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Ing. Petr Sejkot, Ph.D.

2024, Buildings, 14 (2), ISSN 2075-5309

Kevlar fibers are widely used for industrial and military purposes due to their remarkable mechanical properties, such as their high tenacity and high strength-to-weight ratio. In this study, two-layered Kevlar composite specimens were impregnated with 10 wt.% poly (vinyl butyral)/ethanol solution which contained TiO2 nanoparticles as reinforcement. The concentrations of the nanoparticles were 1 wt.% or 2 wt.% with respect to the poly (vinyl butyral), PVB. The single-axial tensile test and three-point bending test of the Kevlar/PVB composites have been performed according to the ASTM D 3039 and ASTM D 790-03 standards, respectively. The tensile and bending properties of the dry and wet Kevlar/PVB composite specimens after a 56-day immersion are examined in this work. Upon the addition of the 2 wt.% TiO2 nanoparticles, the tensile strength and modulus of the dry specimens without reinforcement were increased by 39.8% and 24.3%, respectively. All the submerged specimens’ tensile and flexural property values were lower than those of the dry specimens. After comparing the wet composite specimens to their dry counterparts, the percentage decrease in tensile strength was approximately 20%. The wet Kevlar/PVB specimens with no TiO2 reinforcement showed the greatest reduction in bending strength, 61.4% less than for the dry Kevlar/PVB specimens, due to the degradation of the PVB matrix. In addition, a numerical simulation of the three-point bending test was carried out in Abaqus.

2024, Canadian Journal of Civil Engineering, 51 (7), p. 803-811), ISSN 1208-6029

Engineered wood products along with advanced processing and fabrication are pushing the limits of modern wood construction. Innovative concepts and applications facilitate the development of a new generation of structural systems. However, capacities of such systems are often governed by capabilities of the connections. This paper presents the experimental results with numerical and analytical models of angle brackets for prediction of load bearing capacity, stiffness and ductility. Three types of metal brackets in beam-column connections have been investigated and their performance has been studied in various loading arrangements. Detailed finite-element models of each connection have been developed to gain insights into their behavior. An analytical approach is also adopted to represent the connections. Comparison with test data suggests that the models can reproduce results with good accuracy. The findings confirm feasibility of implementing ductile connections in practical mass timber structures.

2024, Journal of Building Engineering, 96, ISSN 2352-7102

This paper presents a comparative study between Timber-Concrete Composite (TCC) systems with glued-in steel plates and screwed connections. The research focuses on evaluating the structural performance of these connections, with a particular focus on the innovative glued-in steel plate through full-scale four-point bending tests on floor subassemblies with a 7.5 m span, complemented by push-out tests for assessing the behaviour of the slab-beam connection. The study aims to determine if the glued-in steel plate, a more economical option, can replace screws without significantly compromising the TCC system’s stiffness or strength. The findings reveal that while the glued-in steel plate connections exhibited lower stiffness and ductility compared to screwed connections, they demonstrated a significantly higher load-bearing capacity. Specifically, the glued-in connections achieved capacities ranging from 670-720 kN, markedly exceeding the 300-350 kN and 480-520 kN capacities of specimens with fewer and more screws, respectively. These results suggest that glued-in steel plate connections can effectively replace screws in TCC systems, offering a cost-effective alternative without significantly reducing structural integrity. Moreover, the paper evaluates how to design TCC floors by estimating the effective number of connectors and the effective length of the glued-in steel plate using an iterative -method. This is done to account for changes in stiffness, as determined from push-out tests and observed in bending tests. The iterative -method’s findings have been validated against the predictions of a Finite Element model.

