Dissertations / Theses on the topic 'Return of serve'

Tennis är en sport som ständigt förändras. Nya data som analytikern Craig O'Shannessy tagit fram visar att en klar majoritet av alla poäng är korta, det vill säga poäng som innehåller fyra eller färre slag. Den spelare som vinner flest av dessa tenderar även att stå som segrare i slutet av matchen. Målet med det här arbetet är att undersöka WTA-turneringen Sydney International 2018 för att se om samma mönster går att hitta där och, om så är fallet undersöka om det finns några skillnader mellan hur olika grupper presterar. Ett par exempel på grupper är matchvinnare mot förlorare och seedade mot oseedade spelare. Det visade sig att poänglängderna nästan var identiska med O'Shannessys resultat och de hade påverkan på utgången av matcherna. Vidare så låg matchvinnarna högre i de flesta kategorierna som exempelvis andelen servar och returer in och andelen vunna. Gällande serve+1 så höll de en jämn nivå både på forehand- och backhandsidan, jämfört med de förlorande spelarna som hade problem med backhanden. Däremot gick det inte att se några tydliga skillnader i serve- och returmönster, bortsett från andraservar till deuce court. Utifrån resultaten så verkar kvaliteten på slagen vara mer avgörande än var i banan som de landar.
Tennis is an everchanging sport. New data from tennis analyst Craig O'Shannessy show that a vast majority of points are short, i.e. points decided in four or fewer shots. The player who wins most of them also tends to be the winner. The purpose of this paper is to examine the WTA tournament Sydney International 2018 to see if the same pattern occurs there and if that is the case investigate if there is any difference between how different groups are performing. A few examples of groups are match-winners against losers and seeded against unseeded players. It turns out that the lengths of the points were almost identical to O'Shannessy’s results and they influenced the matches outcome. Furthermore, match winners performed better in most categories, for example, the ratio of serves and returns in play and won. Regarding serve+1 they were able to keep an even level on both forehand and backhand side, while the losing players had some problems with their backhands. With that said, there were no clear differences in serve and return patterns, apart from second serves to the deuce court. From the results, it seems like the quality of the shots is more important than where they are landing.

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A presente pesquisa surgiu a partir da inten??o de identificar o perfil dos servidores T?cnico-Administrativos em Educa??o da Universidade Federal do Rio Grande do Norte (UFRN) que solicitaram Vac?ncia por Posse em Outro Cargo Inacumul?vel e Exonera??o dos cargos ocupados na referida Institui??o, no per?odo de 2010 a 2015. Al?m disso, buscou entender os reais motivos que levaram essas pessoas a sa?rem do v?nculo empregat?cio considerando todo o vi?s que envolve as rela??es de trabalho existentes na Organiza??o. A revis?o bibliogr?fica foi centrada em documentos legais que regem a carreira dos servidores T?cnico-Administrativos das Institui??es Federais de Ensino Superior (IFES), a estrutura da UFRN e trabalhos de autores que desenvolveram estudos sobre esse tema, al?m de pesquisas sobre o estado da arte. Trata o presente estudo de uma pesquisa de levantamento descritivo e, para atingir seus objetivos, foi realizada coleta de dados atrav?s da aplica??o de um question?rio online utilizando-se da ferramenta Google docs para chegar ? popula??o pesquisada, que totalizou 268 pessoas. Conclu?da a coleta de dados, obteve-se o retorno de 122 question?rios respondidos, sendo utilizado para tratamento destas informa??es o SPSS (Statistical for Social Sciences). O principal motivo apontado como insatisfa??o no ?rg?o refere-se ? remunera??o inerente ? Carreira dos Servidores T?cnico-Administrativos, representada pelo percentual de 91,2% das respostas e, por essa raz?o, pode-se concluir o entendimento pela busca por novos cargos no servi?o p?blico, confirmada pelo resultado de 77% dos casos. Com base nos dados desse estudo, percebe-se que, apesar do alto ?ndice de insatisfa??o com as quest?es salariais, de modo geral, o clima organizacional e a satisfa??o nas rela??es de trabalho se apresentaram de maneira favor?vel ? Institui??o pesquisada.
