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Rigorous and efficient management of the railway infrastructure is crucial to avoid accidents and reduce operation and maintenance costs. This requires in-depth knowledge of the assets, the interaction among them and the effect that each track parameter has on the overall infrastructure performance. In this study, a large set of studies are carried out, on a previously calibrated finite element slab track model, where the relevant track parameters are varied within their usual ranges. The results are then used to train and validate a series of predictive models based on Machine Learning algorithms. This methodology provides greater understanding and enhanced prediction of the behaviour of tracks, which are composed of multiple variables such as the soil/subgrade, supporting layers, sleepers, pads and rails. The study also considers train axle loads and service speeds, which are other key elements that influence the track performance. The results show that the parameters that have greatest influence on the railway infrastructure are the properties of the soil, characteristics of the rail pads and the axle loads. This work can support the implementation of predictive maintenance procedures for railway tracks and the development of innovative technological solutions, providing responses to the industrial needs of reducing costs and contributing to improve the competitiveness of railway transport. The authors would like to thank the MCIN/AEI/ 10.13039/ FEDER, UE for financing the project Pry PID2021-128031OB-I00 “Development of a System to Monitor Automatically High- Speed Railway Lines Through Machine Learning and Numerical Simulation Algorithms (SMART- Algorithms).

Railway superstructure components are subject to impact forces caused by irregularities in the rail and wheel or variations in the support stiffness. These imperfections lead to the premature and accelerated deterioration of the track and increase maintenance costs. Generally, to mitigate these dynamic loads, elastomeric rail pads are interposed between rails and sleepers. For each track section, the pad choice is made according to its stiffness, impact attenuation and cost. In this experimental study, eight types of rail pads were characterized using two procedures from EN 13146–3. The results revealed that the rail pad material and geometry affect the impact attenuation ability of the pad and the testing procedure affect the results. It was found that the preload applied is one of the parameters that most affects the impact attenuation value. Appling a preload of 50 kN during the impact attenuation test, which is suggested by the standard, affected this parameter by ranging from between ~2 and ~ 20, depending on the rail pad. Based on these results it is considered that EN 13146–3 should be updated.

This paper is aimed at identifying by means of micro-CT the microstructural differences between normal and degenerative mitral marginal chordae tendineae. The control group is composed of 21 normal chords excised from 14 normal mitral valves from heart transplant recipients. The experimental group comprises 22 degenerative fibroelastic chords obtained at surgery from 11 pathological valves after mitral repair or replacement. In the control group the superficial endothelial cells and spongiosa layer remained intact, covering the wavy core collagen. In contrast, in the experimental group the collagen fibers were arranged as straightened thick bundles in a parallel configuration. 100 cross-sections were examined by micro-CT from each chord. Each image was randomized through the K-means machine learning algorithm and then, the global and local Shannon entropies were obtained. The optimum number of clusters, K, was estimated to maximize the differences between normal and degenerative chords in global and local Shannon entropy; the p-value after a nested ANOVA test was chosen as the parameter to be minimized. Optimum results were obtained with global Shannon entropy and 2≤K≤7, providing p < 0.01; for K=3, p = 2.86⋅10-³. These findings open the door to novel perioperative diagnostic methods in order to avoid or reduce postoperative mitral valve regurgitation recurrences. This work is supported by the “Ministerio de Economía, Industria y Competitividad” (MINECO) and the “Instituto de Salud Carlos III” (ISCIII) of Spain, through projects INTRACARDIO (DTS17/00056) and CIBER-BBN (co-financed by FEDER funds) and IDIVAL under project DiCuTen (INNVAL16/02). The technical contributions from the members of the DICUTEN and the financial contribution from the IDIVAL are gratefully acknowledged. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. This study was approved by the Ethical Committee of Clinical Research of Cantabria – IDIVAL (Acta 02/2018).

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 Spain

In oil-immersed power transformers, the insulation system is constituted of a dielectric oil-solid combination. The insulation oil generally used is mineral oil; however, this fluid has started to be substituted by natural and synthetic esters due to their higher biodegradability and flash point. The introduction of a new fluid in the insulation system of power transformers requires kinetic models that can estimate the degradation rate of insulation solids. The aim of this work was to go further in quantifying through different kinetic models the deterioration suffered by a commercial cellulose board (PSP 3055), which is one of the solid materials used in the insulation system of oil-filled transformers. The aging study was extended to cellulose board specimens immersed in two different oils (mineral and synthetic ester). It was obtained that there is a lower degradation when synthetic ester is used in the insulation system. Additionally, it can be concluded that the use of mechanical properties to quantify the degradation of the cellulose board through kinetic models provides information about the different behavior shown by PSP 3055 when different fiber direction angles are considered. This research was funded by State Scientific and Technical Research and Innovation Plan under the PID2019-107126RB-C22/AEI/10.13039/501100011033 grant agreement, financed by the Government of Spain

