| 1. |
- Hallström, Hadar, et al.
(författare)
-
Probiotic supplements and debridement of peri-implant mucositis : a randomized controlled trial
- 2015
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Ingår i: Acta Odontologica Scandinavica. - : Taylor & Francis. - 0001-6357 .- 1502-3850. ; 74:1, s. 60-66
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Tidskriftsartikel (refereegranskat)abstract
- Objective. The aim of this double-blind randomized placebo-controlled trial was to evaluate the effects of probiotic supplements in adjunct to conventional management of peri-implant mucositis.Materials and methods.Forty-nine adult patients with peri-implant mucositis were consecutively recruited after informed consent. After initial mechanical debridement and oral hygiene instructions, the patients received a topical oil application (active or placebo) followed by twice-daily intake of lozenges (active or placebo) for 3 months. The active products contained a mix of two strains of Lactobacillus reuteri. Patients were clinically monitored and sampled at baseline and after 1, 2, 4, 12 and 26 weeks. The clinical end-points were pocket-probing depth (PPD), plaque index (PI) and bleeding on probing (BOP). In addition, the subgingival microbiota was processed with checkerboard DNA-DNA hybridization and samples of gingival crevicular fluid (GCF) were analyzed for selected cytokines with the aid of multiplex immunoassays.Results.After 4 and 12 weeks, all clinical parameters were improved in both the test and the placebo group. PPD and BOP were significantly reduced compared with baseline (p < 0.05), but no significant differences were displayed between the groups. The clinical improvements persisted 3 months after the intervention. No major alterations of the subgingival microflora were disclosed and the levels of inflammatory mediators in GCF did not differ between the groups.Conclusions.Mechanical debridement and oral hygiene reinforcement resulted in clinical improvement of peri-implant mucositis and a reduction in cytokine levels. Probiotic supplements did not provide added benefit to placebo.
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| 2. |
- Wemming, Hannes, 1984-
(författare)
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Experiments and Modelling of Composite–Aluminium Bolted Joints
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aeronautical industry has always strived towards weight-efficient structures since they improve aircraft performance, enable more fuel or payload, reduce cost of manufacturing and operating the aircraft, and reduce the fuel consumption and emissions. Modern airframes are often built in materials with high strength-toweight and stiffness-to-weight ratio, such as aluminium or fibre composites. Fibre composite materials are becoming increasingly common, requiring bolted joints between composite and aluminium parts. Bolted joints are often weak points in the structure, and understanding their mechanical properties in terms of stiffness and strength is of great importance for designing aircraft that are safe to fly. The aim of the research presented in this licentiate thesis is to improve the understanding of shear-loaded, composite–aluminium, bolted joints in terms of structural compliance, deformation, load distribution, strength and failure, by performing experiments and developing simulation models that describe those experiments. Experiments are performed where the optical digital image correlation (DIC) technique is used to measure the deformation of the test specimen surface during quasistatic load application until specimen failure. The DIC method enables more detailed deformation measurements compared to traditional methods. Data processing algorithms for noise reduction of DIC measurement data are developed and the results are used in a novel way to detect beginning bearing damage within the material by observing the specimen surface. The experimental DIC data is used for adapting models, where the goal is to create a structural element that represents a fastener. A structural element is computationally efficient and suitable for implementation in large scale models of airframe structures in an industrial context. A model is proposed and the selection of parameters for this model are investigated. The parameters are identified by fitting to experimental data. This licentiate thesis is divided into two parts, where the first part gives an introduction and background to the research, while the second part consists of two appended papers.
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| 3. |
- Akbarpour, Sahar, 1980-
(författare)
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Enhanced Composite Joint Performance through Interlacement of Metal Inserts
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work in this thesis investigates bolted joints in fibre reinforced composites with particular focus on a novel insert concept. The concept is characterised by replacing all composite plies with stacked metal patches, locally around a bolt hole, so that they jointly form a solid metal reinforcement. An extensive experimental study is presented together with finite element analysis of the studied cases.Reinforcing bolt holes with high-strength metals improves the bearing load capacity of the composite laminates. True enhancement of the joint performance however requires that the open-hole tensile strength is improved as well. The work started with tests of pin-loaded and open-hole tensile specimens with inserts, and significant improvement of the bearing load capacity was found. The initial tests enabled more informed design, and insert configurations having sufficient open-hole tensile strength could thereby be manufactured and tested. In parallel, composite-metal joints were numerically modelled to simulate and analyse the mechanical performance of the joints and gain a better understanding of the governing damage mechanisms.The performance of the joints was eventually investigated by means of experiments on single-shear, single- and double-bolt specimens, with and without inserts. The allowable bolt distance and the influence from the bolt tightening torque were also examined.The initial samples had inserts of stainless steel. Later, specimens with titanium alloy inserts were also included in the test series. Various insert configurations were designed to study the effects of different features in the composite-metal bond lines. The numerical simulations of the composite--metal interfaces were performed with two types of models, one joining the two materials directly to each other, without modelling any adhesive film in between, and the other including an elastic representation of the adhesive layer. The experimental results were then used to support verification of the results from the simulations.The final assessment of the concept was performed on insert configurations designed either for pure tensile loading or for more general (bi-directional) loading conditions, and the bearing load capacity, open-hole tensile strength and the performance of bolted joints were compared for cases with different inserts. While higher bearing strength improvement was achieved when the holes were reinforced with inserts of stainless steel, reinforcement with inserts of titanium was even more successful since it improved virtually all studied aspects of the joints considerably.
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| 4. |
- Akbarpour, Sahar, et al.
(författare)
-
Enhancing the performance of bolted joints in composites by use of patched steel or titanium inserts
- 2021
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A new insert concept that interlaces metal inserts into composite laminates has earlier been shown to improve the relatively poor bearing strength of holes in fibre reinforced polymer composites, and it is here further and more thoroughly investigated. The concept was invented to increase the efficiency of joints with mechanical fasteners in composite materials and this work presents experiments on double bolt joints with inserts made of either stainless steel or a titanium (Ti) alloy. In particular the work compares different implementations of the insert concept by reinforcing one or two holes in double bolt joints, and the effect of using different metals in the inserts. Some complementary tests on pin-loaded specimens and open hole tensile specimens are also performed and compared, also with some results reported previously. Considerable improvements in the bearing load capacity, i.e. 50%-60% or 35%-45%, is attained. The open-hole tensile strength is also improved considerably (almost 30%) when the holes are reinforced with Ti inserts. The fact that the inserts can improve not only the bearing strength but also the performance in open-hole tension implies that the Ti inserts bring nothing but positive effects to the strength of the joints. The test results from single-shear double-bolt specimens with inserts at one hole showed improved strengths of 30% and 20% for specimens with steel and Ti inserts, respectively. Finally, an impressive strength improvement of 40-45% is achieved for single-shear double-bolt specimens having both holes reinforced with inserts of either steel or Ti.
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| 5. |
- Akbarpour, Sahar, et al.
