Formability Evaluation Using Modified Cockcroft Criterion with Strain Paths for Sheet Metal Forming

2014 ◽  
Vol 626 ◽  
pp. 495-501 ◽  
Author(s):  
Rong Shean Lee ◽  
Ta Wei Chien
Keyword(s):  
Tensile Test ◽  
Sheet Metal ◽  
Sheet Metal Forming ◽  
Metal Forming ◽  
Strain Path ◽  
Dome Height ◽  
Bulge Test ◽  
Cruciform Specimen ◽  
Strain Paths ◽  
Good Agreement

In most situations, original Cockcroft criterion underestimates material formability in the first quadrant of FLD. So far, some modified Cockcroft criteria have been reported for different applications. This presentation will focus on the modified Cockcroft criterion which takes strain-path effect into consideration. This paper demonstrates the accuracy of this criterion through limiting dome height test, free bulge test, and the biaxial tensile test using cruciform specimen respectively. The results showed that the modified Cockcroft criterion with strain path effect has good agreement with experimental results.

Stress-Based Forming Limits in Sheet-Metal Forming

2000 ◽  
Vol 123 (4) ◽  
pp. 417-422 ◽  
Author(s):  
Thomas B. Stoughton
Keyword(s):  
Sheet Metal ◽  
Sheet Metal Forming ◽  
Metal Forming ◽  
Strain Path ◽  
Forming Limit ◽  
Stress Strain ◽  
Strain Relation ◽  
Stress Strain Relation ◽  
The Stability ◽  
Definition Of

A strain-based forming limit criterion is widely used throughout the sheet-metal forming industry to gauge the stability of the deformed material with respect to the development of a localized neck prior to fracture. This criterion is strictly valid only when the strain path is linear throughout the deformation process. There is significant data that shows a strong and complex dependence of the limit criterion on the strain path. Unfortunately, the strain path is never linear in secondary forming and hydro-forming processes. Furthermore, the path is often found to be nonlinear in localized critical areas in the first draw die. Therefore, the conventional practice of using a path-independent strain-based forming limit criterion often leads to erroneous assessments of forming severity. Recently it has been reported that a stress-based forming limit criterion appears to exhibit no strain-path dependencies. Subsequently, it has been suggested that this effect is not real, but is due to the saturation of the stress-strain relation. This paper will review and compare the strain-based and stress-based forming limit criteria, looking at a number of factors that are involved in the definition of the stress-based forming limit, including the role of the stress-strain relation.

Finite-Element Modeling of Thermal Forming of Ti-TWBs with Experimental Verification

2014 ◽  
Vol 626 ◽  
pp. 518-523
Author(s):  
C.P. Lai ◽  
Luen Chow Chan
Keyword(s):  
Finite Element ◽  
Sheet Metal ◽  
Experimental Verification ◽  
Sheet Metal Forming ◽  
Metal Forming ◽  
Production Test ◽  
Forming Process ◽  
Tailor Welded Blanks ◽  
Multi Stage ◽  
Strain Paths

The titanium tailor-welded blanks (Ti-TWBs) are being developed in different industries such as automobile and aerospace, combining the advantages of both tailor-welded blanks technology and titanium alloys. In recent decades, computer simulation of sheet metal forming processes has been employed increasingly over conventional production test and adjustment methodology to achieve the optimum and cost-effective operation procedures. Recently, certain amounts of theoretical analysis for the sheet metal forming process have been developed. However, these analyses could not be applied directly to the material under multi-stage forming process. Thus, some researchers have developed a damage-based model to predict the instability and failure of sheet metals, particularly for the above Ti-TWBs, with consideration of material damage under discontinuous or proportional loading strain paths. So far this model has been used and proved to be successful to predict formability of selected sheets of steel and aluminium alloy. However, the application of the damage-coupled model has yet to be extended to the Ti-TWBs under thermal multi-stage forming operation.The main objective of this paper is to investigate numerically the formability of Ti-TWBs under multi-stage forming process with experimental verification. Titanium alloy sheets (Ti-6Al-4V) in thickness of 0.7mm and 1.0mm were selected and laser-welded the specimen of Ti-TWBs. The model based on the damage mechanics is introduced to predict the thermal formability of Ti-TWBs with change of strain paths. In this study, the anisotropic damage model incorporate with the finite element codes and user-define material subroutine were developed to predict the formability of Ti-TWBs with change of strain paths. The mechanical properties and damage parameters of Ti-TWBs for the simulation were measured experimentally. The simulation of Ti-TWB under multi-stage forming process were then conducted and validated experimentally at similar forming conditions. The predicted results have been found to agree well with those obtained from the experiments. This analysis can be applied readily to design and manufacture TWB components or structures so as to satisfy the need of such market demands.

Failure prediction for nonlinear strain paths in sheet metal forming

CIRP Annals ◽  
2012 ◽  
Vol 61 (1) ◽  
pp. 259-262 ◽  
Author(s):  
Wolfram Volk ◽  
Hartmut Hoffmann ◽  
Joungsik Suh ◽  
Jaekun Kim
Keyword(s):  
Sheet Metal ◽  
Sheet Metal Forming ◽  
Metal Forming ◽  
Failure Prediction ◽  
Nonlinear Strain ◽  
Strain Paths

A New Method for Testing Formability in Sheet Metal Forming at Biaxial Tensile State

2014 ◽  
Vol 626 ◽  
pp. 275-280 ◽  
Author(s):  
Rong Shean Lee ◽  
Ta Wei Chien
Keyword(s):  
Experimental Data ◽  
Sheet Metal ◽  
Sheet Metal Forming ◽  
Metal Forming ◽  
Experimental Results ◽  
New Method ◽  
Dome Height ◽  
Uniaxial Tensile ◽  
Biaxial Tensile ◽  
Theoretical Predictions

This paper presents a new method concerning testing formability in sheet metal forming, especially focuses on clarifying the divergence of the experiment and a variety of theoretical predictions on biaxial tensile state. Up to now, there are many different fracture criteria appeared. All researches have presented their experimental data which could justify the criterion they presented. However, the experimental results and predictions in the first quadrant of the forming limit diagram (FLD) often diverge. Today, limiting dome height test is commonly used for FLD experiment, but specimens are rubbed and bended during the test, both influencing the experimental results.In order to provide for convincible experimental data, this paper presents a new experimental method to establish the first quadrant of FLD. In this method, cruciform biaxial tensile specimen and biaxial tensile apparatus have been developed. The proposed specimen has the feature of thickness reduction and contour design to ensure the fracture location is in the central region, so that accurate biaxial tensile state can be obtained. Through this method, there is an opportunity to obtain the whole FLD using uniaxial tensile testing machine, which is a low-cost alternative in compared with limiting dome height test. Besides, the experimental results can be utilized to clarify the divergence between various theoretical predictions and experimental results in the first quadrant of the FLD.

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