Use of explicit FEM models for the structural and parametrical analysis of rockfall protection barriers

This paper illustrates the experimental test procedure and results of two flexible barriers of low and medium energy, the so-called IBT-150 and IBT-500. For this purpose, ETAG 027 European Guideline is used. All the requirements for the tests performance are followed and the two energy-level tests performance requirements have been fulfilled in both rockfall barriers. Numerical modelling helps to understand and predict the behavior of these barriers with different configurations drastically reducing the costs of performing real tests. The results of the real test on IBT-150 and IBT-500 have been taken as references to validate two numerical models using Abaqus Explicit software. Afterwards, a presentation of some alternatives of the barrier IBT-150 are stated, which allow a more economical design removing some components that do not affect the energy level of 150?kJ set by the manufacturer. Also, a parametrical analysis of the IBT-500 numerical model has been performed varying the geometrical characteristics, such as the net grid dimension, the diameter of the perimeter cable, the length of the functional modules and its height. The aim of this analysis is the enhancement of maximum energy capacity of the barrier related with the amount of material used to build it. Following the ETAG recommendation, the maximum energy level (MEL) test is achieved if the barrier is able to retain the block. Thus, the MEL level for each numerical model was determined by increasing the initial speed of the block until it trespasses the barrier

Validation through finite element simulation of the behaviour of a polyurethane shock absorber under in-service and extreme conditions

ABSTRACT: The safety rules for the construction and installation of lifts, currently in force in Europe, include several requirements concerning the behaviour of the shock absorbers when stopping an elevator. In this paper, a finite element model simulating the behaviour of a cellular polyurethane shock absorber has been developed. The material mechanical behaviour was simulated by means of an elastomeric foam theoretical model, previously calibrated in a former paper. Several in-service and extreme condition scenarios have been analysed with this numerical model, thus verifying the fulfilment of the requirements of the standard

Characterization of mechanical properties of a shock absorber polyurethane foam for elevators. Numerical fitting of mechanical behavior models for hyperelastic and elastomeric foam materials

ABSTRACT: The elastic cellular polyurethane elastomer is widely used to manufacture shock absorbers for elevators, due to its excellent conditions for absorption of energy and vibration damping. In this paper, a complete mechanical characterization of this material was performed including the uniaxial compressive test, the planar test, and the volumetric and the simple shear test. From the experimental results, several models of behavior for hyperelastic and elastomeric foam materials have been analyzed by fitting their corresponding material parameters. The scope of this work includes the Ogden model, the Van der Waals model, and polynomial and elastomeric foam forms

A modification of the Norris failure criterion for the prediction of the mechanical failure of the aged paper insulation in the windings of a power transformer

The deterioration of the insulation in the windings of power transformers affects their lifespan. A commercial insulated Continuously Transposed Conductor (CTC) was studied experimentally, numerically and analytically. The purpose was to understand the mechanisms governing the mechanical failure of the insulating paper, and to achieve a criterion for predicting failure under different conditions. Samples of that insulated CTC were extracted from a coil and aged at 150∘C for different durations inside vessels filled with naphthenic oil. Then the degree of polymerisation and tensile, compressive and shear mechanical properties of the insulation were measured/estimated. Aged insulated CTC samples were subjected to three-point bending tests, producing deformations compatible with a short circuit, and the fractures in the insulation were analysed. The bending test over a CTC sample was simulated by means of a FEM Program. The Norris failure model, applicable to a lamina, was adapted to the studied insulation materials. The predictions of that failure criterion agreed with experimental observations

Mechanical behaviour of the cellulosic dielectric materials of windings in power transformers in operation

Power transformers are crucial elements in electrical systems, and the end of their useful life is commonly conditioned by the degradation of the cellulosic insulation materials inside them. These materials are subjected to elevated temperatures and mechanical stresses, generated by electrical solicitations which deform the copper conductors and subsequently the paper, and also to the chemical reactions which take place in the dielectric oil in which the paper is impregnated. In order to better understand the behaviour of cellulosic insulation, we have studied the previous bibliography analysing the mechanical behaviour of cellulosic materials. At present, there are no experimental results analysing how the paper responds to the deformation suffered by the copper conductor in a realistic situation. We have developed a simulation model describing the mechanical behaviour of a standardised copper conductor wrapped with four layers of dielectric paper, using ANSYS Workbench Static Structural, which will be compared with experimental results afterwards

Experimental dataset on the tensile and compressive mechanical properties of plain Kraft and crepe papers used as insulation in power transformers after ageing in mineral oil

The solid insulation in the windings of power transformers, which generally consists of oil-impregnated thin paper, is one of the key elements for the performance and durability of these electrical machines. Insulation paper is subjected to static and dynamic forces of electromagnetic origin, in combination with high temperatures and chemical reactions, during the operating life of a power transformer. The mechanical properties of the cellulosic insulation are relevant parameters because its breakage could result in the electric failure of the transformer. Indeed, paper manufacturers usually provide values of the tensile strength and elongation at breakage of the insulating paper in its two principal material directions, the MD (machine direction) and CD (cross-direction). However, paper is a highly anisotropic material and its material properties evolve as the paper insulation ages. The paper insulation in an operating transformer is subjected to a multiaxial stress state field including compressive and shear stresses. This article reports experimental data on the tensile and compressive mechanical properties of two types of paper, plain Kraft and crepe paper, typically used as insulation in power transformers, under different ageing states (which were induced through accelerated thermal ageing and quantified by means of the degree of polymerisation). These data could be reused for several purposes. They can improve the current understanding of the mechanical response and degradation processes of the cellulosic insulation in power transformers, and give some reference values that can be compared with others obtained in the factory by manufacturers. In the field of engineering failure analysis, those values could be reused for the assessment of mechanical failure of paper materials used in power transformers, see [1]

