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The Zienkiewicz–Zhu (ZZ) super-convergent patch recovery technique based on a node neighborhood patch configuration is used most widely for recovery of the stress field of a finite element analysis. In this study, an improved ZZ recovery technique using element neighborhood patch configuration is proposed. The improved recovery procedure is based on recovery of the stress field in the least-squares sense over an element patch that consists of the union of the elements surrounding the element under consideration. The proposed patch configuration provides more sampling points and improves the performance of the standard ZZ recovery technique. The effectiveness and reliability of the improved ZZ recovery approach is demonstrated through plane elastic and plastic plate problems. The problem domain is discretized with triangular and quadrilateral elements of different sizes. A comparison of the quality of error estimation using the ZZ recovery of derivative field and recovery of the displacement field using similar element neighborhood patch configurations is also presented. The numerical results show that the ZZ recovery technique and the displacement recovery technique, using a modified patch configuration, yield better results, convergence rate, and effectivity as compared with the standard ZZ super-convergent patch recovery technique. It is concluded that the improved ZZ recovery technique-based adaptive finite element analysis is very effective for converging a predefined accuracy with a significantly smaller number of degrees of freedom, especially in an elastic problem. It is also concluded that the improved ZZ recovery technique captures the plastic deformation problem solution errors more reliably than the standard ZZ recovery technique.

Manufacturing systems may be subject to external attacks and failures, so it is important to deal with the recovery of the system after these situations. This paper deals with the problem of recovering a manufacturing system, modeled as a Discrete Event System (DES) using the Supervisory Control Theory (SCT), when the control structure, called supervisor, desynchronizes from the physical plant. The desynchronization may be seen as plant and supervisor being in uncorresponding states. The recovery of the system may be attained if there is a word, the synchronizing word, that regardless the state of each one of them, brings the system and supervisor back to a known state. The concepts of synchronizing automata are used to do so. In this paper we show under what conditions a set of synchronizing plants and specifications leads to a synchronizing supervisor obtained by the Supervisory Control Theory. The problem is extended to cope with multiple supervisors, proposing a local recovery when possible. We also present a simple way to model problems, composed of machines and buffers, as synchronizing automata such that it is always possible do restore synchronization between the control (supervisor) and the plant.

The successful operation of quantum computers relies on protecting qubits from decoherence and noise, which-if uncorrected-will lead to erroneous results. Because these errors accumulate during an algorithm, correcting them is a key requirement for large-scale and fault-tolerant quantum information processors. Besides computational errors, which can be addressed by quantum error correction1-9, the carrier of the information can also be completely lost or the information can leak out of the computational space10-14. It is expected that such loss errors will occur at rates that are comparable to those of computational errors. Here we experimentally implement a full cycle of qubit loss detection and correction on a minimal instance of a topological surface code15,16 in a trapped-ion quantum processor. The key technique used for this correction is a quantum non-demolition measurement performed via an ancillary qubit, which acts as a minimally invasive probe that detects absent qubits while imparting the smallest quantum mechanically possible disturbance to the remaining qubits. Upon detecting qubit loss, a recovery procedure is triggered in real time that maps the logical information onto a new encoding on the remaining qubits. Although the current demonstration is performed in a trapped-ion quantum processor17, the protocol is applicable to other quantum computing architectures and error correcting codes, including leading two- and three-dimensional topological codes. These deterministic methods provide a complete toolbox for the correction of qubit loss that, together with techniques that mitigate computational errors, constitute the building blocks of complete and scalable quantum error correction.

The massive stock of Non-Performing Loans (NPLs) in Europe has forced regulatory and supervisory authorities to promote debate on their management and timely disposal. Simultaneously, the transition to IFRS 9 created the need for higher provisioning and for weighting sale scenarios in the assessment of NPLs. This study, using a scenario analysis based on the Italian experience of the NPL resolution process, focuses on the cost of deleveraging by comparing the alternative strategies of direct sale and securitization. The study highlights the impact of the assumptions derived from the portfolio assessment and the additional cost arising from the uncertainties surrounding the appropriate recovery procedure. It demonstrates that securitization minimises this cost, while estimating the benefit derived from the support of State-backed guarantees. These findings provide useful insights for policy makers, suggesting the promotion of further measures that aim to reduce the transfer of value from banks to third parties.

