Non-linear Strain Paths Research Articles

On the experimental characterization of sheet metal formability and the consistent calibration of the MK model for biaxial stretching in plane stress

Analytical formability models based upon in-plane proportional plane stress loading such as the Marciniak–Kuczynski (MK) model are commonly evaluated against forming limit curves (FLCs) obtained using Nakazima tests with out-of-plane deformation, non-linear strain paths and tool contact. The present study has conducted a comprehensive experimental characterization of a DP1180 advanced high strength steel with relatively low formability to demonstrate that the use of Nakazima FLC data can lead to a physically inconsistent MK model. Although good agreement can be obtained using the MK model with Nakazima data, it is attributed to calibration bias in the hardening model and selection of the imperfection factor. Marciniak tests should be performed to characterize the FLC or process-corrections be applied to the Nakazima FLC to then calibrate the MK imperfection factor. An analysis of the MK framework reveals a false dependence of the imperfection factor on the hardening model due to artefacts in the hardening model calibration. When the Considère constraint for the uniform elongation is imposed during the hardening model calibration, the MK imperfection factor becomes nearly independent of the choice of hardening model. The accuracy of the FLC predicted by the MK model, when appropriately constrained and calibrated in a deterministic manner, was in a reasonable agreement with the experimental data only after accounting for material anisotropy. In contrast, the deterministic Modified Maximum Force Criterion (MMFC) model provided a very close estimate of the experimental forming limit strains of the mildly anisotropic DP1180 using both isotropic and anisotropic yield functions.

Development of a Nakazima Test Suitable for Determining the Formability of Ultra-Thin Copper Sheets

The objective is to propose an accurate method for determining the forming limit curves (FLC) for ultra-thin metal sheets which are complex to obtain with conventional techniques. Nakazima tests are carried out to generate the FLCs of a pure copper and a copper beryllium alloy with a thickness of 0.1 mm. Because of the very small thickness of the sheets, the standard devices and the know-how of this test are no longer valid. Consequently, new tools have been designed in order to limit friction effect. Two different methods are used and compared to estimate the necking: the position-dependent measurement method (ISO Standard 12004-2), and the time-dependent method based on the analysis of the derivatives of the planar strain field. It is shown that the ISO standard method underestimates the forming limit curves. As the results present non linear strain paths, a compensation method is applied to correct the FLCs for the tested materials, which combines the effects of curvature, nonlinear strain paths and pressure. The curvature effect for such thickness and punch diameter on the FLCs is weak. The results show that this procedure enables to obtain FLCs that are close to those determined by the reference Marciniak method, leading to a minimum in major strain that converges to the plane strain state.

Formability analysis of hot-rolled dual-phase steel during the multistage stamping process of wheel disc

For reduction of vehicle weight and higher fatigue property, the hot-rolled dual-phase steel 700DP was developed to produce wheel disc with multistage stamping process. The wheel disc is one critical component of wheel system whose forming precision is extensively concerned. The critical area of the part characterizes nonlinear strain path during the multistage stamping process, which makes it difficult to accurately evaluate the formability through standard forming limit curve (FLC) method. The concept of equivalent plastic strain was reported to be strain path independent. Hence, in this paper, the formability of 700DP steel was studied through tensile test, Nakazima test, and two-step nonlinear formability test. It was certified that the modeling using the equivalent plastic strain–based FLC (ep-FLC) had better agreement with the two-step nonlinear test results. Then based on current die shape, the feasibility of 700DP steel for wheel disc with three-stage stamping process was studied by finite element method (FEM) with both traditional FLC and ep-FLC. Furthermore, the corresponding experiments were conducted for validation. The results show that the location and critical condition for the occurrence of fracture can be more exactly predicted by the ep-FLC compared with the traditional FLC. The process parameters such as the radius of die corner were then optimized by the modeling analysis. Finally, the wheel disc was successfully manufactured with high precision and sound mechanical property.

