In-plane Torsion Test Research Articles

Reaching Large Strains During Simple Shear Experiments Thanks to Sequential Re-Machining of the Free Edges

BackgroundThe simple shear experiment is widely used for the calibration of plasticity models due to straightforward post processing. The specimen can be as simple as a rectangular strip of sheet metal, but the maximum strain is limited by early initiation of fractures from the free edges. Avoiding this drawback has been a major motivation for the development of new specimens with optimized edge geometries or the in-plane torsion test, but at the cost of a more complex analysis of the test and often a reduction of the gauge section.ObjectiveThe objective of the present work is to overcome the initiation of fracture from the free edges during simple shear experiments. Our goal is to double the achievable maximum strain, while keeping the size of the specimen and the post processing simplicity of a standard simple shear test.MethodsA sequential single shear test is proposed, consisting of several two steps sequences on a notched geometry. First, an interrupted shear test is performed up to a specified displacement value. Then, the damaged free edges of the specimen are removed through milling. The specimen is then ready for the following sequence of shear and re-machining.ResultsExperiments are performed on three engineering materials, with up to five loading-machining sequences. The maximum attained effective strain is up to two times the one reached during a monotonic experiment. Numerical simulations are used to validate the shear stress and strain calculations from experimental measurements. Practical recommendations are derived for the choice of the displacement step size and Digital Image Correlation analysis.ConclusionIt is found that the maximum strain attained before the undesired failure of the specimen during simple shear test can be substantially extended through repeated re-machining of the specimen boundaries, enabling behavior identification at larger strains.

Influence of mechanical characterization on the prediction of necking issues during sheet flow forming process

Flow forming is an incremental sheet forming (ISF) process during which a sheet metal is compressed and stretched multiple times by means of one or multiple rotating roller tools. The local tool-workpiece contact zone evolves during the entire process. The necking phenomenon, which corresponds to an uncontrolled thinning of the part wall, is introduced. This phenomenon represents a major issue for ISF processes. A review of the state-of-the-art about ISF processes shows that most studies do not consider the loading path complexity when choosing the mechanical characterization test and its associated constitutive model. Besides, the prediction of necking occurring during sheet flow forming is poorly studied in the literature. In this paper, a finite element analysis (FEA) using the FORGE® software enables a detailed understanding of the loading path (strain and stress states) prevailing during the flow forming operation. Based on the peculiarities of this loading path, different mechanical tests associated with adequate constitutive models are chosen to characterize the material behavior. The ability of each constitutive model used within the FE approach to predict necking is then assessed. Results show that the best prediction of a geometry exhibiting necking issues is obtained with the cyclic in-plane torsion test (ITT) associated with its calibrated isotropic – kinematic hardening model. These results suggest that the behavior characterization under cyclic shear loadings is relevant. Using a simple tensile test with associated power-law provides a faster and conservative necking prediction.

Analytical model of the in-plane torsion test

In research and industry, the in-plane torsion test is applied to investigate the material behaviour at large plastic strains: a sheet is clamped in two concentric circles, the boundaries are twisted against each other applying a torque, and simple shear of the material arises. This deformation is analysed within the scope of finite elasto-plasticity. An additive decomposition of the Almansi strain tensor is derived, valid as an approximation for arbitrary large plastic strains and sufficiently small elastic strains and rotations. Constitutive assumptions are the von Mises yield criterion, an associative flow rule, isotropic hardening, and a physically linear elasticity relation. The incremental formulation of the elasticity relation applies covariant Oldroyd derivatives of the stress and the strain tensors. The assumptions combined with equilibrium conditions lead to evolution equations for the distribution of stresses and accumulated plastic strain. The nonzero circumferential stress must be determined from the equilibrium condition because no deformation is present in tangential direction. As a result, a differential-algebraic-equation (DAE) system is derived, consisting of three ordinary differential equations combined with one algebraic side condition. As an example material, properties of a dual phase steel DP600 are analysed numerically at an accumulated plastic strain of 3.0. Radial normal stresses of 3.1% and tangential normal stresses of 1.0% of the shear stresses are determined. The influence of the additional normal stresses on the determination of the flow curve is 0.024%, which is negligibly small in comparison with other experimental influences and measurement accuracies affecting the experimental flow curve determination.