The accurate computational modelling of railway systems is crucial for analysis and design, which allows for excellent and enduring performance of such systems. It is capable of providing the industry with data for improving speed, comfort, load capacity and reliability. Further, as accurate solutions serve as an aid to improve railway systems, they contribute to quality services, social welfare, cost effectiveness and sustainability. An important component of a railway system is the track, which, in general, requires high investments for construction and maintenance. This work develops calibrated three-dimensional (3D) Finite Element (FE) models for slab track systems which can be employed for analysis and design with great level of reliability. These models are developed based on full-scale dynamic tests performed under the application of loads which simulate the passage of high-speed trains. The components considered are the rails, rail pads, slab track, grout, Hydraulicaly Bonded Layer (HBL), Frost Protection Layer (FPL) and subgrade. The FE models are built and calibrated in order to reproduce the measured displacement and acceleration test results. Due to the uncertainties in some material properties, a parametric analysis is also performed to establish to which material characteristics of the system the model is more sensitive to. It has been found that the Young’s moduli for the FPL layer and subgrade are the most important parameters. Further, the stiffness properties of rail pads play a paramount role in the accuracy of the model.

Slab track is replacing obsolete ballasted track. This work proposes crushing out-of-service ballast and sleepers for use as aggregates for constructing the slab track. Three kinds of recycled self-compacting concretes were designed; the first one with aggregates from crushed ballast, the second one with aggregates from crushed sleepers and the third one with both kinds of aggregates in the proportion they appear in the track. The mechanical and durability properties of these concretes were analysed. The durability test was focused on freeze-thaw and drying-wetting cycles. The results obtained lead to the conclusion that out-of-service track could be used to build the new one, closing the loop. Spanish Ministry of Economy and Competitiveness for financing the project MAT2014-57544-R.

The fastening system that links the rail and the sleeper in railway lines is an element of high responsibility, since its failure may lead to the derailment of the train. This paper combines an experimental approach and a Finite Element model to assess the structural integrity of the spring clip of the SKL-1 fastening system, used assiduously in high-speed lines. Our results prove that fatigue is not the failure mechanism that explains the fractures of clips as pointed out by other authors, provided the following requirements: the material has received the appropriate thermomechanical treatments; the assembly of the clip has been carried out with precision; and the system has not suffered anomalous in-service overloads.

The aim of this work is to analyse the influence of moisture content on the fracture behaviour of notched short glass fibre reinforced polyamide 6 (SGFR-PA6). For this purpose, the study combines two fibre contents (10 wt% and 50 wt%) with three different moisture contents (dry, 2% and 4–5%). Fracture tests were conducted on Single Edge Notched Bending (SENB) specimens, considering five notch radii (from 0 to 2 mm) for each combination of fibre and moisture content. Although notch effect is clearly observed in dry conditions, when the moisture content increases this effect tends to decrease for the notch radii considered here. The Line Method of the Theory of Critical Distances has been calibrated and validated for the conditions studied, providing a reasonable prediction of the apparent fracture toughness of the material in notched conditions. Concerning the fractographic observations, a Scanning Electron Microscopy analysis was performed, revealing the evolution of fracture micromechanisms when the moisture content increases.

Due to the large amount of CO2 emissions produced nowadays by the construction sector, the scientific community is looking for measures that will bring constructions to the be as environmental-friendly as possible. One of the most popular lines of research is the use of construction waste as recycled aggregates for the production of recycled aggregates concrete. Regarding the use of coarse recycled aggregates, there are a large number of studies that validate their use at both a mechanical and durability levels. Regarding the use of fine particles, which are inherent to the existence of coarse particles, it is necessary to increase the existing knowledge before they can be widely used. In this paper, the authors have sought to isolate the weakest part of those concrete mixes with exclusively recycled aggregate, i.e. a mortar made exclusively with fine recycled aggregates, in order to analyse the effect of drying-wetting cycles on mixes with recycled aggregates only. Specifically, three types of fine aggregates were used, one natural silica aggregate, one aggregate from crushed ballast and one from crushed sleepers. The mortars made with these aggregates were subjected to 50 durability cycles in distilled water, seawater and water with sulphates. Mechanical, visual and microstructural tests were carried out to analyse the effect of each of these environments on each of the mortars.

