Continuous-bending-under-tension Research Articles

Origins of improved elongation to fracture in cyclic bending under tension of AA6022-T4 sheets as revealed using crystal plasticity modeling

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 of alloy AZ31 sheets: Influence on elongation-to-fracture and strength

This paper describes the main results from an experimental investigation into the consequence of cyclic bending under tension on elongation-to-fracture (ETF) and strength of AZ31 sheets. The deformation is imparted on test specimens of the alloy using a recently built continuous-bending-under-tension (CBT) apparatus conceived to increase ETF relative to simple tension (ST). The apparatus pulls the specimen in tension with a certain velocity while the specimen reciprocates through a set of three rollers, which impart a given amount of bending. The parameter space consisting of bending depth, crosshead velocity, and sheet thickness is explored to achieve the greatest ETF for the alloy. However, only moderate improvements in ETF are obtained. The small improvements are attributed to the relatively uniform elongation of the alloy during both CBT and ST with a small amount of the remaining ductility to deplete throughout the sheet by CBT after exhausting the uniform elongation. Measurements of the grain structure and texture evolution using electron-backscattered diffraction and neutron diffraction reveal slip dominated deformation with some twinning followed by de-twinning. The behavior of the alloy upon CBT processing to a certain number of CBT cycles followed by heat treatments (HT) is also investigated to achieve tradeoffs in strength and ductility. Results and insights from these investigations are presented and discussed. Significantly, it is shown that the strength of the alloy can be increased for over 30% while preserving at least 5% of its ductility.

Mechanical properties and crystallographic texture of non-oriented electrical steel processed by repetitive bending under tension

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 <110> (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.

Micromechanical origins of remarkable elongation-to-fracture in AHSS TRIP steels via continuous bending under tension

Continuous bending under tension (CBT) is known to achieve elongation-to-failure well above that achieved under a conventional uniaxial simple tension (ST) strain path. However, the detailed mechanism for supplying this increased ductility has not been fully understood. It is clear that the necking that occurs in a typical ST specimen is avoided by constantly moving the region of plastic deformation during the CBT process. The volume of material in which the flow stress is greatest is limited to a moving line where the rollers contact the sheet and superimpose bending stress on the applied tensile load. Hence the condition of a large volume of material experiencing stress greater than the material flow stress, leading to strain localization during ST, is avoided. However, the magnitude of the contribution of this phenomenon to the overall increase in elongation is unclear. In the current set of experiments, an elongation to fracture (ETF) of 4.56x and 3.7x higher than ST was achieved by fine-tuning CBT forming parameters for Q&P 1180 and TBF 1180, respectively. A comparison of maximum local strains near the final point of fracture in ST and CBT sheets via digital image correlation revealed that avoidance of localization of plastic strain during CBT accounts for less than half of the increased elongation in the CBT specimens for two steels containing different amounts of retained austenite (RA). Geometrically necessary dislocation evolution is monitored using high-resolution EBSD (HREBSD) for both strain paths, indicating a lower hardening rate in the CBT samples in the bulk of the sheet, potentially relating to the cyclical nature of the stress in the outer layers of the sheet. Interestingly, the GND evolution in the center of the sheet, which does not experience the same amplitude of cyclic stress, follows the ST behavior more closely than the sheet edges. This appears to contribute to a precipitous drop in residual ductility for the specimens that are pulled in ST after partial CBT processing. The rate of transformation of RA is also tracked in the steels, with a significantly lower rate of transformation during CBT, compared to ST. This suggests that a slower transformation rate achieved under CBT also contributed to higher strain-to-failure levels.

Plasticity and structure evolution of ferrite and martensite in DP 1180 during tension and cyclic bending under tension to large strains

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.