Characterization of Flow Curves for Ultra-Thin Steel Sheets With the In-Plane Torsion Test

AbstractThe in-plane torsion test (IPT) is a shear test that has already been successfully used to determine flow curves up to high strains for thin sheets with thicknesses between 0.5 mm and 3.0 mm. In the same way as with other shear tests, the formation of wrinkles is a major challenge in determining flow curves with the IPT, especially when testing ultra-thin sheets with a thickness between 0.1 mm and 0.5 mm. A new method for suppressing wrinkling is introduced, in which the formation of wrinkles is avoided by arranging and gluing single sheets to multi-layered specimens. The influence of the used adhesive on the determination of flow curves is negligible. The proposed method is used to identify flow curves for two materials, the high-strength steel TH620 and the soft steel TS230, used in the packaging industry. The materials are tested in sheet thicknesses between 0.17 mm and 0.6 mm. The determined equivalent plastic strains for the TH620 with a sheet thickness of 0.20 mm could be increased from 0.38 (bulge-test) to over 0.8 with the new method using four-layered specimens.

Robust characterization of anisotropic shear fracture strains with constant triaxiality using shape optimization of torsional twin bridge specimen

In-plane torsion test has been drawing attention as an effective strategy to obtain good shear properties. Herein, a framework of shape optimization for a twin bridge specimen is presented to improve the performance while ensuring better machinability. The framework includes the consideration of material anisotropy and the determination of design variables. In this study, TRIP1180 sheet was employed as a target material. The optimized design was verified by an actual in-plane torsion test. The experimental results demonstrate that the proposed framework can optimize the shape of the twin bridge for maintaining zero triaxiality up to fracture occurrence.

Large strain flow curves of sheet metals by sheet extrusion

Metal sheets are forward extruded at large plastic strains up to 1.6. The sheet specimens are placed between two half-cylindrical billets and cold-extruded collectively. While extruding the sheets, their central zone is plastically deformed nearly homogeneously under a deviatoric stress state equivalent to simple tension. Tensile test specimens are extracted from the extruded sheets at various extrusion strains delivering flow stresses at discrete large plastic strains of the flow curve. Sheet thicknesses as thin as 0.2 mm could be tested successfully. Steel and aluminum alloys with different strengths were investigated. Results were compared with in-plane torsion test measurements.

Strain hardening under large deformation for AA5182

In this study, an aluminium alloy of AA5182 is taken as the research object to study strain hardening under large plastic deformation. Tensile tests are done for four specimens, including dog-bone specimens, notched specimens, specimens with a central hole and in-plane shear specimens. Bulging tests are also conducted to measure strain hardening under balanced biaxial tension. In addition, an experimental method called in-plane torsion test is also used for shear loading. At least three experiments are completed for each type of specimens along the rolling direction (RD), diagonal direction (TD), and transverse direction (DD). The stroke of each tests is measured by a digital image correlation (DIC) system, and the load-stoke curves were obtained for the tests. Combined with an inverse engineering method, the strain hardening properties are calibrated for the alloy under different loading conditions of shear, uniaxial tension, plane strain tension, and balanced biaxial tension. The strain hardening under various loading conditions is compared and modelled by various yield functions to evaluate their performance. It is concluded that inverse engineering approach is a simple but powerful method to obtain the stress-strain curve up to large plastic deformation. It is also observed that it needs to develop yield functions to model yielding behaviour under complex loading conditions.

Enhanced characterization of the yield behavior of sheet metal at torsional load using digital image correlation methods

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 mild- and 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.

Microstructure and Crystallographic Texture of Silicon Iron Modified by Torsion Under Quasihydrostatic Pressure

The electron backscatter diffraction, X-ray diffraction analysis, electromotive force instantaneous measurement, microhardness and coercive force measurement techniques are used to explore the development of the microstructure, crystallographic texture and physico-mechanical properties of silicon iron (Fe-3% Si) alloy under quasi-hydrostatic pressure in a Bridgman anvil. It is found that the alloy deformation is accompanied by its significant hardening. In-plane torsion test shows that the average grain size rapidly decreases down to 185 nm after a quater turn of the movable anvil. With the number of turns increased up to six the grain size gradually decreases down to 150 nm. At the same time, the average size of subgrains is less dependent on the deformation degree. The crystallographic texture evolution involves the formation of the strong {001} texture component after comparatively small megaplastic deformation followed by the simple shear texture formation. Such a behavior can be correctly described in terms of non-equilibrium thermodynamics.