International audience

How to restore the load of power system after disasters, especially to guarantee the power supply of the important load, is a significant segment of the research on power system resilience. Nowadays, quantities of research existing has been done to reduce the load loss after disasters. However, there is no clear methodologies or metrics available to evaluate the effects of restoration strategies. Here, a method for evaluating the recovery and economic efficiency of the power system post-disaster recovery strategy is explored, and assessment metrics related are defined. The method considers the repair and recovery process of transmission lines, generators, and the distributed generation at load nodes. After given the repair strategies, components state sampling and system power flow calculation will be conducted to analyse the running state of the system, then simulate the component repair and the load recovery procedure, and finally calculate corresponding metrics. This novel method can obtain and compare the expected effects of different restoration strategies in advance, providing more reliable reference information for system decision makers. Finally, the proposed method is applied to IEEE 14-bus system and verifies its effectiveness and application value.

General-relativistic magnetohydrodynamic (GRMHD) simulations are an important tool to study a variety of astrophysical systems such as neutron star mergers, core-collapse supernovae, and accretion onto compact objects. A conservative GRMHD scheme numerically evolves a set of conservation equations for 'conserved' quantities and requires the computation of certain primitive variables at every time step. This recovery procedure constitutes a core part of any conservative GRMHD scheme and it is closely tied to the equation of state (EOS) of the fluid. In the quest to include nuclear physics, weak interactions, and neutrino physics, state-of-the-art GRMHD simulations employ finite-temperature, composition-dependent EOSs. While different schemes have individually been proposed, the recovery problem still remains a major source of error, failure, and inefficiency in GRMHD simulations with advanced microphysics. The strengths and weaknesses of the different schemes when compared to each other remain unclear. Here we present the first systematic comparison of various recovery schemes used in different dynamical spacetime GRMHD codes for both analytic and tabulated microphysical EOSs. We assess the schemes in terms of (i) speed, (ii) accuracy, and (iii) robustness. We find large variations among the different schemes and that there is not a single ideal scheme. While the computationally most efficient schemes are less robust, the most robust schemes are computationally less efficient. More robust schemes may require an order of magnitude more calls to the EOS, which are computationally expensive. We propose an optimal strategy of an efficient three-dimensional Newton-Raphson scheme and a slower but more robust one-dimensional scheme as a fall-back., 13 pages, 3 figures, 2 tables; matches published version

The Run Control System of the Compact Muon Solenoid (CMS) experiment at CERN is a distributed Java web application running on Apache Tomcat servers. During Run-1 of the LHC, many operational procedures have been automated. When detector high voltages are ramped up or down or upon certain beam mode changes of the LHC, the DAQ system is automatically partially reconfigured with new parameters. Certain types of errors such as errors caused by single-event upsets may trigger an automatic recovery procedure. Furthermore, the top-level control node continuously performs cross-checks to detect sub-system actions becoming necessary because of changes in configuration keys, changes in the set of included front-end drivers or because of potential clock instabilities. The operator is guided to perform the necessary actions through graphical indicators displayed next to the relevant command buttons in the user interface. Through these indicators, consistent configuration of CMS is ensured. However, manually following the indicators can still be inefficient at times. A new assistant to the operator has therefore been developed that can automatically perform all the necessary actions in a streamlined order. If additional problems arise, the new assistant tries to automatically recover from these. With the new assistant, a run can be started from any state of the subsystems with a single click. An ongoing run may be recovered with a single click, once the appropriate recovery action has been selected. We review the automation features of the CMS run control system and discuss the new assistant in detail including first operational experience. During Run-1 of the LHC, many operational procedures have been automated in the run control system of the Compact Muon Solenoid (CMS) experiment. When detector high voltages are ramped up or down or upon certain beam mode changes of the LHC, the DAQ system is automatically partially reconfigured with new parameters. Certain types of errors such as errors caused by single-event upsets may trigger an automatic recovery procedure. Furthermore, the top-level control node continuously performs cross-checks to detect sub-system actions becoming necessary because of changes in configuration keys, changes in the set of included front-end drivers or because of potential clock instabilities. The operator is guided to perform the necessary actions through graphical indicators displayed next to the relevant command buttons in the user interface. Through these indicators, consistent configuration of CMS is ensured. However, manually following the indicators can still be inefficient at times. A new assistant to the operator has therefore been developed that can automatically perform all the necessary actions in a streamlined order. If additional problems arise, the new assistant tries to automatically recover from these. With the new assistant, a run can be started from any state of the sub-systems with a single click. An ongoing run may be recovered with a single click, once the appropriate recovery action has been selected. We review the automation features of CMS Run Control and discuss the new assistant in detail including first operational experience.

