Implementation of a modified Graham-Walles viscosity function within a Chaboche viscoplastic constitutive model

peer reviewedThis work provides a numerical framework for the accurate prediction of operational life of metallic components exhibiting a non-classical creep behavior under constant loadings and very high temperature. A modified Graham-Walles type analytical viscoplastic function is implemented into a Chaboche unified viscoplastic constitutive model. The numerical model is integrated into the finite element software Lagamine following a fully-implicit two-step radial return mapping algorithm. The non-linear system of equations is solved using a robust Newton-Raphson method. The computational efficiency of the model is enhanced by implementing a sub-step routine, thereby decreasing the average number of iterations of the finite element software. The validation of the model is performed using experimental data available in the literature on the non-classical creep behavior of Incoloy 800H, a Ni-superalloy exhibiting a two-step creep strain rate minima attributed to multiple complex dislocation-precipitate interactions.Development of a generic MultiScale Creep-Fatigue approach, allowing finite element simulations to predict strains and fracture of metal components at high temperature- application on 800H alloy9. Industry, innovation and infrastructur

Assessment of the influence of creep transition and nitridation in the creep-life prediction of Incoloy 800H

Accurate prediction of creep life of materials has been for years a matter of high research interest. The engineering design of such components is often performed following standardized analytical procedures aimed to empirically correlate state variables (mainly stress and temperature) with the chosen failure criteria (e.g., buckling, time-to-1% strain, time-to-rupture, etc). However, the accuracy of the chosen model ultimately depends on the microstructural properties of the material. As such, changes in thermomechanical treatments and environmental conditions can largely affect the creep behaviour of such components, thus making inadequate the use of simplified analytical models (R. W. Swindeman and D. L. Marriot, “Criteria for design with structural materials in combined-cycle applications above 815°F”, in Journal of Engineering for Gas Turbines and Power, vol. 116, pp. 352-359, 1993). Such is the case of Incoloy 800H, a solution-annealed austenitic Fe-Ni-Cr alloy of high industrial interest as it provides a good balance between production cost and high-temperature mechanical response. Under particularly low-stress and high-temperature loadings, this alloy is reported to exhibit a diffusion-to-dislocation transitional creep. Furthermore, the subsequent large dislocation-driven tertiary creep stage undergoes a nitridation-induced hardening while exposed to high-N environments (V. Guttmann and R. Bürgel, “Creep-structural relationship in steel alloy 800H at 900-1000°C”, in Metal Science, vol. 17, pp. 549-555, 1983). In this work, the creep behaviour of the alloy is modelled using a Chaboche-type constitutive law (H. Morch et al., “Efficient temperature dependence of parameters for thermo-mechanical finite element modelling of alloy 230”, in European Journal of Mechanics / A Solids, vol.85, 2020) implemented in the MSM-team (ULiège) proprietary finite element software Lagamine. The results are later assessed with the aim of proposing a novel and efficient numerical creep micromechanics approach intended for the identification of Chaboche parameters while addressing the underlaying uncertainties that rule the creep behaviour of this alloy: diffusion-dislocation creep transition and nitridation.Development of a generic MultiScale Creep-Fatigue approach, allowing finite element simulations to predict strains and fracture of metal components at high temperature-application on two Ni-Cr alloysMSCreep9. Industry, innovation and infrastructur

A metaheuristic-based method for photovoltaic temperature computation under tropical conditions

Tropical climates have favorable irradiation levels for the development of photovoltaic systems; however, high temperatures have a negative impact on the efficiency of solar cells. Since direct measurement of cell temperature is not common, mathematical models are needed to make predictions. Numerous models have been documented, highlighting the challenge of applying a universal model to different climatic conditions. The main contribution of this study is the proposal of a metaheuristic algorithm to accurately compute the temperature of solar cells. This method is simple and effective in exploring numerous potential states of the reference parameters (i.e., irradiance and ambient temperature). Data collected over a 23-month period in two photovoltaic installations with an output power of 2.2 MW of multicrystalline silicon technology were used to develop the proposed method and validate it. The proposed model was compared with 19 previously reported models in the literature. Compared to the model recommended by the International Electrotechnical Commission (IEC Standard 61215-1), the mean square error, mean absolute error (MAE) and mean absolute percentage error were reduced by 4.9, 4.8, and 2.4 times, respectively. The accuracy of the proposed method is demonstrated by MAE errors ranging from 0.56 °C to 1.88 °C, obtained by considering three different daily profiles of irradiance and ambient temperature. Therefore, the proposed method is recommended to more accurately calculate the temperature of the photovoltaic cell in tropical areas

