Aplicación de métodos basados en análisis de vibraciones para la caracterización de daño en laminados carbono/epoxi

This thesis presents a study in which a method for the detection of barely visible structural damage due to multi-impact in carbon/epoxy composite plates has been developed. The developed method is applicable to structural health monitoring of structures containing this type of elements, such as the fuselages of certain aircraft, during their commercial operation. The method is based on the analysis of the variations in the dynamic response of the laminates, through the study of forced vibrations by means of controlled excitation. In the study, both numerical simulations of this dynamic behaviour have been carried out, through the development of finite element models, and experimental work consisting of impact tests on the plates, modal tests and ultrasonic inspection tests. The process followed was as follows: 1. Dynamic characterisation tests on the intact plates: vibration tests under free boundary conditions, measuring the vibrations induced both by means of accelerometers and high-speed cameras. 2. Processing of the vibration data to obtain the dynamic characteristics of the plates. 3. Development of preliminary numerical models. 4. Models update: correction of some initial parameters of the numerical models based on the modal characteristics estimated experimentally in step 2. 5. Multi-impact tests to simulate the impact of small debris or hail. The spatial distribution of the impacts was random and performed in a range of velocities previously set according to the objective of achieving barely visible damage. 6. Ultrasonic inspection: the impacted plates were inspected by C-Scan to establish the location and extent of the delaminations produced by the impacts. 7. Vibration tests for dynamic characterisation of the damaged plates under the same conditions as those performed in step 1 on the intact plates. 8. Processing of the vibration data of the damaged plates to obtain the dynamic characteristics modified by the damage. 9. Comparison of the dynamic data of intact and damaged plates (both modal and spectral) to establish damage detection rates and explore damage localisation capability. 10. Simulation of damage in models calibrated in step 4: a large number of simulations established trends that can be used as a reference for damage quantification. Damage detection using vibration-based techniques is a large field of research and the techniques described have been in use for some time. The main difference of this research with respect to others is to focus on the detection threshold of barely visible damage in multi-impact composite plates with a small cumulative damage area, proposing and comparing indices capable of detecting it. Additionally, the applicability of these techniques has been demonstrated despite drastically reducing the time required for the development of the detection tests, which is a key aspect for the adoption of the proposed techniques for industries such as aeronautics, where the reduction of the non-flight time of an aircraft due to maintenance is crucial.En esta tesis se presenta un estudio en el que se ha desarrollado un método para la detección de daño estructural apenas visible debido a multi-impacto en placas de material compuesto carbono/epoxi. El método desarrollado es aplicable al seguimiento de la integridad estructural de estructuras que contengan este tipo de elementos, como son los fuselajes de ciertas aeronaves, durante su explotación comercial. El método se basa en el análisis de las variaciones en la respuesta dinámica de los laminados, a través del estudio de las vibraciones forzadas mediante una excitación controlada. En el estudio se han realizado tanto simulaciones numéricas de dicho comportamiento dinámico, mediante el desarrollo de modelos de elementos finitos, como un trabajo experimental consistente en ensayos de impacto sobre las placas, ensayos modales experimentales y de inspección mediante ultrasonidos. El proceso seguido ha sido el siguiente: 1. Ensayos de caracterización dinámica de las placas intactas: ensayos vibratorios en condiciones de contorno libre, midiendo las vibraciones inducidas tanto mediante acelerómetros como con cámaras de alta velocidad. 2. Tratamiento de los datos de vibración para obtener las características dinámicas de las placas. 3. Desarrollo de modelos numéricos preliminares. 4. Ajuste de los modelos: corrección de algunos parámetros iniciales de los modelos numéricos basada en las características modales estimadas experimentalmente en el paso 2. 5. Ensayos de multi-impacto para simular el impacto de pequeños residuos o granizo. La distribución espacial de los impactos fue aleatoria y se realizaron en un rango de velocidades previamente establecido con el objetivo de conseguir daño apenas visible. 6. Inspección por ultrasonidos: las placas impactadas se inspeccionaron mediante C-Scan para establecer la localización y extensión de las delaminaciones producidas por los impactos. 7. Ensayos vibratorios de caracterización dinámica de las placas dañadas en las mismas condiciones que los realizados en el paso 1 sobe las placas intactas. 8. Tratamiento de los datos de vibración de las placas dañadas para obtener las características dinámicas modificadas por el daño. 9. Comparación de los datos dinámicos de placas intactas y dañadas (tanto modales como espectrales) para establecer índices de detección de daño y estudiar la capacidad de localización del daño. 10. Simulación de daño en modelos calibrados en el paso 4: se realizó una serie de ensayos virtuales con presencia de daño aleatorio, mediante técnicas de Montecarlo, y se establecieron tendencias que pueden ser usadas como referencia para la cuantificación de dicho daño. La detección de daños mediante técnicas basadas en vibraciones es un amplio campo de investigación y las técnicas descritas llevan utilizándose desde hace tiempo. La principal diferencia de esta investigación con respecto a otras es centrarse en el umbral de detección de daño apenas visible en placas de material compuesto producidas por multi-impacto con un área de daño acumulado reducida, proponiendo y comparando índices capaces de detectarlo. Adicionalmente, se ha demostrado la aplicabilidad de estas técnicas incluso en el caso de reducción drástica del tiempo necesario para el desarrollo de los ensayos de detección, lo cual es un aspecto clave para la adopción de las técnicas propuestas para industrias como la aeronáutica, en las que la reducción del tiempo de no vuelo de un avión por mantenimiento es crucial.Programa de Doctorado en Ingeniería Mecánica y de Organización Industrial por la Universidad Carlos III de MadridPresidente: Salvador Ivorra Chorro.- Secretario: Marco Antonio Pérez Martínez.- Vocal: Jorge López Puent

