A stress transformation approach to predicting the failure mode of wood

Summary A simple model, based on the use of transformations of second-order tensors, is presented in this paper to predict the failure mode of wood members stressed in various degrees of parallel-and perpendicular-to-grain tension and parallel-to-grain shear. This type of loading is indicative of structural wood members with cross grain or grain deviations in the vicinity of knots subjected to bending or tension. The model is based on the assumptions that failure is dictated by the presence of any of the aforementioned stresses that exceed the clear wood strength in that mode and that failure does not result from stress interactions. The magnitudes of the applied stresses are normalized relative to the wood strength in that mode. The ratio of applied stress to material strength that is greatest at any particular angle of load to grain is presumed to be the failure mode at that angle. To verify model predictions, optical and microscopic analyses of surfaces of failed specimens loaded in uniaxial tension at angles between 0° and 90° to grain were compared to previously obtained, or otherwise known, surfaces of specimens tested in tension and shear. Specimens tested at various angles to grain demonstrated failed surfaces very much like those associated with specimens loaded in the modes predicted by the model.     

Mechanical and physical screw withdrawal properties behavior of agglomerated panels reinforced with coir and pejibaye fibers

The screw withdrawal resistance and the mechanical behavior of agglomerated panels reinforced with coir and pejibaye fibers were evaluated using through and non-through screw withdrawal tests. Widespread commercially materials, such as Pinus solid wood, Medium Density Fiberboard (MDF) and plywood, were also tested in order to compare the behavior and results obtained with the lignocellulosic-reinforced agglomerated panels. Among the conventional materials tested, the highest maximum withdrawal load/thickness ratio was measured in solid wood. Panels reinforced with natural fibers showed lower withdrawal resistance than conventional materials. However, panels reinforced with coir fibers reached about 74% of the maximum strength achieved by MDF samples. Panels reinforced with pejibaye showed the worst mechanical performance. Hybridization between pejibaye and coir fibers resulted, however, in a performance improvement of approximately 50% in the maximum withdrawal load comparing with panels reinforced only with pejibaye fibers. The behavior along screw withdrawal was analyzed and could be split into four different steps. Each of these steps was correlated to the screw withdrawal rate, and also with the interactions between the material and the screw. The mechanisms involved in each step are presented and discussed.     

Nonlinear superposition model for the behavior of bolted joints

Summary A nonlinear superposition model was developed to assess the load-slip behavior of bolted joints consisting of a single bolt subjected to lateral loading at angles of load to grain. This model characterizes the bolted joint as a pair of orthogonal nonlinear springs aligned parallel and perpendicular to the grain of the wood members. The spring stiffnesses are quantified by a logarithmic or exponential function depending upon whether the connection softens or stiffens with increasing slip. The spring deformations are superimposed to determine the movement of each component of the connection. Deformations of connected members are added vectorially to determine their relative displacement. Spring constant were determined experimentally using metal-to-wood connections. Thick steel side plates were employed to limit the system deformation to the wood component. Wood members were evaluated at angles of load to grain ranging from zero to ninety degrees. Once the spring constants had been determined, the model was executed to predict the load-slip behavior of wood-to-wood connections. These predictions were compared to experimentally obtained load-slip values. The results indicate that the nonlinear superposition concept is a valid approach to predict joint deformation at angles of load to grain.The authors express their gratitude to the Fulbright-Hays Foundation and the Colorado State Agricultural Experiment Station for their financial support of this study     

单面交错铺板带翼木托盘的力学性能及试验研究

通过对单面交错铺板带翼木托盘在均布载荷作用下受力变形情况的分析,建立单面交错铺板带翼木托盘的横梁及铺板的弯曲强度和弯曲刚度的力学模型,得出在标准状态下满足弯曲强度和刚度的托盘的各项指标,为单面交错铺板带翼木托盘的设计以及与普通木托盘在承受均布裁荷的托盘的设计和校核提供理论依据.     

