A modified structural stress method for fatigue assessment of welded structures

Fatigue life assessment of weldedstructures is commonly based on the nominal stress method, but more flexible and accurate methods have been introduced. In general, the assessment accuracy is improved as more localized information about the weld is incorporated. The structural hot spot stress method includes the influence of macro geometric effects and structural discontinuities on the design stress but excludes the local features of the weld. In this thesis, the limitations of the structural hot spot stress method are discussed and a modified structural stress method with improved accuracy is developed and verified for selected welded details. The fatigue life of structures in the as-welded state consists mainly of crack growth from pre-existing cracks or defects. Crack growth rate depends on crack geometry and the stress state on the crack face plane. This means that the stress level and shape of the stress distribution in the assumed crack path governs thetotal fatigue life. In many structural details the stress distribution is similar and adequate fatigue life estimates can be obtained just by adjusting the stress level based on a single stress value, i.e., the structural hot spot stress. There are, however, cases for which the structural stress approach is less appropriate because the stress distribution differs significantly from the more common cases. Plate edge attachments and plates on elastic foundations are some examples of structures with this type of stress distribution. The importance of fillet weld size and weld load variation on the stress distribution is another central topic in this thesis. Structural hot spot stress determination is generally based on a procedure that involves extrapolation of plate surface stresses. Other possibilities for determining the structural hot spot stress is to extrapolate stresses through the thickness at the weld toe or to use Dong's method which includes through-thickness extrapolation at some distance from the weld toe. Both of these latter methods are less sensitive to the FE mesh used. Structural stress based on surface extrapolation is sensitive to the extrapolation points selected and to the FE mesh used near these points. Rules for proper meshing, however, are well defined and not difficult to apply. To improve the accuracy of the traditional structural hot spot stress, a multi-linear stress distribution is introduced. The magnitude of the weld toe stress after linearization is dependent on the weld size, weld load and plate thickness. Simple equations have been derived by comparing assessment results based on the local linear stress distribution and LEFM based calculations. The proposed method is called the modified structural stress method (MSHS) since the structural hot spot stress (SHS) value is corrected using information on weld size andweld load. The correction procedure is verified using fatigue test results found in the literature. Also, a test case was conducted comparing the proposed method with other local fatigue assessment methods.

”Stress Monitoring via Wearable Electrodermal Sensors”

Stress is a phenomenon that on some level affects everyone’s lives on a daily basis. The autonomic nervous system controls the varying levels of stress at any given time. The responses of the autonomic nervous system adjust the body to cope with changing external and internal conditions. During high-stress situations the body is forced into a state of heightened alertness, which passes when the stressor is removed. The stressor can be any external or internal event that causes the body to respond. Stress is a very versatile phenomenon that can be both a cause and an indicator of other medical conditions, for example cardiovascular disease. Stress detection can therefore be helpful in identifying these conditions and monitoring the overall emotional state of a person. Electrodermal activity (EDA) is one of the most easily implemented ways to monitor the activity of the autonomic nervous system. EDA describes changes occurring in the various electrical properties of the skin, including skin conductivity and resistance. Increased emotional sweating has been proven to be one possible indication of stress. On the surface of the skin, increased sweating translates to increased skin conductivity, which can be observed through EDA measurements. This makes electrodermal activity a very useful tool in a wide range of applications where it is desirable to observe changes in a person’s stress level. EDA can be recorded by using specialized body sensors placed on specific locations on the body. Most commonly used recording sites are the palms of the hands due to the high sweat gland density on those areas. Measurement is done using at least two electrodes attached to the skin, and recording the electrical conductance between them. This thesis implements a prototype of a wireless EDA measurement system. The feasibility of the prototype is also verified with a small group of test subjects. EDA was recorded from the subjects while they were playing a game of Tetris. The goal was to observe variations in the measured EDA that would indicate changes in the subjects’ stress levels during the game. The analysis of the obtained measurement results confirmed the connection between stress and recorded EDA. During the game, random occurrences of lowered skin resistance were clearly observable, which indicates points in the game where the player felt more anxious. A wireless measurement system has the potential of offering more flexible and comfortable long-term measuring of EDA, and could be utilized in a wide range of applications.

