Search

Integrated leading edge protection (InLEP) is a novel LEP method that involves co-bonding a tough thermoplastic to the blade shell of the wind turbine made of fiber-reinforced thermoset polymer. In the co-bonding process, as a result of the interdiffusion of the bonded thermoplastic and thermoset polymers, an interphase is formed between them. An important factor affecting the level of interdiffusion is the cure temperature. In this work, we investigate the influence of cure temperature on the interphase morphology and bond strength of ABS-polyester/glass and PC-polyester/glass hybrid composites. The hybrid composites are manufactured via vacuum-assisted resin transfer molding. Interphase morphology is observed and the interphase thickness is measured via optical microscopy. Bond strength is tested via climbing drum peel testing and subsequently, fractography analysis is carried out on the fractured samples. It was found that both the interphase thickness and bond strength decrease with an increase of cure temperature. The decrease in bond strength at high temperatures was accompanied by an increase in the extent of interfacial failure, while interphase failure at low temperatures promoted higher bond strength.

Process simulation software for hot press forming requires accurate material characterizations. One of these characterizations is tool-ply friction, for which the methodology is well established. However, the experimental conditions are often not representative for the actual forming process. This research focuses on the effect of release agent and heating time on the tool-ply friction response. UD carbon fiber-reinforced PEKK was forced to slide against metal foils in a benchmarked friction tester at different rates, normal pressures and temperatures. The typical friction response, exhibiting a shear stress overshoot followed by a steady-state region, did not qualitatively change when applying a Marbocote 227CEE release agent on the metal foils. However, the overshoot reduced and, in case of a high normal pressure of 45 kPa, the steady-state response lowered as well. Thus, release agent should be included for a more accurate characterization of tool-ply friction. A longer heating time resulted in a large increase of the overshoot, whereas the steady-state response was nearly unaffected. The same observation was made when testing at a higher temperature, which may suggest that the increase in overshoot is due to increased adhesive bonding. Moreover, a change in adhesive bonding could also explain the lower overshoot observed when a release agent was applied, indicating adhesion as a key mechanism for tool-ply friction.

Data from EXACT -Experiment and Analysis of Aluminum Cup Drawing Test, the first ESAFORM benchmark G. Vincze, A. Santos, M.C. Oliveira, A. B. Lopes, T. Kuwabara, A-M.Habraken, O. Cazacu, F. Barlat Thesedata are the basis of the Benchmark Exact, the first benchmark of the European Scientific Association for material FORMing – ESAFORM, and support the article entitled “Analysis of ESAFORM 2021 cup drawing benchmark of an Al alloy, critical factors for accuracy and efficiency of FE simulations”, published in the International Journal of Material Forming, Special Issue ESAFORM 25 YEARS ON,https://doi.org/10.1007/s12289-022-01672-w How to cite the data If you publish any work using thesedata, please cite theHabrakenet. al., (2022)articleaboveas well asthe dataset in the following recommended format: Vincze et al(2022);Dataof TheFirst ESAFORM benchmark EXACT,10.5281/zenodo.6874577 Benchmark motivation: Numerical simulations are powerful predictive tools for virtual forming process design and contribute to the significant reduction of trial-and-error time and cost in the development of new products. However, realistic numerical predictions are achieved only with a careful consideration of all the model input.In particular, a key ingredient is the choice of consistent constitutive models for the elasto-plastic behavior, identified using sound procedures from reliable and repeatable data sets. At present, the information provided in most benchmarks consists of the hardening law coefficients, along with limited experimental data such as, r-values, yield stresses and ultimate strengths extracted from uniaxial tension in certain loading directions, and possibly the coefficients of certain yield functions available in the material libraries of most commercial software. The identification of constitutive models is usually not included in the work requested to participants. Generally, there is no information provided concerning the experimental scatter because the “raw” data are not made available. While the behavior in tensile tests is essential, other loading paths (e.g. biaxial tension, uniaxial compression) may also be important for certain forming processes. Moreover, other types of data describing the initial material anisotropy and that induced by the deformation process, i.e. initial and post-test texture data, are rarely provided. For example, in a simple case of cup drawing, if the final cup test results are “blind data” describing the final geometry, the participants do not have the opportunity to analyze the quality/performance of their simulation results. In addition, if the tests are done in a single laboratory, the experimental scattering is limited to material variability but not to the characterization method nor to the raw data analysis procedure. This benchmark is a unique opportunity at providing an exhaustive analysis of the characterization and modeling of the elasto-plastic behavior of a sheet metal and its influence on the final virtual product. Specifically, the goal of the EXACT data is to generate in-depth discussions regarding the initial and final states of the material, methods of characterization, modelingand earing prediction in cup drawing. File organization Please, read the file “Identification of the files and folders”&nbsp

