On the prediction of bolted single-lap composite joints

A new set of failure criteria to predict composite failure in single lap bolted joints is proposed. The pres ent failure criteria are an extension of Chang Lessard criteria considering a three dimensional stress field and including out of plane failure modes. The advantage with respect to other three dimensional failure criteria is the consideration of non linear shear stress strain relationship. The failure criteria were imple mented in a finite element model and validated through comparison with experiments in literature. Stresses were calculated by a non linear finite element model developed in ABAQUS/Standard which con siders material and geometric nonlinearities. A progressive damage model was implemented in a USDFLD subroutine. The model predicted the effect of secondary bending and tightening torque showing an excel lent agreement with experimental results. Moreover, results were compared with those reported in lit erature using Hashin failure criteria. In addition, a parametric study was carried out to analyse the influence of friction coefficient and tightening torque.The authors are indebted to the Spanish Comisión Interministe rial de Ciencia y Tecnología (TRA2010 19573) for the financial sup port of this work.Publicad

Estudio numérico de uniones atornilladas en estructuras aeronáuticas

El objetivo principal de este proyecto fue estudiar numéricamente las uniones atornilladas en estructuras aeronáuticas utilizando materiales compuestos, en particular, fibra de carbono. Para esto, se plantearon los siguientes objetivos secundarios. Primeramente se realizó el estudio, simulación y análisis de una unión atornillada formadas por placas de fibra de carbono de ocho láminas y aluminio con tornillo de titanio de cabeza recta. Seguidamente se realizó el estudio, simulación y análisis de una unión atornillada formadas por placas de fibra de carbono de dieciséis láminas y aluminio con tornillo de titanio de cabeza recta. Posteriormente se realizará el estudio, simulación y análisis de una unión atornillada formadas por placas de fibra de carbono de dieciséis láminas y aluminio con tornillo de titanio de cabeza avellanada. Finalmente se analizó la influencia del efecto de pandeo o “secondary bending” en el conjunto de la unión. Este proyecto se dividió en siete capítulos, siendo la introducción el primero de ellos. En el segundo capítulo se hizo una breve introducción a los materiales compuestos, especialmente a los laminados, comentando las características de los constituyentes del material que se estudiaron. Seguidamente se trató de abordar las uniones atornilladas en una amplitud significativa expresando sus características y aspectos más relevantes de cara al proyecto. Se expresó también el método de elementos finitos utilizado. En el tercer capítulo se abordó el material científico más directo utilizado para el desarrollo del presente documento. Es importante el hecho de que este material fue el que aportó el desarrollo del modelo ilustrado en el cuarto y quinto capítulo; y los resultados experimentales de validación obtenidos que han apoyado a los resultados del capítulo sexto. En el cuarto capítulo el proyecto se detuvo en el estudio del modelo numérico empleado para la unión con tornillo de cabeza recta al detalle, es decir, se llevó a cabo un estudio de cada paso realizado en el modelo, teniendo especial importancia la malla y su proceso de afinamiento para la posterior simulación. El quinto capítulo abordó el estudio del modelo numérico empleado para la unión con tornillo de cabeza avellanada. Este capítulo se detuvo tan solo en explicar aquellos puntos de la realización del modelo que fueron diferentes a los del capítulo anterior como es la geometría del tornillo y la placa de fibra de carbono, o la malla empleada para este caso en concreto siendo los demás puntos idénticos. El sexto capítulo se dedicó exclusivamente al análisis de los resultados de una unión atornillada simple de una placa de fibra de carbono con una placa de aluminio mediante tornillo de cabeza recta y avellanada de titanio. Se analizó la influencia del afinamiento de la malla, la variación del número de láminas de la placa de fibra de carbono, el problema de convergencia, el “secondary bending” y los resultados extraídos. Se realizó hincapié en la zona próxima al agujero de las placas. En el séptimo capítulo se han expuesto las conclusiones fundamentales y se han propuesto posibles líneas de desarrollo para trabajos futuros. Por último se ha expuesto la bibliografía consultada para la realización del proyecto.Ingeniería Industria

Desarrollo de modelos predictivos de comportamiento de uniones mecánicas de material compuesto

