Stefan Hallström - Academia.edu (original) (raw)
Papers by Stefan Hallström
Acta Materialia, Jul 1, 2016
Stochastic equilibrium finite element (FE) foam models are used to study the influence of relativ... more Stochastic equilibrium finite element (FE) foam models are used to study the influence of relative density and distribution of solid material between cell walls and edges on the elastic properties of foam materials. It is first established that the models contain a sufficient number of cells to ascertain isotropy and numerically and statistically robust results. It is then found that the elastic moduli are very weakly coupled to cell size variation in the models, when the latter is varied systematically. The influence from relative density and distribution of solid on the elastic parameters is considerably stronger. Analytical estimates from the literature, based on idealized cell models and dimension analysis, are matched by fitting coefficients to the FE results, providing good qualitative but relatively poor quantitative correlation. An expansion of the analytical coupling functions is then suggested in order to reduce their level of idealization. The expanded formulation shows virtually perfect agreement with the numerical results for almost the whole range of relative densities and distributions of solid in the FE parameter study. The presented analytical expression is believed to be general and provide accurate estimates of the elastic properties of a wide range of foam materials, provided that their bulk material properties and micro structure can be established.
The high specific bending stiffness of sandwich structures can with advantage be used in vehicles... more The high specific bending stiffness of sandwich structures can with advantage be used in vehicles to reduce their weight and thereby potentially also their fuel consumption. However, the structure must not only meet the in-service requirements but also provide sufficient protection of the vehicle passengers in a crash situation. The in-plane compressive response of sandwich panels is investigated in this thesis, with the objective to develop a methodology capable of determining if the structural response is likely to be favourable in an energy absorption perspective. Experiments were conducted to identify possible initial failure and collapse modes. The initial failure modes of sandwich panels compressed quasi-statically in the in-plane direction were identified as global buckling, local buckling (wrinkling) and face sheet fracture. Global buckling promotes continued folding of the structure when compressed beyond failure initiation. Face sheet fracture and wrinkling can promote collapse in the form of unstable debond crack growth, stable end-crushing or ductile in-plane shear collapse. Both the unstable debond crack propagation and the stable end-crushing are related to debond crack propagation, whereas the ductile in-plane shear mode is related to microbuckling of the face sheets. The collapse behaviour of sandwich configurations initially failing due to wrinkling or face sheet fracture was investigated, using a finite element model. The model was used to determine if the panels were likely to collapse in unstable debond propagation or in a more stable end-crushing mode, promoting high energy absorption. The collapse behaviour is mainly governed by the relation between the fracture toughness of the core and the bending stiffness and strength of the face sheets. The model was successfully used to design sandwich panels for different collapse behaviour. The proposed method could therefore be used in the design process of sandwich panels subjected to in-plane compressive loads.During a crash situation the accelerations on passengers must be kept below life threatening levels. The extreme peak loads in the structure must therefore be limited. This can be achieved by different kind of triggering features.Panels with either chamfered face sheets or with grooves on the loaded edges were investigated in this thesis. The peak load was reduced with panels incorporating either of the two triggering features. Another positive effect was that the plateau load following failure initiation was increased by the triggers. This clearly illustrates that triggers can be used to promote favourable response in sandwich panels. Vehicles are harmful to the environment not only during in-serve use, but during their entire life-cycle. By use of renewable materials the impact on the environment can be reduced. The in-plane compressive response of bio-based sandwich panels was therefore investigated. Panels with hemp fibre laminates showed potential for high energy absorption and panels with a balsa wood core behaved particular well. The ductile in-plane shear collapse mode of these panels resulted in the highest energy absorption of all investigated sandwich configurations.
Journal of physics, Oct 25, 2013
In-line phase-contrast x-ray imaging is emerging as a method for observing small details when the... more 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.
