Incremental Bending Research Articles

Flexible manufacturing chain with integrated incremental bending and Q-P heat treatment for on-demand production of AHSS safety parts

This work proposes a manufacturing chain that combines the two key technologies Incremental Swivel Bending (ISB) and Quenching and Partitioning (Q-P). While incremental forming methods offer chances for production flexibility, strain hardening often exhausts material’s ductility. Heat treatment processes can potentially renew the material’s capability to deform. Q-P, in particular, not only erases the influence of cold forming but also delivers advanced high strength properties (AHSS). Q-P has already been proposed for the incorporation of hot forming methods; however, integration methods for cold forming techniques represent a key research gap. The combined effects of cold forming of a Q-P suitable material in its initial pearlitic structure and subsequent Q-P heat treatment are investigated in this paper. Profiles of various cross sections are manufactured from a low alloyed, carbon enriched (42SiCr) steel and bent to various geometries by the flexible ISB process. An analytic model to determine the strain distribution caused by ISB is developed and validated within the bent areas by considering strain maps, local material properties and curvature distributions. Furthermore, material properties resulting from different Q-P treatments on the previously bent sections are evaluated with respect to accelerated heating gradients from quenching to partitioning. Tensile test specimens, locally extracted from the bent arcs, confirm the elimination of any remaining strain hardening after the heat treatments. Moreover, due to considerably high ductility at high tensile strength levels, the evaluation of energy dissipated during tensile tests demonstrates the Q-P treated material’s high potential to absorb energy even at elevated strain rates. The suggested manufacturing chain, combining ISB and Q-P, enables high production flexibility and delivers advanced high strength properties for the production of lightweight components, suitable for energy absorption at high strain rates, e.g. crash structures in cars.

A Semianalytical Model for the Determination of Bistability and Curvature of Metallic Cylindrical Shells

Bistable metal shells with a fully closed unfolded geometry are of great interest as lightweight construction parts which could be transported without housing and unfolded at the construction place. In order to achieve the effect of bistability in metallic shells, residual stresses with a specific distribution along the shell thickness are necessary. These residual stresses can be introduced in bending processes. The tools with specific bending radii are used to influence the curvature of the shell in the different stable states and thus determine whether a completely closed profile can be achieved. In addition to the forming process, the shell thickness and the shell material have an effect on the achievable geometries and stability. In order to manufacture bistable metallic cylindrical shells from different materials and shell thicknesses, it is necessary to be able to determine a promising process sequence and corresponding bending radii in advance. For this reason, this article presents a semianalytical model for the calculation of bistability and final curvatures. This model is applied to an incremental die-bending process using two bending operations with bending radii of 6 to 12 mm and a 0.2 mm thick steel shell of grade 1.1274 (AISI 1095). The calculation results show that bistability cannot be reached for all combinations of the two bending radii. Moreover, the model indicates that a bistable and fully closed shell is only achieved for a bending radii combination of R1 = 6 mm and R2 = 6 mm. With the aim of model verification, experiments with a closed-die incremental bending tool were performed. Calculated and experimental results show good correlation regarding bistability and curvature. In addition, X-ray diffraction measurement of the residual stresses shows a good qualitative agreement regarding the calculated and experimental results.

Structural health assessment of prefabricated concrete elements using Acoustic Emission: Towards an optimized damage sensing tool

Abstract The design feasibility and structural integrity of prefabricated construction elements is assessed (concrete cantilever slab carrying thermal insulation break reinforced with steel/basalt rebars and ultra-high-performance concrete) by performing incremental bending tests. The developed damage mechanisms govern failure, therefore their identification appears crucial: concrete cracking, rebars-concrete interface debonding, rebars slippage, concrete crush under compression. However, the complex fracture cannot be effectively characterized by measuring the deterioration of the mechanical properties. In this study, Acoustic Emission (AE) is applied to highlight internal damage progress. It is found based on cumulative AE hits distribution under incremental load and considering the Kaiser and Felicity effects, damage initiation is identified. The damage severity is quantified using Load/Calm ratios. AE sources localization identifies the moment at which the rebars plastically deform; debond from concrete; slip and when concrete cracks and crushes under compression. The damage mechanisms are discriminated based on acoustic signal features distribution as damage evolves (specifically, average frequency and the ratio of rise time over amplitude). It is shown that AE appears an essential and accurate monitoring method that fully characterizes the damage on complex structures. The promising results open new horizons towards the design of fully autonomous prefabricated elements carrying a smart on-line AE sensing tool.