Desarrollo y validación experimental de una metodología para estudiar la propagación de grietas por fatiga en metales

La presencia y propagación de grietas en estructuras y componentes es un reto para sectores industriales como el oil&gas, energía eólica o nuclear. El presente estudio resume la investigación realizada para desarrollar y validar un procedimiento basado en el método de los Elementos Finitos capaz de simular la propagación de grietas tridimensionales. La investigación comprende una parte experimental donde se ha caracterizado la velocidad de propagación de fisuras frente a fatiga del acero grado R5, utilizado en cadenas de fondeo offshore. Para ello se ha determinado la Ley de Paris del material mediante el ensayo de una probeta de flexión en tres puntos. Asimismo, se ha realizado un estudio estadístico para estimar la incertidumbre en la velocidad de propagación de la grieta. Los resultados experimentales han sido empleados como inputs del modelo numérico desarrollado en ANSYS APDL. Este algoritmo permite evaluar numéricamente los parámetros de la mecánica de la fractura en el frente de fisura durante su crecimiento. Esta información alimenta un algoritmo iterativo que proporciona el avance del frente de fisura. Finalmente, se ha realizado un estudio de sensibilidad para evaluar el efecto del tamaño del mallado en los resultados.The existence and propagation of cracks in structures and components is a challenge for industrial sectors such as oil & gas, wind or nuclear. The present study summarizes the research carried out to develop and validate a procedure, based on the finite element method, able to simulate the propagation of three-dimensional cracks. This research comprises an experimental part where the fatigue crack propagation rate of the grade R5 steel, widely used in offshore mooring chains, has been characterized. For this purpose, the Paris Law of the material has been determined by testing a three point bending specimen. Furthermore, a statistical study to estimate the uncertainty in the crack propagation rate was carried out. The experimental results have been used as inputs for the numerical model developed in ANSYS APDL. This algorithm allows numerical evaluating the fracture mechanics parameters in the crack front during its growth. This information feedbacks an iterative algorithm that provides the crack front growth. Finally, a sensitivity study was carried out to evaluate the effect of the meshing size on the results

Experimental and numerical analysis of cellulosic insulation failures of continuously transposed conductors under short circuits and thermal ageing in power transformers

The integrity of the cellulosic insulation in power transformers is considered one of the most relevant parameters that affects their performance and reliability. Electric faults, such as short circuits, have thermal and mechanical effects that degrade the paper and can eventually produce the end-of-life of the transformer. The evolution of the properties of the paper insulation of a commercial continuously transposed conductor due to thermal ageing was characterised through the degree of polymerisation and tensile testing. Failure initiation and propagation in the paper was analysed macroscopically and microscopically using scanning electron microscope. A finite element numerical mechanical model of the conductor was implemented to reproduce the experiments and to obtain the load level and strain state that produce failure at each ageing state, aiming at developing a failure model for the insulation. This model may contribute to an improvement in manufacturing processes and management of the electrical system

Fracture toughness as an alternative approach to quantify the ageing of insulation paper in oil

Oil-immersed transformers use paper and oil as insulation system which degrades slowly during the operation of these machines. Cellulose materials are used generally as insulation solid in power transformers. The degree of polymerization (DP), defined as number of repeating b-glucose residues in the cellulose molecule, is a critical property of cellulosic insulation material used in transformers, since it provides information about paper ageing and its mechanical strength. The fast-developing electric power industry demanding superior performance of electrical insulation materials has led to the development of new materials, as well as different drying techniques performed during transformer manufacturing and service when required. Both developments have caused some practical difficulties in the DP measurement. Moreover, the increasing interest in synthetic dielectric materials replacing cellulose materials requires measuring alternative properties to the DP to quantify the degradation of insulation solids over time. In this sense, this paper proposes the possibility of analyzing paper degradation through fracture toughness. This approach is different from the study of mechanical properties such as tensile strength or strain because it provides a tool for solving most practical problems in engineering mechanics, such as safety and life expectancy estimation of cracked structures and components which cannot to be considered through the traditional assessment of the mechanical resistance of the material. An accelerated thermal ageing of Kraft paper in mineral oil was carried out at 130 ºC during different periods of time, to obtain information on the kinetics of the ageing degradation of the paper. Double-edged notched specimens were tested in tension to study their fracture toughness. The evolution of the load-displacement curves obtained for different ageing times at the ageing temperature of 130 ºC was utilized to the determination of the stress intensity factor. Furthermore, different kinetic models based on this stress intensity factor were applied to relate its evolution over time as a function of the temperature. Finally, the correlation between the DP and stress intensity factor, which depends on the fiber angle, was also defined.The authors are grateful for the funding received to carry out this work from the State Scientific and Technical Research and Innovation Plan under the PID2019-07126RB-C22 grant agreement, financed by the Government of Spai