No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
A European round robin test for the hole expansion test according to ISO 16630
Abstract
The ISO 16630 standard for the hole expansion test was initially published in 2009. The test was originally developed to describe the suitability of metallic sheets for flanging of stamped bolt holes in the production of wheel disks and for similar forming methods. The test is increasingly used to assess the formability of cut edges on punched sheets during the manufacturing of car bodies. In a round robin test organized by Stahlinstitut VDEh, the test was validated with respect to its suitability to characterizing the edge crack sensitivity of steel sheets. The results show a larger amount of scatter which significantly reduces the relevance of the test for the intended purpose.
... Der Lochaufweitversuch (engl.: Hole Expansion Test, HET) nach ISO 16630 [50] wurde ursprünglich entwickelt, um metallische Werkstoffe für Radscheiben auf ihre Rissanfälligkeit bei Aufdornprozessen zu untersuchen [30]. Die ISO-Norm basiert dabei im Wesentlichen auf der bereits seit 1996 gültigen japanischen JFS-T1001-Norm [51]. ...
... Dies liegt zum einen an den vielen Variationsmöglichkeiten, die die Norm bei der Versuchsdurchführung zulässt. Die Einflüsse aus Kantenpräparation, Versuchsdurchführung und Abbruchkriterium sind nach wie vor Gegenstand diverser Untersuchungen und werden beispielsweise in [30] detailliert beschrieben. Zum anderen stellt das Lochaufweitverhältnis einen lokalen Kennwert dar, der sich auf ein sehr geringes Probenvolumen bezieht. ...
... Zum anderen stellt das Lochaufweitverhältnis einen lokalen Kennwert dar, der sich auf ein sehr geringes Probenvolumen bezieht. Entsprechend kann es, verteilt über eine Blechtafel oder ein ganzes Coil, zu Streuungen der Kennwerte von ± 20 % des absoluten Wertes kommen [30]. Bei einem Einsatz zur Qualitätssicherung bei der Werkstoffeingangsprüfung oder als Teil einer Lieferbedingung müssen diese möglichen Unterschiede berücksichtigt werden. ...
Die Herstellung von Karosseriebauteilen aus hoch- oder höchstfesten Stählen wird aufgrund der zumeist ungenügenden Versagensvorhersagegenauigkeit in der FE-Simulation erschwert. Gängige Versagenskriterien bedürfen daher einer Ergänzung. Der Einsatz von Grenzspannungskurven, die in diesem Buch auf Basis einer prozessnahen Werkstoffcharakterisierung und unter Berücksichtigung des Versagensverhaltens unter Biegebelastungen definiert werden, erlaubt dem Anwender die Bewertung des Rissrisikos bereits in einer frühen Phase der Bauteilentwicklung. Es wird über die Entwicklung entsprechender Kurven informiert, sowie anhand konkreter Vorschläge ein Betrag geleistet um die Vergleichbarkeit der dafür eingesetzten technologischen Versuche zu verbessern.
... The (residual) forming capability of cut edges cannot solely be characterized by a hardness measurement. In the publications of [25] and [26], it is reported that the cut surface quality also has an influence on the hole expansion ratio. On the basis of the results, [26] assumes that the non-reproducible pre-damage of the cutting edge indeed disclose a significant influence on the scatter of the hole expansion ratio. ...
... In the publications of [25] and [26], it is reported that the cut surface quality also has an influence on the hole expansion ratio. On the basis of the results, [26] assumes that the non-reproducible pre-damage of the cutting edge indeed disclose a significant influence on the scatter of the hole expansion ratio. Even if the holes are punched with the same tool, the cut surface quality is poorly reproducible. ...
... If set hole expansion test in correlation to edge hardening, it is noticeable that the statements are contradictory, since cutting edges achieve different hole widening ratios with a comparable edge hardening. As [25] and [26] report, the cut surface quality has an influence on the hole expansion Correlation between hole expansion ratio, edge hardening and cut surface quality for the material HC420LA with t = 2 mm ratio. A large smooth cut surface increases the forming capacity of a cutting edge. ...
