Hole Expansion Test Research Articles

Numerical and experimental investigation of hole flangeability of AA6061 alloy

Hole flangeability is a measure of edge formability or local formability, and it is important for forming processes like stretch flanging. Hole flangeability of sheet metals is evaluated by determination of hole expansion ratio (HER) using a standard hole expansion test (HET). In the present work, numerical and experimental investigation of hole flangeability has been carried out on 1.6mm thick AA6061 alloy sheets. The material model plays an important role in the accuracy of predicted results in numerical simulations. The simulations have been carried out using two different work hardening laws (Swift and Voce). The failure has been predicted using the experimentally determined forming limit curve (FLC) of AA6061 alloy sheets. HER and thickness have been predicted along the edge of the expanded hole by simulation. The predicted HER and thinning from FE simulation have been validated experimentally by performing hole expansion tests according to ISO 16630-2009 using a conical punch. The predicted results have been found to be in good agreement with the experimental results.

Evaluation of ductile fracture criteria in combination with a homogenous polynomial yield function for edge splitting damage of DP steels

Abstract 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.

Effects of Edge Heating and Strain Gradient on Stretch Flange Deformation Limit of Steel Sheet

To improve the stretch flange deformation limit of ultrahigh-strength steels, a partial edge heating method was investigated. The effect of the strain gradient on the stretch flange deformation limit when a partial edge heating method was applied was investigated by the hole expansion test and FEM analysis. The stretch flange formability was improved by application of a partial edge heating method because of the recovery of the shearing microstructure and the reduction of the hardness difference of microstructure. It was found that the improvement of the deformation limit strain for the strain gradient with edge heating depends on the strain gradient region. In the low strain gradient region, the improvement margin of the deformation limit strain with edge heating was small. On the other hand, in the high strain gradient region, the deformation limit strain was markedly improved compared with that without heating. It was confirmed that the strain gradient in the direction perpendicular to the maximum principal strain could be used for the prediction of the deformation limit and also the determination of the stretch flange fracture by the FEM analysis of actual parts even under edge heating conditions.

Investigation of surface hardness and roughness on formability of aluminum alloy sheet AA2024-T3 subjected to the shot peening process by silica shots

Shot peening is one type of modified surface treatment that produces a residual compressive stress on the material subsurface and improves surface properties while generating plastic deformation on the surface. This research work aims to improve surface properties, which include the enhanced material formability of aluminum alloy 2024-T3 sheet having 1.2 mm of thickness, by providing residual compressive stress on the surface using the shot peening process, which uses silica particles of 0.1 mm in diameter. First, shot peening was performed using various process parameters: compressed air, distance from nozzle to target, and duration time. Based on the obtained peening sheet, the surface hardness and roughness tests were experimentally performed on the peened surfaces. Additionally, the residual tension created in the sheet after the shot peening is calculated using the X-ray diffraction technique. Consequently, the shot-peened and unpeened sheets were put through hole expansion and Erichsen cupping tests to compare the results of the formability between the shot-peened and unpeened sheets. It was found that peened sheets had a low surface roughness and increased surface hardness, which is better than the unpeened sheet. Moreover, the residual compressive stresses were higher than on the original sheet. Last, the shot peening condition, which changed the surface properties the most, was tested on the hole expansion and Ericshen cupping tests, where the formability results were very significant.

Does friction contribute to formability improvement using servo press?

Servo press forming machines are advanced forming systems that are capable of imparting interrupted punch motion, resulting in enhanced room temperature formability. The exact mechanism of the formability improvement is not yet established. The contribution of interrupted motion in the ductility improvement has been studied through stress relaxation phenomena in uniaxial tensile (UT) tests. However, the reason for improved formability observed when employing servo press is complicated due to the additional contribution from frictional effects. In the present work, an attempt is made to decouple the friction effect on formability improvement numerically. The improved formability is studied using a hole expansion test (HET). The limit of forming during hole expansion is modeled using the Hosford-Coulomb (HC) damage criteria, which is implemented as a user subroutine in a commercial explicit finite element (FE) software. Only the contribution of stress relaxation is accounted for in the evolution of the damage variable during interrupted loading. Therefore, the difference between simulation and experimental hole expansion ratio (HER) can be used to decouple the friction effect from the overall formability improvement during hole expansion. The improvement in HER due to stress relaxation and friction effect is different. The study showed that the model effectively captures the hole expansion deformation process in both monotonic and interrupted loading conditions. Compared to stress relaxation, friction effect played a major role during interrupted HET.