Present research arose from intention to identify the profile of Education Administrative Techinicians of Federal University of Rio Grande do Norte (UFRN) who requested Vacancy by Possession in Another Unaccountable Position and Exoneration, from occupied positions in that Institution between 2010 and 2015. In addition, It sought to understand real reasons that led these people to leave their employment bond considering all the bias that surrounds existing labor relations in the Organization. Bibliographic Review focused on legal documents governing career of the Administrative Techinicians of Federal Institutions of Higher Education (IFES), structure of UFRN and works of authors who have developed studies on this subject, as well as researches on the state of the art. This study concerns a descriptive survey and, in order to achieve its objectives, data collection was performed through the application of an online questionnaire using Google Docs tool to reach surveyed population, which totalized 268 people. 122 questionnaires were responded, and SPSS (Statistical for Social Sciences) was used to treat this information. Main cited reason as dissatisfaction in the Board refers to remuneration inherent to the Career of Administrative Techinicians, represented by the percentage of 91.2% of the answers and, for this reason, one can conclude the understanding for seeking new positions in Public Service, confirmed by the result of 77% of the cases. Based on the data of this study, it is noticed that, in spite of the high index of dissatisfaction with the salary issues, in general, organizational climate and the satisfaction in labor relationships presented a favorable way to researched Institution.

Táto diplomová práca sa zaoberá implementáciou bezpečnostnej funkcie pre elektrický aktuátor, ktorá spočíva v riadení BLDC motora, po výpadku napájacieho napätia, keď je aktuátor poháňaný vstavanou pružinou.. Celé riadenie motora je navrhnuté v prostredí Matlab-Simulink technikou nazývanou návrh systému z modelu. Následne pomocou automatického generovania kódu bol získaný zdrojový kód, ktorý bol použitý v reálnom aktuátore a odtestovaný v reálnych podmienkach. Cieľom týchto testov bolo overiť reálnu možnosť nasadenia vyvinutých algoritmov v reálnych, komerčne dostupných produktoch.

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This Master s Thesis proposes the application of Data Envelopment Analysis DEA to evaluate the performance of sales teams, based on a study of their coverage areas. Data was collected from the company contracted to distribute the products in the state of Cear?. Analyses of thirteen sales coverage areas were performed considering first the output-oriented constant return to scale method (CCR-O), then this method with assurance region (AR-O-C) and finally the method of variable returns to scale with assurance region (AR-O-V). The method used in the first approach is shown to be inappropriate for this study, since it inconveniently generates zero-valued weights, allowing that an area under evaluation obtain the maximal score by not producing. Using weight restrictions, through the assurance region methods AR-O-C and AR-O-V, decreasing returns to scale are identified, meaning that the improvement in performance is not proportional to the size of the areas being analyzed. Observing data generated by the analysis, a study is carried out, aiming to design improvement goals for the inefficient areas. Complementing this study, GDP data for each area was compared with scores obtained using AR-O-V analysis. The results presented in this work show that DEA is a useful methodology for assessing sales team performance and that it may contribute to improvements on the quality of the management process.
Esta tese de mestrado prop?e utilizar a An?lise de Envolt?ria de Dados - DEA (Data Envelopment Analysis) na avalia??o de desempenho das equipes de vendas a partir de um estudo de suas ?reas de atua??o. S?o utilizados dados obtidos junto ? empresa contratada para distribui??o dos produtos no estado do Cear?. As an?lises das treze ?reas de vendas foram realizadas considerando o m?todo de retorno constante de escala orientado a produto (CCR-O), em seguida, esse m?todo com utiliza??o de regi?o de garantia (AR-O-C) e por ?ltimo o m?todo de retorno vari?vel de escala com utiliza??o de regi?o de garantia (AR-O-V). O m?todo utilizado na primeira abordagem mostra-se inapropriado para este estudo pois apresenta o inconveniente de gerar pesos com valor igual a zero, permitindo que uma ?rea avaliada obtenha escore m?ximo n?o produzindo. Utilizando-se restri??es de pesos, atrav?s dos m?todos da Regi?o de Garantia AR-O-C e AR-O-V, identificam-se a exist?ncia de retornos decrescentes de escala significando que a melhoria de desempenho n?o ? proporcional ao tamanho das ?reas analisadas. Atrav?s do m?todo AR-O-V foram encontradas nove ?reas eficientes. Observando os dados gerados pelas an?lises faz -se um estudo para projetar metas de melhoria para as ?reas ineficientes. Complementando o estudo, comparam-se os dados do PIB de cada ?rea com os escores obtidos na an?lise A R-O-V. Os resultados apresentados nesse trabalho garantem a utilidade da DEA como metodologia para a avalia??o de desempenho das equipes de vendas podendo contribuir para a melhoria da qualidade do seu processo de gest?o.

