Papers by Keyword: Material Modeling

Abstract: The present work proposes a novel strategy that significantly shortens Solid-State Foaming (SSF) times, delivering a substantial improvement in process efficiency and paving the way for faster production of customized and functionalized prosthetic components. In particular, the evolution of porosity was evaluated in terms of both volume fraction and mean pore diameter and its subsequent effect on microhardness in a Ti6Al4V-ELI alloy that was initially densified via Hot Isostatic Pressing (HIP) and then subjected to Laser-Induced Solid-State Foaming (LISSF). This acronym was introduced by the authors to underline the originality of this approach, which is not reported in the existing literature. Localized heat treatments were performed using a CO₂ laser source at a target temperature of 1020°C, with three distinct dwell times (120, 240, and 360 seconds). To predict density variations and the resulting mechanical properties, two analytical models were implemented and validated: (i) the Johnson–Mehl–Avrami–Kolmogorov (JMAK) kinetic model, which effectively described the time-dependent evolution of porosity and pore growth under different thermal regimes (based on conduction phenomena vs. direct laser exposure) and (ii) the Ryshkewitch-Duckworth (RD) model, which was used to correlate the exponential decay of microhardness with increasing porosity. The experimental results and regression analyses confirm the high predictive accuracy of both models (R² greater than 0.95), demonstrating the feasibility of the LISSF process for fabricating titanium components with locally controlled porosity for biomedical applications with reference to the manufacturing of customized and functionalized prosthetic components, ensuring both structural reliability and enhanced performance. On the other side, experimental results demonstrated that process parameters play a critical role in the microstructural evolution: specifically, increasing the dwell time to 360 s under direct laser exposure (1020°C) led to a maximum porosity fraction of approximately 30% and a growth in mean pore diameter up to about 35 µm.

Abstract: Accurate modeling of the real material behavior is fundamental to improve the accuracy of the finite element analysis (FEA) of sheet metal forming processes. Classical material models such as Hill’48 or Barlat Yld2000-2d do not consider the material behavior under plane strain and shear, even though these states are the primary cause of failures observed in sheet metal forming. Moreover, yield criteria are conventionally calibrated at the onset of plastic deformation to determine the initial yield locus. Isotropic hardening is subsequently assumed, based on the flow curve under uniaxial tension. However, some modern sheet metals exhibit a pronounced distortional hardening behavior, which cannot be sufficiently mapped by the conventional modeling strategy. Hence, this contribution aims to improve the mapping of the yield locus distortion by considering the plane strain and shear stress states and by performing the parameter calibration at higher plastic strains. Hereby, the yield locus exponent of the Barlat Yld2000-2d is adapted in order to accurately map the material behavior under plane strain or shear. Moreover, the influence of a strain-dependent calibration of the yield locus on the mapping accuracy is investigated. Two materials, AA5182 and DP600, are being investigated. It is observed that the consideration of the plane strain state leads to a reduction of the yield locus exponent while the consideration of the shear stress state is accompanied with an increase of the yield locus exponent.

Abstract: This study proposes a Bayesian data assimilation approach to estimate material model parameters based on deformation fields measured via digital image correlation in a biaxial tensile test using a cruciform specimen. The anisotropy parameters and exponent of the Yld2000-2d yield function for a A5052P-H32 aluminum alloy are identified. The results indicate that the proposed method can estimate parameters with high accuracy—comparable to those identified via conventional multiaxial testing methods—while requiring only a single biaxial test. The proposed method offers an efficient framework for material modeling by minimizing a cost function via Bayesian optimization, enabling parameter identification from a single biaxial tensile test for sheet metal forming applications.

Abstract: Wire arc directed energy deposition (WA-DED) is a cost-efficient additive manufacturing process with high deposition rates, yet the prediction of resulting mechanical properties remains challenging due to repeated thermal cycling and associated microstructural changes. Accordingly, this work aims to validate a hardness prediction model for DIN SG2 by Härtel et al. For this purpose, a demonstrator was designed, manufactured, and simulated using a thermal finite element model in the standard software Simufact Welding 2025. Since the DED module of the software used does not adequately represent active interlayer cooling, four substitute models for the convective heat transfer coefficient were implemented and evaluated. In addition, the original hardness prediction model was refined to consider complex path planning, remelting effects and a material-dependent lower temperature limit for tempering or heat treating the material. Using a substitute model that adjusts the convective heat transfer coefficient over time, the improved hardness prediction the adjusted hardness prediction model achieved an accuracy of ±5% for 81 of 88 evaluated measurement points. In order to enable an efficient and reproducible comparison between simulation and experiment, a Python evaluation script was developed. This tool automatically identifies relevant temperature peaks, correlates them with hardness data, creates individual evaluation diagrams and a comparison diagram, and exports all processed data to an Excel file.