(författare)
-
Enhancing the performance of bolted joints in composites by use of patched steel or titanium inserts
- 2021
-
Ingår i: Composite structures. - : Elsevier BV. - 0263-8223 .- 1879-1085. ; 275
-
Tidskriftsartikel (refereegranskat)abstract
- A new insert concept that interlaces metal inserts into composite laminates has earlier been shown to improve the relatively poor bearing strength of holes in fibre reinforced polymer composites, and it is here further and more thoroughly investigated. The concept was invented to increase the efficiency of joints with mechanical fasteners in composite materials and this work presents experiments on double-bolt joints with inserts made of either stainless steel or a titanium (Ti) alloy. In particular the work compares different implementations of the insert concept by reinforcing one or two holes in double bolt joints, and the effect of using different metals in the inserts. Some complementary tests on pin-loaded specimens and open hole tensile specimens are also performed and compared, partly with results that were reported previously. Considerable improvements of the bearing load capacity are attained, i.e. 50%-60% for steel and 35%-45% for Ti, compared to references. The open-hole tensile strength is also improved considerably (almost 30%) when the holes are reinforced with Ti inserts. The fact that the inserts can improve not only the bearing strength but also the performance in open-hole tension implies that the Ti inserts bring nothing but positive effects to the strength of the joints. The test results from single-shear double-bolt specimens with inserts at one hole showed improved strengths of 30% and 20% for specimens with steel and Ti inserts, respectively. Finally, an impressive strength improvement of 40-45% is achieved for single-shear double-bolt specimens having both holes reinforced with inserts of either steel or Ti.
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| 6. |
- Akbarpour, Sahar, et al.
(författare)
-
Experimental investigation and numerical analysis of multi-material interfaces related to a composite joint concept
- 2021
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- An insert concept for reinforcing bolt-holes with high strength metals was previously introduced by the authors, where inserts are anchored in composite laminates through interlacement of composite plies and thin metal patches. The resulting finger-joints must be strong enough to avoid composite-metal debonding happening before bearing failure at the bolt-hole. The strength of the composite-metal interfaces is thus crucial for successful implementation of the insert concept. The paper presents an experimental study investigating the strength of various interface geometries between a prepreg composite material and stainless steel or titanium alloy inserts. In addition to the experimental work, finite element simulations are performed to analyse the stresses at the interfaces. The results indicate that the stress concentrations at multi-material corner points govern the failure and that the strength can be enhanced by expedient design.
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| 7. |
- Akbarpour, Sahar, et al.
(författare)
-
Reinforcement around holes in composite materials by use of patched metal inserts
- 2019
-
Ingår i: Composite structures. - : Elsevier. - 0263-8223 .- 1879-1085. ; 225
-
Tidskriftsartikel (refereegranskat)abstract
- Metal inserts are sometimes used to improve the load carrying capacity of bolted joints in composite materials. In this paper a new concept is introduced where inserts are built during composite manufacturing by integrating stacked metal patches at locations where holes are to be made after consolidation. Initial tests and a parameter study enable more informed design, and specimens with improved stacked inserts are then produced and tested. The specimens with inserts show up to 60% strength improvement in pin-loaded tests. In addition to the experimental work, finite element analysis is performed to investigate the stress fields and the failure mechanisms. The model indicates that the singular stresses at the multi-material corner points are governing for the strength and give indications of the failure mechanisms. Some basic analytical estimates are also presented.
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| 8. |
- Akbarpour, Sahar, et al.
(författare)
-
Strength improvement of bolted joints in composite materials by use of patched metal inserts
- 2020
-
Ingår i: Composite structures. - : Elsevier BV. - 0263-8223 .- 1879-1085. ; 52
-
Tidskriftsartikel (refereegranskat)abstract
- Metal inserts are occasionally used to improve bearing load capacity of bolted joints in laminated composite materials. This paper investigates a new reinforcement concept where inserts are built by locally replacing composite plies with metal patches of various diameters, surrounding the holes. The inserts are built during composite manufacturing by alternately placing the metal patches through the thickness of the laminate at locations where holes are to be drilled after consolidation. An extensive experimental study including pin‐ loaded, open–hole tensile, and single‐shear testing of bolted specimens is presented. Considerable improve- ment of the bearing strength – 50‐60% – is attained for pin‐loaded specimens with inserts, demonstrating the potential of the reinforcement concept. The open–hole tensile tests show that the by‐pass strength can be maintained or even improved with up to 20% if the inserts are properly designed. Finally, the results from the single‐shear tests of bolted joints show more than 25% improvement in strength for reinforced single‐ and double‐bolt specimens. It is possible that the inserts would maintain clamping pressure over time, which could then almost double the imrovement (47%) for bolted joints.
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| 9. |
- Arnesen, Marcus, et al.
(författare)
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A comparative study of constitutive models for EPS foam under combined compression and shear impact loading for helmet applications
- 2024
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Ingår i: Results in Engineering (RINENG). - : Elsevier BV. - 2590-1230. ; 23
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Tidskriftsartikel (refereegranskat)abstract
- Virtual testing of helmets using finite element (FE) analysis can be a valuable tool during product development. Still, its usefulness is limited by the quality of the constitutive model of the energy-absorbing material, usually foam. Built-in constitutive models in commercial FE software are developed for traditional linear compression loading. However, modern oblique test methods load the foam in combined compression and shear. Therefore, we aim to evaluate to what extent built-in constitutive models in commercial FE software can represent Expanded Polystyrene (EPS) foam during combined compression and shear loading (CCSL). EPS foam is tested experimentally in a newly developed test rig for CCSL (V-test). The response is compared against the simulation using three different constitutive models available in LS-DYNA (M83, M126, and M181). The models are assessed by their ability to capture the correct response, focusing on how well the continuum models can capture the phenomenological events seen in the experiments. The results show that the models perform well in compression, as expected. However, we point out limitations in the shear response and significant limitations in the unloading response, both important for oblique helmet testing. Due to these limitations, we conclude that the existing models are inadequate for accurately simulating oblique helmet impacts. There is a clear need to develop and implement new constitutive models focused on capturing CCSL including the unloading. Additionally, frictional sliding was found to substantially influence the measured response in the V-test method. Minimizing interface sliding is therefore critical for isolating the material behavior.
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| 10. |
- Bull, Peter Hoaas, et al.
(författare)
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Curved sandwich beams with face-core debond subjected to bending moment
- 2004
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Ingår i: Journal of Sandwich Structures and Materials. - : SAGE Publications. - 1099-6362 .- 1530-7972. ; 6:2, s. 115-127
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Tidskriftsartikel (refereegranskat)abstract
- Curved sandwich beams subjected to opening bending moment are studied. Face-core debonds of varying size are introduced at the compressively loaded face sheet and the structural integrity is investigated. Analytical and finite element models are compared in order to identify the governing failure modes of the beams. A simple expression is presented as a tool for getting a quick estimate of the severity of an interface crack in a curved sandwich beam. Five different configurations of beams are tested experimentally in a custom made bending rig.
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| 11. |
- Bull, Peter Hoaas, et al.
(författare)
-
High-velocity and quasi-static impact of large sandwich panels
- 2004
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Ingår i: Journal of Sandwich Structures and Materials. - : Sage Publications. - 1099-6362 .- 1530-7972. ; 6:2, s. 97-113
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Tidskriftsartikel (refereegranskat)abstract
- An investigation of the response of sandwich structures subjected to impact velocities of virtually 0 m/s and approximately 1000 m/s is conducted. The higher velocity exceeds both the longitudinal and the transverse wave propagation velocities of the core material in the sandwich panels. The objective is to investigate the possibility to simulate the damage from ballistic impact of sandwich panels through quasi-static experiments. Panels are impacted using a 40 mm Bofors AA gun and, using a similar projectile, other panels are indented quasi-statically. Energy absorption is measured in both test series. After impact, the panels are tested in in-plane compression together with one undamaged panel for reference. Residual strength of impacted panels is analyzed by finite element analysis. It is shown that the damage from high-velocity impact is limited, and it is possible to regain most of the undamaged strength by repair.
