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  • Loukil, Mohamed Sahbi, 1984- (författare)
  • Experimental and numerical studies of intralaminar cracking in high performance composites
  • 2013
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • The macroscopic failure of composite laminates subjected to tensile increasing load is preceded by initiation and evolution of several microdamage modes. The most common damage mode and the one examined in this thesis is intralaminar cracking in layers. Due to this kind of microdamage the laminate undergoes stiffness reduction when loaded in tension. For example, the elastic modulus in the loading direction and the corresponding Poisson’s ratio will decrease.The degradation of the elastic properties of these materials is caused by reduced stress in the damaged layer which is mainly due to two parameters: crack opening displacement (COD) and crack sliding displacement (CSD). At fixed applied load these parameters depend on the properties of the damaged and surrounding layers, on layer orientation and on thickness. When the number of cracks per unit length is high (high crack density in the layer) the COD and CSD are reduced because of to crack interaction.The main objective of the first paper is to investigate the effect of crack interaction on COD using FEM and to describe the identified dependence on crack density in a simple and accurate form by introducing an interaction function dependent on crack density. This interaction function together with COD of non-interactive crack gives accurate predictions of the damaged laminate properties. The application of this function to more complex laminate lay-ups is demonstrated. All these calculations are performed assuming that cracks are equidistant.However, the crack distribution in the damaged layer is very non-uniform, especially in the initial stage of multiple cracking. In the second paper, the earlier developed model for general symmetric laminates is generalized to account for non-uniform crack distribution. This model is used to calculate the axial modulus of cross-ply laminates with cracks in internal and surface layers. In parametric analysis the COD and CSD are calculated using FEM, considering the smallest versus the average crack spacing ratio as non-uniformity parameter. It is shown that assuming uniform distribution we obtain lower bond to elastic modulus. A “double-periodic” approach presented to calculate the COD of a crack in a non-uniform case as the average of two solutions for periodic crack systems is very accurate for cracks in internal layers, whereas for high crack density in surface layers it underestimates the modulus reduction.In the third paper, the thermo-elastic constants were calculated using shear lag models and variational models in a general calculation approach (GLOB-LOC) for symmetric laminates with transverse cracks in 90° layer. The comparison of these two models with FEM was presented for cross-ply and quasi-isotropic laminates.Using FEM, we assume linear elastic material with ideal crack geometry. Fiber bridging over the crack surface is possible which can affect COD and CSD. The only correct way to validate these assumptions is through experiments.The main objective of the fourth and the fifth paper is to measure these parameters for different laminate lay-ups in this way providing models with valuable information for validation of used assumptions and for defining limits of their application. In particular, the displacement field on the edge of a [90/0]s and [903/0]s carbon fiber/epoxy laminates specimens with multiple intralaminar cracks in the surface layer is studied. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry).
  • Lundgren, Nils, 1962- (författare)
  • Microwave sensors for scanning of sawn timber
  • 2007
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • A microwave imaging sensor that measures the signal transmitted through a board was investigated with respect to the ability to predict the distribution of moisture and density in sawn lumber. Furthermore, the response from the sensor was related to strength properties of the boards. Multivariate statistics was used to relate the measured variables to various properties. A finite element model based on X-ray computed tomography images was developed to describe the interactions between microwaves and wood. The model made it possible to simulate the response from the sensor under varying conditions. The results show that microwaves can be used for prediction of density and moisture content. They can also be used for prediction of strength properties, mainly from the correlation to density, but also from the influence on microwaves of structural variations in the wood. The finite element model is useful in the evaluation of microwave sensors for wood, drying equipment or other applications where electromagnetic waves interact with wood.
  • Bäckström, Mikael, 1963- (författare)
  • On Monitoring and Control of Machining Processes
  • 1999
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • The thesis presents several aspects related to the industrial and academic activities associated with monitoring and control of the machining process and machine tools A survey of the industrial situation identified some key factors for a successful implementation of monitoring and control techniques. Applicable, relatively simple, systems for cutting-process monitoring and adaptive control are available on the commercial market today but the degree of industrial utilisation of the technique is low because the systems are experienced as hard to operate and use, and are at the same time considered unreliable. In order to promote a higher degree of industrial utilisation more work has to be devoted raising the performance of the system solutions. An Integrated Supervisory Process Control (ISPC) concept is presented as an approach dealing with multi-purpose control requirements utilising the individual advantages of several sensors and modelling techniques. The ISPC system is supposed to operate as a conceptual integration of sensors, process models and different control modules. Two prototype systems, based on five-axis machining centres, for real-time control of the machining process are outlined. Different strategies for monitoring and control are developed, implemented and experimentally tried out, and the studies conducted demonstrate results that encourage further research work devoted to verification of the proposed control concept. The results obtained in the initial experiments utilising methods for process control and monitoring, such as double exponential smoothing technique (DES), multivariate monitoring applied to signal tracking and artificial neural networks applied to cutting torque control, reveal that the different techniques have potential to improve the performance of monitoring and control tasks, thus contributing to the creation of more sophisticated and reliable solutions in the ISPC concept.
