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.     

含LT型裂纹木梁起裂载荷确定方法的试验研究

木材裂纹萌生的准确判定对木材损伤断裂的评估具有重要的意义,起裂载荷是标定裂纹萌生的关键参数。本试验以杉木为研究对象,利用声发射技术(AE)、数字图像相关法(DIC)和电测法(EM),对含LT型裂纹木梁的损伤断裂特性进行了试验研究。通过研究木梁在加载过程中声发射参数变化规律以及裂尖区域的表面应变信息的演变,分析木梁裂纹萌生规律并确定起裂载荷Pini。结果表明:声发射累计振铃计数、幅度可有效反映木梁内部损伤的产生和演化,利用声发射参数的变化规律能准确确定含LT型裂纹木梁的起裂载荷Pini;数字图像相关法、电测法可以实时监测木梁表面裂缝尖端区域的应变变化,根据应变演变特征可以有效监测木梁表面裂纹的萌生和扩展。声发射技术、数字图像相关法、电测法在确定木梁起裂载荷Pini方面有较好的适用性,所确定的起裂载荷大小为:电测法>数字图像相关法>声发射。试验结果为研究监测含LT型裂纹木材裂纹萌生的试验方法提供了依据,应用时可结合实际工况选择合适的测量方法。     

木材横纹理断裂及强度准则

以杉木为研究对象从宏观和微观上研究了木才横纹断裂的性质，并阐述了木才的强韧机理。研究表明：木材横纹Ⅰ型裂纹扩展方式是先沿纤维开裂伸展，然后再沿横截面作韧性断裂，其扩展过程分线性、稳定和非稳定性3个阶段；顺纹启裂时的断裂韧性与试件尺寸无关，是木材的固有属性；木材因其多胞及纤维增强的多层胞壁结构，而具有很强的抗横断韧性，不会因裂尖应力奇异性而发生低工作应力破坏，故在对含横纹理裂纹的木构件作安全设计时，建议仍采用传统的强度准则，考虑净尺寸上的常规强度即可。     

Glass fibre reinforced holes in laminated timber beams

Summary An experimental investigation of glass fibre reinforcement of glued laminated timber beams is presented. A polyester resin is used both as matrix and adhesive between the reinforcement and the wood. The main part of the work considers beams with large holes tested in three point bending. Circular and rectangular holes, centred at quarter length of the beams make the strength of wood perpendicular to the grain become critical. Great improvements of strength are obtained with the glass fibres. A comparison between various kinds and combinations of glass fibre reinforcement is made. Further, the reinforcement applied as repair of earlier cracked beams is investigated with positive results. One series of beams without holes is reinforced and tested in four-point bending.This reseach project was initiated to improve the ability of glued laminated timber to compete with steel and concrete, in large free span constructions. The project was funded by the Nordic Industrial Fund (NI), Svenskt Limträ AB and the Swedish National Board for Industrial and Technical Development (NUTEK)     

Ultrastructural features affecting mechanical properties of wood fibres

Abstract

The purpose of this review is to re-examine some of the existing knowledge on the ultrastructure of softwood fibres and modelling of the hygroelastic properties of these fibres. The motivation is that the ultrastructure of wood fibres has a strong influence on fibre properties such as stiffness and hygroexpansion. This structure–property relationship can be modelled with, for instance, composite mechanics to assess the influence of ultrastructure on the fibre properties that in turn control the engineering properties of wood fibre composites and other wood-based materials. Comprehensive information about the ultrastructure is presented that can be useful in modelling the hygroelastic behaviour of wood fibres. Many attempts to model ultrastructure–property relationships that have been carried out over the years are reviewed. Even though models suffer from limiting approximations at some level, they have been useful in revealing valuable insights that can help to clarify experimentally determined behaviour of wood fibres. Still, many modelling approaches in the literature are of limited applicability, not the least when it comes to geometry of the fibre structure. Therefore, an example of finite element modelling of geometrically well-characterized fibres is given. This approach is shown to be useful to asses the influence of the commonly neglected irregular shape on elastic behaviour and stress state in wood fibres. Comparison is also made with an analytical model which assumes cylindrical fibre shape. Predictions of the elastic properties made with analytical modelling of cylindrical fibres and with finite element modelling of geometrically characterized fibres are in concert, but the stress state and failure predictions only show qualitative similarity. It can be concluded that calculations on fibres with the irregular and more realistic geometry combined with experiments on single fibres are necessary for a better and more quantitative understanding of the hygroelastic behaviour and particularly failure of wood fibres. It is hoped that this paper can provide a foundation and an inspiration for modelling, in combination with experiments and microscopy, for better predictions of the mechanical behaviour of wood fibres and wood fibre composites.     