碩士
國立臺灣師範大學
運動競技學系
103
Purpose: This study aims to explore the serve advantages of top women players in single serve game and major return performance of receive in the receive game. Methods: This study takes 15 competitions of women singles from Round 16 to finals in Australian Open 2014 as scope and analyzes the major score methods and return methods of top women players in the receive game with descriptive statistics and pivot table analysis. Results: 1.The average first and second serve scoring rate of top women players in the serve game are 64% and 87% respectively. Those of the serve winning rate are 62% and 51%. The major scoring performance is serve advantage. 2.In the receive game, the average first and the second receive scoring rate is 78% and 88% separately. Those of the receive winning rate are 38% and 49% with the defense and standoff as major return methods. 3.When being broken in the serve game, the first and the second serve scoring rate reduces to 60% and 82% respectively. The first serve winning rate reduces to 35% and that of the second reduces to 26 which are related to reduction of serve advantage scoring. 4.When break successfully in the receive game, the first and the second receive scoring rate is 86% and 94% respectively. The first receive winning rate is 65% and the second receive winning rate is 74% which are related to the increase of offensive return. Conclusions: 1.The major scoring method of top women players is serve advantage in the serve game. The first serve scoring rate, winning rate and serve advantage reach over 60%. However, the first serve scoring rate reduces over 60% below and the second serve scoring rate reduces over 80% below. The first serve winning rate reduces over 40% and the second serve winning rate reduces over 30%. In this way, they are easy to be broken. 2.In the receive game, the major return methods are defense and standoff, but the reasons to break successfully are caused by increase of offensive return. When break successfully, only by reaching over 40% in the second serve winning rate with offensive return can the players have chance to break.

Title: Analysis of tennis single on clay Objectives: The aim of the work was to analyze 10 elite players on clay and also to discover differences in observed aspects of their plays. Methods: The analysis was done by observing video recordings of chosen matches from the French Open 2016 tournament. It was a notational analysis. The results were recorded into record sheets that were prepared in advance. Firstly, descriptive analysis was used to analyze all of the matches. Thereafter, all of the matches were analyzed by descriptive analysis of graphs and measured values. Results: The results show different techniques and styles among male athletes on clay, given the service and its variations, the return and its variations, the length of plays, and other observed aspects; resulting in other tactical differences among the players. Keywords: Tennis, men, differences, serve, return

Title: Comparison of the badminton match in men's and women's singles Objectives: The goal is identify the most effective a choice combinations of the first three shots, leading to the acquisition point in singles between the best Czech male and female players. And also to discover determine and compare which was the usage of different types of serve, return and the third shots. Methods: Comparison of differences in badminton match between genders was based on indirect observation - video analysis. Statistics were used for data processing. Results: The results shower huge difference in tactical approaches among men and women players. Women preferred long high service and most of the service returns were played into back backhand court area and the third shot usually routed into back forehand court area. It means that the most played shot was a clear shot and the most occupied part of court was a back backhand corner. Despite more successful was when women players played drop by their third shot. Domination of short backhand service was seen by men players, the second shot routed the same direction like women players second shot - to the back backhand court, but also to the front part of the court. Third shot was mostly played into back backhand part of the court. If players wanted to be successful,...