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| 12. |
- Burvall, Anna, et al.
(författare)
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Phase-retrieval methods with applications in composite-material tomography
- 2013
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Ingår i: 11th International Conference On X-Ray Microscopy (XRM2012). - : Institute of Physics Publishing (IOPP). ; , s. 012015-
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Konferensbidrag (refereegranskat)abstract
- In-line phase-contrast x-ray imaging is emerging as a method for observing small details when the contrast in absorption x-ray imaging is low. It gives images with strong edge enhancement, and phase retrieval is necessary to obtain quantitative thickness information. In particular for tomography, clarity can be enhanced by phase retrieval, as here demonstrated on a 3D-weave reinforced composite material. Seven suitable phase-retrieval methods are identified and integrated into a single method, where each version is marked by variations in particular steps. The general method and its variations are outlined and a comparison shows which methods are most suitable in different situations.
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| 13. |
- Eckermann, Tomas, et al.
(författare)
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Non-conformance aspects of moulded composite materials and “corresponding” simulation models with 3D textile reinforcement
- 2024
-
Ingår i: Composites. Part A, Applied science and manufacturing. - : Elsevier BV. - 1359-835X .- 1878-5840. ; 186
-
Tidskriftsartikel (refereegranskat)abstract
- Composite materials with 3-dimensional (3D) reinforcement were manufactured and corresponding simulation models were created in parallel. The used simulation approach has earlier been shown to produce close to authentic geometrical representation of the yarn architecture in 3D reinforcement. It is shown that although the as-woven reinforcement pattern can be modelled quite reliably, significant distortion from the nominal fibre arrangement might take place later in manufacturing, primarily related to compression during moulding. Such effects have earlier received significant attention for composites with 2-dimensional reinforcement but not as much for their 3D counterparts. The yarns in the real and the simulated materials are studied and compared, and some of the discrepancies and the mechanisms behind are discussed. The distortions are partly attributed to the relatively sparse weave that allows yarns oriented in the through-thickness direction, in particular, to deviate from their original positions.
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| 14. |
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| 15. |
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| 16. |
- Edström, Kristina, et al.
(författare)
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Mini Workshop - Designing Project-Based Courses for Learning and Cost-Effective Teaching
- 2011
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Ingår i: 2011 Frontiers In Education Conference (FIE). - 9781612844695
-
Konferensbidrag (refereegranskat)abstract
- This workshop draws on experience in the international collaboration for engineering education reform, called the CDIO Initiative, where project-based learning is a key part of the concept. The purpose of project-based courses in engineering education is to provide environments where students can develop a deeper working knowledge of technical fundamentals together with the complex skills necessary for engineering practice, or in short: where students can become engineers. In this workshop, the learning perspective is emphasized, by identifying trade-offs where there are inherent tensions between learning outcomes and other factors in project-based courses (such as project goal, product performance, technical sophistication, teacher popularity, student satisfaction). A set of principles are derived for enhancing learning and teaching in project-based courses, using concrete examples to illustrate thought-provoking implications. Each principle aims to improve both student learning outcomes and cost-effectiveness of teaching. Together the principles constitute a framework for learning-driven course design. The aim is to challenge assumptions and common practices in project-based courses, and provoke fruitful discussion among participants.
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| 17. |
- Ekermann, Tomas, 1984-, et al.
(författare)
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Influence of surface layers on the tensile response of a composite material with 3D woven fibre reinforcement
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The differences between surface and interior parts of a composite material with 3D reinforcement is studied. It is done through experiments on composites with different thicknesses, and thereby varying ratios of interior and surface structure. The influence of the surface layers on the overall mechanical response under tensile loading is investigated and discussed. A finite element model of a meso-scale representative volume element of the material is built and used to relate measured properties to the internal reinforcement topology. Details of the reinforcement structure and the FE model are presented and discussed aiming to identify where in the composite material failure initiates, and how that is related to the surface/interior structure. It is shown that the properties of the surface and interior parts differ significantly. It is also found that failure in tensile loading initiates at the surfaces, that they are matrix dominated and can be well predicted by basic isotropic failure criteria.
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| 18. |
- Ekermann, Tomas, et al.
(författare)
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Mechanical Characterisation Of Composites With 3D-Woven Reinforcement
- 2015
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Ingår i: 20Th International Conference On Composite Materials. - : AALBORG UNIV PRESS.
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Konferensbidrag (refereegranskat)abstract
- Flat specimens made of carbon/epoxy composite material are manufactured by resin transfer moulding, using 3D-woven carbon fibre preforms with a grid of warp yarns interlaced with both horizontal and vertical wefts. The aim of the study is to bring more light to the coupling between the mechanical properties of the composite material and the internal fibre architecture of its 3D-woven reinforcement. Factors that are varied in the fibre architecture are the amount of fibres in the through-thickness reinforcement (vertical weft) and the warp's wavelength. Tensile, compressive, in-plane and out-of-plane shear and peel tests are performed for the mechanical characterisation. Tensile and compressive properties are found to decrease when the crimp of the warp yarns is increased, and even more so in compression. The in-plane shear strength is evaluated through use of a new test specimen, designed for the purpose. Results show that the strength is higher when the shear load is applied across the warp than across the weft, where the difference is attributed to varying fibre content in the two in-plane directions. The out-of-plane shear properties are compared through short beam shear tests and the inter-laminar shear strength (ILSS) is determined. It is shown that the ILSS increases with increasing yarn thickness in the vertical weft, which is intuitive. The peel strength is evaluated by the opening mode I interlaminar fracture toughness (G(Ic)) through double cantilever beam tests. It is shown that G(Ic) is greatly dependent on the amount of reinforcement in the vertical weft.
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| 19. |
- Ekermann, Tomas, 1984-, et al.
(författare)
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Non-conformance aspects of moulded compositematerials and “corresponding” simulation models with 3D textile reinforcement
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Composite materials with 3-dimensional (3D) reinforcement were manufactured and corresponding simulation models were created in parallel. The used simulation approach has earlier been shown to produce close to authentic geometrical representation of the yarn architecture in 3D reinforcement. It is shown that although the as-woven reinforcement pattern can be modelled quite reliably, significant distortion from the nominal fiber arrangement might take place later in manufacturing, primarily related to compression during moulding. Such effects have earlier received significant attention for composites with 2-dimensional reinforcement but not as much for their 3D counterparts. The yarns in the real and the simulated materials are studied and compared, and some of the discrepancies and the mechanisms behind are discussed. The distortions are partly attributed to the relatively sparse weave that allows vertical yarns, in particular, to deviate from their original positions.
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| 20. |
- Ekermann, Tomas, et al.
(författare)
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Pull-off tests of CFRP T-joints with conventional and 3D reinforced fillets
- 2019
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Ingår i: Composite structures. - : ELSEVIER SCI LTD. - 0263-8223 .- 1879-1085. ; 223
-
Tidskriftsartikel (refereegranskat)abstract
- A study of T-joints made of CFRP prepreg is presented where the joints contain either conventional uni-directional (UD) fillets or fillets with three-dimensional (3D) woven reinforcement in the joint cavity. Both pristine and impacted specimens are tested experimentally in a pull-off load case. The T-joints with UD fillets are stronger but also show greater spread in strength than T-joints with 3D fillets. The higher strength is attributed to the UD fillets' ability to deform transversely to their length direction and efficiently adapt to the T-joint cavity before curing. The 3D fillets do not admit the same level of transverse shape adaptability and if their cross sections do not fit the geometry of the T-joint cavity sufficiently well, local stress concentrations could emerge that reduce the strength of the T-joint. The UD fillets on the other hand are believed to be sensitive to manufacturing flaws causing the greater spread in strength. That in turn is attributed to a lack of crack-arresting capability in the UD fillet. The 3D fillets however have excellent crack-arresting properties due to their multidirectional fibre architecture. With a few exceptions the impact damages did not significantly affect the strength of the T-joints tested in this study.