Abstract: The design process of fiber-reinforced plastics (FRP) is a challenging task, especially concerning passenger vehicles in crashworthiness applications where manufacturing limitations and requirements regarding passive safety have to be considered. Numerical optimization can be a helpful tool during the design process, but most available methods are not applicable because analytical sensitivities are not available in crash simulations. The Graph and Heuristic based Topology Optimization (GHT) can be utilized to optimize the topology of cross-sections of crashworthiness structures while fulfilling a wide range of manufacturing constraints, but it has to be extended for composites. Since the topology changes during optimization runs, the stress state changes as well. This demands high predictive capabilities on the material model. This paper presents the necessary adjustments to describe composite profile structures within the GHT method. A commercial material model for LS-Dyna is parameterized and used for the calculation process.

Abstract: The cyclic growth and recovery of warpage were observed in experiments on Si/solder/Cu layered plates subjected to cyclic thermal loading [1]. In the present study, the experiments were analyzed using representative material models for the solder and Cu layers in finite element analysis. The warpage growth/recovery behavior observed was reproduced well in the analysis using the Armstrong-Frederick and Ohno-Wang models for the solder and Cu layers, respectively. Material ratcheting due to non-proportional cyclic loading was found to happen in the solder layer as a consequence of the CTE mismatch, while material ratcheting due to proportional cyclic loading occurred in the Cu layer as a result of the significant temperature dependence of viscoplasticity in the solder layer.

Abstract: Multi-material and hybrid constructions are increasingly used in the automotive industry with the aim of achieving significant weight reductions of conventional car bodies, and thereby lead to effective reductions of fuel consumption. In this respect, the use of aluminum and short fiber reinforced plastics represents an interesting material combination. A full exploitation of such a material combination requires a suitable joining technique. Among different joining techniques, clinching represents one of the most appealing alternatives for automotive applications. This contribution deals with the experimental tests for determination of material behaviour of two representative materials PA6GF30 and EN AW 5754, which are used for parameterization of material models needed for numerical analysis of the clinching process using the FE software LS-DYNA. With regard to the material modeling of the aluminum sheet, an isotropic material model based on the von Mises plasticity implemented in LS-DYNA was chosen. For the description of the strain hardening behaviour of the aluminum sheet at high equivalent plastic strains, the hydraulic bulge test was carried out in addition to the uniaxial tensile test. For modeling of the short fiber reinforced thermoplastic a semi-analytical model for polymers (SAMP-1) available in LS-DYNA was taken. This material model uses an isotropic pressure dependent yield surface for the description of homogeneous materials. Finally, the FE model of clinching process is presented and an outlook of planned activities is given in terms on determination of the yield surface and hardening behaviour of PA6GF30 at high plastic strains.

Abstract: With the development of numerical calculation and precision forming, constitutive equations are required to possess high accuracy and good reliability, rather than simplicity of mathematical form. Due to simple algorithm and constant parameters, the conventional constitutive models can not be suited to describing superplastic flow behavior which represents complex responses with a large strain. In this study, through surface fitting on experimental data from tension tests performed over a wide range of strain rates, tensile velocities and loads, an empirical approach was proposed to establish constitutive equation for complex superplastic behavior of Zn-5%Al alloy at 340 °C. The empirical constitutive equation not only represents the strain dependence and the strain rate dependence of stress, but also reflects the coupling effects of strain and strain rate on stress, which can not be achieved by traditional models. A comparison between the predicted flow stresses and the experimental data verified that the empirical constitutive equation has high accuracy and good reliability on modeling superplastic flow behavior of Zn-5%Al alloy at 340 °C in a wide range of strains 0~2.5 and strain rates 7.0×10^-5~8.0×10^-2 s^-1.

Abstract: The evolution of the mold temperature during squeeze casting of EN-AB46000 aluminum alloy has been correlated with the final mechanical performances of cast ingots. Starting from a material model which expresses hardness and yielding stress of cast aluminum alloys as a function of the cooling rate during the melt solidification, an experimental approach has been used to provide a useful tool for process monitoring. As a result, the mold temperature increase during the melt squeezing phase is directly correlated with the main mechanical and microstructural parameters. Experiments were made by squeeze cast small cylinders (14 mm in diameter and 18 mm height) at different values of squeezing pressure, mold pre-heating temperature, and melt temperature. Microscopic observations of the sample sections were made as well as hardness measurements and indentation tests. In conclusion, because of the material solidification, a temperature gradient has been observed in the sample which can be directly related with the aluminum alloy dendrite size and, in turn, with microhardness and yielding stress.

Abstract: Numerical simulation of machining processes represents a promising tool able to reproduce the cutting conditions without the need to perform a large number of experimental tests. In order to obtain reliable results from the finite element method simulation, is then necessary to properly set up the simulation conditions and to implement the most suitable materials behavior according to the real workpiece characteristics. These data are available in commercial softwares libraries but often they have difficulties to properly represent the machined workpiece behavior. Thus, advanced model are implemented in the software to improve the simulations performance and to obtain realistic results. In this work, the more suitable materials flow stress, within those proposed in literature, is sought to simulate the machining process of Ti6Al4V. The results of the simulations have been compared with those obtained experimentally in terms of temperature, chip morphology and cutting force. The results confirm the need to properly select the materials flow stress model according to the physical sample.