Universiẗat Diss., 2002--Essen.

Thesis (M.S.)--West Virginia University, 2007. / Title from document title page. Document formatted into pages; contains xviii, 141 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 140-141).

This thesis presents the results of research on sandwich panel construction, with the aims of developing tools for modelling sandwich panel fire performance and hence to use the tools to aid the development of sandwich panel construction with improved fire resistance. The research focuses on sandwich panels made of thin steel sheeting and a polyisocyanurate (PIR) foam core. For non-loadbearing sandwich panel construction, fire resistance is measured in terms of thermal insulation and integrity only. However, these two parameters are affected by mechanical performance of sandwich panel construction due to the high distortion and large deformation nature of sandwich panel construction under fire attack. Therefore, it is necessary to consider both thermal and mechanical performances of sandwich panels under fire conditions. The work in this thesis includes development of a thermal conductivity model for PIR foam as this thermal property is one of the key values in determining heat transfer through sandwich panels; this thermal conductivity model is based on the effective thermal conductivity of porous foams proposed by Glicksman (1994) and includes the effects of polymer decomposition and increases in foam cell size. It is validated against fire tests carried out on PIR sandwich panels 80mm and 100mm thick with steel facings of thickness 0.5mm. A large 3D sequentially coupled thermal-stress model of a full scale fire test has been developed in the commercial finite element analysis (FEA) software ABAQUS to provide insight into the way sandwich panels behave in a fire resistance test and also to assess different modelling techniques. Aspects and stages of the simulation that agree well with test data are explained. Limitations of the ABAQUS software for simulating sandwich panel fire tests are highlighted; namely, it is not possible to simulate the correct radiation heat transfer through panel joints, as cavity radiation cannot be specified in a fully coupled thermal-stress analysis. Joints are key components of sandwich panel construction. In order to obtain temperature development data for modelling joints, a number of fire tests have been carried out. These fire tests were conducted with different joint configurations and panel thicknesses under realistic fire conditions using timber cribs. The joint fire tests revealed significant ablation of the foam core within the joints of sandwich panels at high temperatures. At the beginning of fire exposure, the joint temperature on the unexposed surface was lower than that on the panel due to the better insulation property of air compared to the foam. However, as the joint gap increased due to ablation of the foam, the joint temperatures became higher than in the panel. A numerical simulation model has been created to investigate this behaviour. Using the aforementioned thermal model, numerical simulations have been carried out to examine the influences of possible changes to sandwich panel design on sandwich panel construction fire performance. It was suggested that if the maximum gap in the joints can be limited to 5mm, for example, by applying intumescent coating strips within the sandwich panel joints to counter the increasing gap formed due to core ablation, then the joint temperature on the unexposed surface would not exceed that of the panel surface, hence the joint would cease to be the weak link. To increase the panel fire resistance, the use of graphite particles in the PIR foam formulation may be considered to lower the contribution of radiative heat transfer within the foam cells by reducing the transmissivity of the cell walls. Graphite particles may offer considerable increases in the thermal resistance of PIR foam at high temperatures by limiting the radiation contribution which dominates heat transfer above 300oC.