Methods for measuring large shear strains in in-plane torsion tests

The in-plane torsion test achieves true strains far beyond 1.0 for sheet metals, especially using specimens with circular grooves. The accurate measurement of these high strains is a challenge for the conventional digital image correlation (DIC). Thus, the determination of flow curves is limited and fracture strains for very ductile materials cannot be measured. A new grooved specimen is introduced to avoid strain localization. Shear stress and shear strain along a defined area in the groove are constant so that strains can be measured independently of the DIC system setting without error due to strain localization. Furthermore, three methods for the measurement of very high shear strains in the in-plane torsion test are presented: Firstly, the limit of the optical strain measurement is extended by multiple renewal of the digital image correlation (DIC) pattern on the samples. Secondly, the shear strain for the planar specimen is calculated exactly from the rotation angle curve. Lastly, a new incremental method is presented. This method enables to determine shear strains for plane and grooved specimen exactly by only measuring the torque and the angle of rotation. All methods were applied for three steel sheet materials namely DP1000, CP1000 and DC04. The equivalent strain in a grooved in-plane torsion test of sheet steel DC04 was determined as 3.3 with the new incremental method. Such high strains far exceed conventional methods for determining the flow curve.

Influence of the preheating strategy on the deep drawing of extruded magnesium alloy ME20 sheets

Due to the hexagonal crystal structure of magnesium alloys, a high forming limit can only be achieved at elevated temperatures. For the material characterization of extruded magnesium alloy ME20 sheets, at elevated temperatures, the in-plane torsion test and a multi-layer upsetting test were conducted. Also, FLCs were determined at elevated temperatures. For the deep drawing, two different heating strategies are investigated. In the first method, specimens are placed in an oven at 400 °C for around 10 minutes and then rapidly transferred to the tool. In the second method the specimens are directly heated in the deep drawing tool. In both methods the specimens are painted with Bornitrid lubricant. The effect of the preheating on the coefficient of friction is investigated by using strip tensile tests. FEM simulations for the deep drawing comparing two different material models (Barlat 2000, CPB06) are executed. The results show that specimens heated in the tool show a better formability than oven-heated specimens. The numerical results present that there is no significant difference between Barlat 2000 and CPB06 in an isothermal deep drawing condition. The numerical results are in good agreement with the deep drawing experiments which also indicates that the warm FLCs allow for a good failure prediction.

Characterizing plasticity and fracture of sheet metal through a novel in-plane torsion experiment

In-plane shear specimens typically feature free gage section boundaries along which the state of stress deviates from that of pure or simple shear. As a consequence, the geometry of in-plane shear specimens needs to be carefully chosen to avoid any early fracture initiation from the free boundaries, before the actual failure strain for shear is reached at the specimen center. From this perspective, disc specimens for in-plane torsion experiments offer a significant advantage: they do not feature any free boundaries. However, detailed analysis suggests that circular groove need to be introduced (local thickness reduction) to ensure a strain localization away from the clamped specimen shoulders. In most existing in-plane torsion tests, the specimen is clamped on the inner diameter by applying out-of-plane compression to avoid any slipping. In such configurations, it is impossible to monitor the entire sheared circumference with cameras for digital image correlation. It is the goal of the present work to develop in-plane torsion test using grooved specimens with full optical access to the specimen for 2D or 3D DIC measurements. Furthermore, the experimental set-up will be designed for plasticity and fracture characterization at strain rates of up a few 100/s. Its main feature is a new clamping technique. After identifying a suitable specimen geometry through finite element simulations, experiments are performed on specimens extracted from aluminum alloys and steels sheets. The experimental campaign includes proportional loading, reversed loading and strain rate jumps. The full optical access to the sheared gage section area also enables the discussion of the effects of plastic anisotropy on the strain fields in in-plane torsion experiments. The results from the in-plane torsion experiments are also compared with the fracture strain measurements from in-plane shear experiments performed in a conventional uniaxial loading frame.

