Stretch Bending Tests Research Articles

Effects of Non-Associated Flow Rule on AHSS Shear Fracture

Recently, advanced high strength steels (AHSSs) have been widely applied to the structural parts of vehicles thanks to their good combination of strength and ductility. When one makes parts with AHSSs, however, fractures in sharp corners of the parts are frequently observed below forming limit, which is normally defined by strain based FLC(Forming Limit Curve). This phenomenon is well-known as “Shear Fracture”. Recent researches point out that additional numerical techniques should be considered in order to predict it accurately. Kim et al. [1] suggested that shear fracture can be predictable with continuum-based finite elements rather than conventional shell elements, and more constitutive informations for large strain thermo-mechanical simulation are needed to improve accuracy. Luo and Wierzbicki [2] showed that shear fracutre in stretch-bending test can be fully characterized by proposed MMC(Modified Mohr-Coulomb). This paper shows that solid-shell approach based on hyper-elastoplastic material model enables one to properly predict shear fracture pheonomenon without any special failure criteria. Furthermore, the effects of non-associated flow rule on shear fracture will be also discussed with several numerical examples.

Determination of uniaxial large-strain workhardening of high-strength steel sheets from in-plane stretch-bending testing

This paper proposes a novel approach to determining the large-strain uniaxial stress-strain curves of sheet metals by using in-plane stretch-bending test data. In this approach, a stress-strain curve given by uniaxial tension is extrapolated by a constitutive equation of the combined Swift and Voce laws. The weighting coefficient in the combined Swift-Voce equation is determined by minimizing the difference in the stretch-bending mechanical responses, i.e., punch stroke (or load) vs. maximum bending strain, between the finite element (FE) simulation and the corresponding experimental result. This method is verified by performing experiments on four types of high-strength steel (HSS) sheets (the dual-phase type 590Y, 780Y, and 980Y and the precipitation type 590R). For all sheets, stress-strain curves are determined for strain levels of more than twice the achieved uniaxial-tension uniform elongations. The effects of sheet anisotropy and the Bauschinger effect in this method are discussed. The determined workhardening characteristic by this method for 590R at a large strain range is found to differ from that under an equi-biaxial stress state, as obtained from the bulge biaxial test.

Stretch bending - the plane within the sheet where strains reach the forming limit curve

Finite element analysis (FEA) was used to model the angular stretch bend test, where a strip of sheet metal is locked at both ends and a tool with a radius stretches and bends the center of the strip until failure. The FEA program used in the study was Abaqus. The FEA model was verified by experimental work using a dual phase steel (DP600) and with a simplified analytical analysis. The FEA model was used to simulate the experimental test for various frictional conditions and various radii of an upward moving tool. The primary objective of the study was to evaluate the concave-side rule, which states that during stretch bending the forming limit occurs when the strains on the concave surface plane of the bent sheet (i.e. bottom plane) reach the forming limit curve (FLC). The verification with experimental data indicates that the FEA model represents the process very well. Only conditions where failure occurred on or near the tooling are included in the results. The FEA simulations showed that the actual forming limit of the sheet occurs when the strains on the bottom plane of the sheet (i.e. concave side of the bend) reach the forming limit curve for high friction and small tool radii. For lower friction and for larger tool radii the actual forming limit occurs when strains on other planes in the sheet (i.e. mid planes or top surface plane) reach the forming limit curve. The implications of these results suggest that care must be taken in assessing forming operations when both stretch and bending occur. Although it is known that the FLC cannot predict the forming limit for small bend radii, the common assumption that the forming limit occurs when the strains for the middle thickness plane of the sheet reach the forming limit curve or that the concave side rule is often made. Understanding the limits of this assumption needs to be carefully and critically evaluated.

Stretch Bending of Z-section Stainless Steel Profile

The stretch bending properties of a new Z-section stainless steel profile were investigated by simulation. The causes of the forming defects, such as section distortions and poor contour precision, were analyzed, and the corresponding controlling methods were proposed. The results show that the main forming defects for the stretch bending of the Z-section profile were the flange sagging, the sidewall obliquing inward, the bottom surface up warping, and the bad contour accuracy; the cross-section distortions were mainly induced by the shrinkage of the sidewall, which could be eliminated by increasing the sidewall height of the profile reasonably; the poor contour precision was mainly due to springback, which could be controlled by modifying the die surface based on the springback amount; for the investigated bending beam, the proper sidewall height compensation was 2 mm, and the suitable die surface modification amount was 1.2 times of the springback amount, when the elongation was 10% of the initial profile length. Stretch bending tests were conducted on a new type of die with adjustable bending surfaces, and high quality components were achieved, which verified the effectiveness of the defect controlling measures.

