Effects of structural parameters on the buckling and post-buckling behavior of auxetic cellular plates and cylinders

LAUREA MAGISTRALEL'oggetto di questa tesi sono i materiali cellulari, ovvero materiali dotati di microstruttura la quale può essere efficacemente rappresentata da travi piane e gusci. Le strutture celllulari sono dotate di proprietà meccaniche e fisiche che possono essere di maggior vantaggio rispetto ad un solido costituito da una distribuzione continua di materiale, specialmente per quanto concerne i rapporti tra resistenza e peso o proprietà superficiali. Negli ultimi decenni, grazie al continuo sviluppo di metodi di analisi e progettazione, oltre alla grande rivoluzione della stampa 3D, lo studio di questi materiali e delle loro proprietà ha subito un'impennata in molti campi ingegneristici, compreso l'ingegneria biomedica. Lo studio di questi materiali si orienta verso lo sviluppo di dispositivi ad elevate prestazioni meccaniche con peso ridotto, elevate capacità di assorbimento di energia sonora, conduzione del calore e altre proprietà fisiche. Una classe particolare di materiali cellulari è costituita da solidi dotati di microstruttura che mostrano a livello macroscopico un modulo di Poisson apparente minore di zero, circostanza non comune tra i materiali continui. Questi tipi di materiale si espandono lateralmente nella direzione perpendicolare alla direzione di sollecitazione di trazione e viceversa subiscono una contrazione laterale se sono sottoposti ad una sollecitazione di compressione. Un aspetto critico nell'applicazione di sollecitazione su questi materiali è costituito dal fenomento di instabilità a compressione (buckling) che porta a collasso prematuro della struttura. La forza critica che porta al collasso di una struttura cellulare è molto inferiore alla forza critica che porta al collasso per instabilità di una struttura costituita dallo stesso materiale ma continuo (privo di microstruttura). Di conseguenza il dimensionamento di strutture con microstruttura è un aspetto di fondamentale importanza per la corretta funzionalità di un dispositivo. In questo lavoro è stata studiata la risposta al buckling e la fase post-buckling per due diverse geometrie macroscopiche (lastra piana e cilindro) dotate di microstruttura. Sono stati realizzati 84 modelli di lastra piana e 45 modelli cilindrici con una cella unitaria a nido d'ape con pareti rientranti come proposto da Gibson a partire dalla definizione di alcuni parametri geometrici. Il comportamento meccanico di ciascuna microstruttura sottoposta a sollecitazione di compressione è stato simulato mediante un modello ad elementi finti. I risultato ottenuti dalla simulazione numerica sono poi stati messi a confronto con un risultato sperimentale ottenuto sottoponendo a compressione campioni rappresentanti alcune delle geometrie selezionate tra le geometrie analizzate ed ottenute per mezzo della stampa 3D. La risposta meccanica numerica sia in termini di valore del carico critico, sia in termini di evoluzione post-critica segue in maniera molto soddisfacente i risultati sperimentali. I risultati ottenuti su lastre piane cellulari hanno mostrato come il valore di forza critica normalizzata rispetto al peso in funzione del modulo di Poisson macroscopico raggiunge un valore massimo proprio nella zona di auxeticità (ovvero nella zona a modulo di Poisson negativo o vicino allo zero); al contrario, le strutture cilindriche cellulari mostrano una forza critica crescente con il valore del modulo di Poisson macroscopico. Per quanto riguarda il comportamento post-critico, si è osservato che quando il modulo di Poisson è vicino allo zero, esso è poco dipendente dallo spessore della lastra piana. Le strutture cilindriche cellulari hanno mostrato un comportamento sensibile allo spessore specialmente nel range di valori elevati di modulo di Poisson. In conclusione, le strutture cilindriche hanno mostrato che, sebbene ad elevati valori di modulo di Poisson, il valore del carico critico sia elevato, esso mostra un comportamento post-critico sfavorevole. Questo risultato è in contrasto con quanto ottenuto nelle strutture di lastre piane. I risultati ottenuti hanno inoltre mostrato che l'uso di una microstruttura che conferisce un modulo di Poisso vicino allo zero è vantaggioso rispetto ad una struttura continua che mostri il medesimo modulo di Poisson macroscopico, questo fenomeno è osservato maggiormente nel range di piccoli spessori. In generale, tutti i parametri microstrutturali hanno mostrato un ruolo determinante nella determinazione del carico critico e nel comportamento post-critico delle strutture di lastre piane e cilindriche. Infatti, le pendenze delle curve forza spostamento in regime post-elastico sono fortemente influenzate dai parametri microstrutturali i quali possono quindi essere appositamente modificati al fine di conferire alle strutture i comportamenti desiderati.Cellular materials are those materials which is