2024, MecaNano 2nd General Meeting - Book of Abstracts, Vídeň, Technische Universität Wien)

The Kevlar fabric composites are widely applied for body armor and vehicle armor structures. In this study, the specimens were produced of the Kevlar fabrics impregnated with 10 wt.% poly (vinyl butyral)/ethanol solution which contained the ZnO nanoparticles in different concentrations (1 wt.% or 2 wt.% in relation to poly (vinyl butyral), PVB). The two-layered Kevlar/PVB composite samples were fabricated by hot compression. In accordance with the ISO 62 standard, the square Kevlar/PVB composite specimens were immersed for the water uptake measurements. The tensile and flexural properties of the Kevlar/PVB specimens were examined in compliance with the ASTM D 3039 and ASTM D 790-03 standards, respectively. The specimens that had been submerged in distilled water at 40 ◦C for eight weeks were compared to the dry specimens in terms of their tensile and bending characteristics. The addition of ZnO nanoparticles significantly improved the tensile strength and tensile modulus of the dry Kevlar/PVB specimens. In contrast to the tensile test results, the dry specimens with no nanoparticles exhibited the superior flexural qualities (strength and modulus) because of the better bonding between their two impregnated fabric layers. When compared to their dry counterparts, the tensile and bending properties of every immersed Kevlar/PVB composite specimen showed some lower values.

2024, Proceedings of the Canadian Society for Civil Engineering Annual Conference 2023, Volume 11, Berlin & Heidelberg, Springer), p. 287-298), ISBN 978-3-031-61530-6, ISSN 2366-2557

Reinforced angle brackets are commonly used as connectors in wood structures. These connectors are typically installed with ring shank nails or screws. This paper presents the experimental results of cross-laminated timber (CLT) connections which consist of reinforced angle brackets and heavy-duty screws. These screws are one of the stiffest and the strongest options for the fasteners which can be used with the reinforced angle brackets. However, this option has not been extensively studied by many scholars yet. Therefore, this paper contains detailed numerical simulations with aim to be able to predict the failure modes and the load-bearing capacities of experimentally tested connections. Benefits together with the limitations of the presented numerical method are discussed.

2024, Buildings, 14 (6), ISSN 2075-5309

Rooftop solar modules are usually held in place by racks or frames that are mechanically attached to a roof structure and/or by heavyweight, ballasted footing mounts. These mounts ensure that the panel system remains in position against wind load. However, mechanical connectors create penetrations into the water-resistant layer of the roof, whereas ballasted footing mounts cause a significant additional load on the load-bearing structure of roof. For these reasons, adhesive connection seems to be a beneficial solution. Acrylic adhesive tapes, marked as VHB, may provide sufficient strength, and they have no need for mechanical fasteners or ballast. Acrylic adhesive tapes also provide a comfortable, fast, and efficient bonding process with no curing compared to liquid adhesives. On the other hand, resistance to water at load-bearing joints has not been sufficiently studied yet and could be critical for connections exposed to the outdoor environment. The present study aims at the determination of water resistance and durability of the VHBTM tapes from the GPH series, which are typically used to bond a variety of substrates including many metals. The mechanical properties and failure modes are compared for the specimens before and after a 21-day immersion in water. A significant reduction in strength was observed, depending on the substrate material. The study of chemical changes in the acrylic tape and in its leachate through infrared spectroscopy (FT-IR), X-ray fluorescence, and X-ray diffraction analyses clarified the reduction in mechanical properties. The selected VHB tape demonstrated strong resistance to the effects of water. However, the overall strength of the joint after immersion was significantly impacted by the decrease in adhesion to a specific substrate.

2023, Proceedings from the 13th World Conference on Timber Engineering 2023, World Conference on Timber Engineering 2023), p. 1397-1402), ISBN 9781713873297

Beam-on-foundation modelling shows huge potential to substitute the limit analysis and empirical stiffness formulas used to calculate the load bearing capacity of timber connections with dowel-type fasteners like nails and screws. It takes into the account the non-linear behaviour of fastener’s shank subjected to bending as well as the elastic-plastic behaviour of contact between the fastener and surrounding wood. Its another advantage is the capability to calculate both capacity and stiffness of complex connections commonly used in cross-laminated timber structures. This paper describes numerical models of axially and laterally loaded fasteners in wood. Results from modelling are thereafter compared to the experimentally obtained data.