Nonlinear superposition model for the behavior of bolted joints

Summary A previously developed model to predict the load-slip relationship for mechanical joints using one bolt subjected to lateral loading (Part 1) was extended to incorporate the rotational resistance of joints containing two bolts. The rotation is about the long axis direction of the bolts, and considers the wood members oriented at arbitrary angles to the grain. The model utilizes nonlinear translational springs to represent the parallel and perpendicular to grain components of the reaction force present on each bolt resisting the applied moment. A series of experiments were conducted to determine the spring constants of bolted joints in axial loading and to verify the predictions of the mathematical model. Bolted joints subjected to a pure rotation were tested using combinations of steel plates and wood side members experiencing bolt reaction forces at various angles of load to grain. The results indicated an excellent agreement between theoretical predictions and experimentally obtained data.The authors wish to extend thanks to the Colorado State Agricultural Experiment Station for their financial support and to the Fulbright-Hayes Foundation for the educational scholarship and research funding provided to the project     

微米木纤维模压制品握钉结合部载荷分布系数的确定方法

微米木纤维模压制品是通过细胞纵向劈裂而成的微米木纤维模压成型的新型高强度人造木基材料制品.以人造木基材料为主要材料的各种握钉力的计算均以栽荷沿轴向分布的积分方程为基础,而有关其栽荷分布系数的精确计算尤为重要.为此本文首先建立了握钉结合部裁荷分布的力学模型,然后给出了螺纹牙受力变形协调条件,推导出了轴向载荷分布强度与螺纹牙侧面上比压的关系式,分析了微米木纤维铺装纹理方向对其模压制品弹性模量的影响,从而详细地推导出了微米木纤维模压制品握钉结合部载荷分布系数的计算公式.     

Estimation of yield and ultimate strengths of bolted timber joints by nonlinear analysis and yield theory

A finite element nonlinear analysis was conducted on bolted timber joints under lateral loads parallel and perpendicular to the grain. The results obtained from this analysis were compared with the experimental results and calculated values based on the yield theory. The analysis and experiment were performed on double shear bolted joints parallel and perpendicular to the grain with steel side plates and a slotted-in steel plate. It was found from the analysis that the yielding of wood and bolt occurred before the overall yielding of the bolted joint. Shear strength of bolted joints calculated from the yield theory using the embedding yield strength of wood and the yield moment of the bolt showed comparatively good agreement with the shear strength evaluated by 5% offset of the load–slip curve in the experiment and analysis. The shear strength of the bolted joint calculated from the yield theory using the embedding ultimate strength of wood and the ultimate moment of the bolt agreed quite well with the shear strength evaluated by the maximum load up to 15mm slip in the analysis. The former, parallel and perpendicular to the grain, were 11% and 34%, on average smaller than the latter in the experiment.Part of this paper was presented at the 52nd Annual Meeting of the Japan Wood Research Society, Gifu, April 2002; the Annual Meeting of Architectural Institute of Japan 2002, Ishikawa, August 2002; and the World Conference on Timber Engineering 2002, Shah Alam, Malaysia, August 2002     

微米木纤维模压制品螺纹牙受力分布曲线的数学描述

分析了微米木纤维模压制品螺纹连接的受力,提出了其力学分析的基本假设,建立了螺纹牙的力学模型,推导了螺纹牙受力分布函数,对螺纹牙受力分布曲线进行了数学描述,提出了微米木纤维模压制品螺纹牙受力分布的百分比。应用本文提出的理论,可以根据微米木纤维模压制品的刚度和螺纹的几何参数定量计算螺纹牙受力分布,进而计算其螺纹连接的强度,为微米木纤维模压制品螺纹连接的强度计算提供了重要的理论依据。     

Tensile strength of softwood butt end joints. Part 1: Effect of grain angle on adhesive bond strength

To study the effect of grain angle on the adhesive bond strength in wood, three-part Norway spruce wood specimens were bonded and tested in tension. The two axially orientated outer parts of the specimens were joined with the middle part by means of three adhesives typically used for load-bearing constructions, i.e. one-component polyurethane (PUR), melamine-urea-formaldehyde (MUF) and phenol-resorcinol-formaldehyde (PRF). The grain angle of the middle part was varied from 0° (end grain to end grain) to 90° (flat grain to end grain) in incremental steps of 10°. In general, PRF- and MUF-bonded samples exhibited highest tensile strength at end grain to end grain orientation of the three parts, while specimens bonded with PUR showed only 25% of the strength measured for PRF and MUF, respectively. At high grain angles (90°) all specimens showed similar strength values in the range of 30% of maximum strength of MUF- and PRF-bonded specimens. To explain the changing strength levels at different grain angle a composite failure criterion was applied.     