Material modeling and simulation of continuous-bending-under-tension of AA6022-T4

In earlier contributions, we discussed continuous-bending-under-tension (CBT) experiments on AA6022-T4. We found that CBT significantly enhanced the elongation-to-fracture and strength, over uniaxial tension. In the present paper, our understanding of CBT is expanded beyond these experimental observations, with the aid of material modeling and numerical simulations of the process. Cyclic tension-compression experiments were performed on this material, using strain histories that are expected to replicate the loading during CBT, i.e., different combinations of constant strain amplitude and linearly increasing mean value, to failure. During these experiments, a limited but not negligible amount of kinematic hardening was discovered. Some of these experiments are used for calibration of a combined isotropic-kinematic hardening model, while the rest are used for experimental validation of the model. The modeling framework is based on a rate-independent, associated flow rule with the von Mises yield criterion as the plastic potential. Isotropic hardening is introduced by a simple, exponential-decay model of the growth of the yield surface with plastic deformation. Non-linear kinematic hardening is introduced by a 4-term, Chaboche-type model. The large strain hardening curve is identified by extrapolation, an approach that is validated later in the work and contrasted with alternative options. This material modeling framework is introduced in finite element models of the CBT process. The model is meshed with linear, reduced-integration elements, with 7 elements through the thickness. It is found that the numerical model reproduces the experimental force-displacement curve, including the succession of spikes and plateaus typical of CBT, very closely. The model also replicates the development of strain on the surface during CBT, and compares well with post-test strain measurements. After these validations, the model is used to probe the mechanics of the CBT process, e.g., the development of stress and strain through the thickness and per cycle, the location and onset of failure, as well as the failure angle, which in CBT differs from the localized neck angle found in a typical uniaxial tension experiment.

Modeling material behavior during continuous bending under tension for inferring the post-necking strain hardening response of ductile sheet metals: Application to DP 780 steel

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.

Inferring Post-Necking Strain Hardening Behavior of Sheets by a Combination of Continuous Bending Under Tension Testing and Finite Element Modeling

This paper presents a combined experimental and simulation approach to identify post-necking hardening behavior of ductile sheet metal. The method is based on matching a measured load-displacement curve from a continuous bending under tension (CBT) test with the curve simulated using the finite element method (FEM), while adjusting an input flow stress curve into the FEM. The CBT test depletes ductility uniformly throughout the gauge section of a tested sheet, and thus, stretches the sheet far beyond the point of maximum uniform strain in a simple tension (ST) test. Having the extended load-displacement curve, the calibrated flow stress curve is extrapolated beyond the point of necking. The method is used to identify the post-necking hardening behavior of an aluminum alloy, AA6022-T4, and two dual-phase (DP) steels, DP 980 and DP 1180. One measured load-displacement curve is used for the identification of a flow stress curve per material, and then the flow stress curve is used to simulate two additional measured load-displacement curves per material for verification. The predictions demonstrate the utility of the developed CBT-FEM methodology for inferring the post-necking strain hardening behavior of sheets. Furthermore, the results for AA6022-T4 are compared with the hydraulic bulge test data. Unlike the hydraulic bulge test, the CBT-FEM method can predict increasing as well as decreasing anisotropic hardening rate in the post-necking regime. The latter is associated with probing stage IV hardening. The proposed methodology along with the results and these key advantages are presented and discussed in this paper.

Experimental studies into the role of cyclic bending during stretching of dual-phase steel sheets

Continuous-bending-under-tension (CBT) has been conceived as a mechanical test or a forming process imparting cyclic bending during stretching of a metallic sheet or strip in order to increase its elongation to fracture (ETF) relative to simple tension (ST). In a recent work, over five times improved ETF by CBT over ST has been reported for a dual-phase (DP) steel DP 1180. This paper evaluates the behavior in CBT of three additional automotive advanced high strength steels (AHSS), DP 590, DP 780, and DP 980. In doing so, the process parameter space defined in terms of crosshead velocity applying the tensile force, and roller depth imposing the amount of bending to the specimen has been explored to maximize the ETF of these materials. The studied steel sheets had different thicknesses, in addition to intrinsically containing different fractions of ferrite and martensite phases. After establishing the optimal process parameters, significant improvements in ETF are achieved for all studied steels. Based on comprehensive data, it is found that lower martensitic content moderately improves ETF, while increasing of the sheet thickness rapidly deteriorates ETF, under CBT. The behavior in tension of sheets that were subjected to CBT processing under the established optimal process condition is investigated to determine enhancement in strength and any residual ductility of the materials. In addition to testing, a combination of electron microscopy along with electron-backscattered diffraction and neutron diffraction is employed in order to assess the initial microstructure, evolution of crystallographic texture, and fracture mechanisms for the studied steels. Texture evolution in CBT forms a more pronounced {011} fiber along the stretching direction than in ST, revealing that the deformation in CBT could extend to greater strain levels than those reached at the fracture location in ST. Fractured surfaces after CBT are found to consist of fine ductile dimples, while those after ST consist of coarser dimples and some content of brittle flat martensitic regions.