Liikuntasaumaproblematiikan selvittäminen laivan runkorakenteissa

Nykyään laivan kansirakenteet suunnitellaan pääosin kantaviksi rakenteiksi, mikä edellyttää niiltä suurta lujuutta. Jatkuvasti kasvavissa risteilijöissä ja muissa suurissa aluksissa ongelmaksi muodostuu kansirakenteiden suuret jännitykset. Kansirakenteet sijaitsevat kauimpana laivan neutraaliakselilta, jolloin niissä syntyy suuria venymiä. Kansirakenteista pitää näin ollen suunnitella hyvin kestäviä tai vaihtoehtoisesti tarpeeksi lyhyitä rakenteita. Liikuntasaumojen avulla on 1950 -luvulle asti laivoissa katkaistu pitkät kansirakenteet, mutta hitsausmenetelmien kehittyessä kansirakennukset on tehty yhtenäisiksi kansirakenteiksi. Tämä on tähänastisissa risteilijöissäkin toiminut hyvin, mutta laivojen koon kasvaessa on etsittävä keinoja mahdollisiin runkorakenteiden ja varustelun lujuus- ja väsymisongelmiin. Tavoitteena oli saada aikaan työ, joka olisi hyvä ”työkalu” tuleville tutkimuksille liikuntasaumojen soveltamisessa laivarakenteisiin sekä niihin liittyviin varusteluosiin ja -rakenteisiin. Työssä tutustutaan kirjallisuustutkimuksen avulla liikuntasaumasovelluksiin ja esitellään sovelluksia eri aloilta. Kirjallisuusosuuden päätteeksi esitellään muutama laivarakennesovellus, joita löytyy hieman vanhemmista laivarakenteista. FE analyysiosuudessa tutkitaan liikuntasauman pohjan muodon vaikutusta pohjan jännitystasoihin ja liikuntasaumojen vaikutusta laivan kansirakenteiden jännitystasoihin kolmella eri liikuntasaumojen lukumäärällä. Lisäksi kansirakenteiden jännitystasoja tutkittiin kolmella kansirakenteen leveydellä. Esimerkkejä liikuntasaumoista löytyy monelta eri aloilta, joiden ominaisuuksia yhdistelemällä saavutetaan oikea ratkaisu liikuntasaumojen soveltamisessa laivojen kansirakenteisiin. Lisäksi FE -analyysistä voidaan nähdä, että liikuntasaumat laskevat jännitystasoja laivojen kansirakenteissa. Liikuntasaumojen oikea lukumäärä riippuu hyvin paljon siitä, kuinka paljon kansirakenteen jännityksiä halutaan laskea. Liikuntasauman pohjan muotoa on kehitettävä ja se on otettava myös huomioon yhtenä tärkeänä seikkana suunniteltaessa liikuntasaumoja laivojen kansirakenteisiin. Esimerkiksi vahvistelevyillä saadaan jännityksiä laskettua liikuntasauman pohjan läheisyydessä tehokkaasti.

Size Effect and Notch Size Effect in Metal Fatigue

It is a well known phenomenon that the constant amplitude fatigue limit of a large component is lower than the fatigue limit of a small specimen made of the same material. In notched components the opposite occurs: the fatigue limit defined as the maximum stress at the notch is higher than that achieved with smooth specimens. These two effects have been taken into account in most design handbooks with the help of experimental formulas or design curves. The basic idea of this study is that the size effect can mainly be explained by the statistical size effect. A component subjected to an alternating load can be assumed to form a sample of initiated cracks at the end of the crack initiation phase. The size of the sample depends on the size of the specimen in question. The main objective of this study is to develop a statistical model for the estimation of this kind of size effect. It was shown that the size of a sample of initiated cracks shall be based on the stressed surface area of the specimen. In case of varying stress distribution, an effective stress area must be calculated. It is based on the decreasing probability of equally sized initiated cracks at lower stress level. If the distribution function of the parent population of cracks is known, the distribution of the maximum crack size in a sample can be defined. This makes it possible to calculate an estimate of the largest expected crack in any sample size. The estimate of the fatigue limit can now be calculated with the help of the linear elastic fracture mechanics. In notched components another source of size effect has to be taken into account. If we think about two specimens which have similar shape, but the size is different, it can be seen that the stress gradient in the smaller specimen is steeper. If there is an initiated crack in both of them, the stress intensity factor at the crack in the larger specimen is higher. The second goal of this thesis is to create a calculation method for this factor which is called the geometric size effect. The proposed method for the calculation of the geometric size effect is also based on the use of the linear elastic fracture mechanics. It is possible to calculate an accurate value of the stress intensity factor in a non linear stress field using weight functions. The calculated stress intensity factor values at the initiated crack can be compared to the corresponding stress intensity factor due to constant stress. The notch size effect is calculated as the ratio of these stress intensity factors. The presented methods were tested against experimental results taken from three German doctoral works. Two candidates for the parent population of initiated cracks were found: the Weibull distribution and the log normal distribution. Both of them can be used successfully for the prediction of the statistical size effect for smooth specimens. In case of notched components the geometric size effect due to the stress gradient shall be combined with the statistical size effect. The proposed method gives good results as long as the notch in question is blunt enough. For very sharp notches, stress concentration factor about 5 or higher, the method does not give sufficient results. It was shown that the plastic portion of the strain becomes quite high at the root of this kind of notches. The use of the linear elastic fracture mechanics becomes therefore questionable.