Characterization of forming limits at fracture for aluminum alloy 6K21-T4 sheets in non-linear strain paths using a biaxial tension/shear loading test

A good knowledge of forming limits at fracture (FLF) for aluminum sheets exhibiting poor formability at room temperature in various non-linear strain paths (N-LSPs) is essential for fully exploiting their forming ability and avoiding their fracture failure. However, the FLF over a wide range of strain states in N-LSPs with pre-strain paths between uniaxial tension and simple shear cannot be determined through the existing two-stage loading test methods. To solve the problem and fill the gap, a new two-stage loading test method employing an optimized butterfly specimen is proposed in this paper, in which strain path changes are controlled by changing biaxial loading conditions acting on the specimen. To validate the capability of the new test method to characterize the FLF in the N-LSPs and investigate the effect of the loading path changes on aluminum 6K21-T4 sheet formability at fracture, different proportional and non-proportional loading experiments using the butterfly specimen obtained from the aluminum sheets are performed. Experimental results indicate that the forming limit strains at fracture over a wide scope of strain states in the N-LSPs can be effectively determined by utilizing the proposed method, thus filling the gap mentioned above. Additionally, four newly developed ductile fracture criteria (a new model proposed in this paper, Hu−Chen 2017, Lou−Huh 2012 and MMC3) together with a non-linear damage accumulation rule (N-LDAR) are employed to forecast the FLF for the 6K21-T4 sheets in the N-LSPs. The results show that both the proposed new model with the N-LDAR and the Hu−Chen model along with the N-LDAR can provide the highest prediction accuracy for the FLF in the N-LSPs. However, the prediction accuracy of the proposed model is much higher that of the Hu−Chen 2017 model for aluminum alloy 5083-O sheets.

Analysis of forming limit behaviour of high strength steels under non-linear strain paths using a micromechanics damage modelling

The formability characteristics of advanced high strength (AHS) steel grades 780 and 1000 subjected to varying forming histories including strain path changes were described by means of a damage modelling approach. The Marciniak tests of the examined steels were firstly carried out to apply pre-strains under various deformation modes. Afterwards, tensile, stretch-bending and hole expansion test of the pre-strained samples were performed. FE simulations coupled with the Gurson-Tvergaard-Needleman (GTN) ductile damage model were conducted to predict failure occurrences of AHS samples in the two-stage forming tests. The damage model parameters were determined under consideration of pre-strain effects on void evolution mechanisms and its interdependencies with microstructure constituents of steels, for which hybrid methods of metallographic analyses, local strain measurement and representative volume element (RVE) simulations were employed. Then, forming limits of the investigated steels in different forming procedures with non-proportional loading paths were characterized. It was shown that the micromechanics damage simulations in combination with the proposed parameter identification scheme could precisely represent the fracture states of steels under such complex forming conditions.

Experimental and theoretical plasticity analyses of steel materials deformed under a nonlinear strain path

In this study, the macroscopic mechanical behavior of extra-deep drawing quality (EDDQ) and transformation-induced plasticity (TRIP1180) steels subjected to nonlinear-strain-path modes of deformation was investigated. For the nonlinear-strain-path experiments conducted on steels, an optimized experimental approach was proposed for deforming the material using two linear loading segments with a single abrupt change in the strain path. A specific medium-scale uniaxial tension specimen was designed for the pre-strain stage. The advantage of this approach over existing methods was that it enabled testing of various strain-path changes for steels with strengths greater than 1 GPa within a reasonable load range. In addition, this approach produced a more uniform strain field, which is important during the pre-strain stage. After the uniaxial pre-strain, various specimens were extracted from the uniformly deformed area for use in experiments involving changes in the strain path. To predict the material behavior under nonproportional loading, the homogeneous anisotropic hardening (HAH) distortional plasticity model was selected. The predictions of the HAH model were compared with the corresponding experimental results to assess the suitability of the constitutive model to accurately reproduce the observed effects of strain-path changes.

Validation of homogeneous anisotropic hardening model using non-linear strain path experiments

We conducted non-linear strain path experiments on a dual-phase steel-sheet sample. The first strain path in each of these experiments, called pre-strain, was always uniaxial tension in the direction transverse to the sheet (90° from the rolling direction). For this purpose, we clamped large tensile specimens in a suitable gripping system and deformed them to produce a sufficiently large area with a uniform strain distribution. We machined new specimens from this area, and we subjected them to different modes of deformation, namely uniaxial tension in different directions from the transverse direction, simple shear, and biaxial tension with different load ratios. In addition, we performed uniaxial tension-compression tests consisting of either one or three full cycles. We obtained the coefficients of a distortional plasticity model—the homogeneous anisotropic hardening (HAH) model—using an inverse analytical identification tool. First, we calibrated two sets of reverse loading coefficients using the tension-compression stress–strain curves obtained with either one or three full cycles. Then we used the 90° tension–45° tension and the 90° tension–0° simple-shear sequences to determine two sets of cross-loading coefficients. We compared the other independent experimental flow curves with simulations using the HAH model with combinations of the different sets of coefficients. These comparisons showed the influence of the selected input data on the calculations of the coefficients calibrated and demonstrated the advantages and limitations of the present HAH model.