Tensile strength of softwood butt end joints. Part 1: Effect of grain angle on adhesive bond strength

Abstract

To study the effect of grain angle on the adhesive bond strength in wood, three-part Norway spruce wood specimens were bonded and tested in tension. The two axially orientated outer parts of the specimens were joined with the middle part by means of three adhesives typically used for load-bearing constructions, i.e. one-component polyurethane (PUR), melamine–urea–formaldehyde (MUF) and phenol–resorcinol–formaldehyde (PRF). The grain angle of the middle part was varied from 0° (end grain to end grain) to 90° (flat grain to end grain) in incremental steps of 10°. In general, PRF- and MUF-bonded samples exhibited highest tensile strength at end grain to end grain orientation of the three parts, while specimens bonded with PUR showed only 25% of the strength measured for PRF and MUF, respectively. At high grain angles (90°) all specimens showed similar strength values in the range of 30% of maximum strength of MUF- and PRF-bonded specimens. To explain the changing strength levels at different grain angle a composite failure criterion was applied.     

Duration-of-load and creep effects in strand-based wood composite: a creep-rupture model

In order to investigate the creep and duration-of-load (DOL) effects in thick strand-based wood composite products, a creep-rupture model is proposed linking the accumulated damage to creep deformation. Results from long-term constant load tests have been interpreted by means of this creep-rupture model, which is capable of representing the time-dependent deflection and time-to-failure data at different stress levels. The predictions of the model have been verified using results from ramp load test at different rates of loading. The creep-rupture model incorporates the short-term strength of the material, the load history and predicts the deflection history as well as the time-to-failure. As it is a probabilistic model, it allows its incorporation into a time-reliability study of wood composites’ applications.     

热处理对水与木材接触角的影响

以多枝桉和扁柏作供试木材,采用扩大影象法,测定了热处理和空气层对水与木材的接触角的影响。结果表明,热处理温度、时间、木材含水率和在空气中暴露的时间等,对两种试材与水的接触角均有显著的影响。树种不同,其影响程度不一。可根据水与木材的接触角大小,判断各种木材的亲水性。     

Use of Leicester's “rheological model for mechano-sorptive deflections of beams”

Summary The model for mechano-sorptive deformation proposed by Leicester is used to predict stress relaxation of wood drying under constraint from apparent creep of drying wood under constant load. The predictions are compared with measurements on four pairs of beams, one beam in each pair being tested under constant load, the other at constant deflection. Agreement was sufficiently close to demonstrate the value of the model.     

空心胶合木柱轴心受压性能研究

【目的】节能环保理念越来越受关注,木材作为绿色环保的可再生建筑材料可应用在工程中,目前国内木建筑中应用的木柱主要局限于原木圆柱,为了提高木材利用率,同时降低成本,改善受力性能,满足工程需要,提出一种新型的空心胶合木柱,并进行试验研究分析。【方法】使用相同尺寸拱形锯材作为骨架,利用环氧树脂胶粘剂进行胶合,制作3根空心胶合木柱试件进行轴心加载受压试验,研究空心胶合木柱的轴压力学性能,在试验过程中通过仪器记录应变、应力和位移等数据,主要分析木柱的竖向与横向应变、竖向与侧向位移、稳定承载力等特性,并利用ABAQUS有限元软件进行建模对比分析,探讨木柱最终破坏特征。【结果】空心胶合木柱破坏形态主要是整体压屈破坏,达到极限荷载80%左右时,承载能力快速下降,侧向位移随荷载增加而迅速增大,加载过程中存在多个增长台阶;与同截面积原木圆柱比较,理论承载力提高了4.3%,计算得承载能力稳定系数为0.9,材料缺陷对轴心承载力有影响;通过有限元建模分析,材料在弹性阶段理论值与试验值吻合程度较好。【结论】空心胶合木柱应用在实际工程中是可行的,能够满足工程使用需求,充分利用小型锯材,提高了木材利用率,降低了成本,相较于原木圆柱受力性能更好。     

Load-carrying capacity of steel-to-timber joints with a pretensioned bolt

Previous experimental studies reported that bolt pretensioning greatly increases the initial stiffness and load-carrying capacity of bolted joints. It is also a matter of great importance to structural designers to understand the effect of pretension on the load-carrying capacities of bolted joints, and this study presents an extended yield model that considers the fastener’s pretension force. In the extended yield model, the load-carrying capacity was defined as the load at a slip of 15 mm. The ultimate fastener bending angle at the yielded cross section equivalent to this joint slip, which was affected by the fastener’s axial force, was iteratively evaluated in numerical analyses. The introduction of bolt pretensioning largely increased the joint slip resistance at initial loading, but it decreased the ultimate fastener bending angle. This decrease of fastener bending angle resulted in a relatively low stiffness hardening (or secondary stiffness), which is caused by secondary axial forces associated with embedment of steel plates into the wood member. Prediction was verified by the tests of 36 steel-to-timber joints under three different pretension forces and two loading directions relative to the grain. Some of the observed load-carrying capacities of the joints, particularly in loading perpendicular to the grain, however, were not as high as those expected by the numerical analyses considering the given pretension forces.     

