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The following contribution deals with the experimental investigation and theoretical evaluation of fatigue crack growth under isothermal and non-isothermal conditions at the nickel alloy 617. The microstructure and mechanical properties of alloy 617 are influenced significantly by the thermal heat treatment and the following thermal exposure in service. Hence, a solution annealed and a long-time service exposed material condition is studied. The crack growth measurement is carried out by using an alternate current potential drop system, which is integrated into a thermomechanical fatigue (TMF) test facility. The measured fatigue crack growth rates results in a function of material condition, temperature and load waveform. Furthermore, the results of the non-isothermal tests depend on the phase between thermal and mechanical load (in-phase, out-of-phase). A fracture mechanic based, time dependent model is upgraded by an approach to consider environmental effects, where almost all model parameters represent directly measureable values. A consistent description of all results and a good correlation with the experimental data can be achieved.
Instabilities of the interface between two thin liquid films under DC electroosmotic flow are investigated using linear stability analysis followed by an asymptotic analysis in the long-wave limit. The two-liquid system is bounded by two rigid plates which act as substrates. The Boltzmann charge distribution is considered for the two electrolyte solutions and gives rise to a potential distribution in these liquids. The effect of van der Waals interactions in these thin films is incorporated in the momentum equations through the disjoining pressure. Marginal stability and growth rate curves are plotted in order to identify the thresholds for the control parameters when instabilities set in. If the upper liquid is a dielectric, the applied electric field can have stabilizing or destabilizing effects depending on the viscosity ratio due to the competition between viscous and electric forces. For viscosity ratio equal to unity, the stability of the system gets disconnected from the electric parameters like interface zeta potential and electric double-layer thickness. As expected, disjoining pressure has a destabilizing effect, and capillary forces have stabilizing effect. The overall stability trend depends on the complex contest between all the above-mentioned parameters. The present study can be used to tune these parameters according to the stability requirement.
DEM–FEA estimation of pores arrangement effect on the compressive Young’s modulus for Mg foams
(2015)
This work reports the study of the effect of the pore arrangement on the compressive behavior of Mg foams with regular pore size and porosities ranging from 25% to 45%. Pore arrangements were modeled using Finite Element Analysis (FEA), with random and ordered models, and compared to the estimations obtained for a previous work. The coordinates of the random pore arrangements were firstly generated using Discrete Element Method (DEM), and used in a second stage for modeling the pores by FEA. Estimations were also compared to the experimental results for Mg foams obtained by means of powder metallurgy. Results show important drops in the Young’s moduli as the porosity increases for both, experimental results and FEA estimations. Estimations obtained using ordered pore arrangements presented significant differences when compared to the estimations acquired from models with random arrangements. The randomly arranged models represent more accurately the real topologies of the experimental metallic foams. The Young’s moduli estimated using these models were in excellent agreement with the experiments, whilst the estimations obtained using ordered models presented relative errors significantly higher. The importance of the use of more realistic FEA models for improving the predicting ability of this method was probed, for the study of the mechanical properties of metallic foams.
The formation and analysis of ten microporous triazolyl isophthalate based MOFs, including nine isomorphous and one isostructural compound is presented. The compounds 1 M – 3 M with the general formula [ M ( R 1 - R 2 - trz - ia ) ] ∞ 3 ·x H 2 O (M 2+ = Co 2+ , Cu 2+ , Zn 2+ , Cd 2+ ; R 1 = H, Me; R 2 = 2py, 2pym, prz (2py = 2-pyridinyle; 2pym = 2-pyrimidinyle; prz = pyrazinyle)) crystallize with rtl topology. They are available as single crystals and also easily accessible in a multi-gram scale via refluxing the metal salts and the protonated ligands in a solvent. Their isomorphous structures facilitate the synthesis of heteronuclear MOFs; in case of 2 M , Co 2+ ions could be gradually substituted by Cu 2+ ions. The Co 2+ :Cu 2+ ratios were determined by ICP-OES spectroscopy, the distribution of Co 2+ and Cu 2+ in the crystalline samples are investigated by SEM-EDX analysis leading to the conclusions that Cu 2+ is more favorably incorporated into the framework compared to Co 2+ and, moreover, that the distribution of the two metal ions between the crystals and within the crystals is inhomogeneous if the crystals were grown slowly. The various compositions of the heteronuclear materials lead to different colors and the sorption properties for CO 2 and N 2 are dependent on the integrated metal ions.
In this paper, the correlation of the cyclic J-integral, ΔJ, and the cyclic crack-tip opening displacement, ΔCTOD, is studied in the presence of crack closure to assess the question if ΔJ describes the crack-tip opening displacement in this case. To this end, a method is developed to evaluate ΔJ numerically within finite-element calculations. The method is validated for an elastic–plastic material that exhibits Masing behavior. Different strain ranges and strain ratios are considered under fully plastic cyclic conditions including crack closure. It is shown that the cyclic J-integral is the parameter to determine the cyclic crack-tip opening displacement even in cases where crack closure is present.
In this paper, the initial multiaxial yield behavior of three different gray cast iron materials with lamellar shaped graphite inclusions is numerically investigated by means of the finite-element method. Therefore, volume elements including the real microstructure of the materials are loaded bi- and triaxially beyond macroscopic yield. The shape of the obtained yield surfaces are compared to the surfaces of four continuum models which, amongst others, are proposed in literature to describe the inelastic behavior of gray cast iron with lamellar shaped graphite inclusions. It is found that the presented continuum models and the macroscopic yield surfaces obtained with microstructure-based finite-element models deviate. Furthermore, the initial inelastic flow direction is computed at the onset of macroscopic yielding. The analysis show that the inelastic flow is normal to the yield surface.
Demand Side Management for Thermally Activated Building Systems based on Multiple Linear Regression
(2015)
Since 2003, most European countries established heat health warning systems to alert the population to heat load. Heat health warning systems are based on predicted meteorological conditions outdoors. But the majority of the European population spends a substantial amount of time indoors, and indoor thermal conditions can differ substantially from outdoor conditions. The German Meteorological Service (Deutscher Wetterdienst, DWD) extended the existing heat health warning system (HHWS) with a thermal building simulation model to consider heat load indoors. In this study, the thermal building simulation model is used to simulate a standardized building representing a modern nursing home, because elderly and sick people are most sensitive to heat stress. Different types of natural ventilation were simulated. Based on current and future test reference years, changes in the future heat load indoors were analyzed. Results show differences between the various ventilation options and the possibility to minimize the thermal heat stress during summer by using an appropriate ventilation method. Nighttime ventilation for indoor thermal comfort is most important. A fully opened window at nighttime and the 2-h ventilation in the morning and evening are more sufficient to avoid heat stress than a tilted window at nighttime and the 1-h ventilation in the morning and the evening. Especially the ventilation in the morning seems to be effective to keep the heat load indoors low. Comparing the results for the current and the future test reference years, an increase of heat stress on all ventilation types can be recognized.