Mención Internacional en el título de doctorLa presente Tesis Doctoral aborda el estudio de las uniones mecánicas a través del desarrollo de modelos teóricos que sean capaces de predecir el comportamiento de las mismas. Para ello se han abordado los dos tipos de uniones mecánicas más comunes en las estructuras aeronáuticas, como son la unión a solape doble y simple. Para el caso de la unión a solape doble mediante pasador, se ha propuesto un modelo analítico que combina dos submodelos. El primero de ellos predice la rigidez a nivel global en la unión, mediante el empleo de un modelo masas-muelles. El segundo de ellos predice la rotura en la unión, analizando las tensiones producidas en cada una de las láminas del laminado en las inmediaciones del agujero a nivel local. La verificación de un fallo en el segundo submodelo afecta a la rigidez del global del primer submodelo. La interacción entre estas dos vías da un modelo completo que predice el comportamiento de la unión a solape doble con un bajo coste computacional. Cuando la unión se ha diseñado con una disposición a solape simple, se ha propuesto en este caso dos vías para su análisis: La primera es mediante un modelo analítico de masas-muelles para la predicción a nivel global del comportamiento de la unión en el rango elástico. Para ello se ha propuesto considerar la exión secundaria dependiente únicamente de los parámetros geométricos y del material; en lugar de parámetros experimentales como se venido haciendo hasta ahora en la literatura científica. Para la segunda vía se ha desarrollado un modelo numérico completo que pudiese predecir el comportamiento de la unión en todas sus fases, en zona elástica y cuando el daño aparece y además se propaga. El modelo numérico consigue predecir la aparición y propagación del fallo, que con el modelo analítico no se podrán abordar debido a la complejidad en el comportamiento de la unión. Para la caracterización del daño en la unión se ha propuesto un nuevo criterio de rotura, que considera el efecto de las tensiones fuera del plano debidas al par de apriete y a la exión secundaria. Los modelos propuestos han sido validados mediante la realización de ensayos experimentales y la comparación con resultados publicados en la literatura.This PhD Thesis discusses a mechanical joints study through the development of predictive models. The two most common types of mechanical joints in aeronautical structures, double-lap and single-lap joints, have been analysed in this work. In the case of double-lap joint by pin, an analytical model that combines two sub-models is proposed. The first predicts global stiffness by the use of a spring-mass model. The second sub-model predicts how the joint fails, analyzing the stresses field in the vicinity of the hole in each layer. Verifying a presence of failure in the second submodel affect the stiffness of the first submodel. The interaction between these two paths gives a complete model that predicts the behaviour of the double-lap joint by pin with low computational cost. When the joint is designed using a simple-lap configuration, two pathways for analysis are proposed: The first is an analytical spring-mass model for predicting the global behaviour in the elastic range. For this, the model considers that the secondary bending effect is dependent on the geometrical parameters and material; instead of experimental parameters, as it is indicated, until now, in scientific literature. For the second way, a completed numerical model that predicts the joint behaviour, has been developed. The numerical model predicts the damage and its propagation, due to the complexity of the damage modes the analytical model was not able to predict the joint failure. A new set of failure criteria was developed to include the out-of-plane stresses produced by secondary bending and torque. The present models have been validated through the comparison with experimental tests and results published in scientific literature.Programa Oficial de Doctorado en Ingeniería Mecánica y de Organización IndustrialPresidente: Ramón Eulalio Zaera Polo.- Secretario: Luis Castejón Herrer.- Vocal: Filipe Teixeira-Dia

An analytical model for predicting the stiffness and strength of pinned-joints composite laminates

An analytical model to predict bearing failure of pinned-joint composite laminates is proposed. The model combines a mass-spring model to reproduce the joint stiffness and a characteristic curve model to predict bearing damage. When bearing failure was verified at any ply, the corresponding spring element was removed from the model. The accuracy of the analytical model was validated through comparison with experimental results. Analytical model predictions agreed with the load–displacement curves and ultrasonic inspections of experimental tests. The present model predicted the different stages in the bearing failure, considering the consecutive failure of the different plies.The authors are indebted for the financial support of this work to the Ministry of Science and Innovation of Spain (Projects TRA2010_19573 and DPI2010-15123)

Use of artificial neural networks to optimize stacking sequence in UHMWPE protections

This article belongs to the Special Issue Polymeric Materials for Energy Absorption Applications.The aim of the present work is to provide a methodology to evaluate the influence of stacking sequence on the ballistic performance of ultra-high molecular weight polyethylene (UHMWPE) protections. The proposed methodology is based on the combination of experimental tests, numerical modelling, and Artificial Neural Networks (ANN). High-velocity impact experimental tests were conducted to validate the numerical model. The validated Finite Element Method (FEM) model was used to provide data to train and to validate the ANN. Finally, the ANN was used to find the best stacking sequence combining layers of three UHMWPE materials with different qualities. The results showed that the three UHMWPE materials can be properly combined to provide a solution with a better ballistic performance than using only the material with highest quality. These results imply that costs can be reduced increasing the ballistic limit of the UHMWPE protections. When the weight ratios of the three materials remain constant, the optimal results occur when the highest-performance material is placed in the back face. Furthermore, ANN simulation showed that the optimal results occur when the weight ratio of the highest-performance material is 79.2%.This research was funded by Comunidad de Madrid of Spain, grant number IND2017/IND7762 and The APC was funded by this project

Development of numerical model for ballistic resistance evaluation of combat helmet and experimental validation

Modern designing process of combat helmets requires both numerical modeling and experimental validation in order to achieve exigent requirements combining impact resistance and reasonable weight. In this work a finite element model of a combat helmet is presented. Mechanical behaviour of the shell aramid composite under impact conditions was analyzed from experimental Fragment Simulating Projectile (FSP) and Full-Metal Jacketed (FMJ) impact tests on aramid flat plates. Numerical modeling based on finite elements method was used to simulate both impacts in simple plates of the composite and also the simulation of ballistic impact involving real ammunition and the complex geometry of the helmet including inner foam. Experimental work involving impact tests on composite plates and also ballistic test on the helmet with a dummy provided real data for comparison with models predictions and proved the accuracy of the numerical models developed.The authors acknowledge the Ministry of Economy and Competitiveness of Spain and FEDER program under the Project RTC-2015-3887-8 for the financial support of the work

