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Key factors of stretch-flangeability of sheet materials

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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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Key factors of stretch-flangeability of sheet materials
Jae Ik Yoon
, Jaimyun Jung
, Jung Gi Kim
, Seok Su Sohn
, Sunghak Lee
, and Hyoung Seop Kim
Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673,
Republic of Korea
Center for Advanced Aerospace Materials, Pohang University of Science and Technology (POSTECH), Pohang 37673,
Republic of Korea
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
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.
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
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DOI 10.1007/s10853-017-1012-y
J Mater Sci (2017) 52:7808–7823
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). ...
Full-text available
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. ...
Full-text available
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]. ...
Conference Paper
Full-text available
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. ...
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.
Full-text available
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.
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.
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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Conventional hole-flanging by stamping is characterized by low formability. It is common knowledge that formability can be improved by forming at high temperatures. High-speed punch rotation is introduced to conventional hole-flanging to use frictional heat to improve and control formability. Thermomechanical finite element (FE) simulations of conventional hole-flanging and hole-flanging with punch rotation are used to determine the effects of punch rotation on the process temperature. Hot tensile tests were conducted to find the effects of temperature and strain rate on the forming limit of the blank. The Marciniak–Kuczynski (M–K) forming limit model is used to estimate temperature and strain-rate dependent forming limits of the material. Hole flanging experiments were conducted at different punch speeds and feeds to determine process windows that maximize formability. A maximum hole expansion ratio (HER) of 4 was obtained in hole-flanging with punch rotation compared to 1.48 in conventional hole-flanging experiments. In theory, a rise in blank temperature to 400 °C in hole-flanging with punch rotation enhances the HER by 30% based on the FE simulations. However, experiments of hole-flanging with punch rotation reveal a 170% increase in formability. The difference in formability between the experiments and FE simulations is attributed to the influence of high-speed deformation, in-plane shear and non-proportional loading paths. To control formability in hole-flanging with high-speed punch rotation, it seems sufficient to establish a closed-loop control of the process with a pre-defined temperature profile.
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Due to forming processes of the upcoming manufacturing technology of sheet-bulk metal forming, where bulk forming operations are applied to sheet metal, ductile damage occurs due to high triaxial stress and strain states in the material. To consider this type of damage in models for an optimized process design, a description of the critical material state is required, taking the main damage mechanisms into account. In comparison to single-phase materials, where voids nucleate mainly at grain boundaries and triple junctions, in dual-phase materials also phase boundaries or the movement of different hard phases due to forming process contribute to ductile damage. In this study, the mechanisms of nucleation and growth of voids were analyzed in a dual-phase steel DP600 at different strain states and forming temperatures. By specimen preparation using ion-beam slope cutting and scanning electron microscopy, it was possible to observe at nano- and microscale voids, cracks, and elements of deformation reliefs. A correlation between the shape of voids and their location in the crystal lattice was found, whereby the mechanisms of their nucleation, growth, and coalescence could be described.
Full-text available
Stretch-flangeability measured using hole expansion test (HET) represents the ability of a material to form into a complex shaped component. Despite its importance in automotive applications of advanced high strength steels, stretch-flangeability is a less known sheet metal forming property. In this paper, we investigate the factors governing hole expansion ratio (HER) by means of tensile test and HET. We correlate a wide range of tensile properties with HERs of steel sheet specimens because the stress state in the hole edge region during the HET is almost the same as that of the uniaxial tensile test. In order to evaluate an intrinsic HER of steel sheet specimens, the initial hole of the HET specimen is produced using a milling process after punching, which can remove accumulated shearing damage and micro-void in the hole edge region that is present when using the standard HER evaluation method. It was found that the intrinsic HER of steel sheet specimens was proportional to the strain rate sensitivity exponent and post uniform elongation. © 2016, The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht.
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Hardcover ▶ 89,99 € | £81.00 | $119.00 ▶ *96,29 € (D) | 98,99 € (A) | CHF 101.50 eBook Available from your library or ▶ The book covers all types of advanced high strength steels ranging from dual-phase, TRIP. Complex phase, martensitic, TWIP steels to third generation steels, including promising candidates as carbide free bainitic steels, med Mn and Quenching&Partitioning processed steels. The author presents fundamentals of physical metallurgy of key features of structure and relationship of structure constituents with mechanical properties as well as basics of processing AHSS starting from most important features of intercritical heat treatment, with focus on critical phase transformations and influence of alloying and microalloying. This book intends to summarize the existing knowledge to show how it can be utilized for optimization and adaption of steel composition, processing, and for additional improvement of steel properties that should be recommended to engineering personal of steel designers, producers and end users of AHSS as well as to students of colleges and Universities who deal with materials for auto industry.