Strengthening of alloy AA6022-T4 by continuous bending under tension

This paper studies the evolution in strength of alloy AA6022-T4 sheets that have been pre-deformed by a continuous-bending-under-tension (CBT) process. Significant improvements in strength are observed only after a few CBT cycles. Less appreciable improvements in strength are observed with more CBT cycles and with every cycle the ductility reduces. These observations are rationalized by characterizing microstructural evolution using transmission electron microscopy and electron backscattered diffraction. It is found that evolution of texture and grain shape during CBT slightly differ from those in simple tension (ST). Also, the precipitates do not change their shape during CBT or ST. It is, therefore, concluded that these microstructural features have only a secondary effect on the strength behavior of the alloy. Consistent with earlier observations in the literature, we find that dislocation structures form within grains during monotonic ST and that they are disorganized and not as well defined. In contrast, cellular substructures are observed to form very early during CBT processing, even after the first cycle and to evolve from loose tangles of dislocations to well-defined walls during subsequent cycles. These dislocation patterns are found responsible for the observed behavior of the alloy. Therefore, the strength of the material is determined not only by the achieved effective strain level but also by achieved microstructure.

Experimental study of continuous-bending-under-tension of AA6022-T4

This paper presents an experimental setup for continuous-bending-under-tension (CBT) of thin strips and sheets, aimed at the investigation of the observed elongation-to-fracture (ETF) enhancements under such conditions. In particular, the kinematics of the process are described and correlated with the evolution of axial force measured during testing. The main results of the effect of process parameters, such as crosshead velocity and bending depth, on the ETF and the reduction of axial force are presented for the aluminum alloy AA6022-T4. The force vs. displacement response is not found to be a strong function of the crosshead velocity, which is explained by the near rate-independence of the material in the strain-rate range that occurs in CBT. Similarly, ETF does not show a clear dependence on the bending depth or the crosshead velocity. On the other hand, the axial force decreases linearly with the bending depth and slightly increases with the velocity. From the detailed analysis, the optimal parameters for enhancing ductility of AA6022-T4 under CBT are 1.2 mm/s for crosshead velocity and 2.0 for normalized bending depth. Under these conditions, a significant finding is that the strain level achieved throughout the CBT specimen is comparable to that in the necked region of a specimen tested to rupture in uniaxial tension.

Over five-times improved elongation-to-fracture of dual-phase 1180 steel by continuous-bending-under-tension

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.

Formability Improvements of DP 1180 Subjected to Continuous-Bending-Under-Tension

There continues to be a desire to incorporate advanced high strength steels (AHSS) into automotive and other applications in order to reduce the overall weight of the final product. However, these materials exhibit limited ductility prior to fracture and require high forming forces. One means to address these concerns, at least in a laboratory setting, is by subjecting the material to continuous-bending-under-tension (CBT). In this procedure, a set of three rollers reciprocate over the gauge length of the specimen while the material is continuously pulled in tension. The adjustable parameters during CBT testing are the roller depth, the crosshead velocity (which applies the continuous, tensile force to the specimen), and the carriage speed for the reciprocating rollers (which is typically set to the highest value possible while maintaining a safe process). The formability improvements obtained through CBT processing are decreased applied force and increased elongation to fracture. The latter is achieved by assuring that during the CBT process the entire gauge length elongates to the maximum possible value before fracture occurs. Past research has shown that the concentrated deformation in the fracture location of a tensile specimen is similar to the deformation over the entire gauge length of a CBT processed specimen. In this paper, a parameter space study of the DP 1180 AHSS is conducted in order to determine the optimal roller depth and crosshead velocity to achieve the maximum formability improvements for this material. The results demonstrate a promising means to improve displacement (approximately four times over that in simple tension) prior to fracture for this AHSS when subjected to CBT processing.