An improved distortional plasticity framework that describes the anisotropic hardening occurring during strain path changes, such as the Bauschinger and cross-loading effects, is developed. This approach is a modified version of the homogeneous anisotropic hardening (HAH) framework proposed almost a decade ago. In the present formulation, the yield condition includes an alternative description of the distortion suggested by crystal plasticity simulation results. In addition, it incorporates the influence of the hydrostatic pressure, which manifests itself by a higher flow stress in uniaxial compression than in tension. The state variable evolutions are modified compared with the previous HAH version to improve the material response when the strain path changes occur near a pure cross-loading condition. The model is calibrated on a DP780 dual-phase steel sheet sample using the data of a tension-compression test with three full cycles, as well as a sequence of two uniaxial tension segments in different directions. After calibration, predicted and experimental stress-strain curves obtained for independent loading sequences are compared and shown to be good agreement. Finally, theoretical predictions of two-step tension tests using the constitutive coefficients of DP780 are discussed to highlight the improvement offered by this enhanced framework.

This paper reviews the most recent models for description of the anisotropic plastic behavior and formability of sheet metals. After a brief review of classic isotropic yield functions, recent advanced anisotropic criteria for polycrystalline materials of various crystal structures and their applications to cup drawing are presented. Next, the discussion focuses on novel formulations of anisotropic hardening. A brief review of the experimental methods used for characterizing and modeling the anisotropic plastic behavior of metallic sheets and tubes under biaxial loading is presented. The experimental methods and theoretical models used for measuring and predicting the limit strains, development of new tests for determining the Forming Limit Curves (FLC), as well as on studying the influence of various material or process parameters on the limit strains are presented.

This study was performed to estimate the stretch-flangeability of two important advanced high strength steel (AHSS) sheets, i.e. DP980 and TRIP1180, using hole-expansion test (HET). Moreover, the effect of hole edge-quality on stretch-flangeability of both steels was analyzed by adopting two different methods, (i) shearing and (ii) wire cutting, to create the central hole on test specimens. Formability of the sheets was measured in terms of fracture strain and hole expansion ratio, under the influence of two different edge-qualities. A detailed characterization at both micro- and macro-scale was carried out to analyze the fracture behavior of both steels. Experimental results showed that the role of localization and shear in addition to edge-quality is crucial to fracture behavior of both the steels. Apart from the macro- and micro-scale characterization, a finite element study was also carried out to model the HET numerically. Finally, a suitable fracture criterion was used to predict the onset of fracture during the test.

This study investigates the effects of material modeling in high strain range on the forming limit analysis of a 6000-series alloy sheet 6016-T4. A servo-controlled tension-internal pressure testing machine was used to measure the multiaxial plastic deformation behavior in a strain range from initial yield to fracture. Tubular specimens were fabricated with a weld line thicker than the sheet thickness to prevent fracture from occurring in the weld zone. Proportional loading in the first quadrant of the principal stress space was applied to the specimens to measure the forming limit curve (FLC) and forming limit stress curve (FLSC), in addition to the contours of plastic work and the directions of plastic strain rates. Moreover, forming limit strains were measured using a flat-headed punch stretching test (PST). From the multiaxial tube expansion test (MTET) data, although a differential hardening effect was observed, appropriate coefficients including the exponent of the Yld2000-2d yield function (Barlat et al, 2003) were determined and employed in the Marciniak-Kuczyski (1967) forming limit analysis (M-K analysis). The calculated forming limit strains in the vicinity of equibiaxial tension were smaller than the measured ones. The M-K analysis predicted that the forming limit strain in the vicinity of equibiaxial tension becomes minimum when the inclination angle of the localized neck from the rolling direction (RD) is 45° (diagonal direction; DD) while the experimental fracture direction was the RD. With the additional MTET, the plastic deformation behavior in the DD was measured and compared with that calculated from the material model. The reason of the discrepancy in the fracture direction between the experiment and the M-K approach is discussed.