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| 21. |
- Ekermann, Tomas, 1984-
(författare)
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Testing and analysis of composites with 3D woven reinforcement
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Composites with three-dimensional (3D) reinforcement have several benefits, compared to laminated composites. Their through-thickness reinforcement increase the out-of-plane properties significantly and could eliminate problems with delaminations. Also, these composites have proven to have great damage tolerance and energy absorption. However, their complex yarn architectures make it challenging to predict their mechanical response and performance.In this thesis different aspects of composites with 3D woven reinforcement are explored. The focus is on a specific yarn architecture, called fully interlaced 3D weave. The results are however not only limited to that specific 3D reinforcement but could to a certain extent also be applicable to 3D reinforcement in general.Preforms with fully interlaced 3D weaves were manufactured and impregnated with epoxy. These were then examined in great detail with computer tomography (CT) to study the internal yarn architecture after impregnation. Analysis showed that the yarns were quite significantly distorted by the through-thickness compression during impregnation. The distortion was attributed to the relatively sparse weave, not supporting the through-thickness reinforcement, which therefore distorts and brings the rest of the yarns along with it. In parallel, a simulation model of the internal geometry of the manufactured material was developed. The simulation model was however not designed to include the distortions encountered in the physical material.The manufactured material and its corresponding model was tested in a tensile test setup. Two different thicknesses of the material was manufactured as well as a corresponding composite with two-dimensional (2D) reinforcement. Results showed that the material with 2D reinforcement was stiffer and stronger than the ones with 3D reinforcement, which was attributed to the lower crimp in the 2D reinforcement. A difference in stiffness between the two 3D weaves was also found and addressed to the larger amount of surface layer in the thinner weave, where vertical weft yarns are aligned with the warp direction and contributing to the overall stiffness in that direction. Failure analysis of specimens tested in the warp direction showed that initial cracks form in the boundaries of vertical weft yarns, close to the material surface. For specimens tested in the horizontal weft direction, initial cracks were found through the vertical weft yarns at the surfaces. Both these findings were supported by results from the simulation model.An application for composites with fully interlaced 3D weave was also explored, where it was integrated as a fillet in a composite T-joint. The scope here was to make a 3D reinforced fillet, having low transverse thermal expansion which would decrease the residual stresses in the fillet after curing. T-joints with conventional fillets and fillets with 3D woven reinforcement were manufactured and tested in a pull-off test. Results showed that T-joints with conventional fillets had higher strength, but also higher spread, than the ones with 3D reinforcement. The higher strength of T-joints with conventional fillets was attributed to their better ability to adapt to the T-joint cavity, while the fit was not as good for the 3D fillets. The lower spread in strength of the T-joints with 3D fillets was attributed to their lower sensitivity to minor flaws such as voids inside the fillet.
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| 22. |
- Franciscangelis, C., et al.
(författare)
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Vibration measurement on composite material with embedded optical fiber based on phase-OTDR
- 2017
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering Volume 10168, Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2017; 101683Q (2017). - : SPIE. - 9781510608214
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Konferensbidrag (refereegranskat)abstract
- Distributed sensors based on phase-optical time-domain reflectometry (phase-OTDR) are suitable for aircraft health monitoring due to electromagnetic interference immunity, small dimensions, low weight and flexibility. These features allow the fiber embedment into aircraft structures in a nearly non-intrusive way to measure vibrations along its length. The capability of measuring vibrations on avionics structures is of interest for what concerns the study of material fatigue or the occurrence of undesirable phenomena like flutter. In this work, we employed the phase-OTDR technique to measure vibrations ranging from some dozens of Hz to kHz in two layers of composite material board with embedded polyimide coating 0.24 numerical aperture single-mode optical fiber. © 2017 SPIE.
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| 23. |
- Grenestedt, Joakim L, et al.
(författare)
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Crack initiation from homogeneous and bimaterial corners
- 1997
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Ingår i: Journal of applied mechanics. - 0021-8936 .- 1528-9036. ; 64:4, s. 811-818
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Tidskriftsartikel (refereegranskat)abstract
- Fracture initiated at a corner between two different isotropic materials is considered. A "small" crack, well within the region dominated by the asymptotic stress fields of the non-cracked corner, is modelled and the stress intensities associated with the tip of the small crack are determined. Different criteria for the direction of crack propagation are studied.
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| 24. |
- Grenestedt, Joakim L, et al.
(författare)
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On cracks emanating from wedges in expanded PVC foam
- 1996
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Ingår i: Engineering Fracture Mechanics. - 0013-7944 .- 1873-7315. ; 54:4, s. 445-456
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Tidskriftsartikel (refereegranskat)abstract
- An experimental and analytical study was made on the effect of stress singularities on the strength of expanded PVC foam materials of different densities. Experiments were performed on specimens with different wedge geometries ranging from sharp cracks, with the ordinary inverted square root stress singularity, to shallow re-entrant corners with weak singularities. A brittle fracture criterion based on a generalised stress intensity factor, called Q, at the wedge tip was fit to experimental data. The critical stress intensity factor, Qcr, for crack initiation depends on the wedge geometry. This dependence was estimated from simple point-stress criteria and a criterion due to Seweryn [Brittle fracture criterion for structures with sharp notches. Engng Fracture Mech. 47, 673-681 (1994)], and good agreement with experimental data was obtained. When the point-stress criterion was applied to Mode II sharp cracks, poor agreement with published data was found. A critical study of the Mode II crack specimen was therefore initiated, leading to the conclusion that the commonly used specimen gives erroneous values of KIIc and the reason seems to be due to crack surface friction. A new Mode II crack specimen which eliminates crack surface friction was proposed and tested, and good agreement with the point-stress criterion was obtained. A criterion for homogeneous materials proved to be adequate also for the porous PVC foams.
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| 25. |
- Hallström, Hadar, et al.
(författare)
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Effect of probiotic lozenges on inflammatory reactions and oral biofilm during experimental gingivitis
- 2013
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Ingår i: Acta Odontologica Scandinavica. - : Informa UK Limited. - 0001-6357 .- 1502-3850. ; 71:3-4, s. 828-833
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Tidskriftsartikel (refereegranskat)abstract
- Aim. Probiotic bacteria have been introduced for prevention and treatment of periodontal diseases. The aim was to assess if daily oral administration of probiotic bacteria could influence the inflammatory response and the composition of supragingival plaque in an experimental gingivitis model. Materials and methods. Eighteen healthy female adults volunteered after informed consent. A double-blind randomized placebo-controlled cross-over design was used. The buccal surface of first molars was used as experimental sites. A mouth-guard covering the first premolar to second molar was used when brushing, preventing accidental cleaning during 3 weeks of plaque accumulation. Lozenges containing L. reuteri (ATCC55730 and ATCC PTA5289) or placebo were taken twice a day. During the run-in and washout periods, professional tooth cleaning was performed 5 days/week. At baseline and follow-up, plaque index, gingival index and bleeding on probing were recorded. Samples of gingival crevicular fluid (GCF) were analysed for concentration of seven inflammatory mediators. Bacterial samples were processed with checkerboard DNA/DNA-hybridization. Results. All subjects presented a local plaque accumulation and developed manifest gingivitis at the test sites during the intervention periods. The volume of GCF increased in both groups but was statistically significant only in the placebo group (p < 0.05). The concentrations of IL1-beta and IL-18 increased significantly (p < 0.05), while IL-8 and MIP1-beta decreased (p < 0.05). No differences were displayed between test and placebo. Likewise, the microbial composition did not differ between the groups. Conclusion. Daily intake of probiotic lozenges did not seem to significantly affect the plaque accumulation, inflammatory reaction or the composition of the biofilm during experimental gingivitis.