Ultrastructural features affecting mechanical properties of wood fibres

The purpose of this review is to re-examine some of the existing knowledge on the ultrastructure of softwood fibres and modelling of the hygroelastic properties of these fibres. The motivation is that the ultrastructure of wood fibres has a strong influence on fibre properties such as stiffness and hygroexpansion. This structure-property relationship can be modelled with, for instance, composite mechanics to assess the influence of ultrastructure on the fibre properties that in turn control the engineering properties of wood fibre composites and other wood-based materials. Comprehensive information about the ultrastructure is presented that can be useful in modelling the hygroelastic behaviour of wood fibres. Many attempts to model ultrastructure-property relationships that have been carried out over the years are reviewed. Even though models suffer from limiting approximations at some level, they have been useful in revealing valuable insights that can help to clarify experimentally determined behaviour of wood fibres. Still, many modelling approaches in the literature are of limited applicability, not the least when it comes to geometry of the fibre structure. Therefore, an example of finite element modelling of geometrically well-characterized fibres is given. This approach is shown to be useful to asses the influence of the commonly neglected irregular shape on elastic behaviour and stress state in wood fibres. Comparison is also made with an analytical model which assumes cylindrical fibre shape. Predictions of the elastic properties made with analytical modelling of cylindrical fibres and with finite element modelling of geometrically characterized fibres are in concert, but the stress state and failure predictions only show qualitative similarity. It can be concluded that calculations on fibres with the irregular and more realistic geometry combined with experiments on single fibres are necessary for a better and more quantitative understanding of the hygroelastic behaviour and particularly failure of wood fibres. It is hoped that this paper can provide a foundation and an inspiration for modelling, in combination with experiments and microscopy, for better predictions of the mechanical behaviour of wood fibres and wood fibre composites.     

竹板增强单板层积材组合梁受弯性能

单板层积材加工成梁构件应用于建筑结构材时,由于材料本身的强度和刚度不足,其结构构件不能满足现代多、高层以及大跨度建筑的需求。竹集成材是原竹经过切削成竹片、低温干燥、碳化、涂胶,再同方向平面或侧面组坯、热压胶合而成的竹基复合材料,其力学性能与稳定性优于木材。将集成竹材作为增强材料用于加固单板层积材梁是一种简单有效的提高梁极限承载力的方法。通过进行竹板增强单板层积材组合梁四点弯曲试验,研究了集成竹板对单板层积材受弯性能的增强效果。结果表明,在单板层积材受弯构件上下部粘贴集成竹板可提高构件极限承载力10%～50%。同时,考虑单板层积材和集成竹材料的非线性,推导出了适用于组合梁的极限承载力计算公式,计算结果与试验结果吻合良好。     

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.     

Fracture in Norway spruce wood treated with <Emphasis Type="Italic">Physisporinus vitreus</Emphasis>

Changes in the fracture behaviour of Norway spruce tonewood after fungal treatment were studied. Specimens were incubated for 6, 9 and 12 months with Physisporinus (P.) vitreus. Fracture tests were performed in a compact-tension fracture experiment set-up, and the results were compared with the morphological analysis of the degraded wood structure and transverse sections of the crack tip viewed under light and fluorescence microscopy. It was evident that both the failure load and critical stress intensity factors were reduced in wood after prolonged incubation periods. Weight losses were significantly higher in sapwood than in heartwood. With prolonged incubation periods, the frequency of unstable fracture and brittle behaviour of the wood increased. In untreated wood, cracks were initiated in the earlywood. The process involved both delamination of the cells within the middle lamellae and rupture of the cell walls, inducing a zigzag crack tip pattern. In fungally treated wood, cracks often commenced from the intersection between late- and earlywood, resulting in a straight tangential crack line. Micrographic images showed that P. vitreus was more active in the secondary walls of latewood tracheids. In this region of the wood, the cell walls were strongly degraded after 9–12 months of incubation, resulting in a reduction in tensile strength, even though the wood did not show strong features of decay at the macroscopic level.     

