Structural performance of rounded dovetail connections: experimental and numerical investigations

Rounded Dovetail Connections (RDC) are a relatively new wood-to-wood connection concept that, despite the lack of design guidance in standards, has become popular in timber construction due to the widespread of modern milling machinery. Because of the anisotropic nature of wood and the complex stress-strain state in RDC, the question of their dimensioning is very complex. Experimental and numerical investigations were carried out on full scale RDC used to connect two timber members as joist to beam connections subjected to quasi-static shear loading. The influence of two geometric parameters was investigated: the dovetail height (varied between 109 and 189?mm) and the flange angle (varied between 5 and 20°). Both, serviceability and ultimate limit states were studied using analysis of variance. It was found that the joint capacity (i) depends on the dovetail height, with an optimum of approximately 2/3 of the beam height and (ii) can be considered almost independent of the flange angle. The development and implementation of a numerical model for the design process of RDC was examined and good agreement between experimental and numerical load deformation curves validated the model, thus making it suitable for developing a method to predict RDC capacity. The paper proposes a probabilistic method to predict the capacity of RDC taking into account the scale sensitivity of the material strength, which is modelled using Weibull statistics, and considers not only the magnitude of the stress fields, but also the volume over which these stress peaks act. The proposed method has immediate actionable application for the improvement of RDC design.     

Time dependent moisture driven backout of nailplates: experimental investigations and numerical predictions

European Journal of Wood and Wood Products - Nailplates are widely used in domestic and low-rise residential housing markets and have begun forming part of mid-rise developments. There is however a...     

Fracture in spruce: experiment and numerical analysis by linear and non linear fracture mechanics

That paper first presents a simplified Damage Mechanics (DM) model for the simulation of fracture in wood. All damage phenomena are assumed to occur on a surface (or a line in a 2D problem). Then test results of mode I fracture in spruce and fir are given. The size effect is investigated. Linear Elastic Fracture Mechanics (LEFM) and DM are compared for a simulation of three point bending tests, classically used for the determination of the fracture energy (G _f) in tension perpendicular to grain. The study of the observed size effect gives the range of applicability of LEFM. The critical energy release rate (G _c) and the fracture energy (Y _f) that are energy parameters of LEFM and DM respectively, are identified for small specimens and compared with the experimentally dissipated energy to fracture the specimen (G _f). Load-displacement curves are correctly predicted with both methods. G _f can be considered as a material parameter and it is verified that a non-linear approach is necessary for the simulation of fracture of small specimens.     

Tensile strength properties of particle boards at different temperatures and moisture contents

The equilibrium tensile properties of urea-formaldehyde (UF)- and phenol-formaldehyde (PF)-bonded particle boards have been studied at moisture contents between 1 and 33% and at temperatures between ?15°C and +45°C. These conditions may occur e.g. during exterior use of the boards. The tensile strength and the modulus of elasticity decrease slightly while the strain at rupture increases slightly between 1% and 7% moisture content. These effects are similar for UF- and PF-bonded boards. Above 7% moisture content the tensile strength and the modulus of elasticity decrease markedly while the stretch at rupture increases with increasing moisture content. These changes are more pronounced for UF-bonded boards. The bending stiffness is less affected than the modulus of elasticity since the board thickness increases with increasing moisture content. The tensile properties are not significantly affected by the temperature at low moisture contents, but at high moisture contents the combined effects are more pronounced. The effect of moisture is generally stronger than that of temperature, in the range studied.     

Evaluation of density and strength of Norway spruce wood by near infrared reflectance spectroscopy

The non-destructive evaluation of wood properties by Near Infrared Reflectance Spectroscopy (NIR) has been assessed. The surfaces of specimens of clear wood (Picea abies) were NIR-scanned, and the results compared to such properties as moisture content, density, compression strength and chemical and biological degradation. In addition, the NIR-scans of clear wood specimens were compared to the bending strength of the structural timber from which it had been cut. The NIR dependency of surface roughness was investigated and found to be of minor importance. NIR calibrations for moisture content (MC<30%), density, compression strength and chemical degradation proved that the NIR technique is an excellent non-destructive method (r ²≧0.9; independent test set). Even biological degradation was determined with a most promising accuracy (r ²=0.75; independent test set). For the prediction of the bending strength of timber NIR proved less efficient. However, NIR still contributed to timber strength prediction at the same level as annual ring width, the parameter which is presently visually assessed by timber graders. It is concluded that the NIR method is very versatile in the non-destructive evaluation of wood. The results merit further investigations in order to develop proper models and instrumentation for commercial use.     