Evaluation of the VDA 238–100 Tight Radius Bend Test for Plane Strain Fracture Characterization of Automotive Sheet Metals

Accurate characterization of the fracture limit in plane strain tension of automotive sheet metals is critical for the design and crash performance of structural components. Plane strain bending using the VDA 238–100 V-bend test has potential for proportional fracture characterization by avoiding a tensile instability. The VDA 238–100 V-bend test was evaluated using DIC strain measurement to characterize the plane strain fracture limit under proportional plane stress loading and to evaluate the effect of the VDA pre-straining methodology for ductile alloys upon the material response. The load-based failure criterion of the V-bend test was evaluated with DIC to monitor the development of surface cracking. The influence of the non-linear strain path imposed by the pre-straining procedure for ductile materials was then evaluated for three automotive alloys: an advanced high strength dual phase steel, DP1180, a rare-earth magnesium, ZEK100, and an AA5182 aluminum. A fracture criterion based on the load threshold was reasonable for the three alloys considered. Pre-straining in uniaxial tension prior to plane strain bending affected each alloy differently. The DP1180 was not affected by the non-linear strain path whereas the cumulative equivalent strain for the AA5182 and ZEK100 increased by strains of 0.07 and 0.05 strain, respectively. The non-linear strain path within the VDA pre-straining methodology creates ambiguity in comparing the fracture limits of different materials. The plane strain fracture limit for proportional loading can be readily obtained in the V-bend test with DIC strain measurement.

Predictions of Forming Limit Curves of AA6014 Aluminium Alloy at Room Temperature

The aim of present work is to examine D-Bressan mathematical model to accurately predict the surface limit strains by the critical fast shear stress criterion of conventional AA6014 aluminium alloy sheets, after loading with non-linear strain paths at room temperature. Two kinds of limit strain curves can be plotted in the Map of Principal Surface Limit Strains (MPLS): the surface local necking limit, FLC-N, and the fast shear stress fracture limit curve FLC-S. D-Bressan´s critical shear stress criterion model combined with Barlat´s Yld 2000-2D yield criterion is proposed for theoretical prediction of FLC-S curve in sheet metal forming operations and were compared with experimental outcomes of AA6014. In order to calibrate the Bressan-Barlat macroscopic model, tensile tests at two different strain rates on specimens cut at 0o, 45o and 90o to the rolling direction and bulge test were carried out at room temperature to obtain the material coefficients of plastic anisotropy, strain and strain rate hardening law. Experimental bi-linear strain paths were carried out in specimens with 1.04 mm thickness by pre-stretching in uniaxial stress direction up to two strain levels before performing Nakajima testing experiments for obtaining the forming limit strain curves. D-Bressan critical fast shear stress modelling, combined with Barlat´s Yld 2000-2d yield function, predicted quite well both formability of AA6014 sheet as received and the reduced FLC-S after 10% and 20% uniaxial pre-stretching and bi-linear strain path. Comparisons of experimental FLC-S and predictions with Barlat´s Yld 2000-2D and Hill 79 yield criteria were analyzed and discussed. D-Bressan approach with Barlat´s Yld 2000-2d yield function, for exponent m=6, provided better fitting to the experimental FLC-S curves than D-Bressan model combined with Hill 79 yield function.

