A NOVEL MODEL BASED ON VISCOELASTIC THEO- RY TO PREDICT THE TIME-DEPENDENT SPRINGBACK FOR DP600 STEEL SHEET

doi:10.11900/0412.1961.2015.00176

Acta Metall Sin

2015, Vol. 51

Issue (11): 1356-1364 DOI: 10.11900/0412.1961.2015.00176

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A NOVEL MODEL BASED ON VISCOELASTIC THEO- RY TO PREDICT THE TIME-DEPENDENT SPRINGBACK FOR DP600 STEEL SHEET

Shuai SUN,Daxin E(

)

School of Materials, Beijing Institute of Technology, Beijing 100081

Cite this article:

Shuai SUN,Daxin E. A NOVEL MODEL BASED ON VISCOELASTIC THEO- RY TO PREDICT THE TIME-DEPENDENT SPRINGBACK FOR DP600 STEEL SHEET. Acta Metall Sin, 2015, 51(11): 1356-1364.

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Abstract

DP600 steel sheet with high strength has drawn much attention in the automotive industry, but the shape change following forming and unloading has not been known widely. The time-dependent springback of DP600 steel sheet was investigated under different pre-strains by uniaxial tension. According to the viscous behaviors under the elastic and plastic loading tests, the lower limit of integration in the constitutive equation of linear viscoelasticity was modified and the creep compliance was gained from the creep curve at a constant stress level of 309 MPa at room temperature. The predicted curve was acquired by using the superposition of the unloading impulse and the historical loading curve. The results reveal that the strain rates with high initial values gradually decreased following unloading at room temperature. As the pre-strain went up, both the absolute anelastic strain of time-dependent springback and the anelastic proportion of the total springback increased. Meanwhile, the direction of the time-dependent springback was same as that of the initial springback and opposite to the loading direction. In the same springback period, the ratio of the unloading stress to the creep springback strain tended to vary more linearly with the pre-strain than those obtained from immediate unloading. The simulated results using the revised model are in good agreement with the experimental data.

Key words: DP600 steel sheet time-dependent springback creep springback Kelvin model

Fund: Supported by National Natural Science Foundation of China (No.51175044)

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https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00176 OR https://www.ams.org.cn/EN/Y2015/V51/I11/1356

Fig.1 Stress-strain curves in uniaxial tensile test for DP600 steel sheet

Fig.2 E_u varied with pre-strain ε_i under strain e=1.7×10^-⁴

Table1 Initial springback and time-dependent springback without holding time

Fig.3 Measured p as a function of e_i

Fig.4 Measured time-dependent springback strain Δe_r varied with time

Fig.5 Creep curves of DP600 steel sheet under low (a) and high (b) stresses holding for 4 h

Fig.6 Creep springback strain of DP600 steel sheet (a) and loss percentage k for creep springback (b)

Fig.7 Fitting curve of creep compliance for DP600 (a) and detail view of marked area in Fig.7a (b)

Table 2 Initial springback for creep and creep springback after holding for 4 h

Fig.8 Stress rate-time curve (a) and stress-time curve (b) (t_A is the starting point of stress rate function; t_B, t_C, t_D and t_E are the times of unloading for specimens PI-5, PI-8, PI-10 and PI-15)

Fig.9 Convolutions of historical loading effect under different pre-strains (a) and detail view of marked area in Fig.9a (b)

Fig.10 Elastic after effect strain curves (a) and time-dependent springback strain curves for prediction (b)

Fig.11 Experimental strain and predicted strain of time-dependent springback varied with pre-strain e_i

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