Doctor of Philosophy / Department of Statistics / Wei-Wen Hsu / Evaluating heterogeneity in the class of zero-inflated models has attracted considerable attention in the literature, where the heterogeneity refers to the instances of zero counts generated from two different sources. The mixture probability or the so-called mixing weight in the zero-inflated model is used to measure the extent of such heterogeneity in the population. Typically, the homogeneity tests are employed to examine the mixing weight at zero. Various testing procedures for homogeneity in zero-inflated models, such as score test and Wald test, have been well discussed and established in the literature. However, it is well known that these classical tests require the correct model specification in order to provide valid statistical inferences. In practice, the testing procedure could be performed under model misspecification, which could result in biased and invalid inferences. There are two common misspecifications in zero-inflated models, which are the incorrect specification of the baseline distribution and the misspecified mean function of the baseline distribution. As an empirical evidence, intensive simulation studies revealed that the empirical sizes of the homogeneity tests for zero-inflated models might behave extremely liberal and unstable under these misspecifications for both cross-sectional and correlated count data. We propose a robust score statistic to evaluate heterogeneity in cross-sectional zero-inflated data. Technically, the test is developed based on the Poisson-Gamma mixture model which provides a more general framework to incorporate various baseline distributions without specifying their associated mean function. The testing procedure is further extended to correlated count data. We develop a robust Wald test statistic for correlated count data with the use of working independence model assumption coupled with a sandwich estimator to adjust for any misspecification of the covariance structure in the data. The empirical performances of the proposed robust score test and Wald test are evaluated in simulation studies. It is worth to mention that the proposed Wald test can be implemented easily with minimal programming efforts in a routine statistical software such as SAS. Dental caries data from the Detroit Dental Health Project (DDHP) and Girl Scout data from Scouting Nutrition and Activity Program (SNAP) are used to illustrate the proposed methodologies.

This thesis explores the use of lightweight composite materials in road freight trailer design as a means of reducing the emissions of the road freight industry. A comprehensive review of previous lightweight composite trailers and related projects was conducted; it concluded that the application of composites in trailers to-date has largely been limited by relatively high material and production costs. The review highlighted that the trailer industry could learn from the success of composites in the bridge construction industry. A statistical weight analysis of two road freight fleets and an energy consumption estimation, via a drive cycle analysis, were used to identify trailers that are particularly suited to lightweighting. Hardwood trailer decking was identified as a prime subcomponent for composite replacement. However, there is little literature on how conventional hardwood trailer decks react to in-service loadings. This problem was addressed through a comprehensive deck damage study, which was used to benchmark novel lightweight deck systems. Several lightweight replacement composite sandwich panels were designed, built and tested. Two different pultruded GFRP decks were also examined. While pultrusions do not offer the same level of weight savings as sandwich panels, the highly cost-driven nature of the trailer industry could dictate that their integration is the most reasonable first step to introducing composites into structural subcomponents. The final part of the thesis explores options for lightweighting the trailer chassis holistically. Trailer load cases were investigated through finite element modelling in Abaqus. A parametric model of a typical longitudinal trailer I-beam was developed using Python scripting and Abaqus. The model was expanded to analyse composite trailer structures. It showed that approximately 1,300 kg of weight could be saved by shape and material optimisation in a composite trailer. In summary, this research has shown that short-term trailer weight reductions can be effectively achieved through subcomponent replacement, while more significant reductions can be achieved in the long-term by a ‘clean slate’ composite redesign of the trailer chassis. The lightweighting strategies presented here are poised to have an increasingly important role in reducing the emissions of the road freight industry.

Les structures sandwichs dissymétriques sont un cas particulier de la famille des structures sandwichs. Elles présentent une peau dite travaillante qui est chargée en membrane. Celle-ci est stabilisée par une âme en nid d'abeille Nomex et une contre-peau dite peau stabilisatrice. Cette configuration génère un comportement non linéaire géométrique. Plusieurs thèories analytiques des poutres et plaques sandwichs dissymétriques ont été élaborées basées sur le minimum de l'énergie potentielle ou le principe des puissances virtuelles associées à une méthode de discrétisation de Ritz. Ces théories intégrent de plus la modélisation a priori ou a posteriori de la compression de l'âme. Parallélement un montage d'essai sous sollicitations combinées compression/cisaillement a été développé et des essais multiaxiaux ont été réalisés sur éprouvettes neuves et impactées. Une comparaison théorie/expérience est aussi réalisée.