Erichsen cupping test on thermosetting CFRP sheets

The deformation of carbon-fiber-reinforced plastic (CFRP) sheets consisting of a thermosetting resin and continuous fibers has been investigated. The bending of CFRP sheets was achieved under a suitable forming temperature and strain path. Formability indexes and a forming limit diagram (FLD) are indispensable data for showing the process window of CFRP sheets. There are four formability indexes: bendability, deep drawability, stretchability, and stretch-flangeability. Bendability and deep drawability can be evaluated by conducting a stretch-bending test, and stretchability can be evaluated by conducting an Erichsen cupping test. If the process window can be predicted, the range of applications using CFRP can be expanded. In this study, the Erichsen index of CFRP sheets indicates the stretchability of laminated CFRP structures.

Minimum bending radius of Al-Li alloy extrusions in stretch bending

In this investigation, the attention is focused on the minimum bending radii of 2196-T8511 and 2099-T83 Al-Li alloy extrusions. To predict the failure of Al-Li alloys, sheet and extrusion stretch bending tests are developed, carried out and simulated using finite element model. The theoretical minimum bending radius is introduced to derive a safe lower limit for the bending radius which can serve as a guideline for tool and product design. Stretch bending tests of Al-Li alloys are performed using the three-point bending test and displacement-controlled stretch bending test at room temperature. The finite element model incorporates three-dimensional solid elements and ductile damage modeling. The experimental results show that Al-Li alloy extrusions in stretch bending show three types of failures, occurring at the unbent region near the entrance of the jaws, at the region below the exit of the die and within the region in contact with the die, respectively. Comparison between predicted values and experimental results has been made, a consistent agreement being achieved, reflecting the reliability of the present model. The three types of failure mechanisms which compete with each other are tensile localization failure, die-corner failure and shear failure, respectively. Based on the analytical models, experiments and simulations, it appears that the three distinct failures need to be applied to predict the minimum bending radius and range of failures that can occur with 2196-T8511 and 2099-T83 Al-Li alloy extrusions in stretch bending.

Microstructure-Based RVE Approach for Stretch-Bending of Dual-Phase Steels

Fracture behavior and micro-failure mechanism in stretch-bending of dual-phase (DP) steels are still unclear. Representative volume elements (RVE) have been proved to be an applicable approach for describing microstructural deformation in order to reveal the micro-failure mechanism. In this paper, 2D RVE models are built. The deformation behavior of DP steels under stretch-bending is investigated by means of RVE models based on the metallographic graphs with particle geometry, distribution, and morphology. Microstructural failure modes under different loading conditions in stretch-bending tests are studied, and different failure mechanisms in stretch-bending are analyzed. The computational results and stress-strain distribution analysis indicate that in the RVE models, the strain mostly occurs in ferrite phase, while martensite phase undertakes most stress without significant strain. The failure is the results of the deformation inhomogeneity between martensite phase and ferrite phase. The various appearance and growth of initial voids are different depending on the bending radius.

Stretch bending defects control of L-section aluminum components with variable curvatures

To achieve high-precision bent components for rail vehicles, the stretch bending properties of the common L-section aluminum extrusions with variable contour curvatures were investigated by simulation. The causes of the defects, such as bad die fittingness, cross-section distortion of horizontal plate sagging and low contour accuracy, were analyzed, and the corresponding control methods were proposed. The simulation results demonstrate that die fittingness could be significantly improved by increasing the die elongation. The horizontal plate sagging distortion of L-section aluminum components could be eliminated by modifying the die supporting surface curve, i.e., to reduce the depth from the die’s outer surface according to the shrinkage of the profile’s vertical wall. By applying suitable springback compensation to the bending die geometry, the contour accuracy could be enhanced exponentially. Using the optimal process parameters and defect controlling methods, the stretch bending tests were then carried out, and results indicate that the proposed controlling methods were quite effective and the scale production of high-precision bent parts has been achieved.