One of the biggest challenges in facilitating the installation of concrete is the development of fibre-reinforced concrete. Although nowadays fibre reinforced concrete is relatively common, it is still necessary to deepen in the study on its behaviour, especially regarding its fatigue behaviour. This paper proposes a new methodology to analyse the bending fatigue behaviour of notched test specimens. From these tests, it was possible to verify that, despite carrying out the tests with load control, the presence of fibres extends the fatigue life of the concrete after cracking. This effect is of great importance since during the extra lifetime with the cracked concrete, the damage to the concrete will be evident and the corresponding maintenance measures can be carried out. Regarding the analysis of the results, in addition to obtaining a traditional S-N curve, two new criteria have been applied, namely energy and notch growth. From these two new approaches, it was possible to determine critical energy values that can be used as predictive indicators of the collapse of the element. Moreover, from the notch growth analysis, it was possible to determine crack growth rate as a function of the stress conditions for the concrete and the specific geometry. From the comparison among the results obtained from the different tests, a limit cracking index of 0.05 mm can be defined.

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] .

Designing or analysing the influence of fatigue on concrete structures is becoming increasingly important for a number of structural elements. For this reason, it is necessary to define a method capable of determining the concrete´s fatigue limit in the most economical, fast and efficient way. The aim of this study is to compare and correlate different methods found in the literature in order to reduce the number and duration of the tests required to determine the fatigue limit. This reduction will consequently reduce the economic costs of determining the fatigue limit. For these proposes three different types of self-compacting recycled concrete was used. A linear correlation was found between the analysed methods, which means that the methods are capable of providing similar results and that the results obtained by the economical procedure must be selected. This work opens the door to define an optimal procedure in the process of concrete fatigue characterization, which is capable of yielding results up to five times faster and more economical than with other methodologies, so that resources are not wasted. The authors would like to thank the Spanish Ministry of Economy and Competitiveness of Spain for financing the project MAT2014-57544-R.

Train operations generate high impact and fatigue loads that degrade the rail infrastructure and the vehicle components. Rail pads are installed between the rails and the sleepers in order to damp the transmission of vibrations and noise and to provide flexibility to the track. These components play a crucial role in maximizing the durability of the railway assets and minimizing maintenance costs. Rail pads can be fabricated with different polymeric materials that exhibit non-linear mechanical behaviours, which strongly depend on the service conditions. Therefore, it is extremely difficult to estimate their mechanical properties, in particular the dynamic stiffness. In this work, several machine learning methodologies (multilinear regression, K nearest neighbours, regression tree, random forest, gradient boosting, multi-layer perceptron and support vector machine) were used to determine the dynamic stiffness of rail pads depending on their in-service conditions (temperature, frequency, axle load and toe load). 720 experimental tests, under different realistic operating conditions, were performed to produce a dataset that was then used for the training and testing of the machine learning methods. The optimal algorithm was gradient boosting for EPDM (R2 of 0.995 and mean absolute percentage error of 5.08% in the test dataset), TPE (0.994 and 2.32%) and EVA (0.968 and 4.91%) pads. This model was implemented in an application, available for the readers of this journal, developed on the Microsoft .Net platform that allows the dynamic stiffness of the pads study to be estimated as a function of the temperature, frequency, axle load and toe load.

Concrete fatigue behaviour has not been extensively studied, in part because of the difficulty and cost. Some concrete elements subjected to this type of load include the railway superstructure of sleepers or slab track, bridges for both road and rail traffic and the foundations of wind turbine towers or offshore structures. In order to address fatigue problems, a methodology was proposed that reduces the lengthy testing time and high cost by increasing the test frequency up to the resonance frequency of the set formed by the specimen and the test machine. After comparing this test method with conventional frequency tests, it was found that tests performed at a high frequency (90 &plusmn, 5 Hz) were more conservative than those performed at a moderate frequency (10 Hz), this effect was magnified in those concretes with recycled aggregates coming from crushed concrete (RC-S). In addition, it was found that the resonance frequency of the specimen&ndash, test machine set was a parameter capable of identifying whether the specimen was close to failure.