Multi-stage incremental bending to form doubly curved metal plates based on bending limit diagram

Forming complex metal plates is important for many industrial sectors such as shipbuilding, chemical engineering, shell structure construction, etc. This paper presents a multi-stage incremental bending method to form doubly curved metal plates, in which the workpiece is formed by bending operations step after step. The multi-stage bending strategy is adopted to avoid excessive bending and cross-deformation. The concept of bending limit diagram is developed based on the von Mises criterion to guide the bending operation. To validate the new method, the saddle shape metal plates have been formed using a custom-built incremental bending machine. The experimental results show that the new method can be used to form doubly curved metal plates. Forming defects such as plastic hinge and punching dimple can be eliminated by multi-stage bending strategy.

Production of bistable fully closed metallic shells by introducing residual stresses during bending processes

Bistable fully closed metallic shells are of great interest as lightweight construction components, which could be transported without enclosure and unfolded at the construction place. The effect of bistability in metallic shell structures is achieved by a specific distribution of residual stresses over the shell thickness, which can be introduced by bending processes. So far no fully closed structures have been achieved in metallic shells. Hence, the aim of this paper is the identification of suitable bending radii combinations in order to produce a bistable shell with a fully closed tube shape in the deployed state. Therefore, a Finite Element (FE) model was established to determine whether bistability occurs in dependence on the bending radii. The numerical model indicates, that bistability can be achieved for 0.2 mm thick spring steel 1.1274 (AISI 1095) by applying two die-bending operations with small bending radii of 6–8 mm. Accompanying experiments with a closed-die incremental bending tool were performed. A good transferability of the numerical results regarding the occurrence of bistability and the calculated residual stresses is shown. For the second aspect, X-ray diffraction measurements of residual stresses were used for comparison. Finally, the present work presents a process route to generate a bistable shell with a fully closed tube shape structure in the deployed state, having a radius of 40.95 mm.

Experimental studies of adhesive joints in four-point static bending

A new type of specimens, being the subject of the patent application in Poland, was used in the studies. The sample consists of two beams: top one (longer) made of structural material such as steel, aluminium alloy and bottom one (shorter), which is part of the glass, which is tempered, layered or traditional glass. The top beam may have any cross-section, e.g. rectangular, square, c-shaped, etc., which is characterized by different values of the moment of inertia. The bottom beam, meanwhile, is made of rectangular glass. The tests were performed under four-point bending conditions at monotonic incremental bending moment values on the Instron 5965 durability machine. The sample allows bending tests to be performed at bending loads while maintaining a constant bending moment in the glued joint area. The new type of specimen has been developed to allow for the testing of glued joints of different structural rigidity, with special emphasis on the steel-glass, aluminum-glass, steel-aluminum and similar connections. The experimental study of the durability of adhesive joints under four-point bending conditions with the monotonic incremental bending moment allows to determine the values of stresses, whose value is related to initiation of damage of the tested joint.

Experimental studies of glued Aluminum-glass joints

Glued steel-glass or aluminum-glass joints are to be found, among other things, in vehicles (cars, buses, trains, trams) as windscreen assembly pieces for the supporting structure. For the purposes of the experiments, samples were made in which the top beam was made of the AW-2017A aluminum alloy and the bottom beam was made of thermally reinforced soda-lime glass whereas the glued joints were made of one-component polyurethane glue Körapur 175. The tests were performed under four-point bending conditions at monotonic incremental bending moment values on the Instron 5965 durability machine. The experimental study of the durability of glued joints under four-point bending conditions with the monotonic incremental bending moment allows to determine the values of stresses, whose value is related to initiation of damage of the tested joint.