Title: Analysis of the badminton matches in men's and women's singles The goals of essay: Main goal of the essay is to find out differences in first three shots and its most common combinations in single matches of men and women in relation to gaining a point during the rally. Another goal was to find frequency of using backhand and forehand hold in second and third shot. The last goal was to compare the frequency of ending in first, second and third shot between men and women. Methods: Indirect analysis - videoanalysis Results: According to results men and women primarily choose different types of serving. Men most often use short backhand serve, women high forehand serve. For the second shot men choose short shot or lob, women use smash. The third shot is lob in men rally, women choose short shot most often. Generally men uses in the first three shots shots less clears than women do. For both sexes are the most effective shots offensive ones, but men are more successful than women. The most effective direction of return was for men and for women the body of the player, in third shot it was the backhand side too (left middle and left back of the field). Men were more successful with forenhand, women with backhand. The frequenc of endings to third shots shows that women more often end the rally...

Expert tennis players are known to anticipate the serve of an opponent using two sources of information from their environment. Kinematic information sources are cues from the action of the server, while contextual information may be any source of information outside of the visual kinematic cues which the expert considers. While previous research has examined the contribution of these two sources on an expert tennis player’s anticipation capability, it is still unknown how they interact during the return of serve. Consequently, this thesis had two aims: 1. Investigate the temporal interaction of kinematic and contextual information sources considered by expert tennis players when returning serve. 2. Investigate how changes in kinematic or contextual sources of information alters an expert player’s return of serve performance. The two aims of this thesis were considered to address the current gap in the existing literature, and enhance our understanding about the temporal priority of anticipatory information sources expert tennis players are attuned to during the return of serve. The three investigations from this thesis revealed a number of important findings about the temporal interaction of anticipatory information sources. Study 1 determined nine higher order themes from qualitative interviews with expert tennis players about their returning experiences in professional matches. These themes were developed into a temporal model that presented the anticipatory information in order of priority during the return of serve. Study 2 found that although expert tennis players discussed the influence of contextual information on their returning behaviour in Study 1, spatiotemporal data from professional matchplay revealed that this only had an influence on their depth position, and not their lateral position. This study also found that the largest variation in return position occurred at return impact, which was important for confirming that significant changes in return position does not occur until ball flight information is available. The final experiment revealed that although expert tennis players are attuned to contextual and kinematic information, this information does not necessarily improve the quality of the return. Contextual information was the priority anticipatory information source until ball toss information became available. The kinematic information from the ball toss is then prioritised by the returners until ball flight information is available, which was found to be the most influential information source for altering returner behaviour. Furthermore, it was found that expert tennis players were not susceptible to a congruence effect as suggested in a number of previous studies. The conclusion from the experimental series is that a returner’s behaviour is influenced by the most reliable source of information available at each moment in time, with ball flight information the most reliable and heavily prioritised source. This finding is important for tennis players and coaches to consider when implementing training strategies for the returners to recognise and respond to the various information sources during a match.

碩士
國立新竹教育大學
人資處體育碩士專班
100
