Showing papers by "Federico París published in 2018"

A Device for Biaxial Testing in Uniaxial Machines. Design, Manufacturing and Experimental Results Using Cruciform Specimens of Composite Materials

Abstract: The present study deals with the design and manufacturing of a mechanical device to perform biaxial testing in universal (uniaxial) testing machines. A review of previous definitions of similar devices is carried out and a new device is conceived and developed. The main improvement with the present device is that it allows different types of biaxial loadings (tension-compression) to be performed with few manipulations. The device allows variable displacement ratio to be used in each loading direction, giving then rise to variable loading ratios. Biaxial tension-tension tests on cruciform specimens made of composite material were carried out using very brittle samples in which the fibre direction was perpendicular to the loading plane. Strain gages were used to monitor the percentage bending parameter so that the correct alignment of the loading could be checked. Values below 5% for the bending parameter were achieved at the moment of failure.

BEM multiscale modelling involving micromechanical damage in fibrous composites

Abstract: Composite laminates are materials where different scales are required for the understanding of the damage and for the calculation of structures made of these materials. The appearance of ultra thin plies of composites has opened the possibility of delaying the onset of damage, which at a first stage appears at a micromechanical level, debondings in between fibre and matrix in the weakest lamina of the laminate. A multiscale BEM model is developed in the present paper with the final purpose of being able to study the effect that the relative size of the laminas of the laminate plays in the appearance of this initial damage. The model presents many difficulties derived from the different scales involved in it and from the non-linear nature of the problem under study. The approach followed involves the solution of the whole problem, with the different scales involved in it, at once. The solution obtained is checked with another already obtained with a much simpler model. The multiscale model developed has been proved to be very efficient, accurate and robust, having been applied to simulate the first stages of damage in the light of the scale effect that is trying to be studied.

Microscopical observations of interface cracks from inter-fibre failure under compression in composite laminates

Abstract: Matrix/inter-fibre failure is characterized by the appearance at the fibre–matrix interfaces of small debonds that can progress along them until reaching a certain extension, then changing their orientation to kink towards the matrix and, finally, growing through it. The particular case of compressive loading is specially interesting, given the morphology of the interface cracks and the specific angle that the macro-cracks form in the matrix. To date, the analysis of this problem at micro-mechanical level has been carried out mainly by means of Finite Element or Boundary Element models. In this work, the problem is approached from the experimental point of view, observing under optical microscope those coupons previously tested at different loading levels. Several aspects such as the identification of the stages of the failure mechanism, the kinking angle, the extension of the interface cracks and the presence of damage as a function of the loading level are studied.

Experimental mechanical characterization of composite-concrete joints

Abstract: The use of composite materials as a reinforcement for concrete in civil construction has become a significant topic, the quality of the joint between composite and concrete being one of the key parameters to study. The preparation of surfaces involved in the joint and the fabrication procedure are relevant factors in this quality. Two surface treatments of concrete (grinder and grit sandpaper) and three manufacturing techniques of the composite and corresponding joining to the concrete (precured composite, wet hand lay-up and infusion) have been employed. To evaluate the quality of the joint, four tests have been applied: Lap peeling, Lap shear, Pull off and Shear torsion. Once the tests were carried out, all of them were found representative of the quality of the joint and it was observed that concrete cracking predominates over adhesive or cohesive failures, which indicates a satisfactory adhesion between the concrete and the composite. Grinder surface treatment was found the best option, and with reference to the manufacturing technique, the use of pre-fabricated reinforcement was found to lead to undervalues of the properties.