Composite Structures, Oct 1, 2010
The influence of triggering topologies on the peak load and energy absorption of sandwich panels ... more The influence of triggering topologies on the peak load and energy absorption of sandwich panels loaded in in-plane compression is investigated. Sandwich panels with different geometrical triggering features are manufactured and tested experimentally. Damage initiation in panels with grooves is investigated using finite element models.As expected the investigated triggering features reduce the extreme load peaks. A less expected result is that the plateau load following peak load tends to be higher for panels with triggering features. Both results are judged favourable for crash performance of panels in vehicle applications. Analysis suggests that there is a transition in failure mode for the studied panels, where the peak load for panels containing no or few grooves seems to be governed by principles of fracture mechanics while for panels with a high number of grooves it appears to be limited by the average stress.
Composite Structures, Sep 1, 2012
Composite materials reinforced with three-dimensionally (3D) woven carbon fibre textiles are inve... more Composite materials reinforced with three-dimensionally (3D) woven carbon fibre textiles are investigated and the challenge and the driver for the work is to generate numerical models to predict the mechanical behaviour of these composites. The result of the final modelling stage is near authentic finite element (FE) models of representative volume elements (RVEs) of the composites. They are created by using only a small number of input parameters, such as the size of the RVE, the number of yarns and their mutual interlacing, and the yarn crimp. The FE models may then be utilised for various purposes but are here used to derive homogenised elastic mechanical properties of 3D reinforced composite materials. The correlation between the models and experiments is good, both in terms of details in the architecture and mechanical properties. There are however some deviations that could be explained by the models being more regular than the real material.
Thermoplastic composite materials have some advantages over thermoset resin based composites, suc... more Thermoplastic composite materials have some advantages over thermoset resin based composites, such as fast processing, recyclability, reparability etc. In the present study commingled Carbon/Polyamide 12 (PA12) is analyzed as dry yarn and after heating the matrix fibers above the melting temperature and applying ambient pressure. The results are presented as micrographs. The mode I interlaminar fracture toughness of Carbon PA12 composite materials is measured, with and without introducing through-thickness yarns, and it is found to be very high even without 3D reinforcement. The consolidation time is also found to have great effect on the measured fracture toughness.
In the strive towards reduced fuel consumption and lower emissions, low structural weight is beco... more In the strive towards reduced fuel consumption and lower emissions, low structural weight is becoming a key factor in the design of advanced vehicle and aerospace structures. Whereas most traditional construction materials are seemingly reaching their limitations, composite materials with their high specific properties offer possibilities to further reduce weight. In high demand structural applications, the quality of the composite material is of utmost importance, requiring the material to be void free and the matrix well distributed as a binder for the load carrying reinforcement. To achieve proper wetting of the fibres, knowledge of the flow resistance of the porous fibre reinforcement is required. It is normally expressed in terms of permeability.Fibre reinforcements in composite materials are normally regarded as a heterogeneous porous media since both fabric and tows are porous but at different length scales. In order to numerically compute the permeability of such media, one of following two approaches can be used. Either filaments are added one-by-one into the modelled geometry (resolved model) or the tows are considered as porous homogenised media. In the latter case expression for the intra-tow permeability is needed.In this thesis, a porous homogenised tow model is benchmarked with a resolved model to the level of refinement possible without being too expensive computationally. Based on this approach, the permeability of complex three- dimensional (3D) textiles is computed utilizing computational fluid dynamics (CFD) analysis. The effect of inter- and intra-tow porosity on the overall permeability of 2D and 3D structures is analysed and discussed in relation to contradictions found in past studies. A clearer picture of the problem is presented, which will be helpful in future modelling and understanding of the permeability of complex structures. In an experimental study, the overall fibre volume fraction as well as the tow compaction are varied and their influence on the permeability is measured. Experimental studies show good agreement with numerical simulations.The interlaminar shear strength of thermoplastic composite materials is studied and the influence of specimen size is examined. Using finite element (FE) analysis it is shown that size effects may be partly due to statistical effects and partly due to the higher number of composite layers in thicker specimens.The effect of processing on the interlaminar delamination toughness of car-bon/polyamide 12 (C/PA12) is studied. It is observed that processing conditions have vital effect on the interlaminar delamination of thermoplastic composites. The mode I crack energy release rate (GIc) of C/PA12 is found to be 15 times higher than for conventional thermoset based composites and 1.5 times higher than for a thermoset composite with stitched reinforcement through the thickness. The best performing C/PA12 composite is manufactured in a hydraulic press equipped with a cold tool, thereby showing potential for both cost and time efficient manufacturing.
