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Key factors of stretch-flangeability of sheet materials

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Abstract and Figures

Stretch-flangeability evaluated using hole-expansion testing represents the ability of sheet materials to resist edge fracture during complex shape forming. Despite a property imperative for automotive part applications of advanced high-strength steels, factors governing stretch-flangeability are not yet well understood. In this study, the mechanical properties of a selected group of materials with different microstructures were investigated using tensile, fracture toughness, and hole-expansion tests to find the factor governing the stretch-flangeability that is universally applicable to a variety of metallic materials. It was found that the fracture toughness of materials, measured using the fracture initiation energy, is a universal factor governing stretch-flangeability. We verified that fracture toughness is the key factor governing stretch-flangeability, showing that the hole-expansion ratio could be well predicted using finite element analysis associated with a simple ductile damage model, without explicitly taking into account the microstructural complexity of each specimen. This validates the use of the fracture toughness as a key factor of stretch-flangeability.
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METALS
Key factors of stretch-flangeability of sheet materials
Jae Ik Yoon
1
, Jaimyun Jung
1
, Jung Gi Kim
1
, Seok Su Sohn
2
, Sunghak Lee
1,2
, and Hyoung Seop Kim
1,2,3,
*
1
Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673,
Republic of Korea
2
Center for Advanced Aerospace Materials, Pohang University of Science and Technology (POSTECH), Pohang 37673,
Republic of Korea
3
Center for High Entropy Alloys, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
Received: 19 December 2016
Accepted: 15 March 2017
Published online:
20 March 2017
ÓSpringer Science+Business
Media New York 2017
ABSTRACT
Stretch-flangeability evaluated using hole-expansion testing represents the
ability of sheet materials to resist edge fracture during complex shape forming.
Despite a property imperative for automotive part applications of advanced
high-strength steels, factors governing stretch-flangeability are not yet well
understood. In this study, the mechanical properties of a selected group of
materials with different microstructures were investigated using tensile, fracture
toughness, and hole-expansion tests to find the factor governing the stretch-
flangeability that is universally applicable to a variety of metallic materials. It
was found that the fracture toughness of materials, measured using the fracture
initiation energy, is a universal factor governing stretch-flangeability. We veri-
fied that fracture toughness is the key factor governing stretch-flangeability,
showing that the hole-expansion ratio could be well predicted using finite ele-
ment analysis associated with a simple ductile damage model, without explicitly
taking into account the microstructural complexity of each specimen. This val-
idates the use of the fracture toughness as a key factor of stretch-flangeability.
Introduction
Currently, automotive industries are subjected to
strong pressure to reduce exhaust emission of their
automobiles, which is an important cause of air pol-
lution. To address this imposition, automotive
industries have focused on the development of a
lightweight body-in-white using high-strength sheet
steel. The development of lightweight automobile
bodies has a variety of other goals as well, such as
increasing fuel efficiency, increasing strength and
safety, and improving durability [14]. Advanced
high-strength steels (AHSS) with excellent tensile
properties such as dual phase (DP), transformation
induced plasticity (TRIP), twinning induced plastic-
ity (TWIP), quenching and partitioning (Q & P), and
lightweight steels have been developed to meet these
goals [511].
However, formability, which is the ability to be
shaped into a desirable structure without fracture,
becomes a problem in high-strength grade steels.
Generally, the formability of these steels is
Address correspondence to E-mail: hskim@postech.ac.kr
DOI 10.1007/s10853-017-1012-y
J Mater Sci (2017) 52:7808–7823
Metals
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... Barnwal et al. [18] employed the Yld2004-18p yield criterion in conjunction with a stress triaxiality-based yield criterion for the fracture prediction of DP980 and TRIP1180 steels in HET. Yoon et al. [19] used a different approach which established a relation between the fracture toughness and the hole expansion ratio. ...
... with σ 1-3 (MPa): Principal stresses and τ max (MPa): Maximum shear stress. Equation (20) can be obtained by incorporating Equation (16), (17), and (19) into Equation (15). ...
Article
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This study investigates the formability characteristics of dual phase steels (DP600 and DP800) under flange stretching conditions through hole expansion tests. The hole-splitting initiation was numerically predicted using ductile damage functions coupled with an orthotropic plasticity model. Therefore, a polynomial yield criterion coupled with three damage criteria, namely generalized plastic work, void growth model, and a shear ductile fracture model, is implemented into Marc software by the user defined material subroutine. Thus, the fracture stroke, hole expansion ratio, and fracture initiation location were estimated for both steels. The polynomial yield criterion could capture the anisotropic features of the dual phase steels. Furthermore, the stress triaxiality-based criteria were reasonably accurate in stretching limit predictions of both steels’ grades. Nevertheless, plastic work predicted the fracture strokes and hole expansion ratios noticeably lower than the experimental outcomes for both steels. In addition, all the numerical results captured the exact fracture initiation location for DP600, while a slight discrepancy was observed for DP800. All ductile fracture models pointed out the identical crack location, which shows the cruciality of the yield criterion for locating the fracture initiation in hole expansion test. Consequently, both void growth model and shear ductile fracture model showed accurate performances conforming to the stress triaxiality was found to be more dominant than the Lode parameter.
