PSI - Issue 35

Toros Arda Akşen et al. / Procedia Structural Integrity 35 (2022) 82 – 90 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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but also provide sufficient strength and crashworthiness owing to their microstructure. In addition, most of these steels also show good formability performance in uniaxial tensile test (UTT). However, the forming limits obtained from the UTT may lead to inaccurate prediction for different loading modes. Therefore, different tests are required to understand the fracture limit of the material under different loading conditions. Stretch flanging ability is the one of the mechanical property which cannot be easily predicted through the UTT. Determination of this feature is also important field of interest in automotive industry. Hole expansion test (HET) is one of the tests in order to determine the stretch flange formability limit. In HET, a hole at the center of the blank was subjected to enlarge with the motion of a punch. Stress state being dominant at the hole edge is identical to the conditions of UTT. However, differences in fracture strain may be observed between the UTT and HET. The differences in formability limits between these two tests result from the strain localization taking place in UTT earlier (Chung et al (2011), Lee et al. (2018)). The main reason of the delayed instability in HET is that the stress state changes from uniaxial tensile to balanced biaxial rapidly between the hole edge to the periphery of the blank (Parmar and Mellor (1978)). It is postulated that the surrounding region of the hole edge affects the failure mechanism and postpones the strain localization at the hole edge (Lee et al. (2018)). Another challenging issue in HET is the determination of the failure location which is highly depending on the yield criterion. In recent years, several studies were carried out to evaluate the effect of the yield criterion in HETs. Kuwabara et al. (2011) performed the simulations of HET for DP780 steel in order to investigate the effect of the yield criterion on HET. They used Yld2000-2d as yield criterion and obtained compatible results with the experiments. Moreover, they indicated that the biaxial tensile test data was quite important in the definition of the parameters of the yield criterion. Xu et al. (2012) conducted the HET of twinning-induced plasticity steel (TWIP) and performed FEM analyses. They examined the fracture location, and it was recorded that the fracture occurred in rolling direction. In addition, shear fracture was observed in the specimen. Iizuka et al. (2014) investigated the formability of a high strength steel (HSS) experimentally and numerically in HET and they recorded that the yield function is important for the prediction of deformation behavior. Kuwabara et al. (2017) investigated the stretch formability of AA6016-O and AA6016-T4 aluminum alloys in HET using Yld200-2d yield criterion. They also examined the thickness strain in three directions, namely rolling, diagonal and transverse directions (RD, DD and TD). HETs of these two materials were conducted and it was observed that the numerical results were in good agreement with the experimental results. Lee et al. (2018) conducted HETs of zinc coated low carbon steel and performed finite element (FE) simulations to examine the effect of the yield criterion. They implemented Yld2000-2d and Yld2004-18p yield criteria, separately. In addition, they optimized the parameters of these two yield criteria. It is seen that the prediction capability was highly increased with the optimized parameters of these yield criteria. Moreover, there were also several studies which was incorporated a failure criterion into FE analyses. Chung et al. (2011) investigated the formability behavior of TWIP940, TRIP590 and R340 steels in HET, numerically, and experimentally. They used stress triaxiality-based failure criterion as well as Hill48 anisotropic yield criterion. The effect of strain rate sensitivity was also considered and an inverse calibration method for damage criterion was developed in this work. It is recorded that the strain rate is also an important mechanical property for stretch formability. In addition, it was observed that the results obtained from the inverse calibration method were consistent with the experimental results. Apart from these works, Yoon et al. (2017) investigated the effect of the fracture toughness on the prediction of hole expansion ratio (HER) of various AHSS. It was seen that the results were in accordance with the experimental results. Barnwal et al. (2020) conducted another study investigating the fracture behavior of DP980 and TRIP1180 steels during hole expansion test. They implemented a material model consisting Yld2004-18p as a yield criterion in conjunction with a phenomenogical failure criterion proposed by Rice and Tracey. It is reported that the numerical results obtained by the Yld2004-18p yield criterion with Rice and Tracey fracture model were quite reasonable for both steels. Aforementionned works mostly focused on the effect of the linear transformation based yield criteria which have a cumbersome calibration procedure, or a fundamental criteria such as Hill48 which are not capable of representing the r and yield stress ratio directionalities, simultaneously (Kuwabara et al. (2011), Kuwabara et al. (2017), Chung et al (2011)). In addition, simulations which the linear transformation based criteria are adopted, suffer from the higher solution times in the determination process of plastic strain increment (Banabic (2010)). In this work, failure prediction capability of generalized plastic work criterion was assessed in HET. Homogeneous fourth order polynomial-based yield function (HomPol4) having an easy calibration process in comparison to the linear transformation based criteria, was implemented to describe the anisotropy of the material. TRIP590 and TWIP940 advanced high strength steels were used as test material. Firstly, the performance of HomPol4 was investigated on the prediction of HERs for both