The recycling of the fine fractions from construction and demolition wastes is an environmental and technological challenge. In view of its incorporation as recycled aggregate in the manufacture of concretes, this work analyzes its influence on the mechanical response of these concretes to the action of variable loading, which can lead to failures due to fatigue. To do this, 3 different recycled mortar were manufactured using fine aggregate: a standardized siliceous aggregate and two others with recycled fine aggregate, coming from the crushing of out of use basalt ballast and concrete sleepers from the railway superstructure. Plain mortars have been subjected to fatigue compressive loading, following the Locati methodology, and have been characterized both by conventional methods and by micro-Computerized Tomography (µ CT) during the fatigue testing. From the observation made on the specimens, it appears that the cracks are generated, mostly, in the paste-aggregate interface and at the aggregate corner vertex of the flakiest aggregates, propagating through the weakest planes, until the failure of the concrete occurs. The authors would like to thank the Spanish Ministry of Economy and Competitiveness of Spain for financing the project MAT2014-57544-R.

The most recommendable frequency range for concrete fatigue tests is between 1 and 15 Hz. It has been clearly established that performing the tests at low frequency (

Polymeric materials are widely used in applications where the environmental conditions entail their exposure to different sources of irradiation (in most cases ultraviolet or low dose of electromagnetic irradiation for sterilization). In contrast, in this study we have assessed the modifications undergone by a series of polyurethane joints exposed to high radioactive doses of either X-rays or gamma rays (with doses of 20.5, 100, 300 and 900 kGy) or neutron irradiation (with a fluence of 7.23·1010 n/cm2) which are typical of the environment of nuclear reactors. Tensile tests were carried out to assess the change in mechanical properties derived from the radioactive exposure. Three mechanical parameters were used to monitor the evolution of strength, ductility and toughness: the tensile strength (σmax), the strain corresponding to σmax (eσmax) and the density of energy absorbed prior to maximum load (Uσmax). With regards to X and gamma rays, a negative impact of radiation on strength, ductility and toughness was observed. The detailed statistical analysis of the results has shown that a threshold dose of 300 kGy must be overcome to trigger the damage process. For the fluence employed in this study, neutron irradiation produced very little change in the mechanical properties. The SEM fractographic study has allowed the influence of irradiation on the material failure mechanisms to be identified. Thus, the fracture surface of unirradiated samples shows evidence of plastic deformation and ductile tearing. In contrast, the fracture surface of those samples exposed to a dose of 900 kGy corresponds to brittle fracture. In a consistent way, samples exposed to neutron irradiation have a fracture surface similar to that of the non-irradiated material. In summary, electromagnetic radiation for doses above the threshold leads to the embrittlement of polyurethane. Raman spectroscopy was employed to identify the microstructural changes induced by the different sources of radiation at the molecular level. The band corresponding to the vibration of the C-H bending bonds present in the polyurethane was measured as a function of the dose, finding a strong correlation between its vibration frequency and the dose of exposure to electromagnetic radiation. This shift is more sensitive than the mechanical material response since the frequency is affected at doses of 100 kGy, below the threshold previously identified for any of the mechanical properties. This correlation opens the door for the use of Raman spectroscopy as a novel non-destructive tool to characterize the microstructural effect of irradiation on polyurethane.

Fatigue tests until fracture of five notched specimens of short glass fibre reinforced polyamide, with a humidity of 2.5%, were carried out at 23, 28, 33, 38 and 43 °C. A correlation between the environmental temperature and the number of cycles to failure was established. Three different regions of strain were observed, namely, transient, steady-state and pre-failure. Strain and strain-rate during the creep-fatigue process revealed the existence of non-temperature dependent critical strains between these regions. The fractographic study performed after failure suggested the suitability of describing the stress distribution in the remnant ligament through a cohesive model. This is based on the observed fact that, at the crack tip, the matrix and the fibres debond, giving rise to crazing mechanisms so that the ultimate bearing ability of the material relies on the matrix. The calculations involved in the cohesive model were based on the information provided by the fractographic study and the real-time measurements of the crack growth by means of an infrared thermographic camera. Based on this methodology, the cohesive stress of the material was estimated. In a last stage, the surface roughness of the samples was determined after being tested, revealing a reliable correlation between the roughness and the fracture micromechanisms undergone by the underlying material. This correlation makes it possible to use the surface roughness of an in-service component as a parameter to evaluate the level of microstructural damage that it has experienced.