2023, Proceedings from the 13th World Conference on Timber Engineering 2023, World Conference on Timber Engineering 2023), p. 1403-1408), ISBN 9781713873297

Mechanical connections in wood structures typically consist of structural members connected with combination of cold-formed thin-walled steel angle brackets and fasteners in forms of nails or screws. Design of these steel connectors is typically based on short-term monotonic load bearing capacities. Observations from experimental testing indicate that their failure modes and resistance to a cyclic loading should be checked carefully to prevent the thread of sudden collapse of structures using these connections. This paper presents the experimental results of connections of CLT elements connected together by angle brackets and subjected to the external cyclic loading. Special focus is made on connections using heavy duty screws as fasteners. Results of experimental testing are compared to results of testing of similar connections using ring nails as fasteners. In addition, numerical simulations are made with aim to predict the failure modes and the load bearing capacities of experimentally tested connections.

2023, Modern Building Materials, Structures and Techniques, Springer, Cham), p. 268-277), ISBN 978-3-031-44602-3, ISSN 2366-2557

The Kevlar fabric composites have a wide range of applications in the body armor and lightweight vehicle-armor structures. Nanoparticles are one of the most common nanofillers for these structures. In this research, the testing specimens have been made from the Kevlar fabrics impregnated with the poly (vinyl butyral)/ethanol solution which had been reinforced with the ZnO nanoparticles. The two-layered composite samples have been fabricated by means of hot compression. The immersion of the square Kevlar/PVB specimens for the water uptake measurements has been performed according to the ISO 62 standard. The specimens have been immersed in a water bath filled with the distilled water (40 ℃) in the period of 8 weeks. The Kevlar/PVB specimens have been tested in accordance with the ASTM D 3039 standard for the tensile properties and the ASTM D 790–03 standard for the flexural properties. The tensile and bending characteristics of the dry specimens have been compared with the ones that had undergone the water immersion.

2023, ICFC9 9th International Conference on Fatigue of Composites - Scientific Program and Book of Abstracts, Padova, University of Padova), p. 48-48)

In this research, the testing specimens were made from the Kevlar fabrics impregnated with 10 wt.% poly (vinyl butyral)/ethanol solution but with the addition of the TiO2 nanoparticles. The tensile and bending properties of the dry specimens were compared with the ones that had been immersed in a water bath filled with distilled water (40°C) in the period of 8 weeks. The PVB/fabric weight ratio for the impregnation of the woven Kevlar fabric was 20 wt.%. The Kevlar/PVB fabric composites were reinforced with the TiO2 nanoparticles in different concentrations (1 wt.% or 2 wt.% of reinforcement regarding poly (vinyl butyral), PVB). The two layers of composite fabrics were hot-pressed by using the digital press at a temperature of 170 °C for 30 minutes. The immersion of the square Kevlar/PVB composite specimens for the water weight gain measurements was performed according to the ISO 62 standard. The Kevlar/PVB specimens were tested in accordance with the ASTM D 3039 standard for the tensile properties and ASTM D 790-03 standard for the flexural properties. During the tensile and the flexural test, the complete fracture of the specimens did not occur. The addition of 2 wt.% TiO2 nanoparticles produced 39.8% and 24.3% improvement in the tensile strength and tensile modulus, respectively, compared to the dry Kevlar/PVB specimens without reinforcement. The tensile properties of all the immersed Kevlar/PVB composite specimens had decreased values compared to the ones of their dry specimens. Unlike the tensile test results, the best flexural properties (strength and modulus) were achieved with the dry specimens with no particles due to the better bonding between their two impregnated fabric layers since the presence of TiO2 nanoparticles reduced the shear connection between the layers. However, there was not such a decline in the properties of the wet specimens with nanoparticles which even made some improvement in the bending strength compared to the wet Kevlar/PVB specimens.