Analysis of Lateral Displacements in Large Railway Viaducts Under Traffic Loads. Impact on Ride Safety and Passenger Comfort.

The increasing design speed of the new high speed lines and the stringent requirements on track alignment parameters are leading to a sustained increase of the number of railway viaducts. The relevant standards impose limiting values on lateral vibrations. Both the Spanish and European standards establish a minimum value for the first natural frequency of lateral vibration of a span, that should not be lower than fh0 = 1,2 Hz. This limit was originally proposed by ERRI committee D181, which assessed the lateral forces in railway bridges. This limit was proposed in order to avoid lateral resonance in railway vehicles going across the structure, taking into account that the frequencies of lateral vibration of railway vehicles are, in general, not greater than 1,0 Hz. In the case of large continuous viaducts with high piers, the lateral deformations occurring during a train pass-by can be significant and the natural frequencies of the first mode of vibration of the deck can be very low. In these cases it is not clear whether the required verifications must be applied to spans considered independently, to several successive spans or to the whole viaduct. There is currently no analysis methodology allowing to assess this situation and check the viaduct design against the requirements of ride safety and passenger comfort. This paper analyzes the lateral deformations of a large continuous viaduct and the infrastructure vehicle interaction effects due to the circulation of freight trains and several types of high speed train sat different speeds. The application of this methodology will allow an optimized design of viaducts with significant lateral deformations that cannot be justified only by using the simplified criteria of the current applicable standards. In such cases, the compliance with standards may lead to over dimensioning or in other cases to neglect the limits without the adequate verification of the proper infrastructure behavior once it has been commissioned. As it is the case for vertical deformations, for which the European standards require the assessment of dynamic effects, we stress the need for a dynamic analysis of the effects of lateral deformations in large railway viaducts

Model updating of uncertain parameters of carbon/epoxy composite plates using digital image correlation for full-field vibration measurement