  • Isaksson, Per (författare)
  • On crack growth under compressive stresses
  • 2001
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • This thesis concerns fractures subjected to compressive stresses. In the four papers appended fracture behavior in brittle as well as ductile materials is studied. In the first paper, an expression for the mode II stress intensity factor at a straight extended kink has been calculated under the condition that crack opening is suppressed during crack growth. The expression has been found as a function of the mode II stress intensity factor K2 at the parent crack, the direction and length of the kink, and the difference between the remote compressive normal stresses perpendicular to, and parallel with, the plane of the parent crack. Crack growth directions have been suggested based on the result. At a sufficiently high non-isotropic compressive normal stress, so that the crack remains closed, the crack will propagate along a curved path maximizing the mode II stress intensity factor. Only at an isotropic compressive normal stress will the crack continue straight ahead in its original plane without directional change. By analyzing experimental crack growth patterns in paper two, the conclusion is that crack paths experimentally observed indicate that mode II crack growth under compression in some brittle materials follow a propagation path described by a function y=gx^b. In fact, the agreement between the experiments and the propagation path prescribed by the model, in which b=3/2, is astonishingly good since b was found in the interval [1.43-1.58] in all the experiments studied. Further, the investigation of the curvature parameter g has revealed that g also agree with the simplified model, even though not as good as the exponent b. However, the experimentally observed g differs in general less than 15% from the theoretical value predicted by the analytical model discussed in paper I. In paper three, a directional crack growth criterion in a compressed elastic perfectly-plastic material is considered. A slip-line solution is derived for evaluation of the stresses at the crack tip, which considers hydrostatic pressure and friction between the crack surfaces. Based upon the slip-line solution a projection stress based model is discussed for prediction of the direction of initiated crack growth. The opening displacement of an extended kink has been examined in paper four, using a finite element procedure. The conclusion is that an over-critical pressure in the plastic zone surrounding the crack tip suppresses crack opening regardless the direction of crack growth. The only possibility seems to be shear mode crack growth, which occur straight ahead in the crack plane if the crack is assumed to follow the plane of maximum shear stress. At a sub-critical hydrostatic pressure, or lower friction between the crack surfaces, the crack can extend via a kink subjected to local opening mode. An expression for the critical value determining fracture mode has been found as a function of hydrostatic pressure and friction between the crack surfaces assuming the fracture process to be predominantly controlled by local tensile stresses at the crack tip. The crack growth directions predicted by the projection stress based criterion in paper three are comparable with the directions maximizing the opening displacement of an extended kink computed in paper four.
  • Johansson Falck, Marlene, et al. (författare)
  • Technology, Language and Thought : Extensions of Meaning in the English Lexicon
  • 2005
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • In this thesis, the relationship between technological innovation and the development of language and thought is analysed. For this purpose, three different fields of technology are investigated: 1) the steam engine, 2) electricity, and 3) motor vehicles, roads and ways. They have all either played an extremely important part in people’s lives, or they are still essential to us. The overall aim is to find out in what ways these inventions and discoveries have helped people to develop abstract thinking and given speakers of English new possibilities to express themselves. Questions being asked are a) if the correlations in experience between the inventions and other domains have motivated new conceptual mappings? b) if the experiences that they provide people with may be used to re-experience certain conceptual mappings, and hence make them more deeply entrenched in people’s minds? and c) if the uses of them as cognitive tools have resulted in meaning extension in the English lexicon? The study is based on metaphoric and metonymic phrases collected from a number of different dictionaries. In the material a large number of metaphorical and metonymic expressions including terms connected to the inventions and discoveries that are part of this thesis are found. As is clear from the expressions, the steam engine, electricity, motor vehicles, roads and ways have all provided us with ample tools for structuring our thoughts, and for conveying our thoughts to others. Primarily, it seems to be the different functions of the discoveries and inventions, or the effects that they have on other objects that have motivated the mappings. In addition to analysing the cognitive role of the inventions that are part of this thesis, some general conclusions concerning the relationship between language, thought and world are suggested.