2.16 Micromechanics of Interfacial Damage in Composites

Abstract: One of the reasons for the application of composites to mobile structures is their capacity to absorb energy under impact As their components have a very reduced interval of deformation till breakage, the mechanism of absorption of energy cannot be based on plasticity but on the capacity of generating free surfaces inside the material, the debonding between fibers and matrix being the most direct materialization of the mechanism of absortion of energy in composites In this chapter the onset and propagation of damage in between fiber and matrix in a fibrous composite, particularly under transverse loading is studied and characterized The tools to apply are interfacial fracture mechanics (IFM), linear elastic brittle interface model, finite fracture mechanics (FFM), and cohesive zone model (CZM), combined with a numerical tool, in this case the boundary element method (BEM) Different loading situations are analyzed, single tension and compression, bi-dimensional loading as well as curing effects Finally single fiber, two fibers, and multi-fiber configurations are considered in this chapter, the key being the evolution of the energy released by the debonding crack representing damage in between fiber and matrix Numerical predictions are compared with experimental evidence to support the adequacy of the approaches followed

Ensayos numéricos de uniones adhesivas composite-composite para la determinación de la tenacidad a la fractura

Abstract: Nowadays, the use of composite materials in primary structures has considerably increased, in particular in what refers to primary structures and bonded joints. Thus, numerical tools and experimental tests have to be implemented in order to understand failure modes and critical loads in this kind of structures. Usually, bonded joints quality is tested based on the interlaminar fracture test DCB (G1c). Nevertheless, these tests have some limitations. The aim of the present study is to create a feasible and accurate numerical model which allows the quality of a bonded joint as function of the peeling load to be determined. The idea is then an actual alternative to offer in-situ test by means of a transportable and easy to use device. In this study, a research analyzing two tests able to identify the quality of a bonded joints, as an alternative to the interlaminar fracture test, based on a classic peeling test on carbon laminates, is presented. The numerical simulation is based on a Finite Element Analysis using the commercial software Abaqus®. Onset and propagation of adhesive damage is taken into account by means of a cohesive zone model. A virtual test of the peeling test is done and its behavior is compared with the experimental results from an extensive laboratory test campaign. Numerical results will allow us to verify the feasibility to compute the value of G1c from experimental results.

A study of the reproducibility and reliability of the musculo-articular stiffness of the ankle joint.

Abstract: The objective of this work was to evaluate the reproducibility, reliability and usefulness of the musculo-articular stiffness (MAS) of the ankle joint, measuring it by the free vibration technique. Seventeen (nine males and eight females) healthy university students were included in the study. Force (f), MAS (k) and unitary MAS (ku) (defined as the ratio between the value of stiffness k obtained in the test (absolute terms) and the value of force (f)) were obtained. A test-retest protocol was designed and performed on the same day to determine the short-term reproducibility of f, k and ku. Short-term reproducibility of k and ku on 1 day in absolute terms (

Numerical Study of the Progression of the Micromechanical Debonding Damage in Composites

Abstract: This paper deals with the study of the actual progression of the damage in the 90 degrees lamina of a composite. It has been proved and observed that isolated debondings between fibres and matrix are the first manifestation of damage in the weakest lamina, the 90 degrees lamina in a [0,90]S laminate. It was also numerically supported that this first phase was independent of the thickness of the 90 degrees lamina, not being then affected by the “scale effect”. The continuation of this first phase of damage is the objective of the present paper. To this end, a multiscale model is created involving the debonding between fibre and matrix and studying the kink of this crack, abandoning the fibre-matrix interface and entering into the matrix to produce a meso-transverse crack in the 90 degrees ply. The study is based on the application of Fracture Mechanics to an incipient kinked crack that starts from a debonding between fibre and matrix. It is concluded that this second phase of damage, playing with the thickness of the 90 degrees lamina, is not affected by the scale effect, as the variation of the energy release rate of the kinked crack is not significantly influenced by the variation of the thickness of the lamina.

