A hierarchically coupled cellular automata (CA) model, crystal plasticity finite element method (CPFEM), and thermal finite element (FE) model is developed to predict the softening kinetics of the ...bulged steel tube during non-isothermal annealing. Through the developed model, the kinetics of softening mechanisms including static recovery (SRV) and static recrystallization (SRX), as well as the recrystallization texture are predicted. Later, the Johnson-Mehl-Avrami-Kohnogorov (JMAK) model based on the predicted SRX data is developed to interpret the recrystallization behavior of the material. To perform this study, diverse experimental tests including tube hydroforming (THF), annealing, uniaxial tensile test, hardness test, as well as microstructure observations through optical microscopy and Electron Backscatter Diffraction (EBSD) tests on steel tube are performed. The obtained experimental data are utilized to calibrate and verify the implemented CPFEM model for simulation of THF process, thermal FE model for prediction of the local temperature over annealing time, and CA algorithm for modeling of the softening kinetics and texture evolution throughout the annealing process. The study shows that the predicted deformation characteristics, softening kinetics, recrystallization texture and temperature profile during non-isothermal annealing are in good agreement with experimental data. During the annealing process, a total of four stages for the kinetics of softening mechanisms is observed: No softening; SRV only; SRV dominant; and SRX dominant. During the progress of SRX, the behavior of recrystallization is abruptly changed, confirming that two different mechanisms are controlling the kinetics of transformation.
A modeling setup based on the cellular automata (CA) method was developed to model the kinetics of static recrystallization (SRX) in hydroformed steel tubes undergoing the annealing process. In ...addition, the impact of multiaxial deformations on the kinetics of SRX within the hydroformed steel tube was investigated. First, hoop and axial strains developing at the pole of the steel tube during hydroforming were obtained from the digital image correlation measurements. Then, an exact analytical solution was employed to calculate the corresponding hoop and axial stresses at the pole of the bulged tube. Using these biaxial stress–strain curves, the stored energy was calculated for the hydroformed tube specimen. Second, the actual grain topology and crystallographic orientation of grains in the deformed specimen were obtained with the electron backscatter diffraction. Third, the calculated stored energy as well as the microstructural and crystallographic data was incorporated into the CA model as initial conditions to predict the kinetics of SRX in the specimen during annealing. Finally, to assess the accuracy of the CA model, experimental and predicted results were compared in terms of grain topology data including the grain size and aspect ratio distributions, as well as the rate of recrystallization during annealing. A reasonable agreement between the experimental and CA predictions in partial and fully recrystallized specimens was achieved inferring the validity of the developed algorithm and the modeling setup to predict the SRX kinetics during the post-tube hydroforming annealing process.
Crystal plasticity finite element (CPFEM) modeling of metals that can be age hardened consisting of second phase particles is extensively performed based on representative volume element (RVE) ...models. The RVE model is generated for ferritic low-carbon steel using the data obtained from microstructural observation through optical microscopy (OM) and electron backscatter diffraction (EBSD). The generated RVE is required to statistically represent the original material in terms of grain topology and texture in microscale, as well as the configuration of second phase particles in submicron scale. The multiscale, multi-phase nature of the generated RVE leads to a computationally expensive modeling procedure. To overcome this issue, an alternative multiscale modeling approach based on a homogenization scheme is proposed, in which the effect of second phase particles on deformation behavior is accounted for with no need for the explicit presence of particles in RVE. Lastly, a thorough parametric analysis is performed to investigate the sensitivity of the mechanical properties to the second phase particles in terms of size, volume fraction, geometrical distribution, and deformable or non-deformable properties of precipitates in the investigated material. The results show that the proposed multiscale modeling approach successfully accounts for the effect of second phase particles on deformation behavior, while the computational cost is reduced by more than 99%. In addition, the simulations show that the configuration of second phase particles at a microscale plays an important role in defining the mechanical behavior of the material.
