Marciniak Test Research Articles

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.

A type of cruciform specimen applied to evaluate forming limits for boron steel under hot stamping conditions

The ultra-high strength boron steel has been intensively used in the hot stamping process to produce complex-shaped structural components in transportation industries. Forming limit diagram (FLD) is a fundamental and useful tool to evaluate the formability of metallic materials under various forming conditions. Since the standardized Nakajima test and Marciniak test are not applicable to perform formability tests for hot stamping applications due to the complex heating and cooling processes required, an in-plane testing method, in which cruciform specimens are deformed under hot stamping conditions in a Gleeble materials simulator combined with a multi-axial tensile rig to convert an input force to an output biaxial force, has been successfully applied to assess the formability of aluminium alloys at elevated temperatures. However, it is challenging to apply this in-plane testing method for boron steel due to higher nonuniformity of temperature distribution in gauge region of the cruciform specimen at a higher temperature. In this paper, a new type of cruciform specimen, together with a new specimen heating strategy, has been proposed to improve the temperature distribution in the gauge region. The dimensions of the newly-designed cruciform specimen have been optimised by a thermo-electrical finite element model embedded with a UAMP subroutine in ABAQUS to improve the uniformity of temperature distribution in the gauge region. In order to validate the new design of cruciform specimen, biaxial tensile tests were conducted under hot stamping conditions by using the in-plane testing method.

Laser-induced toughening inhibits cut-edge failure in multi-phase steel

The as-cut microstructures and the subsequent microstructural deformation characteristics of dual-phase steel specimens were analyzed using in-situ biaxial Marciniak tests, microscopic digital-image-correlation and nano-indentation, for two industrially relevant cutting processes: laser cutting and blanking. Interestingly, the strain-to-failure of the former is almost twice that of the latter, even though microstructural damage initiates twice as early (at 8% strain) in the ∼60 µm-thick, fully-martensitic surface layer of the laser-cut affected zone. However, its ∼145 µm-thick, tempered-martensite sub-surface layer provides the toughness to delay micro-damage propagation, arrest the crack growth, and ultimately provide the high strain-to-failure. These observations reveal guidelines to avoid cut-edge failure.

Feasibility evaluation of failure models for predicting forming limit of metal foils

Forming limit of metal foil is an important index to evaluate its formability, and is of considerable significance to improve the quality of products. The ductile fracture (DF) behavior in microscale plastic deformation is remarkably affected by the geometry and grain size. To explore the size-dependent forming limit curve (FLC), the Holmberg and Marciniak tests of SUS304 foils with the thicknesses of less than 0.1 mm and diverse grain sizes were performed. In addition, the validity and feasibility of three types of existing failure models including Swift/Hill, Marciniak-Kuczynski (M-K) and DF criteria for predicting the micro-scaled FLCs were discussed. It is found that the Swift/Hill model possesses the worst accuracy with predicting deviation above 50%. Four classical DF criteria including Freudenthal, Ayada, Brozzo and Oh show great difference, and Oh model considering plastic anisotropy presents the best precision. The predicted deviation of M-K model is aggravated with increasing grain size and decreasing foil thickness, which is attributed to the intensified free surface roughening and transformation of fracture mechanism with miniaturization. This research thus provides a deeper understanding and valuable reference for the widespread application of FLC in microforming.

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.

A New Compression Test for Determining Free Surface Roughness Evolution in Thin Sheet Metals

In sheet microforming processes, in-surface principal strain rates may be compressive such that the thickness of the sheet increases in the process of deformation. In general, the evolution of free surface roughness depends on the sense of the principal strain normal to the free surface. Therefore, in order to predict the evolution of free surface roughness in processes in which this normal principal strain is positive by means of empirical equations, it is necessary to carry out experiments in which the thickness of the sheet increases. Conventional experiments, such as the Marciniak test, do not provide such strain paths. In general, it is rather difficult to induce a sufficiently uniform state of strain in thin sheets of increasing thickness throughout the process of deformation because instability occurs at the very beginning of the process. The present paper proposes a compression test for thin sheets. Teflon sheets are placed between support jigs and the metallic sheet tested to prevent the occurrence of instability and significantly reduce the effect of the support jigs on the evolution of surface roughness. The test is used to determine the evolution of surface roughness in thin sheets made of C1220-O under three strain paths.