Objective: The aim of this study was to test if performance of airline pilots, in performing an aerodynamic stall recovery procedure, decreases when they are surprised, compared to when they anticipate a stall event.Background: New flight-safety regulations for commercial aviation recommend the introduction of surprise and startle in upset prevention and recovery training. This calls for more evidence on the effects of surprise on pilot performance, as well as methods to effectively induce surprise in training simulators.Method: The study took place in a motion-base simulator with a poststall aerodynamic model. Using a within-subjects design, the recovery performance of 20 pilots was tested in 2 conditions: 1 anticipated condition, and 1 surprise condition. In addition to flight parameters, subjective and physiological data relating to surprise and startle were measured.Results: Pilots had significantly more difficulties with adhering to the recovery procedure in the surprise condition compared to the anticipation condition. The subjective and physiological measures confirmed that the manipulation mainly increased surprise, and to a lesser extent also startle.Conclusion: The results suggest that pilots have more difficulty in managing an upset situation (i.e., an aerodynamic stall) when this situation is presented unexpectedly, underlining that upset prevention and recovery training should include elements of surprise.

The present investigation concludes the triad of papers by the first three authors concerning the 2D-unconstrained third order shear deformation theory for shell-like structures. Here, the static behavior of functionally graded spherical shells and panels subjected to uniform loadings at the extreme surfaces is studied. The material properties are graded in the thickness direction according to a four parameter power law. The structural model involves the a posteriori stress and strain recovery procedure. The obtained governing equations are solved by means of the GDQ numerical technique. An extensive numerical investigation is carried out to characterize the effect of material parameters on the stress, strain and displacement profiles along the thickness direction. The second order equilibrium operators, of the fundamental system of equations for functionally graded spherical shells and panels, are reported in the extended form.

One of the main problems encountered in manual assembly workstations is human error in performing the operations. Several approaches are currently used to face this problem, such as intensive training of personnel, poka-yoke devices or invasive sensing systems (e.g. sensing gloves) used for monitoring the process and detect wrong procedures or errors in joining the parts. This paper proposes an innovative system based on the interaction between a force sensor and an augmented reality (AR) equipment used to give to the worker the necessary information about the correct assembly sequence and to alert him in case of errors. The force sensor is placed under the workbench and it is used to monitor the assembly process by collecting force and torque data with respect to an XYZ reference system; a pattern recognition technique allows the error identification and the selection of the appropriate recovery procedure. Two AR devices have been tested in this application: a video-mixing spatial display and an optical see-through apparatus, comparing the pro and cons of these two solutions. The first device includes a CCD camera positioned over the workstation and an LCD display used by the worker as a support for the correct execution of assembly operations and receiving instructions about recovery procedures. The latter consists of a head mounted display (HMD) having the capability of reflecting projected images in front of the worker's eyes, allowing a real-world view with the superimposition of virtual objects. The CCD camera is also used for identifying errors that are not detectable by the force sensor. At the end, a case study concerning a typical assembly procedure is presented and discussed.

A compressed sensing/sparse-recovery procedure is adopted to obtain enhanced range resolution capability from the processing of data acquired with narrow-band SFCW radars. A mathematical formulation for the proposed approach is reported and validity limitations are fully discussed, by demonstrating the ability to identify a great number of targets, up to 20, in the range direction. Both numerical and experimental validations are presented, by assuming also noise conditions. The proposed method can be usefully applied for the accurate detection of parameters with very small variations, such as those involved in the monitoring of soil deformations or biological objects.

FSCQ is the first file system with a machine-checkable proof (using the Coq proof assistant) that its implementation meets its specification and whose specification includes crashes. FSCQ provably avoids bugs that have plagued previous file systems, such as performing disk writes without sufficient barriers or forgetting to zero out directory blocks. If a crash happens at an inopportune time, these bugs can lead to data loss. FSCQ's theorems prove that, under any sequence of crashes followed by reboots, FSCQ will recover the file system correctly without losing data. To state FSCQ's theorems, this paper introduces the Crash Hoare logic (CHL), which extends traditional Hoare logic with a crash condition, a recovery procedure, and logical address spaces for specifying disk states at different abstraction levels. CHL also reduces the proof effort for developers through proof automation. Using CHL, we developed, specified, and proved the correctness of the FSCQ file system. Although FSCQ's design is relatively simple, experiments with FSCQ running as a user-level file system show that it is sufficient to run Unix applications with usable performance. FSCQ's specifications and proofs required significantly more work than the implementation, but the work was manageable even for a small team of a few researchers.