This experimental study is focused on characterisation of edge hardening of sheet metal and remaining formability of differently prepared cutted edges. Edge cracking sensitivity of counter cutted, shear cutted, recutted and water-jet cutted components are compared and evaluated. Subsequently, edge hardening and hole expansion ratio were correlated for material HC420 LA with sheet thickness of t = 2 mm. As other studies show, the cutting edge surface quality influences the hole expansion ratio: a high clear cut surface increases formability of cutting edges, whereas micro cracks and rough surfaces result into a large fracture surface, which impact remaining formability noticeably. Thus, cutting edges with lower edge hardening behaviour in conjunction with a higher clear cut surface exhibit higher hole expansion ratios. Counter cutting and the recutting do show a similar effect on edge hardening. Using the hole expansion test, it was possible to prove that counter cutted components show a significantly lower edge cracking sensitivity in comparison to conventionally shear cutted components. The hole expansion ratio of counter cutted specimens looks balanced and is comparable to the hole expansion ratio measured from specimens with recutted or water jet cutted edges. The significant difference of the investigated cutting processes is characterized by size of clear cutting area. This area of recutted edges emerges larger than the area of counter cutted specimens, which evidently leads to an increased hole expansion ratio of recutted specimens compared to conventionally shear cutted ones. However, it is important to note that the hole expansion ratio of counter cutted and recutted specimens appear fairly balanced, but counter cutted samples indeed can be produced burr-free. Using counter cutting technology, it is possible to produce burr free surfaces with high edge formability.
... Hole expansion testing is fraught by excessive uncertainty and variation, and the results of which can be heavily influenced by specimen preparation technique and human subjectivity. For example in 2013, a group of European steel researchers [3] reported "an unacceptably large difference between labs" with reference to the ISO Standard 16630 HE test.They ultimately concluded that the "difference is too large for the method to be useful in practice." It was also shown that inter-laboratory differences may be reduced if the number of test specimens is increased from three to ten; however, intra-laboratory scatter remained large. ...
... Once the data are sorted, the second-lowest value (λ 2 ) is assumed to be the 25% quartile (Q 1 ), and the second-highest value (λ 4 ) is assumed to be the 75% quartile (Q 3 ). The inter-quartile range (IQR) is therefore (3) Low and high outliers may be identified by their relative positioning with respect to statistical "fence posts", where the left fence post (LFP) and right fence post (RFP) are commonly defined as (4) and it follows from Equation 3 and Equation 4 that (5) If the minimum measured λ value lies to the "left" of the left fence post (λ 1 < LFP), it is considered a low outlier and may be discarded. Similarly, if the maximum measured λ value is to the "right" of the right fence post (λ 5 > RFP), it may be discarded. ...
... Once the data are sorted, the second-lowest value (λ 2 ) is assumed to be the 25% quartile (Q 1 ), and the second-highest value (λ 4 ) is assumed to be the 75% quartile (Q 3 ). The inter-quartile range (IQR) is therefore (3) Low and high outliers may be identified by their relative positioning with respect to statistical "fence posts", where the left fence post (LFP) and right fence post (RFP) are commonly defined as (4) and it follows from Equation 3 and Equation 4 that (5) If the minimum measured λ value lies to the "left" of the left fence post (λ 1 < LFP), it is considered a low outlier and may be discarded. Similarly, if the maximum measured λ value is to the "right" of the right fence post (λ 5 > RFP), it may be discarded. ...
Until now the hole expansion ratio has been generally regarded as a relative “local formability” parameter with limited application to edge-cracking analysis and prediction. In this study a constrained statistical test data analysis methodology is introduced, where the lower-bound hole expansion ratio is the basis for three practical edge-cracking failure criteria. The Maximum Edge Stretch Criterion is directly compatible with CAE simulation. The Edge Thinning Limit Criterion and the Critical Thickness Criterion are more useful in field work and post mortem laboratory failure analysis. Two case studies are described, where hole expansion test data are used to analyze edge cracking of Advanced High Strength Steel (AHSS) in real-world automotive seating applications. Finally the intrinsic local and global formability parameters of the subject AHSS materials were explored, and a linear relationship between the hole expansion ratio and reduction of area at fracture in tension was demonstrated.