An application of the Tsaï criterion as a plastic flow law for timber bolted joint modelling

Summary This paper describes a modelling approach to predict the behaviour of an elementary thin timber bolted joint. The application concerns principally joints with steel side members; bolts have a constant 12 mm diameter with two ratios of end distance to bolt diameter and two bolt clearances. The behaviour of the bolted joints is characterized by a double non linearity; the first one is due to the contact area evolution between the bolt and the hole of the jointed elements. The second one is owing to the evolution of plasticity on the wood. A spring element compatible with isoparametric plane finite elements represents the contact evolution. The elastic-plastic wood is provided with a plastic flow rule according to the Tsa? criterion. This study allows an investigation on the parameters characterizing the Tsa? criterion, particularly F₁₂ which represents the interaction between the principal axis of orthotropy. A two-dimensional model is used. It permits the assessment of the clearance bolt, joint dimensions, wood plasticity and wood grain angle effect on the joint behaviour. The wood grain angle has a non negligible effect on the plastic strains distribution and it can create a parasite loading because the joint tends to rotate even for an axial loading. The results showed a good agreement between experimental values given by some authors and numerically-predicted stresses on the joint. So, the applications concern a two-dimensional joint with anisotropic plastic material. The generalization in the three-dimensional modelling is desirable to take into account the interaction between the wood and a metallic fastener in thick joints with different geometric characteristics.     

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     

胶合木-钢夹板螺栓连接滞回性能试验

[目的]为探明胶合木-钢夹板螺栓连接的动力性能和抗震性能,确保连接件在车辆、机械振动等动力荷载下的可靠性.[方法]针对胶合木-钢夹板螺栓连接的构造特点,考虑胶合木厚度和螺栓直径之比(厚径比)、螺栓顺纹间距、螺栓并列和错列布置方式等参数的影响,设计制作了4类13组共39个胶合木-钢夹板螺栓连接件,在低周反复荷载作用下进行...     

Crash simulation of wood and composite wood for future automotive engineering

ABSTRACT

Multi material mix is a promising approach to reduce weight and the carbon footprint in automotive engineering at competitive costs. As a result, automotive industry is getting more venturous, exploring and applying other materials than metals and plastics – e.g. fibre reinforced plastics (FRPs). In this context, engineered wood products (EWPs) and wood composites should be considered: Wood composites provide high stiffness, strength, excellent damping, high resistance against fatigue and a very low density at low material costs. It is hypothesized that modern wood composites are competitive to metals and artificial fibre-reinforced materials when designed and applied properly. The application of wood and wood composites in automotive engineering calls for precise and reliable material data, required for initial material selection and later in numerical simulation. In this study, a material model normally used for modelling FRPs was adopted. A material database was generated for three hardwood species, to establish the required material parameters and validate material model. Results prove that wooden components can be simulated in crash situations and the selected material model is applicable, even in full vehicle simulation.     

Crack influence on load-bearing capacity of glued laminated timber using extended finite element modelling

Abstract

Most of the cracks are caused by changes in temperature and relative humidity which lead to shrinkage and swelling of the wood and thereby induce stresses in the structure. How these cracks influence the strength of the wooden structure, especially the shear strength, is not well understood. However, it is reasonable to expect that cracks have an impact on the shear strength as they preferably run along the beams in the direction of grain and bond lines. The purpose of this study was to investigate the load-bearing capacity of cracked glulam beams and to find a model that could predict the failure load of the beams due to the cracks. Three-point bending tests were used on glulam beams of different sizes with pre-manufactured cracks. An orthotropic elastic model and extended finite element method was used to model the behaviour of the cracked beams and to estimate the load-bearing capacity. The conclusions were validated by numerical simulations of the mechanical behaviour of three-point bending of glulam beams with different crack locations. The crack initiation load was recorded as the failure load and compared to the experimental failure load. The results of the compaction simulations agree well with the experimental results.