The dielectric paper of power transformers is subjected to ageing caused by high temperatures and chemical reactions in the dielectric oil. The electromagnetic forces that affect the conductor can cause the breakage of the paper and even the end of the useful life of the transformer. To improve the understanding of those phenomena, a continuously transposed conductor with four layers of dielectric paper (plain Kraft and crepe papers) was impregnated in naphthenic oil and aged during different periods, representative of its whole lifespan. Samples of the conductor were subjected to bending deformations compatible with a short circuit, and the cracks generated in the paper were analysed. The aged dielectric paper was mechanically characterised (in tensile, compressive and shear modes) and the bending tests of the conductors were simulated in ANSYS Static Structural, to estimate the strains in the paper. The Norris failure criterion was modified to predict the regions in the dielectric paper susceptible to fail for a particular ageing state and deformation level, obtaining a good agreement with the experimental results and thus validating this new version of the Norris approach.

Oil-immersed transformers, whose lifespan is defined by cellulose insulation's lifetime, utilize frequently mineral oil. However, this insulating fluid is being replaced by alternative liquids such as natural and synthetic esters. This replacement requires to guarantee a similar behavior of solid insulating materials immersed in them. Although there are different authors who have concluded that Kraft paper reduces its deterioration rate when it is immersed in biodegradable fluids, there are few works that have analyzed the effect of insulation liquids on the mechanical properties of other cellulosic materials such as diamond dotted paper (DDP) during laboratory tests. This paper shows a comparative analysis of four paper/oil specimens (a standard Kraft paper and a diamond dotted paper aged in both mineral oil and natural ester) under controlled laboratory accelerated thermal ageing. This work focuses on changes in mechanical properties such as the energy consumed per unit volume of the failure zone (E R ), rupture strength (σ R ) and strain under ultimate strength (ε cm ). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 823969-BIOTRAFO

Resumen Objetivo: Las propiedades de los materiales que constituyen el tejido óseo son determinantes en su resistencia mecánica pero los factores que influyen sobre ella son parcialmente desconocidos en la actualidad. Material y métodos: En esta investigación medimos la dureza ósea mediante ensayos de ultra-microindentación con punta tipo Berkovich y una carga de 150 mN en fémures de ratas Sprague-Dawley sometidas a una fractura transversal o a una osteotomía de sustracción, y comparamos los resultados en diferentes localizaciones óseas y grupos experimentales. El estudio comprende los siguientes cuatro grupos experimentales, cada uno de ellos constituido por cuatro ratas: a) fractura diafisiana estándar; b) fractura más osteotomía de 2 mm; c) osteotomía tratada con hormona paratiroidea humana, PTH (1-84); d) osteotomía tratada con ranelato de estroncio. Resultados: Encontramos que la dureza del material era consistentemente mayor en el hueso cortical que en el hueso trabecular. También fue consistentemente más alto en las epífisis femorales superiores que en las epífisis inferiores (diferencia de 1,2 desviaciones estándar). La cirugía redujo la dureza en el fémur operado (diferencia de 0,3 desviaciones estándar, p=5,5 x10-2.) El tratamiento con PTH indujo un aumento leve pero consistente de la dureza en todos los sitios(p=1,8x10-5) mientras que el efecto del ranelato de estroncio fue inconsistente. Conclusiones: Estos datos muestran que la microdureza tisular está influida por una variedad de factores, incluyendo la anatomía, el tipo de tejido óseo, la lesión esquelética y la terapia farmacológica. Por lo tanto, los estudios futuros sobre la calidad del tejido deberían diseñarse cuidadosamente teniendo en cuenta estos factores.

Train operations generate high impact and fatigue loads that degrade the rail infrastructure and vehicle components. Rail pads are installed between the rails and the sleepers to damp the transmission of vibrations and noise and to provide flexibility to the track. These components play a crucial role to maximize the durability of railway assets and to minimize the maintenance costs. The non-linear mechanical response of this type of materials make it extremely difficult to estimate their mechanical properties, such as the dynamic stiffness. In this work, several machine learning algorithms were used to determine the dynamic stiffness of pads depending on their in-service conditions (temperature, frequency, axle-load and toe-load). 720 experimental tests were performed under different realistic operating conditions; this information was used for the training, validation and testing of the algorithms. It was observed that the optimal algorithm was gradient boosting for EPDM (R2 of 0.995 and mean absolute percentage error of 5.08% in test dataset), TPE (0.994 and 2.32%) and EVA (0.968 and 4.91%) pads. This algorithm was implemented in an application, developed on Microsoft. Net platform, that provides the dynamic stiffness of the pads characterized in this study as function of material, temperature, frequency, axle-load and toe-load.

