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Sökning: L4X0:1402 1544 > (2015-2019)

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361.
  • Zhuang, Linqi (författare)
  • Effects of Non-uniform Fiber Distribution on Fiber/matrix Interface Crack Propagation in Polymeric Composites
  • 2017
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Fiber/matrix interface cracking plays an important role in determining the final failureof unidirectional (UD) composites. When subjected to longitudinally tensile loading,fiber/matrix interface debonds originate from fiber breaks or initial defects propagatealong loading direction. Depending on the quality of fiber/matrix interface, debondscould keep growing longitudinally which leads to the degradation of compositestiffness or kink out of interface and connect with neighboring debonds or fiberbreaks that forms a so called critical fracture plane which leads to the final failure ofUD composite. For UD composite subjected to transversely tensile loading, theinitiation, growth and coalesce of arc-shape fiber/matrix interface debonds result inthe formation of macro-size transverse cracks, the propagation and multiplication ofthese transverse cracks, although would not directly lead to the final failure ofcomposite, could cause significant stiffness degradation of composite structures.In the presence thesis, the growth of a fiber/matrix interface debond of a UDcomposite with hexagonal fiber packing under longitudinal and transverse tensileloading was investigated numerically, with the special focus on the influence ofneighboring fibers. In the current study, energy release rate (ERR) is considered as thedriving force for the debond growth and was calculated based on J Integral andVirtual Crack Closure Technique (VCCT) using finite element software ANSSY.Papers A – C in the present thesis deal with the influence of neighboring fibers on theERR of a debond emanating from a fiber break under longitudinal loading condition.In longitudinal loading case, debond growth is mode II dominated. In paper A, anaxisymmetric model consisting 5 concentric cylinders that represent broken fiber withdebond, surrounding matrix, neighboring fibers, surrounding matrix and effectivecomposite was generated. It’s found that there are two stages of debond growth, thefirst stage is when debond length is short, the ERR decreases with increasing debondlength, and the presence of neighboring fibers significantly increase the ERR ofdebond. For relatively long debond, the debond growth is steady when ERR is almostconstant regardless of debond length. In steady state of debond growth, the presenceof neighboring fibers have little effect on the ERR. In papers B and C, a 3-D modelwas generated with broken fiber and its 6 nearest fibers in a hexagonal packed UDcomposite were modelled explicitly, surrounded by the homogenized composite.Based on the obtained results, it’s shown that ERR is varying along debond front, andhas its maximum at the circumferential location where the distance between two fibercenter is the smallest. This indicates that the debond front is not a circle. For steadystate debond, the presence of neighboring fibers have little effect on averaged ERR(averages of ERR along debond front). For short debond, the presences ofneighboring fibers increases the averaged ERR, and that increase is more significantwhen inter-fiber distance is the smallest. Paper D investigates the growth of afiber/matrix debond along fiber circumference under transverse loading. It’s foundthat debond growth in this case is mixed-mode, and both mode I and mode II ERRcomponents increase with increasing debond angle and then decreases. Debondgrowth is mode I dominated for small debond angle and then switch to mode IIdominated. The presence of neighboring fibers have an enhancement effect on debondgrowth up to certain small debond angle and then changes to a protective effect. InPaper E, the interaction between two arc-size debond under transverse loading isinvestigated. It’s found that when two debonds are close to each other, the interactionbetween two debond becomes much stronger, and that interaction leads to the increaseof ERR of each debond significantly, which facilitates further growth for bothdebond.
362.
  • Zrida, Hana (författare)
  • Composites with bundle mesostructure: Elastic properties and Damage
  • 2016
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Many types of composite materials are today used in various types of load carrying structures, due to their excellent strength and stiffness to weight ratio. Simplicity, reliability and low cost of the material processing are important factors affecting the final selection.With the textile reinforced composites, the cost-efficiency is reached by using dry preforms which are impregnated by resin infusion, resin transfer molding etc.; this have made a break-through and have been widely used. Textile composites with bundle meso-structure have been studied in this thesis for elastic properties and damage investigations. The first part of this thesis deals with elastic properties modeling for Non-crimp fabric (NCF) based composites for investigating the effect of meso-structure defects on mechanical properties degradation. The objective of the work is to formulate a model for the NCF composite mesostructure in an attempt to investigate the effect of the waviness on stiffness reduction. Moreover, the stiffness calculation methods for the complex geometry are explained and justified and finally, the different geometrical parameters changes are taken into consideration and included in the calculation.The damage initiation and development is presented is the second part, where woven fabric composites designated for high temperature application were investigated under severe thermal conditions to study their thermal stability and their resistance to thermal damage. The mechanical performance of the same composites was studied. The effect of aging was also investigated. 3D models were realized with Finite elements in order to explain the edge effect on the evolution of the cracks observed during the tensile tests. In addition, the differences and similarities in cracking in different layers were analysed using probabilistic approaches (a simple one as well as Monte Carlo simulations with Hashin’s and also shear lag model) and fracture mechanics arguments.