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| 26. |
- Hallström, Hadar, et al.
(författare)
-
Open flap debridement of peri-implantitis with or without adjunctive systemic antibiotics : A randomized clinical trial
- 2017
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Ingår i: Journal of Clinical Periodontology. - : John Wiley & Sons. - 0303-6979 .- 1600-051X. ; 44:12, s. 1285-1293
-
Tidskriftsartikel (refereegranskat)abstract
- Aims: To investigate clinical, radiographic and microbiological outcome over 12months following open flap debridement of peri-implantitis with or without antibiotics. Materials and methods: Peri-implantitis was surgically treated with or without Zithromax((R)) in 19 control and 20 test individuals. Probing pocket depth (PPD), gingival inflammation (BOP), intra-oral radiographs and microbial samples were studied. Per protocol and intent-to-treat analyses were performed. Results: The mean difference (reduction) in PPD values between baseline and month 12 in the test and control groups was 1.7mm (SD1.1, 95% CI: 1.1, 2.3, p<.001) and 1.6mm (SD +/- 1.5, 95% CI: 0.8, 2,4, p<.001), respectively. Data analysis failed to show study group differences for BOP, PPD, radiographic bone level and microbial load. Successful treatment (per protocol: PPD5mm, no BOP, no suppuration and no bone loss 0.5mm) at 12months in test and control groups was 7/15 (46.7%) and 4/16 (25.0%). Bacterial load reduction was similar in study groups with a temporary reduction following treatment. Conclusions: Surgical treatment of peri-implantitis with adjunctive systemic azithromycin did not provide 1-year clinical benefits in comparison with those only receiving open flap debridement.
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| 27. |
- Hallström, Hadar, et al.
(författare)
-
Probiotic supplements and debridement of peri-implant mucositis : A randomized controlled trial
- 2016
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Ingår i: Acta Odontologica Scandinavica. - : Informa Healthcare. - 0001-6357 .- 1502-3850. ; 74:1, s. 60-66
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Tidskriftsartikel (refereegranskat)abstract
- Objective. The aim of this double-blind randomized placebo-controlled trial was to evaluate the effects of probiotic supplements in adjunct to conventional management of peri-implant mucositis. Materials and methods. Forty-nine adult patients with peri-implant mucositis were consecutively recruited after informed consent. After initial mechanical debridement and oral hygiene instructions, the patients received a topical oil application (active or placebo) followed by twice-daily intake of lozenges (active or placebo) for 3 months. The active products contained a mix of two strains of Lactobacillus reuteri. Patients were clinically monitored and sampled at baseline and after 1, 2, 4, 12 and 26 weeks. The clinical end-points were pocket-probing depth (PPD), plaque index (PI) and bleeding on probing (BOP). In addition, the subgingival microbiota was processed with checkerboard DNA-DNA hybridization and samples of gingival crevicular fluid (GCF) were analyzed for selected cytokines with the aid of multiplex immunoassays. Results. After 4 and 12 weeks, all clinical parameters were improved in both the test and the placebo group. PPD and BOP were significantly reduced compared with baseline (p < 0.05), but no significant differences were displayed between the groups. The clinical improvements persisted 3 months after the intervention. No major alterations of the subgingival microflora were disclosed and the levels of inflammatory mediators in GCF did not differ between the groups. Conclusions. Mechanical debridement and oral hygiene reinforcement resulted in clinical improvement of peri-implant mucositis and a reduction in cytokine levels. Probiotic supplements did not provide added benefit to placebo.
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| 28. |
- Hallström, Hadar, et al.
(författare)
-
Reproducibility of subgingival bacterial samples from patients with peri-implant mucositis
- 2015
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Ingår i: Clinical Oral Investigations. - : Springer Science+Business Media B.V.. - 1432-6981 .- 1436-3771. ; 19:5, s. 1063-1068
-
Tidskriftsartikel (refereegranskat)abstract
- Objective The aim of the present study was to investigate the reproducibility of bacterial enumeration from subsequent subgingival samples collected from patients with peri-implant mucositis. Material and methods Duplicate microbial samples from 222 unique implant sites in 45 adult subjects were collected with paper points and analyzed using the checkerboard DNA-DNA hybridization technique. Whole genomic probes of 74 preselected bacterial species were used. Based on the bacterial scores, Cohen's kappa coefficient was used to calculate the inter-annotator agreement for categorical data. The percentage agreement was considered as "good" when the two samples showed the same score or differed by 1 to the power of 10. Results Moderate to fair kappa values were displayed for all bacterial species in the test panel (range 0.21-0.58). There were no significant differences between Gram-positive and Gram-negative species. The percentage of good agreement between the first and second samples averaged 74.7 % (n=74; range 56-83 %), while the proportion of poor agreement ranged from 1 to 19 % for the various strains. Conclusion While an acceptable clinical agreement was obtained in most cases, diverging bacterial scores may appear in subgingival samples collected at the same time point from patients with peri-implant mucositis. Clinical relevance The broad bulky base of implant crowns may present an obstacle for the collection of reproducible subgingival samples with paper points.
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| 29. |
- Hallström, Hadar, et al.
(författare)
-
Systemic antibiotics and debridement of peri-implant mucositis : a randomized clinical trial
- 2012
-
Ingår i: Journal of Clinical Periodontology. - 0303-6979 .- 1600-051X. ; 39:6, s. 574-581
-
Tidskriftsartikel (refereegranskat)abstract
- Background This RCT compared non-surgical treatment of peri-implant mucositis with or without systemic antibiotics. Materials and Methods Forty-eight subjects received non-surgical debridement with or without systemic Azithromax ® (4 days), and were followed during 6 months. The checkerboard DNA-DNA hybridization method was used to analyse the microbiological material. Results Five subjects were excluded due to antibiotic medication during follow-up. At baseline,1 and 3 months no group differences were found. Statistical analysis failed to demonstrate differences in probing pocket depths (PPD) values at 6 months (Mean diff PPD: 0.5 mm, SE: ±0.4 mm, 95% CI: −0.2, 1.3, p = 0.16). Mean% implant bleeding decreased between baseline and month 6 from 82.6% to 27.3% in the test, and from 80.0% to 47.5% in the control group (p < 0.02). Throughout the study, no study group differences in bacterial counts were found. Conclusion No short-term differences were found between study groups. The clinical improvements observed at 6 months may be attributed to improvements in oral hygiene. The present study does not provide evidence for the use of systemic antibiotics in treatment of peri-implant mucositis.
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| 30. |
- Hallström, Hadar, et al.