A Study Investigating the Area of Interest of Returned Serve for Table Tennis Players with Different Skill Levels in Elementary School Abstract Purpose: To analyze the AOI (Area of Interest) of returned serve for table tennis of the male players in elementary school, and the difference among players’ AOI with different skill levels.Methods: Using eye tracker (iView X HED4, 50 Hz), notebook computer（IBM）to record the eye situation of 8 participants with excellent abilities the elite group, 7 participants with fair abilities the average group and 8 beginners. Then, using BeGaze 3.0 software to analyze the data of the percentage duration time of AOI. All the variables were tested by t-test and one-way ANOVA which was calculated by SPSS for Windows. Results: 1.The AOI of the elite group is the racket and the ball. The AOI of the average group is the ball. The AOI of the beginners is the ball. 2. The aiming time on the racket of the elite group is obviously higher than the average group and the beginners. 3. The aiming time on other things of the beginners is obviously higher than the average group and the elite group. Key words: Area of Interest, table tennis, returned serve

碩士
國立屏東教育大學
體育學系
99
The purpose of the research is the different reaction time ( RT ) and movement time ( MT ) of retuned serve for the table tennis robots with different gender and dissimilarity stimulates source from university student while catching the serving ball from left side or right side . Do the“pilot study”first to select two table tennis robots and position to set of the“pactical research ”. The participants of the “practical research ” are 15 male and 15 female physical education major students of National Pintung University of Education who are practicing and studying the table tennis, The right-handed participants will repeatedly accept trials of the RT and MT under response sets and stimulus-response compatibility including: 2 (table tennis robot: right or left) x 2 (stimulus-response compatibility) x 12 (deliver ball number) = 48. The RT and MT of the participants are analyzed by using the Laboratory Virtual Instrument Workbench Accelerometer infrared-inductance.Taking gender difference , dissimilarity stimulatessource and stimulus-response compatibility as the independent variables and the RT and MT as the dependent variables , to examine the three-way mixed design ANOVA are used to verify their differences. If the interaction is above the statistical significant level the simple interaction verification will be carried out and the simple simple main effect verification will be carried out on those above the significant level. After discussing the outcomes of the study,the conclusions are as follows： 1. The interaction doesn''t exist between the influences of gender difference, and gender difference,dissimilarity stimulates source and stimulus-response compatibility on RT.To gender difference says, the reaction time of the female is quicker than the male, and With compatibility and without compatibility the reaction time of the female is quicker than the male.Dissimilarity stimulates source and stimulus-response compatibility have not remarkable difference to RT influence. 2. The interaction doesn''t exist between the influences of gender difference, and gender difference,dissimilarity stimulates source and stimulus-response compatibility on MT. Dissimilarity stimulates source and stimulus-response compatibility have not remarkable difference to MT influence.

碩士
國立臺東大學
體育教學碩士在職專班
104
The study mainly investigated the table tennis strategy of Taiwanese high school table tennis players by three stage skill analysis. The study analysed players who are from the quarter final in the 2014 National High School Athletic Games(all about Fu Cheng Senior High School players). Games were recorded by video and record those players’ gripping mode from preliminaries to the final game. The gripped mode, three stage skill, and strategy were recorded by 3 A-level coaches. By this way, we can analse high school students’ skill in actual situation. Moreover, we can review the advantage and disadvantage for future training, and enhance their level of skill. The scoring rates of Three-Part Skill are as follow:No.1234 player 48.9％、39.1％、12％, No.1235 player 40.6％、28.1％、31.3％,and No.1240 player 33.1％、35％、32％. The usage rates of Three-Part Skill are as follow:No.1234 player 42.6％、43.3％、14.1％, No.1235 player 38.4％、33.2％、28.2％,and No.1240 player 28.2％、35.1％、36.6％.The overall average of usage rate showed that Three-Part Skill is used more in the section of attack after serve part and attack on return serve part. Conclusion: The research shows that players of Kaohsiung Municipal Fu Cheng Senior High School mainly use quick attack combining top spin and top spin combining quick attack. If they could participate more national events,and train themselves to strengthen their advantages and improve shortcomings. Someday, they can greatly enhance their personal achievement.

This work reports on virtualization and cloud computing technologies. It evaluates their strengths and weaknesses as well as describes the examples of its deployment. The document serves as a concise and comprehensive guide, describing the necessary theory along with the practical use. It is all demonstrated on a real project. The practical part refers to the data environment of the Fire Rescue Service of South Bohemia Region and represents the usage of studied technologies. The analysis also includes project´s financial and technical aspects.

