Improving the magnetic properties of non-oriented electrical steel (NOES) through the optimization of crystallographic texture has been an on-going research activity for decades. However, using ...traditional rolling and annealing procedures, the obtained final textures were usually very similar, i.e., exhibiting the {111} (γ) and (α) fibres, which were not the desired {001} texture (θ-fibre) for optimal magnetic quality. In the current work, a 1.8 wt% Si NOES was processed using a new sheet metal deformation method, i.e., repetitive bending under tension (R-BUT), also known as continuous bending under tension (C-BUT), to modify the texture of the electrical steel. The hot-rolled and annealed NOES plates were repeatedly bent and unbent when they were pulled under tension. The deformed plates were then heat treated at different temperatures for various times. Neutron diffraction and electron backscatter diffraction (EBSD) characterisation of the macro- and micro-textures proved that the R-BUT process significantly reduced the undesired {111} texture while promoting the {001} texture. The cube texture, which rarely formed after conventional rolling and annealing, was also seen in the R-BUT samples after annealing. It was shown that, the shear plastic deformation (induced by R-BUT) played a significant role in promoting the desired textures. In addition, the results indicated that the NOES processed by R-BUT could be deformed beyond its common formability limit, which may provide a method to address the poor workability challenge of high silicon electrical steels.
•Over 3-times improved elongation of cp-Ti sheets is achieved by cyclic-bending-under-tension relative to simple tension.•Tradeoffs in strength and ductility of cp-Ti can be optimized using ...cyclic-bending-under-tension and annealing.•Strength increases owing to stored dislocations since insignificant twinning and grain refinement were observed.•Measurements of the grain structure and texture evolution reveal slip dominated deformation in the material.•Directional hardening and underlying microstructural changes during cyclic-bending-under-tension can reduce anisotropy.
This paper describes results acquired in an investigation into determining the influence of cyclic-bending-under-tension (CBT) and annealing on improving elongation-to-fracture (ETF) and optimizing strength and ductility of commercially pure titanium (cp-Ti) sheets. The space of process parameter involving crosshead velocity and bending depth along with sheet thickness was explored to establish a set of optimal parameters providing the greatest ETF for cp-Ti. Enhancements in ETF of about 3× were achieved using CBT relative to simple tension. Given the uniform elongation facilitated by CBT to very large strains, tradeoffs in strength and ductility of the material were examined by subjecting a set of sheets to a certain number of CBT cycles under the optimized parameters and annealing. In doing so, strength of the material increased by a factor of 1.6 along the sheet softest direction, while by a factor of 1.3 along the sheet strongest direction reducing the anisotropy. Microstructural evolution was characterized using electron-backscattered diffraction, while texture evolution was measured using neutron diffraction. These results revealed slip dominated deformation with minor activity of twinning. The role of CBT in preserving integrity of the sheets to large plastic strains is discussed by comparing measured and simulated geometries and mechanical fields.
This study investigates the deformation and fracture mechanisms of two testing methods, tension under cyclic bending (TCB) and tension under cyclic bending plus compression (TCBC) and their ...relationship to single point (SPIF) and double-sided (DSIF) incremental sheet forming processes. Experimental tests were carried out by using a bespoke TCBC test rig and a DSIF machine with grade 1 pure Ti samples. The results show the elongation-to-fracture has a high relevance to the bending depth and compression, which leads to detailed investigation to the stress and strain evolutions in the local bending region using finite element (FE) method. A new Gurson-Tvergaard-Needleman (GTN) model is proposed with a modified shear damage mechanism utilising experimental fracture strain loci to calibrate the Lode angle effect under low stress triaxiality. It is found the bending and reverse-bending stages correspond to different stress states and significantly affect the fracture occurrence in TCB, TCBC and SPIF, DSIF processes. For the first time, the stress paths in the plane of stress triaxiality and Lode parameter are used to reveal the transition of deformation modes from equi-biaxial to plane strain tension in SPIF and DSIF, as compared to the plane stress tension in TCB and TCBC. Using the new GTN model, the simulation gives accurate predictions to the elongation-to-fracture in TCB and TCBC, and the fracture depth in SPIF and DSIF with an error of less than 8% in comparison to the experimental results. Although there is a distinction between the equi-biaxial and uniaxial tension deformations, the study concludes that the TCB and TCBC tests provide an insight into the formability improvement and represent intrinsic deformation mechanisms of SPIF and DSIF processes, an ongoing research question, which has drawn considerable attention in recent years.
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•Report a comprehensive study on the fundamental mechanisms of incremental sheet forming.•Present a new Gurson-type model to capture damage from macro-scale point of view.•Show deformation mode evolutions using stress paths of stress triaxiality and Lode parameter.•Reveal local bending and compression as key factors to fracture occurrence.•Establish correlation of cyclic bending and tension plus compression to ISF.