Numerical and experimental investigation of formability enhancement during continuous-bending-under-tension (CBT) of AA6022-T4

Abstract Continuous-bending-under-tension (CBT) is a stabilized tension test, whereby a strip is bent around three reciprocating rollers while being loaded remotely with an axial force. This subjects the strip to plastic bending while under tension, and has the beneficial effect of suppressing the instability that, in a conventional tension test, leads to localization of deformation and rupture. CBT experiments were performed on strips extracted from cold-rolled, automotive-grade AA6022-T4 sheet using a custom CBT testing machine. The CBT experiments revealed that the remote axial force necessary to plastically deform the materials was lower than the yield force under uniaxial tension, due to the synergy between tension and plastic bending. Furthermore, the force-displacement curves exhibited a saw-tooth like pattern, which can be explained from the kinematics of the process. A parametric investigation identified the combination of bending depth, crosshead velocity and roller velocity that yielded the maximum elongation-to-fracture. However, the experiments also revealed that the elongation-to-fracture was different between the rolling and transverse directions of the sheet, and significantly more than the uniaxial testing would indicate. The CBT experiments were then simulated using the non-linear finite element code Abaqus. Solid, linear, full-integration elements were used in the simulations, which replicated only half of the strip width, due to the symmetries present. The key for successful simulations is the use of an appropriate material model. Cyclic tension-compression tests on the AA6022-T4 were performed, and the results were captured accurately by a combined isotropic/non-linear kinematic hardening model, using a simple, exponentially-decaying, expansion of the yield surface and a 4-term Chaboche model, respectively. The force-displacement response from the simulations matched the experimental results well. In the future, the simulations will be used to probe the stress and strain development during each CBT cycle, to improve the understanding of the stabilization of deformation that the CBT loading mode provides.

An analytical study of new material test method for tension under bending and compression in double side incremental forming

Abstract Incremental sheet forming (ISF) has attracted considerable research interests owing to its unique advantages. Double side incremental forming (DSIF) was proposed to further improve the forming accuracy and material formability. Compared with conventional sheet forming technologies, ISF provides greater process flexibility and achieves an enhanced formability. At the same time, however, ISF has exhibited a far more complicated material deformation behavior for formability enhancement. It is now widely acknowledged that the material deformation during ISF consists of stretching, bending, and shearing with cyclic effects. Continuous bending under tension (CBT) testing method was proposed by Emmens et al. [1], which proved the cyclic stretch-bending effect for formability enhancement in single point incremental forming (SPIF). However, limited research had been reported to investigate the material deformation mechanism leading to the formability improvement in DSIF. An analytical model of a new material test method, Tension Under Bending and Compression (TUBC), is proposed in this study to investigate the material deformation leading to the formability enhancement in DSIF. Under TUBC condition, a specimen is stretched by the pulling force on both ends, while multiple rollers, in contact with the strip on both sides, move backward and forward continuously to create both cyclic bending and compression loading at a localized area. The analytical model is used to investigate the maximum stable elongations under TUBC condition. Key test variables, bending depth and compressive force, imposed by the rollers, are introduced to consider the effects of continuous bending, compression, and contact between rollers and workpiece. From the results obtained, it is clear that bending and compression have determinant effects on the formability enhancement of DSIF. However, the results show varied degrees of sensitivity of formability to different test variables. The findings correlate well with experimental observations and help to explain the formability enhancement of DSIF.

Fundamental Studies of Continuous Bending under Tension (CBT) and Potential Automotive Forming Applications

A new sheet metal testing procedure has been developed that is simple to perform and that can serve as an adjunct to tensile testing for evaluating the formability of a wide range of sheet metals. The method is called Continuous Bending under Tension (CBT) and results in percentage elongations that significantly exceed those obtained in tensile testing and correlate better with the type and amount of deformation found in certain sheet metal forming operations. The mechanism by which CBT enhances formability can be exploited in innovative processes that are especially useful in automotive manufacturing. A novel CBT testing machine has been developed and preliminary experiments have been performed on an automotive aluminium alloy (AA6022-T4). These experiments demonstrated doubling of the potential strain that can be induced before failure.