(författare)
-
Systemic antibiotics and debridement of peri-implant mucositis. A randomized clinical trial
- 2012
-
Ingår i: Journal of Clinical Periodontology. - : Wiley Blackwell. - 0303-6979 .- 1600-051X. ; 39:6, s. 574-581
-
Tidskriftsartikel (refereegranskat)abstract
- Background This RCT compared non-surgical treatment of peri-implant mucositis with or without systemic antibiotics. Materials and Methods Forty-eight subjects received non-surgical debridement with or without systemic Azithromax (R) (4 similar to days), and were followed during 6 similar to months. The checkerboard DNA-DNA hybridization method was used to analyse the microbiological material. Results Five subjects were excluded due to antibiotic medication during follow-up. At baseline,1 and 3 similar to months no group differences were found. Statistical analysis failed to demonstrate differences in probing pocket depths (PPD) values at 6 similar to months (Mean diff PPD: 0.5 similar to mm, SE: +/- 0.4 similar to mm, 95% CI: -0.2, 1.3, p similar to 0.16). Mean% implant bleeding decreased between baseline and month 6 from 82.6% to 27.3% in the test, and from 80.0% to 47.5% in the control group (p similar to 0.02). Throughout the study, no study group differences in bacterial counts were found. Conclusion No short-term differences were found between study groups. The clinical improvements observed at 6 similar to months may be attributed to improvements in oral hygiene. The present study does not provide evidence for the use of systemic antibiotics in treatment of peri-implant mucositis.
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| 31. |
- Hallström, Hadar, et al.
(författare)
-
Virus vid lokal aggresiv parodontit studeras
- 2009
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Ingår i: Tandläkartidningen. - 0039-6982. ; 101:3, s. 58-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Under det senaste decenniet har virus roll i parodontitsjukdomens uppkomst och progression studerats. Data har publicerats som visar att virus förekommer och eventuellt har betydelse för sjukdomsutvecklingen vid vissa typer av parodontit. Vi har därför undersökt förekomsten av bakterier och virus hos ett antal unga patienter med lokal aggressiv parodontit. Vår studie kan dock varken belägga eller avvisa hypotesen om virusförekomstens betydelse.
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| 32. |
- Hallström, Hadar, et al.
(författare)
-
Virus vid lokal aggresiv parodontit studeras
- 2009
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Ingår i: Tandläkartidningen. - 0039-6982. ; 101:3
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Under det senaste decenniet har virus roll i parodontitsjukdomens uppkomst och progression studerats. Data har publicerats som visar att virus förekommer och eventuellt har betydelse för sjukdomsutvecklingen vid vissa typer av parodontit. Vi har därför undersökt förekomsten av bakterier och virus hos ett antal unga patienter med lokal aggressiv parodontit. Vår studie kan dock varken belägga eller avvisa hypotesen om virusförekomstens betydelse.
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| 33. |
- Hallström, Stefan
(författare)
-
3D-textile reinforcement in composites- mechanics, modelling, pros and cons
- 2012
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Ingår i: ECCM 2012 - Composites at Venice, Proceedings of the 15th European Conference on Composite Materials. - : European Conference on Composite Materials, ECCM. - 9788888785332
-
Konferensbidrag (refereegranskat)abstract
- Fibre reinforced composite materials are quite rightfully accused of suffering from poor out-of-plane mechanical properties, and delamination concerns are known to strongly reduce their competitiveness in certain types of applications. Several methods have been invented and explored aiming at improving their 3D performance but none of them has yet received broad acceptance and utilisation in practical use. One of the reasons is likely that improved out-of-plane performance by means of through-thickness reinforcement inherently compromises the in-plane performance. The paper predominantly discusses 3D reinforcement from a general point of view but some more specific results from recent work on composites containing 3D-woven fibre reinforcement are also presented. Potential benefits and drawbacks of using 3D textiles in composites are discussed and partly quantified. The tradeoffs that become accentuated when introducing 3D reinforcement in composite materials can then be approached through use of more informed design principles.
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| 34. |
- Hallström, Stefan, 1968-, et al.
(författare)
-
Architecture and properties of stochastic foam models
- 2015
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Ingår i: 20th International Conference on Composite Materials.
-
Konferensbidrag (refereegranskat)abstract
- Recent results from numerical analysis of the microstructure and constitutive properties of foams are presented and relations between these properties and various topological differences in the cellular architecture are illustrated and discussed. The focus is on closed cell foams but parts of the work concern effects of distribution of solid between the cell walls and edges, which then also include open cell foams.The study is based on a modelling scheme where Voronoi partitioning is conducted, based on seed point distributions coming from random packings of hard spheres. The models are then brought to surface energy equilibrium through use of the Surface Evolver software. Sphere packing, partitioning of space and relaxation of the cellular models are conducted on periodic representative volume elements (RVE), typically containing in the order of 100 cells. Finite element models of the foam architecture is then performed with various sets of periodic boundary conditions in order to study the model topology, convergence rates and extract representative constitutive properties for the studied cases.The results are compared with other simulation results from the literature, experimental data and measurements made from SEM and CT micrographs. Some differences between evaluations of 2D and 3D representations of the foam materials are also illustrated and discussed.Aspects of the generation of the models are illustrated, indicating that some model results are virtually unaffected by most of the model details while other results depend strongly on the input parameters and differences between modelling techniques.Some references are also made to classical foam property estimates coming from dimension analysis of idealised foam structures where the relatively simplistic models in many cases provide surprisingly good estimates of the mechanical properties of the foam materials. There are however also some significant differences that are highlighted and explained in the presentation.
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| 35. |
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| 36. |
- Hallström, Stefan, 1968-, et al.
(författare)
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Failure analysis of laminated timber beams reinforced with glass fibre composites
- 1997
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Ingår i: Wood Science and Technology. - 0043-7719 .- 1432-5225. ; 31, s. 17-34
-
Tidskriftsartikel (refereegranskat)abstract
- A qualitative analysis is presented of failure, perpendicular to the grain, in laminated timber reinforced with a glass fibre composite. The study is focused on beams with holes of different shape. The stresses by corners, infinitesimal cracks and finite cracks are investigated. An initial crack model is suggested that brings about some of the phenomena observed in earlier performed experiments. A crack appears to propagate in the wood but is retarded in the reinforced beams. Eventually, the composite will fracture and failure of the beam follows. Finite element computations suggest that the reinforcement decreases the stress intensity at cracks in the wood and acts as a crack stopper. The reinforcing effect increases with the crack length. A point-stress criterion is used to predict failure in the fibre composite.
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| 37. |
- Hallström, Stefan, 1968-
(författare)
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Glass fibre reinforced holes in laminated timber
- 1996
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Ingår i: Wood Science and Technology. - 0043-7719 .- 1432-5225. ; 30, s. 323-337
-
Tidskriftsartikel (refereegranskat)abstract
- An experimental investigation of glass fibre reinforcement of glued laminated timber beams is presented. A polyester resin is used both as matrix and adhesive between the reinforcement and the wood. The main part of the work considers beams with large holes tested in three-point bending. Circular and rectangular holes, centred at quarter length of the beams make the strength of wood perpendicular to the grain become critical. Great improvements of strength are obtained with the glass fibres. A comparison between various kinds and combinations of glass fibre reinforcement is made. Further, the reinforcement applied as repair of earlier cracked beams is investigated with positive results. One series of beams without holes is reinforced and tested in four-point bending.
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| 38. |
- Hallström, Stefan, 1968-, et al.