2022, Composite Interfaces, 29 (12), p. 1309-1324), ISSN 1568-5543

Carbon fiber reinforced polymer (CFRP) composites have many applications in civil engineering, aircraft industry and in many other areas. They are lightweight composite materials with outstanding mechanical properties. In this study, the testing specimens were made from the carbon fabrics impregnated with epoxy resin which had been reinforced with the silicon carbide (SiC) microparticles or SiC microfibers. The four-layered composite samples were fabricated by means of hot compression and curing of epoxy resin. The thermal, tensile and impact properties of the untreated specimens were compared with the ones that underwent the water absorption in duration of 72 hours, followed by desorption. The treated specimens containing the SiC particles and SiC microfibers demonstrated 3.4% and 8.8% of the reduction in the tensile energy absorption, correspondingly, compared to their untreated counterparts. Opposed to this trend of results, the tensile energy absorption value of the treated carbon/epoxy specimens, experienced an increase of 84.4% compared to the dry carbon/epoxy specimens. The additional test results showed that the water treated carbon/epoxy composites went into a certain decline in their thermal, tensile and impact properties. The obtained results provide some significant pieces of information for the future practical use of the presented composites.

2022, CertBond COST Action CA18120 – Training School 2 – Book of Abstracts, TECHNISCHE UNIVERSITEIT DELFT), p. 40-40)

Carbon fiber reinforced polymer (CFRP) composites are extensively employed in aircraft industry, and in other areas where outstanding mechanical performance is a requirement. In this study, the composite samples were made from the carbon fabrics impregnated with epoxy resin reinforced with the silicon carbide (SiC) microparticles or microfibers. The thermal, tensile and impact properties of the untreated specimens were compared with the ones that underwent the water absorption in duration of 72 h (immersion or high humidity) followed by desorption. The glass transition temperatures (Tg) of the specimens were determined using the differential scanning calorimeter (DSC) device. The carbon/epoxy specimens were tested in accordance with the ASTM D5942 standard for their impact properties and ASTM D3039 standard for the tensile properties of the composite materials with the polymer matrix. The dry carbon/epoxy specimens with the SiC microfibers or SiC particles showed a large increase in the tensile strength and tensile energy absorption (TEA), respectively, in comparison with the dry carbon/epoxy specimens while the specimens with the SiC particles had the highest value of the impact toughness. The carbon/epoxy specimens with the SiC reinforcement, which were exposed to 70% humidity or water immersed, saw the decline in Tg. The tensile strength and TEA of the water immersed carbon/epoxy specimens with the SiC reinforcement had the values decreased in comparison with the ones of their dry specimens. Contrary to this trend of results, the tensile properties values achieved a significant increase in the immersed carbon/epoxy specimens (with no reinforcement), in comparison with the dry ones. The decrease of the elastic modulus was determined in all the specimens after water immersion followed by desorption. The impact toughness values of all the specimens decreased after exposing them to the elevated humidity as well as to water immersion, compared to the values in their untreated counterparts.

2021, CSCE 2021 Annual Conference – Inspired by Nature – Inspiré par la Nature, Montreal, Canadian Society for Civil Engineering)

In some instances, algebraic hysteresis models may exhibit some advantages compared to differential hysteresis models. In this paper, the authors present a novel transcendental hysteresis model based on the piece-wise definition of arctangent (Atan) functions. The definition of the shape parameters of the arctangent functions originates from the features of the experimental hysteresis curves directly. In the first step, the authors present an application of this model to the experimental cyclic response of a Light-Timber frame (LTF) shear wall. The proposed model originates from the analysis of the experimental response of wood joints. However, it can be of more general application, and the particular reference towood joints does not preclude other application fields.

2021, Earthquake Engineering and Structural Dynamics, 50 (9), p. 2277-2293), ISSN 0098-8847

The authors present a novel transcendental hysteresis model based on the piecewise definition of arctangent functions. The definition of the shape parameters of the arctangent functions originates from the features of the experimental hysteresis curves directly. In the first step, the authors present this model's application to the experimental cyclic response of cross‐laminated timber (CLT), light‐timber frame (LTF) shear walls, and a steel angle bracket. Then, the response of a plywood‐coupled laminated‐veneer lumber (LVL) shear wall is used for validation purposes. The paper introduces the arctangent‐based model, labeled Atan model, by illustrating some possible shortcomings of hysteresis models with pinching. The paper concludes with a simple demonstration of these issues in the case of a Bouc‐Wen class hysteresis model, the extended energy‐dependent generalized Bouc‐Wen (eegbw) model. The numerical instabilities of the eegbw model are used to endorse the advantages of the proposed formulation in modeling complex structural arrangements, like wood joints and structural systems. The proposed model originates from the analysis of the experimental response of wood joints. However, it can be of more general application, and the particular reference to wood joints does not preclude other application fields.