Effect of Cut-Off Side Stiffness on Stretch Flangeability in High-Strength Steel

In high-strength steel (HSS), shear edge failure due to stretch-flange formability is a serious problem. Shear edge failure is strongly affected by cutting methods. As countermeasures, cut-off punching and double punching of the entire circumference have conventionally been considered. However, it has not been shown whether these processes can be applied to the stretch-flange forming part of the outer circumference of automotive parts. In this study, the effect of the punching allowance and the method of improving the double punching process on the outer circumferences of automotive parts were investigated using high-strength hot-rolled steel sheets. The results of hole expansion tests and observations of shear edge faces and metal flow show that partial double punching reduces the rigidity of the cut-off side and increases the collapse probability. Also, the crack propagation caused by high tensile stress near the upper blade is accelerated. In comparison with double punching of the entire circumference of a 590 MPa or 780 MPa-class high-strength steel sheet, the effect of improving the stretch-flange formability is approximately 1.5 times the hole expansion test value, as indicated by the index, up to the area where the punching exceeds the size of the sheet thickness. However, it is considered that the stiffness of the cut-off side varies depending on the curvature and the thickness of the plane, and the effect of such variations will be confirmed in a wide range of the curvature and the thickness in the future.

Machine Learning for Predicting Fracture Strain in Sheet Metal Forming

Machine learning models are built to predict the strain values for which edge cracking occurs in hole expansion tests. The samples from this test play the role of sheet metal components to be manufactured, in which edge cracking often occurs associated with a uniaxial tension stress state at the critical edges of components. For the construction of the models, a dataset was obtained experimentally for rolled ferritic carbon steel sheets of different qualities and thicknesses. Two types of tests were performed: tensile and hole expansion tests. In the tensile test, the yield stress, the tensile strength, the strain at maximum load and the elongation after fracture were determined in the rolling and transverse directions. In the hole expansion test, the strain for which edge cracking occurs, was determined. It is intended that the models can predict the strain at fracture in this test, based on the knowledge of the tensile test data. The machine learning algorithms used were Multilayer Perceptron, Gaussian Processes, Support Vector Regression and Random Forest. The traditional polynomial regression that fits a 2nd order polynomial function was also used for comparison. It is shown that machine learning-based predictive models outperform the traditional polynomial regression method; in particular, Gaussian Processes and Support Vector Regression were found to be the best machine learning algorithms that enable the most robust predictive models.

Experimental investigation of hole expansion ratio for automotive-grade steels

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.

Effect of Different Conical Punch Angle Geometries and the Initial Hole Diameters on the Hole Expansion Ratio of DP steels

In the recent decade, advanced high strength steels (AHSS) have gained a great popularity in the automotive manufacturing industries due to their high strength to weight ratio, which significantly improves the safety of the manufactured automobiles while reducing the weight and thus, enabling to improve the fuel efficiency. However, it is known that some types of AHSSs, especially DP steels, are highly susceptible to edge cracking behaviour during the forming operations. Edge cracking behaviour is generally investigated with a 600 conical punch as suggested by the ISO 16630 standard. However, in this study, to observe the behaviour of edge cracking ability of DP steels under different conical punch angles for different initial hole diameters, hole expansion tests have been performed with conical punches with three different angles (300 ,600, 900) for three different initial hole diameters (14, 16, 18 mm). The results have shown that the hole expansion ratio (HER) does not differ considerably with the variation of the conical punch angle and the initial hole diameter due to low fracture strain of DP steels observed after hole expansion tests. The major factor for the edge stretching ability of DP steels have been observed to be microstructure rather than geometrical factors such as conical punch angles.

Improving the stretch flangeability of ultra-high strength TRIP-assisted steels by introducing banded structure

A novel ultra-high strength transformation induced plasticity (TRIP)-assisted steel with a better combination of ductility and stretch flangeability was fabricated by introducing a banded structure into the resulting microstructure. The corresponding relationships between microstructures and mechanical properties were studied in detail by the tensile test and the hole expansion test. These tests reveal that some pancake-like ferrite grains obtained in cold rolling are retained during intercritical annealing and finally form a banded structure together with martensite/austenite (M-A) islands along the rolling direction. The steel can be further strengthened in the subsequent punching process, while uniformly-distributed bainite matrix with fine M-A islands dominates the overall microstructure after annealing at higher annealing temperatures. The banded structure of the soft ferrite phase will promote crack propagation along the rolling direction rather than in the thickness direction, thereby favoring improvement of the hole expansion ratio (HER). Instead, the mixed bainite-M-A islands microstructure led to increased micro-voids density at the edge of the shear affected zone (SAZ) after the hole punching process and a great number of cracks rapidly propagated through the thickness direction in the matrix, which is responsible for a decrease in HER. Excellent mechanical properties were obtained in the low-temperature annealed sample with an HER of 29–35%, elongation of 21–26%, tensile strength of 941–1016 MPa, and yield strength of 426–566 MPa.