363.
  • Åkerfeldt, Pia (författare)
  • Additive Manufacturing of Ti-6Al-4V: Relationship between Microstructure, Defects and Mechanical Properties
  • 2016
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Additive manufacturing (AM) is a relatively new technology that is labelled to be innovative, disruptive, near-net shaping, enabling manufacturing of complex and customised products, for limitless number of applications, directly from the CAD model into real physical parts. For titanium alloys in aerospace applications, AM moreover stands for a reduced material cost, but also for large challenges when considering consistency and qualification of material properties and components in serial production. In the AM process the feedstock material is melted by a heat source that moves according to a building sequence defined by the CAD model. Layer-by-layer the material solidifies into the wanted shape and accordingly the microstructure forms,which determines the average mechanical properties of the manufactured component. However, even if the AM process seems to be very straight forward, the prediction of mechanical and metallurgical properties is complex, partly because of its building in layer nature which generates a complex thermal history dictating the mechanical properties, and partly because of the number of parameters involved during the AM process itself. The objective of the present work was to increase the fundamental understanding of the relationship between microstructure, defects and mechanicalproperties of AM:ed Ti-6Al-4V. Three AM techniques were investigated, namely laser metal-wire deposition (LMwD), electron beam melting (EBM), and gas tungsten arc welding (GTAW) wire feed AM, with the main focus on LMwD. The different techniques were evaluated with regard to microstructure and tensile and fatigue properties. In addition, the EBM Ti-6Al-4V was tested in a hydrogen atmosphere to simulate the working environment for a certain engine application. One of the core findings in the present work was that AM:ed Ti-6Al-4V exhibited a columnar microstructure with elongated prior beta grains growing through several layers following the temperature gradient direction in the built material. To cover the different characteristics of the columnar microstructure, the mechanical properties were evaluated in two orientations of the built Ti-6Al-4V. The mechanical properties, both static and dynamic, were found to be anisotropic, which was further evaluated indetail with respect to the microstructure evolution and defects generated by the AM process. Among the results, when different process conditions were tested, it was concluded that the thickness of the grain boundary alpha along the prior beta grain boundary did not influence the level of anisotropy. However, the prior beta grain boundary was observed to be the weakest microconstituent when the load was applied perpendicular to its prevalence in both tensile and LCF testing. In order to get a better understanding of how the columnar microstructure influences the fatigue properties, the fatigue crack propagation characteristics were investigated with respect to the columnar prior beta grains and crystal orientation. An extensive fractographic study was carried out on all tested specimens. Lack of fusion (LoF) defects were concluded to be the individually most detrimental type of defect to the material properties. The influence of the LoF defects was further concluded to be very dependent on its prevalence in relation to the loading direction; the largest impact on the fatigue life was observed when the LoF defect wasperpendicular to the loading direction. Finally, a part of the aim of the present work was to support the development of a microstructure model that will be implemented in a thermo-mechanical model when simulating AM of Ti-6Al-4V. In order to validate the material model developed, the alpha lath thickness and the fraction of grain boundary alpha were quantified atspecific locations in single and multiple bead walls of GTAW wire feed AM:ed Ti-6Al-4V and compared with the results of the simulated AM process of Ti-6Al-4V.
364.