Modelado por Elementos Finitos de las deformaciones térmicas en componentes de Material Compuesto

Abstract: A comparative study of four different finite element (FE) models is presented, to determine the thermal deformations in a typical aeronautical structure. A detailed FE model with solid elements is used as a reference to determine the accuracy given by three FE model with shell elements. The component consists in four planar laminates with (relatively small) curved zones at the joints. The analysis shows that a correct modelling of the curved laminates is needed for obtaining accurate results. If the FE model with shell elements ignores the existence of the curved zones, as is typically done in models used for stress calculations under pressures and loads, unsatisfactory results are obtained. Precision is not significantly increased if the curved zones are modelled in detail and the properties of the shell elements are defined as in the planar zones. For this reason, a technique for modifying the in-plane thermal properties of the elements modelling the curved laminates, that forces them to follow the same shape changes observed in the real curved laminates is presented. The FE model with shell elements obtained with this technique provides the same accuracy than the FE model with solid elements, with a significantly lower computational cost.

Finite element modeling of thermal deformations in composite materials

Abstract: A comparative study of four different finite elements (FE) models to determine the thermal deformations in a typical aeronautical structure is presented. A detailed FE model with solid elements is used as a reference to determine the accuracy given by three FE model with shell elements. The component consists of three planar laminates with (relatively small) curved zones at the joints. The analysis shows that a correct modelling of the curved laminates is needed for obtaining accurate results. If the FE model with shell elements ignores the existence of the curved zones, as it is typically done in models used for stress calculations, unsatisfactory results are obtained. Precision is not significantly increased if the curved zones are modelled in detail and the properties of the shell elements in the curved zones are defined as it is done in the planar zones. For this reason, a technique for modifying the in-plane thermal properties of the elements modelling the curved laminates that forces them to follow the same shape changes observed in the real curved laminates is presented. The FE model with shell elements obtained with these curvature-modified coefficients of thermal expansion provides similar accuracy to that obtained with the FE model with solid elements, with a significantly lower computational cost.

Estudio Micromecánico del Efecto Escala en laminados de Material Compuesto

Abstract: From the beginning of the application of composite materials, different theories to predict their behaviour and failure have been developed. The authors have already made a previous study, analyzing at the macroscopic level (both from an experimental and theoretical point of view), the failure of non-conventional laminates changing the thickness of the laminas that form it, in order to evaluate the “scale effect” (also known as “in-situ strength”) present in them. Results of this study proved the strength variation of the laminate with its configuration (scale effect) and it was found that the presence of laminas with fibres oriented in the direction of the load diminishes this effect. This paper is focused on the understanding of the aforementioned effect, conducting a study at micromechanical level (the level where the damages leading to the failure are generated) of the scale effect with an energetic approach, in order to progress in the understanding of this phenomenom. To this end, the Boundary Element Method (BEM) has been used. It has been applied to a laminate model with [0, 90n]S stacking sequence, where the influence of n at different stages of the generation and progression of damage has been studied: the beginning of the damage (that takes place when the debonding between fibre and matrix happens) and during the change of direction of the debonding crack towards the matrix (kinking). The models developed are multiscale, involving a mesoscale representation of the laminas of the laminates (modelled as homogeneous) and with micromechanics cells that simulate fibre and matrix and therefore with the capacity for collecting information about damage mechanisms observed experimentally.

Eliminación de singularidades en ensayos estándar de materiales compuestos

Abstract: In many standard tests involving composite material, specially those needing tabs at the ends of the sample, configurations of multimaterial corners, with abrupt changes in geometry and material properties, typically appear giving rise to stress fields which achieve very high stresses and theoretically are unbounded These stress intensifications may promote premature failures, or simply, alterations of the uniform stress state which is expected to occur in the specimen, and the test strength, calculated as a certain load divided by a certain failure area, may not be representative of the real strength of the material under analysis In the present work three examples are presented, in which, by means of an addecuate selection of the geometrical parameters defining the test configuration which do not affect the mechanical property to be measured, the stress singularities at these multimaterial corners are removed (or at least reduced to a minimum) The three cases are: a) tests in tension and shear (Iosipescu) in bimaterial coupons, b) the off-axis test for the intralaminar shear strength in composite materials and c) a compression test of thick laminates in composite materials In two of the three cases experimental results are available to corroborate that the removal of the stress singularities gives rise to higher strength values than those obtained in the original test configurations