High responsibility thermoplastic components subjected to dynamic forces show serious in-service thermal limitations. This paper shows that adding the adequate material with a phase-change, PCM, to a thermoplastic matrix component significantly increases its fatigue life. To do this, the fatigue resistance of short fibre-glass reinforced polyamide is analysed by means of monotonous fatigue tests on tensile specimens. Next, the design of the pieces of this material, used in high-speed rail fastenings, is optimised for fatigue efforts by means of accelerated single-piece tests. Finally, it is verified that the fatigue resistance of these pieces endowed with hydrated salt, Na2S2O3·5H2O, which acts as a PCM, increases by a surprising 400% with respect to the reference pieces without PCM.

The track has a crucial role in the performance of the rail network as it provides support and guidance to the rolling stock. During train operation, the vehicle-track interaction generates high impact loads and fatigue, which lead to degradation of vehicle components and rail infrastructure. These loads tend to increase the maintenance needs and, consequently, the life-cycle-costs of the rail assets. In order to minimize these consequences, rail pads are generally used between the rails and the sleepers in order to provide flexibility to the track and to damp the transmission of noise and vibrations. In ballasted tracks, this flexibility is a combination of the mechanical properties of the ballast and the rail pads. However, in slab tracks, the flexibility of the infrastructure is almost exclusively dependant on the rail pads. These materials exhibit non-linear, dissipative characteristics that are affected by the service conditions such as temperature, frequency, toe load and axle load. This work aims to investigate experimentally different pad materials widely used in the rail industry in order to characterize the influence of these factors on the mechanical characteristics of the rail supporting elements. The detailed characterization of the rail pads enables not only better understanding of their performance in realistic service conditions, but also provides good perspectives for use of these well quantified mechanical properties in studying the vehicle-track dynamic behaviour in different scenarios and predicting the long-term performance of the infrastructure components.

Self-compacting concrete modifies its workability with small variations in the amount of superplasticizer additive. For this reason, large number of tests are required to monitor its workability. In order to determine the appropriate amount of additive for a single mix, the evolution of the power consumption of the concrete mixer during the addition of small amounts of additive to the concrete was analyzed. These results were compared both with typical workability characterization for self-compacting concrete and with the saturation point results, determined by the Marsh cone method. After this comparison, a good correlation was obtained between the results from the traditional tests and the proposed &ldquo, concrete mixer method&rdquo

Research and development of technology for railways has found new impetus as society continues to search for cost effective and sustainable means of transport. This tasks engineers with using the state-of-the-art science and engineering for rolling stock development and advanced technologies for building high performance, reliable and cost-effective rail infrastructures. The main goal of this work is to develop detailed and validated three-dimensional slab track models using a finite element formulation, which include all components of the infrastructure. For this purpose, the parameters of the computational models are identified by performing full-scale tests of the fastening system and of the slab track, including all its material layers. The computational model proposed here is calibrated using this approach and a good agreement is obtained between experimental and numerical results. This work opens good perspectives to use this reliable track model to study the interaction with railway vehicles in realistic operation scenarios in order to assess the dynamic behaviour of the trains and to predict the long-term performance of the infrastructure and of its components.

Vegetal oils are considered a suitable substitute for mineral oil that is widely used in power transformers as insulation and cooling medium. Due to the fact that more and more power transformers containing this alternative insulating fluid are being constructed a mathematical ageing models is needed to help predict transformer failures. The continued performance of power transformers depends on the condition of its paper insulation mainly. In this sense, this paper analyses Kraft paper degradation through the loss of its mechanical strength. Accelerated thermal ageing test of the paper in two different vegetal oils were carried out at three temperatures during diverse periods of time, in order to obtain information on the kinetics of the ageing degradation of the paper. The evolution of the mechanical properties of paper failure are analysed as a function of temperature and ageing time. In addition, the results obtained are compared with the traditional method of degradation analysis, based on the measurement of the degree of polymerisation. 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 DPI2013-43897-P grant agreement, financed by the Government of Spain.

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.

Power transformers use Kraft paper, thermally upgraded Kraft and other polymeric papers (ex. Nomex) as the main solid insulation between the winding conductors. Dielectric oil used in transformers as an insulating and cooling fluid typically has an operating temperature range of 60–90 °C. These service temperatures can cause slow degradation of both the oil and the insulating paper winding, with a loss of mechanical and dielectric properties. In this sense, this work analyzes paper degradation through Young’s Modulus, yield stress, rupture strength and strain under ultimate strength. An accelerated thermal ageing of the paper in mineral oil was carried out at temperatures of 110, 130 and 150 °C over different periods of time, in order to obtain information on the kinetics of the ageing degradation of the paper. The evolution of the mechanical properties and micro mechanisms of paper failure are analysed as a function of temperature and ageing time. Finally, the results obtained are compared with the traditional method of degradation analysis, based on the degree of polymerisation measurement. 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 DPI2013-43897-P Grant Agreement, financed by the Government of Spain.