Determination of mechanical properties in bulged specimens has its own challenges due to their nonhomogeneous geometry. In the present study, a set of empirical hardness-strength-hardening law ...correlations is proposed to estimate the flow behavior of the bulged specimens. To do so, comprehensive empirical strength-hardness correlations applicable for all aluminum alloys were developed by collecting datasets from 16 individual studies. These relationships were then compared with hardness/strength experimental data obtained from heat-treated AA6061 and AA7075 tubular specimens. Then, a set of empirical models was developed to calibrate the coefficients of the Holloman and Voce type hardening laws by utilizing the measured hardness and strength values. Finally, to assure the validity of the proposed model, Rockwell hardness test, conducted on the bulged AA6061 and AA7075, was simulated. To do so, the generated flow behavior of bulged specimens based on hardness measurement was used as input plastic flow data in Abaqus commercial FE software, and the indentation geometry on bulged Al specimens were then compared to the experiments. Comprehensive linear and second-degree polynomial relationships were regressed on compiled data for ultimate tensile and yield strengths, respectively. Reasonable agreement was achieved between hardness-strength relationships obtained for all Al alloys and processed AA6061 and AA7075 tubular samples, confirming that these relationships are applicable for any Al alloys regardless of product type and process design. Finally, although both Holloman and Voce type hardening laws gave reasonable estimation of flow behavior in Al tubular specimens particularly at high strain values, Voce model provided a better estimation in comparison to Holloman hardening law. The FE analysis of Rockwell hardness test confirmed that the proposed hardness-strength-hardening law relationship based on Voce type hardening rule was sufficiently accurate in predicting the flow curve in bulged 6061 and 7075 Al alloys.
Satellite systems such as Low Earth Orbiting (LEO) networks play an important role in the next generation 5G networks. To facilitate the integration of satellite and terrestrial networks, ...software-defined networking (SDN) is embraced which brings flexibility, user-customized services and reduces the cost of network configurations. However, it has been long known that communications via LEO satellite network suffer from long delay and frequent ground-satellite handovers, both are problematic for TCP connections. The emergence of Multipath TCP (MPTCP) provides a new solution to these challenges. In this paper, we study the performance of MPTCP over SDN-enabled LEO satellite networks. MPTCP maintains multiple simultaneous subflows in space to increase throughput. In anticipation of handover, MPTCP creates subflows that run in backup mode and shifts traffic smoothly. To support MPTCP, we design an SDN controller that identifies MPTCP subflows attached to the same MPTCP session and splits them to disjoint paths. The SDN architecture centralizes the routing logic, so the system is more scalable and on-board processing is minimized. Simulations are run to evaluate the proposed MPTCP-SDN framework. It is shown that compared to previous solutions, our strategy significantly improves throughput performance and prevents the interruption of transmission during handover.
The martensite phase transformation dependence upon deformation modes and strain paths in QP980 steel formed into a T-shape panel was studied through combination of a 3D digital image correlation ...(DIC) and neutron diffraction. The T-shape emulates the rocker-end of a generic automotive component B-pillar. QP980 belongs to the third-generation advanced high strength steel, which is chosen for two reasons: (1) deformation-induced phase transformation from austenite to martensite occurs in QP980; (2) the initial retained austenite volume fraction (RAVF) of approximately 12% enables the examination of martensite phase transformation at various deformation levels before fracture. Strain fields obtained from DIC enable an investigation of the effects of linear, bi-linear, and nonlinear strain paths together with deformation modes, such as tension, plane strain, biaxial tension and equibiaxial tension. The measured RAVF values from neutron diffraction were extracted from ten specific locations on the formed T-shape panels, which provide the martensite phase transformation percentage at every forming depth. The coupled results between strain field and RAVF reveal significant martensite phase transformation dependence on deformation mode and strain path before fracture. The most phase transformation generally occurs under biaxial tension, while the least happens under plane strain deformation. Moreover, six different forming cases are included in this study with various draw and stretch depths to investigate the forming effects on the martensite phase transformation.