Curvature Based Forming Limit Prediction of High-Strength Steel Components with Superimposed Stretching and Bending in the Deep Drawing Process

The state of deformation in deep drawing operations is characterized by superimposed stretching and bending (i.e. stretch-bending). Bending effects, especially for Advanced High Strength Steels (AHSS) are known to influence the material formability. Traditional formability measures such as the Forming Limit Curve (FLC) fail to reliably predict stretch-bending formability. Consequently, to ensure an efficient and economical use of AHSS in the industrial application, current research work is focusing on the reliable numerical prediction of stretch-bending formability of AHSS sheets.Within this work, a phenomenological concept to predict the forming limit (e.g. the onset of necking) in deep drawing processes taking bending effects into account is presented. The proposed concept is based on curvature-dependent (i.e. regarding the principle curvatures κ1 and κ2 of the stretch-bend (convex) sheet surface) forming limit surfaces representing the probability of failure and is calibrated with experimental results from stretch-bending tests and conventional forming test such as a Nakazima test. The results of the phenomenological forming limit criterion are promising and show a more accurate prediction of the drawing depth at failure than the conventional FLC approach. The method contributes also to a probabilistic view on the forming limit of deep drawing parts.

Constitutive modelling effect on the numerical prediction of springback due to a stretch-bending test applied on titanium T40 alloy

Nowadays, numerical simulation by finite element analysis is an essential tool that allows performing virtually sheet metal forming processes, and therefore to reproduce various phenomena such as springback (SB) and necking that are generated by plastic deformation. However, the quality of the model used to represent the mechanical behaviour is a determining factor for the realism of numerical simulations. To perform well, the model must reproduce all the properties of the material such as the anisotropy and the strain hardening induced by plastic deformation. The main purpose of this work is to show, by means of numerical simulations, the influence of constitutive modelling on the prediction of the degree of SB in the case of a stretch bending test. Tests have been carried out on titanium sheets which have a wide range of applications for high tech industries because of specific mechanical and physical properties. At the same time, we have investigated the dependence of some process parameters such as the clamping force on SB. In order to prove the accuracy and reliability of the proposed finite element model, experimental data were used to compare with the numerical results.

Suitable structure of thermosetting CFRP sheet for cold/warm forming

The forming processes at room temperature and under a warm condition have been investigated for carbon-fiber-reinforced plastic (CFRP) sheets consisting of thermosetting resin and continuous fibers used for mass production. While CFRPs consisting of thermosetting resin have the advantage of high strength, to subject them to press forming, in contrast to carbon-fiber-reinforced thermoplastics (CFRTPs), which consist of thermoplastic resin. When CFRP sheets are formed into the desired shape through plastic deformation, a higher-strength structural material easily applicable to mass production is obtained. To improve the formability of thermosetting CFRP sheets while retaining their strength, a suitable structure allowing plastic deformation under warm condition is proposed. The tensile stress obtained by a tensile test and the bending properties obtained by a stretch-bending test indicate the strength and formability, respectively. A stretch-bending test is a bending test in which a tensile load is placed on a sheet, and it has the characteristics of a bending test and a deep drawing test. A suitable structure containing prepreg layers to allow plastic deformation based on the relationship between the bending load and the results of specimen observations is revealed.

Shear Fracture of Advanced High Strength Steels

Failure experiments were carried out through a stretch-bending test system for advanced high strength steels, i.e. dual-phase (DP) steels and martensitic steels (MS). The die radius in this system was designed from 1 to 15 mm to investigate the failure mode under different geometries. Two failure modes were observed during the experiments. As a result, critical relative radii (the ratio of inner bending radius R to sheet thickness t) for DP590 and DP780 steels were obtained. The stretch-bending tests of DP980 display some trends unlike DP590 and DP780 steels, and curve of DP980 in different thicknesses does not coincide well. High blank holder force exhibits more possibility of shear fracture tendency than low blank holder force. The unique character of high strength martensitic steel (1500MS) is that no shear fracture is found especially over small bending radius (R = 2 mm) under the same experimental conditions. Microstructure analysis indicates that there are obviously elongated grains on shear fracture surface. It shows smaller diameter and shallower depth of the dimples than the necking failure.

Development of a Plane Strain Tensile Geometry to Assess Shear Fracture in Dual Phase Steels

A geometrically modified sample capable of generating a triaxial stress state when tested on a standard uniaxial tensile frame was developed to replicate shear fractures observed during stretch bend tests and industrial sheet stamping operations. Seven commercially produced dual phase (DP) steels were tested using the geometrically modified sample, and the modified sample successfully produced shear fractures on a unique shear plane for all steels. For each steel, void densities were determined, based on metallographic analyses, as a function of imposed displacement. Microstructural properties of ferrite and martensite grain size, martensite volume fraction (MVF), retained austenite content, Vickers hardness, average nanoindentation hardness, average ferrite and martensite constituent hardness, and tensile properties were obtained in order to evaluate potential correlations with void data. A linear correlation was observed between Vickers hardness and the average nanoindentation hardness, verifying the ability of nanoindentation to produce data consistent with more traditional hardness measurement techniques. A linear relationship was observed between the number of voids present at 90% failure displacement and the martensite/ferrite hardness ratio, indicating that a decrease in relative hardness difference in a microstructure can suppress void formation, and potentially extend formability limits. The void population appeared independent of MVF, grain size, and tensile properties suggesting that constituent hardness may be a dominant parameter when considering suppression of void nucleation in DP steels.