International audience; Most of Wireless Sensor Networks are deployed to monitor a set of targets over a specified area. The lifetime of such a network is defined as the time duration from the network deployment till the time when one target is no longer covered. Thus, this lifetime is limited by the energy resource of sensor nodes. In order to maximize the lifetime of the network, only a subset of nodes capable of covering all targets are activated at a time while the others are put in sleep mode to save their energy. When an active sensor fails, a recovery procedure should be executed to keep all targets covered. In this paper we propose a new self-healing method for network reconfiguration in case of failure of an active node. Simulation results show that this method increases the network dependability by reducing the network unavailability time up to 90 % % compared to the dynamic maintenance for networks with more than 200 sensor nodes of sensing range equals to 10, uniformly deployed over a 50 × 50 square to cover 50 targets also uniformly deployed over the same area.

The present work aims to offer a contribute for the evaluation of the opportunity to adopt strategies based on state support or, alternatively, on the managerial autonomy of local bodies in financial trouble. The research, after analyzing the main guidelines at European level and mentioning possible solutions currently laid down in Italian legislation, presents the results of an empirical investigation on the economic effects produced on the local economy by the collapse of the Municipality of Taranto. The choice of the city of Taranto is due both to the significant magnitude of the ascertained debt, which was one of the highest in the history of Italy, and to the considerable economic effects caused by the recovery procedure. Specifically, have been firstly examined the reasons which led to the collapse of the local public authority and the main aspects characterizing the administrative management of the passive mass besides the possible solutions for the definitive implementation of the recovery plan.

Optical transport networks typically deploy dynamic restoration mechanisms in order to automatically recover optical connections disrupted by network failures. Elastic optical networks (EONs), currently emerging as the next-generation technology to be adopted in optical transport, introduce new challenges for traditional generic multiprotocol label-switching (GMPLS)-based restoration that may seriously impact the achievable recovery time. At the same time, the software-defined networking (SDN) framework is emerging as an alternative control plane. It is therefore important to investigate possible benefits provided by SDN in the implementation of restoration mechanisms for EONs. This paper proposes a dynamic restoration scheme for EONs based on the SDN framework. The proposed scheme contemporarily exploits centralized path computation and node configuration to avoid contentions during the recovery procedure with the final aim of minimizing the recovery time. The performance of the proposed scheme is evaluated by means of simulations in terms of recovery time and restoration blocking probability and compared against three reference schemes based on GMPLS and SDN.

The compressed sensing paradigm allows to efficiently represent sparse signals by means of their linear measurements. However, the problem of transmitting these measurements to a receiver over a channel potentially prone to packet losses has received little attention so far. In this paper, we propose novel methods to generate multiple descriptions from compressed sensing measurements to increase the robustness over unreliable channels. In particular, we exploit the democracy property of compressive measurements to generate descriptions in a simple manner by partitioning the measurement vector and properly allocating bit-rate, outperforming classical methods like the multiple description scalar quantizer. In addition, we propose a modified version of the Basis Pursuit Denoising recovery procedure that is specifically tailored to the proposed methods. Experimental results show significant performance gains with respect to existing methods.

International audience; The linear quadratic Gaussian-loop transfer recovery procedure is a classical method to desensibilise a system in closed loop with respect to disturbances and system uncertainty. Here an extension is discussed, which avoids the usual loss of performance in LTR, and which is also applicable for non-minimum phase systems. It is also shown how the idea can be extended to other control structures. In particular, it is shown how proportional integral derivative controllers can be desensibilised with this new approach. The method is tested on several examples, including in particular the lateral flight control of an F-16 aircraft.