... They applied Cockcroft and Latham ductile damage model combined with Hill48 yield function and predicted HERs of these materials. Yoon et al. [28] applied a different method to predict HER. They considered fracture toughness of the materials and predicted HERs by applying a strain-based fracture criterion along with Hill48 yield criterion. ...
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The main purpose of this study is to exhibit failure prediction capability of polynomial-based yield functions with a basic damage model. For this purpose, a constitutive model considering anisotropic plasticity and ductile fracture was developed. In this model, anisotropic plastic behavior of dual phase steels, namely DP600 and DP800, was described by quadratic Hill48 and non-quadratic anisotropic homogeneous the fourth-order polynomial (HomPol4) stress potentials and the gener�alized plastic work criterion from ductile damage models was used for the prediction of fracture initiation. The model has been implemented into an implicit fnite element (FE) code. The parameters of the constitutive model were calibrated with uniaxial tensile tests performed in diferent directions with respect to the rolling direction of the materials and anisotropic stress potentials were evaluated by comparison of the predicted in-plane variations of the plastic properties (yield stress ratios and Lankford coefcients), and yield locus contours with experimental data. The calibrated model was frstly applied to uniaxial tensile test and then to a hole expansion test to predict fracture. The stroke values at fracture, hole expansion ratios (HER) and fracture locations were investigated. Any signifcant diference between the anisotropic stress potentials was not observed in terms of HER predictions, however plastic work criterion in conjunction with HomPol4 function predicted the crack initiation locations accurately on the fractured samples. Afterward, the Lode parameter and stress triaxiality efects were investigated in fracture stroke prediction. Since the HomPol4 predictions of fracture initiation locations are accurate, the predicted HomPol4 results from the generalized plastic work criterion were compared with the modifed Mohr-Coulomb ductile fracture model results. A signifcant improvement was observed in the fracture displacement predictions. However, it is seen that the failure location predictions of both models were the same. From these results, it can be concluded that the HomPol4 yield criterion has an efective potential to predict the failure locations even though with a basic damage model. In the current study, the out-of-plane anisotropy efect was assessed as well. To this end, Hill48’s parameter correlated with the out-of-plane shear components were adjusted. It was found that the out-of-plane anisotropy has a negligible efect on the predictions of HER and fracture initiation location
... No entanto, formar um furo flangeado em aços de alta resistência é uma tarefa mais complexa do que formar um furo em aços macios convencionais. À medida que a resistência da chapa de aço aumenta, a tendência a ocorrência de trincas na ponta do flange, mesmo sob baixa deformação, aumenta devido à redução da deformabilidade de borda [8,9]. Portanto, é difícil empregar essa abordagem para componentes estruturais compostos de aços endurecidos por estampagem a quente [5]. ...
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Neste estudo, chapas de 1,5mm de espessura do aço ao boro 22MnB5 em diferentes condições foram submetidas ao ensaio de expansão de furo. Paralelamente, análises numéricas através do método dos elementos finitos foram realizadas em condições idênticas aos ensaios executados. Por fim, microscopia óptica foi utilizada para investigar a influência da microestrutura na deformabilidade de borda. Palavras-chave-Deformabilidade, Flangeamento, 22MnB5, Aços Avançados de Alta Resistência.
... The mechanical properties have predominantly been investigated for their correlation with anisotropy [23], strain-rate sensitivity [23,24], and strain-hardening exponent [25], which can be obtained via a uniaxial tensile test. However, Yoon et al. [26,27] reported a comparison of the mechanical properties and HER of various alloys and demonstrated that the HER has a distinct correlation with fracture toughness rather than tensile properties. These works directly compared the fracture toughness and HER of various alloys and corroborated a linear correlation. ...