... The average standard deviation of measuring HEC three times on the same steel grade is AE9%, which is calculated from the work by Chen et al. [22]. Meanwhile, there is also research showing a standard deviation of 15% on HEC values for martensitic steels [23]. ...
... The HEC prediction accuracy of the deep learning model is AE16%. Comparing the hole expansion testing error range of the experimental data acquired by Chen et al. [22], where the average standard deviation of testing three times on the same steel grade is AE9%, and the 15% standard deviation of experimental HEC values for martensitic steels [23], due to various testing conditions, such as edge surface quality and first crack determination timing, it can be concluded that deep learning predictions reach a similar degree of accuracy as experiments, where the 9% accuracy for the training dataset indicates an experimental accuracy of that magnitude. In Fig. 8, the deep learning-predicted HEC is plotted against the experimental HEC, with the experimental test error shown in the bottom-right corner. ...
The relationship between microstructure features and mechanical properties plays an important role in the design of materials and improvement of properties. Hole expansion capacity plays a fundamental role in defining the formability of metal sheets. Due to the complexity of the experimental procedure of testing hole expansion capacity, where many influencing factors contribute to the resulting values, the relationship between microstructure features and hole expansion capacity and the complexity of this relation is not yet fully understood. In the present study, an experimental dataset containing the phase constituents of 55 microstructures as well as corresponding properties, such as hole expansion capacity and yield strength, is collected from the literature. Statistical analysis of these data is conducted with the focus on hole expansion capacity in relation to individual phases, combinations of phases and number of phases. In addition, different machine learning methods contribute to the prediction of hole expansion capacity based on both phase fractions and chemical content. Deep learning gives the best prediction accuracy of hole expansion capacity based on phase fractions and chemical composition. Meanwhile, the influence of different microstructure features on hole expansion capacity is revealed.
Graphical abstract
... The hole expansion test according to ISO 16630 [33] for characterizing these stretch-flangeability or rather edge-crack sensitivity itself displays several disadvantages and was therefore controversially discussed in recent research: In [34] the influences on the hole expansion ratio (HER) from the operator over the tool up to the evaluation procedure are presented. A European round robin test [35] revealed high deviations among several laboratories in testing of high strength steels because of the non-reproducible pre-damaged edge condition and the influence of several operators. Exemplarily, the HER showed a standard deviation of 14.6% with an average value of 58.8% for a martensitic steel [35]. ...
... A European round robin test [35] revealed high deviations among several laboratories in testing of high strength steels because of the non-reproducible pre-damaged edge condition and the influence of several operators. Exemplarily, the HER showed a standard deviation of 14.6% with an average value of 58.8% for a martensitic steel [35]. Larour et al. [36] evaluated therefore alternative stretch flangeability testing methods. ...
The usage of high-strength steels for structural components and reinforcement parts is inevitable for modern car-body manufacture in reaching lightweight design as well as increasing passive safety. Depending on their microstructure these steels show differing damage mechanisms and various mechanical properties which cannot be classified comprehensively via classical uniaxial tensile testing. In this research, damage initiation, evolution and final material failure are characterized for commercially produced complex-phase (CP) and dual-phase (DP) steels in a strength range between 600 and 1000 MPa. Based on these investigations CP steels with their homogeneous microstructure are characterized as damage tolerant and hence less edge-crack sensitive than DP steels. As final fracture occurs after a combination of ductile damage evolution and local shear band localization in ferrite grains at a characteristic thickness strain, this strain measure is introduced as a new parameter for local formability. In terms of global formability DP steels display advantages because of their microstructural composition of soft ferrite matrix including hard martensite particles. Combining true uniform elongation as a measure for global formability with the true thickness strain at fracture for local formability the mechanical material response can be assessed on basis of uniaxial tensile testing incorporating all microstructural characteristics on a macroscopic scale. Based on these findings a new classification scheme for the recently developed high-strength multiphase steels with significantly better formability resulting of complex underlying microstructures is introduced. The scheme overcomes the steel designations using microstructural concepts, which provide no information about design and production properties.
... Standardized ISO 16630 hole expansion test [4] has been widely criticized due to its large inherent scatter [5], but it is currently still the most used method to estimate suitability of sheet metals for forming processes that involve stretching of cut edges. The test also considers only a single result value, the limiting hole expansion ratio (HER). ...
