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Evaluation of formability and fracture of pure titanium in incremental sheet forming

Gatea, Shakir; Xu, Dongkai; Ou, Hengan; McCartney, Graham

Authors

Shakir Gatea

Dongkai Xu

Hengan Ou

Graham McCartney



Abstract

A forming limit diagram (FLD) is commonly used as a useful means for characterizing the formability of sheet metal forming processes. In this study, the Nakajima test was used to construct the forming limit curve at necking (FLCN) and fracture (FLCF). The results of the FLCF are compared with incremental sheet forming (ISF) to evaluate the ability of the Nakajima test to describe the fracture in ISF. Tests were carried to construct the forming limit diagram at necking and fracture to cover the strain states from uniaxial tension to equi-biaxial tension with different stress triaxialities - from 0.33 for uniaxial tension to 0.67 for equi-biaxial tension. Due to the fact that the Gurson–Tvergaard- Needleman (GTN) model can be used to capture fracture occurrence at high stress triaxiality, and the shear modified GTN model (Nahshon-Hutchinson’s shear mechanism) was developed to predict the fracture at zero stress or even negative stress triaxiality, the original GTN model and shear modified GTN model may be not suitable to predict the fracture in all samples of the Nakajima test as some samples are deformed under moderate stress triaxiality. In this study, the fractures are compared using either the original GTN model, shear modified GTN model or Nielsen-Tvergaard model with regard to stress triaxiality. To validate the ability of these models, and to assess which model is more accurate in predicting the fracture with different stress triaxialities, finite element (FE) simulations of the Nakajima test were compared with an experimental results to evaluate the applicability of the Nakajima test to characterise the fracture from ISF. The experimental and FE results showed that the shear modified GTN model could predict the fracture accurately with samples under uniaxial tension condition due to low stress triaxiality and that the original model is suitable for an equi-biaxial strain state (high stress triaxiality), whereas the stress triaxiality modified GTN model should be considered for samples which have moderate stress triaxiality (from plain strain to biaxial strain). The numerical and experimental FLCF of pure titanium from the Nakajima test showed good agreement with the experimental and numerical results of ISF.

Journal Article Type Article
Journal International Journal of Advanced Manufacturing Technology
Print ISSN 0268-3768
Electronic ISSN 1433-3015
Publisher Springer Verlag
Peer Reviewed Peer Reviewed
APA6 Citation Gatea, S., Xu, D., Ou, H., & McCartney, G. (in press). Evaluation of formability and fracture of pure titanium in incremental sheet forming. International Journal of Advanced Manufacturing Technology, https://doi.org/10.1007/s00170-017-1195-z
DOI https://doi.org/10.1007/s00170-017-1195-z
Keywords Nakajima test, ISF, FLD, Stress triaxiality, GTN model
Publisher URL https://link.springer.com/article/10.1007%2Fs00170-017-1195-z
Copyright Statement Copyright information regarding this work can be found at the following address: http://creativecommons.org/licenses/by/4.0

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Copyright Statement
Copyright information regarding this work can be found at the following address: http://creativecommons.org/licenses/by/4.0





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