| 1. |
- Sterley, Magdalena, 1959-
(författare)
-
Characterisation of green-glued wood adhesive bonds
- 2012
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The gluing of unseasoned wood, called green gluing, is a relatively new sawmill process, implying a radically changed order of material flow in the production of value-added wood-based products. It facilitates the enhancement of raw material recovery and value yield by integrating defect elimination and gluing already before kiln drying. The present study evaluates green glued adhesive bonds in flatwise glued beams and finger joints. The main part of this work deals with green gluing using a moisture curing polyurethane adhesive (PUR). Standardised test methods and specially designed, small scale, specimens were used for the determination of the strength, fracture energy and the ductility of both dry- and green glued bonds in tension and in shear. Using the small scale specimens it was possible to capture the complete stress versus deformation curves, including also their unloading part. An optical system for deformation measurement was used for the analysis of bond behaviour. The influence of moisture content during curing and temperature after curing on the adhesive chemical composition and on the mechanical properties was investigated. Furthermore, the moisture transport through the adhesive bond during curing was tested. Finally, microscopy studies were performed for analysis of bond morphology and fracture. The results show that two significant factors influence the shear strength of green glued bonds: wood density and adhesive spread rate. Bonds which fulfil the requirements according to EN 386 could be obtained within a wide range of process parameters. The small specimen tests showed that green glued PUR bonds can reach the same strength and fracture energy, both in shear and in tension, as dry glued bonds with the same adhesive amount. The local material properties of the bonds could be determined, thanks to the failure in the tests taking place within the adhesive bond itself and not in the wood. Following process factors were shown to cause lower bond strength: a) a low adhesive spread rate, b) high pressure and c) short pressing time in combination with low wood density and high moisture content. Moreover, the heat treatment of the cured PUR adhesive during drying influenced the chemical composition of the adhesive, providing for higher strength, stiffness and Tg of the adhesive, caused by an increased amount of highly ordered bidentate urea.
|
|
| 2. |
- Akter, Shaheda T.
(författare)
-
Experimental characterization and numerical modeling of compression perpendicular to the grain in wood and cross-laminated timber
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Compression perpendicular to the grain (CPG) of wood is a typical loading situation in timber structures. It has been an extensively studied research topic for decades, due to the highly ductile behavior of wood under such loading, the large variations in mechanical properties, and the relevance of these properties in structural design. Among others, the main influencing factors for CPG properties are stressed volume, load and support configurations, and annual ring orientations to the loading direction. After the innovation of the massive, engineered wood based product, cross-laminated timber (CLT) and its application in high rise buildings, CPG of wood has gained further importance. The development of a non-homogeneous, undesired and combined stress state under CPG in solid wood, due to the material anisotropy in the radial-tangential plane, can build up a complex multi-axial stress state in CLT. As a comparatively new product, the study of the influencing factors for CPG properties of CLT, and an understanding of the local material behavior under such loading, is essential for product characterization and for the development of design guidelines to ensure safe and efficient design.The main aim of the doctoral thesis is to establish a relationship betweenthe anisotropic behavior of clear wood in the transverse plane and the structural response of CLT under CPG loading. Both experimental and numerical studies were adopted herein, to enhance the understanding of the basic material behavior and the product and structural behavior. On the clear wood scale, the focus was on developing a test setup for uniaxial and biaxial loading in the radial-tangential(RT) plane. The potential of the developed test setup for the biaxial testing in the transverse plane was exploited for the investigation of the moisture and time dependent behavior of clear wood under radial compression and rolling shear loading. For data acquisition, in addition to the force and displacement data measured by the internal actuators of the testing machine and an external load cell,a contact-free digital image correlation (DIC) system was used in the experimental investigations. A numerical model was developed, which can describe the elasto-plastic behavior of wood under compression in the transverse plane and predict the structural behavior of solid wood and CLT. For that purpose, a novel Quadratic multi-surface (QMS) failure criterion and a simplified Hoffman failure criterion were implemented in a user-subroutine in the finite