The existing analytical solutions for determining an average coefficient of friction in bending under tension tests are revisited. The most commonly used equations are based on the assumption of ...uniform pressure in the contact interface, which is proven to be a statically inadmissible assumption. New analytical solutions are presented based on the assumption of concentrated forces, which are shown to be statically admissible. Existing literature supports the existence of localized peaks in the pressure distribution in terms of both numerical simulations and experimental results. Due to the inhomogeneous pressure distribution, bending under tension tests are not considered appropriate for determining the coefficient of friction, but rather a qualitative test capable of relative comparison of tribo-systems.
The material condition of a workpiece changes as it is formed. Typically, the workpiece hardens and undergoes geometrical and topographical changes. In progressive forming, where workpieces are ...formed in multiple steps, this means that the workpiece in subsequent steps is different from the as-received material. In this work, strip reduction of an as-received strip and of a pre-strained strip are compared. EN1.4404 stainless steel is pre-strained by bending-under-tension. This leads to changes in the material condition and the surface topography of the strip. Two different coatings are tested in a strip-reduction test to evaluate the effect of these changes to the workpiece state on the tribological performance of the tool coatings. A coating that performed poorly when used to reduce the as-received material exhibited an improved performance when used to reduce the pre-strained material.
This paper reports the main results from an experimental investigation into the improved elongation-to-fracture (ETF) of the advanced high strength steel (AHSS) dual-phase (DP) 1180 by subjecting the ...material to the continuous-bending-under-tension (CBT) process. The investigation is carried out using a recently developed testing apparatus, where the specimen flows in a reciprocating fashion through a set of three rollers while it is continuously pulled in tension. The process parameters such as the roller depth defining the amount of bending and wrapping around the rollers and crosshead velocity applying the tensile force to the specimen are varied to maximize the ETF of the material. From the recorded force vs. displacement curves along the rolling direction (RD), 45°, and transverse direction (TD), ETF of DP 1180 is found to improve with the crosshead velocity and with the bending depth up to a certain level, after which it decreases. The optimal parameters of 1.35 mm/s for the crosshead velocity and 3.5 for the normalized bending depth improve ETF of the material over five times in every testing direction relative to simple tension tests. The role of CBT in preserving high integrity of the material to large plastic strains is discussed.
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•Elongation-to-fracture of DP 1180 steel under continuous-bending-under-tension is investigated.•Over five-times improved elongation of DP 1180 is achieved by continuous-bending-under-tension relative to simple tension.•Continuous-bending-under-tension facilitates uniform depletion of ductility preventing a localized flow followed by necking.•Strength of the material can be significantly improved by continuous-bending-under-tension processing.
•Cyclic bending under tension improves elongation independent of the specimen width.•Elongation in cyclic bending under tension reduces as specimen width increases.•Wider specimens undergo greater ...thickness strains reducing elongation in the test.
In recent publications, we have shown that repeated bending and unbending during tension can significantly increase the percent elongation-to-fracture (ETF) of metallic sheets. A custom-built machine for cyclic bending under tension (CBT) testing was designed and used to test several sheet metals. In particular, the improvements in ETF using CBT were over 3× relative to simple tension (ST) for commercially pure titanium (cp-Ti) sheets. The present paper evaluates the influence of specimen width on the achieved ETF in CBT of cp-Ti sheets. To facilitate the study, the CBT machine was furnished with wide grips to enable processing of sheets in addition to testing narrow strips. The ETF was observed to reduce with the sheet width. To rationalize the observations, the strain rate-independent plasticity theory of von Mises (J2) with isotropic expansion of yield surfaces of cp-Ti was used in finite elements (FE) to perform a set of simulations. To facilitate the FE simulations of the CBT process stretching the sheets of cp-Ti far beyond the point of maximum uniform strain in ST, the post-necking hardening behavior was inferred along RD, TD, and 45 sheet directions using an established methodology combining ST tests, CBT tests, and modeling material behavior during CBT. Simulated geometries and mechanical fields were found to be in good agreement with corresponding measurements for every specimen width. While predicted axial strains were similar for every width, the strain path shifted from uniaxial tension for the narrow specimen toward plane-strain tension for the wider specimens. As such, as plane-strain tension was approached via progressively wider specimens, the width strains reduced, while the thinning strains increased. The increase in the thinning strain was established as the primary reason for the reduced ETF with increasing sheet width. Experimental and FE simulation results along with the insights into the influence of specimen width on the ETF in CBT of cp-Ti are presented and discussed.