We study classic streaming and sparse recovery problems using deterministic linear sketches, including l1/l1 and linf/l1 sparse recovery problems (the latter also being known as l1-heavy hitters), norm estimation, and approximate inner product. We focus on devising a fixed matrix A in R^{m x n} and a deterministic recovery/estimation procedure which work for all possible input vectors simultaneously. Our results improve upon existing work, the following being our main contributions: * A proof that linf/l1 sparse recovery and inner product estimation are equivalent, and that incoherent matrices can be used to solve both problems. Our upper bound for the number of measurements is m=O(eps^{-2}*min{log n, (log n / log(1/eps))^2}). We can also obtain fast sketching and recovery algorithms by making use of the Fast Johnson-Lindenstrauss transform. Both our running times and number of measurements improve upon previous work. We can also obtain better error guarantees than previous work in terms of a smaller tail of the input vector. * A new lower bound for the number of linear measurements required to solve l1/l1 sparse recovery. We show Omega(k/eps^2 + klog(n/k)/eps) measurements are required to recover an x' with |x - x'|_1

Conical and cylindrical shells are the principal object of many numerical works in the literature background based on the shear deformation theories of composite shells[1]. As it is well known, the first order shear deformation theory of Reissner and Mindlin releases a five parameter displacement model. A third order expansion of the displacement field gives a quadratic variation of transverse strains and stresses, and require no “shear correction factor” compared to the first order theory[2]. In order not to violate the boundary condition while assuming a cubic expansion for the displacement field, the unconstrained model of the third order can become constrained, as suggested by Reedy[3]. Matsunaga [4] used the stress recovery technique for determining the accurate distributions of transverse displacement and stress - strain along the thickness direction for graded shells. In this work the authors focused on the static analyses of functionally graded conical, cylindrical panels and shells, by means of the generalized third order unconstrained theory with the contribution of the initial curvature, joined with the stress - strain and displacement recovery technique. The present theory starts with the definition of the displacement field which includes higher order terms and the components of strains are derived. The graded material via a four parameters power exponent law and the elastic engineering stiffness, the relations between stresses and strains, as well as between internal actions and generalized components of displacement and the definition of external applied loads are concerned in the formulation. Then, by applying the principle of virtual displacements seven indefinite equations of equilibrium are determined. Using the constitutive equations expressed in terms of generalized components of displacement, the fundamental equations are finally obtained. These equations are discretized via the G.D.Q technique and by means of the 2D numerical solution the membrane stress strain and displacement response of graded shells is determined along the thickness direction. The resting part of the mechanical response concerning with the transverse stress - strain and displacement distributions is accurately worked out via the recovery technique.

Purpose - The purpose of this paper is to assess the effect of the statistical admissibility of the recovered solution and the ability of the recovered solution to represent the singular solution; also the accuracy, local and global effectivity of recovery-based error estimators for enriched finite element methods (e.g. the extended finite element method, XFEM). Design/methodology/approach - The authors study the performance of two recovery techniques. The first is a recently developed superconvergent patch recovery procedure with equilibration and enrichment (SPR-CX). The second is known as the extended moving least squares recovery (XMLS), which enriches the recovered solutions but does not enforce equilibrium constraints. Both are extended recovery techniques as the polynomial basis used in the recovery process is enriched with singular terms for a better description of the singular nature of the solution. Findings - Numerical results comparing the convergence and the effectivity index of both techniques with those obtained without the enrichment enhancement clearly show the need for the use of extended recovery techniques in Zienkiewicz-Zhu type error estimators for this class of problems. The results also reveal significant improvements in the effectivities yielded by statistically admissible recovered solutions. Originality/value - The paper shows that both extended recovery procedures and statistical admissibility are key to an accurate assessment of the quality of enriched finite element approximations. © Emerald Group Publishing Limited.

In this paper we address the application of recovery procedures in advanced problems in structural mechanics. The attention is focused on the recovery by compatibility in patches procedure (RCP) and shear deformable plate structures. The formulation of RCP procedure is extended to shear deformable plate problems (Reissner---Mindlin theory) and is applied to recover stresses from mixed and hybrid stress finite elements. These elements offer new possibilities, for recovery procedures in general, which deserve to be discussed. A comprehensive investigation on which finite element solution can be used as input for the recovery procedures is given through standard benchmark problems, obtained for several values of the thickness on structured and unstructured meshes. The numerical results confirm the effectiveness of the recovery procedure extended to plates problems.