Experimental and numerical study on one flexible incremental bending process

In the shipbuilding industry, the traditional manufacturing method for the complicated ship hull metal plates is line heating method, which is not only labor intensive but also inefficient. Considerable efforts have been taken to develop the new automated manufacturing processes at a reasonable quality, efficiency and price during the last few decades. This paper presents a novel flexible forming process called incremental bending to achieve complicated plates. In this method, the initial metal plate is supported by some rotatable hydraulic cylinders and the punches move according to the given path, which is decided by minimum energy method and springback compensation method, to achieve the final plate. Taking one variable curved metal plate as one example, this paper investigates the formability of this novel process based on experiment and numerical simulation. During the numerical simulation, the implicit method is used and the gravity of the plate is considered mainly for the automatic positioning of the plate during each springback process. Results show that the incremental bending process can achieve variable curved metal plates with good accuracy, high efficiency, and relatively small punch load. In comparison with the existing methods, the new process has greater potentiality in the ship hull manufacturing industry.

Springback law study and application in incremental bending process

One incremental bending process has been proposed for manufacturing the complex and thick ship-hull plates. The accuracy and efficiency for this novel process is mainly dependent on the loading path and thus the unavoidable springback behavior should be considered in the loading path determination. In this paper, firstly, the numerical simulation method is verified by the corresponding experiment, and then the springback law during the incremental bending process is investigated based on numerical simulation, and later the loading path based on the springback law and the minimum energy method is achieved for specific machining shape. Comparison between the designed curve based on springback law and the new simulation results verifies that the springback law obtained by numerical simulation is believable, so this study provides a new perspective for the further research on incremental bending process.

Experimental study of springback behavior in incremental bending process

Manufacturing complicated and thick ship-hull plates at reasonable price and efficiency has always been a challenge in shipbuilding industry. One widely used method is line heating process, which is not only labor intensive but also ineffective. Recently many flexible forming processes have been developed while they all have avoidable disadvantages. In order to reduce the forming force, our group proposed one flexible incremental bending technology, during which the flat blank was bended into the required shape by gradually stamping at various positions. During the incremental bending process, the minimum energy method is used to determine the proper loading path. However, there is huge springback phenomenon after each bending. The shape change after each springback process should be considered in the loading path determination to make sure the accurate shape can be obtained.Here in this paper, the springback behavior of the carbon steel Q235 during the incremental bending process was investigated by experiments. In the first stage, the flat plates were bended several times at the same position while the final bending depth is the same, the final bending shapes after springback were compared to each other to investigate the influence of loading path on the springback behavior of the material. In the second stage, the flat plates were bended once at various positions and bending depths to study the springback behavior in one step bending process. It was found that loading path has little influence on the springback behavior, so during the incremental bending process, the springback law obtained from each single bending process can be used to combine with the minimum energy method to obtain the proper punch moving trajectory.

A new flexible sheet metal forming method of incremental bending

Flexible sheet metal forming is important for many industrial sectors such as shipbuilding, chemical engineering and shell structure construction. For thin and small sheet metal, incremental forming can be used. But for thick and large sheet metal, it is not applicable. We present a new flexible sheet metal forming method of incremental bending based on minimum energy principle. In the presented method, sheet metal is formed by continuous bending operations. The minimum energy principle is used to calculate bending trajectory. In this paper multiple tools bending strategy is presented to improve forming efficiency. The presented method is tested in a customized prototype. A kind of variable curvature sheet metal has been formed. The experimental results show that the presented method is effective and efficient.

A Novel Incremental Sheet Bending Process of Complex Curved Steel Plate

Bending complex curved steel plates for constructing ship hull has long been a challenge in shipbuilding industry. This paper presents a novel incremental bending process to obtain complicated curved steel plates by a series of sequential and layered punches. Taking advantage of this process, the blank plate that is fixed and held by a flexible supporting system can incrementally be bent into the target shape by a press tool along a planned tool path step by step and layer by layer. Acting as a “lower die,” the flexible supporting system can provide flexible and multifunctional supports for the work piece during the forming process, whose four general motion modes are demonstrated in this paper. Meanwhile, the procedures of tool path planning and forming layering are also explained in detail. In addition, aiming at different motion modes of the flexible supporting system, two springback compensation methods are given. Furthermore, according to the forming principle presented in this paper, an original incremental prototype equipment was designed and manufactured, which is mainly composed of a three-axis computer numerical control (CNC) machine, a flexible supporting system, and a three-dimensional (3D) scanning feedback system. A series of forming experiments focusing on a gradual curvature shape were carried out using this prototype to investigate the feasibility and validity of this forming process.