  • Kristén, Lars (författare)
  • Possibilities offered by interventional sports programmes to children and adolescents with physical disabilities : an explorative and evaluative study
  • 2003
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Today, sport is a topic that interests and engages many people. However, pursuing sporting activities and being a member of a sports club is not a matter of course for children and adolescents with physical disabilities. The overall aim of this thesis is to study the possibilities offered by interventional sports programmes to children and adolescents with physical disabilities and to describe in what way they can be offered meaningful physical activities as well as active participation in sport clubs, with a view to improving their health and socialisation through sport. The thesis includes three descriptive and one evaluative study. The results of the thesis rest on a combination of methods: practical- empirical experiences based on theoretical reasoning, interviews in line with a method inspired by phenomenography, and data from questionnaires analysed by descriptive and analytical statistics. The results are also based on a variety of data sources i.e. children, young people and parents as well as theory triangulation i.e. socialisation and health. According to the findings, sports programmes offered possibilities to children and adolescents with physical disabilities for improving their health and socialisation. An important component of the sports programmes were sports clubs that offered meaningful physical activities as well as active participation. Guidelines for co-operation in and the organisation of sports programme for children and adolescents with physical disabilities were of great importance and should involve the families as well as sports organisations and local authorities. The children and adolescents regarded sport as a form of social arena to enhance their participation in society and as a means for achieving better health. They also stressed the importance of knowledge and experiencing nature as factors that promote an active and healthy lifestyle. The parents regarded sport as a form of health education and as a means for their children to achieve increased participation in society. The parents also stressed the fact that the learning process was important for empowering the children to influence their life situation. The findings also illustrate the importance of building co-operation between the habilitation centre, sports clubs and families of children with physical disabilities by means of a working team, as well as the importance of the sports movement opening up its activities to children and adolescents with functional disabilities and each sports club appointing a contact person with special responsibility for this target group. Continued external support was regarded as necessary in order to assist the sports clubs in implementing permanent and well-functioning activities in partnership with a personal sport and health counsellor, who advises and supports the children and adolescents to participate in physical activity.
  • Wennergren, Ann-Christine, 1957- (författare)
  • Dialogkompetens i skolans vardag : En aktionsforskningsstudie i hörselklassmiljö
  • 2007
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • In classes for hard-of-hearing pupils there are children and young people with different degrees of impaired hearing, all of whom need hearing aids and technical equipment inthe environment. The initial problem area of the present thesis concerned the pattern of communication in the classroom. Previous research has shown that such communication is strongly affected by the chosen technical solution. The research questions developed successively from the assumption that it is only the teachers that can change the learning environment. In order to achieve a combination of scientific results and improvement,this study has an action research study approach. The overall aim was to illuminate and describe dialogues between different actors in a national school improvement project. Actors in the context were teachers, their pupils and the researcher. The thesis is based on improvement work in which the researcher cooperated with fifty teachers from five schools located in different parts of Sweden. This work involved parallel processes of change, for the pupils in the classrooms and for the participating teachers. The thesis comprises three substudies, two of which are focused on the teachers’ dialogues and one on the pupils’ listening environments. In the last-mentioned substudy, 165 pupils were asked to draw and describe their best listening environment at school. The result showed that the listening role required different environmental conditions, such as a cleaned upsound environment, visual support, conversation rules and comfortable surroundings. A central conclusion was that pupils need to be offered opportunities to verbalise their individual needs in the school environment. The result further showed that the teachers worked in different ways to improve the learning environment and that they introduced structures to support the dialogue between pupils. This happened at the same time as the teachers were trying out tools for their own learning in order to take part in dialogues with colleagues based on confirmations and challenges. Tools used in the learning processes were a logbook, shadowing, facilitating and a net-based dialogue. The analyses showed explicit differences between using the tools and learning through them in the zone of proximal development. In their improvement processes the teachers depended on critical friends in order to be challenged as knowledge developers. Difficulties in giving a balanced response were evident from the net-based forum in which the teachers reported different attempts at change. The study has drawn on sociocultural perspectives on learning in which dialogue competence have been central to learning in the classroom and in the teachers’ occasional communities of practice. The results indicate that teachers, for their professional improvement, require critical friends in alternative forms of learning processes and that pupils as actors require alternative listening environments.