Proceedings of SPIE, Apr 12, 2017
Distributed sensors based on phase-optical time-domain reflectometry (phase-OTDR) are suitable fo... more 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.
Key Engineering Materials, Jul 1, 2019
Lightweight and strong composite material beams are increasingly sought to quickly, easily, and c... more Lightweight and strong composite material beams are increasingly sought to quickly, easily, and cost-effectively transport and setup a variety of constructions such as bridges, cabins/ stores/shelters, vehicles etc. For structural beams produced as conventional laminated composite materials, their weak areas tend to occur at intersections such as web-flange junctions due to absence of fibres bridging the interconnections. This drawback can however be overcome with development of profiled 3D textile reinforcements having combination architectures and constituent web-flange parts inherently mutually interconnected through fibre interlacement. In addition to general strength improvement, beams containing such novel reinforcement architectures also show increased energy absorption capability due to the mutual web-flange integration at the junctions. An ‘I’ and a ‘flanged-triangle’ cross-section beams were produced by a novel non-conventional weaving method, using carbon fibres as reinforcement, and their energy absorption capabilities were tested. These beams respectively absorbed over 50% and 300% more energy per weight in bending, compared to metal counterparts. This paper presents some relevant aspects of these innovative beams.
Mechanics of Materials, Sep 1, 2021
The use of 3D-woven composite materials has shown promising results. Along with weight-efficient ... more The use of 3D-woven composite materials has shown promising results. Along with weight-efficient stiffness and strength, they have demonstrated encouraging out of plane properties, damage tolerance and energy absorption capabilities. The widespread adoption of 3D-woven composites in industry however, requires the development of efficient computational models that can capture the material behaviour. The following work proposes a framework for modelling the mechanical response of 3D-woven composites on the macroscale. This flexible and thermodynamically consistent framework, decomposes the stress and strain tensors into two main parts motivated by the material architecture. The first is governed by the material behaviour along the reinforcement directions while the second is driven by shear behaviours. This division allows for the straightforward addition and modification of various inelastic phenomena observed in 3D-woven composites. In order to demonstrate the applicability of the framework, focus is given to predicting the material response of a 3D glass fibre reinforced epoxy composite. Prominent non-linearities are noted under shear loading and loading along the horizontal weft yarns. The behaviour under tensile loading along the weft yarns is captured using a Norton style viscoelasticity model. The non-linear shear response is introduced using a crystal plasticity inspired approach. Specifically, viscoelasticity is driven on localised slip planes defined by the material architecture. The viscous parameters are calibrated against experimental results and off axis tensile tests are used to validate the model.
Journal of Cellular Plastics, Jul 28, 2016
Stochastic foam models are generated from Voronoi spatial partitioning, using the centers of equi... more Stochastic foam models are generated from Voronoi spatial partitioning, using the centers of equi-sized hard spheres in random periodic distributions as seed points. Models with different levels of polydispersity are generated by varying the packing of the spheres. Subsequent relaxation is then performed with the Surface Evolver software which minimizes the surface area for better resemblance with real foam structures. The polydispersity of the Voronoi precursors is conserved when the models are converted into equilibrium models. The relation between the sphere packing fraction and the resulting degree of volumetric polydispersity is examined and the relations between the polydispersity and a number of associated morphology parameters are then investigated for both the Voronoi and the equilibrium models. Comparisons with data from real foams in the literature indicate that the used method is somewhat limited in terms of spread in cell volume but it provides a very controlled way of varying the foam morphology while keeping it periodic and truly stochastic. The study shows several strikingly consistent relations between the spread in cell volume and other geometric parameters, considering the stochastic nature of the models.
Composites Part A-applied Science and Manufacturing, Jun 1, 2021
The influence from moisture on shape distortion of curved thermoset composite laminates is invest... more 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 contentthe 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.
