Experimental studies of necking and fracture limits of boron steel sheet under hot stamping conditions

Zhang, Ruiqiang; Shi, Zhusheng; Yardley, Victoria A.; Lin, Jianguo

doi:10.1016/j.jmatprotec.2021.117481

Cited by 17 publications

(12 citation statements)

References 43 publications

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“…The times taken to reach localised necking tnormalLN and fracture tnormalF in plane-strain stretching (Figure 4(b)) are shorter than those obtained either in uniaxial (Figure 4(a)) or biaxial stretching (Figure 4(c)). This agrees with the experimental observations in the formability tests in Zhang et al. (2022) used for the calibration of the CDM-based constitutive model.…”

Section: Model Implementation and Discussionsupporting

confidence: 89%

“…FLCs) and at fracture (i.e. FFLCs) (Zhang et al., 2022) obtained under the conditions given in Table 1. These true stress-true strain curves of boron steel were obtained by carrying out uniaxial tensile tests under hot stamping conditions using a Gleeble thermal-mechanical simulator, together with the digital image correlation (DIC) technique for full-field strain measurement.…”

Section: Model Implementation and Discussionmentioning

confidence: 99%

“…Due to the current limitations of the biaxial test rig, which is able to produce three distinct displacement ratios only, only a few experimental limit strain data points are available for constructing FLCs and FFLCs (Zhang et al., 2022). Therefore, the curve fitting was performed by trial and error, in combination with a thorough understanding of the physical meaning of each variable and associated material constants in the equations.…”

Section: Model Implementation and Discussionmentioning

confidence: 99%

“…The fracture forming limit diagram (FFLD) is an extension of the FLD (Atkins, 1996), in which fracture forming limit curves (FFLCs) are constructed by connecting the limit strains at fracture in different strain states for a given temperature and strain rate. According to the experimental data (Figure 3 in Section Determination of material constants from experimental data) on boron steel under hot stamping conditions reported by Zhang et al (2022), there is a large difference between the limit strains at the onset of localised necking and those at fracture. From this experimental work, it has also been observed that with increasing deformation, necking progresses continuously, terminating at fracture.…”

Section: Introductionmentioning

confidence: 99%

“…This is partly due to the lack of available experimental data on FLCs and FFLCs for the calibration of material models. Recently, both FLCs and FFLCs for 22MnB5 boron steel under hot stamping conditions were determined for the first time using a biaxial test method developed by Zhang et al (2022). This provides fundamental experimental data on FLCs and FFLCs for the calibration of the model developed in the present study.…”

Section: Introductionmentioning

confidence: 99%

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A CDM-based unified viscoplastic constitutive model of FLCs and FFLCs for boron steel under hot stamping conditions

Zhang

Shi

Yardley

et al. 2022

International Journal of Damage Mechanics

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Forming limit curves (FLCs), which are constructed using the limit strains at localised necking, are the most widely used tools for the evaluation of the formability of sheet metals. Fracture forming limit curves (FFLCs) are more recently developed, complementary tools for formability evaluation which are instead constructed using the limit strains at fracture. Since the formability depends strongly on forming conditions such as strain state, temperature and strain rate, models for predicting FLCs and FFLCs are essential for the optimisation and further application of hot forming processes in which these forming conditions vary significantly with both position and time. However, no model has so far been developed to predict FFLCs either alone or in conjunction with FLCs for sheet metals such as boron steel under hot stamping conditions. In this study, a set of unified viscoplastic constitutive equations for the prediction of both FLCs and FFLCs based on continuum damage mechanics (CDM) has been formulated from a set of recently developed constitutive equations for dislocation-based hardening, in combination with two novel coupled variables characterising the accumulated damage leading to localised necking and fracture. The novel variables take into account the effects of strain state, temperature and strain rate on the formability of sheet metals. The material constants in the CDM-based constitutive equations have been calibrated using experimental data comprising true stress-true strain curves and limit strains of a 22MnB5 boron steel obtained at a range of temperatures and strain rates. Investigation of the effect of varying selected parameters in the coupled damage variables on the resulting computed FLCs and FFLCs has demonstrated the flexibility of the model in enabling curves of different shapes and numerical values to be constructed. This indicates the potential of the CDM-based constitutive model for application to other materials for warm or hot stamping processes.

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