A new method for the representation and evolution of three dimensional discontinuity surfaces in XFEM/GFEM

The ability of the extended and generalized finite element methods of modeling discontinuities independent of mesh alignment requires a suitable representation for the discontinuity surfaces. In the present paper a method for constructing level set functions based on vector data and geometric operations in three dimensions is presented. In contrast to classical level set methods, the proposed approach does not require the integration of differential evolution equations, resulting in a particularly simple structure and easy implementatio

Finite Element Quadrature of Regularized Discontinuous and Singular Level Set Functions in 3D Problems

Regularized Heaviside and Dirac delta function are used in several fields of computational physics and mechanics. Hence the issue of the quadrature of integrals of discontinuous and singular functions arises. In order to avoid ad-hoc quadrature procedures, regularization of the discontinuous and the singular fields is often carried out. In particular, weight functions of the signed distance with respect to the discontinuity interface are exploited. Tornberg and Engquist (Journal of Scientific Computing, 2003,19: 527-552) proved that the use of compact support weight function is not suitable because it leads to errors that do not vanish for decreasing mesh size. They proposed the adoption of non-compact support weight functions. In the present contribution, the relationship between the Fourier transform of the weight functions and the accuracy of the regularization procedure is exploited. The proposed regularized approach was implemented in the eXtended Finite Element Method. As a three-dimensional example, we study a slender solid characterized by an inclined interface across which the displacement is discontinuous. The accuracy is evaluated for varying position of the discontinuity interfaces with respect to the underlying mesh. A procedure for the choice of the regularization parameters is propose

Peculiar and Unusual Drowning in Waste Oil from Motor Vehicles: Case Report

Drowning is one of the most frequent causes of accidental or suicidal death, and more rarely it is associated with a homicide. Cases of drowning in water or in the sea are common. The authors report an unusual and peculiar case of drowning, that of a woman who accidentally fell inside a collection tank of waste oil of motor vehicles

A unilateral nonlocal tensile damage model for masonry structures

Abstract In the present paper, a constitutive nonlocal damage model is proposed for the non-linear incremental finite element analysis of masonry structures. The mechanical model is based on the assumptions of linear elasticity under compression and softening behaviour under tension, described by the adoption of a unique strain-driven nonlocal damage variable. Specifically, non-locality of the integral type is introduced in order to prevent spurious strain localization. It can be noted that the unilateral nature of the model is suitable to contemplate both diffused macro-cracks induced by the tensile damage process and the stiffness recovery in the transition from tension to compression, considering the anisotropy induced by the damage process as well. This is performed by realizing a decomposition of the strain tensor in its positive and negative components, and accounting for stiffness degradation only along tensile direction. The assumption of a linear elastic behaviour in compression is motivated by the fact that the main interest of the model is represented by investigating the response of masonry structures under service loads, condition in which very low compressive states are usually predominant. Consequently, the number of constitutive parameters is more limited with respect to other models that include a damage criterion also in compression. Finally, the validation of the proposed damage model is carried out with reference to a plane problem, in order to check the capability of the model to treat damage in an anisotropic way as well as the almost null dependence of the results on the discretization

A new experimental test for the characterization of the masonry shear parameters

The shear properties evaluationon existing unreinforced masonry structure is usually performed through destructive tests. However, these tests have the characteristics of being very expensive and result in significant damage, not only to the samples but also on the portion of wall surrounding them. The present work illustrates the design of a new testing procedure for the characterisation of the shear properties in masonry panels for application in routine testing. In the aim of preserving the integrity of the area under testing, and to reduce the cost of the new testing procedure, it has been decided to use flat jacks. The numerical analyses used to design the test are presented, as well as the results ofa first application of the procedure

Considerations on the Glide Snow Avalanches based on the Stauchwall Model

Avalanches are natural events that can have consequences such as silvicultural losses, infrastructural damages, fatalities. In this paper, the attention is given to glide avalanches starting by a glide crack, a tensile crack that propagates at the crown – the upper release limit – due to the internal stress variation. However, the presence of a glide crack does not always give rise to a glide avalanche. In fact, when the slab starts to move, interacts with the stauchwall (the downslope boundary of the slab) which can fail or withstand.The Stauchwall model was adopted in order to verify if the gliding avalanche is triggered or not, by analyzing the dynamic stability of a slab subjected to an initial perturbation. In this paper, the model has been expanded by coupling it with a stress failure criterion. Thanks to this new failure criterion, it is possible to investigate the possible causes of subsequent glide avalanches triggering (in terms of hours or even days) after the crack propagation. In addition, the effect of a skier’s fall/brake on the slab stability is analyzed. Finally, a sensitivity analysis of the model pointed out the important role played by the basal snow/soil friction. Therefore, it is shown that actions meant to increase this characteristic may be taken into account to effectively prevent glide avalanche