  • Barth, Henrik, 1971- (författare)
  • Barriers to growth and development in small firms
  • 2004
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • The purpose of the extended summary is to develop the understanding of the existence and reasons behind barriers to growth and development in small firms. More specifically, four questions are addressed: (1) What do we know about the term barrier, how can it be defined? (2) What types of barriers to growth and development have been identified? (3) How can barriers to growth and development be addressed in a framework? and (4) How do the results of the appended papers (Papers I-V) contribute to the research field of barriers to growth and development?A literature review was undertaken to answer the first two questions. The literature review was conducted in two phases. The conceptual and operational definition of barriers was addressed in the first phase. The results reveal diverse ways of focusing on barriers, which made it necessary to address similar terms, such as problem and causes of business failure in the second phase of the literature review. The different terms used in the papers of the literature review are discussed in order to determine whether or not the understanding of their content is coherent and conceptual. Based on the conceptual understanding and definition of barriers, the different aspects focused on in the literature were discussed, which led to the second question, What are the various aspects of barriers to growth and development?The knowledge gained from the literature review and the development of the understanding of the barrier concept led to the third question, Can barriers to growth and development be addressed in a framework? The discussion takes its starting point in the studies identified in the literature review and is related to some of the appended papers (Papers I and II). Similarities and differences between frameworks used in the papers identified in the literature review are discussed and, based on this knowledge, a framework for barriers to growth and development in small firms is presented. The framework distinguishes two types of barriers, namely internal/external barriers and tangible/intangible barriers.The knowledge available in the reviewed papers within the field of barriers to growth and development in small firms in combination with the results from the appended papers (Papers I-V) resulted in the fourth question. Part of the contribution of the appended papers concerns the operational definition, for example that barriers to innovation should be categorised as either occurring or preventing firms from innovating (Paper II), while other contributions concern specific types of barriers, for example the importance of managerial skills (Paper V). Overall, the contribution focuses on conditional factors, such as strategy and planning, organisation and management, and institution and policy factors, in relation to growth and development.
  • Broberg, Patrik, 1983- (författare)
  • Imaging and analysis methods for automated weld inspection
  • 2014
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • All welding processes can give rise to defects, which weakens the joint and can eventually lead to the failure of the welded structure. In order to inspect welds for detects, without affecting the usability of the product, non-destructive testing (NDT) is needed. NDT includes a wide range of different techniques, based on different physical principles, each with its advantages and disadvantages. The testing is often performed manually by a skilled operator and in many cases only as spot-checks. Today the trend in industry is to move towards thinner material, in order to save weight for cost and for environmental reasons. The need for inspection of a larger portion of welds therefore increases and there is an increasing demand for fully automated inspection, including both the mechanised testing and the automatic analysis of the result. Compared to manual inspection, an automated solution has advantages when it comes to speed, cost and reliability. A comparison of several NDT methods was therefore first performed in order to determine which methods have most potential for automated weld inspection. Automated analysis of NDT data poses several difficulties compared to manual data evaluation. It is often possible for an operator to detect defects even in noisy data, through experience and knowledge about the part being tested. Automatic analysis algorithms on the other hand suffer greatly from both random noise as well as indications that originate from geometrical variations. The solution to this problem is not always obvious. Some NDT techniques might not be suitable for automated inspection and will have to be replaced by other, better adapted methods. One such method that has been developed during this work is thermography for the detection of surface cracks. This technique offers several advantages, in terms of automation, compared to existing methods. Some techniques on the other hand cannot be easily replaced. Here the focus is instead to prepare the data for automated analysis, using various pre-processing algorithms, in order to reduce noise and remove indications from sources other than defects. One such method is ultrasonic testing, which has a good ability for detecting internal defects but suffers from noisy signals with low spatial resolution. Work was here done in order to separate indications from corners from other indications. This can also help to improve positioning of the data and thereby classification of defects. The problem of low resolution was handled by using a deconvolution algorithm in order to reduce the effect of the spread of the beam.The next step in an automated analysis system is to go beyond just detection and start characterising defects. Using knowledge of the physical principles behind the NDT method in question and how the properties of a defect affect the measurement, it is sometimes possible to develop methods for determining properties such as the size and shape of a defect. This kind of characterisation of a defect is often difficult to do in the raw data, and is therefore an area where automated analysis can go beyond what is possible for an operator during manual inspection. This was shown for flash thermography, where an analysis method was developed that could determine the size, shape and depth of a defect. Similarly for laser ultrasound, a method was developed for determining the size of a defect.
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