International journal of material forming 15(5), 61 (2022). doi:10.1007/s12289-022-01672-w special issue: "ESAFORM 25 Years On", Published by Springer, Paris [u.a.]

In the present work, the constitutive description of a 0.1-mm-thick ferritic stainless steel sheet was optimized for application to springback simulations after bending. Springback simulations require Bauschinger effects to be characterized carefully, which is a challenging task with an ultrathin sheet. The coefficients of the homogeneous anisotropic hardening (HAH) distortional plasticity model were calibrated with Zang's novel approach based on three-point bending conducted on pre-strained sheets (Zang et al., 2014). The Hockett–Sherby isotropic hardening law and the Yld2000-2d non-quadratic yield function were considered in this work to complete the HAH approach. Moreover, the degradation of elastic modulus was also accounted for in the finite-element simulations. The coefficients of the HAH model were calibrated using an inverse method by minimizing the difference between experimental and predicted specimen profiles after three-point bending and springback of pre-strained sheets. To validate the coefficients determined with this three-point bending test, U-draw bending tests were conducted and finite-element simulations were carried out. The springback predictions were found to agree well with the experimental results for all three pre-strains investigated, namely, 0% (as-received material) and 7.5% and 12.5% pre-strained ferritic stainless steel sheets.

The advanced high strength steels (AHSS) have gained major attention from research groups across the world especially in the last two decades. The exact description of the forming behavior of these steels has several challenges associated with it. The origin of these challenges is related to complex forming behavior and failure characteristics of AHSS sheets at macro and micro scale. In the present study, the fracture characteristics of two important AHSSs, i.e. DP980 and TRIP1180, were investigated using uniaxial tension and shear tests. The critical strains were calculated considering both necking and fracture using digital image correlation (DIC) technique. The deformed specimens were examined at macro and micro scale to understand the fracture mechanisms of both steels. Numerical simulations were performed for identification and validation of constitutive parameters. A simple fracture law was used to predict the fracture and the results were compared with the experiments. Reasonable fracture prediction was achieved for both DP980 and TRIP1180.

The hole expansion of a low carbon steel sheet shows an interesting feature that localized thinning and subsequent crack initiation are observed inside the specimen, and not at the hole edge as is typically expected. The present work investigated a numerical modeling approach to predict this localization behavior within the framework of a finite element (FE) analysis. Plastic anisotropy of the sheet was taken into account using the anisotropic yield functions Yld2000-2d and Yld2004-18p for the plane stress and three-dimensional elements, respectively. Careful examination of the FE model revealed that the influence of the out-of-plane stress is very small, suggesting that shell elements can be efficiently used in the analysis. The influence of friction was also found to be negligibly small. However, the constitutive description exhibited a significant influence in that even a slight change in the yield function parameters resulted in a considerable difference in the prediction. For this reason, several sets of parameters were obtained based on the different material properties, and their influences on the hole expansion simulation were analyzed. In particular, the prediction accuracy could be greatly improved when the yield function parameters were optimized such that the flow stresses and plastic strain rate ratios in uniaxial and plane strain states were well captured.

The dislocation density-based model after Kocks–Mecking–Estrin (KME) is widely used to characterize the thermally activated plastic deformation and dislocation kinetics. According to the model, the slope of the stress–strain curve decreases linearly with stress, which contradicts the experimental observation. In the current study, the evolution of dislocation density in the model is generalized to account for the nonlinearity of the slope.

This presentation reviews the key features of a macroscopic plasticity framework applicable to numerical simulations of practical forming processes in industry. The constitutive relationships are developed for anisotropic materials with an anisotropic hardening assumption. The framework relies on mathematical concepts, physical understanding of deformation mechanisms, and experimental results. Lower scale simulation results are essential for the introduction of relevant features in the macroscopic framework. The experimental investigations on plasticity conducted by Professor Tozawa in the 1960s and 1970s is of fundamental importance for the development and validation of the theory.