AI based bridge health assessment

Starting from the data extracted from a long-term monitoring system installed on a steel bridge, it has been possible to outline the undamaged behaviour of the structure. The structure under monitoring is a steel suspended arch bridge of long span that has been instrumented with several types of sensors, e.g. triaxial accelerometers, load cells and environmental sensors. The records of the measurements during the first period of structural life and the lack of construction problems ensure the good respect of the structural nominal conditions. The accelerometric data stored during this period have been used to extrapolate the dynamic characteristics of the bridge: natural frequencies, damping ratios and modal shapes. The use of a specific stochastic subspace technique (SSI-UPCX), allowed to obtain not only the modal parameters but also their uncertainty. In this way, the range of variation of modal parameters, e.g. affected by environmental factors, has been calculated and a minimum and maximum threshold for each parameter has been determined. Consequently, the assessment and control of structural health is updated and linked to these ranges of variation. In addition, a promising modern approach to tackle the problem is the use of machine learning techniques within the broad field of AI. After the selection/reduction of the parameters that better represent the data, signal detection has been used and the obtained outcomes compared. In the light of both the above approaches, albeit in a different way, it is possible to create a model of the normal operating condition of the structure and consider the deviations from the pattern as an anomaly. The work represents a first step and a benchmark for the wider damage and ageing identification problem to figure out which method is the most appropriate and effective for this specific case of structural assessment, in terms of effort and accuracy

2D finite elements for the computational analysis of crack propagation in brittle materials and the handling of double discontinuities

Crack growth simulations by way of the traditional Finite Element Method claim progressive remeshing to fit the geometry of the fracture, severely increasing the computational effort. Methods such as the eXtended Finite Element Method (XFEM) allow to overcome this limitation by means of nodal shape functions multiplied by Heaviside step function to enrich finite element nodes. Through the medium of a discontinuous field, the entire geometry of the discontinuity can be modelled regardless of the mesh, avoiding remeshing. In this paper two shell-type XFEM elements (a three-node triangular element and a four-node quadrangular element) to evaluate crack propagation in brittle materials are presented. These elements have been implemented into the widespread opensource framework OpenSees to evaluate crack propagation into a plane shell subjected to monotonically increasing loads. Moreover, in the perspective of fracture propagation simulations, the problem of managing multiple cracks without remeshing or operating subdivisions on the integration domain has been investigated and a four-node quadrangular finite element for the computational analysis of double crossed discontinuities by the means of equivalent polynomials is presented in this paper. Equivalent polynomials allow to overcome inaccuracies on the results when performing standard numerical integration (e.g. Gauss-Legendre quadrature rule) over the entire domain of XFEM elements, without the need of defining integration subdomains. The presented work and the computational strategy behind it may be extremely useful not only in the field of fracture mechanics, but also to solve complex geometry problems or material discontinuities

Real-time evaluation and management of extreme traffic load risk on main road’s bridges

The risk induced by extreme traffic loads on bridges was rarely investigated and the existing methods require computationally expensive elaborations that are not compatible with a real time risk management. Traditional approaches to reduce risk suggested the optimisation of bridge maintenance plans. Conversely, approaches that real-time evaluate and manage the risk are missing. Moreover, the integration of emerging prediction models, such as Artificial Neural Networks, was never explored. This study fills the previous gaps by proposing a three-block methodology. It adopts Weight-In-Motion systems to collect site-traffic load data, formulates a probabilistic Risk Prediction Model to estimate frequency and severity of bridge failure events according to Eurocodes, and simulates an Intelligent Transportation System (ITS) architecture to apply real time management actions. The methodology was tested on 2.5M+ vehicles raw WIM data gathered along the ring road of Brescia (Italy). Bridge failure events resulted significantly more frequent than that prescribed by Eurocode, and factors of compliance with Traffic Code mass limits prescriptions had the more significant effect on risk predictions. The findings suggest a greater attention when permits for extremely overweighed vehicles are issued, as well as the implementation of enforcement strategies and ITS-based architectures for the real time risk management