Dualities Near the Horizon

In 4-dimensional supergravity theories, covariant under symplectic electric-magnetic duality rotations, a significant role is played by the symplectic matrix M({\phi}), related to the coupling of scalars {\phi} to vector field-strengths. In particular, this matrix enters the twisted self-duality condition for 2-form field strengths in the symplectic formulation of generalized Maxwell equations in the presence of scalar fields. In this investigation, we compute several properties of this matrix in relation to the attractor mechanism of extremal (asymptotically flat) black holes. At the attractor points with no flat directions (as in the N = 2 BPS case), this matrix enjoys a universal form in terms of the dyonic charge vector Q and the invariants of the corresponding symplectic representation RQ of the duality group G, whenever the scalar manifold is a symmetric space with G simple and non-degenerate of type E7. At attractors with flat directions, M still depends on flat directions, but not MQ, defining the so-called Freudenthal dual of Q itself. This allows for a universal expression of the symplectic vector field strengths in terms of Q, in the near-horizon Bertotti-Robinson black hole geometry.Comment: 1+31 pages, 1 Table; v2 : completely reorganized (and shortened) structure, some Sections removed; various clarifications, formulae, and Refs. added; to appear on JHE

Transition from small to large cracks in Ti-6Al-4V specimens

Light alloys allow us to have light components with interesting mechanical features. Ti-6Al-4V is a bimodal titanium alloy employed in aerospace, automotive, maritime and biomedical applications. This alloy has also a good corrosion strength which can be reduced by damages on the passivating surface layer. These damages can be due to an inadequate adhesion of the surface layer, variable loads and interactions with aggressive media [1,2]. For these reasons, quasi-static and fatigue tests on Ti-6Al-4V specimens in different inert, aggressive and very aggressive environments were carried out in the past. Air, air + beeswax coating, paraffin oil, 3.5wt.% NaCl-water solution and water-methanol solutions with several percentages were investigated in order to evaluate the chemical and mechanical forces of the corrosion fatigue phenomena [3]. Stress corrosion cracking (SCC) tests on a low strength steel [4] showed that the threshold of dJ/dt decreases with decreasing deformation rate and that the electrochemical energy contribution on the crack growth is independent from the displacement rate but dependent from the electrochemical conditions at the crack tip. As stated in [5] the crack size effect must be considered because small cracks have very high growth rate. Slow strain rate tensile and low-amplitude cyclic tests on micro-notched high strength low alloy steel specimens showed that crack growth strongly depends on the notch-tip plastic zone and hydrogen activity itself. High cycle fatigue tests with different notch shapes showed in [7] that the maximum stress and gradient increase with decreasing defect size. The stress state at the notch root is a function of the geometry. In [7] an elasto-plastic FE modelling with a multiaxial fatigue criterion and a correction for the stress gradient is also shown. Tanaka et al. [8] applied the fracture mechanics approach to fatigue crack initiation for also small notch-tip radius. In [9] micro-notched 316L steel specimens in a chloride medium gave the crack propagation rate in function of the global loading. A FE model was also developed in [9]. Micro-notched Ti-6Al-4V specimens were machined to get various notch lengths (up to about 100 ) & tested under static & cyclic loads to obtain fracture properties, incubation times. The notches were made using Electro Discharge Machining (EDM) in order to reach various values of stress concentration factor ( ) without notch tip plasticity. The tests were carried out on a testing machine previously designed by the Structural Mechanics Laboratory (SM-Lab) of the University of Bergamo and now modified so that bigger specimens can be used and easier setting can be reached. During the tests an axial load was applied with a fixed increment of it every fixed time. The machine has specific grips in order to avoid unwanted bending moment on the specimens. Test results are plotted on Kitagawa diagram to analyze the role of environment on static and cyclic applied loads. At the threshold i.e. at the endurance limit (horizontal line in Kitagawa diagram) we calculated: . [1] Brown B.F. 1972: 147–244, Naval Research Laboratory, Washington, D.C. [2] Lee E.U., Vasudevan A.K., Sadananda K. International Journal of Fatigue 2005; 27: 1597-1607. [3] Baragetti S., Villa F. Fracture and Structural Integrity 30 (2014), 84-94. [4] Sarioğlu, F. and Doruk, M. Conf.Proc., Advances in Fracture Research (ICF-7), Houston, Texas, USA, ed. K. Salama, 1989, 1, pp. 259–65. [5] Gangloff RP. Met Trans A 1985;16A (5):953-69. [6] Bosch, C., Delafosse, D., Longaygue, X. European Corrosion Congress 2010 (Eurocorr2010), Sep 2010, Moscou, Russia. 2, pp.1558-1570, 2010. [7] Franck Morel, Anne Morel, Yves Nadot. International Journal of Fatigue,Elsevier, 2009, 31 (2), pp.263-275. [8] Tanaka K., Nakai, Y., Kawashima, R. Engineering Fracture Mechanics 18, 5, pp.1011-1023,1983. Tinnes, J.P., Delafosse, D., Bosch, C., Raquet, O., Santarini, G. Euroorr 2004