element software Abaqus®, and their suitability was compared with the Abaqus implemented Hill’s criterion.The validation of the material models was based on the experimental investigations of failure behavior of clear wood under stress perpendicular to the grain with rolling shear interaction. The material models were further utilized to predict the structural response of solid wood and CLT wall-to-floor connections under CPG loading. The predicted response of CLT connections under CPG by using the above-mentioned material models was compared with experiments, which investigated the influences of different connection types, wall and floor thicknesses, positions of walls, and outer deck layer orientations. The models were then applied to investigate the influence of the pith location in the boards, the number of layers and the thickness of walls and the floor on the stiffness and strength of CLT connections. Moreover, the CLT connection’s rotational rigidity as a consequence of compressive force from the upper floor in a multi-story building was studied by means of finite element calculations.The DIC measured strain fields from the experiments on clear wood confirmed the dependence of strain field on the curvature of the annual rings. As regards the material models, Hill’s model resulted in significantly higher force carrying capacity than experiments on clear wood, whereas Hoffman’s and QMS models predicted reasonably well the force-displacement relationships as found in experiments. The Hoffman’s and QMS models predicted stiffness was about 5–10% higher than corresponding experimental results on clear wood, and about 25% higher for CLT connections. The higher difference in the latter case is due to the difference in material properties of clear wood and structural timber, and the contact behavior between the structural members. The results from CLT wall-to-floor connections revealed a strong influence of loading and supporting configurations, wall thickness and pith locations on their stiffness and strength. A compressive loading on the CLT wall showed a positive effect on the rotational stiffness of CLT wall-to-floor connections, which considerably reduces the CLT floor mid-span deflection in comparison to a simply-supported floor.The thesis work contributes to an enhanced understanding of the anisotropic material behavior of wood in the RT-plane and of its effects on structural timber and CLT under CPG loading. The outcomes of the thesis are beneficial to the product design and standardization of CLT and can be applied in further product development and in optimized structural design.
|
|
| 3. |
- Lundstedt, Karin, 1963-
(författare)
-
Thermal stresses in load-bearing glass-timber components
- 2012
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Thermal stresses can arise in glass used in architectonic realisations in buildings as a result of solar irradiation: that is, sunlight. The magnitude of these thermal stresses depends on the temperature conditions in the glass in terms of the temperature differences between warm and cold areas. The work described here has verified the existence and distribution of thermal stresses in glass, both theoretically and experimentally, in a load-bearing element made of glass and wood, when exposed to incident solar radiation and various degrees of shading. The finite element method was used in order to simulate the temperature distribution in the glass, together with the resulting associated thermal stresses. Parts of these model results were then compared with results and data from experimental investigations in a solar simulator. The experimental trials were carried out on a plain glass sheet and on a wood-framed glass sheet. This frame assembly saw the glass fitted in a wooden frame such that its edge was flush with the edges of the frame and therefore freely exposed to the surroundings. Two different grades of glass were analysed in the theoretical modelling and used in the experimental solar exposure: a clear glass, with a low absorption coefficient, and a tinted glass having a relatively high absorption coefficient. The experimental part of the work included two different finishes to the glass edge: as-cut, with no further attention, and cut and smoothed. The results from the finite element method calculations agree relatively well with the experimental results. When simulating the temperature values and distributions, the value of the coefficient of heat transfer is an important factor in determining the results. The coefficient can be given either a constant value, taken from a standard, or a calculated value that varies depending on the surface temperature and ambient temperature at every instant. For the clear glass with a low coefficient of absorption, the calculated temperatures did not differ significantly depending on which method had been used to provide a value for the coefficient of heat transfer. However, for the glass with a high coefficient of absorption, and when exposed to high solar intensity, a calculated value of coefficient of heat transfer should be used in order to arrive at relevant values of surface temperatures and stresses. Thermal tensile stresses have more effect on the total stress level than have stresses arising from typical in-service vertical loads.
