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TopIntroduction
In sheet metal forming process, cyclic loading occurs due to bending and unbending of material as in the die draw bead and when the sheet is drawn over a die shoulder corner as shown in Figure 1 (Hosford and Caddell 1993; Sanchez 2010; Yoshida et al. 2002). The bending-unbending deformation causes an effect where the yield stress during reversal loading is lower than the yield stress during forward loading. This effect is known as the Bauschinger effect.
Figure 1. Description of cyclic loading: (a) Draw-bend, (b) Springback, (c) Stress-strain path (Yoshida et al. 2002)
The Bauschinger effect, by definition, is the reduction of yield stress on the reversal of loading when compared to the forward loading. The Bauschinger effect factor (BEF) has been used to quantify the Bauschinger effect according to the following formula:
(1) where
Y1 and
Y2 are shown in Figure 2. A zero BEF value indicates that no Bauschinger effect is present in the loading and unloading deformation (Weinmann et al., 1988).
Figure 2. Weinmann’s cyclic loading effect (Weinmann et al., 1988)
Yoshida described this cyclic process as having four distinct features: load reversal and Bauschinger point, transient behaviour, work-hardening stagnation, and permanent softening as shown in Figure 3 (Yoshida and Uemori, 2003).
Figure 3. Features of cyclic loading in bending (Yoshida and Uemori 2003)
To improve sheet metal forming process simulation, there is a need to incorporate an appropriate constitutive equation capable of describing the Bauschinger effect and the so-called cyclic transient, derived from a near to actual sheet metal forming process testing tool. This has motivated several researchers to develop bending testers, such as Weinmann et al. (1988), Yoshida et al. (1998), Geng et al. (2002), Zhao and Lee (2002), Omerspahic et al. (2006), Carbonniere et al. (2009) and Boers et al. (2010).
The objective of this paper is to investigate Bauschinger effect of low carbon steel and stainless steel using a newly developed bending and unbending tool. Other characteristics such as transient behaviour and work-hardening stagnation are also being monitored.
TopMethodology
The mechanical properties of low carbon steel and stainless-steel specimen used for this study are shown in Table 1 and Table 2.
Table 1. Cold rolled low carbon steel
| Standard and grade | EN10130:1999: DC01
BS1449:1991: CR4 |
| Chemical composition, % | C = 0.120, P = 0.045, S = 0.045, MN = 0.6 |
| Mechanical properties | Yield strength: 140–280 MPa
Tensile strength: 270–410 MPa
Young’s modulus: 207 GPa
Total elongation: 30%
Strength coefficient: 500 MPa
Strain hardening exponent: 0.25 |