2021, World Conference on Timber Engineering (WCTE 2021) Proceedings, Centro UC de Innovación en Madera), p. 3026-3032), ISBN 978-1-7138-4097-8

Mechanical connections in wood structures typically consist of structural members connected with combination of thin-walled steel angle brackets and fasteners in forms of nails or screws. Design of these steel connectors is typically based on short-term load bearing capacities. Observations from experimental testing indicate that both the angle bracket and the fasteners possess properties such the connections can be designed to behave in a ductile manner. This paper presents numerical simulations of ductile angle bracket connections calibrated against experimental results and subsequently validated. Special focus is on single nail behaviour when the jointed timber members are subjected to cyclic loading. Simulations of global behaviour of connections show that acceptable match between the simulation and the experimental results can be achieved despite relatively simple nail model is used for both loading and unloading behaviour.

2021, World Conference on Timber Engineering (WCTE 2021) Proceedings, Centro UC de Innovación en Madera), p. 2007-2013), ISBN 978-1-7138-4097-8

Timber roof structures built up by several trusses designed with structural timber members and jointed together by punched metal plate fasteners are considerably effective structures for long spans. They are considered cost-effective and can serve their purpose well during the lifespan of a roof. Lately, the limits of how long spans they can withstand for a normal loading situation have been pushed. Failures that often relate to lateral instability in the top chord or in other compressed members of the truss have been reported. The effect of various parameters on the load carrying capacity of the top chord is studied. Analytical solutions based on the method available in Eurocode 5 and a method based on the assumption of an elastic foundation is used in combination with finite element simulations. Results show that, in a Nordic context, the approach suggested in Eurocode is problematic due to the suggested bay length which is supposed to develop between lateral supports. Simulation results also suggests that load-displacement curves based on experiments performed on the connections involved should be used in simulations of the top chord rather than an elastic spring.

2021, Current Applied Physics, 26, p. 16-23), ISSN 1567-1739

Para-aramid fibers (Kolon) are high performance polymeric fibres characterized by their high tenacity and impact resistance. They are used for the soft body armor structures in ballistics. In this study, the testing specimens were made from multilayered Kolon fabrics impregnated with epoxy resin where silicon carbide (SiC) microparticles or SiC nanofibers were added as reinforcement. The laminated composite samples were fabricated by hot compression and curing of epoxy resin. The tensile and impact strengths of the untreated specimens were compared with the ones that underwent water absorption in duration of 72h (immersion or humidity) followed by desorption. The immersion of the specimens in water and exposure to high humidity (70%) were performed according to the ISO 62 standard while the tensile test was carried out in accordance with the ASTM D 3039 standard. In the end, the tensile test simulation of the laminated composite by using software Abaqus® was accomplished.

2021, 15th International conference on materials chemistry (MC15), Cambridge, Royal Society of Chemistry)

The para-aramid fibres (Kevlar, Twaron, Kolon) are high performance polymeric fibres characterized by their high tenacity, strength to weight ratio and impact resistance. They are used for the soft body armour structures in ballistics and for the manufacture of parts in aerospace and automotive industry. In this study, the specimens were made from the two-layered Kolon fabrics impregnated with epoxy resin where some silicon carbide (SiC) microparticles or SiC nanofibres were added as reinforcement. These composite specimens were fabricated by hot compression and curing of epoxy resin. Despite their good mechanical strength it is a well-known fact that the mechanical properties of epoxy resins are affected by moisture absorption. The tensile and impact properties of the untreated specimens were compared with the ones that underwent water absorption in the duration of 72h (immersion or humidity) followed by desorption. The immersion of the specimens in water and the exposure to high humidity (70%) were completed in accordance with the ISO 62 standard while the tensile test was performed according to the ASTM D 3039 standard. Furthermore, the tensile test simulation of the Kolon/epoxy composite by using software Abaqus® was accomplished. For the determination of the absorbed impact energy and the impact toughness of the composite specimens, an impact test was performed by using the Charpy impact pendulum in accordance with the ASTM D5942 standard.