Integrated assessment of diciplinary and interpersonal skills - student perceptions of a novel le... more Integrated assessment of diciplinary and interpersonal skills - student perceptions of a novel learning experience
Journal of Cellular Plastics, Oct 23, 2013
Stochastic cellular models of rigid foam based on Voronoi spatial partitioning are generated and ... more Stochastic cellular models of rigid foam based on Voronoi spatial partitioning are generated and investigated for potential use in numerical analysis using finite element methods. Such partitions are deterministic once a distribution of cell nuclei has been defined. A drawback is that the models tend to exhibit a significant share of short edges and small faces. Such small geometrical features are not likely to occur in real foams since they are unfavorable from a surface energy point of view and they also generate problems in numerical analysis due to associated meshing challenges. Through minimization of the surface area, using the computer software Surface Evolver, the Voronoi models are brought to better resemblance with ideal dry foam and the occurrence of small geometrical features is strongly reduced. It is generally seen that different seed point distribution algorithms result in different model topologies. The presented methodology is systematic, parameterized and the results are very promising. Good grounds are provided for modeling of real rigid foam materials, that do not necessarily fully resemble ideal dry foam.
Composites Science and Technology, Sep 1, 2009
Fully interlaced 3D fabric is produced by a new weaving technology, and it is here utilised to pr... more Fully interlaced 3D fabric is produced by a new weaving technology, and it is here utilised to produce woven carbon fibre preforms, which are then used as reinforcement in composite materials. The purpose of this study is to assess the mechanical performance of this new type of composite material. A prototype loom was used to weave preforms with a rectangular cross section where all warp and weft yarns were fully interlaced in plain weave. Tensile, compressive, out-of-plane, shear and flexural properties of the composite flat-beam specimens were tested. The in-plane stiffness and strength were found to be lower, while the out-of-plane properties were higher compared to conventional 2D laminates. In terms of strength, it was not possible to quantify the difference, since the specimens with 3D woven material exhibited other failure modes than those tested for.
Composite Structures, Apr 1, 2012
ABSTRACT
In the strive towards reduced fuel consumption and lower emissions, low structural weight is beco... more In the strive towards reduced fuel consumption and lower emissions, low structural weight is becoming a key factor in the design of advanced vehicle and aerospace structures. Whereas most traditional construction materials are seemingly reaching their limitations, composite materials with their high specific properties offer possibilities to further reduce weight. In high demand structural applications, the quality of the composite material is of utmost importance, requiring the material to be void free and the matrix well distributed as a binder for the load carrying reinforcement. To achieve proper wetting of the fibres, knowledge of the flow resistance of the porous fibre reinforcement is required. It is normally expressed in terms of permeability.Fibre reinforcements in composite materials are normally regarded as a heterogeneous porous media since both fabric and tows are porous but at different length scales. In order to numerically compute the permeability of such media, one of following two approaches can be used. Either filaments are added one-by-one into the modelled geometry (resolved model) or the tows are considered as porous homogenised media. In the latter case expression for the intra-tow permeability is needed.In this thesis, a porous homogenised tow model is benchmarked with a resolved model to the level of refinement possible without being too expensive computationally. Based on this approach, the permeability of complex three- dimensional (3D) textiles is computed utilizing computational fluid dynamics (CFD) analysis. The effect of inter- and intra-tow porosity on the overall permeability of 2D and 3D structures is analysed and discussed in relation to contradictions found in past studies. A clearer picture of the problem is presented, which will be helpful in future modelling and understanding of the permeability of complex structures. In an experimental study, the overall fibre volume fraction as well as the tow compaction are varied and their influence on the permeability is measured. Experimental studies show good agreement with numerical simulations.The interlaminar shear strength of thermoplastic composite materials is studied and the influence of specimen size is examined. Using finite element (FE) analysis it is shown that size effects may be partly due to statistical effects and partly due to the higher number of composite layers in thicker specimens.The effect of processing on the interlaminar delamination toughness of car-bon/polyamide 12 (C/PA12) is studied. It is observed that processing conditions have vital effect on the interlaminar delamination of thermoplastic composites. The mode I crack energy release rate (GIc) of C/PA12 is found to be 15 times higher than for conventional thermoset based composites and 1.5 times higher than for a thermoset composite with stitched reinforcement through the thickness. The best performing C/PA12 composite is manufactured in a hydraulic press equipped with a cold tool, thereby showing potential for both cost and time efficient manufacturing.
