Abstract

La presente monografía está basada en el texto de la tesis doctoral homónima defendida
en la Universitat Politècnica de Catalunya el 6 de febrero de 2012, en el marco
del programa de doctorado de Análisis Estructural del Departament de Resistència de
Materials i Estructures a l’Enginyeria. En este estudio se analiza la viabilidad del uso
de las técnicas experimentales y numéricas basadas en la respuesta dinámica, para la
detección y localización del daño inducido, la cuantificación del grado de severidad y
la predicción de las propiedades mecánicas residuales de laminados de material compuesto
tras ser sometidos a un impacto a baja velocidad. El trabajo de investigación
comprende un enfoque mixto experimental y numérico. Por un lado constituye un
riguroso estudio experimental que incluye la evaluación de la resistencia a impacto,
la caracterización del daño inducido, la cuantificación de los efectos en la respuesta
dinámica y la evaluación de la capacidad portante residual. El estudio experimental
se completa con un minucioso análisis del efecto inducido por deslaminaciones artificiales en la respuesta dinámica y en la capacidad portante residual de los laminados.

Por otro lado, se ha simulado el fenómeno estimando principalmente la iniciación y la
propagación del daño interlaminar e igualmente los efectos inducidos en la respuesta
dinámica. En el enfoque numérico se trata el material compuesto como un sistema
microestructural, en el cual los fallos surgen en la interacción entre los materiales
constituyentes. Para reproducir la degradación se emplea una estrategia de reducción
localizada de la rigidez elástica del material que no demanda una intervención durante
el preproceso.
Los resultados empíricos aportan conclusiones relevantes en relación al grado de
sensibilidad y a la adecuación de los diferentes criterios de correlación modal para la
identificación del daño inducido por un impacto. Los nuevos criterios de correlación
definidos y el análisis conjunto de las propiedades estáticas y dinámicas residuales, han
permitido acotar el intervalo de incertidumbre y reducir la limitación actual en relación
a la deformación máxima admisible a compresión de los laminados. La herramienta
de cálculo desarrollada permite simular el comportamiento vibratorio de laminados
compuestos definiendo el material y su estado de degradación en la microescala. Los
resultados corroboran la viabilidad del enfoque microestructural para la simulación
del fenómeno.

Abstract

The use of hydrogen as an energy source is a promising strategy to replace fossil fuels, in addition to batteries and biofuels. The European aerospace industry is already working on the development of propulsion technologies with low or zero emissions and heavily considers hydrogen technology. It is favoured in aeronautics to store H2 in liquid form, LH2, at temperatures of 20K to have the highest possible energy density of the fuel. This means that all materials in contact with LH2 need to be characterized at this temperature.

INTA began to carry out cryogenic tests on composite materials 27 years ago. Initially in the field of characterization of composite materials for tanks for reusable launchers. Later, the structural monitoring of LH2 tanks, the detection of hydrogen leaks and fuel cells have also been investigated. Currently, INTA is involved in the characterization of composite materials in LH2 tank projects and the composite material structures that these tanks carry for sustainable aviation.

The article gives an overview of the activities carried out in the field of cryogenic tests of composite materials and provides data obtained in these tests, comparing the behaviour of different materials and different types of tests. In addition, the new lines of research that are being carried out at INTA in the characterization of composite materials and their behaviour before and after being in contact with hydrogen will be explained.

Abstract

The post-impact strength of composite materials is one of the main design parameters of aeronautical structures in terms of damage tolerance. During the low-velocity impact test, from a threshold energy, the laminate only partially returns the energy received during the impact to the indenter (elastic recovery). The remaining energy is absorbed by the laminate and dissipated in the form of damage (interlaminar and intralaminar), plastic deformation of the polymer matrix and breakage of the carbon fibers. To date, few authors have attempted to quantify the participation of each of the damage mechanisms in the overall energy absorption process of the laminate due to their experimental difficulty. In this work, a methodology has been developed capable of performing damage of similar extent and location to that produced in a low-velocity impact, but without damaging the fibers, through the application of local induction heating. For this purpose, the residual strength and stiffness of AS4/PEEK laminates, subjected to impacts over a wide range of energies (30-70J), have been compared with those obtained in laminates damaged by electromagnetic currents, for equivalent damage extensions. The results reveal that the breakage of carbon fibers has a great influence on the loss of stiffness of the laminate, but not on its strength, confirming the role of delamination as the main responsible for the loss of the strength capacity of the damaged material.

Abstract

It is essential to understand the behavior of composite laminates against fatigue crack initiation and growth in order to accurately predict component service life and to establish safe maintenance periods.

In this work, an extrapolation procedure to characterize the mode II fatigue behavior based on the J_0i-C_0i and Δ_0i-C_0i master curves is presented and validated.

The extrapolation procedure, based on the compliance variation, assumes that all effects associated with damage are included in the equivalent crack length. In this method, new factored expressions for flexibility (C_0i), Integral-J (J_0i) and crack tip displacement (Δ_0i) are defined and according to their polynomial expressions with respect to the equivalent crack length, invariant relationships between J_0i-C_0i and Δ_0i-C_0i are obtained for a given material system and test configuration.

Once the master curves have been calibrated, the extrapolation procedure allows characterizing the fatigue behavior of a material system by determining the Paris law during the fatigue test, monitoring only the test flexibility and the maximum load with respect to the number of cycles.

Abstract

The behaviour of composite materials under dynamic loading conditions has been a subject of research for the last two decades. When subjected to dynamic conditions, composite materials show a significant sensitivity to strain rate. This is critical for a correct simulation of the behaviour of composite structures under impact. Currently, a few studies have been carried out on the analysis of strain rate sensitivity of the fracture toughness. These studies have reported different conclusions for the same material (IM7/8552), indicating that the optimal technique for characterising this property under high strain rate loading remains unresolved. In this work, it has been carried out dynamic characterization test on the fracture toughness using the Split Hopkinson Pressure Bar. It has been developed a new methodology for obtaining an accurate synchronization between Hopkinson bar data and high speed camera. Using the proposed methodology a good repeatability has been obtained for dynamic tests.