Optimizing laser power of directed energy deposition process for homogeneous AISI M4 steel microstructure

peer reviewedA finite element model of directed energy deposition (DED) process predicts the thermal history during the manufacturing of high speed steel cuboid samples. The simulation result validation relies on comparisons between measured and predicted data such as temperature histories within the substrate and the melt pool depth of the last coating layer. Integrated within an optimization loop, these DED simulations identify two variable laser power functions able to generate a constant melt pool size. These functions are expected to provide a homogeneous microstructure over layers. The computed thermal fields and the microstructure generated by three AISI M4 experiments performed with the constant laser power case and the two optimized functions at three points of interest located at different depths within the deposit are correlated. The effect of the melt superheating temperature and the thermal cyclic history on micro and nanohardness measurements is observed. As a result, the optimized laser power functions provide samples with more homogeneous microhardness than the constant laser power function, however, the homogeneity of microstructure is not fully confirmed by the nanohardness map throughout the deposited M4 steel layers

Nanomechanical Characterization of the Deformation Response of Orthotropic Ti–6Al–4V

The nanoindentation‐induced mechanical deformation response is applied to identify the orthotropic elastic moduli using the Delafargue and Ulm method as well as to validate the asymmetric orthotropic CPB06 nonlinear plasticity model required in simulations of nonuniform macroscopic mechanical response of the Ti–6Al–4V alloy. Scanning electron microscope (SEM) technique allows to select the maximum penetration depth for the indentation in the deformed alpha phase and alpha–beta interphase, α and α/β, respectively. The apparent macromechanical response can be successfully derived from several residual imprints conducted at micro‐ and/or submicrometric length scale and distributed throughout samples of the investigated bulk alloy, as demonstrated by correlation with finite element simulations based on the orthotropic elastoplastic model. The accurate numerical response obtained validates the material model and the Delafargue and Ulm approach, opening a window for next generation identification methods of macromechanical plasticity models with hybrid experimental–numerical method based on instrumented indentation and the use of SEM technique

Sigma Phase Stabilization by Nb Doping in a New High-Entropy Alloy in the FeCrMnNiCu System: A Study of Phase Prediction and Nanomechanical Response

peer reviewedThe development of high-entropy alloys has been hampered by the challenge of effectively and verifiably predicting phases using predictive methods for functional design. This study validates remarkable phase prediction capability in complex multicomponent alloys by microstructurally predicting two novel high-entropy alloys in the FCC + BCC and FCC + BCC + IM systems using a novel analytical method based on valence electron concentration (VEC). The results are compared with machine learning, CALPHAD, and experimental data. The key findings highlight the high predictive accuracy of the analytical method and its strong correlation with more intricate prediction methods such as random forest machine learning and CALPHAD. Furthermore, the experimental results validate the predictions with a range of techniques, including SEM-BSE, EDS, elemental mapping, XRD, microhardness, and nanohardness measurements. This study reveals that the addition of Nb enhances the formation of the sigma (σ) intermetallic phase, resulting in increased alloy strength, as demonstrated by microhardness and nanohardness measurements. Lastly, the overlapping VEC ranges in high-entropy alloys are identified as potential indicators of phase transitions at elevated temperatures

Assessment of damage and anisotropic plasticity models to predict Ti-6Al-4V behavior