Characterization of kinematic hardening with a hydraulic bulge test

The increase of the mapping accuracy in numerical simulations of sheet metal forming processes is an important field of research today. In this context, the modelling of the kinematic hardening plays a major role when it comes to nonlinear strain paths during the forming process. The kinematic hardening leads, beside other effects, to a lower increase of the hardening after a change of the linear strain path. Characterization of the Bauschinger coefficient, an indicator of the portion of kinematic hardening, is normally done with cyclic tension-compression tests. The testing setup is quite challenging in the strain measurement and the specimen handling due to the small geometry and the risk of buckling during compression. Thus, a hydraulic bulge test with an optimized geometry is used for the characterization of the kinematic hardening with a strain path change for the materials DC06 and DP600. According to literature, DC06 has no significant kinematic hardening, while the high strength steel DP600 is sensitive to this hardening mechanism. The strain path change is realized by a secondary element in the die. In the beginning of the test, a conventional circular die induces an equi-biaxial strain. After this preforming step, an elliptical geometry is used to induce plane strain in the specimen. The resulting strain path changes from equi-biaxial to plane strain. The flow curves out of the experiment show the kinematic hardening effect after the strain path change for the DP600 while no change is observable for the DC06. Numerical simulation with the identified kinematic factor should lead to a better mapping accuracy regarding the springback behavior.

Non-linear strain path experiment and modeling for very high strength material

The goal of this work is to develop experimental methods and modeling of very high strength steel sheets subjected to non-linear strain paths. A 1.6 mm thick TRIP steel sheet with a tensile strength of 1180 MPa was investigated. A suitable test for the uniaxial tension pre-strain was developed. A constraint was to achieve an as uniform as possible strain field over the entire gauge area. The pre-strain was conducted on a 500kN MTS tensile testing machine. The geometry of a new specimen was optimized through finite element simulations and experiments on the TRIP1180 material. Then, different types of subsequent specimens were extracted from the pre-strained material by electro-discharge machining (EDM) for the second deformation step leading to a strain path change. The subsequent plastic behavior was measured and modeled using the HAH distortional plasticity model. Comparisons between the experimental and predicted behavior are discussed.

Developments of the Marciniak-Kuczynski model for sheet metal formability: A review

Abstract This paper presents a historical perspective on the development of the model that was initially proposed by Prof. Marciniak in 1965 for calculating the Forming Limit Curves (FLC's) of metallic sheets. Based on a pre-existing geometric and/or structural non-homogeneity of the metallic sheet, it is at present the most widely used framework for the theoretical prediction of FLC’s. This is a consequence of the fact that this modelling approach is well funded from the mechanical and intuitive points of view. By consulting lesser known papers (in English and Polish), the authors revisit the origins of this model that has become commonly known as the Marciniak-Kuczynski (M-K) model in the literature. To put these developments in historical context, the first experimental test for assessing the formability of metallic sheets, the evolution of the Forming Limit Diagram (FLD) concept and the first models for evaluating limit strains (Swift and Hill's models) are summarized as an introduction. The model, initially published in 1965 by Marciniak as sole author, was further developed by Marciniak, Kuczynski and co-workers as explained next in this review. Since then, the following lines of development of the M-K model have been identified and are systematically addressed in this review: implementation of new constitutive equations, polycrystalline and ductile damage models, extending the models to take into account new material or process parameters, taking into account of non-planar stress states and non-linear strain-paths.

A comparative investigation into the influence of the constitutive model on the prediction of in-plane formability for Nakazima and Marciniak tests

The present study investigates the use of both Marciniak and Nakazima tests to generate the forming limit curves (FLCs) for a power-law hardening dual phase steel, DP980, and an aluminum-magnesium alloy, AA5182, that exhibits dynamic hardening and saturation-type behavior. The recently proposed methodology of Min et al. (2016) to compensate for the so-called process effects of non-linear strain paths (NLSP) and contact pressure was evaluated and applied to the Nakazima FLCs to enable comparison with the Marciniak FLCs and the formability predictions of the Modified Maximum Force Criterion (MMFC) of Hora et al. (1994, 2013) for in-plane stretching. An emphasis was placed upon the experimental determination of the constitutive model to plastic strains in excess of 0.5 using tensile and shear tests. A flexible variant of the Hockett-Sherby (1975) hardening model for large strain levels along with constraints upon the hardening model calibration are proposed. It is demonstrated that the choice of hardening model, test data, and the calibration procedure can have a marked influence on the Nakazima process corrections and the predicted FLC using the MMFC model. An extension to the Linear Best Fit (LBF) limit strain detection methodology by Volk and Hora (2011) was developed to account for bend severity and the material hardening response and is compared with the ISO 12004-2 limit strains. If the hardening model is accurately calibrated to large strain levels, the analytical MMFC predictions were in excellent agreement with the process-corrected Nakazima and Marciniak FLCs for DP980. The results for the AA5182 were found to be strongly dependent upon the choice of hardening model that influences both the MMFC and the contact pressure correction.

