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We tested the MOF framework Cu-BTC for natural gas (NG) storage. Adsorption isotherms of C1–C4 alkanes were simulated applying the Grand Canonical ensemble and the Monte Carlo algorithm in a classical molecular mechanics approach. Experimental monocomponent isotherm of the alkanes was used to validate the force field. We performed multicomponent adsorptions calculations for three different quaternary mixtures of C1–C4 alkanes, matching typical NG streams composition, and predicted theoretical storage capacities, efficiency and accumulation of the NG within that composition. Despite being one of the frameworks with greatest storage capacity of methane, we found that Cu-BTC presented great sensitivity to the variation of the heavier alkanes in NG composition. When we increase the percentage of butane from 0.1% to 0.7% in the mixture, the mass of components retained in the discharge pressure (1 bar) increases from 35 to 60%. We also perform siting and interaction energy investigations and compare the NG storage performance of the Cu-BTC with that of activated carbons. To our knowledge, this is the first study regarding the efficiency of the NG storage in Cu-BTC.
Cast iron materials are used as materials for cylinder heads for heavy duty internal combustion engines. These components must withstand severe cyclic mechanical and thermal loads throughout their service life. While high-cycle fatigue (HCF) is dominant for the material in the water jacket region, the combination of thermal transients with mechanical load cycles results in thermomechanical fatigue (TMF) of the material in the fire deck region, even including superimposed TMF and HCF loads. Increasing the efficiency of the engines directly leads to increasing combustion pressure and temperature and, thus, lower safety margins for the currently used cast iron materials or alternatively the need for superior cast iron materials. In this paper (Part I), the TMF properties of the lamellar graphite cast iron GJL250 and the vermicular graphite cast iron GJV450 are characterized in uniaxial tests and a mechanism-based model for TMF life prediction is developed for both materials. The model can be used to estimate the fatigue life of components by means of finite-element calculations (Part II of the paper) and supports engineers in finding the appropriate material and design. Furthermore, the effect of the elastic, plastic and creep properties of the materials on the fatigue life can be evaluated with the model. However, for a material selection also the thermophysical properties, controlling to a high level the thermal stresses in the component, must be considered. Hence, the need for integral concepts for material characterization and selection from a multitude of existing and soon-to-be developed cast iron materials is discussed.
A complete thermomechanical fatigue (TMF) life prediction methodology is developed for predicting the TMF life of cast iron cylinder heads for efficient heavy duty internal combustion engines. The methodology uses transient temperature fields as thermal loads for the non-linear structural finite-element analysis (FEA). To obtain reliable stress and strain histories in the FEA for cast iron materials, a time and temperature dependent plasticity model which accounts for viscous effects, non-linear kinematic hardening and tensioncompression asymmetry is required. For this purpose a unified elasto-viscoplastic Chaboche model coupled with damage is developed and implemented as a user material model (USERMAT) in the general purpose FEA program ANSYS. In addition, the mechanismbased DTMF model for TMF life prediction developed in Part I of the paper is extended to three-dimensional stress states under transient non-proportional loading conditions. The material properties of the plasticity model are determined for lamellar graphite cast iron GJL250 and vermicular graphite cast iron GJV450 from isothermal and non-isothermal uniaxial tests. The methodology is applied to obtain a TMF life prediction on two cast iron cylinder heads for heavy duty diesel engine applications made from both cast iron materials. It is shown that the life predictions using the developed methodology correlate very well with observed lives from two bench tests in terms of location as well as number of cycles to failure.
Warmumformwerkzeuge unterliegen während des Betriebes komplexen thermischen und mechanischen Beanspruchungen. In kritischen Bereichen können dadurch lokal Spannungen entstehen, die die Fließgrenze überschreiten. Bei der Serienproduktion führt dies zu zyklischen plastischen Verformungen und zur thermomechanischen Ermüdung, welche die Lebensdauer der Warmumformwerkzeuge maßgeblich bestimmen kann. Zur Bewertung der thermomechanischen Ermüdung der Warmumformwerkzeuge gibt es jedoch heute keine etablierten Konzepte, da dieser Aspekt erst durch die Notwendigkeit einer höheren Ressourcen- und Energieeffizienz und optimierter Produktionsprozesse (beispielsweise im Rahmen von Industrie 4.0) eine höhere Aufmerksamkeit erreicht. In dieser Arbeit wird zum einen die aktuell industriell angewandte Vorgehensweise zur Auslegung von Warmumformwerkzeugen hinsichtlich der Lebensdauer erläutert. Des Weiteren wird ein Überblick über existierende Plastizitätsmodelle und Lebensdauermodelle gegeben. Dabei wird zwischen rein phänomenologischen und mechanismenbasierten Modellen unterschieden. Aus der betriebenen Recherche wird ersichtlich, dass weiterer Forschungsbedarf auf diesem Gebiet notwendig ist.