Approach for modelling the Taylor-Quinney coefficient of high strength steels

Abstract Precise knowledge of the temperature that arises in the material during plastic forming is of crucial importance, as it has a significant influence on material behaviour and therefore on the forming process. In order to describe the amount of heat that is generated during plastic forming accurately, the Taylor-Quinney coefficient β was introduced as the ratio of dissipated heat to plastic work and generally assumed to be a constant value. However, recent studies have shown that there is a dependency on material and process-specific parameters. In this study, the Taylor-Quinney coefficient β is shown as a function of strain and being influenced by the test specific strain rate and stress state. The tested material is a dual-phase steel HCT980X. The uniaxial tensile test and the Marciniak test with different tallied specimen at forming-relevant global strain rates were investigated. By means of thermographic and optical measuring systems the temperature and local strains were recorded during the tests. Based on an approach similar to the finite volume method, both experimental setups were modelled taking heat transfer effects into account. As a result, the Taylor-Quinney coefficient is calculated by means of experimental data. It is shown that the Taylor-Quinney coefficient is a variable value depending on the flow behaviour of the steel. The local strain rate and the specimen geometries of Marciniak test have a significant influence on the arising heat conduction. The stress state, however, has minor influence on β.

Determination of forming limit for aluminium alloy sheet eliminating the interferences of through-thickness stress and non-linear strain path

The accuracy of the forming limit diagram (FLD) determined through Nakazima test is often influenced by non-linear strain path (NLSP) and through-thickness stress. The influence of these factors on measurement accuracy can be weakened or avoided in Marciniak test. But meanwhile, Marciniak test is difficult in operation and possesses poor repeatability. Therefore, the Nakazima test eliminating the influencing factors of accuracy is still an attractive method due to the easy operability and good repeatability. In the present research, two FLDs of aluminium alloy 6014-T4 that were established through Nakazima and Marciniak tests were compared. In order to obtain an accurate FLD in plane-stress state and linear strain paths via Nakazima test, a modified compensation method was applied in Nakazima test to eliminate the interferences of NLSP and through-thickness stress on measuring forming limits. The ultimate strains measured in Nakazima test were transformed to stress-based forming limits through constitutive models to remove the influence of through-thickness stress. Then, the corrected ultimate stresses were transformed back to the limit strains along linear strain paths. Compensated results of Nakazima test showed a good agreement with the experimental results from Marciniak test, proving the validity of this method. Thus, through the easy-operate Nakazima test, an FLD with comparable accuracy to that based on Marciniak test was established.

Metallographic analysis of failure mechanisms during Nakajima tests for the evaluation of forming limits on a dual-phase steel

The material characterization is the first step for an accurate numerical design of forming operations. Since its first definition in the 1960s, the forming limit curve (FLC), usually evaluated with Nakajima or Marciniak tests, is still one of the most used criteria in the industrial and scientific field for the determination of the forming limits under different stress conditions. Despite the several signs of progress in terms of measurement techniques, thanks to the introduction of optical measurement systems, the European standard method for the evaluation of the FLC remains the cross-section-method of 2008. This method is basically suitable for materials with a pronounced necking evolution. For materials with an abrupt localization, like modern high-strength materials, the standard evaluation shows weaknesses. Investigations in 2015 based on pattern recognition on Nakajima tests show that the pattern evolution of the strain distribution during the test can be used for the prediction of the material failure. However, a definition of the pattern development is needed. In order to investigate new possibilities for the determination of the FLC, the knowledge about the failure mechanisms during Nakajima tests for these materials has to be increased. The aim of the present work is an analysis of the changes on the surface and the microstructure during the Nakajima tests at different stress states and drawing depths. The correlation between material modifications and failure behavior is conducted on the dual-phase steel DP800. For the analysis of the surface and the thickness, the scanning electron microscope (SEM) is employed. Moreover, considerations about the forming mechanisms of the DP800 at different stress conditions are given and compared with the forming limit prediction of the FLC.