Neutron Transverse-Momentum Distributions and Polarized 3He within Light-Front Hamiltonian Dynamics

The possibility to extract the quark transverse-momentum distributions in the neutron from semi-inclusive deep inelastic electron scattering off polarized 3He is illustrated through an impulse approximation analysis in the Bjorken limit. The generalization of the analysis at finite momentum transfers in a Poincare' covariant framework is outlined. The definition of the light-front spin-dependent spectral function of a J=1/2 system allows us to show that within the light-front dynamics only three of the six leading twist T-even transverse-momentum distributions are independent.Comment: 4 pages, 2 figures, Presented at the 20th International IUPAP Conference on Few-Body Problems in Physics, 20 - 25 August, 2012, Fukuoka, Japa

Towards an improved description of SiDIS by a polarized 3He target

The possibility of improving the description of the semi-inclusive deep inelastic electron scattering off polarized 3He, that provides information on the neutron single spin asymmetries, is illustrated. In particular, the analysis at finite momentum transfers in a Poincare' covariant framework is outlined and a generalized eikonal approach to include final state interaction is presented.Comment: 4 pages, 2 figures, presented at The 22nd European Conference on Few Body Problems in Physics, Krakow, Poland, 9 - 13 September 201

study on a new mobile anti terror barrier

Abstract The vehicle-ramming terror attacks in Berlin, Barcelona, London and Nice highlighted our vulnerability: all of us could be wounded or killed during a walk in a crowded place, it is sufficient a car, a van or a truck. The authors of this paper designed a planter full of water and mainly made of steel and cast iron. For this reason, this device serves as both mobile anti-terror barrier and street furniture. This barrier can stop a 3500 kg vehicle running at 64 km/h and the system itself in less than five meters as demonstrated by the experimental crash test. Starting from these considerations, a simplified mathematical model of the impact was developed and a finite element model was calibrated. The first one points out the main features needed by the obstacle; the second one is a good base for further analyses

Quantifying Credit Portfolio sensitivity to asset correlations with interpretable generative neural networks

In this research, we propose a novel approach for the quantification of credit portfolio Value-at-Risk (VaR) sensitivity to asset correlations with the use of synthetic financial correlation matrices generated with deep learning models. In previous work Generative Adversarial Networks (GANs) were employed to demonstrate the generation of plausible correlation matrices, that capture the essential characteristics observed in empirical correlation matrices estimated on asset returns. Instead of GANs, we employ Variational Autoencoders (VAE) to achieve a more interpretable latent space representation. Through our analysis, we reveal that the VAE latent space can be a useful tool to capture the crucial factors impacting portfolio diversification, particularly in relation to credit portfolio sensitivity to asset correlations changes