Critical analysis of necking and fracture limit strains and forming forces in single-point incremental forming

Single-Point Incremental Forming (SPIF) is an emerging manufacturing process especially suitable to produce small batches of metal parts. Moreover, the enhanced formability of metal sheets deformed by SPIF makes this technology useful to those industrial applications requiring high deformation levels. In this sense, the precise setting of limit strains in SPIF in relation to the conventional formability limits of the material, as well as the influence of the process parameters on these strains, are essential variables to understand how and how much can be deformed the metal sheets in real production. On the other hand, the forming force in SPIF is an essential variable, especially for the design of dedicated equipment or for the safe use of adapted machinery. This paper revisits failure in SPIF by means of an experimental analysis of the influence of process parameters, such as the tool diameter, the spindle speed and the step down, on the formability in SPIF (spifability) of AISI 304 metal sheets, studied in the light of circle grid analysis. The work also involves the independent determination of conventional formability limits by necking and fracture under laboratory conditions by using stretching tests (Nakazima tests), in conjunction with stretch-bending tests performed in order to quantify the influence of the bending induced by the tool radius. Failure strains are experimentally obtained and compared in stretch-bending and SPIF tests, being the failure mode discussed in each case. Finally, the axial forming force evolution was recorded with the aim of analyzing the range of process parameters that would guarantee the safely utilization of the non-dedicated process equipment.

Study on Instability and Forming Limit of Sheet Metal under Stretch-Bending

Under stretch-bending conditions, a significant tensile stress gradient through sheet thickness is induced, especially for a small punch radius. The traditional instability theories were developed assuming a uniform tensile stress / strain distribution through thickness; hence, may lead to unreliable prediction of stretch-bending formability. In this study, the instability behavior of sheet metal under stretch-bending is analyzed via FE-simulation of an Angular Stretch-Bend Test (ASBT). In order to reflect the influence of bending, contact normal stress etc., solid elements are used in the simulation. Three deformation stages are identified: (a). stable deformation; (b). strain localization through sheet thickness; (c). localized necking. Based on the instability characteristics, a localized necking criterion is proposed for predicting forming limits of sheet metal under stretch-bending. By combining the proposed criterion and solid element simulation, good agreement between numerical and experimental results is indicated. This work provides a new approach for predicting stretch-bend formability with sufficient accuracy and convenience.

New approaches to detect the onset of localised necking in sheets under through-thickness strain gradients

The standard ISO 12004-2:2008 provides a position-dependent methodology to estimate the limit strains in Nakazima- and Marciniak-type tests. However, this method is not applicable, at least in its current formulation, when there are significant strain gradients across the sheet thickness, such as when using relatively small punch radii or in stretch-bending operations very commonly in industrial practice. This paper analyses two physically-based methodologies, a time-dependent method and a time-position-dependent method (called here flat-valley method), to detect the onset of necking and to evaluate the limit strains under significant strain gradients through the sheet thickness. The digital image correlation (DIC) technique is used to compute the displacement and strain evolutions at the outer surface of the tested specimens using the commercial software ARAMIS®. A detailed analysis and validation of both methodologies, and comparison with other local methods recently published in the literature, are carried out in the light of a series of Nakazima tests and stretch-bending tests for different cylindrical punch radii in AA7075-O.

Deformation Scenarios of Combined Stretching and Bending in Complex Shaped Deep Drawing Parts

