Key Engineering Materials Vol. 794

Abstract: In the plastic deformation of hexagonal metals, deformation twinning plays an important role as well as slip deformation. Therefore, a modelling of deformation twinning is essential in the crystal plasticity modeling. In this study, a model considering the volume fraction of deformation twinning is presented in the framework of crystal plasticity, and it is combined with a finite element-based homogenization scheme to represent the polycrystalline behavior. The presented model is adopted to a sheet necking formulation. Plastic flow behaviors under several strain paths are evaluated using the present framework, and the effect of volume fraction of deformation twinning on the formability of hexagonal metal is discussed.

Abstract: High-strength steel is a type of alloy steel that provides better mechanical properties or greater resistance to corrosion than carbon steel. Strip shape is an important factor affecting the strip quality significantly for the rolled products. Because of the complex influence factors of plate shape and profile, shape detection and control technology have not been solved, especially for high strength steel rolling. In this paper, a novel three dimensional finite element simulation of the strip shape and flatness of high strength steel has been proposed. The material constitutive model has been built up based on experimental results through the Gleeble 3800 Thermal Simulator under different temperatures and stain rates. The modelling of roll elastic deformation system, roll gap profile and edge drop has been set up systematically considering the influence of the work roll transverse shifting and roll bending. Results have shown that both higher bending force and more roll shifting will significantly reduce the strip crown, and obtain improved edge drop distribution as well. The proposed numerical model has been validated through hot rolling experiments in 4-high rolling mills.

Abstract: Metallic materials usually have microscopically heterogeneous structures, such as polycrystalline structures, affecting macroscopic mechanical characteristics. Both macroscopic and microscopic non-uniform deformations of polycrystalline pure copper under a moderate stress gradient were investigated. In this study, macroscopic and microscopic non-uniform deformations under higher stress gradients are investigated. Uniaxial tensile tests using three-curve specimens with different curvatures and grain sizes were performed. In order to evaluate the heterogeneous strain field in the specimen surface, the development of the displacement field was measured using the digital image correlation method (DIC). The stress field was evaluated by coupling the DIC and finite-element methods. In smaller-grain specimens, a strong strain concentration was generated in the minimum cross-section area. Although a strong strain concentration was also confirmed in a larger-grain specimen, the strain field depended not only on the specimen shape but also on the microscopic heterogeneity. This microstructure-driven non-uniform deformation was also observed in the specimen with a larger curvature radius. These results indicated that the macroscopic non-uniform deformation should be estimated by the material parameter related to the microscopic heterogeneity.

Abstract: In this paper a novel Cohesive Zone Model (CZM) is derived within the framework of continuum thermodynamics to describe cracking and delamination behaviour of coatings at high-temperatures. The separation variable in the Traction-Separation-Law (TSL) is decomposed into elastic and inelastic part. For evolution of inelastic separation, a power-law in combination with a damage evolution law is used to consider the tertiary stage of inelastic separation of the interface, additionally. Thereby, damage evolution is related to the corresponding thermodynamic driving force and the inelastic opening rate. For reasons of simplicity the resulting thermo-mechanical problem only considers heat conduction through the interface. Due to the fact that standard Newton-Raphson procedure gets unstable (e.g. snap-back) when softening occurs which is the case by using a CZM, this model is enhanced with the damage gradient, similar to approaches in phase field modelling. Further on, this extension is done to investigate if it is possible to overcome the size dependence of CZMs. Finally, the model is reduced to pure Mode I opening and an example for a Double Cantilever Beam (DCB) is analysed by the finite difference method.

Abstract: A sixth order yield function was used to analyze the anisotropic plasticity behavior of sheet metal forming. Based on a complete sixth order homogenous polynomial in plane stress, the yield function was implemented as user material subroutines in the FE code ABAQUS Explicit and Standard. The associated flow rule and isotropic hardening were assumed. Material parameter values in the yield function were decided by uniaxial yield stresses and plastic strain ratios along 7 different loading orientations and plane strain yield and equal biaxial stresses and plastic strain ratio. To show the superiority of the sixth order yield function, the hole expansion test by Kuwabara et al.[1] was considered. The results of finite element simulation using the sixth order yield function showed a better agreement with the test results than YLD2000-2D yield function with M=6.

Abstract: Roll forming has been widely used to produce steel sheet with low formability such as Ultra High Strength Steel (UHSS). It allows the steel sheet to be formed through successive bending process into a desired shape which even cannot be formed by press brake forming. Although the process effectively improves the formability of UHSS, there still the remains accuracy issue such as springback, flair, bow and so on. Especially, springback of UHSS is one of the major challenges in roll forming process as much as press forming process. In this paper, the springback of 1.5 GPa grade steel in roll forming process was numerically investigated for automotive sill-side inner component. The material behavior was described by using the selected hardening models: isotropic hardening (Piecewise linear model), linear kinematic hardening (Prager model [6]), nonlinear kinematic hardening model (Yoshida-Uemori model [7]). A commercial software LS-DYNA was utilized for the analysis. Eighteen successive roll stages were modelled for the simulation. From the results, it was found that the springback prediction during roll forming process could be successfully achieved when the complicated material behaviors including Bauschinger effect, nonlinear transient hardening, and changeable unloading modulus are taken into account for the Finite Element (FE) simulation.