Sheared edge formability can play a critical role in cold forming of high-strength hot-rolled steels due to susceptibility
to edge cracking. Standardized ISO 16630 hole expansion test is currently the most widely used
method to estimate sheared edge formability of steel sheets, however, there has been increasing interest in the
industry to reduce the amount of required testing. The aim of this paper was to compare hole expansion performance
of hot-rolled strip steels to various uniaxial tensile test results. The five investigated steels were
selected to have 3 mm thickness and similar (838±10 MPa) longitudinal tensile strength level while consisting of
different microstructures. Tensile properties were measured in 0°, 45°, and 90° angle relative to the rolling
direction to investigate anisotropy. Plastic strain ratios were determined by digital image correlation measurement.
Local ductility was evaluated with thickness strain measurements (εp_amin) on fracture surfaces of the
tensile specimens. To investigate stretch-flangeability, ISO 16630 conforming hole expansion tests were performed.
Test holes were produced by punching and by wire electrical discharge machining (W-EDM) to evaluate
edge damage tolerance of the materials. The hole expansion ratios (HER) were not observed to correlate with
majority of the investigated tensile test results, such as uniform elongation and yield strength. However, a good
correlation between HER and planar anisotropy was observed. Planar anisotropy is seen to affect strain localization
tendency around the hole. The HER values of neither punched nor W-EDM specimens correlated clearly
with εp_amin measurement results. The results show that at a similar strength level, anisotropy is highly influencing
the hole expansion performance of hot-rolled steels, and that the HER is not reliably predicted by a single
local formability parameter.
... Two other common methods for characterization of local ductility are the hole expansion test (HET) as per ISO 16630 [3], and the hole tensile test (HTT) [11,12]. The HET has a disadvantage of producing results with high scattering due to the influence of the stamping process and individual interpretation of crack initiation -this results in a poor gauge repeatability and reproducibility (GRR) value [13]. To avoid any potential pre-damage at the edge, it is possible to conduct the HTT using a mechanically machined or electrically discharge machined hole. ...
Local ductility has become a useful and significant mechanical property of sheet metal, particularly for advanced-high-strength-steels (AHSS) [1, 2]. The widely accepted hole expansion test (HET) [3] has two problems: It mixes up the effects of shear cutting with the intrinsic material ductility, and the variance of test results is large. As a complementary approach, a workgroup consisting of universities, steel producers, automotive suppliers and OEMs have developed VDA-guideline 238-110 [4] for industrial application of the TTS (True Thickness Strain at fracture at minimum thickness) and TFS (True Fracture Strain from area reduction), to quantify local ductility based on the postmortem microscope analysis of the fracture surface of tensile test samples. In 18 laboratories, more than 1440 tensile tests have been performed on a sample batch of 3 AHSS grades (CR440Y780T-DH, CR780Y980T-CP, and HR660Y760T-CP). Using ASTM, JIS and ISO tensile specimens, each laboratory determined the conventional mechanical properties as well as TTS and TFS. For comparison HET were also performed. The measurement results were analyzed statistically in terms of gauge repeatability and reproducibility (GRR). Measurement variance for TTS is comparable to elongation at breakage and significantly lower than HET. TTS results clearly show superior local ductility of the -CP steels over -DH. An analysis of fracture morphology shows differences between the hot-rolled and cold-rolled -CP grades and a tendency for the -DH steel to fail at the edge. For -CP steels, TTS and TFS results are indistinguishable, for the -DH grade, the TFS value is slightly higher than TTS, likely a side-effect of higher global ductility.
... The hole expansion test (HET) is an experimental procedure mainly intended to determine the formability of a material, see, e.g., Paul (2020), for a recent review, and Park et al (2021) for a physics-based simulation framework. While its usability with respect to fracture prediction has been put into question, Atzema et al (2012), due to repeatability and reproducibility issues, see also the recent work of Park et al (2023) where it was observed that crack's directions are not always associated with maximum thinning directions, it is nevertheless useful in assessing the anisotropy of the material, especially for loading in the biaxial stretching regime, Ha and Korkolis (2021). ...