Model updating is usually based on the contrast between the modal characteristics predicted by the models and those experimentally identified. Traditional experimental methods are based on the use of contacting sensors, but more recently other techniques as 3D Digital Image Correlation (DIC) have also been used successfully. In this paper the results obtained by applying these alternative techniques are compared, to obtain physically-sound models of carbon/epoxy composite plates. Primarily a roving hammer exciting the plates at evenly distributed degrees of freedom (DoF), and a mono-axial accelerometer attached to a single DoF reference point, have been used for modal identification. Alternatively, high speed cameras were applied to measure full-field vibrations of the plates. 3D DIC allowed obtaining a lower number of natural frequencies but much smoother mode shapes and similar results for model updating. The experimental setup has been benchmarked using two different sets of plates varying thickness and ply stacking.This research was done with the financial support of the Spanish Ministry of Economy and Competitiveness under Project reference DPI2013-41094-R, and the Vicerrectorado de Política Científica UC3M (Projects 2014/00006/002 and 2013/00413/003)

A study of the lateral dynamic behaviour of high speed railway viaducts and its effect on vehicle ride comfort and stability

The study of the lateral behaviour of railway bridges and vehicles is an important issue on bridges with low lateral stiffness, which has been defined by ERRI (1996) as those with lateral natural frequencies below 1.2 Hz. This limit applies to the deformation of the deck in one span, and was demonstrated to be a real issue on measurements and models of bridges with open deck sections and supporting trusses, of low lateral bending stiffness for the deck. Although not included in the above category, modern long viaducts for HSR with continuous decks on tall piers may also exhibit very low lateral stiffness and frequencies, which could produce undesired effects for the comfort or even the stability of the railway vehicles. In this work a simple model has been developed and applied to consider worst-case scenarios in a representative bridge, the “Arroyo de las Piedras” viaduct in Spain. The trains considered are representative of those circulating in the Spanish HSR network, as well as a freight wagon. Threedimensional dynamic models were developed with finite elements. The actions considered include the lateral deformation of the bridge in response to vertical eccentric loads, track alignment irregularities and finally lateral motion of vehicles due to conicity of wheel-rail contact. The results show that there is, at least in this case, no cause for concern. However, for some scenarios the results in terms of lateral motion and forces are not negligible and should be considered in the design

Model updating of uncertain parameters of carbon/epoxy composite plates from experimental modal data

This work presents a methodology to obtain physically-sound models of composite structure laminates using a combination of modal analysis, numerical modelling and parameter updating, avoiding the common uncertainties on the constructions of similar numerical models. Moreover this model establishes the baseline (pristine situation) of the dynamic behaviour of the set of composite plates. Therefore it could be applied for condition assessment or quality manufacturing control of existing structures through a non-destructive Structural Health Monitoring (SHM), and hence it could help to detect degradation or defects of the composite components. The driven data of the methodology were the modal frequencies and shapes of composite plates. To obtain these values an extensive experimental campaign of modal analysis has been performed on a set of carbon/epoxy laminates. A multiple input single output technique has been applied, using a roving hammer exciting the plates at evenly distributed Degrees of Freedom (DoF), and a mono-axial accelerometer attached to a single DoF reference point. The use of a high dense grid of points has allowed to identify a number of natural frequencies greater than usual in similar works, as well as improving the smoothness of the mode shape. Modal characteristics numerically obtained from a Finite Element Method (FEM) model based on manufacturer reference data were compared with experimental results. This baseline model was updated through a gradient based optimization algorithm. Before the process of model updating, a sensitivity analysis has been performed to identify the driven uncertain parameters using a Montecarlo approach. This technique reduces the number of parameters to be optimized to a small set increasing the efficiency of the methodology. As a result of the whole process, a physically more accurate model is obtained on which discrepancies with the corresponding experimentally measured modal parameters are drastically reduced. AnalyThis research was done with the financial support of the Spanish Ministry of Economy and Competitiveness under Project reference DPI2013-41094-R, and the Vicerrectorado de Política Científica UC3M (Projects 2014/00006/002 and 2013/00413/003)