made by interconnection of solid beam, truss plates that form the edges of the unit cells. Cellular structures possess superior properties over their solid counterpart such as strength-to-weight ratio and surface area properties. Apart from the mentioned advantages, these structures due to their high performance, high strength-to-weight ratio, low heat conduction properties, superior energy absorption and excellent thermal and acoustic insulation have attracted much attention in the engineering industries especially medical field. Cellular auxetic material are a type of cellular materials which provide unique properties due to their negative Poisson’s ratio. These sort of material expand laterally in the direction perpendicular to the stretching loading direction. On the other hand if they are compressed longitudinally they contract in transverse directions. One of the biggest weaknesses of all cellular structures including conventional and auxetic one is their buckling instability when they are under the action of compressive stress, which leads to premature collapse in cellular structures. The critical compressive stress required for the collapse of the cellular structure is far less than the critical compressive stress of the material that makes up the cellular structure. Structural parameters of cellular structures play an important role in the performance of these structures. They can help improve the static and dynamic properties of the structure or, conversely, their improper design can jeopardize the stability of the structure. As a result, determining the optimal cellular structure parameters is essential for a particular application. In order to investigate more precisely the effect of structural parameters on the buckling and post-buckling behavior of cellular structures, two different geometry of plate and cylinder have been studied. 84 plate models and 45 cylindrical models have been designed by exploiting the re-entrant honeycomb model proposed by Gibson. While defining the structural parameters of the cellular structure, we will describe the models created by the variation of these parameters. All models were analyzed by finite element method and the accuracy of the results obtained from the numerical method has been investigated by fabricating reference models (three reference models for plate geometry and two reference models for cylindrical geometry). All five reference models are fabricated by additive manufacturing technology and buckling mechanical test have been conducted on them. The experimental results for all reference models follow the simulation results so well, and there is a perfect agreement between the two results. The results show that in the cellular plate structure, the stiffness-weight-normalized critical stress in cellular plates with different Poisson's ratio gains its maximum value generally in the auxetic region and close to Poisson's ratio equals to zero. On the other side, in a cylindrical structure, the normalized critical stress increases with increasing Poisson's ratio. It can also be interpreted that for sufficient slender cellular plates the post-buckling behavior of the cellular plate with a Poisson's ratio close to zero has the least dependence on the out of plane thickness of the plate and its dependence on the out of plane thickness increases by moving away from zero Poisson’s ratio. On the other side, the results of the data of the cylindrical structure show that the post-buckling behavior of this structure for the maximum Poisson's ratio is the most affected by the change in membrane thickness. The structure also has a larger negative slope for the maximum Poisson's ratio. As a result, although the cellular cylinder with maximum Poisson's ratio has a higher buckling resistance, it is more unstable in the post-buckling regime. Moreover it seems that for plate models when the ratio of the in-plane thickness of the struts to the scaling parameter increases, the auxeticity has practically less effect on the post-buckling behavior of the structure. The results have also reveals the superiority of using a cellular plate with zero Poisson’s ratio over a bulk plate with the same Poisson’s ratio especially in smaller thicknesses. In general, all structural parameters including the Poisson's ratio (for both plate and cylindrical models), the out-of-plane thickness (for plate models), the membrane thickness (for cylindrical models), the in-plan thickness of the struts (for plate models), the width (for plate models), the scaling parameter of the unit cells (for both plate and cylinder models) and the ratio a/b have undeniable effects on the critical buckling load and post-buckling behavior of structures. In fact, by changing the geometric parameters, a wide range of post-buckling slopes of the structure can be achieved in both plate and cylindrical geometry and the post-buckling behavior of the structure can be tuned by designing geometric parameters