In this study, a U-shaped channel formed using a double drawing process (double stage U-draw bending) was proposed to reduce the amount of springback in the AHSS sheets. The performance of the double stage U-draw bending process in reducing the amount of springback was compared with that of the conventional U-draw bending process. The process was simulated using a finite element (FE) analysis with two different types of anisotropic hardening models, namely, isotropic-kinematic and distortional models, to describe the Bauschinger effect and associated anisotropic hardening transients during the strain-path changes. Moreover, plastic anisotropy, captured by different yield functions, and the degradation of the elastic modulus were taken into account. In addition to the basic mechanical characterization tests conducted to identify the material coefficients, in-plane compression–tension experiments were conducted. The experimental and FE simulated results of the double stage U-draw bending process were compared and analyzed to understand the effect of anisotropic hardening on the springback under non-proportional loading.

The strength differential effect (SDE), i.e., the difference between the flow stress of uniaxial tension and compression, of a dual-phase steel sheet with a tensile strength of 980 MPa (DP980) was measured using an in-plane uniaxial tension–compression test apparatus for sheet metals. The SDE data were obtained for a logarithmic total strain range of 0 ≤ | e | ≤ 0.086 in both the rolling (RD) and transverse (TD) directions of the test material. It was observed that the magnitude of the compressive flow stress, σ C , was consistently higher than that of the tensile flow stress, σ T , for a strain range of | e | > 0.009 in both the RD and TD; the amount of SDE, defined as σ C − σ T / σ T × 100 (%), at | e | = 0.086 was 5.2% in the RD and 6.2% in the TD. Moreover, in order to validate the SDE, a novel test apparatus for obtaining the pure bending moment M versus curvature κ (i.e., M − κ ) diagram was designed and manufactured. The obtained M − κ curves were consistent with those based on the elastoplastic numerical analysis considering the SDE. Thus, the SDE of the test material is experimentally validated using the pure bending test.

In this study, a numerical sensitivity analysis of the springback prediction was performed using advanced strain hardening models. In particular, the springback in U-draw bending for dual-phase 780 steel sheets was investigated while focusing on the effect of the initial yield stress determined from the cyclic loading tests. The anisotropic hardening models could reproduce the flow stress behavior under the non-proportional loading condition for the considered parametric cases. However, various identification schemes for determining the yield stress of the anisotropic hardening models significantly influenced the springback prediction. The deviations from the measured springback varied from 4% to 13.5% depending on the identification method.

This article investigates the macroscopic behavior of uniaxially pre-strained large-scale DP780 and CHSP45R sheet specimens. A novel grip system was designed for the pre-straining of the large tensile specimens. After this first loading, the uniformly strained gauge-section of the large-scale specimen was machined to smaller standard specimens. After pre-strain, further uniaxial tension tests at 45° and 90° from the pre-tensile direction, compression test and in-plane biaxial tension tests at 1:1, 2:1 and 1:2 force ratios were conducted. The flow curves and the instantaneous r-values of the pre-strained steel in the aforementioned uniaxial loading directions were compared with their monotonic response. In addition, the pre-strained compression test was incorporated to the π-plane at several incremental offset strains. The yield function- and isotropic hardening-based homogeneous anisotropic hardening (HAH) model (Barlat et al., 2014) was selected to predict the material response after non-linear strain path change. The Swift law characterized the isotropic hardening, while the Yld2000-2d anisotropic yield function (Barlat et al., 2003a) represented the yield locus. All the coefficients of the distortional plasticity model were manually determined and validated with an optimization algorithm. The HAH-predictions of the pre-strained flow curves and yield loci at several offset strains were in reasonable agreement with the experimental data, while the instantaneous r-value evolution was qualitatively well captured.