Responses of pipeline to side-by-side twin tunnelling at different depths: 3D centrifuge tests and numerical modelling

The tunnel-pipeline-soil interaction has gotten momentum recently, but most of the previous studies were focused on the responses of pipeline to single tunnelling and simplified the interaction as a plane strain problem. Up to date, there is still a lack of research on the responses of a pipeline to twin tunnelling at different depths. This study presents a series of three-dimensional centrifuge model tests which were performed to investigate the effects of side-by-side twin tunnelling at varying depths on an existing buried pipeline in dry sand. In the centrifuge model tests, both the volume loss and the weight loss were simulated in-flight using a novel technique. To gain better understanding of the tunnel-pipeline interaction, the centrifuge tests were back-analysed numerically based on an advanced hypoplastic model, which accounts for strain- and path-dependent soil stiffness at small strains. Based on the physical and numerical investigations, it was revealed that the distribution of pipeline maximum shear force is greatly affected by the second tunnelling. The shifted distance of the pipeline maximum shear force induced by the second tunnelling in the case of cover-to-diameter ratio (C/D) of 2 was 80% larger than that in the case of C/D of 4. The major influence zone on the pipeline induced by tunnelling is within 1.2D (tunnel diameter) ahead of and after the tunnel face. When the pipeline is located within the major influence zone due to tunnelling, there was a sharp increase in the relative pipeline-soil stiffness and the additional bending strain in the pipeline. The tests results were compared with the results from a dimensionless chart and an analytic solution in the literature. Although these two approaches can reasonably predict the bending strain in the pipeline due to the first tunnelling, they fail to accurately capture the incremental bending strain solely caused by the second tunnelling. This implies that one should be cautious in using the superposition principle to predict the bending strain in a pipeline subjected to twin tunnelling. Because the deviatoric strain of the soil around the pipeline was increased by the second tunnelling, leading to further degradation of the soil stiffness around the pipeline.

Incremental Bending of Three-Dimensional Free Form Metal Plates Using Minimum Energy Principle and Model-Less Control

Bending 3D free form metal plates is a common process used in many heavy industries such as shipbuilding. The traditional method is the so-called line heating method, which is not only labor intensive but also inefficient and error-prone. This paper presents a new incremental bending method based on minimum energy principle and model-less control. First, the sheet metal is discretized into a number of strips connected through virtual springs. Next, by applying the minimum energy principle, the punching and supporting points are calculated for the strip. Then, the bended shape of the strip is computed based on the beam bending theory. This process is continued until the final shape is reached. To compensate the bending error, the computer vision-based model-less control is applied. The computer vision detects the bending error based on which additional bending steps are calculated. The new method is tested in a custom build incremental bending machine. Different metal plates are formed. For a metal plate of 1000 × 800 × 5 mm3, the average bending error is less than 3 mm. In comparison with the existing methods, the new method has a number of advantages, including simple, fast, and highly energy efficient.

A NEW TYPE OF INCREMENTAL BENDING METHOD FOR COMPLICATED CURVED SHEET METAL

A new type of incremental bending process for complicated curved sheet metal is proposed in the paper. The blank sheet is bended incrementally step by step. To validate the forming process, an incremental bending prototype is designed and manufactured, which was composed of a 3-DOF working table, a flexible supporting system and a 3D scanning system. The forming trajectory based on the theory of the minimum energy is planned according to the designed model of the sheet metal. Several experiments are carried out and the designed part is manufactured, which validated the proposed incremental forming method was successful.