Motivations of undertaking CSR initiatives by independent hotels: a holistic approach

Purpose - The aim of this study is to explore how senior managers of independent hotels perceive the notion of CSR through a holistic analysis of motivations of undertaking both social and environmental initiatives.Design/methodology/approach – This study conducts qualitative interviews with 22 Malaysian senior hotel managers together with ethnographic observations.Findings - Emergent themes show that CSR is a dynamic concept involving complex struggles and trade-offs between fulfilling business objectives, paying heed to personal ethical values and considering cultural norms when making decisions regarding the adoption of a range of environmental and social initiatives.Research limitations/implications - Our study contributes to legitimacy theory by highlighting that, in the absence of pressure from key stakeholders for responsible initiatives, managers still proactively engage in CSR initiatives. More surprising still is that they set expectations instead of simply aligning or responding to the key stakeholders’ pre-set demands. Meanwhile, the uniqueness, size and purposeful selection of the sample, limit the applicability of the findings to wider geographical and cultural locations.Practical implications – While Malaysian hotel managers are willing to employ CSR practices to demonstrate their commitment to their employees as well as the local communities where they operate, it is equally important that they utilize other media (such as their websites or social media) for both promotional and legitimacy building purposes. Within the context of growing tourism for economic development, the Malaysian government can develop more effective strategies, rewards or incentives for encouraging businesses to undertake and improve CSR adoption for sustainability of this industry.Originality/value - By extending the scope of CSR studies beyond firm performance issues, we attempt to show the wider set of motivations and contexts considered important in determining hotel engagement in CSR programs

Strategies to improve energy and carbon efficiency of luxury hotels in Iran

Luxury hotels generate substantial carbon footprint and scholarly research is urgently required to better understand how it could be effectively mitigated. This study adopts a method of life cycle energy analysis (LCEA) to assess the energy and carbon performance of six luxury, five star, hotels located in Iran. The results of the energy and carbon assessment of luxury hotels in Iran are compared against the energy and carbon values reported in past hotel research. This current study finds that luxury hotels in Iran are up to 3–4 times more energy- and 7 times more carbon-intense than similar hotels examined in past research. Low cost of fossil fuels, international trade sanctions and the lack of governmental and corporate energy conservation targets discourage Iranian hoteliers from carbon footprint mitigation. To counteract poor energy and carbon efficiency of luxury hotels in Iran, it is important to relax economic sanctions, develop alternative energy sources, refine corporate energy conservation targets, regularly benchmark hotel energy performance and enable exchange of good practices amongst Iranian hoteliers

Measuring Perceived Service Quality In The Hotel Industry: A Study On Luxury Hotels In Malaysia

The present research intends to study the expectations and perceptions of luxury hotels guests who come from either advanced or emerging/developing economies to Malaysia. The research found that the hotel customers\' perceptions of the hotel industry\'s service quality were higher than their expectations. Moreover, the difference between male and female guests from advanced and emerging/developing economies was significant in terms of their preferences of service quality dimensions

An Integrated Framework for Ergonomic Assessment of Modular Residential Construction Tasks

Despite increasing levels of automation in modular residential construction, workers are frequently exposed to highly labour-intensive tasks involving prolonged standing, bending, stooping, and material handling; these activities increase the potential risk of work-related musculoskeletal disorders (WRMSDs), which may worsen over time, resulting in permanent disability and, consequently, the loss of ability to work. The research presented in this thesis comprises the development of several models, each addressing specific research sub-objectives, to suit the ergonomic assessment requirements of a wide range of typical daily residential construction tasks. The primary objective is to develop a comprehensive framework for assessing ergonomic risk factors, including physical and non-physical risk factors. Thus, the research framework includes two main sections, physical ergonomics and non-physical ergonomics, through which both identification and mitigation aspects of risks are addressed. In most workplaces, ergonomic principles are not adequately addressed by managers due to their demanding schedules and their lack of knowledge about possible ergonomic risks associated with specific tasks as well as the major body parts affected during a given task. The physical ergonomics aspect of this research encompasses the unsafe and risky jobsite conditions and task designs that may expose workers to a combination of physical risk factors. Considering the effectiveness of information visualization and limitations of existing ergonomic evaluation tools, the first section of the research aims at developing a system dynamics-based ergonomic assessment tool capable of visualizing the evaluation process. Such a tool can provide a graphical illustration of links between cause and effects, thereby increasing the knowledge of how ergonomic risk can develop while performing task cycles. As a result of conducting ergonomic assessment using the developed tool, the risk level or classification of risk associated with the task with respect to each body part exposed to the risk can be identified. Often, actions taken to control ergonomic risks at the company level are limited due to limited knowledge of the actions required to manage such risks. Since manual material handling (MMH) is reported to be the most common contributor to occupational fatigue and lower-back pain, the second part of physical ergonomics research presented in this thesis, aims at addressing the gap in the knowledge of mitigation practices, and thereby examines two different hypotheses from the physiological perspective for effective intervention during MMH tasks. To this end, surface electromyography (sEMG) is used to examine the effectiveness of proposed solutions. Overall, participants in this study performed a total of 1,410 cycles while wearing sEMG electrodes. The results provide an incentive strategy for managers and job designers that by raising the lifting height and applying work pressure, not only can a time savings be achieved due to faster pace of work, but also the muscle activity in the lower back area can be reduced, leading to lower risk of low back pain and injury at work. Additionally, a microbreak stretching policy is introduced and its effectiveness is examined using the collected data. The results indicate that breaks, even for a brief period of one minute, combined with stretching exercises can effectively mitigate the risk of fatigue development in the lower-back muscles (multifidus) during an MMH construction task. The second main section of the present research focuses on the non-physical aspects of ergonomics, psychosocial risk factors. Despite the increase in research on psychosocial hazards, studies are unable to evaluate the inter-relationships among psychosocial risks from a risk network perspective, and their findings are usually limited to bivariate relationships between psychosocial risks and their adverse outcomes. Also, they are unable to draw the pathways by which psychosocial risks can affect the health of workers leading to impaired job performance. The present study performs a comprehensive literature review on the psychosocial risk-related research and provides an evidence-based list of major psychosocial factors. In addition, Social Network Analysis (SNA) is implemented for developing the psychosocial risk network for further evaluation of the interactions between variables leading to the identification of key psychosocial risks that play important roles in structuring the entire risk network. In the next step, an evidence-based System Dynamics (SD) model is developed to provide the mechanism underlying the development of MSDs and productivity loss due to exposure to psychosocial stressors. The method can be used by managers and job designers to evaluate the impact of implementing different policies and to understand the point at which workers are more likely to develop adverse effects