Magíster en Ciencias de la Ingeniería, Mención Mecánica / El objetivo de esta investigación es desarrollar un algoritmo capaz de detectar, cuantificar y localizar daño en tiempo real en estructuras de material compuesto tipo panal de abeja, buscando delaminaciones entre las paredes y el núcleo. Para llevar a cabo este objetivo es necesario recopilar datos experimentales de frecuencias naturales y modos de vibración de placas con y sin delaminación, los que servirán para validar un modelo numérico de la estructura. El modelo numérico se construye en MATLAB. La estructura compuesta se modela con tres paneles unidos por resortes que representan a la capa de adhesivo. La delaminación se introduce como una disminución de rigidez local en los resortes. Las características mecánicas del modelo numérico son obtenidas mediante un método de ajuste de modelos que utiliza los datos experimentales de las placas con y sin delaminación. Con este modelo, se generó una base de datos de placas con distintos escenarios de delaminación que sirvieron para los métodos de identificación de daño evaluados. Se proponen dos métodos de aprendizaje supervisado para la identificación de daño, las redes neuronales artificiales y un método de interpolación lineal basado en el principio de máxima entropía. Ambos métodos de identificación de daño fueron evaluados por separado para así determinar cuál es el que identifica de mejor manera la existencia, magnitud y ubicación del daño. Para evaluar los métodos de identificación de daño, se realizó un estudio para determinar cuál es el o los parámetros que mejor determinan la delaminación. Los resultados indican que es necesario combinar la información de las frecuencias naturales y de los modos de vibración de la estructura. Para evaluar el método basado en redes neuronales, primero se estudió cual era la mejor configuración en cuanto a funciones de transferencia y porcentaje de utilización de datos para entrenamiento y validación de la red entre 12 posibles opciones. Luego de encontrada la configuración óptima se estudió cuál es el número de neuronas de la capa oculta que entrega mejores resultados. Se encontró que la mejor configuración correspondía a: 80 neuronas en la capa oculta, una proporción de casos de la base de datos para entrenamiento y validación de 90/10, función de transferencia entre el input y la capa oculta de Logsig y función de transferencia entre la capa oculta y output de Satlins. Se determinó que el entrenamiento de redes es un proceso que requiere de altos recursos computacionales y sin embargo no entrega resultados confiables. Por otra parte se trabajó con un algoritmo de aproximación lineal basado en el principio de máxima entropía, que utiliza la misma base de datos que las redes neuronales pero no requiere entrenamiento y que entrega resultados con la misma velocidad que las redes. Se obtuvo que este algoritmo permite encontrar con bastante exactitud el sector de la placa que se encuentra delaminado, con hasta un 10,5% de error en la ubicación y un 22,9% de error en el tamaño de la delaminación. Además, contrario a los resultados obtenido con las redes neuronales, el método de aproximación lineal no detecta delaminaciones en lugares que no corresponden (falsos positivos). Se decide entonces optar por el algoritmo de aproximación lineal basado en el principio de máxima entropía para la detección de daño ya que utiliza menos recursos, entrega información más acertada que las redes neuronales artificiales, y se detecta el daño con bastante precisión.

Honeycomb-cored sandwich structures are widely used in transport for their high strength-to-mass ratio. Their inherent high stiffness and lightweight properties make them prone to high vibration cycles which can incur deleterious damage to transport vehicles. This PhD thesis investigates the performance of a novel passive damping treatment for honeycomb-cored sandwich structures, namely the Double Shear Lap-Joint (DSLJ) damper. It consists of a passive damping construct which constrains a viscoelastic polymer in shear, thus dissipating vibrational energy. A finite element model of such DSLJ damper inserted in the void of a hexagonal honeycomb cell is proposed and compared against a simplified analytical model. The damping efficiency of the DSLJ damper in sandwich beams and plates is benchmarked against that of the Constrained Layer Damper (CLD), a commonly used passive damping treatment. The DSLJ damper is capable of achieving a higher damping for a smaller additional mass in the host structure compared to the optimised CLD solutions found in the literature. The location and orientation of DSLJ inserts in honeycomb sandwich plates are then optimised with the objective of damping the first two modes using a simple parametric approach. This method is simple and quick but is not robust enough to account for mode veering occurring during the optimisation process. A more complex and computationally demanding evolutionary algorithm is subsequently adopted to identify optimal configurations of DSLJ in honeycomb sandwich plates. Some alterations to the original algorithm are successfully implemented for this optimisation problem in an effort to increase the convergence rate of the optimisation process. The optimised designs identified are manufactured and the modal tests carried out show an acceptable correlation in the trends identified by the numerical simulations, both in terms of damping per added mass and natural frequencies.

