Análisis y modelización de vigas sándwich sometidas a impactos de baja velocidad

En esta Tesis Doctoral se ha estudiado el comportamiento frente a impactos de baja velocidad de vigas sándwich constituidas por pieles de material compuesto de tipo laminado y núcleo de nido de abeja de aluminio. Existen tres clases de metodologías para hacer frente a este estudio: el análisis experimental, los modelos analíticos y las simulaciones numéricas. La realización de un estudio experimental exhaustivo supone una gran inversión, tanto en términos económicos como temporales, por lo que la utilización de modelos analíticos y herramientas de simulación representa una alternativa eficiente. Los modelos analíticos permiten evaluar variables globales como la fuerza o el desplazamiento pero sin llegar a profundizar en el proceso de fallo de pieles y núcleo. Las simulaciones numéricas suelen lograr descripciones más precisas y detalladas de la respuesta a impacto y su flexibilidad en relación a las variaciones de condiciones de contorno, geometría y material, es muy apreciada en el campo de la investigación. Disponer de modelos numéricos permite profundizar en el conocimiento del comportamiento a impacto de las estructuras sándwich, proporcionando información que es difícilmente extraíble de forma experimental. No obstante, ambos tipos de modelos exigen la realización de ensayos experimentales para su validación. Con este objetivo se han llevado a cabo una serie de ensayos experimentales en torre de caída, donde se evaluaron la fuerza máxima de contacto, el tiempo de contacto del ensayo, el desplazamiento máximo de ambas pieles y la energía absorbida de las vigas sándwich. Se ha desarrollado un modelo analítico que permite estimar la fuerza de contacto que se produce en un impacto de baja velocidad sobre vigas sándwich de pieles de material compuesto. Este modelo, validado experimentalmente, se ha formulado en variables adimensionalizadas lo que ha permitido determinar los grupos adimensionales que dominan la dinámica del sistema, pudiendo realizar variaciones de los mismos para analizar su influencia sobre el fenómeno de impacto estudiado. Asimismo se ha realizado un modelo de elementos finitos para simular el impacto a baja velocidad sobre vigas sándwich, estudiando la absorción de energía en función de la energía de impacto, así como el comportamiento de las pieles de material compuesto y del núcleo de nido de abeja durante el fenómeno de impacto. El modelo constitutivo utilizado para las pieles ha sido ampliamente utilizado en trabajos previos del Departamento. El comportamiento asumido para simular el núcleo de nido de abeja se ha validado mediante la realización de ensayos experimentales de compresión. Debido a que la compresión del núcleo es un factor clave en la capacidad de absorción de energía de la viga sándwich, se ha realizado un estudio numérico de la influencia que tiene la variación de ciertos parámetros del modelo (espesor de pared de celda, el tamaño de la celda hexagonal, el valor del límite elástico del material y la altura de núcleo).In this PhD Thesis, the low-velocity impact behaviour of sandwich beams with woven carbon fibre-epoxy face-sheets and aluminium honeycomb core is studied. There are three ways to deal with this study: experimental analysis, analytical modelling and numerical simulations. An intensive experimental test program can be both time and cost intensive; therefore, analytical and finite-element models may represent an efficient alternative to reproduce those impact events. Analytical approaches can assess global variables such as force or displacement; however it is needed to explore in more detail the failure process of the facesheets and the core. Finite-element models are usually developed to achieve more precise and detailed descriptions of impact events. Finite-element modelling seems to be more flexible than the analytical approach, as several variables such as material, geometry, and boundary conditions can be easily modified. In this context, simulations have allowed a deeper understanding of the impact response of sandwich structures, and can also give access to information hardly obtainable by experiments. Modelling require experimental testing for validation. In line with this objective, impact tests of the composite sandwich beams at low-velocity impact were conducted in an instrumented drop-weight tower apparatus. The experimental tests were evaluated in terms of maximum contact force, contact time, displacement of the two face-sheets, and absorbed energy. It was developed an analytical model for estimating the contact force that occurs in a low-velocity impact on sandwich beams with composite face-sheets. This analytical model, validated experimentally, has been formulated using a dimensionless method and there were determined several dimensionless groups, which dominate the dynamics of the impact. It was analysed the effect of varying the dimensionless groups and their influence on the studied impact phenomenon. A finite-element model of the honeycomb cored composite sandwich beams under low-velocity impact was developed with Abaqus/Explicit code. The absorbed energy as a function of the impact energy, the behaviour of both face-sheets and the honeycomb core during the impact event were studied. The face-sheet behaviour was modelled through a user subroutine, and has been widely used in previous works. The aluminium honeycomb core behaviour was validated through a set of experimental tests. Due to the influence of several core parameters on the crush behaviour and on the energy-absorption capacity of honeycomb core structures, a threedimensional finite-element model of a honeycomb-core structure was developed, and virtual compressive tests were simulated

