Investigation on the strain-path dependency of stress-based forming limit curves

This article analyzes the formability data sets for aluminum killed steel (Laukonis, J. V., and Ghosh, A. K., 1978, “Effects of Strain Path Changes on the Formability of Sheet Metals,” Metall. Trans. A., 9, pp. 1849–1856), for Al 2008-T4 (Graf, A., and Hosford, W., 1993, “Effect of Changing Strain Paths on Forming Limit Diagrams of Al 2008-T4,” Metall. Trans. A, 24A, pp. 2503–2512) and for Al 6111-T4 (Graf, A., and Hosford, W., 1994, “The Influence of Strain-Path Changes on Forming Limit Diagrams of Al 6111 T4,” Int. J. Mech. Sci., 36, pp. 897–910). These articles present strain-based forming limit curves (ϵFLCs) for both as-received and prestrained sheets. Using phenomenological yield functions, and assuming isotropic hardening, the ϵFLCs are transformed into principal stress space to obtain stress-based forming limit curves (σFLCs) and the principal stresses are transformed into effective stress and mean stress space to obtain the extended stress-based forming limit curves (XSFLCs). A definition of path dependence for the σFLC and XSFLC is proposed and used to classify the obtained limit curves as path dependent or independent. The path dependence of forming limit stresses is observed for some of the prestrain paths. Based on the results, a novel criterion that, with a knowledge of the forming limit stresses of the as-received material, can be used to predict whether the limit stresses are path dependent or independent for a given prestrain path is proposed. The results also suggest that kinematic hardening and transient hardening effects may explain the path dependence observed in some of the prestrain paths.

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Investigations of the effect of strain path changes on forming limit curves using an in-plane biaxial tensile test

International Journal of Mechanical Sciences ◽

10.1016/j.ijmecsci.2015.05.007 ◽

2015 ◽

Vol 99 ◽

pp. 21-28 ◽

Cited By ~ 36

Author(s):

Lionel Leotoing ◽

Dominique Guines

Keyword(s):

Tensile Test ◽

Strain Path ◽

Forming Limit ◽

Biaxial Tensile Test ◽

Biaxial Tensile ◽

Strain Path Changes ◽

Forming Limit Curves

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Forming Limit Curves for Complex Strain Paths

Archives of Metallurgy and Materials ◽

10.2478/amm-2013-0042 ◽

2013 ◽

Vol 58 (2) ◽

pp. 587-593 ◽

Cited By ~ 4

Author(s):

J. Rojek ◽

D. Lumelskyy ◽

R. Pęcherski ◽

F. Grosman ◽

M. Tkocz ◽

...

Keyword(s):

Plastic Strain ◽

Strain Path ◽

Experimental Studies ◽

Polar Coordinates ◽

Forming Limit ◽

Effective Plastic Strain ◽

Steel Blank ◽

Strain Paths ◽

Complex Strain ◽

Forming Limit Curves

This paper presents results of experimental studies of forming limit curves (FLC) for sheet forming under complex strain paths. The Nakazima-type formability tests have been performed for the as-received steel blank and for the blank pre-strained by13%. Prestraining leads to abrupt change of strain path in the blank deformation influencing the forming limit curve. The experimental FLC of the pre-strained blank has been compared with the FLC constructed by transformation of the as-received FLC. Quite a good agreement has been found out. The concept of strain-path independent FLCs in polar coordinates has been verified. Two types of the polar diagrams have been considered, the first one with the strain-path angle and effective plastic strain as the polar coordinates, and the second one originally proposed in this work in which the thickness strain has been used instead of the effective plastic strain as one of the polar coordinates. The second transformation based on our own concept has given a better agreement between the transformed FLCs, which allows us to propose this type of polar diagrams as a new strain-path in dependent criterion to predict sheet failure in forming processes.

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