Work-family conflict (WFC) is a stressor that can cause negative consequences, including deleterious effects on an individual's health and well-being. This study examined this relationship through the lens of the Job Demands-Resources (JD-R) Model. This study expanded on two recent applicable studies to examine role quality as a moderator of the WFC-well-being relationship. Well-being was operationalized by three constructs: overall health, depression, and life satisfaction. Hierarchical regression analyses were used to test the hypotheses that a negative relationship existed between WFC and the three well-being outcomes over time, and that this negative relationship between WFC and well-being could be reduced when role quality is high, compared to when role quality is low. These hypotheses were tested using a sample of 234 working, married individuals caring for both children and parents. The use of this sample allowed examination of four types of role quality as potential moderators: job, spousal, child-care and parent-care. Results showed that WFC had a significant and positive relationship with depression over time. Two of the proposed moderators, spousal role quality and child-care role quality, significantly buffered the effects of work-to-family conflict on life satisfaction. Theoretical and practical implications of the findings are discussed.

SAFRAN Aircelle produit des panneaux sandwiches constitués de peaux composites collées sur une âme nid d’abeille (« Nida ») en aluminium pour des applications de nacelle de moteur d’avion. Des décollements locaux peuvent exister à l’interface peau/Nida et réduire significativement la capacité structurale de ces sandwiches. L’objectif de cette thèse entre l’ISMANS et SAFRAN Aircelle réside dans la mise en place d’une approche de type « Virtual testing » pour réduire les coûts de validation et de vérification des sandwiches possédant des décollements.L’originalité de ces travaux vient du choix de la représentation des sandwiches à travers deux familles de modèles : les modèles « âme pleine » (modélisation volumique) prévus pour une utilisation en bureau d’études et les modèles « âme creuse » (modélisation physique des clinquants) permettant d’analyser finement les mécanismes de propagation des décollements peau/Nida. Une technique de sous-structuration a été mise en oeuvre avec succès et a permis de réduire significativement les temps de calcul du modèle « âme creuse ».La mécanique de l’endommagement, avec des éléments à zone cohésive, a été choisie pour déterminer numériquement le seuil de propagation des décollements, après une étude comparative avec la mécanique linéaire élastique de la rupture et une méthode de recalage essais. Pour alimenter les lois d’endommagement disponibles dans les codes industriels, une méthode expérimentale a été proposée. Des essais de type DCB sandwiches ont permis de remonter aux propriétés mécaniques de l’interface en mode d’ouverture et en mode mixte, avec un seul montage. Ces essais ont été recalés avec succès, notamment en mode I pur. / SAFRAN Aircelle manufactures sandwich structures made of composite skins bonded to aluminium honeycomb core for aircraft’s engines nacelles applications. Local disbonds may occur at the skins/core interface and lead to significant strength reduction under in-service loadings. The present work was done with ISMANS and SAFRAN Aircelle and deals with the introduction of a “Virtual Testing” approach in order to reduce substantiation and validation cost of sandwiches structures with embedded disbond.In this study we have two kinds of models at two scales to describe the sandwich behaviour: the “homogeneized” model, which provide an industrial tool for design offices and the “detailed” model, which provide a specific tool for accurate analysis of disbond growth initiation at skin/core interface. Due to the numerical cost of the second type of model, the superelement technique has been successfully used which permits to gain computational costs without altering the result quality.Damage mechanics, with cohesive zone elements, have been chosen to numerically determine the disbond growth threshold after a comparative study with linear fracture mechanics and a full experimental method. Inputs data for industrial code using cohesive zone elements have been studied through an experimental investigation. DCB type tests on reinforced sandwiches were perfomed in order to determine the energy release rate at the skin/core interface. Various mode-mixity and pure opening mode are available using the same testing tool. The latter has been successfully modelled.