An inverse approach based on the virtual fields method (VFM) is presented to identify the material hardening parameters under dynamic deformation. This dynamic-VFM (D-VFM) method does not require load information for the parameter identification. Instead, it utilizes acceleration fields in a specimen’s gage region. To investigate the feasibility of the proposed inverse approach for dynamic deformation, the virtual experiments using dynamic finite element simulations were conducted. The simulation could provide all the necessary data for the identification such as displacement, strain, and acceleration fields. The accuracy of the identification results was evaluated by changing several parameters such as specimen geometry, velocity, and traction boundary conditions. The analysis clearly shows that the D-VFM which utilizes acceleration fields can be a good alternative to the conventional identification procedure that uses load information. Also, it was found that proper deformation conditions are required for generating sufficient acceleration fields during dynamic deformation to enhance the identification accuracy with the D-VFM.

The aim of this study was to investigate the differences between shell and solid elements in the numerical simulation of a 2.85 thick DP780 sheet. It was observed the stress-strain response with shell elements was very different from the experiment after maximum load while, solid element showed a much better agreement. For shell elements, stress triaxiality had the tendency to abruptly converge at certain value while, with solid elements, the triaxiality was continuously increasing. To compensate for the limitation of shell element, the strain hardening curve was modified after maximum load. The method was validated with plane strain tension simulation.

Crash analysis simulation is now very important in automotive industry to assess automotivecrashworthiness and safety. In order to acquire reliable crash simulation results, precise materialbehaviors at intermediate strain rates should be used as input data. To derive the stress-strain curves atvarious strain rates, a large number of experiments are needed, which is costly and time consuming.The present study aims at determining the stress-strain curves of sheet metals at various strain ratesfrom a single dynamic experiment. A new type of high-speed tensile tester for sheet metal specimenswas built and high-speed tensile tests were carried out. Full-field heterogeneous strain fields weremeasured by a digital image correlation technique using a high-speed camera. The load data wasacquired from strain gauges attached to the elastic deformation region on the specimen. Then, an inverseidentification scheme with a rate dependent hardening law was applied to retrieve dynamic parameters.The stress-strain curves of three advanced high strength steels at intermediate strain rates (100 s-1 –300 s-1) were successfully obtained from a single experiment.

Constitutive models developed for simulating plastic response upon strain path changes are combined: 1) a macro-mechanical model based on anisotropic yield function, associated flow rule and distortional hardening using Homogeneous Anisotropic Hardening (HAH) approach; 2) a micro-mechanical model using self-consistent crystal plasticity in conjunction with crystallographic dislocation-density based hardening. The micro-mechanical model is employed to probe the yield surface in order to gain the insight required to construct empirical rules appropriate for the macro-mechanical model. Simulation results of the micro-mechanical model under various loading conditions involving strain path changes and different crystallographic textures are presented. The trends captured in the yield surface evolution predicted by the micro-mechanical model were used to validate and improve the empirical rules used in the HAH model.

A crystal plasticity finite element analysis was conducted for predicting forming limits of a ferritic stainless steel. A virtual microstructure crystal plasticity finite element model, or a representative volume element (RVE), was developed, in which the behavior of grains was governed by a rate-dependent viscoplastic crystal plasticity model. To account for the variation in hardening of different slip modes, the model parameters of different slip modes in the bcc metal were obtained from the uniaxial stress-strain, longitudinal-transverse strains, and hydraulic bulge test data using an inverse method. A multi-scale framework, which combines the virtual microstructure models and the Marciniak-Kuczynski procedure, was developed and applied to the forming limit analysis of a ferritic stainless steel. Forming limits predicted using the combined slip mode compared well with the measurements when the {110} and {112} slip systems are operating and the {123} slip systems are inactive. The influence of the slip system activity on the yield criteria and forming limits is discussed.

A multi-surface approach is suggested to describe nonlinear and hysteretic unloading-reloading behaviors of sheet metals, adopting the concept of multiple yield surfaces in the Mroz model. This approach divides the elastic domain into many fields that have different values of elastic modulus, resulting in a piecewise linear, hysteretic unloading-reloading stress-strain curve. Because this approach simply divides the elastic domain, it can be used in conjunction with any phenomenological plasticity models. The proposed model was implemented into a commercial finite element code and applied to springback simulations and stiffness analyses, demonstrating that its computational efficiency is comparable (1.66 times) to that required for linear elasticity and its accuracy is as good as the nonlinear elasticity model. It was further verified that the proposed model provides a stable solution even when the numerical simulation involves small stress oscillations during unloading or reloading.