Alternative approach to model ductile fracture by incorporating anisotropic yield function

Due to the texture formed in cold/hot rolled forming process, anisotropy is a key issue not only in modeling of plastic deformation but also in characterization of fracture behavior. In this study, an anisotropic ductile fracture criterion is developed by introducing anisotropic parameters into the weight function of an uncoupled shear ductile fracture criterion. The proposed anisotropic ductile fracture is applied to describe the anisotropic characteristics in the ductile fracture of AA6082-T6. Ductile fracture behavior of AA6082 is experimentally investigated at the different loading conditions: shear by in-plane torsion test, uniaxial tension by specimens with a central hole, plane strain tension by notched specimens, and the balanced biaxial tension by the Nakajima test. In-plane torsion and tension tests with a central hole and notch are conducted along three directions: rolling direction, diagonal direction and transverse direction. Specimen deformations during the tests are recorded and fracture strains are measured by digital image correlation approach. The measured fracture strains are then utilized to calibrate the parameters in the proposed ductile fracture criterion. With the calibrated ductile fracture criterion, the fracture locus and anisotropic ductile fracture in various loading conditions are predicted and compared with experimental measurement and those predicted by linearly transformed anisotropic fracture model to investigate the predictability of the proposed ductile fracture criterion. The comparison demonstrates that the anisotropic fracture of AA6082 is predicted by the proposed criterion with good agreement in the different loading directions of shear, uniaxial tension, plane strain tension, and the balanced biaxial tension. Considering the high accuracy of the proposed ductile fracture criterion, it is expected that the proposed anisotropic ductile fracture criterion can improve the reliability of failure prediction in metal forming for materials with strong directionality in fracture.

Material characterization for plane and curved sheets using the in-plane torsion test – An overview

The in-plane torsion test offers a broad range of applications for the characterization of mechanical properties of sheet metal materials and components. True plastic strains beyond 1.0 can be achieved. Such data can be used for the numerical analysis without extrapolation of the flow curve. The stress and strain state correspond to truely ideal simple shear during the entire process. The full-field strain and stress analysis of the test area makes it possible to determine an almost arbitrary number of cyclic flow curves with only one single specimen. By using a grooved specimen, the ideal simple shear state can be obtained until fracture of the material without loosing the shear state. New investigations show that the in-plane torsion test is also suitable for the determination of flow curves of sheets with curved surfaces. Investigations on curved rotationally symmetrical as well as a tubular shape sheets were performed. Finally, first results for the determination of the local strength at arbitrary positions of a sheet component are presented.

Influence of manufacturing processes on material characterization with the grooved in-plane torsion test

In-plane torsion tests offer advantages such as a proportional loading path and a homogeneous stress and strain distribution when characterizing the material behavior in the state of in-plane shear. The use of a grooved specimen is mandatory for the characterization of the damage behavior. The manufacturing of the groove by turning, milling, and electrical discharge machining for a DP600, DP1000, and CP1000 showed a strong influence on the experimentally measurable strain values at failure. Fine machining by milling displayed good results for all tested materials. The notch-effect in turned grooves led to an early fracture initiation. A straightforward parameter identification scheme for the Hosford–Coulomb fracture criterion as proposed by [16] was used to show the sensitivity of fracture curves on the experimental database.

Material characterization for plane and curved sheets using the in-plane torsion test – an overview

Abstract The in-plane torsion test offers a broad range of applications for the characterization of mechanical properties of sheet metal materials and components. True plastic strains up to 1.0 can be achieved. Such data can be used for the numerical analysis without extrapolation of the flow curve. The stress and strain state correspond to ideal simple shear during the entire process. The application range of the plane torsion test has been constantly expanded by different approaches and a more sophisticated evaluation has been developed. The full-field analysis of the test area makes it possible to determine an almost arbitrary number of cyclic flow curves with only one single specimen. By using a grooved specimen, the ideal simple shear state can be obtained until fracture of the material without inhomogeneities. New investigations show that the in-plane torsion test is also suitable for the determination of flow curves for curved surfaces. Investigations on curved rotationally symmetrical as well as a tubular shape were performed. Finally, first results for the local determination of the strength’ arbitrary locations of a component are presented.

