Quasi-non-linear fracture mechanics analysis of the splitting failure of single dowel joints loaded perpendicular to grain

A quasi-non-linear fracture mechanics model based on beam on elastic foundation theory is applied for analysis of dowel joints with a single dowel loaded perpendicular to grain. The properties of the elastic foundation are chosen so that the perpendicular-to-grain tensile strength and fracture energy properties of the wood are correctly represented. It is shown that this particular choice of foundation stiffness makes a conventional maximum stress failure criterion lead to the same solution as the compliance method of fracture mechanics. Results of linear elastic fracture mechanics are obtained as a special case by assuming an infinitely large value of the foundation stiffness. Results of tests on so-called plate joints are compared with theoretical predictions, showing good agreement for variations in initial crack length as well as edge distance.     

Fracture of multiply-bolted joints under lateral force perpendicular to wood grain

The crack initiation and propagation of multiplybolted joints subjected to lateral forces perpendicular to the grain were analyzed. Two types of bolted joint were subjected to lateral loads perpendicular to the grain. One had joints of two bolts aligned with the wood grain (type H), and the other had joints of two or three bolts aligned perpendicular to the grain (type V). The crack initiation and propagation were analyzed by means of the average stress method (ASM) and linear elastic fracture mechanics (LEFM), respectively. The maximum loads calculated by LEFM agreed comparatively well with the experimental results, and it was proved that the LEFM was an appropriate tool to analyze the fracture of multiply-bolted joints subjected to a force perpendicular to the grain. It was also found that the multiply-bolted joints failed with the fracture of the wood before the joints yielded, and that it caused a considerable decrease of the maximum loads. The reduction of strength should be considered in the design of multiply-bolted joints subjected to lateral forces perpendicular to the grain.Part of this work was presented at the annual meeting of the Architectural Institute of Japan, Hikone, September 1996     

Quasi-non-linear fracture mechanics analysis of splitting failure in simply supported beams loaded perpendicular to grain by dowel joints

A quasi-non-linear fracture mechanics model based on beam on elastic foundation theory is applied for analysis of the splitting failure of dowel joints loaded perpendicular to grain. Simply supported beams symmetrically loaded by two dowels are considered, and the effects of edge distance, dowel spacing, and distance between dowels and supports are accounted for. The foundation modulus used in the beam on elastic foundation model is chosen so that the perpendicular-to-grain tensile strength and fracture energy properties of the wood are correctly represented. This ensures that a conventional stress analysis and failure criterion lead to the same solution as the compliance method of fracture mechanics. A semiempirical efficiency factor is proposed to account for the influence of the total beam depth, which does not enter the beam on elastic foundation model, but the effect of which is evident from tests. It is shown that the so-called Van der Put/Leijten model, which recently has been adopted in Eurocode 5, appears as a special case of the model presented. Tests on simply supported beams with a single dowel joint at midspan are compared with the theoretical predictions. Various edge distances, beam depths, and spans were tested.     

线弹性断裂力学原理在木材中应用的特殊性与木材顺纹理断裂

阐述了线弹性断裂力学 (LEFM)的原理以及在木材中应用的特殊性 ,并以杉木和马尾松为研究对象 ,采用不同试样和方法测定了木材的顺纹断裂韧性KTLIC 。研究表明 ,建立在各向同性体之上的LEFM原理对木材裂纹顺纹扩张是适用的 ,其顺纹断裂韧性是木材的固有属性     

Fracture analysis of perpendicular to grain loaded dowel-type connections using a 3D cohesive zone model

The strength and fracture behavior of dowel-type connections with stiff dowels loaded perpendicular to grain was studied by nonlinear 3D finite element (FE)-analysis. A cohesive zone model was used to model the perpendicular to grain fracture of the wood, i.e., failure by wood splitting along the grain. The influence of load eccentricity and dowel-to-loaded-edge distance was studied for a plate type of geometry loaded in tension and for a simply supported beam loaded in bending. The strength found from the FE-analysis is compared to strength from experimental tests with centric loading, showing overall good agreement. Numerical results for centric loading are further compared to strength predictions according to the linear elastic fracture mechanics (LEFM)-based design criterion present in Eurocode 5 (EN 1995-1-1:2004). The comparison showed good agreement regarding the relative influence of connection geometry, but the design criterion appears, however, to yield unconservative strength predictions. The results of the FE-analyses regarding dowel load eccentricity showed that such loading conditions may yield significantly lower strengths compared to centric loading. An approximate engineering method to account for the strength reduction due to load eccentricity is, furthermore, presented.     