Loss Modelling of Three-Level Inverters controlled with Space Vector Modulation Technique

The objective of this master’s thesis is to investigate the loss behavior of three-level ANPC inverter and compare it with conventional NPC inverter. The both inverters are controlled with mature space vector modulation strategy. In order to provide the comparison both accurate and detailed enough NPC and ANPC simulation models should be obtained. The similar control model of SVM is utilized for both NPC and ANPC inverter models. The principles of control algorithms, the structure and description of models are clarified. The power loss calculation model is based on practical calculation approaches with certain assumptions. The comparison between NPC and ANPC topologies is presented based on results obtained for each semiconductor device, their switching and conduction losses and efficiency of the inverters. Alternative switching states of ANPC topology allow distributing losses among the switches more evenly, than in NPC inverter. Obviously, the losses of a switching device depend on its position in the topology. Losses distribution among the components in ANPC topology allows reducing the stress on certain switches, thus losses are equally distributed among the semiconductors, however the efficiency of the inverters is the same. As a new contribution to earlier studies, the obtained models of SVM control, NPC and ANPC inverters have been built. Thus, this thesis can be used in further more complicated modelling of full-power converters for modern multi-megawatt wind energy conversion systems.

Thermal Stress in a Roll-to-Roll Printed Electronics Substrate

The Roll-to-Roll process makes it possible to print electronic products continuously onto a uniform substrate. Printing components on flexible surfaces can bring down the costs of simple electronic devices such as RFID tags, antennas and transistors. The possibility of quickly printing flexible electronic components opens up a wide array of novel products previously too expensive to produce on a large scale. Several different printing methods can be used in Roll-to-Roll printing, such as gravure, spray, offset, flexographic and others. Most of the methods can also be mixed in one production line. Most of them still require years of research to reach a significant commercial level. The research for this thesis was carried out at the Konkuk University Flexible Display Research Center (KU-FDRC) in Seoul, Korea. A system using Roll-to-Roll printing requires that the motion of the web can be controlled in every direction in order to align different layers of ink properly. Between printers the ink is dried with hot air. The effects of thermal expansion on the tension of the web are studied in this work, and a mathematical model was constructed on Matlab and Simulink. Simulations and experiments lead to the conclusion that the thermal expansion of the web has a great influence on the tension of the web. Also, experimental evidence was gained that the particular printing machine used for these experiments at KU-FDRC may have a problem in controlling the speeds of the cylinders which pull the web.

From genes to communities : stress tolerance in eelgrass (Zostera marina)

Shallow coastal areas are dynamic habitats that are affected by a variety of abiotic and biotic factors. In addition to the natural environmental stress, estuarine and coastal seagrass ecosystems are exposed to effects of climate change and other anthropogenic impacts. In this thesis the effect of different abiotic (shading stress, salinity and temperature) and biotic stressors (presence of co-occurring species) and different levels and combinations of stressors on the performance and survival of eelgrass (Zostera marina) was assessed. To investigate the importance of scale for stress responses, varying levels of biological organization (genotype, life stage, population and plant community) were studied in field and aquarium experiments. Light limitation, decreased salinity and increased temperature affected eelgrass performance negatively in papers I, II and III, respectively. While co-occurring plant species had no notable effect on eelgrass in paper IV, the presence of eelgrass increased the biomass of Potamogeton perfoliatus. The findings in papers II and III confirmed that more extreme levels of salinity and temperature had stronger impacts on plant performance compared to intermediate levels, but intermediate levels also had more severe effects on plants when they were exposed to several stressors, as illustrated in paper II. Thus, multiple stressors had negative synergetic effects. The results in papers I, II and III indicate that future changes in light climate, salinity and temperature can have serious impacts on eelgrass performance and survival. Stress responses were found to vary among genotypes, life stages and populations in papers I, II and III, respectively, emphasizing the importance of study scale. The results demonstrate that while stress in general affects seagrass productivity negatively, the severity of effects can vary substantially depending on the studied scale or level of biological organization. Eelgrass genotypes can differ in their stress and recovery processes, as observed in paper I. In paper II, eelgrass seedlings were less prone to abiotic stress compared to adult plants, but stress also decreased their survival considerably. This indicates that recruitment and re-colonization through seeds might be threatened in the future. Variation among population responses observed in paper III indicates that long-term local adaptation under differing selection pressures has caused divergence in salinity tolerance between Baltic eelgrass populations. This variability in stress tolerance observed in papers I and III suggests that some eelgrass genotypes and populations have a better capacity to adapt to changes and survive in a changing environment. Multiple stressors and biological level-specific responses demonstrate the uncertainty in predicting eelgrass responses in a changing environment. As eelgrass populations may differ in their stress tolerance both within and across regions, conservation strategies at both local and regional scales are urgently needed in order to ensure the survival of these important ecosystems.

