameter ◽  
Effect Function ◽  
Sheet Materials ◽  
Ductile Failure Criterion

In this study, the forming limit of aluminum alloy sheet materials are predicted by developing a Ductile Failure Criterion (DFAC). In the DFAC, the damage growth is defined by Mclintock formula, stretching failure is defined at Localized Necking (LN) or Fracture without LN, while the critical damage is defined by a so-called effect function, which reflects the effect of strain path and initial sheet thickness. In the first part of this study, the DFAC is used to predict Forming Limit Curves of six different aluminum sheet materials at room temperature. Then, the DFAC is further developed for elevated temperature condition by introducing an improved Zener-Hollomon parameter (Z′), which is proposed to provide enhanced representation of the strain rate and temperature effect on limit strain. In warm forming condition, the improved DFAC is used to predict the FLCs of Al5083-O and failure in a rectangular cup warm draw process on Al5182+Mn. Comparison shows that all the prediction matches quite well with experimental measurement. Thanks to the proposal of effect function, the DFAC only needs a calibration at uniaxial tension and thus provides a promising potential to predict forming limit with reduced efforts.

Extension of a Failure Criterion for Hemming Applications in the FEM Simulation

2012 ◽  
Vol 504-506 ◽  
pp. 765-770 ◽  
Author(s):  
Martin Zubeil ◽  
Karl Roll ◽  
Marion Merklein
Keyword(s):  
Failure Criterion ◽  
Stress Ratio ◽  
Equivalent Plastic Strain ◽  
Fem Simulation ◽  
Sheet Thickness ◽  
Forming Limit Diagram ◽  
Shell Element ◽  
Forming Limit ◽  
Separate Analysis ◽  
Loading Conditions

In the FEM calculation of sheet metal forming processes to determine the failure case, the forming limit diagram is basically used. To determine the failure case at bending condition, the forming limit diagram can not be used. This behaviour was shown by many authors. Bending tests with an aluminium material (AC170PX) have shown that a high deformation ratio can be achieved without failure. Based on the loading conditions and the previous strain path through the deep-drawing process, a resulting bendability at a certain point can be obtained. Depending on the pre-damage and the mentioned loading conditions of the material failure will be occurring during bending at different times. Current developments of failure criteria consider the failure as in ductile fracture or shear fracture, which must be considered separately in the simulation. To rule out a separate analysis of the mode of failure in the post-processing, an existing failure criterion is extended and will be presented in this work. For the applications flanging and hemming the following extension of a stress-based failure criterion is proposed. Based on the triaxiality and the equivalent plastic strain a monitoring of the stress ratio is implemented in the FEM simulation. During the forming simulation the monitoring system observe the stress ratio based on the principal stresses resulted from the integration (Gauss) point of the shell element. According to the evaluation of the stress ratio evolution, a relevant definition will take into account how the damage will be accumulated. If the critical value of damage in the integration point of the shell element is reached, failure will be occur based on the position of the sheet thickness.

Prediction of Forming Limit Diagrams for Aluminum Alloy Sheet Using Finite Element Analysis

2006 ◽  
Author(s):  
Bing Li ◽  
Tim J. Nye
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Aluminum Alloy ◽  
Sheet Thickness ◽  
Alloy Sheet ◽  
Forming Limit ◽  
Aluminum Alloy Sheet ◽  
Element Analysis ◽  
Anisotropic Parameter ◽  
Forming Limit Diagrams

Prediction of forming limit diagram (FLD) for aluminum alloy sheet using finite element analysis without implementing pre-defined geometrical imperfections or material imperfections was studied. The limit strains of the FLD were determined by applying a new proposed localization criterion in the dome stretching test. The elements just outside the necking area, where their major and minor principal strains have no simultaneous change after localized necking happens, were chosen as the reference elements for measurement of limit strains. Simulations were carried out for various strain paths ranging from balanced biaxial stretching to uniaxial stretching. The effects of material properties, sheet thickness, anisotropic parameter and friction coefficient at the sheet punch interface on the locus of FLD were investigated. It was found that the material yield stress and average anisotropic parameter value has almost no effect on forming limits; larger strain-hardening exponent and higher sheet thickness result in higher level of forming limit strains; the friction coefficient has little influence on the locus of FLD but does affect the strain path taken during the deformation. The predicted FLD of AA 5182-O was compared with an experimentally determined FLD and very good agreement has been achieved. It was demonstrated that forming limit diagrams can be predicted by the finite element method without requiring any assumed geometric or material imperfections in the numerical model.

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