Determination of the Paris' law constants requires the continuous measure of the crack length. This measure has usually been performed using methods based on electric potential differences or by means of correlations between crack length and the compliance of the specimen. Bearing in mind that the crack tip undergoes a temperature rise due to the corresponding stress concentration and the generation and growth of a plastic zone, it is possible, using infrared thermographic techniques, to quantify the crack growth. Thus, in this paper, the fatigue crack propagation behaviour of a polyamide has been characterised by determining the Paris' law constants with the aid of thermographic tools. The values obtained by means of this methodology have been compared with those resulting from the use of conventional measurement techniques.

Power transformers are electrical machines that allow us to transport electric energy, with reduced losses, from generation stations to consumption points. This definition gives us an idea of the number of transformers that are used in power distribution systems worldwide. The main problem that may affect power transformers is the operation at high temperature. This paper summarizes the results of a postmortem temperature estimation carried out on an 800 kVA distribution transformer. Two methodologies are considered for estimating the temperature distribution in the windings of the machine. The first is based on the calculation of the degree of polymerization and the second is obtained from the tensile strength index. By knowing the value of these two magnitudes for a new and for an aged paper, and the period of operation of a transformer, the temperature distribution along the height of the windings can be estimated. None other previous works have used the tensile strength of winding paper for temperature distribution estimation. With these results and the loading regime records, similar transformers still in operation can be operated in an alternative manner and future designs can be improved.

This paper presents the Theory of Critical Distances (TCDs), and more precisely, the Point Method (PM), as a methodology for the prediction of the load-bearing capacity of structural components containing any kind of stress risers and subjected to static conditions. The main aim is to validate this application in Al7075-T651 specimens with different types of defects, geometries and loading conditions, beyond its application to fracture mechanics specimens, covering the different situations appearing in structural components. It is also intended to analyse those situations where the TCD provides accurate results and those where this is not the case, as well as to provide criteria about the predictive capacity of the TCD. The results show that the PM provides accurate results as long as the Neuber number, defined as the notch radius divided by the critical distance ( L ), is sufficiently low (20 is taken as a reference value). If it is intended to extend the validity limits of the PM to situations where the Neuber number is higher, it is then necessary to include in the calibration process of L fracture notched specimens with similar radii to those being analysed, and to use the value of L which provides the corresponding lower envelope of the experimental calibration results (here called L LE ).

The Theory of Critical Distances is actually a group of methodologies, all of them using a characteristic material length parameter when performing fracture assessments on any kind of stress risers. In this paper, the application of the Theory of Critical Distances on Polymethyl methacrylate is analysed, the parameters involved in the analysis are calibrated, and the different versions of this theory are applied, with satisfactory results, to the prediction of the notch effect on the apparent fracture toughness measured on notched specimens. Also, a Scanning Electron Microscopy analysis is performed, establishing relations between the fracture mechanisms and the predictions provided by the Theory of Critical Distances.

This article proposes an analysis method for the identification of the damage caused in a reinforced polyamide fatigue process through the evolution of the generated energy per cycle. Monotonous fatigue tests have been carried out at different stress levels using Polyamide 6 reinforced with short fibre glass tensile specimens. The loss of mechanical properties is measured using a damage parameter, defined as the variation in energy per cycle with respect to the initial conditions. Three clearly differentiated zones can be distinguished in the evolution of damage as a function of the energy per cycle. The first zone is one of deaccelerated rapid growth; in the second, the damage grows linearly, so that the damage growth speed can be determined as a function of the potential of variation of the stress applied; and finally, the third is a zone of accelerated growth of the damage until fracture.

The Point Method and the Line Method are two approaches derived from the Theory of Critical Distances, used in conjunction with a characteristic material length parameter when performing fracture assessments on any kind of stress risers, including notches. This paper analyses the application of both the Point Method and the Line Method for the prediction of the notch effect on the aluminium alloy Al7075 in T651 conditions. The parameters involved in the analysis are calibrated through a combination of experimental tests and finite elements modelling, and a comparison is made between the experimental results and the predictions provided by these two approaches. Also, a Scanning Electron Microscopy analysis is performed, establishing relations between the fracture mechanisms and the predictions provided by the Theory of Critical Distances. The results obtained demonstrate the suitability of the Point Method and the Line Method for the analysis of notches.