Acta Materialia, Jul 1, 2016
Stochastic equilibrium finite element (FE) foam models are used to study the influence of relativ... more Stochastic equilibrium finite element (FE) foam models are used to study the influence of relative density and distribution of solid material between cell walls and edges on the elastic properties of foam materials. It is first established that the models contain a sufficient number of cells to ascertain isotropy and numerically and statistically robust results. It is then found that the elastic moduli are very weakly coupled to cell size variation in the models, when the latter is varied systematically. The influence from relative density and distribution of solid on the elastic parameters is considerably stronger. Analytical estimates from the literature, based on idealized cell models and dimension analysis, are matched by fitting coefficients to the FE results, providing good qualitative but relatively poor quantitative correlation. An expansion of the analytical coupling functions is then suggested in order to reduce their level of idealization. The expanded formulation shows virtually perfect agreement with the numerical results for almost the whole range of relative densities and distributions of solid in the FE parameter study. The presented analytical expression is believed to be general and provide accurate estimates of the elastic properties of a wide range of foam materials, provided that their bulk material properties and micro structure can be established.
The high specific bending stiffness of sandwich structures can with advantage be used in vehicles... more The high specific bending stiffness of sandwich structures can with advantage be used in vehicles to reduce their weight and thereby potentially also their fuel consumption. However, the structure must not only meet the in-service requirements but also provide sufficient protection of the vehicle passengers in a crash situation. The in-plane compressive response of sandwich panels is investigated in this thesis, with the objective to develop a methodology capable of determining if the structural response is likely to be favourable in an energy absorption perspective. Experiments were conducted to identify possible initial failure and collapse modes. The initial failure modes of sandwich panels compressed quasi-statically in the in-plane direction were identified as global buckling, local buckling (wrinkling) and face sheet fracture. Global buckling promotes continued folding of the structure when compressed beyond failure initiation. Face sheet fracture and wrinkling can promote collapse in the form of unstable debond crack growth, stable end-crushing or ductile in-plane shear collapse. Both the unstable debond crack propagation and the stable end-crushing are related to debond crack propagation, whereas the ductile in-plane shear mode is related to microbuckling of the face sheets. The collapse behaviour of sandwich configurations initially failing due to wrinkling or face sheet fracture was investigated, using a finite element model. The model was used to determine if the panels were likely to collapse in unstable debond propagation or in a more stable end-crushing mode, promoting high energy absorption. The collapse behaviour is mainly governed by the relation between the fracture toughness of the core and the bending stiffness and strength of the face sheets. The model was successfully used to design sandwich panels for different collapse behaviour. The proposed method could therefore be used in the design process of sandwich panels subjected to in-plane compressive loads.During a crash situation the accelerations on passengers must be kept below life threatening levels. The extreme peak loads in the structure must therefore be limited. This can be achieved by different kind of triggering features.Panels with either chamfered face sheets or with grooves on the loaded edges were investigated in this thesis. The peak load was reduced with panels incorporating either of the two triggering features. Another positive effect was that the plateau load following failure initiation was increased by the triggers. This clearly illustrates that triggers can be used to promote favourable response in sandwich panels. Vehicles are harmful to the environment not only during in-serve use, but during their entire life-cycle. By use of renewable materials the impact on the environment can be reduced. The in-plane compressive response of bio-based sandwich panels was therefore investigated. Panels with hemp fibre laminates showed potential for high energy absorption and panels with a balsa wood core behaved particular well. The ductile in-plane shear collapse mode of these panels resulted in the highest energy absorption of all investigated sandwich configurations.
Journal of physics, Oct 25, 2013
In-line phase-contrast x-ray imaging is emerging as a method for observing small details when the... more 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.
Composite Structures, Oct 1, 2010
The influence of triggering topologies on the peak load and energy absorption of sandwich panels ... more The influence of triggering topologies on the peak load and energy absorption of sandwich panels loaded in in-plane compression is investigated. Sandwich panels with different geometrical triggering features are manufactured and tested experimentally. Damage initiation in panels with grooves is investigated using finite element models.As expected the investigated triggering features reduce the extreme load peaks. A less expected result is that the plateau load following peak load tends to be higher for panels with triggering features. Both results are judged favourable for crash performance of panels in vehicle applications. Analysis suggests that there is a transition in failure mode for the studied panels, where the peak load for panels containing no or few grooves seems to be governed by principles of fracture mechanics while for panels with a high number of grooves it appears to be limited by the average stress.