Abstract: Currently, common inefficient trial-and-error procedures are used in designing bulk forming dies. Numerous iterations, consisting of numerical simulations and subsequent real tests, are needed to achieve accurate parts. During the compensation cycles, manual redesign in CAD environments is necessary to transform discrete data into parametric descriptions causing approximation errors. Automation of these surface reconstruction processes is barely possible. To address these issues, different data-driven numerical strategies have been deduced based on either displacement or force. In this work, a material point tracking method in forming simulation between die and part geometry is presented. Based on this, enhanced displacement-based and stress-based methods for compensation of bulk forming parts are compared. The convergence behavior of both methods is analyzed with respect to the compensation factor. Finally, the material point tracking approach is validated and verified by compensating a two-dimensional bulk-formed component.

Abstract: Stamping tools are prone to an adhesive wear mode called galling. Adhesive wear on stamping tools can degrade the product quality and can affect the mass production. Even a small improvement in the maintenance process is beneficial for the stamping industry. Therefore, this study will focus on understanding and detecting the initiation of tool wear at the microscopic level in sheet metal stamping using acoustic emission sensors. Stamping tests were performed using a semi-industrial stamping process, which can perform clamping, piercing, stamping and trimming in a single cycle. The stamping test was performed using a high strength low alloy sheet steel and D2 tool steel for dry and lubricated conditions. The acoustic emission signal was recorded for each stamped part until severe wear on the dies was observed. These acoustic emission signals were later analyzed using time and frequency domain features. The time domain features such as peak, RMS, kurtosis and skewness could identify significant changes in the acoustic emission signal only when the severe wear was observed on the stamped parts for both dry and lubricated conditions. However, this study has identified that a frequency feature – known as mean-frequency estimate – could identify early stages of wear initiation at the microscopic level. Evidence of this early stage of wear on the part surfaces was not clearly visible to the naked eye, and could only be clearly observed via surface measurement instruments such as an optical profilometer. The sidewalls of the stamped parts corresponding to the initial change in AE mean-frequency trend were qualitatively correlated with 3D profilometer scans of the stamped parts, to show that AE mean-frequency can indicate the initial minor scratches on the sidewalls of the stamped parts due to the galling wear on the die radii surfaces. The results from this study can be used to develop a methodology to determine the very early stages of stamping tool wear, providing a strong basis for condition monitoring in the stamping industry.

Abstract: Based on robust numerical formulations and various material models, finite element (FE) analysis becomes a powerful tool in conventional sheet metal forming process. Unfortunately, the present constitutive equations irrelevant to thickness that describe well conventional sheet deformation modes have difficulties being applied directly to ultra-thin sheet deformation modes. In the present study, a constitutive equation considering size effect is established by introducing a scale factor that represents size effects through thickness and width directions. Uniaxial tensile tests were used to evaluate the scale factor of different thicknesses together with the parameter identification. The developed constitutive equation reveals that thickness is the most important factor effecting on the constitutive relation of ultra-thin sheet. 2D draw forming process of C7035 ultra-thin sheet is analyzed using JSTAMP/NV introducing the developed constitutive equation. The analysis results show that there are obvious differences in the punch forces and loading geometries according to the size effect through thickness direction. Specimen width has slight effect on the flow stress although specimen thickness has strong effect on the flow stress. It is expected that the proposed constitutive equation gives good applicability to FE analysis of micro-scale forming.

Abstract: The effect of the applied state-of-stress on the processing maps depicting the mechanisms for hot working of hot extruded Mg-3Al-1Zn alloy has been evaluated. Flow stresses at various temperatures in the range 300 – 500 °C and strain rates in the range 0.0003 – 1 s^-1 have been measured by deforming in compression and in tension. Processing maps have been developed from the respective flow stress data at a strain of 0.1. The maps are essentially similar irrespective of the mode of deformation – compression or tension, and exhibited two domains in the temperature and strain rate ranges: (1) 375 – 500 °C and 0.0003 – 0.01 s^-1, and (2) 450 – 500 °C and 0.1 – 1 s^-1. On the basis of slower strain rates, high tensile ductility, and the apparent activation energy (152 kJ/mole closer to that for self-diffusion), Domain #1 is interpreted in terms of the occurrence of climb controlled dynamic recrystallization. In Domain #2, which occurs at higher strain rates and has an apparent activation energy near to 165 kJ/mole, dynamic recrystallization occurs that involves second order pyramidal slip {11-22} <11-2-3> and recovery by cross-slip of screw dislocations. The state-of-stress imposed on the specimen (compression or tension) does not have any significant effect on the processing maps or the kinetics of hot deformation.