Numerical study of damaged micro-lattice blocks subjected to uniaxial compressive loading

This article presents a numerical study on the mechanical behaviour of damaged micro-lattice (ML) blocks submitted to uniaxial compressible loads. The numerical model is implemented in the commercial finite-element code Abaqus/Standard. From the finite-element model, the initial stiffness and the Yield stress of ML blocks are calculated. The damage in ML blocks is modelled as manufacturing defects, that are included in the ML structures using a random algorithm implemented in MatLab. The numerical model is validated with experimental data from uniaxial compression tests carried out on ML intact blocks by other authors. The analysis of the mechanical behaviour of ML blocks is presented in terms of variations of damage percentage, cell type and cell size.The authors wish to gratefully acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness under Projects DPI2011-24068 and DPI2011-23191.Publicad

Analytical study of the low-velocity impact response of composite sandwich beams

In this work the low-velocity impact response of composite sandwich beams was studied by an analytical model. A dimensional analysis was carried out in order to identify the key parameters that influence the dynamic beam response, and to assess the effect of the dimensionless groups on the contact force and contact time. Low-velocity impact tests were conducted to validate the theoretical model. The predicted results were in good agreement with experimental data in terms of maximum contact force, contact time, and contact force time curves. It was shown that the groups with more influence on maximum contact force and contact time are the dimensionless global stiffness, the dimensionless local stiffness, and the dimensionless impact velocity

FEM analysis of dynamic flexural behaviour of composite sandwich beams with foam core

7 pages, 11 figuresThe dynamic flexural behaviour of sandwich beams, with composite face-sheets and a foam core, was analysed by developing a 3D finite-element model. To model the core behaviour, a crushable foam model was used. The Hou criteria were used to predict the failure of the face-sheets. Dynamic bending tests were performed to validate the numerical model. The comparison between numerical and experimental results in terms of contact-force histories, peak-force values, absorbed energy, and maximum displacement of both face-sheets was satisfactory. It was revealed that the collapse of the foam core under the impact region favoured the failure of the upper face-sheet.The authors are indebted to the Autonomous Community of Madrid and University Carlos III of Madrid (Project CCG08 UC3M/DPI 4348) for the financial support of this work.Publicad

Influence of the cohesive law shape on the composite adhesively-bonded patch repair behaviour

In this study, the cohesive failure of the adhesive layer of an adhesively-bonded joint under uniaxial tensile loads in static conditions is discussed as an approximation to the behaviour of adhesively-bonded repairs. A three-dimensional finite-element model of a single-lap joint was developed using the commercial code Abaqus. Cohesive Zone Models (CZM) coupled to Finite Element Analysis, were used to study the failure strength of the joint. They allowed the prediction of the initiation of the crack and its growth. CZM are governed by a traction-separation law, which can acquire different shapes. The numerical model, considering a linear cohesive law, was validated with 2D numerical and experimental results available in the literature. The effect of different cohesive law shapes, such as exponential and trapezoidal, on the failure load of the joint was studied. In addition, a cohesive parametric analysis was performed, varying the adhesive toughness and cohesive strength. The most suitable cohesive law was the trapezoidal, since the failure load results were close to the experimental data taken from the literature. The cohesive strength is identified as the most influential parameter on the studied variable.The authors are indebted for the financial support of this work to the Ministry of Economy and Competitiveness of Spain (project DPI2013 42240 R)