Effect of phase interface on stretch-flangeability of metastable ferrous medium-entropy alloys

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.

STUDY ON DEEP DRAWING BEHAVIOR OF THE INTERSTITIAL FREE STEEL IN CORRELATION WITH THE MICROSTRUCTURE

The objective of this paper was to present results obtained about the behaviour of the IF steel associated with high deformation. To evaluate the deep drawing aptitude, samples were subjected to tensile test, n and r values determination, hole expansion and Erichsen tests. The results were correlated with a microstructural study. Through thermodynamic simulation, the phase transformation temperature (Tg®a) and the precipitates formation were estimated. The Tg®a value was also determined by dilatometry test. The failure of the material during deep drawing or stamping process is evidenced through the formation of cracks in areas with critical angles changes. It was relevant to understand the mechanisms of fracture nucleation and propagation of the IF steel. A fractography study was carried out and was possible to verify the presence of cavitation mechanism as results of the superplastic flow at high deformation conditions, promoting necking and fracture after a high plastic deformation achieved.

Influence of Sheet Metal Pre-forming on Edge Crack Sensitivity using an AHSS Steel Grade

Especially in the automotive sector, high-strength sheet materials are processed in the manufacturing industry. These steels often show a pronounced sensitivity to edge cracks. Because of this, many edge crack testing methods for a wide variety of stress conditions have been developed to describe the edge crack sensitivity of a material. Only the hole expansion test according to ISO 16630 has been standardized. However, the standardization has some gaps in the process description, which has resulted in test modifications. Another disadvantage is the dependence of the results on the machine operator. In the past, the influence of the shear cutting parameters die clearance, cutting edge geometry, and type of cutting line on the edge crack sensitivity was only calculated for undeformed sheet materials. Not only are shear cutting operations carried out on undeformed sheet blanks in the context of the manufacturing of sheet metal components, but more and more pre-formed sheets are mechanically separated and subsequently further formed. Therefore, it is essential to consider the influence of the type and amount of pre-forming introduced on the sensitivity of a material to edge cracks. The discrete types of pre-forming, uniaxial tension, plane strain, and equi-biaxial stretch forming were introduced to sheet metal blanks using dual-phase steel. The Edge-Fracture-Tensile-Test was used to identify the residual formability of the undeformed and pre-formed specimens. The Edge-Crack-Sensitivity-Factor{K}_{mathrm{ec}}, which can be used to predict edge cracks in a finite element forming simulation, was determined from the recorded major strains for selected parameter configurations.

Evaluation of hole expansion formability of high strength AA7075 alloy under varying temper conditions

There has been a widespread increase in the use of aluminum alloys in automotive industries for meeting ever-growing light-weighting requirements. However, edge formability is a critical manufacturing challenge that restricts their widespread use. Edge formability of sheet metal is determined using a hole expansion test (HET) and is evaluated by the hole expansion ratio (HER). The present study investigates the effect of temper conditions on the edge formability of AA7075 alloy sheets. Hole expansion tests were conducted in different temper such as W-temper (super saturated solid solution followed by water quenching), under aged (UA), and peak aged (PA) conditions. Two different hole preparation techniques, a punching and a drilling process, were used to prepare samples with varying edge conditions. The results demonstrate that the W-temper has the highest edge formability irrespective of hole edge conditions. Researchers have reported that uniaxial stress state prevails at the hole edge during the HET. Consequently, uniaxial tensile tests were conducted on for each temper condition and various tensile properties such as yield stress (YS), ultimate tensile strength (UTS), ratio of yield stress to ultimate tensile strength (YS/UTS) were determined to evaluate edge formability. Furthermore, microstructural and failure analysis of the failed specimens were performed to explain the deformation behavior during the HET.

A Study of the Boundary Conditions in the ISO-16630 Hole Expansion Test

As new and more advanced sheet metal materials are introduced to the market, more accurate techniques for determination of failure limits are needed. One area that needs attention is edge formability, where the ISO-16630 standardized Hole Expansion Test currently is used to express this through the Hole Expansion Ratio. Over the years, this standard has been criticized for producing a large scatter in repeated tests. This paper investigates a new setup for the Hole Expansion Test which introduces draw beads into the setup to ensure sufficient restraining of the specimen during the test in an effort to reduced the scatter. In total 62 tests of a DP800 steel alloy were executed, but a large scatter in the results were still seen. It was therefore concluded that a lack of restraining force in the Hole Expansion Test was not the primary cause of the reported scatter seen in other tests.

