Issues associated with the use of Yoshida nonlinear isotropic/kinematic hardening material model in Advanced High Strength Steels

The Yoshida nonlinear isotropic/kinematic hardening material model is often selected in forming simulations where an accurate springback prediction is required. Many successful application cases in the industrial scale automotive components using advanced high strength steels (AHSS) have been reported to give better springback predictions. Several issues have been raised recently in the use of the model for higher strength AHSS including the use of two C vs. one C material parameters in the Armstrong and Frederick model (AF model), the original Yoshida model vs. Original Yoshida model with modified hardening law, and constant Young’s Modulus vs. decayed Young’s Modulus as a function of plastic strain. In this paper, an industrial scale automotive component using 980 MPa strength materials is selected to study the effect of two C and one C material parameters in the AF model on both forming and springback prediction using the Yoshida model with and without the modified hardening law. The effect of decayed Young’s Modulus on the springback prediction for AHSS is also evaluated. In addition, the limitations of the material parameters determined from tension and compression tests without multiple cycle tests are also discussed for components undergoing several bending and unbending deformations. 1.0 Introduction Advanced high strength steels (AHSS) are widely used in automotive body-in-white (BIW) components due to their potential weight-saving properties, while maintaining component performance in crashworthiness and structural durability. The application of AHSS in a typical vehicle will be increased from an average of 254 lbs in 2014 to an average of 483 lbs by 2025 based on a forecast by Ducker [1]. The driving force behind this forecast is the ever-increasing innovations in new steel development. Gen-3 steels aim to achieve a balance between press hardening steels (PHS) and Gen-1 dual phase steels by having higher elongation than PHS and higher strength than dual phase steels. The advantage of having higher elongation is that it is possible to cold stamp those steels using conventional stamping presses, thereby reducing fabrication cost. An additional promising feature of Gen-3 steel is its high fracture resistance compared to PHS, which makes the usage of Gen-3 steels in body-in-white (BIW) structure components attractive. Nevertheless, research has shown that for BIW applications, steels with yield strengths higher than 1.3 GPa are required for significant weight savings. However, one of the significant challenges during cold forming those steels is springback control. The ability to accurately predict springback after forming and trimming is essential in Numisheet IOP Publishing Journal of Physics: Conference Series 734 (2016) 032118 doi:10.1088/1742-6596/734/3/032118 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 minimizing the number of die re-cuts and developing compensated dies with minimal springback and springback variation. Typically, AHSS including those Gen-3 AHSS show a significant Bauschinger effect during reverse loading and a nonlinear combined isotropic and kinematic hardening law is required to capture the Bauschinger effect of those materials. The use of the nonlinear isotropic/kinematic hardening material model proposed by Yoshida, et al [2] is useful in forming simulations where an accurate springback prediction is required. For materials such as steels showing continuous strain hardening behaviour, the Yoshida model with a modified hardening law proposed by Shi, et al [3] is often used. Many successful application cases in industrial scale automotive components using advanced high strength steels (AHSS) have been reported to give better springback predictions [4]. Several issues have been raised recently in the use of the model for higher strength AHSS including the use of two C vs. one C material parameters in the Armstrong and Frederick model (AF model), the original Yoshida model vs. Original Yoshida model with modified hardening law, and constant Young’s Modulus vs. decayed Young’s Modulus as a function of plastic strain. In this paper, the consistency of the material model parameters determined from the tension and compression test is verified using a one-element simulation for a 980 MPa material. Using an industrial scale automotive component, the difference in forming and springback results is addressed for two C and one C material parameters in the AF model using the Yoshida model with and without the modified hardening law. The effect of decayed Young’s Modulus on springback prediction for AHSS is also evaluated. In addition, the limitations of the material parameters determined from the tension and compression tests without multiple cycle tests are discussed for those components undergoing several bending and unbending deformations. 2.0 Determinations and verifications of Yoshida model parameters Two different types of tension and compression tests were carried out: a full cycle test and a multiple cycle test, as shown in Figure 1. The multiple cycle tests were designed to capture material hardening behaviour in loading after the material was subjected to one or more tension and compression cycles. The pure tension data were also included in Figure 1, where the pure tension data coincide with the initial tension portion of the data in the tension and compression tests, as expected. Figure 1 Tension and compression test data and data fitting of one C vs. two C models Table 1 Material Constitutive Parameters for Original and Modified Yoshida Models The material constitutive parameters were determined using an optimisation algorithm using the data shown in Figure 1(a), as shown in Table 1 for both original Yoshida model [2] and modified model -1500 -1000 -500 0 50