Forming limit analysis of Mg-2Zn-1.2Al-0.2Ca-0.2RE alloy sheet using ductile fracture models

This work was aimed to experimentally and theoretically investigate the formability of a new magnesium alloy sheet at room temperature. The fracture forming limit diagram was predicted by MMC3 and DF2014 models, where the non-linear strain path effect was taken into account by means of damage accumulation law. In order to obtain the instantaneous values of the stress triaxiality and the Lode parameter during the deformation process, strains tracked by digital image correlation technique were transformed into stresses based on the constitutive equations. The fracture forming limit diagram predicted by the fracture models was compared with the forming limits obtained by ball punch deformation tests. The prediction errors were evaluated by the accumulative damage values, which verified the advantages of ductile fracture models in predicting the forming limits of the magnesium alloy sheets.

Pre-strain effect on twist springback of a 3D P-channel in deep drawing

Abstract With the widely use of high-strength steel sheets in the automotive industry, the twist springback phenomenon of the steel sheets under multi-step forming conditions has received extensive attention. In this work, the Dual Phase steel DP500 is taken as the research object to investigate the complex non-linear elastoplastic behaviors and twist springback under two-step loading paths. The large specimen with a pre-strain of 4% true strain in rolling direction is carried out on a large tensile testing machine, and several specific blanks are extracted from it at different directions for a subsequent P-channel forming. The influence of twist springback associated with the pre-strain is analyzed. The finite element model based on the non-linear elastic model and the homogeneous anisotropic hardening model (HAH) is also established for the springback prediction and stress analysis. The results indicate that the pre-strain has a considerable impact on the twist springback. The non-linear strain path changes resulted from pre-straining not only influence the residual stress but also affect the elastic modulus distribution.

Influence of Pre-Stretching Levels on the Forming Limit Strain and Stress Curves of High Strength Steel Sheet

In this work, Forming Limit Curves (FLCs) of the conventional and pre-stretched High Strength Steel (HSS) sheet grade 440 (SCGA440-45) were investigated. The conventional forming limit curve was experimentally determined by using the Nakajima stretching test. Subsequently, the non-linear strain path FLCs were precisely developed through the Nakajima stretching test after the specimens were pre-stretched in biaxial direction up to several levels on the Marciniak In-plane stretching test. The gained non-linear strain path FLCs were compared with the conventional FLC.Additionally, the experimental Forming Limit Stress Curve (FLSCs) were calculated using the experimental FLC and non-linear strain path FLCs data from investigated steel sheet. The yield criterion Hill’48 was employed in combination with the Swift strain hardening law to describe anisotropic deformation and plastic flow behavior of the HSS sheet, respectively. Hereby, the influence of pre-stretching levels on the experimentally determined the FLCs and FLSCs were examined. The results prove a significant influence of the pre-stretching levels on the both FLCs and FLSCs of the investigated HSS sheet. For a low pre-stretching in biaxial loading the FLCs demonstrated a reduced formability and the FLSCs exhibited the limited stress levels depending on the experimental FLC data.

Effects of Strain Path Changes on the Kinematics and the Intrinsic Dissipation Accompanying PLC Bands in Al-Mg Alloys

Plastic instabilities, such as the Portevin-Le Chatelier (PLC) effect, reduce material ductility and induce surface roughness during sheet metal forming. The formation and propagation of PLC bands have been extensively studied by using the uniaxial tension test. However, this strain state differs from the complex strain paths encountered in most metal forming operations. In this work, the linear and non-linear strain path effects on the kinematics of PLC instabilities are investigated in an AA5086-H111 Al-Mg alloy, at strain rates between 0.1 and 0.5 s− 1. This is the first study on the spatio-temporal distribution analysis of heat produced by PLC bands during non-linear loadings. Non-linear strain paths are generated with an innovative one-step procedure, without unloading. The strain path changes are controlled by the displacements along the two perpendicular directions of a cruciform specimen loaded with a planar biaxial tensile device. For a given linear or non-linear strain path, full kinematic and thermal fields on the specimen surface were characterized by using Digital Image Correlation (DIC) and infrared thermography (IRT). Heat source fields were reconstructed from the temperature fields and the heat diffusion equation. The calorimetric response, which mainly corresponds to the intrinsic dissipation produced by the material, permits the kinematics of PLC bands to be investigated. It is shown that the strain state (uniaxial, plane strain or equibiaxial) strongly affects the formation and propagation of such plastic instabilities. For each strain path, a band typology is clearly identified. For two-step non-linear strain paths, the change in the strain state induces an instantaneous modification of the kinematics. The band kinematics is directly linked to the current strain path and the plastic deformation history does not appear to influence the typology of bands.

