Wedge splitting experiments on three-layered particleboard and consequences for cutting

Three layered particleboard is a widely used and important plate material used for construction and furniture production. This implies that material parameters such as strength and elasticity in bending are determined. However, usually little or nothing is known about the fracture properties. This paper has the goal to investigate the material fracture properties in the three layers and discuss the linear and non-linear fracture mechanical properties with regard to board production and its consequences for cutting. It could be shown that the fracture properties of the solid boards are dominated by the bigger chips in the core of the plate and the fracture properties of boards are best characterized by the specific fracture energy. The linear elastic- and the non-linear elastic fracture mechanics approach of the outer layers shows essentially similar results, represented by brittle fracture. For the prediction of the cutting performance, linear elastic fracture mechanics parameters are considered to be more appropriate because these parameters describe the limit of linear elastic behaviour and therefore the case of smallest deformation zone. High stress concentration at the tool tip and low permanent deformation in the bulk material give the best cutting quality.     

涤纶基材碱减量处理对建筑膜材力学性能的影响

建筑膜材是发展相当迅速的一类复合材料。为提高膜材的剥离强度,采用碱减量的方法对涤纶基材进行处理,研究了碱减量处理工艺对其膜材机械性能的影响。实验发现,通过对涤纶基材进行碱减量处理,可有效地提高其建筑膜材的剥离强度,但对拉伸断裂强度有一定的影响。     

考虑滑移效应的钢-混凝土组合梁弹性受弯承载力计算

以冷弯薄壁U型钢-混凝土组合梁为研究对象,对组合梁进行了理论分析,建立了考虑滑移效应的弹性抗弯强度计算公式,并计算了8根简支冷弯薄壁U型钢-混凝土组合梁弹性抗弯承载力.将理论计算值与相关文献中的试验结果进行对比,结果表明:考虑滑移效应的弹性弯矩计算值与试验值吻合较好,滑移效应对组合梁弹性抗弯承载力降低的影响不能忽略.     

气门弹簧钢丝的疲劳强度、寿命预测及损伤容限

讨论气门弹簧钢丝的疲劳强度、寿命预测及其影响因素。疲劳断裂是气门弹簧失效的主要模式，它是疲劳裂纹萌生及其扩展的结果。疲劳寿命一般包括疲劳裂纹萌生寿命和裂纹扩展寿命两部分。在介绍一些传统的寿命预测方法的基础上，重点论述从断裂力学与（）方程来预测弹簧（材料）的疲劳寿命。针对气门弹簧钢丝的发展方向及质量要求提出建议。     

Fracture mechanics models applied to delayed failure of LVL beams

Several theories for modelling fracture and slow growth of a crack in wood have been developed. The various models may be differentiated by the specifically regarded stress levels, failure mechanisms and averaging procedures. This paper deals with the application of viscoelastic fracture mechanics models to predict delayed failure of a timber element in bending. Simulations are compared to experimental results of bending tests carried out on LVL (Laminated Veneer Lumber) notched beams. This analysis emphasizes the influence of the geometry and of the size of the beam as well as of the damage area on the delayed failure.     

Moment resisting connections composed of friction-welded spruce boards: experimental investigations and numerical strength prediction

Research on friction welded wood-to-wood connections has shown high potential for further development regarding the industrial application of timber construction. This paper addresses the question to which extent welding of wood can be applied to fabrication of cross-laminated timber panels (CLT). Those panels can be used as wall elements, in which load transfer occurs by transmission of moments and shear stresses. For this purpose, experimental investigations on friction welded L-shaped specimens were carried out. The welded bond between the boards, arranged perpendicularly to their fibre directions, represents points of intersection of those CLT panels. The results allowed for (a) the determination of a fracture mechanism of welded timber connections under torsion, (b) the definition of a set of design parameters influencing the resulting joint strength and (c) the comparison between the performance of welded timber joints and those of commonly glued alternatives. The experimental investigations were compared to results obtained by numerical modelling, thus allowing a probabilistic strength prediction algorithm for predicting the joint capacity as a function of the surface size.     