Tese de doutoramento em cotutela, na área de Engenharia Mecânica, no ramo de Tecnologias de Produção, apresentada ao Departamento de Engenharia Mecânica da Faculdade de Ciências e Tecnologia da Universidade de Coimbra e à Université de Bretagne-Sud, França
The transport industry faces sustainability challenges that demand vehicles’ weight reduction, while safety requirements impose the increase of their structural strength. One of the approaches adopted to address this goal is the continuous development of lightweight materials and their manufacturing technologies. Within lightweight materials, the aluminum alloys are characterized by presenting medium strength and good formability. However, when compared with the metallic alloys traditionally used in sheet metal forming operations, they present lower formability and higher springback. In this context, warm forming was proposed as a solution to solve these problems, with good acceptance, first, in the research community and, nowadays, in industry. However, the warm forming of heat treatable aluminum alloys is still a challenge, since the recommended range of temperature is similar to the one used for heat treatments. Thus, warm conditions can result in heat treatment changes during the forming operation, which need to be predicted during the process virtual try-out to avoid defects in the production line. Moreover, heat treatable alloys are prone to natural aging, which can lead to variability and defects in the production line. Thus, the main goal of this work is to analyze the warm forming conditions of heat treatment aluminum alloys, taking into account the natural aging, in order to propose solutions that can contribute to the increase of robustness of sheet metal forming operations. Two heat treatable Al-Mg-Si (6xxx series) alloys were selected for the study: the EN AW 6016-T4 (natural aging) and the EN AW 6061-T6 (artificial aging). Their thermo-mechanical behavior was characterized using uniaxial tensile tests between room temperature (RT) and 300 ºC at 0, 45 and 90 º to the rolling direction, in a strain rate range from 2x10-4 to 2x10-2s-1, with heating time of 20 seconds. This tests were performed in a Gleeble 3500 machine under monotonic load and with stress-relaxation stages. Additional uniaxial monotonic tensile tests were performed in an Instron 4505 machine coupled with a classical furnace, at RT and 200 ºC, enabling the analysis of heat-holding times of 10 and 30 minutes. The strain field was measured using optical 3D deformation analysis (gom-aramis 4M system). The cylindrical cup tests were performed in a Zwick/Roell Amsler BUP200 sheet metal testing equipment, adapted with specific tools for warm forming. The adoption of this geometry enabled the study springback using the split ring test. All warm forming tests were performed considering non-isothermal conditions, with the die and the blank-holder heated up to the desired temperature, while the punch is refrigerated to keep its temperature close to RT. The cylindrical cup tests were performed between RT and 250 ºC, for punch (ram) speeds between 0.1 and 10 mm/s and heat-holding times of 1, 10 and 30 minutes. During each test, the punch force and its displacement, the blank-holder force and the temperature were acquired, as a function of time. After the forming operation the cup thickness and the cup height were measured, as well as the springback. Finally, the influence of natural aging was evaluated for the period from 1 to 18 months, which required the duplication of some tests. Globally, it is possible to correlate the thermo-mechanical results with the ones obtained in the forming tests, validating the macroscopic approach adopted for the results analysis. The EN AW 6061-T6 mechanical behavior shows a small natural aging effect, while it has a strong effect for the EN AW 6016-T4 alloy, leading to an increase of the yield stress and tensile strength and, consequently, of the springback. In this context, warm forming tests considering a heat-holding time of 1 minute, at 200 and 250 ºC, prove to be an effective solution to reduce the variability caused by natural aging in: forming forces, formability and springback. The orthotropic behavior is not affected by the temperature increase or natural aging. Both alloys present a negligible strain rate sensitivity at RT, while it is positive for warm temperatures. The warm forming tests performed at 200 ºC (heat-holding time of 1 minute), show that high punch speeds are advantageous. Formability and springback remained stable or improved with the punch speed increased. In fact, low punch speeds lead to high exposure time and, consequently, to tools thermal equilibrium that counteract the effects of the non-isothermal conditions, and promotes dynamic precipitation hardening on the EN AW 6016-T4 alloy that leads to strength and springback increase. The tensile tests with stress-relaxation stages were extremely useful on understanding the dynamic precipitation phenomena at warm temperature. Moreover, whatever the punch speed adopted the variability caused by natural aging was negligible at warm forming, which validate the effectiveness of this solution. Concerning the analysis of heat-holding time at 200 ºC, globally the results show that for the EN AW 6061-T6, the increase of the heat-holding time up to 50 minutes has a negligible impact on the material behavior and, consequently, the formability and springback warm forming results are independent of this process variable. For the EN AW 6016-T4 the increase of the heat-holding time up to 30 minutes leads to an increase the yield stress and tensile strength, while the total elongation reduces, as a result of a heat treatment change from natural aged to artificial aging condition. Consequently, the warm forming test performed with similar conditions shows a reduced formability and a springback increase. Moreover, a heat-holding time of 30 minutes leads to the known negative-effect of natural aging in artificial aging, for a storage time of 18 months. Thus, the heating system selected for the warm forming must guarantee a high heating rate, which is also advantageous from the production point of view. Throughout this work the split-ring test was used to evaluate springback. In this context, a numerical study was performed, in order to improve knowledge concerning the impact of the ironing stage in the circumferential residual stresses in the ring. Globally, the results show that the introduction of an ironing stage changes the characteristic distribution of the residual stress component throughout the vertical wall, even for relatively small ironing strains. This change affects the trend observed for the opening value of the rings located at different heights. This study was important to the results analysis since it explained the lower sensitivity of the ring located in the top of the cup (ironed zone) to the changes in material and process conditions. The analysis of the other rings clearly shows that warm forming contributes to springback reduction, when performed under non-isothermal conditions, with high punch speeds and heating rates.