Influence of cross-section and knot assessment on the strength of visually graded Norway spruce

The strength of graded timber is determined by a multitude of parameters. Properties of interest are the shape of the cross-section and the wood quality. With regard to strength, wood quality is primarily expressed in terms of knots and knot clusters which, together with the cross-section of the timber, are used to calculate knot ratios. By applying the visual grading rules as given in the German standard DIN 4074-1, the influence of different timber sizes on grading results has been analysed. Different grading approaches for joists and boards exist and are taken into account in the assessment of 5,665 specimens originating from various parts of Europe. It was shown that both the cross-section and the grading method have a major influence on the characteristic strength values of Norway spruce. Limitations of the current standard with respect to its applicability to certain cross-sections are exposed. Alternative, simple grading approaches for boards are proposed. They ensure equal strength values and yields comparable to the rather complicated board rules used nowadays.     

食品药品冷冻干燥数值模拟的现状与展望

数值模拟用于食品药品的冷冻干燥,可以对其进行指导和预测,以提高冷冻干燥效率。本文介绍了国内外冷冻干燥数值模拟的研究现状并着重介绍了冷冻干燥模型一些新的改进,分类归纳了各种模型的假设条件及优缺点,回顾并总结了针对各种数学物理模型的求解方法并分析了各种方法的优缺点。对冷冻干燥数值模拟的发展方向进行了展望,指出:结合各种测量途径建立专门的模型是冷冻干燥数值模拟的一个发展方向。     

Moisture diffusivity in pears: experimental determination and derivation of a mathematical prediction model

In the present work the dependence of moisture diffusivity on pear composition (in particular, water and sugar concentrations) was examined from the drying rate data measured experimentally. A model for predicting diffusivity that accounts for the effects of three variables (temperature, moisture content and sugar concentration) was then developed, and compared with the model presented by Zogzas and Maroulis (Drying Technology 1996;14:1543), which accounts only for the influence of temperature and moisture concentration. It was observed that the value for the activation energy predicted by the model that includes the influence of the sugar concentration is higher, showing that the amount of sugar present influences the diffusion of moisture through the pear. From the results obtained it was possible to conclude that at constant temperature the diffusivity is increased for higher water contents and lower sugar concentrations. Furthermore, it was possible to observe that higher temperatures have a much more pronounced influence on the diffusivity, compared with lower temperatures.     

Free fall analysis of orange fruit using numerical and experimental methods

In this research, the behavior of orange fruit under impact cases was investigated by finite-element modeling in SolidWorks simulation and experimental dropping tests. The values of 0.584 MPa and 0.24 were experimentally determined for modulus of elasticity and Poisson’s ratio as a pre-requirement for simulation. In the simulation tests, orange fruits were dropped from two different heights of 2 and 3 m with a drop angle of 45° relatives to a rigid plane. Experimental tests were also conducted to validate simulation results. Maximum equivalent stress and contact force were obtained from simulation results 0.5826 MPa and 14.95 N, respectively.     

Experimental and numerical validation of the total trapping number for prediction of DNAPL mobilization

The total trapping number (N(T)), quantifying the balance of gravitational, viscous, and capillaryforces acting on an entrapped dense nonaqueous phase liquid (DNAPL) droplet was originally developed as a criterion to predict the onset and extent of residual DNAPL mobilization in porous media. The ability of this approach to predict mobilization behavior, however, has not been rigorously validated in multidimensional systems. In this work, experimental observations of residual tetrachloroethene (PCE) mobilization in rectangular columns are compared to predictions obtained using a multiphase compositional finite-element simulator that was modified to incorporate the dependence of entrapped residual,flow, and transport parameters on the total trapping number. Consistent with calculated NT values (1.21 x 10(-3)-1.10 x 10(-2)), residual PCE-DNAPL was mobilized immediately upon contact with a low-interfacial tension (IFT) surfactant solution and rapidly migrated downward to form a bank of mobile DNAPL. The numerical model accurately captured the onset and extent of PCE-DNAPL mobilization, the angle and migration of the DNAPL bank, the swept path of the surfactant solution, and cumulative PCE recovery. These findings demonstrate the utility of the total trapping number for prediction of DNAPL mobilization behavior during low-IFT flushing.     

An experimental and numerical study of shape stability in laminated timber columns

The study concerns the question of how the shape stability features of laminated columns of Norway spruce can be improved in terms of twist through optimal orientation of the individual laminates. Both experimental testing and numerical simulations were employed for evaluating twist stability. In all the columns studied, deformations were measured experimentally at different moisture content levels. A number of columns were also selected for numerical analysis in order to obtain a more thorough understanding of the twist behavior involved, their geometries and material properties of interest being determined. The experimental results showed the twist stability of the columns to be highly dependant upon the internal orientation of the individual laminates. It was also found that high quality columns in terms of shape stability could be manufactured, even when the center-core material has a strong twist tendency. The numerical simulations performed were in close agreement with the experimental results.     