2020, Engineering Structures, 204, ISSN 0141-0296

According to the harmonized European design code for timber structures, Eurocode 5, all pitched timber trusses are designed as an in-plane structure, meaning that the bracing systems used are assumed to prevent the out-of plane failure of the truss if sufficient strength and stiffness are provided. The present paper studies how the stiffness of a wooden bracing system contributes to the out-of-plane stability of a trussed roof structure. Results from numerical simulations indicate that significant bracing forces may occur in compressed structural members for long-span timber structures. As well, the values obtained from the calculations according to Eurocode 5 are occasionally far from the results obtained by numerical simulations.

2019, 2019 CSCE Annual Conference Proceedings, Université Laval in Quebec City)

Typical connections in wood structures consist of structural members connected with thin-walled steel elements, commonly angle brackets. The design of these steel elements is typically based on short-term load bearing capacity experimental testing. Experiences from the experimental testing indicate that both steel plate and connectors possess properties such that these connections can be designed to behave in ductile manner. Following the capacity design principle, during seismic loading the structural members remain undamaged while the damage and most of the energy dissipation is confined within the steel elements. However, there is insufficient support in Building Design Codes to design any building for a seismic situation in such a manner. This paper presents development of numerical models of the connections and subsequent validation by experimental results. Their applications in replicating performance of a number of metal bracketed connections with these characteristics are illustrated. Two sources of ductility for the overall arrangement, namely deformations in the metal connectors and in nails (bending and pulling out) due to cyclic loading have been identified. The steel plate absorbs significantly less energy after the first circle while the nails can dissipate energy more consistently over the cycles, characterized by pinching and elasto-plastic behavior respectively. The models offer insights into detailing ductile connections in wood and possibility in the optimization of the connectors for seismic design.

2019, CONSTRUCTION and REAL ESTATE: EXPERTISE and APPRAISAL, ASN Experts & Consultans, s.r.o.), p. 261-262), ISBN 978-5-9903774-6-2

Kolapsy dřevěných konstrukcí a prvků z důvodu stability jsou poměrně častým jevem i přes četná doporučení, jak jim zabránit. Tyto kolapsy jsou typicky doprovázeny náhlou deformací vedoucí k porušení jednoho nosného prvku či přímo celé konstrukce.

2018, WCTE 2018 Proceedings, Seoul, National Institute of Forest Science), ISBN 979-11-6019-235-3

According to the harmonized European design code for timber structures, Eurocode 5, all pitched timber trusses used in load bearing roofs are designed as in-plane structures which means that a bracing system must be designed and put in place to prevent the out-of-plane instability. Results from numerical 3D stability analyses of the whole roof structure indicate that the out of plane stability is often the critical factor. Therefore, influence of stiffness properties of that system is studied in detail herein for long-span timber roofs. Focus is put on how the stiffness of the mechanical connections in the roof structure influences the load carrying capacity of the roof. The punched metal plate connections are modelled as non-coupled spring elements connecting the various beam elements in the timber truss respectively. The spring stiffness of the connections is derived from full-scale tests, which were made for all in- and out-of-plane degrees of freedom. To evaluate the experimental testing, a digital image correlation method was used. The results from the digital image correlation tests were compared with numerical simulations of the experimentally tested connections to check the potential of using the numerical simulations instead of the experimental testing to get the stiffness properties of various connections used in the whole roof stability analysis. Based on such analysis, punched metal plate fasteners showed to be an important contributor to the roof stability because of its relatively high stiffness in all six degrees of freedom.