(författare)
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Mixed mode fracture of cracks and wedge shaped notches in expanded PVC foam
- 1998
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Ingår i: International Journal of Fracture. - 0376-9429 .- 1573-2673. ; 88:4, s. 343-358
-
Tidskriftsartikel (refereegranskat)abstract
- Fracture initiated from a sharp crack or wedge shaped notch in a homogeneous material, subjected to different loading is considered. Singularities in the stress fields at edges and vertices are discussed. A point-stress criterion is used to predict fracture for sharp cracks as well as 90° wedge notches in expanded PVC foam. The point-stress criterion is formulated in a manner allowing failure predictions in general 3D stress situations. The influence of non-singular T-stress at cracks is discussed and substantiated by experimental results.
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| 39. |
- Hallström, Stefan, 1968-, et al.
(författare)
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Stochastic finite element models of foam materials
- 2005
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Ingår i: SANDWICH STRUCTURES7. - Berlin/Heidelberg : Springer-Verlag. - 140203444X ; , s. 935-943
-
Konferensbidrag (refereegranskat)abstract
- Driven by the desire to understand the influence of various mechanical properties and geometric features of foam materials, on the stiffness, failure mechanisms and fatigue life of sandwich materials, 3D finite element models of amorphous cellular structures have recently been developed and analyzed. Different algorithms were used to generate seed points of Voronoi tessellations, such as randomly distorted regular lattice distributions and totally random distributions. Periodic boundary conditions were used on representative volume elements containing hundreds of cells. Some comparison with experimental data from real foam materials was made and the agreement was found to be good. In the presented paper discrepancies between the used models and real foams are discussed and it is concluded that the adopted approach has substantial potential although there are some obvious routes for further improvement.
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| 40. |
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| 41. |
- Hylén, Mia, et al.
(författare)
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Access to health care perceived by parents caring for their child at home supported by eHealth—a directed approach introducing aperture
- 2022
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Ingår i: BMC Health Services Research. - : Springer Science and Business Media LLC. - 1472-6963. ; 22:1
-
Tidskriftsartikel (refereegranskat)abstract
- Background: In recent years a variety of eHealth solutions has been introduced to enhance efficiency and to empower patients, leading to a more accessible and equitable health care system. Within pediatric care eHealth has been advocated to reduce emergency and hospital outpatient visits, with many parents preferring eHealth to physical visits following the transition from hospital to home. Still, not many studies have focused on access from the parental perspective. Therefore, the aim of the study was to analyze access to health care as perceived by parents when caring for their child at home, with conventional care supported by eHealth following pediatric surgery or preterm birth. Methods: Twenty-five parents who went home with their child following hospitalization and received conventional care supported by eHealth (a tablet) were interviewed in this qualitative study. Directed content analysis was used, guided by a framework for dimensions of access previously described as: approachability, acceptability, affordability, appropriateness, and availability. Results: All dimensions of access were present in the material with the dimensions of approachability, appropriateness and acceptability most frequently emphasized. The dimensions highlighted a strong acceptance of eHealth, which was perceived by the parents as beneficial, particularly access to communication with health care personnel familiar to them. The chat function of the tablet was often mentioned as positive. A new dimension was also identified: “aperture.” It is defined by the pathways by which communication is transmitted in cyberspace, and these pathways are not easily visualized for parents submitting information, therefore generating concerns. Conclusions: Parents generally experienced good access to the eHealth-supported health care. Describing access through its dimensions complemented previous descriptions of eHealth in pediatric care and gave new insights. As such, the new dimension of “aperture”, the indeterminate opening of pathways of communication reflecting the uncertainty of not comprehending cyberspace, could be further evaluated. The dimensional framework of access is recommended when evaluating eHealth in the future. Trial registration: ClinicalTrials.gov identifier: NCT04150120.
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| 42. |
- Hörberg, Erik, 1977-
(författare)
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An Experimental Investigation of Shape Distortions in Aerospace Composites
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Composite materials are increasingly used in primary structure of modern commercial aircraft. Its excellent material characteristics enables reduction of structural weight compared to traditional metal solutions and thereby offers reduction of fuel consumption and carbon dioxide (CO2) emissions. In the aerospace industry, carbon fibre reinforced plastics or CFRP is the most commonly used composite material, where the reinforcement is held together by a thermoset resin, often epoxy, referred to as the matrix.When manufacturing aircraft composite parts, the curing temperature is usually in-between 120°C to 180°C. As the constituents, i.e. fibre and matrix, have significantly different thermal expansion, the temperature difference from manufacturing of parts to assembly and in-service use results in shape distortions and/or development of residual stresses. With an increased size and complexity of structural parts used in modern aircraft, the development of efficient methods for shape distortion analysis are therefore becoming increasingly important. Shape distortions come from numerous sources and some of them like thermal expansion and chemical shrinkage during curing are fairly well studied and understood. The focus of this thesis is on less researched parameters such as the laminate bending stiffness and effects of moisture content.The bending stiffness of a laminate can be controlled by varying the thickness of the laminate, or by changing the layup sequence of individual plies. Paper A presents an experimental study on shape distortion were the effect of laminate bending stiffness is separated from that of the laminate thickness. The results show that it is possible to tailor the laminate layup in a way that is beneficial for in-plane loads, while still reducing the built-in stresses that occur in a composite component due to shape distortions.The second parameter investigated in this thesis is the laminate moisture content. Composite materials used in aircraft structures will be exposed to environmental effects such as varying temperatures and moisture. The exposure is seldom constant but varies over time, depending on seasonal change and geographical area of aircraft operation. In Paper B, the influence of laminate moisture content on shape distortions is experimentally investigated. It becomes clear that laminate moisture content has such a strong effect on shape distortions that it is important to control and predict for all composite structures.The results presented in this thesis show that both laminate bending stiffness and laminate moisture content have a great influence on shape distortions, and that further research and development is needed to improve the simulation methodology used within the aerospace industry. This is key to future cost-efficient production and assembly of large composite parts.
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| 43. |
- Hörberg, Erik, 1977-, et al.
(författare)
-
Moisture effect on shape distortions of curved prepreg laminates
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The influence from moisture content on shape distortion of curved thermoset composite laminates is investigated experimentally. The particular objects of study are L-shaped carbon/epoxy specimens with a quasi-isotropic layup and the thickness varying between 1 and 12 mm. The effect on the shape distortion is quantified by means of spring-in angle measurements vs. accelerated moisture uptake utilising a climate chamber at 90°C and 95% relative humidity. The results clearly show a strong dependence on spring-in angles from laminate moisture content – the effect is in fact in the same order of magnitude as the spring-in from thermal and chemical shrinkage during curing. Moisture does thus not only affect shape distortions but also has to be taken into consideration and be carefully controlled when assessing spring-in due to other parameters. Finally, a model based on the experimental results predicting the spring-in angle is presented.
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| 44. |
- Hörberg, Erik, 1977-, et al.
(författare)
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Moisture effect on shape distortions of curved quasi-isotropic prepreg composite laminates
- 2021
-
Ingår i: Composites. Part A, Applied science and manufacturing. - : Elsevier BV. - 1359-835X .- 1878-5840. ; 145
-
Tidskriftsartikel (refereegranskat)abstract
- The influence from moisture on shape distortion of curved thermoset composite laminates is investigated experimentally. The objects of study are L-shaped carbon/epoxy specimens with a quasi-isotropic layup and a thickness varying between 1 and 12 mm. The effect on the shape distortion is quantified by means of angle measurements vs. accelerated moisture uptake utilising a climate chamber at 90 ?C and 95% relative humidity. The results show a strong dependence from laminate moisture content ? the effect is in fact in the same order of magnitude as the spring-in from thermal and chemical shrinkage during curing. Moisture does thus not only affect the spring-in angle but also has to be taken into consideration, and be carefully controlled, when assessing shape distortions due to other parameters. Finally, a closed form expression based on the experimental results is presented, predicting how the bracket angle varies with the specimen thickness and moisture content.