A combined experimental and computational method was developed to characterize the yield stress surface and the conjugate plastic strain-rate potential of the AA7075-O extruded tube under biaxial ...tension and biaxial tension-compression stress states based on the plastic work equivalency assumption. In all the previous experimental investigations, the focus was on developing tube bulging experimental setup with PID capability to control pressure and axial feed to generate real-time proportional deformation in the stress space. In this paper, a more practical method is introduced in which the PID control is implemented into a finite element model with the von Mises yield function to generate the necessary pressure and axial feed that result in any desired proportional strain path in the mid-section of the tube in the tube bulging simulation. As expected, the application of FEM + PID generated boundary conditions in the tube bulging experiments resulted in slightly different deformation paths. However, the plastic work equivalency assumption was used to construct the yield stress surface and the plastic strain-rate potential for the tube at different plastic work levels. Finally, the parameters of the Yld2004–18p yield function were calibrated for AA7075-O aluminum tube with the measured principal stress and plastic strain rate ratios for three levels of accumulated plastic work, and it was shown that this tubular material displays nearly isotropic plasticity under tension-compression, and anisotropic plasticity under tension-tension stress states.
•FEM+PID numerical model was developed for tube bulging experiments boundary conditions generation.•Experimental setup was developed to characterize AA7075 extruded tube under biaxial tension and tension-compression.•Yield surface and the conjugate plastic strain-rate potential of the AA7075 tube was characterized.•Yld2004–18p yield function was calibrated based on the characterized yield surface and the conjugate strain-rate potential.•The material showed nearly isotropic plasticity under tension-compression, and anisotropic plasticity under tension-tension.
We propose Software Defined Networking (SDN) framework to a fleet of naval ships that relies on multiple satellite communication systems for onboard communication. Our solution addresses practical ...issues in current shipboard naval networks such as sharing and load balancing of multiple satellite communication links as well as overcoming limited bandwidth constraints. To ameliorate link intermittence and outage, we propose Multi-Path Transmission Control Protocol (MPTCP), which improves end-to-end data delivery by creating several subflows under one TCP session. In our SDN framework, each ship is an SDN switch with multiple SATCOM connections. The management and classification of MPTCP subflows are handled by a remote SDN controller. The cooperation between MPTCP and SDN controller leads to an agile, bandwidth efficient, robust naval network. System analysis and numerical evaluation validate the feasibility and efficacy of our SDN-based solution for such a network.
Incremental sheet forming was developed several decades ago as a cost-effective forming process for low volume production. The deformation mechanism of this process is completely different from the ...conventional sheet metal–forming processes. Applying a three-dimensional (3D) yield function such as Yld2004-18p for the simulation of incremental sheet forming is necessary to account for the significant anisotropy and out-of-plane shears that develop in the sheet metal. Since it is difficult to experimentally measure out-of-plane shear stresses, a set of virtual experiments was conducted with crystal plasticity finite element method (CPFEM) to obtain the required data. To that end, five different representative volume elements (RVEs) were constructed and simulated with a rate-independent CPFEM model to assess which model best predicts the anisotropy of the AA7075 aluminum sheet. Of the four CPFEM models based on the
associate flow rule
, three used RVEs accounting for grain orientations and grain size distributions, while the fourth model used only the grain orientation information (Taylor’s assumption). The fifth CPFEM model was modified based on the Hill’s 1948
non-associated flow rule
and used the Taylor’s assumption. It was verified by experimental data that the fifth CPFEM model (Taylor + Hill) provides the most computationally efficient and accurate prediction of flow stresses and R-values as a function of the accumulated plastic work. The results from the Taylor + Hill model were then used to calibrate the Yld2004 yield function used in the simulation of the single-point incremental forming (SPIF) of 45
°
and 67
°
conical shapes with AA7075 sheet metal. It was found that simulation results obtained with Yld2004 yield function well predicts the deformation characteristics of both cone shapes when compared with experimental results. Also, the yield locus of AA7075 sheet metal after the SPIF process was predicted based on evolved crystal orientations and the critical resolved shear stress.