Forming limit evaluation by considering through-thickness normal stress: Theory and modeling

For hydroforming process and incremental forming, the plane stress assumption would be invalid to predict material formability. To consider the influence of normal stress on the forming limit strains, the instability perturbation approach proposed by Hu et al. is extended with normal stress. The M–K model with normal stress is chosen to compare with perturbation approach by implementing Hill’48 and Yld2000-2d yield criteria and swift and modified voce hardening laws. To guarantee the convergence of solving process in M–K model with normal stress, the modified increment method is implemented into it. Through comparing the experimental forming limit strains of AA5754-O under plane stress state and the corresponding theoretical values predicted by perturbation approach and M–K model, the suitable yield criterion and hardening law for these two methods are determined. The influences of through-thickness normal stress on the predicted forming limit strains under different stress states are investigated. The results show that forming limit curves (FLCs) in both forms of traditional Forming Limit Diagram (FLD) and equivalent plastic strain (EPS) based FLD (epFLD) enhance with increasing through-thickness normal stress under linear and nonlinear strain paths. The sensitivity of FLD to normal stress is related to the hardening law and the range of strain path in FLD is determined by the yield criterion.

Formability prediction of advanced high-strength steel sheets by means of combined experimental and numerical approaches

Abstract In this work, a group of four capable prediction tools—Classical Forming Limit Curve (FLC), Forming Limit Stress Curve (FLSC), Damage Curve (DC) associating stress-triaxiality with plastic strain and Nonlinear Strain Path Forming Limit Curve (nonlinear strain path FLC) based on an innovative punch originally designed by the Institute for Metal Forming Technology (IFU), University of Stuttgart—are employed to evaluate formability of DP590 advanced high-strength steel (AHSS). The results are subsequently compared. A classical FLC is created with restriction to the Nakajima stretching-test guideline, in which a 100-Tonne universal tensile testing machine is operated to press 1-mm thick DP590 sheets. A nonlinear strain path FLC is generated with an application of an IFU-designed punch to realise nonlinear strain paths under a particular standardised Nakajima testing environment. A stress-based FLC is computationally obtained by transforming the classical FLC, realised in a major-minor strain space, into an FLSC in a principal-stress space using the Hill’48 yield criterion equipped with the Swift strain-hardening law. A DC is arithmetically determined through a combination of the plastic flow rule and Hill’48 yield criterion. To verify and compare those gained curves, both hole-expansion and Fukui stretch-drawing tests are conducted both experimentally and numerically. Regarding the simulation model for numerical investigation, material flow behaviour and deformation anisotropy are modelled by the Swift hardening law and Hill’48 yield criterion, respectively. This research has once again proved that FLSC, DC and nonlinear strain path FLC can obviously identify the forming limit of DP590 AHSS more realistically than the classical FLC can.

Analysis of the failure of H240LA steel sheets subjected to stretch-bending conditions

Industrial sheet metal forming processes with certain complexity involve intrinsically non-linear strain paths, abrupt changes in the local strain ratio (β) due to successive stages during the manufacturing operation and/or the existence of strain/stress gradients through the sheet thickness due to the simultaneous action of stretching and bending. Under these situations, both the use of classical failure criteria assuming a uniform strain/stress distribution across the sheet thickness and the use of strain-path dependent metrics, such as the conventional FLC (Forming Limit Curve), for assessing sheet formability are highly questionable. In this work a failure model for stretch-bending which combines both the use of a path-independent formability diagram, based on the polar equivalent plastic strain curve (epFLC), along with different rules to account for the influence of the stress/strain gradient across the sheet thickness, particularly the Mid-Plane Rule (MPR), Concave-Side Rule (CSR) and Convex-Side Rule (CxSR), is explored. Their capabilities to predict failure by necking in a series of stretch-bending tests over H204LA steel sheets of 1.2 mm thickness are discussed in the light of experimental results. To this end, a 3D calibrated numerical model was used to precisely describe the realistic distribution of stresses/strains across the sheet thickness.