A grooved in-plane torsion test for the investigation of shear fracture in sheet materials

The grooved in-plane torsion test is proposed as a shear fracture test for sheet materials. Unlike conventional simple shear tests, which are prone to incipient cracking at the free edges, this test uses radially continuous specimens, as firstly introduced by Marciniak and Kołodziejski (1972). In order to control the fracture position, a radial groove is cut out which allows to keep the fracture away from the clamping area. Thus, this test is able to create material fracture under ideal shear conditions i.e., the condition of vanishing triaxiality at the observable region of the test. Accordingly, the recent shear extended damage and fracture models for the selected material classes can be validated and/or quantified. With the help of finite element analysis (FEA), the corresponding fracture strains for the steel DP1000 were investigated using the proposed shear test and, additionally, three tensile tests conducted on notched specimens which cause fracture at moderate to high triaxialities. These are used to fit the fracture loci of some shear enhanced fracture criteria which have recently been proposed in the literature. The FEA shows that the proposed test provides fracture development under constantly zero triaxiality and zero Lode parameter conditions. Moreover, among the selected criteria, the model proposed by Lou et al. (2012) delivers the best results for selected experimental set. The developed test is ideally suitable for fracture parameter identification of sheet materials which do not show pronounced in-plane anisotropy, e.g. dual phase steels. Furthermore, this test is not limited to metallic materials.

Determining cyclic flow curves using the in-plane torsion test

A new method of determining cyclic stress–strain curves for characterizing kinematic hardening of sheet materials is introduced utilizing the in-plane torsion test and optical strain measurement. The test enables the simultaneous recording of multiple cyclic stress–strain curves with different amplitudes of plastic strain at load reversal in one single test and with one single specimen. Cyclic flow curves are obtained for DP600, DP800, DX54D, and AA5182, and compared with the Miyauchi shear test. Material parameters for a typical kinematic hardening model are determined. Multiple cyclic stress–strain curves result significantly different model parameters as obtained by a single cyclic curve.

An experimental and numerical investigation of different shear test configurations for sheet metal characterization

Simple shear tests are widely used for material characterization especially for sheet metals to achieve large deformations without plastic instability. This work describes three different shear tests for sheet metals in order to enhance the knowledge of the material behavior under shear conditions. The test setups are different in terms of the specimen geometry and the fixtures. A shear test setup as proposed by Miyauchi, according to the ASTM standard sample, as well as an in-plane torsion test are compared in this study. A detailed analysis of the experimental strain distribution measured by digital image correlation is discussed for each test. Finite element simulations are carried out to evaluate the effect of specimen geometries on the stress distributions in the shear zones. The experimental macroscopic flow stress vs. strain behavior shows no significant influence of the specimen geometry when similar strain measurements and evaluation schemes are used. Minor differences in terms of the stress distribution in the shear zone can be detected in the numerical results. This work attempts to give a unique overview and a detailed study of the most commonly used shear tests for sheet metal characterization. It also provides information on the applicability of each test for the observation of the material behavior under shear stress with a view to material modeling for finite element simulations.

Achieving High Strains in Sheet Metal Characterization Using the In-Plane Torsion Test

The in-plane torsion test is used to determine plastic flow curves for sheet metals. Very high strains of up to an equivalent strain of 1.0 can be measured since there are no edge effects in a plane torsion specimen. In combination with optical strain measurement, an efficient evaluation method for this test was developed. However, the achievable strain varies for each material. The slippage between the inner clamps and the specimen was found to be one main limiting effect. In order to improve the clamping capability, different surface corrugations are applied at the inner clamping tool. Four sheet materials, DC06, DP600, AA6016, and AA5182 are selected for testing this new clamping setup. While the flow curve of DC06 is determined until a strain of 1.0 and above, such high values cannot be achieved for the other materials. It can be shown that the measurable strain can be increased by the choice of the surface corrugation features at the inner clamping. For the DP steel and the aluminum alloys, the flow curve can be determined until equivalent plastic strains of 0.5 to 0.6, which is also a significant improvement compared to many other sheet metal testing methods.