This paper illustrates the need to develop specific methodologies for the assessment of notched structural components. These are usually analysed under the assumption that notches behave as cracks, providing results that may be overconservative. The proposal consists, on the one hand, in the application of the Theory of Critical Distances for the estimation of the apparent fracture toughness, and for the conversion of the notched situation into an equivalent cracked situation in which the material develops a higher fracture resistance (the apparent fracture toughness). On the other hand, once the apparent fracture toughness has been defined, the assessment is performed using the Failure Assessment Diagram methodology, and assuming that the notch effect on the plastic collapse load is negligible. The methodology has been applied to notched fracture specimens made of PMMA and Al7075-T651, providing satisfactory results and a noticeable reduction in the overconservatism derived from analyses in which the notch effect is not considered.

This study involves the determination of the mechanical and fracture properties of polyamide 6 reinforced with short glass fibres (35 wt. %) at five different moisture contents (0, 1, 2, 3, and 6 wt. %, respectively). The mechanical characterisation consisted of tensile tests on plane specimens to determine the influence of moisture content on the material’s mechanical parameters. The study of fracture properties was carried out by performing single-edge notch bending tests on specimens with the levels of moisture mentioned above in order to obtain the critical stress intensity factor, K Ic . The experimental results showed that the material fracture behaviour cannot be properly described following this methodology; for this reason, a more ambitious scope was established, consisting of determining the J-integral during stable propagation by combining the experimental force vs. displacement curves with the results of a finite element numerical model. The analysis clearly demonstrates the improvement in the material’s toughness as humidity increases. The subsequent fractographic study performed through scanning electron microscopy allowed the influence of moisture content on the fracture micro-mechanisms developed during fracture and propagation to be determined.

Thermoplastic elastomer railway pads, placed between the base of the steel rails and the prestressed concrete sleepers, play a major role in the general maintenance of the state of the overall structure of a railway line. These rail setting pads provide the elasticity of the line and damp the vibrations which the rail transmits to the sleeper, thus avoiding the cracking of the concrete sleepers and preventing the wear and tear of the ballast. They also provide electrical insulation between the rails. This paper describes a test methodology for determining the dynamic behaviour of railway fastening setting pads in working conditions, which may involve a wide range of temperatures associated to the wear and the hardening they undergo during the cyclical load process applied on them caused by the passing of the trains. To this end, measurements of the evolution of the stiffness of the pads have been made, both static and dynamic. Also associated energy parameters have been measured, establishing an index of the degree of deterioration undergone by these pads.

The optimal viscosity for railway fastening parts, injected with a thermoplastic polymeric matrix of PA 6.6 (Nylon 6.6) reinforced with a 35% weight of short glass fibre, guarantees that they work properly during the mechanical actions applied on them in relation with the forces transmitted by the rails. Under the previous optimum material injection criteria, verified in this work, a fatigue characterization for the polymeric parts using the Locati method has been developed in this paper. A model is proposed for the identification of the critical conditions through a study of the evolution of various parameters during the fatigue process: the stored and dissipated energy and the phase lag between the load and strain signals These parameters will indicate the situation for which the material temperature reaches the glass transition value, as a consequence of the internal heat generated by the fatigue. From this point the material increases its deformation continuously due to the material damage leading to the fatigue failure.

This paper presents the practical application of cm artificial neural network (ANN) to the problem of concrete mix in a factory. After a brief introduction to the complex problem of concrete mixes design and a quick review of the fundamental basis of neurocomputation, an optimal neural network model has been developed to cope with such a problem. For training the net, several control mixes have been fabricated recording in all cases both the characteristic 28 days compressive strength and the workability measured in terms of the slump of the Abrams' cone. After the training process of the net, the power of its predictive ability is checked by comparison of the results obtained with those corresponding to four reference mixes; in this way, it is shown that the considered approach can be used in multicriterial search for optimal concrete mixes.

This paper presents the analysis of the notch effect in two ferritc–pearlitic steels: S275JR and S355J2. The research is based on the development and analysis of an experimental programme composed of 336 CT specimens, combining 6 different notch radii, and testing temperatures from the lower shelf up to the upper shelf of the two materials. The notch effect is analysed through the evolution of both the load bearing capacity and the apparent fracture toughness, and also through the relation between these two variables and the failure micromechanisms. The results reveal a clear notch effect in both materials. In the case of the load bearing capacity, this notch effect has its maximum at lower shelf temperatures. The notch effect in the apparent fracture presents a maximum at the lower temperatures of the ductile-to-brittle transition zone. Finally, the Scanning Electron Microscopy fractographies have justified the previous observations.