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Continuous bending under tension (CBT) testing can stretch ductile sheet metals significantly more than simple tension (ST) testing. Strength and plastic strain levels increase with ...the number of CBT cycles and substantially exceed those achieved in ST. Taking advantages of the improved elongation-to-fracture (ETF) achieved by CBT, samples of DP 780 steel are pre-deformed to several strain levels by interrupting their CBT testing. Sub-size specimens are machined from the CBT interrupted specimens and tested in ST. The flow curves from these secondary ST tests are then shifted according to the axial strain accumulated during pre-deformation by CBT to determine a large strain (post-necking) flow curve of the material. The identified large strain flow curve is validated with two approaches. First, the large strain flow curve is used to simulate the load versus displacement curve during CBT to large number of CBT cycles. Second, the curve is used to simulate the flow curves based on the secondary ST testing. The simulations are performed using isotropic hardening (IH) and combined isotropic-kinematic hardening (CIKH) material models. The latter model is calibrated using a set of large strain cyclic tension-compression data. An automated procedure is developed for the creation of sub-size specimens from the finite element (FE) mesh of the interrupted CBT specimen to facilitate efficient secondary ST simulations. The procedure involves cutting of the FE mesh into the sub-size specimen shape by Boolean operations in Abaqus software followed by generation of volume FE mesh and mapping of the state variables. The state variables are mapped to simulate deflection of the specimens, as the residual stress field from the prior CBT simulation evolves to reach a new equilibrium. It is found that the model reproduces the experimental load versus displacement curve, including the succession of spikes and plateaus typical of CBT, very closely. The model also predicts yielding of the interrupted sub-size specimens further verifying the identified curve. The predictions demonstrate the utility of the developed methodology for inferring the post-necking strain hardening behavior of sheets.
Plastic instabilities can be suppressed by imposing cyclic bending during tension to increase elongation-to-fracture (ETF) of metallic sheets. This work is a combined experimental and modeling study ...into the origins of the improved ETF in cyclic bending during tension facilitated by a continuous-bending-under-tension (CBT) test. Load-displacement is measured during the CBT test under an optimized combination of band depth and pull speed process parameters for aluminum alloy (AA) 6022-T4. Monotonic and load reversal response of the alloy is also characterized under strain histories that resemble the history in CBT. Electron-backscattered diffraction and neutron diffraction are employed to characterize the initial microstructure and texture evolution of the alloy after CBT and simple tension (ST). Monotonic, load reversal, and texture data are used to adjust and verify the parameters of an elasto-plastic self-consistent crystal plasticity-based finite element model for simulating CBT. Additionally, a flow stress curve based on a bulge test is used to calibrate the model to capture the large strain hardening behavior of the alloy. The model is then used to simulate the CBT test while predicting the load-displacement and texture evolution data. The model successfully reproduced the succession of spikes and plateaus during CBT validating the prediction of the extrapolated hardening. After the validation, the development of stress and strain is probed through the sheet thickness to determine the underlying profiles as well as along the sheet to elucidate the location and onset of failure axially. The model reveals the development of more strain and dislocation density at the surface then at the center of the sheet and correctly predicts the strain localizations. Comparisons of texture evolution and mechanical fields developed in ST and CBT reveal very similar mechanical states in the sheet. These results in combination with experimental results suggest that the origins of improved ETF during CBT pertain primarily to the incremental deformation structural effects and secondary to the microstructural evolution effects.
•Cyclic bending under tension is modeled using crystal plasticity to clarify origins of improved elongation to fracture.•Accommodation of plastic strain and underlying texture evolution during CBT and ST are predicted to be similar.•Primary origin of the improved ETF during CBT is in the incremental deformation underneath the rollers.•Secondary origin of the improved ETF during CBT is in more uniform plastic deformation in CBT then in ST.•Formability of metallic sheets can be improved by inducing a fiber texture in the sheets using CBT.
This paper describes the main results obtained from a detailed experimental investigation into the dislocation and twinned structures that develop in dual phase (DP) steel sheets. In particular, the ...morphology and defect-structures present in DP 1180 steel having 55% ferrite and 45% martensite, were examined using transmission electron microscopy (TEM). Microstructure of the as-received DP 1180 consists of a twin-free martensite phase in the form of lath and island morphologies and a ferrite phase exhibiting a relatively low dislocation content (3.41 × 108 cm−2). The steel was deformed in simple tension (ST) to fracture and in continuous-bending-under-tension (CBT) to six cycles. As the CBT test facilitates stretching of the sheet considerably beyond the point of necking, strength increases and substantially exceeds that achieved in ST at fracture. As a consequence of plastic deformation, nano-twins are found in the island-shaped martensite, while the lath-shaped martensite remains twin-free. Additionally, some martensite regions develop a needle-like morphology after CBT. As dislocation density in the ferrite matrix increases with plastic strain, dislocation structures and accumulations of dislocations near the ferrite/martensite interface develop in the ferrite. Unlike in the as-received and ST-deformed DP 1180 samples, high density of dislocations (1.47 × 109 cm−2) and dislocation tangles are observed in ferrite regions of the CBT processed sample. We propose that the primary mechanisms enabling the achievement of high strength, while maintaining residual ductility upon CBT are intense plastic slip in the ferrite and profuse nano-twinning in the martensite regions.
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