Confining forces

We discuss the forces on the internal constituents of the hadrons based on the bag model. The ground state of the hadrons forms a color singlet so that the effects of the colored internal states are neutralized. From the breaking of the dilatation and conformal symmetries under the strong interactions the corresponding currents are not conserved. These currents give rise to the forces changing the motion of the internal particles which causes confinement.Comment: 8 pages, 2 figure

Risk estimation for LCF crack initiation

An accurate risk assessment for fatigue damage is of vital importance for the design and service of today's turbomachinery components. We present an approach for quantifying the probability of crack initiation due to surface driven low-cycle fatigue (LCF). This approach is based on the theory of failure-time processes and takes inhomogeneous stress fields and size effects into account. The method has been implemented as a finite-element postprocessor which uses quadrature formulae of higher order. Results of applying this new approach to an example case of a gas-turbine compressor disk are discussed.Comment: 12 pages, 6 figure

Taking Informed Action on Student Activity in MOOCs

This paper presents a novel approach to understand specific student behavior in MOOCs. Instructors currently perceive participants only as one homogeneous group. In order to improve learning outcomes, they encourage students to get active in the discussion forum and remind them of approaching deadlines. While these actions are most likely helpful, their actual impact is often not measured. Additionally, it is uncertain whether such generic approaches sometimes cause the opposite effect, as some participants are bothered with irrelevant information. On the basis of fine granular events emitted by our learning platform, we derive metrics and enable teachers to employ clustering, in order to divide the vast field of participants into meaningful subgroups to be addressed individually

Probabilistic Analysis of LCF Crack Initiation Life of a Turbine Blade under Thermomechanical Loading

An accurate assessment for fatigue damage as a function of activation and deactivation cycles is vital for the design of many engineering parts. In this paper we extend the probabilistic and local approach to this problem proposed in [1,2] and [3] to the case of non-constant temperature fields and thermomechanical loading. The method has been implemented as a finite element postprocessor and applied to an example case of a gas-turbine blade which is made of a conventionally cast nickel base superalloy.Comment: 8 pages, 3 figure

A Probabilistic Model for LCF

Fatigue life of components or test specimens often exhibit a significant scatter. Furthermore, size effects have a non-negligible influence on fatigue life of parts with different geometries. We present a new probabilistic model for low-cycle fatigue (LCF) in the context of polycrystalline metal. The model takes size effects and inhomogeneous strain fields into account by means of the Poisson point process (PPP). This approach is based on the assumption of independently occurring LCF cracks and the Coffin-Manson-Basquin (CMB) equation. Within the probabilistic model, we give a new and more physical interpretation of the CMB parameters which are in the original approach no material parameters in a strict sense, as they depend on the specimen geometry. Calibration and validation of the proposed model is performed using results of strain controlled LCF tests of specimens with different surface areas. The test specimens are made of the nickel base superalloy RENE 80.Comment: 20 pages, 6 figure

Electronic Structure of LiMnO2_{2}: A Comparative Study of the LSDA and LSDA+U methods

A first-principles electronic structure study of orthorhombic, monoclinic and rhombohedral LiMnO$_{2}$ has been carried out using the full-potential linearized augmented plane-wave method. The exchange and correlations have been treated within the local spin-density approximation (LSDA) and the LSDA+U methods. In the LSDA, the stable ground state is antiferromagnetic insulator for the orthorhombic and monoclinic structures but is ferromagnetic metal for the rhombohedral structure. The LSDA+U, on the other hand, predicts the ground state to be an antiferromagnetic insulator for all structures. We find that strong correlations change the density of states dramatically around the Fermi level. The LSDA+U predicts the nature of band gap to be a mixture of charge transfer and Mn $d \leftrightarrow d$ like excitations for orthorhombic and monoclinic LiMnO$_{2}$ and Mott-insulator for rhombohedral LiMnO$_{2}$ in agreement with the available experimental results. The inclusion of U increases the magnetic moment on Mn and gives a value in better agreement with experiment. However, Mn valency is not affected by the inclusion of U. We have also calculated X-ray emission photoelectron spectra for the orthorhombic and monoclinic LiMnO$_{2}$ by the LSDA and the LSDA+U methods. We find that LSDA+U gives better agreement with the available experimental results.Comment: 18 pages, 7 figure