Abstract. Bending effects, especially for Advanced High Strength Steels (AHSS), are known to influence the material formability when stretching and bending is combined in sheet forming. Traditional formability measures (e.g. the conventional forming limit curve (FLC)) fail to reliably predict formability when bending is added. Consequently, in order to take full advantage of the available forming potential of AHSS sheets in industrial applications and to ensure a reliable failure assessment at the same time, current research is focusing on the experimental characterization and modeling of the influence of bending effects on the AHSS sheets formability in forming scenarios of combined stretching and bending. It is expected that aside parameters such as bending radius or strain ratio, individual deformation scenarios of combined stretching and bending may affect the material formability too. Due to tool geometry and the resulting material flow in deep drawing various complex scenarios of combined stretching and bending can occur. For example, a material element is subjected to a complex deformation history of in-plane stretching with subsequent stretch-bending over a cylindrical tool contour, followed by unbending under tension. Another material element of the same drawing part presumably starts also with in-plane stretching but is consequently stretch-bent over a doubly curved tool geometry. Consequently, comprehensive knowledge on the stretch-bending deformation scenarios prevailing in deep drawing is crucial for a more reliable formability assessment. This work aims to identify and characterize the stretch-bending deformation scenarios to occur in different complex deep drawing parts (i.e. B-pillar, cross-die test) and small scale tests (i.e. Angular Stretch-Bend Test (ASBT)). For this reason, this investigation uses an innovative approach recently developed by some of the authors and published elsewhere to categorize the stretch-bending scenarios in industrial deep drawing processes. The approach consists of a stretch-bending categorization schema and a procedure to categorize the forming scenarios in deep drawing parts using data of finite element (FE) simulations. Results of the categorization can directly be plotted on the FE mesh of the deep drawing part (i.e. map type plot of deformation scenarios). The categorization approach mentioned uses results of conventional shell-type FE forming simulation and is therefore applicable in an industrial environment. The FE forming simulation results of the parts selected were analyzed using the stretch-bending categorization approach to identify which stretch-bending scenarios occur in deep drawing parts, to quantify which scenarios to prevail and to show that the conventional ASBT does not represent all the relevant deformation scenarios that prevail in typical deep drawing parts. Furthermore, with the use of experimental observations from real part forming, the stretch-bending scenarios which are the most critical (i.e. the scenarios under which failure occurs) in forming the cross-die geometry are identified. Results of these analysis are presented and discussed in detail.

Experimental Study of the Formability of H240LA Steel Sheets under Stretch-Bending Conditions

The present work discusses the effect of the punch radius on the formability of H240LA steel sheets of 1.2mm thickness. A series of hemispherical punch tests (Nakazima tests) and stretch-bending tests with cylindrical punches of different diameters have been carried out in order to characterize the influence of the strain gradient in the sheet failure. The limit strains have been obtained using a recently proposed time-dependent methodology, which is applicable not only in conventional Marciniak and Nakazima tests, but also in situations with a severe strain gradient through the sheet thickness. The results show that formability of H240LA steel sheets increases as the t0/R ratio decreases.

The soft impact response of composite laminate beams

The quasi-static and dynamic responses of laminated beams of equal areal mass, made from monolithic CFRP and Ultra high molecular weight Polyethylene (UHMWPE), have been measured. The end-clamped beams were impacted at mid-span by metal foam projectiles to simulate localised blast loading. The effect of clamping geometry on the response was investigated by comparing the response of beams bolted into the supports with the response of beams whose ends were wrapped around the supports. The effect of laminate shear strength upon the static and dynamic responses was investigated by testing two grades of each of the CFRP and UHMWPE beams: (i) CFRP beams with a cured matrix and uncured matrix, and (ii) UHMWPE laminates with matrices of two different shear strengths. Quasi-static stretch-bend tests indicated that the load carrying capacity of the UHWMPE beams exceeds that of the CFRP beams, increases with diminishing shear strength of matrix, and increases when the ends are wrapped rather than through-bolted. The dynamic deformation mode of the beams is qualitatively different from that observed in the quasi-static stretch-bend tests. In the dynamic case, travelling hinges emanate from the impact location and propagate towards the supports; the beams finally fail by tensile fibre fracture at the supports. The UHMWPE beams outperform the CFRP beams in terms of a lower mid-span deflection for a given impulse, and a higher failure impulse. Also, the maximum attainable impulse increases with decreasing shear strength for both the UHMWPE and CFRP beams. The ranking of the beams for load carrying capacity in the quasi-static stretch-bend tests is identical to that for failure impulse in the impact tests. Thus, the static tests can be used to gauge the relative dynamic performances of the beams.

Formability of Automotive H240LA Steel Sheets in Stretch– Bending Processes

This paper discusses the influence of punch radius on the formability of 1.2mm thick H240LA steel sheets. A series of Nakazima tests with hemispherical punch and stretch–bending tests with cylindrical punches of variable diameter were performed in order to assess the influence of the strain gradient on sheet failure. Necking limit strains were obtained by using a recently reported time-dependent methodology that can be used not only in conventional Marciniak and Nakazima tests, but also in the presence of a severe strain gradient across the sheet thickness. Based on the results, formability in H240LA steel sheets increases with decreasing t0/R ratio. Fracture limit strains in both test series were evaluated by using an optical strain measurement system and measuring specimen thickness after fracture.