Stakes and solutions for in-plane sheet-metal formability assessment

In body design, Finite Element Analysis becomes an unavoidable step in optimizing forming processes to ensure the feasibility of a specific designed shape. Different failure criteria exist but the Forming Limit Diagram remains the most used criterion. It can be built in a wide variety of forms but the most usual one is composed only of a Forming Limit Curve (FLC) which represents the onset of localized necking limit of sheet metal. FLC is determined experimentally by standardized Nakajima or Marciniak tests. However, both present lots of roadblocks in the accurate determination of product formability limits due to the use of counter-blanks, no linear strain paths and because they are not adapted for high ductility steels. Tensile tests were performed in the past to determine the left hand side of the FLCs. They were not included into the ISO 12004-2 standard because of technical reasons although they present lots of advantages (frictionless, no curvature effect and planar configuration). Now, thanks to the current advanced technologies and tools, these issues are overcome. In this paper, the advantages of tensile tests compared to Nakajima or Marciniak ones are briefly discussed. The design and conceptualization of specific jaws to perform plane strain tensile tests on AHSS are presented. A wide range of AHSS was characterized through plane strain tensile tests and results were compared to formability limits determined by the usual practice using Nakajima tests. Different evaluation strategies were used to determine the maximum formability: the position dependent method, the time dependent one and close to fracture.

Development of novel forming limit curve testing method

A new concept of the forming limit curve testing method has been developed for evaluating sheet metal formability. This new method complies with the current ISO 12004-2 standard, and has been verified by the numerical analysis in terms of specimen deformation, strain path characterization and lubrication effect. It combines the advantages from both the standardized Marciniak and Nakajima tests, deforming the sheet metal materials under a complicated deformation mode following the linear strain path without using a carry blank. By comparing the simulation results with the experimental measurements, it is proven that this testing method works with the thinner and thicker sheet metals for a variety of blank geometries.

Sheet Thickness Reduction Influence on Fracture Strain Determination

Standardly, the FLC curve [11] is determined based on ISO12004 according to Nakajima test or Marciniak test [1]. There are two basic influences: the friction between the punch and the specimen and at the same time the pushing the punch on the specimen, which affects the strain distribution over the sheet thickness [17]. The main disadvantage of these tests is the measurement of the linear deformation paths only, but the real forming processes are usually formed in multiple steps processes and the loading path is strongly non-linear. The current trend for sheets formability assessment is investigation of non-linear deformation paths performed on cruciform specimens [2, 3] or using a combination of Marciniak and Nakajima tests [6], [7]. The cruciform specimens should have reduced thickness in the central part in order to assure main plastic deformation and fracture in this region [18]. However, there are indications that the thickness reduction can influence results of the formability assessment. The main goal of this present paper, is experimental investigation of effect of local material thickness reduction on FLC and on Fracture Forming Limit Curve (FFLC). The cruciform specimens with a reduced thickness is investigated with the use of the biaxial stand without friction and pushing the punch. All results are compared to the FLC values.

Prediction of plane-strain specimen geometry to efficiently obtain a forming limit diagram by Marciniak test

Plane-strain forming limit strain (also known as FLD0) is an important data point on a forming limit diagram (FLD). The effects of friction coefficients and material parameters on the specimen width associated with the FLD0 (\(W_{{{\text{FLD}}_0}}\)) in Marciniak test were studied by finite element simulation. \(W_{{{\text{FLD}}_0}}\) was expressed as a function of the Lankford coefficients, n-value, k-value and sheet thickness and validated with various sheet materials. The determination of \(W_{{{\text{FLD}}_0}}\) is of significance not only to reduce iterative attempts to accurately obtain FLD0, but also to obtain a full valid FLD with the least number of test specimens, which largely increases the efficiency and reduces cost to experimentally measure valid FLDs.