Fracturing of wood under superimposed tension and torsion

A testing method using circumferentially notched round bars for investigating mixed mode behaviour under loading in tension and torsion is applied to wood. The applicability of the method to anisotropic materials is investigated for two types of wood, beech and spruce, considering the longitudinal and radial orientation with respect to the stem axis of the tree. The strong anisotropy of wood requires different evaluation procedures for radial and longitudinal sample orientation. The K-concept of linear elastic fracture mechanics (LEFM) and concepts of non-linear elastic fracture mechanics (NLEFM) were used for the evaluation of radial and longitudinal samples, respectively. Differences between the investigated wood types under radial orientation, in their durability to withstand torsional loads, could be observed by examining ratios of the values of the fracture toughness in mode III against mode I. Micrographs of the fracture surfaces support the assumption that the higher amount of wood rays in beech is responsible for the higher toughness under torsion. In case of longitudinal specimen geometry it was found that at very high levels of torsional deformation beech and spruce reach similar values in their specific fracture energy in mode I.     

Effect of decay on shear performance of dowel-type timber joints

Monotonic and reversed cyclic loading tests were conducted on dowel-type timber joints with varying degrees of wood decay due to Fomitopsis palustris (Berk. et Curt.), a brown rot fungus, and the effect of decay on various shear performances of dowel-type joints was investigated. For joints affected by the brown rot fungus, the initial stiffness, yield load, and maximum load of dowel-type joints were significantly decreased, even with a small mass loss of wood. The reductions in shear performance were the largest for initial stiffness, followed by yield load and maximum load, in that order. For a 1% reduction of the yield load, initial stiffness and maximum load showed reductions of 1.15% and 0.77%, respectively. When dowel-type joints that had been exposed to the brown rot fungus were subjected to reversed cyclic loading, the gap between the dowel and the lead hole of the wood was increased and equivalent viscous damping was decreased. These results indicate that decay around the dowel lead hole especially affects the load-displacement behavior at small displacement level, and dowel-type joints under cyclic loading have very low resistance to forces acting on the wooden structure. Part of this report was presented at the 5th Symposium on Timber Bridges of the Japan Society of Civil Engineers, Tokyo, July 2006; the 56th Annual Meeting of the Japan Wood Research Society, Akita, August 2006; and the Annual Meeting of the Architectural Institute of Japan, Fukuoka, August 2007     

Elastic layer model for application to crack propagation problems in timber engineering

A fracture mechanics model for analysis of crack initiation and propagation in wood is defined and applied. The model has the advantage of being simple, yet it enables reasonably general and accurate analysis commonly associated with more complex models. The present applied calculations are made by means of the finite element method and relate to progressive cleavage fracture along grain. The calculations concern a tapered double cantilever beam specimen and an end-notched beam. Comparisons are made of experimental test results. The fracture properties of the wood are modelled by means of a very thin linear elastic layer located along the crack propagation path. The properties of the layer are such that the strength and fracture energy of the wood are represented correctly. This makes a single linear elastic calculation sufficient for strength prediction. Both crack development and pre-existing cracks can be analyzed. Both material strength and fracture energy and stiffness are taken into account, their relative influence on structural strength being different for different elements. The fracture layer is in the finite element context represented by joint elements. Propagation of a crack can be analyzed either by a series of elastic calculations corresponding to different crack lengths or by use of a finite element code for non-linear analysis. The computational results include sensitivity analysis with respect to the influence of the various material parameters on structural strength.     

Quasi-non-linear fracture mechanics analysis of splitting failure in moment-resisting dowel joints

This article addresses the splitting failure of moment-resisting dowel-type fastener joints, in which the failure may be attributed to the perpendicular-to-grain loading of one single dowel located close to the end of a beam. A quasi-non-linear fracture mechanics model based on beam on elastic foundation theory is applied. A simple approximation suitable for practical design is also proposed. Model predictions of the influence of edge distances and end distances are compared with test results.     

木榫旋转焊接节点剪切试验研究与理论分析

为研究无胶层合木梁中旋转焊接木榫的抗剪性能,选用山毛榉木榫旋转焊接SPF(spruce-pine-fir)层板,制备20个单榫焊接节点试件开展双剪试验,获得其破坏模式及荷载-位移曲线.试件的破坏模式为木榫受剪出现类双铰破坏,同时开孔附近基材受木榫挤压破坏.试件的平均峰值位移为4.460 mm,平均峰值荷载为6.127 ...     