Knowledge of the factors that influence the survival of plant remains in archaeological contexts, increases the quality and utility of palaeobotanical data. This paper describes the main points needed to plan a sampling strategy and recovery procedure before beginning field work., Conocer el tipo de información que aportan los macrorrestos vegetales, las formas en las que pueden pervivir en el registro arqueológico y la naturaleza del sedimento, incrementan la calidad y utilidad de los resultados. El artículo trata de evaluar los puntos clave a la hora de planificar la estrategia de muestreo y el sistema de recuperación más adecuado para un yacimiento antes de iniciar el trabajo de campo.

This paper deals with a multisensor scheme based on set theoretic principles, whereby different invariant sets that characterize healthy and faulty functioning of system components are computed offline. Such sets allow to partition the ensemble of sensors into ‘healthy’, ‘faulty’ and ‘under recovery’ subclasses. Fault detection and isolation consists of online set-membership verifications with low computational complexity. Sensors that are deemed healthy are utilized in the computation of the feedback control law, while sensors that are deemed ‘faulty’ or ‘under recovery’ are prevented from participating in the feedback control action. The main focus of this paper is on the reintegration of ‘under recovery’ sensors, that is to say, the transition of sensors from the ‘under recovery’ to the ‘healthy’ sensor subclass. This transition, in contrast to all other possible transitions, is particularly difficult to evaluate since it involves set membership conditions based on unmeasurable quantities. This difficulty is circumvented by resorting to necessary and sufficient conditions for the recognition of recovery, which are based exclusively upon measurable quantities. The interplay between the necessary conditions and the sufficient conditions, together with the particular system structure and fault detection mechanism, allows to obtain further important improvements in the recovery procedure in terms of transient times and sensitivity to the topology of the invariant sets.

Human memory is not perfect - people constantly memorize new facts and forget old ones. One example is forgetting a password, a common problem raised at IT help desks. We present several protocols that allow a user to automatically recover a password from a server using partial knowledge of the password. These protocols can be easily adapted to the personal entropy setting [7], where a user can recover a password only if he can answer a large enough subset of personal questions. We introduce client-server password recovery methods, in which the recovery data are stored at the server, and the recovery procedures are integrated into the login procedures. These methods apply to two of the most common types of password based authentication systems. The security of these solutions is significantly better than the security of presently proposed password recovery schemes. For our protocols we propose a variation of threshold encryption [5, 8, 16] that might be of independent interest. © Springer-Verlag 2009.

The present work deals with an a posteriori error estimator for linear finite element analysis, based on a stress recovery procedure called Recovery by Compatibility in Patches. The key idea of this procedure is to recover improved stresses by minimizing the complementary energy over patches of elements. Displacements computed by the finite element analysis are prescribed on the boundary of the patch. Here, a new form of this recovery procedure is presented. Adopting a different patch configuration, centred upon an element instead of a node, allows to drastically simplify the recovery process, thus improving efficiency and making the implementation in finite element codes much easier. The robustness tests demonstrate that the error estimator associated to the new form of the recovery procedure retains the very good properties of the original one, such as superconvergence. The numerical results on two common benchmark problems confirm the effectiveness of the proposed error estimator, which appears to be competitive with those currently available. Copyright © 2006 John Wiley & Sons, Ltd.

Can we recover a signal f ∈R N from a small number of linear measurements? A series of recent papers developed a collection of results showing that it is surprisingly possible to reconstruct certain types of signals accurately from limited measurements. In a nutshell, suppose that f is compressible in the sense that it is well-approximated by a linear combination of M vectors taken from a known basis Ψ. Then not knowing anything in advance about the signal, f can (very nearly) be recovered from about M log N generic nonadaptive measurements only. The recovery procedure is concrete and consists in solving a simple convex optimization program. In this paper, we show that these ideas are of practical significance. Inspired by theoretical developments, we propose a series of practical recovery procedures and test them on a series of signals and images which are known to be well approximated in wavelet bases. We demonstrate that it is empirically possible to recover an object from about 3 M -5 M projections onto generically chosen vectors with an accuracy which is as good as that obtained by the ideal M -term wavelet approximation. We briefly discuss possible implications in the areas of data compression and medical imaging.