It is well known that polycrystal aggregates obeying Schmid's law at grain level have an overall (macro) constitutive response defined by an associate flow rule (AFR), i.e., the rate of plastic deformation is along the exterior normal of the yield surface. However, the complex structures generated by alloying elements in modern materials are likely to induce deviations from Schmid's law at grain level and hence a non-associate flow rule (NAFR) at macro level cannot be excluded a priori. Two fundamental issues regarding the applicability of the NAFR constitutive model in practice are treated in this work. The first is concerned with the internal mathematical coherence of the model. We show that a common-sense constitutive assumption renders the NAFR-model a mathematical consistency similar to that of the classical AFR-model. The second is about the development of the two main constitutive elements of the NAFR-model-the yield function and the plastic potential. We propose a new methodology, allowing for a thorough control of the impact of both elements on the constitutive response, and illustrate it in the finite element simulation of the hole expansion test and of cup drawing.
... Edge ductility is most often described as a hole-expansion ratio according to the ISO 16630 holeexpansion test [4]. During the past 15 years, this method has been comprehensively investigated but also majorly criticized due to its large scatter, unreliable results and the difficulty of obtaining the same results between different laboratories [5][6][7][8][9]. One major limitation of the ISO 16630 test is that it covers 2 only one stress-strain state of cut edge forming [9,10]. ...
One of the challenges of utilizing advanced high-strength steels is their limited ability to withstand the forming of cut edges. Large production quantities of parts often lead to mechanical punching/shearing processes in blank preparation, providing a challenging starting point for forming processes. The most commonly used edge ductility test is the ISO 16630 hole-expansion test to describe stretch-flangeability properties. However, this method has been widely criticized for its large-scatter, unreliable results and the fact that it covers only a certain stress-strain state of cut edge forming. In addition, it does not provide enough data to be reliably used in forming simulations to predict edge failures. This paper presents an approach to create a more comprehensive way of describing overall edge ductility. Multiple edge forming test methods coupled with digital image correlation (DIC) were selected in order to investigate different edge loading scenarios in both open and closed trim line forming situations. Data regarding limiting local strain before cracking was collected for several steel grades, and results are gathered in 2D and 3D “Edge FLC”-figures. Attempts to utilize these results for simulation purposes are also presented. Results indicate that this approach can be useful to evaluate overall edge forming limits.
... However, when correlating the hole expansion test to edge hardening, it was found that cutting edges achieve different hole widening ratios for comparable edge hardening. As reported in [25,26], the cut surface quality has an influence on the hole expansion ratio. Thus, a large smooth cut surface increases the forming capacity of a cutting edge. ...
Shear cutting induces high strains and work hardening into the shear-affected zone, thus reducing the formability of the sheet metal material during subsequent forming operations. A common method for increasing the residual formability of shear-cut component edges is to shave the surfaces. The shaving process allows for the removal of highly hardened areas from a pre-cut contour, resulting in a comparatively high residual formability of the cutting surface. However, as known from conventional cutting, a burr remains on the shear surface when shaving. Such burr formation is undesirable and therefore usually has to be removed afterward. For this reason, a new process has been developed which combines two processes within the same stroke of punch, the counter-cutting, and shaving process, enabling the production of burr-free shear surfaces showing a high portion of clean-cut proportions and significantly 1.6 times higher hole expansion ratios than conventional cutting surfaces. The present paper deals with numerical and experimental investigations carried out in the course of the process development with the high-strength sheet metal material DP600.
... HER determination is key to the quantification of the edge formability of materials, but very difficult to characterize. Reliance on visual inspection (operator subjectivity) [13], to determine the onset of cracks can also be difficult since it alters the HER values, leaving large variations between repeated test results. The GOM ARAMIS system provides a better alternative by providing the option of tracking images frame by frame to determine the exact point of crack inception. ...
Titanium and its alloys are difficult to form, particularly at room temperature, due to their crystallographic structure and limited availability of slip systems. Such limited formability could be exacerbated by virtue of the technique used to cut the sheet. Forming limit diagrams will not necessarily recognize such effects, which can lead to failures during forming trials. An example of a situation where this could be demonstrated is in sheet with pre-fabricated holes. This work used a hemispherical punch to stretch in-plane a 20mm diameter hole prepared with laser, EDM and AWJ cutting techniques in order to quantify the edge formability of the material. It was identified that, the edge surface conditions have a major impact on the edge formability of the material. The edges of the material prepared with EDM showed very high formability tendencies compared with AWJ and laser cutting. The work proposed an alternative characterization method that could be adopted for edge formability assessment.