Forming Limits of a Sheet Metal After Continuous-Bending-Under-Tension Loading

Forming limit diagrams (FLD) have been widely used as a powerful tool for predicting sheet metal forming failure in the industry. The common assumption for forming limits is that the deformation is limited to in-plane loading and through-thickness bending effects are negligible. In practical sheet metal applications, however, a sheet metal blank normally undergoes a combination of stretching, bending, and unbending, so the deformation is invariably three-dimensional. To understand the localized necking phenomenon under this condition, a new extended Marciniak–Kuczynski (M–K) model is proposed in this paper, which combines the FLD theoretical model with finite element analysis to predict the forming limits after a sheet metal undergoes under continuous-bending-under-tension (CBT) loading. In this hybrid approach, a finite element model is constructed to simulate the CBT process. The deformation variables after the sheet metal reaches steady state are then extracted from the simulation. They are carried over as the initial condition of the extended M–K analysis for forming limit predictions. The obtained results from proposed model are compared with experimental data from Yoshida et al. (2005, “Fracture Limits of Sheet Metals Under Stretch Bending,” Int. J. Mech. Sci., 47(12), pp. 1885–1986) under plane strain deformation mode and the Hutchinson and Neale's (1978(a), “Sheet Necking—II: Time-Independent Behavior,” Mech. Sheet Metal Forming, pp. 127–150) M–K model under in-plane deformation assumption. Several cases are studied, and the results under the CBT loading condition show that the forming limits of post-die-entry material largely depends on the strain, stress, and hardening distributions through the thickness direction. Reduced forming limits are observed for small die radius case. Furthermore, the proposed M–K analysis provides a new understanding of the FLD after this complex bending-unbending-stretching loading condition, which also can be used to evaluate the real process design of sheet metal stamping, especially when the ratio of die entry radii to the metal thickness becomes small.

Material Characterization at High Strain by Adapted Tensile Tests

The strength of materials at high strain levels has been determined using the so-called Continuous-Bending-under-Tension (CBT) test. This is a modified tensile test where the specimen is subjected to repetitive bending at the same time. This test enables to create high levels of uniform strain. A wide variety of materials has been tested this way. The strength of the material after CBT testing has been measured in different ways: by secondary tensile tests, by interrupted CBT tests, and directly from the fracture in the CBT test. All methods yield similar results: the strength is largely unaffected by the cyclic pre-deformation and mainly depends on the overall increase in length. Only for multi-phase materials the strength shows a minor influence of CBT test conditions. The hardening follows the extrapolated hardening observed in a conventional tensile test, except for brass. This test method can potentially be used for measuring hardening curves at high strain levels.

Cyclic stretch-bending: Mechanics, stability and formability

Cyclic stretch-bending has been studied using the so-called Continuous-Bending-under-Tension (CBT) test. This is a modified tensile test where the specimen is subjected to repetitive bending at the same time. A wide variety of materials have been tested this way. A simple mechanical model is presented incorporating non-constant bending radius and cyclic material behaviour that is verified experimentally. The increased stability allowing high levels of uniform straining is discussed, a stability parameter is presented. The mechanisms of failure are discussed as are the different types of fracture that are observed. The overall conclusion is that many observed results are determined by the test method, and are as such independent of the material tested.

Incremental forming by continuous bending under tension—An experimental investigation

The Continuous Bending under Tension (CBT) test has been applied to study aspects of incremental forming. Effects of experimental conditions like speed and bending angle have been studied in particular. The results illustrate an essential aspect of incremental sheet forming (ISF): localized deformation. The actual bending radius is the most important influencing factor and this turns out to be controlled by both the pulling force and the bending angle (depth setting). Material thickness had only a minor effect. The maximum elongation before fracture of mild steel was significantly better than that of aluminium. The material is subjected to additional repetitive bending; this does affect material behaviour in general. The aspects of bending under tension as a governing mechanism in incremental sheet forming are discussed.