Influence of fibre strength and yarn structural characteristics on tensile failure of blended spun yarns: a prediction model

The study reports on the static failure behaviour of P/V blended ring, rotor and air-jet spun yarns explained on the basis of fibre failure coefficient. Fibre failure coefficient is an index introduced to represent fibre break and slip in combination occurring during tensile failure. Fibre break/slip during tensile failure is found dependent on fibre strength, fibre cohesiveness and internal structural developments in yarns. Tensile failure behaviour of ring, rotor and air-jet yarns found to be different owing to their difference in fibre consolidation mechanism. The contribution of individual components towards fibre failure coefficient varies with the spinning technology. An attempt has been made to develop mathematical models to explain the spun yarn failure behaviour under static condition. The developed mathematical models have incorporated the fibre property (fibre strength) and few structural characteristics of yarns which are strategically selected to justify the essence of models to enhance the prediction capability. Individual models are developed for ring, rotor and air-jet yarns owing to their structural changes caused by their inherent fibre consolidation mechanism. The developed mathematical models are free from assumptions and based on pure applied mathematics and have very high potential for prediction of spun yarn failure behaviour.     

Modelling mechanical properties of spruce and Douglas fir timber by means of X-ray and grain angle measurements for strength grading purpose

This study proposes a model using data from a scanner (X-ray and grain angle measurements) to perform strength grading. The research also includes global measurements of modulus of elasticity (obtained by vibrations and ultrasound methods), static bending stiffness and bending strength of 805 boards of Douglas fir and 437 boards of spruce. This model can be used in an industrial context since it requires low computational time. The results of this study show that the developed model gives better results than the global non-destructive measurements of the elastic modulus commonly used in the industry. It also shows that this improvement is particularly higher in the case of Douglas fir than for spruce. The comparison has been made on both the quality of the mechanical properties assessment and on the improvement of the grading process according to the European standards by using different index.     

An experimental and numerical study of the effect of friction in single dowel joints

The effect of friction between the dowel and the surrounding timber was studied for single dowel-type joints. The joints tested were divided into two groups of joints, where the surface of the dowels differed. For the first group, the dowels had a smooth surface and for the second group the dowels had a rough surface. A contact-free measurement technique was used in the experimental investigation. In addition to that, numerical simulations were carried out aimed at predicting the load-bearing capacity of the joints tested as well as estimating the coefficient of friction between the dowel and the surrounding timber. Important conclusions from this study, which are supported by previous research, are that the load-bearing capacity of single dowel-type joints increases when the surface roughness of the dowel increases. A very small scatter in the results, in terms of the load-displacement behavior, was seen in the tests with rough surface dowels. For the joints with smooth surface dowels, the elastic response as well as the plastic response varied considerably between different tests. The current version of the European timber code EC5 does not explicitly take into account the effect of friction. In order to take the effect into account embedding tests have to be performed in order to be able to consider the benefits of using dowels with rough surface.     

Comparing dry bond strength of spruce and beech wood glued with different adhesives by means of scarf- and lap joint testing method

Dry bond strength of different adhesives on spruce and beech wood was tested by means of the former DIN standard 53253 (1964)scarf joint and the operative standard lap joint testing method according to EN 302-1 (2004). The failure mode in standard lap joint tests was mainly wood failure, whereas scarf joints failed in the bond line. Due to the high proportion of wood failure, no clear effect of the different adhesives, but a clear effect of wood species on the bond strength was observed for lap joint samples. By contrast, scarf joint testing was far less dependent on wood species and delivered clear differences in the strength of different adhesive systems.     

Model for the prediction of the tensile strength and tensile stiffness of knot clusters within structural timber

In the present paper, a model for the prediction of the local strength and stiffness properties is developed. Compared to existing models, here the local material properties are described according to their morphological characteristics; i.e. the timber boards are subdivided into sections containing knots (knot sections) and sections without knots (clear wood sections). The strains of the corresponding sections are measured during non-destructive tensile tests using an optical camera device. Based on these measurements the tensile stiffness of each particular section is estimated. For the estimation of the tensile strength, destructive tensile tests are performed. Herewith, the tensile strength of the entire timber board is measured. The strength of the other knot clusters are estimated using censored regression analysis. Taking into account the results of the experimental investigation, material models are developed to predict the tensile strength and the tensile stiffness of knot clusters.