Analysis of the bending effects and the biaxial pre-straining in sheet metal stretch forming processes for the determination of the forming limits

The most common failure criterion in sheet metal forming operations is the forming limit diagram (FLD). Typically, the FLD is evaluated with the Nakajima test, which is a stretch forming-based test that allows the analysis of stretching under different stress conditions. The results of the Nakajima tests are considered ideally achieved under linear strain path deformation. However, due to the hemispherical form of the punch, the induced strain path starts in biaxial condition and switches to a linear path after the first contact between punch and specimen. In addition, the strain distribution is evaluated on the outer surface of the specimen. Consequently, the results are affected by a non-linear strain path and a bending component due to the sheet thickness and punch diameter. These effects have a consequence on the FLD, and thus it is not directly comparable with the ideal FLD that is analytically evaluated since the analytical models in the literature do not consider the process effects like bending and the biaxial pre-straining. In this paper, a study of biaxial pre-straining and bending effects is observed for different stress conditions and materials. The investigation of the bending will be considered under the experimental and numerical aspect. Nakajima tests are conducted with different punch diameters to discover the effects of different material thickness to punch radius ratios. The results are also compared with the outcomes of a numerical model, used for the analysis of the strains on the outer, middle and inner layers. The biaxial pre-stretching is analysed by comparing the outcomes of the FLD achieved by the Nakajima punch with the results of the Marciniak test. In this paper, an efficient method based on the correlation coefficient for the evaluation of pre-straining is presented. The biaxial pre-stretching will be compared to the strain paths and the tendencies will be analyzed. Finally, considerations about the phenomenology of the effects in dependency of the material classes are given.

Numerical analysis of damage evolution and formability of DP600 sheet with an extended Rousselier damage model

Dual phase (DP) steel sheets are increasingly being used for automotive applications due to relatively high strength and formability. In the current work, Marciniak formability tests were carried out to determine the forming limit curve (FLC) of DP600 steel sheet, and an extended version of Rousselier’s ductile damage model, which accounts for void nucleation, growth and coalescence was used to simulate the tests and predict strain localization and failure for three different strain paths: uniaxial tension, plane strain and biaxial tension. In addition, a combination of flat rolling and uniaxial tension tests were used to generate the extended flow curve of the material. Damage evolution in terms of Rousselier scalar damage variable and void volume fraction was assessed for each simulation condition. The FLC as well as neck and fracture morphologies and geometries were obtained from finite element simulations of the Marciniak tests and compared to experimental results. The sensitivity and dependency of the predicted necking limits, damage distribution and geometry predicted by the Rousselier damage model to the type of hardening model, strain path, void nucleation function and void coalescence criterion are discussed. The modified Rousselier model was shown to successfully predict the FLC, damage distribution and the final damage geometry of DP600 sheets.

Microscopic investigation of failure mechanisms in AA5182-O sheets subjected to electro-hydraulic forming

In this study, damage mechanisms in AA5182-O aluminium sheets are investigated using quasi-static (QS) Marciniak tests and high strain-rate electro-hydraulic forming (EHF) process. The results confirm that void nucleation, growth and coalescence are the main damage mechanisms of AA5182-O at both high and low strain rates. The EDS analysis suggests that cracking of Al3(Fe-Mn) intermetallic particles is the main source of void nucleation, whereas Mg2Si particles do not majorly influence void formation. Void growth analysis suggests that specimens deformed under QS conditions contained more voids in areas away from the sub-fracture surface but specimens deformed at high strain rate exhibit more significant rate of void growth close to sub-fracture areas. Void formation is suppressed by increasing the applied energy in EHF. And more significantly, the growth of voids is suppressed due to the high-velocity impact of the sheet against the die which plays an important role in increasing formability of AA5182-O aluminium sheet in EHF process. When the void percentage increase remains less than about 0.5% AA5182-O can be formed safely. However, when the void percentage increases beyond 0.6–0.8% fracture becomes inevitable.

An improved curvature method of detecting the onset of localized necking in Marciniak tests and its extension to Nakazima tests