Mechanical behaviour of carbon nanotubes under combined twisting–bending

The main objective of this paper is to investigate the mechanical behaviour (strength and stiffness) of carbon nanotubes (CNTs) under combinations of bending and twisting. In order to achieve this goal, molecular dynamics (MD) simulations of bended and twisted CNTs are performed. The LAMMPS code is used, the AIREBO potential is considered for CC bonds, the temperature is kept at 300K and incremental bending and twisting rotations are imposed to the CNT. Two types of CNTs are analyzed, including zig-zag (8,0) and armchair (5,5) CNTs with similar radius and length. The CNTs are also analyzed for pure bending and pure twisting. The main results are shown in the form of diagrams of energy and moment against imposed rotations. Some relevant conclusions are drawn concerning the influence of loading (bending and twisting) on the stiffness, strength and failure of CNTs: namely, it is concluded that armchair CNTs possess higher strength and fracture toughness under twisting–bending loading than zigzag CNTs; additionally, it is found that both CNTs (armchair and zigzag) still support moderate-to-high bending levels without failure after being extremely twisted and torsionally buckled, even for twisting angles four times those corresponding to torsional buckling; finally, the results prove that CNTs, mostly armchair ones, exhibit very high twisting–bending stiffness and strength and can be used with confidence as torsional spring elements in nanoelectromechanical systems (NEMS).

BP artificial neural network modeling for accurate radius prediction and application in incremental in-plane bending

Incremental in-plane bending (IIB) is a new and advanced flexible manufacturing technology for small-lot production of strip with various bending radii. The strip of sheet metal is bent incrementally by a beating inclined punch. The bending radius is strongly affected by mechanical properties of the material, geometry of the strip, and processing parameters. It is difficult to predict the bending radius due to the complex synergistic effects of the controlling parameters. How to predict the bending radius accurately has therefore become a key point to be urgently solved in the development of this advanced forming technology. In this paper, a model based on a back propagation neural network (BPNN) is introduced to reveal the relationship of bending radius with angle of die α, indentation s, pitch p, and width of strip w. Out of 14 different BPNN architectures trained, the 4-9-9-1 BPNN with two hidden layers having nine neurons trained with the Levenberg-Marquardt algorithm (trainlm) is found to be the optimum network model, and the prediction error is less than 2 % on average. Otherwise, a 1-9-9-4 reverse BPNN is developed to build the processing window for a given bending radius. Meanwhile, taking section moment of inertia I as a quantitative index of forming stability, α, p, s, w0 are optimized as design variables in order to make objective functions of I maximized simultaneously. Finally, to verify its predictive capability, the present approach is applied to a case study, and the optimal combination of parameters for stable forming during IIB is obtained.

Experimental study into the correlation between the incremental forming and the nature of springback in automotive steels

Bending in a V-die has been well covered in the literature and the results have been used to indicate the out-come of bending in cold roll forming. However, recent work comparing springback between roll forming and single step bending has found lower springback in the roll forming process compared to single step bending. Roll forming is an incremental bending process and in this study a V-section was formed in a single operation and in multiple steps and the springback determined. The springback in V-die forming was significantly reduced by incremental forming. This suggests that the lower springback determined in roll forming compared to single step bending may be related to the incremental nature of the roll forming process.

Study of Cold Rotary Forming by Using Rigid-Plastic Finite Element Method

In this project, a rigid plastic Finite Element Method (FEM) simulator was developed for simulating the plastic flow material in rotary forming. The simulation result was able to predict the cup height of the final product up to an accuracy of 80% and the localized deformation which leads to failure in the deformation process. The workability of the aluminum disc was also studied to understand the material flow behavior under incremental bending and flow forming by utilizing a lathe machine.

Research on Bending Process of Integral Panel Skins

With the advantages such as light weight, high structure intensity and good rigidity, integral panel skins have been widely applied in aerospace industry. The forming process of mechanical milling-filling auxiliary roll bending and the incremental bending of integral panel skins were proposed based on the finite element method and the experiments technique. The research shows that the shapes of panel skins were binded by the ribs plastic deformation. The roll bending was suitable to the panel skins with the low ribs or the simple structure. The incremental forming was suitable to the panel skins with the high ribs or the involutes structure.