The effect of elevated temperature exposure on the fracture toughness of solid wood and structural wood composites

Fracture toughness of wood and wood composites has traditionally been characterized by a stress intensity factor, an initiation strain energy release rate (G _init) or a total energy to fracture (G _f). These parameters provide incomplete fracture characterization for these materials because the toughness changes as the crack propagates. Thus, for materials such as wood, oriented strand board (OSB), plywood and laminated veneer lumber (LVL), it is essential to characterize the fracture properties during crack propagation by measuring a full crack resistant or R curve. This study used energy methods during crack propagation to measure full R curves and then compared the fracture properties of wood and various wood-based composites such as, OSB, LVL and plywood. The effect of exposure to elevated temperature on fracture properties of these materials was also studied. The steady-state energy release rate (G _SS) of wood was lower than that of wood composites such as LVL, plywood and OSB. The resin in wood composites provides them with a higher fracture toughness compared to solid lumber. Depending upon the internal structure of the material, the mode of failure also varied. With exposure to elevated temperatures, G _SS for all materials decreased while the failure mode remained the same. The scatter associated with conventional bond strength tests, such as internal bond and bond classification tests, renders any statistical comparison using those tests difficult. In contrast, fracture tests with R curve analysis may provide an improved tool for characterization of bond quality in wood composites.     

木结构销钉类紧固件连接试验方法简介

介绍了欧美木结构中确定销钉类紧固件连接性能的试验方法,主要包括木质材料抗拔握钉力试验、横向承载试验、抗拉穿性能试验和销槽承压强度试验,以推动我国木结构方面的相关研究。     

Three-dimensional finite element modelling of heat transfer for linear friction welding of Scots pine

Finite element numerical analyses were performed to determine three-dimensional heat flux generated by friction to wood pieces during linear friction welding. The objective was to develop a computational model to explain the thermal behaviour of welded wood material rather than experimental methods, which are usually expensive and time consuming. This model serves as a prediction tool for welding parameters, leading to optimal thermo-mechanical performance of welded joints. The energy produced by the friction welding of small wood specimens of Scots pine (Pinus sylvestris L.) was determined by thermocouples and used as input data in the model. The model is based on anisotropic elasticity and the thermal properties were modelled as isotropic. This numerical simulation gave information on the distribution of the temperature in the welding interface during the entire welding process. A good agreement between the simulation and experimental results showed the appropriateness of the model for planning welded wood manufacture and prediction of thermal behaviour of wood during other mechanically induced vibration processes. The specimens presented in this model required a heat flux of 11 kW/m² to achieve a satisfactory welding joint.     

Influence of span/depth ratio on the measurement of mode II fracture toughness of wood by end-notched flexure test

The influence of the span/depth ratio when measuring the mode II fracture toughness of wood by endnotched flexure (ENF) tests was examined. Western hemlock (Tsuga heterophylla Sarg.) was used for the specimens. The ENF tests were conducted by varying the span/depth ratios; and the fracture toughness at the beginning of crack propagation G_IIc was calculated by two equations that require the load-deflection compliance or Young's modulus. Additionally, the influence of the span/ depth ratio on the load-deflection compliance was analyzed by Timoshenko's bending theory in which additional deflection caused by the shearing force is taken into account. The following results were obtained: (1) When the span/depth ratio was small, the fracture toughness calculated with the data of load-deflection compliance was large. In contrast, the fracture toughness calculated with the equation containing Young's modulus tended to be constant. (2) In the small span/depth ratio range, the load-deflection compliance was estimated to be larger than that predicted by Timoshenko's bending theory. (3) To obtain the proper fracture toughness of wood with a single load-deflection relation, the span/depth ratio should be larger than that determined in several standards for the simple bending test method of wood, 12:16.     

Numerical analysis on tensile performance of bolted glulam joints with initial local cracks

Under varying climate conditions, cracks are commonly observed in bolted joints, owing to the shrinkage of wood and confinement from slotted-in steel plates and bolts. A three-dimensional finite element model was developed to investigate the mechanical behavior of bolted glulam joints with initial cracks. Wood foundation was prescribed in the model to simulate the local crushing behavior of wood surrounding the bolts. The behavior of wood in compression and the foundation were defined as transversely isotropic plastic in the software package ANSYS. Cohesive zone model was applied in the numerical analysis to consider the propagation of initial cracks and brittle failure of wood in the bolted joints under tension load. The numerical model was validated by the experiments conducted on full-scale specimens and it is indicated that the numerical model has good ability in predicting the failure modes and capacity of tension joints with local cracks. To further investigate the influence of crack number, length and locations, a parametric study was conducted with the verified model. Moreover, to study the effects of cracks on the behavior of bolted joints with different failure modes, another bolted joint including bolts with different strength grades and diameters was designed and analyzed in the parametric study, which was expected to have bolt yielding failure mode. It was found that the initial cracks can decrease the capacity and initial stiffness of tension joints by up to 16.5 and 34.8%, respectively.     