The plastic behavior of the Ti-6Al-4V alloy includes several features as strength differential effect, anisotropy and yield strength sensitivity to temperature and strain rate. Monotonic tensions in the three orthogonal directions of the material are performed to identify the Hill ’48 yield criterion. Monotonic compression and plane strain tensile tests are also included in the experimental campaign to identify the orthotropic yield criterion of CPB06. An assessment of the two models is done by comparing the yield loci and the experimental data points for different levels of plastic work. A first approach of the damage modelling of the Ti-6AL-4V alloy is investigated with an extended Gurson-Tvergaard-Needleman damage model based on Hill ’48 yield criterion. Finite element simulations of the experiments are performed and numerical results allows checking force-displacement curves until rupture and local information like displacement and strain fields. The prediction ability of the Hill ’48, CPB and extended Gurson models are assessed on simple shear and notched tensile tests until fracture

Evaluation of the Orthotropic Behavior in an Auxetic Structure Based on a Novel Design Parameter of a Square Cell with Re-Entrant Struts

In this research, a three-dimensional auxetic configuration based on a known re-entrant cell is proposed. The 3D auxetic cell is configured from a new design parameter that produces an internal rotation angle to its re-entrant elements to study elastic properties in its three orthogonal directions. Through a topological analysis using Timoshenko beam theory, the bending of its re-entrant struts is modeled as a function of the new design parameter to manipulate Poisson’s ratio and Young’s modulus. Experimental samples were fabricated using a fused filament fabrication system using ABS and subsequently tested under quasi-static compression and bending tests. Additionally, an orthotropy factor is applied that allows for measuring the deviation between the mechanical properties of each structure. The experimental results validate the theoretical design and show that this new unit cell can transmit an orthotropic mechanical behavior to the macrostructure. In addition, the proposed structure can provide a different bending stiffness behavior in up to three working directions, which allows the application under different conditions of external forces, such as a prosthetic ankle

Design and Characterization of Asymmetric Cell Structure of Auxetic Material for Predictable Directional Mechanical Response.

peer reviewedA three-dimensional auxetic structure based on a known planar configuration including a design parameter producing asymmetry is proposed in this study. The auxetic cell is designed by topology analysis using classical Timoshenko beam theory in order to obtain the required orthotropic elastic properties. Samples of the structure are fabricated using the ABSplus fused filament technique and subsequently tested under quasi-static compression to statistically determine the Poisson's ratio and Young's modulus. The experimental results show good agreement with the topological analysis and reveal that the proposed structure can adequately provide different elastic properties in its three orthogonal directions. In addition, three point bending tests were carried out to determine the mechanical behavior of this cellular structure. The results show that this auxetic cell influences the macrostructure to exhibit different stiffness behavior in three working directions

Experimental characterization of the compressive mechanicalbehaviour of Ti6Al4V alloy at constant strain rates over the fullelastoplastic range

Full range constant strain rate tests are required for accurately characterizing initial yield point, strength differential effect and direct identification of constitutive laws describing the plastic behavior of materials. These tests require the use of a closed-loop control in order to achieve the constant strain rate, however this feature is not available in many laboratories. An alternative method is proposed here for full range constant strain rate with testing machines that can be configured for user-defined displacements of the cross head prior to testing. Tests performed at a constant die speed include a variable strain rate response for the specimen involved. Significant deformation rate variation occurs between the elastic and plastic range with consequences for initial yield point identification. To overcome this drawback, appropriate user-defined displacements can be computed and applied, allowing for both tensile and compression tests to be performed at a constant strain rate. The method is validated using a compression test of Ti6Al4V alloy at room temperature, as well as a 3D digital image correlation (DIC) system exhibiting a constant strain rate value equal to 10-3 s-1, for both elastic and plastic ranges. A non-negligible inhomogeneous strain field was measured on the surface of the compression specimen using DIC and was corroborated by numerical modeling. Results identified the source of the non-homogeneous strain field, thereby proposing a quantitative indicator of plastic anisotropy. The initial yield stress and strain hardening rates of the alloy at several temperatures were obtained with both testing method, conventional constant cross-head speed, and the constant strain rate; these were then used to determine the influence of the small strain rate variations on the mechanical response of Ti6Al4V alloy.DOMAC