Composite Structures, Sep 1, 2012
Composite materials reinforced with three-dimensionally (3D) woven carbon fibre textiles are inve... more Composite materials reinforced with three-dimensionally (3D) woven carbon fibre textiles are investigated and the challenge and the driver for the work is to generate numerical models to predict the mechanical behaviour of these composites. The result of the final modelling stage is near authentic finite element (FE) models of representative volume elements (RVEs) of the composites. They are created by using only a small number of input parameters, such as the size of the RVE, the number of yarns and their mutual interlacing, and the yarn crimp. The FE models may then be utilised for various purposes but are here used to derive homogenised elastic mechanical properties of 3D reinforced composite materials. The correlation between the models and experiments is good, both in terms of details in the architecture and mechanical properties. There are however some deviations that could be explained by the models being more regular than the real material.
Thermoplastic composite materials have some advantages over thermoset resin based composites, suc... more Thermoplastic composite materials have some advantages over thermoset resin based composites, such as fast processing, recyclability, reparability etc. In the present study commingled Carbon/Polyamide 12 (PA12) is analyzed as dry yarn and after heating the matrix fibers above the melting temperature and applying ambient pressure. The results are presented as micrographs. The mode I interlaminar fracture toughness of Carbon PA12 composite materials is measured, with and without introducing through-thickness yarns, and it is found to be very high even without 3D reinforcement. The consolidation time is also found to have great effect on the measured fracture toughness.
In the strive towards reduced fuel consumption and lower emissions, low structural weight is beco... more In the strive towards reduced fuel consumption and lower emissions, low structural weight is becoming a key factor in the design of advanced vehicle and aerospace structures. Whereas most traditional construction materials are seemingly reaching their limitations, composite materials with their high specific properties offer possibilities to further reduce weight. In high demand structural applications, the quality of the composite material is of utmost importance, requiring the material to be void free and the matrix well distributed as a binder for the load carrying reinforcement. To achieve proper wetting of the fibres, knowledge of the flow resistance of the porous fibre reinforcement is required. It is normally expressed in terms of permeability.Fibre reinforcements in composite materials are normally regarded as a heterogeneous porous media since both fabric and tows are porous but at different length scales. In order to numerically compute the permeability of such media, one of following two approaches can be used. Either filaments are added one-by-one into the modelled geometry (resolved model) or the tows are considered as porous homogenised media. In the latter case expression for the intra-tow permeability is needed.In this thesis, a porous homogenised tow model is benchmarked with a resolved model to the level of refinement possible without being too expensive computationally. Based on this approach, the permeability of complex three- dimensional (3D) textiles is computed utilizing computational fluid dynamics (CFD) analysis. The effect of inter- and intra-tow porosity on the overall permeability of 2D and 3D structures is analysed and discussed in relation to contradictions found in past studies. A clearer picture of the problem is presented, which will be helpful in future modelling and understanding of the permeability of complex structures. In an experimental study, the overall fibre volume fraction as well as the tow compaction are varied and their influence on the permeability is measured. Experimental studies show good agreement with numerical simulations.The interlaminar shear strength of thermoplastic composite materials is studied and the influence of specimen size is examined. Using finite element (FE) analysis it is shown that size effects may be partly due to statistical effects and partly due to the higher number of composite layers in thicker specimens.The effect of processing on the interlaminar delamination toughness of car-bon/polyamide 12 (C/PA12) is studied. It is observed that processing conditions have vital effect on the interlaminar delamination of thermoplastic composites. The mode I crack energy release rate (GIc) of C/PA12 is found to be 15 times higher than for conventional thermoset based composites and 1.5 times higher than for a thermoset composite with stitched reinforcement through the thickness. The best performing C/PA12 composite is manufactured in a hydraulic press equipped with a cold tool, thereby showing potential for both cost and time efficient manufacturing.