Einfluß der Bauteilgröße in der linearen und nichtlinearen (Holz-)Bruchmechanik

Fracture mechanics models for assessment of load bearing capacity of (wood) structures imply an influence of structure size such that larger structures show lower capacities compared to smaller ones of alike shape. The size effect scale depends on whether principles of linear or nonlinear fracture mechanics (LEFM resp. NLFM) are adequate. Following a brief survey of linear fracture mechanics the LEFM size effect law is given and compared to Weibull theory. The necessity of an LEFM application limit along with smaller sizes is demonstrated. next, basics of nonlinear fracture mechanics especially of strain softening materials such as wood are outlined and terms like fracture process zones and characteristic lengths are discussed. It is shown how Bazant's NLFM size effect law enables the link between classic strength approaches and linear fracture mechanics.     

Einfluß des querzugbeanspruchten Volumens auf die Tragfähigkeit gekrümmter Träger und Satteldachträger aus Brettschichtholz

To study the influence of volume on the load carrying capacity of glued-laminated curved beams and curved pitched beams tests with full-scale specimens have been carried out. The distribution of the tension stresses perpendicular to the grain was analysed using a finite element computer programme. The relationships between the tensile strength and the stressed volume, the shape of the beams and the load conditions is evaluated on the basis of Weibull's theory of brittle fracture. A comparison of the results with the current design proposals in the draft of EUROCODE 5 “Structural timber design code (1987)” indicated higher characteristic ultimate tensile stresses and for some cases new stress distribution factorsk _dis.     

Quasi-brittle fracture and size effect of glued laminated timber beams

The load bearing capacity of glued laminated timber beams is size dependent. A material based size effect occurs due to the natural variability of the timber properties and can be explained with the Weibull weakest link theory. The Weibull type size effect has received much attention in the timber research community in the past as it is the dominating size effect regarding the load bearing capacity of timber beams. In this paper, it is shown that a further deterministic size effect, which originates from the timber fracture behavior, influences the load bearing capacity of glued laminated timber beams. It is the size effect associated with quasi-brittle behavior. The paper covers a short theoretical review on fracture related size effects and a study comparing experimentally tested beams of several sizes with detailed numerical simulations considering the natural growth characteristics of timber. Further, an analytical model illustrating the features of the fracture related size effect on the load bearing capacity of glued laminated timber beams is presented. It is demonstrated that the simulated load bearing capacity of the tested glued laminated timber beams distinctively improves if the quasi-brittleness is taken into account by using a softening constitutive law.     

改性对大豆分离蛋白可降解材料力学性能的影响

利用环氧氯丙烷对大豆分离蛋白(SPI)进行改性制备可生物降解材料,探讨了改性制备因素对大豆分离蛋白可降解材料力学性能的影响.结果表明,改性制备因素对材料的力学性能有显著影响;材料形成过程中的抗拉强度与分子表面巯基含量的变化没有必然的联系;经改性后可在SPI分子间形成交联大分子,同时维持蛋白分子空间构象的作用力也会对材料的力学性能产生影响.     

Nichtlineares Bruchmechanik-Maßstabsgesetz für Fichte bei Zugbeanspruchung senkrecht zur Faserrichtung

A report is given on investigations, concerning fracture mechanics based size effect of clear spruce wood loaded in mode I in RL-crack system. The tests were performed with single notched, geometrically almost 2D-similar SENB bending specimens. The beam depths,d, varied from 10 to 320 mm, the notch lengths were constant 0,6 timesd. The entity of test results could be approximized well by means of a steady nonlinear fracture mechanics size effect law by Bazant, whereas the Weibull weak link hypothesis, according to fracture mechanics based failure, is rather inapt. The received size effect conformed nearly exactly to the size effect law of linear fracture mechanics (LEFM) with the larger pecimens (d≥160mm). Along with, increasingly smaller sizes a reduced size effect was abtained agreeing well with assumptions of nonlinear fracture mechanics (NLFM), i.e. growing influence of softening fracture prozess zone. The experimental transition region between NLFM and LEFM can be specified via brittleness number β～10, a fracture mechanics parameter independent of individual test configuration.     