|
|
| 4. |
- Sjödin, Johan, 1977-
(författare)
-
Strength and Moisture Aspects of Steel Timber Dowel Joints in Glulam Structures : An Experimental and Numerical Study
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Joints are critical parts of timber structures, transmitting static and dynamic forces between structural members. The ultimate behavior of e.g. a building depends strongly on the structural configuration and the capacity of its joints. The complete collapse of a building or other less extensive accidents that may occur usually start as a local failure inside or in the vicinity of a joint. Such serious failures have recently occurred in the Nordic countries. Especially the collapses of two large glued-laminated timber (glulam) structures clearly indicate the need of an improved joint design. The trend toward larger and more complex structures even further increases the importance of a safer design of the joints.One aim of this partly experimentally and partly numerically based work has been to investigate if the short term capacity of steel-timber dowel joints loaded parallel to the grain is affected by an initial drying exposure. The experimental results showed that the load-bearing capacity of the joints is indeed reduced by such moisture changes. Moisture induced stresses was mentioned to be the explanation. The key point is that the climates chosen in the present work (20°C / 65% RH and 20°C / 20% RH) are equivalent to service class 1 according to EC5 (Eurocode 5 2004). Thus, EC5 predicts no decrease in load-bearing capacity, in relation to the standard climate used during testing. A decrease in load-bearing capacity in the range of 5-20%, which was found in the present work, is of course not negligible and, therefore, there could be a need to introduce the effect of drying in design codes. Because similar results were also observed for a double-tapered glulam beam, further work should consider timber structures in general.Two numerical methods in order to predict the capacity of multiple steel-timber dowel joints loaded parallel to the grain were tested in the thesis. For the first method, where fracture mechanics (LEFM) concepts were implemented, a good correlation with the experimental results was seen. Also for the second method, where the capacity for a single dowel-type joint as given in EC5 was used as a failure criterion, a good correlation to traditional EC5 calculations of multiple dowel-type joints was seen. One advantage of using numerical methods in design is that the capacity of the joint can be calculated also for cases when the dowels are placed in more complex patterns. From both a structural and an architectural point of view this can be very important. In addition, such numerical methods are effective tools for the structural engineer when considering complicated loading situations in joints, i.e. eccentric loading giving moments in the joint.
|
|
| 5. |
- Blyberg, Louise
(författare)
-
Timber/Glass Adhesive Bonds for Structural Applications
- 2011
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Timber with its natural appearance and glass with its transparency may be appealing material for architects and users of modern buildings. Glass is a brittle material, but it is about six times stiffer than timber. Combined appropriately, the materials could form different types of composite products, e.g. beams or shear walls, that can be included in the load-carrying structure of buildings. e knowledge on load- carrying timber/glass components is limited. e intention of this research has been to contribute to the knowledge required for the industry to be willing to produce timber/glass components for the market.The thesis includes experimental testing accompanied with complementary nite element simulations, which provide more details and information about the test results. Tests were performed on small-scale specimens with a bond area of 800 mm2 as well as on I-beam and shear wall prototypes. For the small-scale specimens tested in standard climate, three different adhesives were used for the bond line between timber and glass. ese specimens were tested in both tension and shear. In addition, one of the adhesives was used for small-scale shear specimens which were exposed to different humidity levels before the tests were performed. e 4 m long I-beam prototypes designed with a web of glass and wooden anges were tested in four- point bending. e shear wall prototypes were tested by applying either a vertical load, a horizontal load or a combination of these, all being applied in the plane of the shear wall.Of the three adhesives used in the small-scale testing, an acrylate adhesive had the largest strength, both in tension and in shear. e study on the effect of humidity was performed with this adhesive. is study indicates that the adhesive properties do not change dramatically in indoor climate. is adhesive was also used for twelve of the fourteen tested I-beams. e results from the beams show that a signi cant redundancy is obtained; the load at the nal failure was around 240 % of the load when the rst crack in the glass web appeared. e shear walls were glued using the acrylate adhesive and for a few cases a 2-component silicone based adhesive. e results from the shear wall tests showed the shear wall to behave in a much more brittle manner, without any noticeable redundancy.
|
|
| 6. |
- Walander, Tomas
(författare)
-
Cohesive modelling of the temperature dependence of epoxy based adhesives in Mode I and Mode II loading
- 2013
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this work, the influence of the temperature on the cohesive laws for two epoxy adhesives is studied at temperatures below the glass transition temperature for both Mode I and Mode II loading. Cohesive laws are measured experimentally under quasi-static loading conditions in the temperature range -30≤T≤80"C" . Three parameters of the cohesive laws are studied in detail: the elastic stiffness, the peak stress and the fracture energy. Methods for determining the elastic stiffness in Mode I and Mode II are derived and evaluated. With these methods, the results in this work show that it is possible to measure all three parameters for each pure mode loading case by the use of only the DCB- and the ENF-test specimens. Even though the measures tend to spread in values, this can significantly reduce the cost for performing experiments. It is shown that most of the cohesive parameters are decreasing with an increasing temperature in both loading modes and for both adhesives. An exception is the Mode I fracture energy for one of the adhesives. This is shown to be independent of the temperature in the studied temperature range. For the same adhesive, the Mode II fracture energy is shown to be continuously decreasing with an increasing temperature. The experimental results are verified by finite element analyses. The simulations only consider uncoupled cohesive behaviours. By use of the experimental results, simplified bi-linear cohesive laws to be used at any temperature within the studied temperature range are derived for one adhesive in both loading modes. This is desired in order to simulate adhesively bonded structures that suffer a wide range in temperature.
|
|