The aim of this study is to investigate the path-dependent elastic-plastic behavior of dual-phase steel and its influence on the prediction of twist springback in the channel forming process. The anisotropic hardening responses of the sheet material for non-proportional loading, as well as the degradation of the elastic modulus in uniaxial and biaxial tension are investigated. A recently proposed distortional plasticity model combined with a dislocation density-based hardening approach was adopted to describe the flow behavior of the material. The results indicate that the present model simultaneously reproduces all of the experimentally observed features for both load reversal and changes of the principal strain axis. This constitutive description is employed in the finite element analysis of the forming of two channels with obvious twist springback characteristics. The complex strain path changes during the forming process are then analyzed using a proposed indicator. Finally, the relevance of the load changes and the stress distribution in the channel regarding twist springback predictions are discussed. The influence of loading path-dependent elastic modulus degradation for the prediction of twist springback is also assessed based on two different application geometries.

In the present paper, an inverse problem solution so called the virtual fields method (VFM) is implemented to identify the parameters of the homogeneous anisotropic hardening (HAH) model, a distortional plasticity-based model that describes the material plastic behavior when subjected to strain path changes. The framework of the identification method that combines the formulation of the yield condition, the constitutive stress–strain relation and the principle of virtual work is presented. For validation purpose, the proposed identification method was first attempted on finite element (FE) generated data for a forward-reverse simple shear test to investigate its capability in retrieving the input constitutive parameters. The influence of noise was also evaluated. Then, the identification method was applied to a selection of advanced high strength steel (AHSS), namely DP600, TRIP780 and TWIP980, sheet specimens, subjected to a small number of forward-reverse simple shear cycles. The material constitutive parameters were identified using the VFM based on which shear stress–strain curves were calculated and compared with their experimental counterparts. Good agreement was found between the calculated and the experimental curves despite the larger discrepancies observed in the reverse loading paths. To adjust these discrepancies, the original HAH model was modified with respect to the permanent softening related state variables. After modification, the model was simplified with only one state variable related to permanent softening. It was found that the discrepancies observed in the reverse loading paths were reduced with the modified HAH model.

This study investigates the influence of heat treatment on the anisotropic plastic deformation behaviors of 6016-O and 6016-T4 aluminum alloy sheets. The two material samples were fabricated from the same lot and, therefore, have the same grain size and crystallographic texture. Biaxial tensile tests using both cruciform and tubular specimens are performed for many proportional stress paths in the first quadrant of stress space. The test results reveal that the degree of differential hardening (DH) is much larger in 6016-T4 than in 6016-O. It is shown that the work contour shape of 6016-O is controlled by crystallographic texture, whereas that of 6016-T4 presumably depends on GP-zones as well. From the biaxial stress test data, an appropriate yield function for each material is determined and employed in the finite element analysis of the hole expansion forming process. It was found that the Yld2000-2d yield function provides proper material representations of the plastic behavior of both material samples in the sense that it correctly predicts the fracture or localized neck locations, which occurs in the hole edge vicinity. For 6016-O, the thickness strain profile predicted with the Yld2000-2d yield function, which accounts for the DH of the material, is in better agreement with the experimental results than that obtained with the isotropic hardening model. For 6016-T4, the Yld2000-2d yield function with an exponent of 8 with the isotropic hardening assumption leads to a fair prediction of the experimental data. In order to enhance the accuracy of forming simulations for 6016-T4, it is necessary to develop a material model that is capable of reproducing the significant DH resulting from the GP-zones.