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In order to significantly extend the limit of the measurable strain range to obtain true stress-strain curves of materials using tensile testing, digital image correlation is used to measure local strains. In this study, a method to measure reliable true stress-strain curves in a wide range of strains covering the post-necking stage with reducing gage lengths has been investigated. In tensile testing, by decreasing the gage length for the strain measurements, not only accurate true strain but also reliable true stress can be obtained until sample fractures. This experimental method for accurate true stress-strain measurements is verified by comparing the experimental results and the finite element method simulated load-displacement curve, geometries, and strain distributions. The new method can be used to easily obtain the measurement of accurate and extended true stress-strain curves, which can assist in experimental and theoretical investigations of the mechanical behavior of materials. This simple method has shed new light on the measurement of the mechanical properties of materials for both experimentalists and simulation engineers, who need a wide range of true stress-strain curves.
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The aim of the present study is to investigate the effect of punch speed on forming limit diagram (FLD) and the formability of austenitic stainless steel type 304L. Effect of strain rate on the height of dome is studied using the hemispherical punch test. Results of this study show that strain rate has significant effect on FLD in this material and high formability obtains at low strain rate. The safe area of FLD between major and minor strains is extended under low strain rate. It is seen that at low punch speed, failure and fracture occur at the pole region (top of the dome), whereas at higher forming rates, failure occurs close to the flange region. Modeling studies are also carried out using Ls-Dyna to know the region of high stress concentration and to predict the location of fracture. There is good agreement between simulation and experimental results.
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A novel low-alloy high-strength steel [Fe–0.20C–1.65Mn–1.40Si–1.50Al–1.30Cu–1.05Ni–1.07Co (wt%)] has been thermo-mechanically processed with a finish rolling temperature of 850 °C, followed by air cooling and water quenching in order to obtain a good combination of strength and ductility. Phase transformations of the above steel at different cooling rates have been studied and continuous cooling transformation (CCT) diagram has been constructed using data, obtained from dilatometric study. The phase field of CCT diagram indicates microstructure changes from a mixture of ferrite and bainite to fully martensite accompanied with the enhancement of hardness with increasing cooling rate. The microstructural investigation at lower cooling rate (≤5 °C/s) suggests the possibility of achieving pearlite-free microstructure by direct air cooling from the austenite region. Directly air-cooled steel has demonstrated primarily ferrite–bainite microstructure, which shows attractive tensile strength (>1050 MPa) and ductility (>15 %). On the other hand, directly water-quenched steels reveal predominantly lath martensitic microstructure with high dislocation density which exhibits higher tensile strength (>1600 MPa) and lower ductility (~12 %). The multiple stages of strain hardening behaviour of the investigated steel under different cooling conditions have been examined with respect to microstructural evolution.
Stretch-flangeability representing the capability of a sheet material to form into a complex shaped part is not a well-known sheet metal forming property. We correlate mechanical properties with stretch-flangeability of various advanced high strength steels (AHSSs) to capture the stretch-flanging phenomenon and improve the stretch-flangeability of steel sheet materials. The stretch-flangeability of materials is usually evaluated using a hole expansion test. During the hole expansion test, the stress state in the hole edge part of the specimen is almost the same as that of the uniaxial tensile test. However, a single parameter in tensile properties of the AHSSs exhibits no clear correlation with flangeability estimated as the hole expansion ratio (HER). Because micro-cracks in the hole edge region of the hole expansion testing samples play a significant role in HER values, we propose and demonstrate that fracture toughness is the key factor governing the HER of AHSSs.
JFE Steel has developed new technologies in order to predict stretch flange fracture and to reduce springback for applying high tensile strength steel sheet to more diverse automobile parts. The technology which uses maximum principal strain gradient was developed to predict stretch flange fracture. By this technology, the accurate prediction of stretch flange fracture, which cannot be predicted by forming limit diagram, was obtained. The factor analysis technology of springback was developed to reduce springback in high strength steel press parts. The analysis specifies the area of parts which affects most on springback. By using the analysis, it became possible to obtain the effective solution to reduce the springback.
The effect of cooling on the mechanical properties of hut-rolled high strength steels was investigated in order to improve the stretch-flangeability of conventional TS 590 MPa grade for the automotive parts through laboratory simulation and mill-scale production. The low temperature coiling method using a 3-step controlled cooling pattern after hot rolling was very effective for producing Nb-bearing high strength steel with high stretch- flangeability. It was suggested that the suppressed precipitation of grain boundary cementites and the decreased hardness difference between the ferrite matrix and bainite phases cause the excellent stretch-flangeability of ferrite-bainite duplex microstructure steel. Therefore, the formation and propagation of microcracks were suppressed relative to conventional HSLA Steel with the ferrite and pearlite microstructure. In addition, the elongation improved compared with that of hot-rolled steel sheets using the conventional early cooling pattern because the volume fraction of polygonal ferrite increased.