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| 45. |
- Hörberg, Erik, 1977-, et al.
(författare)
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Thickness effect on spring-in of prepreg composite L-profiles – An experimental study
- 2019
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Ingår i: Composite structures. - : Elsevier. - 0263-8223 .- 1879-1085. ; 209, s. 499-507
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents the results and analysis of an experimental study of laminate thickness effects on the springin and shape distortion of thermoset composite L profiles. The primary objective is to achieve a broader understanding of how shape distortion is affected by laminate bending stiffness and part thickness of L-shaped laminates whose thickness varies between 1 and 12 mm. The larger thicknesses in particular have not received much attention in previous research. This work further aims at distinguishing the pure (geometrical) thickness effect from that of the coupled laminate bending stiffness by comparing laminates with different lay-ups. The work is performed on test specimens subjected to both a standard cure cycle and post-cure heat treatment at elevated temperatures. In parallel, finite element (FE) analysis is performed to evaluate if variation in the bending stiffness or the laminate thickness affects the predicted spring-in angle. The results clearly show springin dependence on laminate thickness and bending stiffness, whereas this dependence is not well predicted by the FE approaches. It is concluded that both effects exist and that shape distortions are more strongly related to bending stiffness than to laminate thickness.
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| 46. |
- Juntikka, Rickard, et al.
(författare)
-
Optimization of single skin surfaces for head injury prevention - a comparison of optima calculated for global versus local injury thresholds
- 2004
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Ingår i: International Journal of Crashworthiness. - : Informa UK Limited. - 1358-8265 .- 1754-2111. ; 9:4, s. 365-379
-
Tidskriftsartikel (refereegranskat)abstract
- This paper describes optimizations of material properties for a bonnet-like plate using finite element calculations and the Euro-NCAP pedestrian head impact test. Four different head models were used for the impact simulations, a Euro-NCAP dummy head, a Hybrid III dummy head and two biomechanical head models exhibiting different mechanical properties for the brain tissue. The objective function was to minimize the displacement of the bonnet plate while satisfying constraints on the head injury criterion (HIC), the resultant contact force and, for the human head models, the strain in the brain tissue. An investigation was also conducted of the kinematics of the head models during impact, evaluating the energy distribution and the apparent mass. The analysis gave at hand that optimization of the plate with respect to impact with the Euro-NCAP and Hybrid III head models reached substantially, different results compared to impact with the biomechanical head models. For the latter case, the stiffness of the brain tissue influenced which constraints were active in the final solution. The investigation of the kinematics at impact showed that a substantial portion of energy was confined within the brain during impact for the biomechanical head models. The apparent mass at impact coincided with the actual mass for the rigid dummy heads while for the human head models it was roughly the mass of the skull only.
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| 47. |
- Juntikka, Rickard, et al.
(författare)
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Selection of energy absorbing materials for automotive head impact countermeasures
- 2004
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Ingår i: Cellular polymers. - : SAGE Publications. - 0262-4893 .- 1478-2421. ; 23:5, s. 263-297
-
Tidskriftsartikel (refereegranskat)abstract
- Material candidates for energy absorption in head impact countermeasures for automotive applications are evaluated using both quasi-static and dynamic test methods. Ranking of different materials turns out to be difficult since the mechanical response of a material could vary considerably with temperature, especially for polymers. Twenty-eight selected materials, including foams, honeycombs and balsa wood are tested and evaluated. The materials are subjected to a sequence of tests in order to thin out the array systematically. Quasi-static uni-axial compression is used for initial mapping of the selected materials, followed by quasi-static shear and dynamic uni-axial compression. The quasi-static test results show that balsa wood has by far the highest energy absorption capacity per unit weight but the yield strength is too high to make it suitable for the current application. The subsequent dynamic compression tests are performed for strain rates between 56 s(-1) and 120 s(-1) (impact velocities between 1.4 and 3 m/s) and temperatures in the range -20 - 60 degreesC. The test results emphasize the necessity of including both strain rate and temperature dependency to acquire reliable results from computer simulations of the selected materials.
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| 48. |
- Juntikka, Rickard, et al.
(författare)
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Shear characterization of sandwich core materials using four-point bending
- 2007
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Ingår i: Journal of Sandwich Structures and Materials. - : SAGE Publications. - 1099-6362 .- 1530-7972. ; 9:1, s. 67-94
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Tidskriftsartikel (refereegranskat)abstract
- A new shear test method for sandwich core materials is proposed and evaluated. Sandwich beams are loaded in four-point bending, and the shear deformation is measured with two rotary sensors. Conditions of idealized sandwich theory are assumed to prevail, and the accuracy of the proposed methodology is thus dependent on a few mechanical and geometric relations between the sandwich constituents. The stress-strain responses for two polymer foam core materials, one relatively brittle and one relatively ductile, are extracted and compared with results from single-block shear tests of the same material batch. The new method provides several benefits with respect to the block shear test. It does not suffer from extreme stress concentrations and the specimens are tested under in-service conditions. Problems arise, however, for the ductile material, predominantly related to large deformations during the test eventually resulting in bending failure of the face sheet instead of shear failure of the core.
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| 49. |
- Juntikka, Rickard, et al.
(författare)
-
Weight-balanced drop test method for characterization of dynamic properties of cellular materials
- 2004
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Ingår i: International Journal of Impact Engineering. - 0734-743X .- 1879-3509. ; 30:5, s. 541-554
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Tidskriftsartikel (refereegranskat)abstract
- A novel weight-balanced drop rig used to evaluate the response of cellular materials subject to dynamic compression is presented. The testing method utilizes approximately constant velocity throughout the major part of the compression phase and the results compare well with results from other methods, reported in the literature. The repetitiveness is excellent, the rig is simple and the results are easily extracted. The applicability of the method for determination of elastic modulus is however limited to materials with relatively low stiffness. Accurate modulus measurements for stiff materials at high strain-rates require a very rigid and lightweight test set-up.
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| 50. |
- Kapur, Rick, et al.
(författare)
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Gastrointestinal microbiota contributes to the development of murine transfusion-related acute lung injury
- 2018
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Ingår i: Blood Advances. - : American Society of Hematology. - 2473-9529 .- 2473-9537. ; 2:13, s. 1651-1663
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Tidskriftsartikel (refereegranskat)abstract
- Transfusion-related acute lung injury (TRALI) is a syndrome of respiratory distress upon blood transfusion and is the leading cause of transfusion-related fatalities. Whether the gut microbiota plays any role in the development of TRALI is currently unknown. We observed that untreated barrier-free (BF) mice suffered from severe antibody-mediated acute lung injury, whereas the more sterile housed specific pathogen-free (SPF) mice and gut flora-depleted BF mice were both protected from lung injury. The prevention of TRALI in the SPF mice and gut flora-depleted BF mice was associated with decreased plasma macrophage inflammatory protein-2 levels as well as decreased pulmonary neutrophil accumulation. DNA sequencing of amplicons of the 16S ribosomal RNA gene revealed a varying gastrointestinal bacterial composition between BF and SPF mice. BF fecal matter transferred into SPF mice significantly restored TRALI susceptibility in SPF mice. These data reveal a link between the gut flora composition and the development of antibody-mediated TRALI in mice. Assessment of gut microbial composition may help in TRALI risk assessment before transfusion.
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