A indústria dos transportes enfrenta desafios de sustentabilidade que exigem a redução de peso dos veículos, ao mesmo tempo que os requisitos de segurança impõem o aumento da resistência estrutural. Uma das abordagens adotadas para atingir estes objetivos é o contínuo desenvolvimento de materiais de elevada resistência e das suas tecnologias de fabrico. As ligas de alumínio pertencem a este conjunto de materiais, onde se caracterizam por apresentarem uma resistência média e boa formabilidade. No entanto, quando comparadas com as ligas metálicas tradicionalmente utilizadas nas operações de conformação de chapas metálicas, apresentam menor formabilidade e maior retorno. A conformação a temperaturas moderadas foi proposta como alternativa para ultrapassar estes problemas, primeiro na comunidade científica e, atualmente, na indústria. No entanto, a utilização deste processo para ligas de alumínio tratáveis termicamente é ainda um desafio, uma vez que a gama de temperaturas recomendada é semelhante à utilizada nos seus tratamentos térmicos. Assim, a conformação nesta gama de temperaturas pode conduzir a alterações do tratamento térmico, que devem ser previstas na conceção virtual para evitar a ocorrência de defeitos na linha de produção. Além disso, as ligas tratáveis termicamente são propensas a envelhecimento natural, que pode introduzir variabilidade e defeitos na linha de produção. O principal objetivo deste trabalho é analisar as condições de conformação a temperaturas moderadas deste tipo de ligas de alumínio, incluindo o efeito de envelhecimento natural, a fim de propor soluções que possam contribuir para aumentar a robustez das operações de estampagem de chapas metálicas. Foram selecionadas duas ligas Al-Mg-Si (série 6xxx) tratáveis termicamente: a EN AW 6016-T4 (envelhecimento natural) e a EN AW 6061-T6 (envelhecimento artificial). O seu comportamento termo-mecânico foi caracterizado com recurso a ensaios de tração uniaxial, realizados entre a temperatura ambiente (TA) e 300 ºC, com provetes orientados a 0, 45 e 90 º, em relação à direção de laminagem, para uma gama de velocidades de deformação de 2x10-4 a 2x10-2s-1 e um tempo de aquecimento de 20 segundos. Estes ensaios fora realizados num equipamento Gleebe, em condições de carga monótona crescente e incluindo estágios de relaxação de tensão. Foram também realizados ensaios monótonos de tração uniaxial numa máquina Instron, equipada com um forno, a TA e a 200 ºC, o que permitiu estudar tempos de permanência a 200 ºC de 10 e 30 minutos. O campo de deformação foi medido com um sistema ótico de análise (gom-aramis 4M system). Foram ainda realizados ensaios de estampagem de taças cilíndricas, num equipamento Zwick/Roell Amsler BUP200, com recurso a ferramentas adaptadas para conformação a temperaturas moderadas. A adoção desta geometria permitiu o estudo do retorno elástico, com base no ensaio de corte de anel. Todos os ensaios foram realizados em condições não-isotérmicas, com a matriz e o cerra-chapas aquecidos, até à temperatura desejada, e o punção refrigerado para manter uma temperatura próxima da TA. Estes ensaios de estampagem foram realizados entre TA e 250 ºC, para velocidades de deslocamento do punção compreendidas entre 0.1 e 10 mm/s e tempos de manutenção à temperatura de aquecimento de 1, 10 e 30 minutos. Em cada ensaio, foram adquiridos os resultados correspondentes à evolução com o tempo, da força e do deslocamento do punção, da força do cerra-chapas e da temperatura. Após a operação de conformação, foi medida a evolução da espessura ao longo da parede da taça, bem como o perfil das orelhas e o retorno elástico. Finalmente, foi avaliada a influência do envelhecimento natural para o período de 1 a 18 meses, o que exigiu a duplicação de alguns ensaios. Globalmente, a abordagem macroscópica adotada para a análise de resultados foi validada pela