Análisis de daño y estudio de tensiones residuales en componentes mecánicos

Conferencia invitadaUnderstanding the effect of residual stresses in critical for the structural integrity of railway and other mechanical components [1–3]. The most important methods for residual stress measurements are introduced, together with their main advantages and disadvantages [4]. Neutron and synchrotron strain measurements performed in European Synchrotron Radiation Facility (France) [5], Institut Laue Langevin (France) [1], Diamond Light Source (UK) [6,7] and ISIS Neutron and Muon Source (UK) are described. They allow the in-plane components of the stress tensor acting in cross-sectional rail slices to be mapped [8–10]. Alternative techniques such as laboratory X-rays and magnetic measurement systems MAPS are also depicted, showing coarser detail but similar trends than previous methods. Stress balancing appears to be worst in zones where significant plastic deformation takes place [11,12]. In addition, the measurements are complemented with contour method analysis to map the longitudinal stress components. REFERENCES [1] J.F. Kelleher, Residual Stress in Railway Rails, PhD thesis, University of Manchester, School of Materials, 2006. [2] B. Moreno, A. Martin, P. Lopez-Crespo, J. Zapatero, J. Dominguez, Estimations of fatigue life and variability under random loading in aluminum Al-2024T351 using strip yield models from NASGRO, International Journal of Fatigue. 91 (2016) 414–422. [3] C.A. Simpson, S. Kozuki, P. Lopez-Crespo, M. Mostafavi, T. Connolley, P.J. Withers, Quantifying fatigue overload retardation mechanisms by energy dispersive X-ray diffraction, Journal of the Mechanics and Physics of Solids. 124 (2019) 392–410. [4] P.J. Withers, H.K.D.H. Bhadeshia, Residual stress. Part 1 - Measurement techniques, Materials Science and Technology. 17 (2001) 355–365.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Departamento de Ingeniería Civil, de Materiales y Fabricació

Difracción de rayos X y difracción de neutrones en estudios de fatiga y fractura de materiales

Conferencia invitada de Joseph F KelleherThe fatigue behaviour of polycrystalline metals is often studied through crack propagation analysis [1,2]. Nevertheless, understanding the mechanical processes that take place right at the crack tip [3,4] would also involve considering the deformation developing at the plastic zone and the contact between the crack faces over a portion of the loading cycle [5–7]. Paris law or newer models such as Forman equation are commonly used to interpret growth data [8], but cannot be used to generalise for complex loading scenarios, such as multiaxial loads [9–11] or variable amplitude loads [2]. Diffraction methods are a powerful tool to characterise crack tip strains and stresses [12]. The basics principles of neutron and synchrotron diffraction for measuring bulk properties are discussed [13,14], with special emphasis on grain size effects [15,16], transition between plane stress and plain strain conditions [17], measurement of the plastic zone and development of shielding effects at the crack tip [18]. REFERENCES [1] P. Lopez-Crespo, P.J. Withers, F. Yusof, H. Dai, A. Steuwer, J.F. Kelleher, T. Buslaps, Overload effects on fatigue crack-tip fields under plane stress conditions: surface and bulk analysis, Fatigue and Fracture of Engineering Materials and Structures. 36 (2013) 75–84. [2] B. Moreno, A. Martin, P. Lopez-Crespo, J. Zapatero, J. Dominguez, Estimations of fatigue life and variability under random loading in aluminum Al-2024T351 using strip yield models from NASGRO, International Journal of Fatigue. 91 (2016) 414–422. [3] C. Bathias, Retrospective view on the role of the plastic zone at a fatigue crack tip, Fatigue and Fracture of Engineering Materials and Structures. 19 (1996) 1301–1306. [4] P. Lopez-Crespo, D. Camas, F. V Antunes, J.R. Yates, A study of the evolution of crack tip plasticity along a crack front, Theoretical and Applied Fracture Mechanics. 98 (2018) 59–66.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Departamento de Ingeniería Civil, de Materiales y Fabricació