Article
The demand for high-strength, lightweight structural materials has been increasing; however, alloys with high mechanical performance often have limitations such as unfavorable bendability and stretch-flangeability. In terms of stretch-flangeability, an unexpected crack occurring on the trim line during sheet metal forming has recently been considered as an issue of concern by the industry and academia, but its cause and solution have not been determined yet. This study investigated the effect of the phase interface generated by deformation-induced martensitic transformation on the stretch-flangeability of metastable ferrous medium-entropy alloys. We prepared three alloys that exhibited different transformation-induced plasticity behaviors by controlling their grain sizes and chemical compositions. It was observed that the transformation activity, which is dependent on the microstructure and phase stability, considerably affects the hole expansion ratio obtained from the hole expansion test. Microstructural analyses revealed that deformation-induced martensite formed a layered structure and the phase interface between the face-centered cubic (FCC) and body-centered cubic (BCC) phases served as a site vulnerable to cracking during stretch-flanging. The decreased transformation activity by grain refinement resulted in an increase in the hole expansion ratio (HER) from 8.6% to 28.4%. On the other hand, when the phase stability is low enough to form athermal martensite during quenching, most of the FCC grains are easily transformed into BCC phase and the phase interface is also decreased during stretch-flanging, resulting in the HER of 22.9%. Therefore, the increment of phase interfaces leads to lower stretch-flangeability. This study sheds lights on expanding the applicability of TRIP-utilized materials to industry by suggesting ways to improve stretch-flangeability.
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Hole expansion ratio is widely used to estimate the stretch flangeability of sheet metals which is a critical property of formability and to evaluate the efficiency of a forming process. Although many experiments were conducted in the past to identify the key tensile properties affecting hole expansion ratio, their results failed due to the data scarcity. This work demonstrates a machine learning framework coupled with imputation methods to augment both the quantity and quality of collected experimental data. Especially, a generative adversarial imputation network (GAIN) is used to impute the missing tensile properties in the collected experimental data. With the imputed data, the hole expansion ratio is predicted through an extra tree regressor. In terms of the imputation performance, GAIN resulted in the lowest root mean square error of 0.09146 when 50 known tensile properties are randomly removed and imputed with GAIN. In terms of the hole expansion ratio prediction performance, the extra tree regressor showed the lowest root mean square error of 0.124 compared to other machine learning models. Finally, the influence of each tensile property on the hole expansion ratio is analyzed using Shapley additive explanations, an explainable artificial intelligence technique. In this study, the influences of various tensile properties on hole expansion ratio were quantitatively determined for the first time via machine learning and this analysis will accelerate the exploration of sheet metals with high formability performances.
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Hole expansion ratio (HER) is widely used to quantify the stretch-flangeability of sheet metal. HER is determined from the maximum limit of successful expansion of a central hole by a conical punch. The central hole edge is prepared by the punching process. Around 45° cracks are noticed at the central hole edge after successfully completing the hole expansion test. HER of punched hole correlates with uniaxial tensile properties like yield strength, ultimate tensile strength, total elongation, post-uniform elongation, and coefficient of normal anisotropy. Comparisons of strength (yield strength, ultimate tensile strength), anisotropy (coefficient of normal anisotropy) and deformation (total elongation, post-uniform elongation) parameters among steel grades are essential to relate HER among steel grades. Interstitial free steel is the highest, and SPFH steel is the lowest HER among the four steel grades. HER correlates nicely with the notch mouth opening displacement at peak load.
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The hole expansion formability of W-tempered aluminum 7075 sheet, which is prepared by solution heat treatment and rapid cooling, is investigated comparatively with the peak aged T6 tempered alloy. The W temper heat treatment has been known to be a potential application to cold forming of high strength aluminum including 7075 alloy as an alternative to the warm or hot forming process. The hole expansion tests are designed with a conical punch and the holes are fabricated using wire-cut and punching. Basic mechanical properties and microstructure analyses are performed to study the effect of the strength and ductility in tension on the hole expansion ratios of specimens with different tempers and hole conditions. From the experimental study, the following conclusions are mainly reached. (1) The W-tempered sheets show much improved HER than T6 tempered sheets; i.e., 31 (T6) vs. 58% (W) for wire-cut hole and 19 (T6) vs. 57% (W) for punched hole. (2) The HER of W-tempered sheets show very similar HER values between wire-cut and punched hole specimens, which has not been commonly reported. (3) The initiation of cracks at hole edges is different depending on hole preparation; i.e., RD or TD (wire-cut T6 and wire-cut and punched W) vs. RD, DD, and TD (punched T6). (4) The KAM map validates the cause of lower HER of punched specimen attributes to earlier crack initiation by prior plastic deformation during punching, but the strengthening of shear affected zone has limited effect on HER. (5) The HERs of T6 and W tempered sheets are well correlated to the yield strength, ultimate tensile strength, and total elongation. However, the effect of post uniform elongation on HER is not correlated to the existing report.
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