Enhanced characterization of the yield behavior of sheet metal at torsional load using digital image correlation methods
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O R I G I NA L A RT I C L E
Markus Grillenberger · Martin Schagerl
Enhanced characterization of the yield behavior of sheet metal at torsional load using digital image correlation methods
Received: 31 October 2019 / Accepted: 18 August 2020 © The Author(s) 2020
Abstract This work describes an approach for strain determination at the “in-plane” torsional test using digital image correlation (DIC) without brushing a statistical pattern on the specimen. It is well known that the in-plane torsion test represents a appropriate test method for material characterization of sheet metal in terms of yielding and kinematic hardening (Wagner et al. in Application of the in-plane torsion test in an industrial environment—recent advances and remaining challenges. In: Conference Paper of the 12th Forming Technology Forum, Herrsching, Germany, 2019). However, the gained measurement data do not allow the exact reading of material models for direct use in numerical analysis. The determination of shear values represents thereby the main challenge. Besides well-established methods for stress–strain analysis mentioned in this contribution, the applicability of DIC methods for the direct reading of distortion on the specimen has been investigated. Therefore, a torsion test rig has been developed and torsion tests using specimen out of mildand high-strength steel have been conducted. The in-plane situation allows the distortion tracking of single points on the specimen by using a 2D image correlation software. The needed pictures were taken by using a common digital lens reflex camera. It has been shown that the resolution of the inherent specimen pattern is sufficient for the use of common 2D image correlation software. Torsional test results in the form of computed yield curves of mild- and advanced high-strength steels are aimed to be presented in this contribution. Keywords In-plane torsion test · Digital image correlation · Back-calculation · Enhanced material testing 1 Introduction Calibration and identification of parameter of material models for forming and crash simulation have become a major issue in the material development. To get comprehensive simulation results, knowledge of the material behavior in the form of material models is required. Different kinds of material tests are available for calibration of material models. However, the in-plane torsion test gives therefore the advantage of gaining test data far beyond uniform elongation as seen in the uniaxial tensile test [1,2]. This available amount of data from the in-plane torsional test lifts the prediction quality of calibrated flow curves to a more precise level. Additionally, the in-plane torsional test provides the possibility of load reversal, without the usage of any stabilizing devices as usually needed at common tensile–compression tests [3], and enables furthermore the determination of shear fracture strains for calibration of damage models [4]. In order to perform in-plane torsional tests, a test rig has been designed at the Institute of Structural Lightwe
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