correlação entre os resultados dos ensaios de caracterização termo-mecânica e os de conformação. O comportamento mecânico da liga 6061-T6 é pouco sensível ao envelhecimento natural, enquanto a liga 6061-T6 apresenta um aumento da tensão limite de elasticidade e máxima não desprezável, que resulta no aumento do retorno elástico. Neste contexto, a conformação a quente a 200 e 250 ºC, com um tempo de manutenção de 1 minuto, revela-se uma solução eficaz para reduzir a variabilidade induzida pelo envelhecimento natural na força de estampagem, na formabilidade e no retorno elástico. O comportamento ortotrópico não é afetado pelo aumento da temperatura nem pelo envelhecimento natural. As ligas apresentam uma sensibilidade à velocidade de deformação negligenciável à TA, que passa a ser positiva para 200 ºC. Os testes de conformação realizados a esta temperatura (tempo de manutenção de 1 minuto) mostram que a utilização de uma velocidade do punção elevada pode ser vantajosa, uma vez que a formabilidade e o retorno elástico permanecem estáveis ou melhoram. De facto, velocidades do punção reduzidas conduzem a tempos de exposição elevados que conduz ao equilíbrio térmico das ferramentas, o que neutraliza os efeitos pretendidos com as condições não-isotérmicas e promove o endurecimento por precipitação dinâmica da liga EN AW 6016-T4, i.e. a força e o retorno elástico aumentam. Os ensaios de tração com estágios de relaxação de tensão permitem uma melhor compreensão dos fenómenos de precipitação dinâmica em função da temperatura. Além disso, qualquer que seja a velocidade seccionada para o punção, a variabilidade causada pelo envelhecimento natural é negligenciável para 200 ºC, o que valida a eficácia desta solução. Em relação à influência do tempo de manutenção a 200 ºC, para a liga EN AW 6061-T6, o aumento deste tempo até 50 minutos tem um impacto negligenciável no comportamento mecânico do material e, consequentemente, a formabilidade e retorno elástico são independentes desta variável de processo. Para a liga EN AW 6016-T4, o aumento do tempo de manutenção até 30 minutos resulta num aumento da tensão limite de elasticidade e máxima, com a redução do alongamento total, como resultado da alteração do tratamento térmico de envelhecimento natural para artificial. Logo, o ensaio de conformação realizado em condições semelhantes apresenta uma formabilidade reduzida e um aumento de retorno elástico. Além disso, um tempo de manutenção de 30 minutos resulta no efeito negativo do envelhecimento natural no envelhecimento artificial, para a liga EN AW 6016-T4 com um tempo de armazenamento de 18 meses. Assim, o sistema de aquecimento selecionado para a conformação a temperaturas moderadas deve garantir uma velocidade de aquecimento elevada, o que também é vantajoso do ponto de vista industrial. Ao longo deste trabalho, o ensaio de corte de anel foi utilizado para avaliar o retorno elástico. Neste contexto, foi realizado um estudo numérico, para melhorar o conhecimento sobre o impacto da etapa de estiramento na distribuição das tensões residuais circunferenciais no anel. Os resultados mostram que a introdução da etapa de estiramento altera a distribuição característica das tensões residuais ao longo da parede vertical, mesmo para deformações reduzidas. Esta alteração afeta a tendência observada para o valor de abertura dos anéis cortados a diferentes alturas. Este estudo foi importante para a análise dos resultados, uma vez que justifica a menor sensibilidade do anel localizado no topo do copo (zona estirada) às mudanças nas condições de comportamento do material e de processo. A análise dos outros anéis mostra claramente que a conformação a temperaturas moderadas, em condições não isotérmicas, contribui para a redução do retorno elástico, se realizada com velocidades do punção e de aquecimento elevadas.
French Ministry of Higher Education