Analytical cracking load estimation of Laminated Veneer Lumber (LVL) beams with holes

Predicting the load-carrying capacity of timber beams with holes cannot be performed by usual analysis methods if the failure of the beam is governed by the crack initiation and propagation around the hole at low load levels. Predicting the cracking load is an important design issue because it corresponds to the load-carrying capacity of the timber beam before the crack propagation. One of the models that can be used for the fracture formulation is that of a beam on elastic foundation. In this model a part of the beam is assumed to follow the conditions of beam with elastic foundation which has spring stiffness equal to the fracture properties of the material in the crack surface. Based on beam on elastic foundation model, the cracking load prediction is the target of the paper. Some closed form solutions for the beam with hole are presented. The formulation has been derived for two cases of pure shear and pure bending moment. Finally a semi-empirical formulation for combination of shear and bending moment in the section is presented. The model predictions are compared with the results of an experimental program showing good correlation. The analytical model can therefore be proposed for future revisions of codes of practice such as the Eurocode 5.     

Deflections in sawn timber beams with stochastic properties

A stochastic model is proposed to study the behavior of structural sawn beams of Argentinean Eucalyptus grandis with the aim of improving the predictability of the elastic deformations. The enhancement of the mid-span deflection calculation is based on a probabilistic model of the Modulus of Elasticity (MOE) and the representation of its lengthwise variability through a random field. The standard model that uses a MOE variable assumed random from piece to piece but deterministic (constant) within each piece is obtained as a particular case. In order to obtain a statistical representation of the MOE, the Principle of Maximum Entropy (PME) is employed. Experimental data obtained from bending tests are employed to find the parameters of the derived Probability Density Function (PDF). The PDF of the mid-span deformations is numerically obtained through the Stochastic Finite Element Method (SFEM) and Monte Carlo Simulations (MCS). Numerical results are validated with experimental values. Deflections of structural sized beams under usual loads are obtained. Finally, the stochastic model is used to compare with the serviceability requirements established in the Argentinean design code. It is shown that the structural performance of timber beams is found through a more realistic material approach.     

竹纤维/低熔点涤纶复合毡的性能研究

采用物理方法将竹纤维与双组份低熔点涤纶按一定比例混合,经过开松、梳理、铺网等工序制备出多层纤维网,利用热风烘压方法将多层纤维网制成毡基材料。利用扫描电镜观察并表征了竹纤维和复合毡的形态;测试分析了竹纤维的结晶度、官能团结构、纤维直径及其分布、拉伸断裂强力性能;对双组份低熔点涤纶的长度与熔点进行了表征分析;对竹纤维/双组份低熔点涤纶复合毡的拉伸断裂强力性能、透气性与传热性能进行了分析评价。结果表明,纤维复合毡的性能与竹纤维的含量相关,断裂强力与传热性能及竹纤维含量呈正相关,透气性随着竹纤维含量的增加先变大后变小。     

Determination of characteristic strength and stiffness values in glued laminated beams of Argentinean <Emphasis Type="Italic">Eucalyptus grandis</Emphasis> according to European standards

The present paper reports the results of an investigation regarding the determination of characteristic strength and stiffness values in glued laminated beams of fast-growth Argentinean Eucalyptus grandis according to the criteria established in European standards. An empirical research project with 100 beams in structural sizes and 191 finger-jointed laminations was carried out. Test results allow to analyse the mechanical property values of this glued laminated timber in comparison with those adopted by the strength class system established in the European standard EN 1194 (1999), and they reveal a very high ratio of modulus of elasticity to density. Test results were also compared with values obtained by means of the equations provided by EN 1194 (1999). The effectiveness of the criteria established in this European standard for determining mechanical properties in beams of this deciduous species is analysed through the discussion of test results and those obtained from calculations based on lamination properties.     

