Micromechanical origins of remarkable elongation-to-fracture in AHSS TRIP steels via continuous bending under tension

A ubiquitous experiment to characterize the formability of sheet metal is the standard uniaxial tension test. Past research [1–3] has shown that if the material is repeatedly bent and unbent during this test (termed Continuous-Bending-under-Tension, or CBT), the percent elongation at failure increases significantly (e.g., from 22% to 290% for an AISI 1006 steel [1]). However, past experiments have been conducted with a fixed stroke of the CBT device, which limits the formability improvements. This phenomenon has also been empirically observed in industry; the failure strains of a sheet which is passed through a drawbead (i.e., that has been bent and unbent three times before entering the die) are higher than those of the original sheet. Thus, the residual formability of the material after a specified number of CBT passes is of interest, to determine if multiple drawbeads would be beneficial in the process. Also of interest is the localization of the deformation during the process as this will provide a better physical understanding of the improved formability observed. In this paper, numerical simulations are presented to assess these effects. Results show that the formability during CBT is dictated by the uniaxial response of the material until the standard elongation at failure is exceeded. This limit can be exceeded by the CBT process. However, failure then occurs as soon as the CBT process is terminated. Also, the deformation is more uniformly distributed over the entire gauge length during the CBT process which leads to the increased elongations observed.

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Forming Limits of a Sheet Metal After Continuous-Bending-Under-Tension Loading

Journal of Engineering Materials and Technology ◽

10.1115/1.4023676 ◽

2013 ◽

Vol 135 (3) ◽

Cited By ~ 22

Author(s):

Ji He ◽

Z. Cedric Xia ◽

Danielle Zeng ◽

Shuhui Li

Keyword(s):

Finite Element ◽

Sheet Metal ◽

Sheet Metal Forming ◽

Metal Forming ◽

Loading Condition ◽

Hybrid Approach ◽

Forming Limit ◽

Forming Limits ◽

Continuous Bending ◽

Bending Under Tension

Forming limit diagrams (FLD) have been widely used as a powerful tool for predicting sheet metal forming failure in the industry. The common assumption for forming limits is that the deformation is limited to in-plane loading and through-thickness bending effects are negligible. In practical sheet metal applications, however, a sheet metal blank normally undergoes a combination of stretching, bending, and unbending, so the deformation is invariably three-dimensional. To understand the localized necking phenomenon under this condition, a new extended Marciniak–Kuczynski (M–K) model is proposed in this paper, which combines the FLD theoretical model with finite element analysis to predict the forming limits after a sheet metal undergoes under continuous-bending-under-tension (CBT) loading. In this hybrid approach, a finite element model is constructed to simulate the CBT process. The deformation variables after the sheet metal reaches steady state are then extracted from the simulation. They are carried over as the initial condition of the extended M–K analysis for forming limit predictions. The obtained results from proposed model are compared with experimental data from Yoshida et al. (2005, “Fracture Limits of Sheet Metals Under Stretch Bending,” Int. J. Mech. Sci., 47(12), pp. 1885–1986) under plane strain deformation mode and the Hutchinson and Neale's (1978(a), “Sheet Necking—II: Time-Independent Behavior,” Mech. Sheet Metal Forming, pp. 127–150) M–K model under in-plane deformation assumption. Several cases are studied, and the results under the CBT loading condition show that the forming limits of post-die-entry material largely depends on the strain, stress, and hardening distributions through the thickness direction. Reduced forming limits are observed for small die radius case. Furthermore, the proposed M–K analysis provides a new understanding of the FLD after this complex bending-unbending-stretching loading condition, which also can be used to evaluate the real process design of sheet metal stamping, especially when the ratio of die entry radii to the metal thickness becomes small.

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Experimental Investigation of Key Process Parameters During Continuous-Bending-Under-Tension of AA6022-T4

Volume 1: Processes ◽

10.1115/msec2017-3045 ◽

2017 ◽

Cited By ~ 1

Author(s):

Edward M. Momanyi ◽

Timothy J. Roemer ◽

Brad L. Kinsey ◽

Yannis P. Korkolis

Keyword(s):

Experimental Investigation ◽

Aluminum Alloys ◽

Process Parameters ◽

Tensile Tests ◽

Load Cycle ◽

Gauge Length ◽

Uniaxial Tensile ◽

Plastic Bending ◽

Continuous Bending ◽

Bending Under Tension

Continuous-Bending-Under-Tension (CBT) is an experimental technique that has been shown to increase elongation-to-fracture by over 100% in aluminum alloys and over 300% in steel as compared to uniaxial tensile tests [1]. This procedure is a modified form of a tensile test in which a specimen experiences 3 point plastic bending, induced by traversing 3 rollers back and forth over the gauge length, while simultaneously being pulled in tension. This process is able to delay the occurrence of necking in pure tension by suppressing the instability. Thus, significantly more elongation is achieved in the specimen prior to fracture. In this paper, an experimental investigation of key process parameters, i.e., bending depth and pulling speed, during CBT testing of AA6022-T4 is presented. The load cycle during a CBT test will also be discussed along with the strain induced throughout the gauge length.

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