Programmable Control of Nucleation for Algorithmic Self-Assembly

Algorithmic self-assembly, a generalization of crystal growth processes, has been proposed as a mechanism for autonomous DNA computation and for bottom-up fabrication of complex nanostructures. A `program' for growing a desired structure consists of a set of molecular `tiles' designed to have specific binding interactions. A key challenge to making algorithmic self-assembly practical is designing tile set programs that make assembly robust to errors that occur during initiation and growth. One method for the controlled initiation of assembly, often seen in biology, is the use of a seed or catalyst molecule that reduces an otherwise large kinetic barrier to nucleation. Here we show how to program algorithmic self-assembly similarly, such that seeded assembly proceeds quickly but there is an arbitrarily large kinetic barrier to unseeded growth. We demonstrate this technique by introducing a family of tile sets for which we rigorously prove that, under the right physical conditions, linearly increasing the size of the tile set exponentially reduces the rate of spurious nucleation. Simulations of these `zig-zag' tile sets suggest that under plausible experimental conditions, it is possible to grow large seeded crystals in just a few hours such that less than 1 percent of crystals are spuriously nucleated. Simulation results also suggest that zig-zag tile sets could be used for detection of single DNA strands. Together with prior work showing that tile sets can be made robust to errors during properly initiated growth, this work demonstrates that growth of objects via algorithmic self-assembly can proceed both efficiently and with an arbitrarily low error rate, even in a model where local growth rules are probabilistic.Comment: 37 pages, 14 figure

Scale-invariance in gravity and implications for the cosmological constant

Recently a scale invariant theory of gravity was constructed by imposing a conformal symmetry on general relativity. The imposition of this symmetry changed the configuration space from superspace - the space of all Riemannian 3-metrics modulo diffeomorphisms - to conformal superspace - the space of all Riemannian 3-metrics modulo diffeomorphisms and conformal transformations. However, despite numerous attractive features, the theory suffers from at least one major problem: the volume of the universe is no longer a dynamical variable. In attempting to resolve this problem a new theory is found which has several surprising and atractive features from both quantisation and cosmological perspectives. Furthermore, it is an extremely restrictive theory and thus may provide testable predictions quickly and easily. One particularly interesting feature of the theory is the resolution of the cosmological constant problem.Comment: Replaced with final version: minor changes to text; references adde

The physical gravitational degrees of freedom

When constructing general relativity (GR), Einstein required 4D general covariance. In contrast, we derive GR (in the compact, without boundary case) as a theory of evolving 3-dimensional conformal Riemannian geometries obtained by imposing two general principles: 1) time is derived from change; 2) motion and size are relative. We write down an explicit action based on them. We obtain not only GR in the CMC gauge, in its Hamiltonian 3 + 1 reformulation but also all the equations used in York's conformal technique for solving the initial-value problem. This shows that the independent gravitational degrees of freedom obtained by York do not arise from a gauge fixing but from hitherto unrecognized fundamental symmetry principles. They can therefore be identified as the long-sought Hamiltonian physical gravitational degrees of freedom.Comment: Replaced with published version (minor changes and added references

2D mapping of plane stress crack-tip fields following an overload

The evolution of crack-tip strain fields in a thin (plane stress) compact tension sample following an overload (OL) event has been studied using two different experimental techniques. Surface behaviour has been characterised by Digital Image Correlation (DIC), while the bulk behaviour has been characterised by means of synchrotron X-ray diffraction (XRD). The combination of both surface and bulk information allowed us to visualise the through-thickness evolution of the strain fields before the OL event, during the overload event, just after OL and at various stages after it. Unlike previous work, complete 2D maps of strains around the crack-tip were acquired at 60m spatial resolution by XRD. The DIC shows less crack opening after overload and the XRD a lower crack-tip peak stress after OL until the crack has grown past the compressive crack-tip residual stress introduced by the overload after which the behaviour returned to that for the baseline fatigue response. While the peak crack-tip stress is supressed by the compressive residual stress, the crack-tip stress field changes over each cycle are nevertheless the same for all Kmax cycles except at OL

Polarization observables in deuteron photodisintegration below 360 MeV

High precision measurements of induced and transferred recoil proton polarization in d((gamma) over right arrow, (p) over right arrow )n have been performed for photon energies of 277-357 MeV and theta(cm) = 20 degrees-120 degrees. The measurements were motivated by a longstanding discrepancy between meson-baryon model calculations and data at higher energies. At the low energies of this experiment, theory continues to fail to reproduce the data, indicating that either something is missing in the calculations and/or there is a problem with the accuracy of the nucleon-nucleon potential being used. (C) 2011 Elsevier B.V. All rights reserved