... In order to ensure reduced variation in the attained HER values, the test must be terminated at the onset of edge fracture. However, complete reliance on visual inspection to terminate the test has been known to be a major source of data scatter in recorded HER values [15]. Various ways to reduce the variations in HER values have also been devised in some works. ...
Despite the good properties of titanium, which have drawn the interest of various industries over the years, one of the major drawbacks of this material is its poor machinability. This has largely been attributed to its low thermal conductivity and elastic modulus. The ability to attain the optimum sheet edge performance during forming is dependent on the quality of the edges produced. Also, the demanding nature of aerospace part design has provoked the interest of both industry and academia to continually explore avenues tailored at enhancing part performance. The sort of edge surface integrity produced for aerospace part fabrication thus becomes a vital consideration in the quest to ensuring prime performance of components. This work seeks to study the influence of different machining-induced surface defects on the sheet edge performance of CP-Ti (grade 2) at room temperature. Hole expansion test was used to assess the edge surface formability of CP-Ti with different machining-induced edge defects. The research found that machining-induced surface defects act as stress concentration sites during the hole expansion test and have a major impact on the material flow. Electro-discharge machined edges were observed to exhibit high edge formability compared to laser and abrasive water jet cut edges due to the impact of machining-induced microstructural changes.
... Hole expansion test [HET] according to ISO 16630 [1] is worldwide accepted for the evaluation of the forming behavior of AHSS sheared edges and the obtained hole expansion ratio (HER) has become an important parameter to consider in AHSS formability. However, the reliability of HET is often questioned due to the large data scattering and the poor repeatability observed in many research works and round robin tests [2][3][4][5]. Accordingly, additional edge cracking resistance tests based on optical strain measurements have been proposed in an attempt to improve edge formability prediction [2,[4][5][6][7]. ...
Edge fracture prediction in high strength cold formed components still being a challenge for automotive part manufacturers. Even though several experimental methodologies have been proposed in the last years to assess edge formability, the material properties governing edge cracking sensitivity of high strength sheet materials are not clearly defined. This work investigates the correlation between the fracture toughness of various 1000 MPa Dual Phase and Complex Phase steel grades and their edge fracture resistance, evaluated by means of hole expansion tests according to ISO 16630. The good linear correlation observed between these parameters shows that fracture toughness is a reliable indicator of edge cracking resistance in advanced high strength steel sheets. However, it is well known that edge formability does not only depends on the material properties but also on the edge quality. In order to evaluate the effect of the edge condition on edge formability, additional hole tension tests are performed in some of the investigated steel grades with different punch to die clearances. It is shown that steels with greater fracture toughness present higher strain at fracture and lower cutting clearance sensitivity. According to these results, the fracture toughness is proposed as a relevant material property to understand the edge formability of high strength metal sheets.
... Hayashi was certainly the first to highlight the different influencing factors on Hole Expansion limit and the necessity to define a standard [4]. Lots of work were done to examine various factors in the Hole Expanding test [5] and [6]. The large variations in the testing results have caused the concerns of several steel product development people and testing engineers [7]. ...
There is an increasing interest in the steelmaking and automotive industries to evaluate the edge cracking sensitivity of Advanced High Strength Steel sheets used in car body manufacturing. Currently, the Hole Expansion test is the only test procedure that is defined by norms (JFS and ISO). This test is increasingly used to assess the formability of cut-edges on punched sheets because it is relatively simple. However, it has been already shown that there can be large differences in Hole Expansion Ratio (HER) values generated by different testing facilities. Among the main sources of variability: punched hole quality and hole expansion termination point. Hole punching operation has a detrimental effect on cut-edge quality and HER values. However, current standards do not give any recommendation. No technical specification is given for the tooling, quality control system and punching speed. It has been stated that the press speed difference in the hole punching operation are significantly different from one laboratory to another one and some steel microstructures sensitive to it. However, very few papers and data are available. As a result, an experimental study was conducted to examine this important issue. A specific 4-post assembly tool was designed to guarantee the best punched hole quality. Different punching speeds, from 0.2mm/s to 367mm/s were tested on different steel sheets (mild steel, AHSS including TWIP and 3rd Gen steel) to emphasize or not the influence on final HER values.