Proceedings of SPIE, Apr 12, 2017
Distributed sensors based on phase-optical time-domain reflectometry (phase-OTDR) are suitable fo... more 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.
Key Engineering Materials, Jul 1, 2019
Lightweight and strong composite material beams are increasingly sought to quickly, easily, and c... more Lightweight and strong composite material beams are increasingly sought to quickly, easily, and cost-effectively transport and setup a variety of constructions such as bridges, cabins/ stores/shelters, vehicles etc. For structural beams produced as conventional laminated composite materials, their weak areas tend to occur at intersections such as web-flange junctions due to absence of fibres bridging the interconnections. This drawback can however be overcome with development of profiled 3D textile reinforcements having combination architectures and constituent web-flange parts inherently mutually interconnected through fibre interlacement. In addition to general strength improvement, beams containing such novel reinforcement architectures also show increased energy absorption capability due to the mutual web-flange integration at the junctions. An ‘I’ and a ‘flanged-triangle’ cross-section beams were produced by a novel non-conventional weaving method, using carbon fibres as reinforcement, and their energy absorption capabilities were tested. These beams respectively absorbed over 50% and 300% more energy per weight in bending, compared to metal counterparts. This paper presents some relevant aspects of these innovative beams.
Mechanics of Materials, Sep 1, 2021
The use of 3D-woven composite materials has shown promising results. Along with weight-efficient ... more The use of 3D-woven composite materials has shown promising results. Along with weight-efficient stiffness and strength, they have demonstrated encouraging out of plane properties, damage tolerance and energy absorption capabilities. The widespread adoption of 3D-woven composites in industry however, requires the development of efficient computational models that can capture the material behaviour. The following work proposes a framework for modelling the mechanical response of 3D-woven composites on the macroscale. This flexible and thermodynamically consistent framework, decomposes the stress and strain tensors into two main parts motivated by the material architecture. The first is governed by the material behaviour along the reinforcement directions while the second is driven by shear behaviours. This division allows for the straightforward addition and modification of various inelastic phenomena observed in 3D-woven composites. In order to demonstrate the applicability of the framework, focus is given to predicting the material response of a 3D glass fibre reinforced epoxy composite. Prominent non-linearities are noted under shear loading and loading along the horizontal weft yarns. The behaviour under tensile loading along the weft yarns is captured using a Norton style viscoelasticity model. The non-linear shear response is introduced using a crystal plasticity inspired approach. Specifically, viscoelasticity is driven on localised slip planes defined by the material architecture. The viscous parameters are calibrated against experimental results and off axis tensile tests are used to validate the model.
Journal of Cellular Plastics, Jul 28, 2016
Stochastic foam models are generated from Voronoi spatial partitioning, using the centers of equi... more Stochastic foam models are generated from Voronoi spatial partitioning, using the centers of equi-sized hard spheres in random periodic distributions as seed points. Models with different levels of polydispersity are generated by varying the packing of the spheres. Subsequent relaxation is then performed with the Surface Evolver software which minimizes the surface area for better resemblance with real foam structures. The polydispersity of the Voronoi precursors is conserved when the models are converted into equilibrium models. The relation between the sphere packing fraction and the resulting degree of volumetric polydispersity is examined and the relations between the polydispersity and a number of associated morphology parameters are then investigated for both the Voronoi and the equilibrium models. Comparisons with data from real foams in the literature indicate that the used method is somewhat limited in terms of spread in cell volume but it provides a very controlled way of varying the foam morphology while keeping it periodic and truly stochastic. The study shows several strikingly consistent relations between the spread in cell volume and other geometric parameters, considering the stochastic nature of the models.
Composites Part A-applied Science and Manufacturing, Jun 1, 2021
The influence from moisture on shape distortion of curved thermoset composite laminates is invest... more 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 contentthe 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.