Delamination prediction in orthogonal machining of carbon long fiber-reinforced polymer composites

Machining processes of composites are common operations in industry involving elevated risk of damage generation in the workpiece. Long fiber reinforced polymer composites used in high-responsibility applications require safety machining operations guaranteeing workpiece integrity. Modeling techniques would help in the improvement of machining processes definition; however, they are still poorly developed for composites. The aim of this paper is advancing in the prediction of damage mechanisms involved during cutting, including out-of-plane failure causing delamination. Only few works have focused on three-dimensional simulation of cutting; however, this approach is required for accurate reproduction of the complex geometries of tool and workpiece during cutting processes. On the other hand, cohesive interactions have proved its ability to simulate out-of-plane failure of composites under dynamic loads, as impact events. However, this interlaminar interaction has not been used up to date to model out-of-plane failure induced during chip removal. In this paper, both a classical damage model and cohesive interactions are implemented in a three-dimensional model based on finite elements, in order to analyze intralaminar and interlaminar damage generation in the simplified case of orthogonal cutting of carbon LFRP composite. More realistic damage predictions using cohesive interactions were observed. The strong influence of the stacking sequence on interlaminar damage has been demonstrated.Financial support for this work has been provided by the Ministry of Science and Innovation of Spain under the projects DPI2011-25999 and TRA2010-19573.Publicad

Ballistic performance of aramid composite combat helmet for protection against small projectiles

This paper focuses on the ballistic performance of aramid composite combat helmet commonly worn by military and security corps, against small projectiles threat. We propose a numerical finite element model for aramid composite protections, considering a multi-layer architecture, able to predict its ballistic behaviour and damage extension. The aim is determining the minimum number of layers required for a correct protection against a given ballistic thread. The constitutive aramid behaviour has been calibrated by means of experimental tests with FSP (Fragment Simulate Projectiles) projectiles and steel spheres on aramid flat plates. Once calibrated, a predictive numerical model of the helmet against different small projectiles and impacted localisations was developed and compared with experimental tests performed in the real head protection. The results calculated for the absorbed impact energy by the helmet and the induced damage due to small projectiles at different impact location, are in good agreement with experimental results and postmortem helmet analysis, validating the proposed numerical model. The numerical model is thus validated for the design of optimized head protections based on aramid compositeThe authors acknowledge the Ministry of Economy and Competitiveness of Spain and the European Regional Development Fund, (FEDER) program under the Project RTC-2015-3887-8 and the Project DPI2017-88166-R for the financial support of the work

Evaluation of Combat Helmet Behavior under Blunt Impact

This article belongs to the Special Issue Armour and Protection SystemsNew threats are a challenge for the design and manufacture of modern combat helmets. These helmets must satisfy a wide range of impact velocities from ballistic impacts to blunt impacts. In this paper, we analyze European Regulation ECE R22.05 using a standard surrogate head and a human head model to evaluate combat helmet performance. Two critical parameters on traumatic brain analysis are studied for different impact locations, i.e., peak linear acceleration value and head injury criterion (HIC). The results obtained are compared with different injury criteria to determine the severity level of damage induced. Furthermore, based on different impact scenarios, analyses of the influence of impact velocity and the geometry impact surface are performed. The results show that the risks associated with a blunt impact can lead to a mild traumatic brain injury at high impact velocities and some impact locations, despite satisfying the different criteria established by the ECE R22.05 standard. The results reveal that the use of a human head for the estimation of brain injuries differs slightly from the results obtained using a surrogate head. Therefore, the current combat helmet configuration must be improved for blunt impacts. Further standards should take this into account and, consequently, combat helmet manufacturers on their design process.This work has been carried out within the framework of the research project DPI2017-88166-R of the FEDER program financed by the Ministerio de Economía, Industria y Competitividad and the Spanish Ministry of Education, Culture and Sports for the professor's mobility program José Castillejo's 2018 grant (CAS18/00292)

Behaviour of a new combat helmet design against ballistic impact: Experimental and numerical analysis

This work focuses on the finite element analysis (FEA) of a new combat helmet design developed by the Spanish Company FECSA S.A. with the aim of optimizing the final design. The helmet was made of aramid/phenolic resin. Mechanical behavior under impact conditions were analyzed through the experimental Fragment Simulating Projectile (FSP) and Full-Metal Jacketed (FMJ) impact tests on aramid flat plates. From this point, the helmet numerical model, developed using the commercial code ABAQUS/Explicit, was optimized in order to reach the required ballistic limit. Since the aramid composite mechanical properties depends on fiber orientation, a user-defined subroutine VUMAT was developed to simulate the composite behavior under ballistic penetration. For the final validation of the proposed numerical model, experimental impact tests on helmets were performed showing good agreement with the numerical predictions.The authors acknowledge the Ministry of Economy and Competitiveness of Spain under the Project RTC-2015-3887-8 for the financial support of the work