A method using surface curvatures to detect the onset of localized necking in forming limit tests [Min J, Stoughton TB, Carsley JE, Lin J. A Method of Detecting the Onset of Localized Necking Based on Surface Geometry Measurements. Exp Mech 2016; DOI:10.1007/s11340-016–0232-4.] is improved to be more robust and operator-independent. The improved method is referred to as the 3D curvature method, whereby the curvature is fitted on the basis of a small narrow surface encompassing the neck band, and the onset of localized necking is still detected by the sudden increase of the fitted curvature according to the neck expansion theory proposed by Min et al. [Min J, Stoughton TB, Carsley JE, Lin J. A Method of Detecting the Onset of Localized Necking Based on Surface Geometry Measurements. Exp Mech 2016; DOI:10.1007/s11340-016–0232-4.] Appropriate widths and lengths of the small narrow area on the specimen surface are determined by investigation of their effects on the curvature fitting procedure and on the curvature increase that signals the onset of localized necking. In addition, the 3D curvature method is successfully extended to detect the onset of localized necking in Nakazima tests, where the deformation is characterized as out-of-plane, as opposed to the nearly in-plane deformation of Marciniak testing. As a result, forming limit strains associated with the onset of localized necking can be obtained from both commonly used Marciniak and Nakazima tests on an equivalent basis of geometrical change of specimen surface. This is significant for the reconciliation of forming limits from the two tests after compensation for the effects of non-linear strain path, bending and contact pressure described by Min et al. [Min J, Stoughton TB, Carsley JE, Lin J. Compensation for process-dependent effects in the determination of localized necking limits. Int J Mech Sci 2016; 117: 115–134.]

Formability enhancement of DP600 steel sheets in electro-hydraulic die forming

The objectives of this paper are to quantify the increase in formability of DP600 steel sheets in electrohydraulic die forming (EHDF). The conventional (quasi-static) and high strain rate forming limits of this sheet material were experimentally obtained from specimens deformed using Marciniak tests and EHDF tests, respectively. A numerical model of EHDF was developed in order to determine the strain path of sheet metal experiencing large strains. Forming limits obtained from EHDF specimens formed in plane strain into a V-shaped die indicated that, locally near the top edge of the specimen, more than 120% formability improvement was achieved. Moreover, a formability enhancement of more than 60% was achieved in EHDF in biaxial tension using a conical die, provided sufficient discharge energy was applied. It was found that the significant increases in formability in EHDF are associated with the high-velocity impact against the die wall and the consequent high effective strain rate.

Influence of hydrogen on formability and bendability of DP1180 steel for car body application

In order to reach future light weight targets, it is increasing necessary to use advanced high strength steels with tensile strength 980 MPa or higher in automotive body-inwhite structures. Due to the sensitivity to hydrogen embrittlement and the limited understanding of various aspects of hydrogen embrittlement on processing and function, the wide application of these steels is still limited.In the current work, the influence of hydrogen on the multiaxial forming behavior was investigated by determining the forming limit curve and bending limit curve of DP1180 steel. Hydrogen concentration in the material was modified by cathodic charging. Then Nakajima tests on hydrogen uncharged and pre-charged samples were carried out in order to adjust and study different strain states resulting in the forming limit curve. In the study of bending limit curve, the steel sheets were pre-strained by Marciniak test. Bending load on the uncharged and pre-charged samples was introduced by VDA238-100 bending tests. The experimental results indicated that the presence of hydrogen affected the formability and bendability of DP1180 steel. A clear difference in the influence of hydrogen at different strain states was observed. When formed in a biaxial strain state via the Nakajima test, the material showed the highest degradation in formability. Moreover, the samples with biaxial pre-loading showed more degradation in bendability comparing to those pre-strained in plane strain and uni-axial paths. Fractography by scanning electron microscope gave evidence of hydrogen-induced cleavage fracture on pre-charged Nakajima samples. Thus this investigation improves the understanding of influences of hydrogen on forming processes and provides important evidence for further studies on HE susceptibility of AHSS for the application on car body constructions.

A Method of Detecting the Onset of Localized Necking Based on Surface Geometry Measurements

To more accurately capture the onset of localized necking and obtain necking limit strains, this paper proposes a method of detecting the onset of localized necking in the Marciniak test (i.e. under in-plane deformation). The method is merely based on the measured surface geometry of the test specimen using digital image correlation (DIC) techniques. It was inspired by the observation of a sudden increase of the surface curvature obtained from 2D curvature fits along the direction across the surface of the sheet. This increase of the surface curvature is detected just before the dimples, which form the final localized neck, become obvious in the DIC measurements. The appearance of this signal is explained by a neck expansion theory defined by propagation of the instability along the direction of the neck, which is a physical behavior of materials.