Estimation of failure lifetime in plywood-to-timber joints with nails and screws under cyclic loading

The performance of plywood-sheathed shear walls is determined at the plywood-to-timber joints. In joints with dowel-type fasteners, such as nails and screws, the fastener is fractured under reversed cyclic loading (e.g., seismic force), reducing the ductility of the joint. The fracture is caused by low-cycle fatigue due to the reversed cyclic bending of the fastener. Therefore, evaluating the fatigue life is important for estimating the ultimate displacement. The main objective of this study is to estimate the ultimate displacement of the joints and to enable load–displacement calculation of single shear joints under reversed cyclic displacement when bending fatigue failure of the fastener occurs. Single shear tests were conducted under different loading protocols, and the damage performances of the fasteners were determined by subjecting them to reversed cyclic bending tests. Based on the results, the failure lifetimes of joints with dowel-type fasteners were estimated. In addition, the fracture mechanism of these dowel-type fasteners was elucidated. CN50-type nails and wood screws with dimensions of 4.1?×?38 and 4.5?×?50 mm were used as fasteners. The single shear tests showed that the smaller the displacements per cycle, the lower are the ultimate displacement and ductilities of the joints. Moreover, load–displacement relationship up to fastener failure can be approximately estimated by combining the yield model and failure lifetime.     

典型直榫梁柱节点在现代木结构中的应用研究

基于Abaqus有限元分析,对不同榫长的实木榫卯连接梁柱节点和榫长为75 mm的不同层数的正交胶合木(CLT)榫卯连接梁柱节点进行数值模拟,分析其受力状态并提取荷载-位移曲线,与使用金属连接件连接的梁柱节点承载力进行对比,探究将榫卯连接应用在现代木结构中的理论依据。研究表明:竖向荷载作用下,榫长75 mm的榫卯连接梁柱节点承载能力处于M8和M10螺栓连接梁柱节点的理论承载力之间;当榫长增大时,节点的最大承载力逐渐提高,榫长150 mm的榫卯节点承载力已经略微超过M12螺栓连接时的理论承载强度,说明榫卯连接梁柱节点在承载能力上可以满足现代木结构建筑的使用要求。由于榫卯节点的承载力主要受到顺纹抗压强度的影响,因此相比于实木连接榫卯节点,CLT榫卯节点的承载力无显著提高。     

Influence of crossing-beam shoulder and wood species on moment-carrying capacity in a Korean traditional dovetail joint

This study investigated the interaction effects of a crossing beam on the moment-carrying capacity of a Korean traditional dovetail joint. In particular, the length of the crossing-beam shoulder (B _s ) and the wood species were varied as important factors. Clearly, the B _s acts as a fastener that improves the performance of timber joints by preventing splitting failure parallel to the grain. All the specimens experienced tension failure by tension force in the direction perpendicular to the grain; therefore, the tension strength perpendicular to the grain could be considered an important property, and standard values could be determined to develop a formula for predicting the structural behavior of the joints or the structural design codes of the joints. The results of the tests indicated that the moment resistance of the joints increased as the length of the crossing B _s and the density of the wood species increased. Joint stiffness results also indicated that the joints became stiffer when the crossing beam had shoulders, but the results were not affected by the length of the B _s . In addition, the joint stiffness was proportional to the density of the wood species.     

Shear strength of beam splice joints with glued-in rods

Splitting failure in beam splice joints with glued-in rods parallel to grain in endwood subjected to pure shear is considered. A simple theoretical expression based on beam-on-elastic-foundation theory and quasi-non-linear fracture mechanics is presented for calculation of the joint strength. Tests were conducted on jointed beams in a four-point bending test setup in which the joints were located at the point of pure shear force. Hardwood dowels with a diameter of 12mm and a glued-in length of 120mm were used as rods, and various beam cross sections and dowel configurations were tested. The theory presented is found to agree well with test results in all cases in which the edge distance of the glued-in rods is relatively small. Some test results indicate that the theory may be conservative in case of large edge distances.     

Quasi-non-linear fracture mechanics analysis of the double cantilever beam specimen

A quasi-non-linear fracture mechanics model based on beam on elastic foundation theory is applied for analysis of the double cantilever beam (DCB) specimen for determination of fracture energy of wood. The properties of the elastic foundation are chosen so that the perpendicular-to-grain tensile strength and fracture energy properties of the wood are correctly represented. It is shown that this particular choice of foundation stiffness makes a conventional maximum stress failure criterion lead to the same solution as the fracture mechanics compliance method. Results of linear elastic fracture mechanics are obtained as a special case by assuming an infinitely large value of the perpendicular-to-grain tensile strength. The quasi-non-linear fracture mechanics model is compared with other models and with results of tests conducted to reveal the influence of the geometrical properties of the DCB specimen. In addition, the appropriateness of choice of the foundation stiffness is investigated.