Integrated assessment of diciplinary and interpersonal skills - student perceptions of a novel le... more Integrated assessment of diciplinary and interpersonal skills - student perceptions of a novel learning experience
Journal of Cellular Plastics, Oct 23, 2013
Stochastic cellular models of rigid foam based on Voronoi spatial partitioning are generated and ... more Stochastic cellular models of rigid foam based on Voronoi spatial partitioning are generated and investigated for potential use in numerical analysis using finite element methods. Such partitions are deterministic once a distribution of cell nuclei has been defined. A drawback is that the models tend to exhibit a significant share of short edges and small faces. Such small geometrical features are not likely to occur in real foams since they are unfavorable from a surface energy point of view and they also generate problems in numerical analysis due to associated meshing challenges. Through minimization of the surface area, using the computer software Surface Evolver, the Voronoi models are brought to better resemblance with ideal dry foam and the occurrence of small geometrical features is strongly reduced. It is generally seen that different seed point distribution algorithms result in different model topologies. The presented methodology is systematic, parameterized and the results are very promising. Good grounds are provided for modeling of real rigid foam materials, that do not necessarily fully resemble ideal dry foam.
Composites Science and Technology, Sep 1, 2009
Fully interlaced 3D fabric is produced by a new weaving technology, and it is here utilised to pr... more Fully interlaced 3D fabric is produced by a new weaving technology, and it is here utilised to produce woven carbon fibre preforms, which are then used as reinforcement in composite materials. The purpose of this study is to assess the mechanical performance of this new type of composite material. A prototype loom was used to weave preforms with a rectangular cross section where all warp and weft yarns were fully interlaced in plain weave. Tensile, compressive, out-of-plane, shear and flexural properties of the composite flat-beam specimens were tested. The in-plane stiffness and strength were found to be lower, while the out-of-plane properties were higher compared to conventional 2D laminates. In terms of strength, it was not possible to quantify the difference, since the specimens with 3D woven material exhibited other failure modes than those tested for.
Composite Structures, Apr 1, 2012
ABSTRACT
In the strive towards reduced fuel consumption and lower emissions, low structural weight is beco... more In the strive towards reduced fuel consumption and lower emissions, low structural weight is becoming a key factor in the design of advanced vehicle and aerospace structures. Whereas most traditional construction materials are seemingly reaching their limitations, composite materials with their high specific properties offer possibilities to further reduce weight. In high demand structural applications, the quality of the composite material is of utmost importance, requiring the material to be void free and the matrix well distributed as a binder for the load carrying reinforcement. To achieve proper wetting of the fibres, knowledge of the flow resistance of the porous fibre reinforcement is required. It is normally expressed in terms of permeability.Fibre reinforcements in composite materials are normally regarded as a heterogeneous porous media since both fabric and tows are porous but at different length scales. In order to numerically compute the permeability of such media, one of following two approaches can be used. Either filaments are added one-by-one into the modelled geometry (resolved model) or the tows are considered as porous homogenised media. In the latter case expression for the intra-tow permeability is needed.In this thesis, a porous homogenised tow model is benchmarked with a resolved model to the level of refinement possible without being too expensive computationally. Based on this approach, the permeability of complex three- dimensional (3D) textiles is computed utilizing computational fluid dynamics (CFD) analysis. The effect of inter- and intra-tow porosity on the overall permeability of 2D and 3D structures is analysed and discussed in relation to contradictions found in past studies. A clearer picture of the problem is presented, which will be helpful in future modelling and understanding of the permeability of complex structures. In an experimental study, the overall fibre volume fraction as well as the tow compaction are varied and their influence on the permeability is measured. Experimental studies show good agreement with numerical simulations.The interlaminar shear strength of thermoplastic composite materials is studied and the influence of specimen size is examined. Using finite element (FE) analysis it is shown that size effects may be partly due to statistical effects and partly due to the higher number of composite layers in thicker specimens.The effect of processing on the interlaminar delamination toughness of car-bon/polyamide 12 (C/PA12) is studied. It is observed that processing conditions have vital effect on the interlaminar delamination of thermoplastic composites. The mode I crack energy release rate (GIc) of C/PA12 is found to be 15 times higher than for conventional thermoset based composites and 1.5 times higher than for a thermoset composite with stitched reinforcement through the thickness. The best performing C/PA12 composite is manufactured in a hydraulic press equipped with a cold tool, thereby showing potential for both cost and time efficient manufacturing.