Numerical modelling of foam-cored sandwich plates under high-velocity impact

This paper studies the high velocity impact response of sandwich plates, with E glass fibre/polyester face sheets and foam core, using finite element models developed in ABAQUS/explicit code. The failure of the face sheets was predicted by implementing Hou failure criteria and a procedure to degrade mate rial properties in a user subroutine (VUMAT). The foam core was modelled as a crushable foam material. The numerical models were validated with experimental data obtained from scientific literature. The contribution of the foam core on the impact behaviour was evaluated by the analysis of the residual velocity, ballistic limit, and damaged area.The authors are indebted to the Spanish Comisión Interministe rial de Ciencia y Tecnología (Project TRA2007 66555) for the finan cial support of this work.Publicad

Modelling of woven CFRP plates subjected to oblique high-velocity impact and membrane loads

In this work, an analytical model was employed to model the perforation of woven plates made of AS4/8552 (carbon/epoxy) laminates exposed to oblique impact and preload in plane. The model assumed six mechanisms for the absorption of the kinetic energy of projectile: the laminate acceleration, elastic deformation of fibres, fibres failure, shear plugging, delamination and matrix cracking. The model was validated with numerical and experimental data from previous works and, was used to determine the influence of an oblique and perpendicular impacts on laminates with and without the in-plane preload.This work has been supported by the proyect "Modelos de daño en composites en problemas dinámicos" of the Universidad Carlos III de Madrid (2010/00309/003), and by the Madrid Government (Comunidad de Madrid) under the Multiannual Agreement with UC3M in the line of "Fostering Young Doctors Research" (PAMACOM-CM-UC3M), and in the context of the V PRICIT (Regional Programme of Research and Technological Innovation)

Experimental response of agglomerated cork under multi-impact loads

In contrast to other cellular materials, which deform by crushing under impact and develop permanent deformation, agglomerated cork has a viscoelastic response and is environmentally friendly; therefore, it could be a good alternative to be used in engineering applications subjected to more than one impact, and thus further understanding of the energy-absorption capabilities of this material under multi-impact conditions is needed. In this work the multi-impact behaviour of agglomerated cork was studied experimentally by performing several consecutive impacts in a drop-weight tower on specimens of different thickness and at two impact energy levels. The maximum contact force, maximum strain, and the absorbed energy were evaluated in each test. The results show the great capability of agglomerated cork to continue absorbing energy after several consecutive impacts.Publicad

Evaluation of the geometry of single-lap adhesive joints in composite laminates

In this work, the influence of geometry on the mechanical behaviour of single-lap adhesive joints has been analysed in order to evaluate its mechanical behaviour. Four configurations have been studied and compared: 1) single-lap joint, 2) adherends chamfering, 3) adhesive chamfering, and 4) adhesive and adherends chamfering. A 2D finite-element model has been developed using Abaqus/Standard, in which the adhesive behaviour has been defined by a Cohesive Zone Model (CZM). The analysis has been carried out in terms of failure load, peak peel stress, and peel stress distributionThe authors are indebted for the financial support of this work to the Ministry of Economy and Competitiveness of Spain (project DPI2013-42240-R)

The oblique impact response of composite sandwich plates

The following work focus on the experimental study of low-velocity oblique impact on composite sandwich plates. Several impact angles and impact energies are selected to study their influence on the maximum contact force, maximum contact time, absorbed energy, maximum displacement of the impactor, and damaged area. Peak load and energy absorption rise with increasing impact energy and impact angle, while the contact time remains almost constant. No major differences in the results are shown for impact angles lower than 15 degrees. In addition, a numerical model is developed to reproduce the experimental results and study the evolution of the main impact results for impact angles difficult to perform experimentally (up to 50 degrees). A good correlation has been found in terms of peak force and contact time, allowing further analysis of the maximum contact force at higher impact angles. Maximum contact force decreases with increasing impact angles, whereas it increases with impact energy until a certain value in which remains almost constant