... poor gage R&R. For example a group of European steel researchers [8] reported "an unacceptably large difference between labs" with reference to hole expansion testing per ISO 16630 [5]. They ultimately concluded that the "difference is too large for the method to be useful in practice". ...
For advanced high strength steel (AHSS), application-specific material design and selection require a thorough understanding of intrinsic deformation behavior and fracture resistance. In this analysis, formability parameters are derived from standard uniaxial tension tests, and a novel local/global formability map concept is introduced. Performance expectations are distinguished by the relationships between true fracture strain (TFS) and true uniform strain, and the foundation for a formability classification and rating system is proposed. Additionally, the limiting bend ratio (r/t)-measured by 90° V-bend testing-was shown to be a combined function of both local and global formability parameters. This paper features a series of commercially-available and experimental AHSS grades with 980 MPa minimum tensile strength and a range of microstructures, compositions and mechanical properties.
A novel in-plane bending test was used to study edge ductility in DP800 as a common advanced high-strength steel in the car industry. The test utilized the digital image correlation technique to measure the local and average fracture strain values along the edge of the specimen. In contrast to the widely used hole expansion capacity test, the impact of punch friction, contact stress, and out-of-plane strain on edge ductility is eliminated by removing the punch. Also, the strain gradient inherent to the beam bending provides a controlled crack propagation path, making crack tracking easier than the sheared edge tensile test. The proposed bending test was utilized to investigate the influence of material orientation, cutting parameters, and global strain gradient on edge fracture strain. A correlation was observed between edge ductility, material orientation, and cutting tool sharpness, while the average fracture strain was independent of the strain gradient. The outcome shows that the in-plane bending test is reliable for determining edge ductility in any desired material orientation.
A novel criterion for microcrack resistance of multi-phase steels based on property gradient maps is proposed. Two industrial sheets of steel were processed to obtain dual-phase and complex-phase microstructures with exactly the same chemical composition. Experimental investigations showed characteristic differences for the tensile tests, hole expansion and the local plastic behavior during deformation. An innovative full-field modeling approach that explicitly predicts mechanical property gradients as a function of microstructural gradients during forming was developed and validated. This allowed to form a new criterion for evaluation of structure–property relationship in nano-structured multi-phase steels and can reveal the formability limitations.
The edge cracking sensitivity of AHSS and UHSS is quite challenging in the cold forming process. Expanding cut holes during flanging operations is rather common in automotive components. During these flanging operations the pierced hole is stretched that its diameter is increased. These flanging operations stretch material that has already been subjected to large amounts of plastic deformation, therefore forming problems may occur. An innovative cutting process decreases micro cracks in the cutting surface and facilitates the subsequent cold forming process. That cutting process consists of two stages, which produces close dimensional tolerance and smooth edges. As a result the hole expanding ratio was increased by nearly 100% when using thick high strength steels for suspension components
The ISO16630 hole expansion test is compared with alternative testing methods for a CR570Y780T-CP (CP800) cold rolled AHSS at 10% cutting clearance. Conical ISO 16630 and flat hole expansion tests, biaxial Nakajima and Marciniak tests with a punched 10/20mm hole as well as punched Nakajima and Marciniak tensile tests with 20/30mm sample width have been performed with Aramis® strain measurement system. Nakajima/Marciniak FLD uniaxial tensile samples with punched edges without holes fail through necking without edge cracks. Flat and Marciniak hole expansion tests and in a lesser extent Nakajima hole test deliver plane strain cracks starting outside the hole edge. Those tests are therefore not suitable for edge crack characterisation for this particular necking controlled material. ISO16630 conical test delivers an edge crack without necking, however at significantly higher failure strain values in comparison to the FLD uniaxial necking strain. Whatever testing method considered, the CP800 voestalpine steel grade investigated is not edge crack sensitive.
ResearchGate has not been able to resolve any references for this publication.