The Role of Oxidative Stress in Environmental Responses of Fennoscandian Animals

All aerobic organisms have to deal with the toxicity of oxygen. Oxygen enables more efficient energy production compared to anaerobic respiration or fermentation, but at the same time reactive oxygen species (ROS) are being formed. ROS can also be produced by external factors such as UV-radiation and contamination. ROS can cause damage to biomolecules such as DNA, lipids and proteins and organisms try to keep the damage as small as possible by repairing biomolecules and metabolizing ROS. All ROS are not harmful, because they are used as signaling molecules. To cope against ROS organism have an antioxidant (AOX) system which consists both enzymatic and non-enzymatic AOX defense. Some AOX are produced by the organism itself and some are gained via diet. In this thesis I studied environmentally caused changes in the redox regulation of different wild vertebrate animals to gain knowledge on the temporal, spatial and pollution-derived-effects on the AOX systems. As study species I used barn swallow, ringed seal and the Baltic salmon. For the barn swallow the main interest was the seasonal fluctuation in the redox regulation and its connection to migration and breeding. The more contaminated ringed seals of the Baltic Sea were compared to seals from cleaner Svalbard to investigate whether they suffered from contaminant induced oxidative stress. The regional and temporal variation in redox regulation and regional variation in mRNA and protein expressions of Baltic salmon were studied to gain knowledge if the salmon from different areas are equally stressed. As a comparative aspect the redox responses of these different species were investigated to see which parts of the AOX system are substantial in which species. Certain parts of AOX system were connected to breeding and others to migration in barn swallows, there was also differences in biotransformation between birds caught from Africa and Finland. The Baltic ringed seal did not differ much from the seals from Svalbard, despite the difference in contaminant load. A possible explanation to this could be the enhanced AOX mechanisms against dive-associated oxidative stress in diving air-breathing animals, which also helps to cope with ROS derived from other sourses. The Baltic salmon from Gulf of Finland (GoF) showed higher activities in their AOX defense enzymes and more oxidative damage than fish from other areas. Also on mRNA and proteomic level, stress related metabolic changes were most profound in in the fish from GoF. Mainly my findings on species related differences followed the pattern of mammals showing highest activities and least damage and birds showing lower activities and most damage, fish being intermediate. In general, the glutathione recycling-related enzymes and the ratio of oxidized and reduced glutathione seemed to be the most affected parameters in all of the species.

STRESS AND INTERNATIONAL ENTREPRENEURHSIP: The dark side of international entrepreneurship

Entrepreneurship attracts people with high job satisfaction and financial independence. Unfortunately, being deceived by this image, people do not pay attention to side effects of entrepreneurship. This ignorance usually turns into devastating results for entrepreneur’s health and venture performance. Therefore, it is required to seek ways to avoid these situations. The interest of the study lies in understanding of stress influence on international entrepreneurs by considering stress as a negative side effect of international entrepreneurship. To cover the concept of entrepreneurial stress completely, the study was divided into three section presented by following research questions: 1. What are the antecedents of entrepreneurial stress? 2. What are the consequences of entrepreneurial stress? 3. What coping strategies are applied to address entrepreneurial stress? Systematic literature review has been chosen as scientific approach to answer above questions due to the reason that it enables to minimise inconsistencies of both concepts of international entrepreneurship and stress. This method has afforded an opportunity to distinguish such stress causes as role conflict, overload, and ambiguity. Additionally, the study has covered the notion of stress moderators. The author argues that entrepreneurial traits, venture environment, and social support can have influence on degree of stress perception. Further, it has been proven that unaddressed stress could lead to reduction of entrepreneur’s psychological and physiological health. It should be taken into consideration that degree of both job satisfaction and performance would depend on the level of perceived stress. The last part of study emphasises the coping strategies. The author argues that it is important for an international entrepreneur to comprehend his or her and others’ emotions in order to overcome negative consequences of stress. In addition, the author suggests that an international entrepreneur needs to practise job sharing to reduce the amount of work to be completed. Moreover, it is believed that job sharing can help to overcome work-family conflict that prevails among entrepreneurs. The author anticipates that results of study can be beneficial for entrepreneurs who aim to achieve great results.