Mechanically driven mass diffusion in solids plays an important role in many technological processes. One of the most known effects is the so called hydrogen embrittlement, which may occur in metals containing an initially uniform dilute concentration of hydrogen. The phenomenon is characterized by a two-way interaction between mechanical and diffusive quantities: changes in mass concentration induce strains in the solid (swelling effect) and strain gradients induce mass fluxes (piezo-diffusive effect). The aim of this paper is to present some recent advances in the numerical analysis of these problems in the linear, steady state case. In the standard finite element setting based on assumed concentration and displacement, the presence of the strain gradient in the piezo-diffusive coupling term demands C1 continuous displacement interpolation, which is difficult to construct in spatial dimension higher than one. To avoid C1 continuity various techniques can be used. A mixed approach where displacements and volumetric strain are interpolated as independent variables was early explored by Girrens & Smith (1987). However, the computational effort increases and some inconsistency issues may arise. An alternative approach is to establish a staggered solution strategy in conjunction with a smoothing procedure. This method was employed by Garikipati et al. (2001) using the standard L2 projection. Here, a more efficient and accurate solution scheme is presented by resorting to a superconvergent patch-based recovery procedure, recently proposed by Ubertini (2004). Another possibility to avoid C1 continuity is to operate within the numerical setting of discontinuous Galerkin methods. Here, a new formulation which requires standard C0 interpolation for both displacement and concentration is proposed. The three approaches are discussed and compared by a benchmark test. The numerical results show that both the recovery-based and the discontinuous approach can be successfully applied.

The present work deals with the recovery of the in-boundary stress on smooth boundary parts in Boundary Element Method (BEM) in two dimensions. First, two Boundary Integral Representations (BIRs) of tangential derivative of boundary displacements, whose integral kernels multiplying either tangential derivative of displacements or displacements are smooth and bounded, are presented. Two procedures for an in-boundary stress recovery based on these BIRs are developed and analyzed. The first procedure, which directly uses the results obtained from a BEM analysis, displacements and tractions, requires the performing of integrations involved in these BIRs either over the real boundary of the solid or over a smooth approximation of the boundary part where the evaluation point is placed. The second procedure, which can be applied on a non-smooth (e.g. polygonal) approximation of the boundary, requires the use, in the first BIR introduced here, of an integral density which is previously smoothed by a local smoothing procedure. The third recovery procedure considered in this work is directly given by a local smoothing procedure of this kind. Results obtained by these three recovery procedures are compared in a series of numerical tests, the best results being obtained by the first procedure developed in this work. Ministerio de Educación Cultura PB98-1118

This paper presents a checkpointing–recovery scheme for Time Warp parallel simulation. The scheme relies on a checkpointing protocol, namely mixed state saving, embedding both sparse and incremental state saving modes, and on a state recovery procedure embedding both forward and backward recovery modes. This scheme is a generalization of many previous solutions, which can be obtained as particular instances of it by selecting appropriate values for the checkpointing protocol parameters. We also present two regulating algorithms to adaptively tune the checkpointing protocol parameters, in order to make the protocol reacting to variable rollback behavior. A synthetic benchmark in several different configurations has been used for evaluating and comparing our scheme with previous solutions. The obtained data show that our solution allows faster execution and, in addition, keeps quite low the amount of memory used for recording state information; this allows the scheme to not adversely affect performance when memory is a critical resource.

Wireless sensor and actor networks (WSANs) have various applications in safety and mission critical systems. Sensors are used for sensing the information whereas actors for taking intelligent decisions. Developing and modeling algorithms for WSANs have raised several research issues which have captured attention of the research community. Maintaining inter-actor connectivity or failure recovery is a critical issue in WSANs because these are deployed in harsh and inhospitable environment which may result into physical damage to actors loosing inter-actor connectivity. In case of failure of inter-actor connectivity, the topology of the network may be affected that might be inefficient to recover. Therefore an efficient subnet-based failure recovery algorithm (SFRA) is proposed in this work. It is assumed the partitioning of WSAN into subnets which localizes the failure recovery procedure at subnet level achieving objective of efficiency. Moreover, algorithm is hybrid as it assumes pre-failure planning and post-failure recovery. The proposed model is presented as a graph-based model to represent static part of the network topology. The graph model is transformed into a formal model using Vienna development method-specification language (VDM-SL). The static model is described by defining formal specification of subnets, network topology, sensors, actors and gateways as composite objects. The state space of the WSANs is described in the form of functions and operations as dynamic part of the model. Invariants are defined over the data types in static model for ensuring safety criteria and pre/post conditions are defined in functions and operations for changing state space of the system. The proposed model is validated and verified using VDM-SL Toolbox.