Sheared Edge Formability Characterization of High Strength Aluminum Alloys at Room and Elevated Temperatures using Hole Expansion Tests

High strength 6000- and 7000-series aluminum alloys have significant potential for automotive light weighting due to their high strength-to-weight ratios. Alternate thermal processing routes and warm forming operations with rapid heating to 150 - 300°C are being explored to expand their forming process windows. While the constitutive and formability behavior of these alloys at elevated temperatures is an active area of research, limited data is available on their sheared edge stretchability, particularly under elevated temperature forming conditions. To this end, the influence of cutting clearance on the edge fracture limits of two precipitation hardening alloys, AA6013-T4 and AA7075-T6, and a non-heat treatable AA5182-O were first investigated at room temperature using the conical hole expansion test. A 12% clearance was found to be suitable for the three materials and used to process the hole expansion samples prior to warm forming. Microhardness tests were used to characterize the depth and severity of the shear affected zone (SAZ) for the AA7075-T6 before and after the warm forming cycle. The forming temperature was found to increase the hole expansion ratio by approximately 400%, 150%, and 520% for the AA5182-O, AA6013-T4, and AA7075-T6 respectively relative to the room temperature tests. The edge stretchability during W-temper forming of the AA6013 and AA7075 at room temperature was also assessed. It was observed that W-temper forming negatively influenced the hole expansion of AA6013 by approximately 30% relative to the T4 condition. Conversely, the hole expansion ratio for AA7075 when formed in W-temper increased by over 35% with respect to the T6 temper.

Sheared edge formability characterization of cold-rolled advanced high strength steels for automotive applications

Edge cracking has become a limiting factor in the use of some advanced high strength steels (AHSS) for high-performance automotive applications. This fact has motivated the development of a multitude of experimental tests for edge formability prediction over the last years. In this sense, the Hole Expansion Test (HET) according to ISO16630 has been established in the automotive industry as a standard procedure for edge cracking sensitivity ranking. However, whereas it may be useful for rapid material screening, the results are often not accurate and reliable enough. Consequently, alternative methods based on Digital Image Correlation (DIC) have been proposed aimed at improving the prediction of edge cracking occurrence during forming and obtaining useful strain data that can be implemented in forming simulations. This paper explores the applicability of different DIC-based methods, such as Half-Specimen Dome Tests, Sheared Edge Tensile Tests, and KWI hole expansion tests with a flat nosed punch, for characterizing the edge formability of three cold-rolled AHSS sheets. The results obtained from the different testing methods are compared and validated with a laboratory-scale demonstrator. Finally, the limitations and advantages of the different methods are discussed.

Adjustment of fracture locus to improve edge crack resistance

Edge crack sensitivity of advanced high strength steels is posing challenges in forming the sheets with shear-cut edges. In fact, cutting process reduces formability of material by leaving residual damage at the cut edges and makes it prone to edge cracking. The difficulty of the problem mainly lies in the complex loading path that sheets undergo through shear-cutting and the subsequent forming processes. In other words, the stress state evolves from pure-shear to plane-strain during cutting process and changes to mostly uniaxial tension during the following hole-expansion test. The present study aimed to investigate the physical mechanism underlying the formation of edge cracks. The overall aim of the study is to tailor an improved microstructural configuration leading to a significantly reduced edge crack sensitivity. As a first step in this process, an artificial fracture locus is iteratively identified which promises to improve edge crack resistance. In this regard, a dual phase steel was considered as reference material and its fracture locus was calibrated based on uncoupled Bai-Wierzbicki fracture model. The artificial fracture locus representing a virtual material with improved edge crack resistance changed in a way that lower residual damage was inherited from the cutting process, which consequently improved the hole-expansion ratio.

An approach to describe edge ductility

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.

An Investigation into the Influence of Interrupted Loading in Improving the Stretch-Flangeability of Dual Phase Steel

Materials resistance to edge failure during sheet metal flanging operations is known as stretch-flangeability. It is one of the important concerns in the current automotive sector. Stretch flangeability of sheet metal is estimated by hole expansion test and commonly it is represented by hole expansion ratio (HER). The objective of the present work is to comprehend the hole expansion deformation behavior of DP600 steel. Firstly, finite element analysis was performed to understand the stress state at the edge during the hole expansion test. Thereafter, the effect of stress relaxation was studied by conducting hole expansion tests (HET) in monotonic and interrupted mode. Considerable improvement in the HER was observed. The HER was found to increase with the pre-strain. The combined effect of friction and stress relaxation played a crucial role in delaying the edge failure, resulting in the enhanced HER.