Micromechanical-based finite element simulations were carried out to investigate the transient plastic strain rate evolutions of ferrite and martensite dual-phase steel during strain path changes. A representative volume element (RVE) was generated through a three-dimensional (3D) reconstruction of microstructure images which were acquired from sequential polishing of a small material volume. The 10 × 10 × 10 μm3 3D RVEs consisted of martensite islands embedded in a ferrite base matrix. Each phase was assumed to exhibit distinct mechanical properties but the grain and phase boundary effects were ignored in this work. The effective mechanical properties for the constituent phases were assumed to be well defined by the von Mises or Hill 1948 yield criteria, the associated flow rule, and an empirical isotropic hardening equation based on chemical composition. This model was applied to investigate the transient behavior of the r-value (Lankford coefficient) in uniaxial tension when the loading direction changed. In addition to monotonic tension, compression-tension, and tension-orthogonal tension, sequences were considered. The simulation results captured well in a qualitative manner the experimental r-value evolutions in terms of a temporary transition and asymptotic limit. The evolutions of stress states in ferrite and martensite were analyzed to explain the r-value behavior that resulted from three factors: (1) r-value differences between ferrite and martensite, (2) martensite configuration-induced stress state in phases, and (3) stress partitioning and its evolution during non-proportional loading. Finally, an analytical relationship between the stress evolution in the constituent phases and the relevant r-value changes is suggested.

This paper aims to identify the mechanisms associated to the transient hardening behaviour of dual phase steels under strain path changes, and to capture the observed material behaviours with appropriate constitutive models. First, three DP steel sheets with different amounts of martensite were tested under monotonic and various strain path changes. Second, microstructural analysis of the materials before and after strain path change were performed by means of SEM, TEM, and EBSD. The contribution of texture evolution on the mechanical behaviour was also assessed using the visco-plastic self-consistent (VPSC) polycrystal plasticity model. Transient hardening behaviour and permanent softening were observed in the tension–tension tests for all the studied DP steels. These behaviours were explained by the development of strain gradients during the first load resulting from strain accommodation incompatibilities between the ferrite and martensite phases. For the purpose of describing the macroscopic material behaviours, the enhanced homogeneous anisotropic hardening (HAH) model (Barlat et al., 2014) integrated with the Yld2000-2d anisotropic yield function were adopted for constitutive modelling. The simulation results were discussed in view of the microstructure evolution.

Multi-stage micro-channel forming by stamping, as a method for cost effective and efficient for mass production, was performed for ultra-thin ferritic stainless steel sheets with thicknesses of 0.1 and 0.075 mm, as a good substitute for traditional graphite bipolar plates of proton exchange membrane fuel cell. Attention was directed to enhance the final forming depth and minimize localized thinning, extremely important aspects of the micro-channel on bipolar plate, by the proposed forming process. A forming depth at the first forming stage was chosen as a process variable, and its effect on the formability of the micro-channel at the second forming stage was experimentally investigated. Finite element simulations for the two-stage forming process were conducted to optimize the punch radius and forming depth at the first stage for improving the formability. The comparative study between the simulations and the experimental results could validate improvements in the formability by the proposed approach. In particular, this study could support the existence of an optimum forming depth at the first forming stage. Based on the simulation results, a mathematical model was established to identify the dominant factor needed for formability improvement and to propose a methodology for the process optimization of the multi-stage forming.

In the present study, the damage mechanisms operating during the tensile deformation of intercritically annealed Fe-0.3%C-6.0%Mn-3%Al-1.5%Si medium Mn steel were investigated. The steel was annealed at different temperatures to obtain a range of strain hardening properties in uniaxial tension by activating the twinning-induced plasticity and transformation-induced plasticity effects. The initial microstructure consisted of coarse δ-ferrite grains and an ultra-fine grained (UFG) constituent containing ferrite (α) and austenite (γ). However, the volume fraction of martensite (α′) increased significantly by phase transformation from austenite as the material deformed plastically. The internal damage and the fracture appearance after monotonic standard uniaxial tension tests and in-situ interrupted tensile experiments were characterized at macro- and micro-scale. The fracture features were analyzed as a function of the intercritical annealing temperature, which is the most important processing parameter for medium Mn steel. Three mechanisms contributed to the damage that developed in these materials. First, nucleation and growth of voids occurred at non-metallic inclusions. Second, debonding of the α-α′ and α′-α′ interfaces due to a local loss of interfacial strength was observed in the UFG constituent. While the voids initiated at non-metallic inclusions were in the order of several microns, the size of those initiated at the α-α′ and α′-α′ interfaces in the UFG constituent was limited by the initial grain size, with little or no growth. Finally, in addition to void damage, longitudinal cleavage-like cracks formed along the δ-ferrite layers, and parallel to the sheet plane, were observed in the fractured specimens. These longitudinal cleavage-like cracks were the consequence, but not the cause, of a fracture process triggered by plastic flow localization during uniaxial tension testing.