  • Öberg, Maria, 1969- (författare)
  • Governance for sustainable development of major European transport corridors The Scandinavian-Mediterranean TEN-T core network corridor
  • 2017
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Intermodal road, rail, air and sea transport networks across the European Union (EU) calledTrans-European Networks for Transport (TEN-T) are currently being implemented. Keyelements, which should be implemented by 2030, are the most strategic TEN-T Core networkcorridors. These nine corridors are expected to enhance smoothness and sustainability oftransport across the EU, thereby enhancing cohesion between EU nations, and relate to non-EU nations. In the EU Regulation 1315/2013 a governance framework for implementation ofthe Core network corridors is outlined, which mainly involves appointment of a Europeancoordinator for each corridor, establishment of Corridor forums for consultations, andformulation of work plans and follow-up measures.This framework is now being implemented in practice. A recognised challenge in the processis engaging the numerous, diverse stakeholders. A possible solution is to introduceappropriate elements of complementary governance. Thus, in this doctoral research, suitablecomplementary governance to support the sustainable development of a Core networkcorridor was investigated. Complementary governance, a concept that was developed andrefined during the project, refers here to additional governance in relation to the governanceframework described for the EU Core network corridors in the EU Regulation 1315/2013.To address an identified gap in knowledge on the nature and utility of complementarygovernance a series of studies was carried out. The studies included a study of relevantacademic literature, studies of EU transport policy and implementation documents regardinggovernance and sustainability, a survey of views of Scandinavian-Mediterranean Corridorforum participants, and interviews with stakeholders in the Baltic Sea region. These studiescollectively constitute a case study, focusing on the Scandinavian-Mediterranean Corenetwork corridor, of the need for, and if a need is present the optimal design of,complementary governance in the process of sustainable development of a Core networkcorridor.The main findings of the studies are that the implementation process of the Core networkcorridors could benefit from complementary governance measures, mainly to increaseinclusiveness, which is crucial for a sustainable development. Complementary governancecould both deepen and broaden stakeholder awareness and involvement. This may haveseveral advantages, such as increasing possibilities for stakeholders to influence the processand benefit from the corridor, align activities, and promote cooperation and learning betweenstakeholders. Moreover, including stakeholders who are based outside the corridors mayimprove distributive aspects of the investments and reduce risks of focusing attention tooclosely on the corridors per se. However, broadening stakeholder inclusion also hasdisadvantages, as it increases the complexity of the process, which may impair itseffectiveness, and may lead to unrepresentative involvement due to differences instakeholders’ interests, administrative capacities and power. It also increases requirements forcoordination.The nature of complementary governance is multifaceted and related to diverse dimensions ofgovernance, such as processes, stakeholder participation and implementation documents. For complementary governance measures a flexible and task-specific multi-level design isemphasised, which may involve various types of engagement mechanisms. Further, thefindings showed a need to extend previously recognised types of public engagementmechanisms based on communication, consultation and participation in the decision-makingstage, by recognising another category, named stakeholder exchange. This category ischaracterised by stakeholder cooperation and collaboration in an implementation stage wheremain goals and policies already are in place.Areas for complementary governance in the implementation process of the Core networkcorridors were indicated in the interview study. They consisted of extended involvement ofprivate stakeholders, extended involvement of regional and local stakeholders, extendedinvolvement of stakeholders geographically external to the immediate corridor, and extendedinformation both generally and specifically (i.e. directed information to specific stakeholders).Several complementary governance measures that could potentially improve implementationof the Core network corridors were raised in the interviews, ranging from minor adjustment inthe existing process to entirely new solutions. These ideas correspond well to the indicatedareas and provide robust foundations for further discussion.Thus, findings from the research suggest several governance measures to systematicallyinvolve and engage diverse stakeholders in the Core network corridor implementationprocess. However, before introducing such measures, they should be assessed together withexisting governance measures, to avoid overlapping or contradictory initiatives, possiblyusing policy packaging approaches, where policies are combined and analysed together toachieve a desired overall outcome. Proposals for future research and practices are presented.
365.
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366.