Verifikation von Festigkeitsmodellen für die Brettschichtholz-Biegefestigkeit

The aim of the study was to validate predictive values of new strength models for the characteristic glulam bending strength. Softwood glulam beams were tested in bending. In order to exactly assess the structural and mechanical properties of the laminations used for the production of glulam beams, representative samples were taken from the boards and the finger joints and tested in tension. The experimental investigation was conducted on two different strength levels in order to obtain two independent comparisons between predictive and empirical values. The experimental values of the board and finger joints were used as input data for the strength models to calculate the predictive values of the glulam beams. The predictive values for the characteristic glulam bending strength and the experimental ones differed not more than??6%. This indicates that the new strength models are suitable to reliably determine the characteristic glulam bending strength.     

Beigefestigkeit von Brettschichtholzträgern in Abhängigkeit von den festigkeitsrelevanten Einflußgrößen

The bending strengths of glulam beams with wood failure and failure due to finger joints depending on beam size and load configuration was investigated. Size effects are more pronounced in case of beams with finger joint failure. This may be explained by the higher variation of strength values in case of “material” finger joint. On the basis of the strength values of the corresponding standard beams, these investigations make it possible to determine the “true” strength distributions of the two influencing “materials” wood and finger joint for any beam size and load configuration.     

基于断裂力学的立式杀菌锅全寿命疲劳分析

目的:研究一种新型立式杀菌锅在热机循环载荷下的疲劳强度,特别是在锅体出现裂纹后的剩余疲劳寿命以及影响杀菌锅疲劳裂纹扩展的因素.方法:从杀菌锅结构完好的设计疲劳寿命和有裂纹后的剩余疲劳寿命两个方面对其进行全寿命疲劳分析.采用Workbench分析杀菌和3种循环载荷下杀菌锅的力学特性;基于SGN曲线研究杀菌锅在3种交变应力下的设计疲劳寿命;基于断裂力学原理研究初始裂纹尺寸、压力、温度对有裂纹杀菌锅应力强度因子和剩余疲劳寿命的影响.结果:此类立式杀菌锅的设计疲劳寿命为5×10⁵次,满足设计需要且有一定安全余量;基于断裂力学分析得出杀菌锅裂纹尺寸寿命曲线,对含缺陷杀菌锅剩余寿命进行预测,具有一定创新性.结论:使用过程中应关注锅体内部裂纹的产生和扩展情况,可以根据试验提出的方法对杀菌锅裂纹缺陷进行强度分析和寿命预测.     

风电机组用高强度螺栓断裂分析

风电机组用M39×315高强度螺栓在安装时断裂。通过化学成分分析,宏观和微观检验等方法对断裂原因进行分析。结果表明:螺栓断口为典型的在拉伸载荷作用下发生的混合型断裂,以脆性断裂为主,断口区域大小为400～550μm。螺栓心部存在少量沿晶分布的小块状铁素体,表明螺栓在热处理时加热不足。断裂螺栓钢中表面与心部非金属夹杂物无明显差别,D类环状氧化物夹杂物级别约为粗D2～D2.5级。断裂源区呈现的沿晶断裂和解理断口主要是螺栓热轧盘条存在不良组织缺陷所致。建议加强对螺栓B7钢原材料热轧态组织的进厂检验,分析其带状组织及热处理组织缺陷。对每批螺栓淬火后检查其淬透情况,以确保高强度螺栓心部马氏体组织体积分数达到90%。     

采用ABAQUS的粘胶机织物拉伸力学性能仿真

为预测织物拉伸性能,采用有限元方法对织物拉伸力学进行数值模拟分析。在实测织物几何结构参数的基础上,借助纺织建模软件Texgen建立了织物模型;利用有限元软件ABAQUS模拟织物拉伸环境,设置材料属性、相互作用和边界条件,得到织物拉伸变形后应力分布云图以及拉伸时应力—应变曲线图等数值模拟结果;最后通过织物拉伸强力测试实验对数值模拟结果进行了验证。结果显示：模拟所得应力—应变曲线和实验所得拉力—伸长曲线上升趋势大致相同;模拟所得最大拉伸应力与实验所得拉伸应力平均误差为3.03%,证明了采用ABAQUS有限元软件模拟粘胶织物拉伸力学性能的是可行的。