The Multi-Breakage Phenomenon in Air Bending Process

Vorkov, Vitalii; Aerens, Richard; Vandepitte, Dirk; Duflou, Joost R.

doi:10.4028/www.scientific.net/kem.611-612.1047

Cited by 10 publications

(8 citation statements)

References 2 publications

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“…Similar results on the effect of friction on multi-breakage and gap formation were also obtained in an FE-simulation by Burchitz [17]. Vorkov et al [2,15,18] studied the contact point shift in multibreakage and its effects on the moment distribution, bending force, springback and bend allowance. Ruoppa et al [19] measured the inner sheet radii and punch-sheet gaps among other cross-sectional measurements in large radius bending of UHSS to 90° angles.…”

Section: Introductionsupporting

confidence: 60%

“…However, bending with large radius punches introduces other challenges and disadvantages such as increased required bending force, larger springback and a tendency for a loss of contact between the sheet and the punch nose. In this phenomenon, also known as "multibreakage", the inner radius of the sheet becomes smaller than the punch radius and the loading scheme changes from 3-point bending to something akin to 4-point bending [1,2]. Multi-breakage and severe punch-sheet separation has been previously associated with a flat-top (or "nut-like") bend geometry [3][4][5][6].…”

Section: Introductionmentioning

confidence: 99%

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Strain distribution during air bending of ultra-high strength steels

Pokka¹,

Arola²,

Kaijalainen³

et al. 2021

Esaform 2021

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Air bending is a widely used method for forming ultra-high strength steels (UHSS). However, the limited formability of UHSS poses some challenges for the bending process in the form of strain localisation, surface defects, punch detachment (multi-breakage) and pseudo-polygonal “nut-like” shape of the bend. In this study, the bendability of three UHSS grades (700, 900 and 1100 MPa) is investigated with 3-point bending tests, utilising Digital Image Correlation (DIC) for measuring the strain distributions on the outer curvature. The differences in the extent of multi-breakage and the bend shapes are also studied, and these observations are correlated with the findings from the bending force and strain measurements. The differences between the investigated UHSS grades are significant. The 900 MPa grade produces more localised strain distributions and pronounced multi-breakage compared to the other grades, along with a more polygonal “nut-like” geometry. The reasons and effects of the multi-breakage phenomenon, as well as the causes for the observed differences in the behaviour of the materials are discussed in this paper. The presented results and the measurement data provide more information about the behaviour of the investigated materials in bending, and can be used for improving bending simulation, numerical models, and workshop instructions.

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Section: Introductionsupporting

confidence: 60%

Section: Introductionmentioning

confidence: 99%

Strain distribution during air bending of ultra-high strength steels

Pokka¹,

Arola²,

Kaijalainen³

et al. 2021

Esaform 2021

View full text Add to dashboard Cite

show abstract

“…where K is a material-based constant, Rm the ultimate strength, t the plate thickness, w0 the die opening. Equation 2does not take into account the change of loading scheme throughout the forming stage due to the presence of the multi-breakage effect [15]. Figure 7 shows the four point bending scheme -a typical representation of large radius bending.…”

Section: Bending Forcementioning

confidence: 99%

“…While there are several existing closed form analytical solutions for air bending, they are not able to predict large radius air bending accurately due to its peculiarities (e.g. multi-breakage effect [15]). Regression analysis provides a flexible and robust way to get a valid prediction of large radius air bending.…”

Section: Introductionmentioning

confidence: 99%

Two regression approaches for prediction of large radius air bending

et al. 2018

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The industrial importance of air bending applications with high-strength steels has increased in recent years. However, the limited cold formability of high-strength steels requires the application of punches with large radii. The usage of large radii tooling leads to a change of the loading scheme from traditional 3-point to 4-point bending. Therefore, the conventional formulas used for the prediction of 3-point bending operations fail to deliver accurate results. In the current work, two regression approaches are proposed to achieve an accurate prediction of industrially important bending parameters. The first model is based on phenomenological observations and fitting of well-established formulas. The second model utilizes the circular approximation model as base for the regression prediction. Large radius bending is used mostly for high-strength steels. Therefore, Strenx 700 MC and Strenx 1300 with thicknesses of 4 and 6 mm were used for the investigation. The following bending characteristics have been studied: position of contact points, bend allowance, bending force and springback. The predicted values are compared with the experimental results to assess the prediction accuracy of the proposed methods. Neither the phenomenological, nor the circular model delivers superior overall prediction quality for all output parameters, thus a combination of the two regression models is an attractive option for a better prediction of large radius air bending.To be able to reduce the post-forming operations, an accurate prediction of any forming process is required. A number of methods can be used for that purpose, but in this work, we focus on regression

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“…The focus of this work is on the analytical determination of the most important bending characteristics, namely: bending force, springback, and bend allowance. During the bending process with large radius punches the loading scheme changes gradually due to the multi-breakage effect [20], this effect is important and must be taken into account in the prediction model because it drastically affects the bending moment and consequently the bending force and springback. This effect is described by the distance lc between the contact points (Figure 1a) and by the distribution of the bending moment, as represented in Figure 2.…”

Section: Bending Characteristicsmentioning

confidence: 99%

Analytical Prediction of Large Radius Bending by Circular Approximation

Vorkov

Aerens

Vandepitte

et al. 2018

Journal of Manufacturing Science and Engineering

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An accurate analytical method is normally the preferred choice in engineering practice since this approach usually does not require additional software and can be applied for different situations. A number of analytical methods have been proposed for the air bending process, however, none of them has the capacity to deal with large radius bending. Large radius bending is characterized by a high ratio of the punch radius to the die opening and it is often applied for high-strength steels because of their limited bendability. This bending mode is used to fulfill the imposed level of maximum strain during the forming process. This contribution develops an analytical solution based on the assumption that the bent plate profile can be represented by two straight lines and a circular segment. Three different hardening laws—linear, Swift, and Aerens—are used for the bending moment calculation. Unit moment measurements are used in order to avoid extrapolation of hardening curves obtained by tensile testing. The model is compared with a wide range of experiments using the coefficient of determination, relative and absolute average errors, in addition to the mean standard error. The analytical prediction based on the circular approximation is found to be an accurate and robust tool for the calculation of the major bending characteristics for large radius air bending of high-strength steels.

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The Multi-Breakage Phenomenon in Air Bending Process

Cited by 10 publications

References 2 publications

Strain distribution during air bending of ultra-high strength steels

Strain distribution during air bending of ultra-high strength steels

Two regression approaches for prediction of large radius air bending

Analytical Prediction of Large Radius Bending by Circular Approximation

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