Finite element design and manufacturing of a nylon-string guitar soundboard from sandwich-structured composites

The aim of this project was to use the Finite Element Method (FEM) to design and manufacture the soundboard of a nylon-string guitar from sandwich-structured composites, with reference to an existing wooden soundboard, and to evaluate the accuracy of the numerical models of the wooden soundboard, the brace-less composite top plate and the braced composite soundboard by means of experimental modal analysis. The modal behaviour of the existing wooden soundboard was studied through experimental modal analysis and numerical simulation. Using FEM, the effects of varying certain physical, geometric and elastic properties of the materials used in the soundboard were determined on its natural frequencies under free and hinged Boundary Conditions (BCs). The composite soundboard that was determined to have natural frequencies relatively similar to those of the wooden soundboard under hinged BCs, and could be built from commercially available materials was constructed. To verify the results predicted numerically, experimental modal analyses were performed on the brace-less composite top plate and the braced composite soundboard under free BCs. The experimental natural frequencies and mode-shapes of the constructed brace-less top plate were found to match those predicted by the simulation in the frequency range below 200 [Hz], while slightly diverging in the higher frequency range. The experimental results for the braced composite soundboard were also found to be relatively similar to the numerically predicted values, with most mode-shapes matching, and some differences in the mode-frequencies, mostly in the low and mid-frequency ranges. Overall, a reasonable agreement was achieved between the numerical and the experimental results.L’objectif de ce projet était d'utiliser la Méthode des Éléments Finis (FEM) pour concevoir et fabriquer une table d'harmonie de guitare à cordes en nylon à partir de composites sandwiches en se référant à une table en bois existante, et d'évaluer la validité des modèles numériques de la table d'harmonie en bois, de la plaque supérieure en composite sans renforts et de la table d'harmonie composite renforcée au moyen d'une analyse modale expérimentale.Le comportement modal de la table d'harmonie existante en bois a été étudié au moyen de l'analyse modale expérimentale et de la simulation numérique. En utilisant la MEF, l'effet de la variation de certaines propriétés physiques, géométriques et élastiques des matériaux utilisés dans la table d'harmonie a été déterminé sur ses fréquences propres en utilisant des Conditions aux Limites (CL) soit libres soit immobiles (c. à-d., sans translations). La table d'harmonie en composite, dont on a déterminé qu'elle avait des fréquences propres relativement similaires à celles de la table d'harmonie en bois sous CL immobiles, et qui peut être construite à partir de matériaux disponibles dans le commerce, a été produite. Pour vérifier les résultats prédits numériquement, des analyses modales expérimentales ont été effectuées sur la plaque supérieure composite et la table d'harmonie en composites sous CL non contraintes.Les fréquences propres expérimentales et les différents modes propres de la plaque supérieure construite sans renforts correspondent à celles prédites par la simulation dans la gamme de fréquences inférieures à 200 [Hz], tout en divergeant dans la plage de fréquences plus élevées. Les résultats expérimentaux pour la table d'harmonie composite avec renforts se sont également révélés relativement similaires aux valeurs prédites numériquement, la plupart des formes de modes propres correspondantes, et certaines différences dans les fréquences propres, principalement dans les plages de basses et moyennes fréquences. Dans l'ensemble, un accord satisfaisant a été obtenu entre les résultats numériques et les résultats expérimentaux