  • Öqvist, Rikard, 1982- (författare)
  • Measurement and perception of sound insulation from 20 Hz between dwellings
  • 2017
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Population growth and urbanization are projected by the United Nations to add 2,5 billionpeople to the world’s urban population by 2050. We need to construct buildings in anunprecedented scale to meet global housing demand. Sustainable development is critical.Compared to traditional heavy constructions, lightweight wooden constructions are moreenvironmentally friendly and will play a key role in meeting future demands. However, thereare two major problems with lightweight constructions that need to be addressed: 1) Limitedlow frequency sound insulation and 2) Variations in sound insulation.Annoyance from walking sound tend to be higher in lightweight than in heavy constructionseven with the same measured sound insulation. The Swedish research program AkuLiteindicated that the correlation between measured sound insulation and annoyance wassignificantly improved by extending current evaluation methods from 50 Hz down to 20 Hz.Secondly, large variations in sound insulation between nominally identical lightweightconstructions are common, which leads to larger safety margins. By identifying and quantifyingunderlying causes, production costs can be minimized and the performance can be improved.The aim of the thesis is to develop a new evaluation method for impact sound insulation thatbetter correspond to rated annoyance, and to identify and control underlying causes forvariations in sound insulation. The thesis contains six papers.In Paper I and II, sound insulation measurements were carried out in a large number ofnominally identical rooms of two different industrially prefabricated lightweight woodenconstructions. The purpose was to assess and quantify the variations in impact and airbornesound insulation. In Paper I, 30 nominally identical apartments of a volume based system wasevaluated. The apartments on the highest floor achieved significantly better sound insulationdue to the extra weight on lower floors affecting the elastic connections between stories. InPaper II, 18 rooms of a cross-laminated timber system of plate elements were evaluated.Additionally, several potential parameters related to measurement uncertainty wereinvestigated.Paper III deals with measurement uncertainty. An empirical study of reverberation timemeasurements showed that current methods need to be improved, if sound insulationrequirements are to be extended to 20 Hz.Paper IV and V verified that the frequency range 20-50 Hz is important for walking soundannoyance, and that alternative frequency adaptation terms can improve the correlation betweenmeasured impact sound insulation and annoyance ratings. In Paper IV, the methodology was toperform extensive field measurements in apartment buildings of various construction types andto perform questionnaire surveys among the residents. In Paper V, the methodology was toevaluate annoyance based on binaural recordings of walking sound in a two-part listening test.In Paper VI, 70 measurements in a lightweight wooden system were evaluated to quantify thetotal variations in impact and airborne sound insulation from 20 and 50 Hz, respectively. It wasconcluded that the proposed metrics of impact sound insulation were primarily determined bythe impact sound level 20-40 Hz and that the measurement methods must be evaluatedthoroughly to avoid excessive safety margins.A new evaluation method for impact sound insulation from 25 Hz, that correspond to the ratedannoyance for both heavy and lightweight constructions is proposed. By using the proposedmethod and attending the specific causes for variations, the lightweight industry will be able todevelop improved multi-story dwellings with higher perceived acoustic quality.
367.
  • Östlund, Rickard (författare)
  • Microstructure based modelling of ductile fracture in quench-hardenable boron steel
  • 2015
  • Doktorsavhandling (övrigt vetenskapligt)abstract
    • Reduction of fuel consumption and emissions by vehicle weight minimization constitute a major driving force for the development of new materials and manufacturing processes in the automotive industry. Simultaneously formed and quenched boron steel components have higher strength to weight ratio than conventional mild steel components. Additionally, hot formed components can be tailored to have regions with lower strength and higher ductility, improving their crash performance. This is often realized via dierential in-die cooling rates, thus yielding a variable microstructure compositiongiving rise to distributed mechanical properties. Predicting the performance envelopes of these types of components poses some challenges in terms of constitutive modelling, due to the dierential material composition and mechanical properties. Moreover, fractureinitiation is often a limiting design factor. This thesis aims to contribute to the constitutive and ductile fracture modelling of quench-hardenable boron steels, with reference to microstructure composition and hence process history. Modelling techniques which in an approximate manner can estimate the eective material properties based on the properties of the constituents in combination with ductile fracture models are presented.Computational issues concerning numerical nite element modelling of material instabilities are also addressed, essentially via two dierent methods. Introducing a discretization dependent parameter in the constitutive description, or by kinematic enhancements with respect to the localization problem. Both aim to reduce mesh sensitivity and provide improved predictions of post-instability response with industrially relevant mesh sizes.Additionally, an experimental investigation on the ow and fracture properties of boron steel, with a comprehensive range of dierent microstructure compositions, is presented. A full-eld measurement technique enabled the direct evaluation of mechanical properties and fracture relevant data from tensile tests. These results have supported the establishment of models and enabled their calibration, and they provide further insight to the inuence of microstructure and processing conditions on the ductile fracture properties. Comparisons between simulations and experiments indicate that useful predictions of the overall hardening behaviour and fracture elongations can be obtained by the suggested microstructure based modelling approach.
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