The tension-compression asymmetry (TCA) of a dual phase steel sheet with a tensile strength of 980 MPa is measured using an in-plane uniaxial tension-compression testing apparatus. The TCA data within a logarithmic strain range of 0≤| e |≤ 0.086 are observed both in the rolling and transverse directions of the test material. Moreover, a novel testing apparatus for measuring a pure bending moment-curvature (M-K) diagram is designed and manufactured. The measured M-K curves are consistent with those calculated taking into account the TCA of the stress-strain curve both in the rolling and transverse directions.

In order to clarify the deformation behavior of an ultra-thin austenitic stainless steel sheet (SUS301) used for manufacturing electronic parts a new testing devise is designed and built. The test material is 0.2 mm thick and has a 0.2 % proof stress of 1800 MPa. The testing apparatus is equipped with comb-type die couples to measure the stress-strain curves of the sample under tension-compression cyclic loading without buckling for a strain amplitude of ±0.017. It is found that the stresses are higher in tension than in compression in the rolling direction (RD) for a strain range of |e|≥ 0.002, while in the transverse direction (TD) the stresses are higher in compression than in tension, and that the test material showed significant difference in the cyclic loading behavior between the RD and TD.

Wrought Al-Ge-Si alloys were designed and produced to ensure dislocation bypass strengthening (“hard pin” precipitates) without significant precipitate cutting/shearing (“soft pin” precipitates). These unusual alloys were processed from the melt, solution heat treated and aged. Aging curves at temperatures of 120, 160, 200 and 240 °C were established and the corresponding precipitate spacings, sizes, and morphologies were measured using TEM. The role of non-shearable precipitates in determining the magnitude of Bauschinger was revealed using large-strain compression/tension tests. The effect of precipitates on the Bauschinger response was stronger than that of grain boundaries, even for these dilute alloys. The Bauschinger effect increases dramatically from the under-aged to the peak aged condition and remains constant or decreases slowly through over-aging. This is consistent with reported behavior for Al-Cu alloys (maximum effect at peak aging) and for other Al alloys (increasing through over-aging) such as Al-Cu-Li, Al 6111, Al 2524, and Al 6013. The Al-Ge-Si alloy response was simulated with three microstructural models, including a novel SD (SuperDislocation) model, to reveal the origins of the Bauschinger effect in dilute precipitation-hardened / bypass alloys. The dominant mechanism is related to the elastic interaction of polarized dislocation arrays (generalized pile-up or bow-out model) at precipitate obstacles. Such effects are ignored in continuum and crystal plasticity models.

Crash analysis simulation is very important in automotive industry to assess automotive crashworthiness and safety. In the FE simulation, accurate dynamic hardening behavior should be used as input data to provide reliable results. But, it is difficult to obtain precise hardening properties at intermediate or high strain rates due to inaccurate measurement of load caused by the inertial effect. In this study, a new methodology was applied to retrieve dynamic strain hardening properties of sheet metal specimens. The virtual fields method (VFM) was adopted as an inverse method to identify hardening parameters without load information. As an initial study, Swift model for a rate independent hardening law was selected for an elasto-plastic constitutive model. In order to validate the proposed methodology in the experiments, a new type of high speed tensile tester for sheet metal specimens was built and high speed tensile tests were performed. Digital image correlation technique using a high-speed camera was utilized to measure strain and acceleration fields so that the identification is carried out from the measured quantities. The validation of the proposed VFM identification procedure using the acceleration will be performed by comparing with the conventional procedure using a load-cell.

A succinct description of advanced constitutive models for applications to forming process simulations is provided. These models are continuum-based because they are more efficient in terms of computation time than microstructure–based models. However, they are so–called advanced because they are considered in many scientific studies but rather scarcely used in industrial applications. In addition, the relationship between these continuum constitutive models and multi-scale approaches based on crystal plasticity, dislocation dynamics and mechanics of multi-phase materials, such as advanced high strength steels, is substantiated.