Three-dimensional modelling of wood fracture in mode I,perpendicular to the grain direction at fibre level

Abstract

A coupled experimental and numerical modelling approach was used to investigate the mechanism of softwood fracture at the fibre level. First, a three-dimensional mixed lattice–continuum fracture model was developed to investigate the mechanism of wood fracture, taking into account the porosity of its structure and its heterogeneities at the fibre level. The critical volumes in the specimen where crack propagation was more probable were modelled by a lattice that could show the alternation of earlywood and latewood fibres, and the other regions were considered as the continuum medium. The proposed model was used to investigate the mode I fracture of a small softwood sample in RL orientation. Secondly, a method was developed for microscopic observation of the crack trajectory and investigating the mechanism of initiation and propagation of cracks. This approach was used for microscopic investigation of the fracture behaviour of spruce specimens in mode I and RL orientation. The results of the numerical study were compared with the experimental results. The prepeak and postpeak behaviour of the obtained stress–displacement curve and also the crack opening trajectory in cross-section and longitudinal section in the model and experiments were in good agreement. Both the model and the microscopic observation showed that in mode I fracture and RL orientation, the main trajectory of the crack propagates in the earlywood ring.     

Three-dimensional modelling of wood fracture in mode I, perpendicular to the grain direction at fibre level

A coupled experimental and numerical modelling approach was used to investigate the mechanism of softwood fracture at the fibre level. First, a three-dimensional mixed lattice-continuum fracture model was developed to investigate the mechanism of wood fracture, taking into account the porosity of its structure and its heterogeneities at the fibre level. The critical volumes in the specimen where crack propagation was more probable were modelled by a lattice that could show the alternation of earlywood and latewood fibres, and the other regions were considered as the continuum medium. The proposed model was used to investigate the mode I fracture of a small softwood sample in RL orientation. Secondly, a method was developed for microscopic observation of the crack trajectory and investigating the mechanism of initiation and propagation of cracks. This approach was used for microscopic investigation of the fracture behaviour of spruce specimens in mode I and RL orientation. The results of the numerical study were compared with the experimental results. The prepeak and postpeak behaviour of the obtained stress-displacement curve and also the crack opening trajectory in cross-section and longitudinal section in the model and experiments were in good agreement. Both the model and the microscopic observation showed that in mode I fracture and RL orientation, the main trajectory of the crack propagates in the earlywood ring.     

木材干燥后的裂纹与断裂

按照传统的强度理论，对干燥后用作建筑或农村房舍结构的木材进行校核和设计时，常忽视木材内部存在的初始干燥裂纹。本文阐述了木材的正交各向异性特征和其干燥后裂纹，以及裂纹时木材强度的影响，并结合木材正交各向特征时其裂纹与断裂力学行为的研究方法提出了初步的构想。     

The fracture behaviour of single wood fibres is governed by geometrical constraints: in situ ESEM studies on three fibre types

In situ tensile tests were performed in an environmental scanning electron microscope (ESEM) on earlywood, transition wood and latewood cells of Norway spruce (Picea abies [L.] Karst.). In order to examine the single wood fibres in a wet state, a specially designed tensile testing stage with a cooling device was built. The fracture behaviour of the cell types was studied at high resolution while straining. Different failure mechanisms were observed for the three tissue types. The thin-walled earlywood fibres showed tension buckling which gave rise to crack initiation and resulted in low tensile strength, whereas thick-walled latewood fibres predominately failed by transverse crack propagation without fibre folding.     

Damage detection of wood beams using the differences in local modal flexibility

Local damage such as knots, decay, and cracks can result in a reduction of service life due to mechanical and environmental loadings. In wood construction, it is very important to evaluate the weakest location and to detect damage at the earliest possible stage to avoid future catastrophic failure. In this study, modal testing was carried out on wood beams to generate the first two mode shapes. A novel statistical algorithm was proposed to extract the damage indicator by computing the local modal flexibility before and after damage in timbers. Different damage severities, damage locations, and damage counts were simulated by removing mass from intact beams to verify the algorithm. The results indicated that the proposed statistical algorithm is effective and suitable for the damage scenarios considered. The algorithm was reliable for detecting and locating local damage under different damage scenarios. The peak values of the damage indicators computed from the first two mode shapes were sensitive to different damage severities and locations. This approach was also reliable for detecting multiple defects.     

基于模态柔度差曲率的云杉锯材梁缺陷检测

在机械和环境荷载作用下,木质构件和结构的局部缺陷如节疤、腐朽和裂缝等会导致使用寿命的缩减.在木结构工程领域,如果能够尽早对结构和构件的最薄弱位置进行检出和评估,可避免因突然破坏而导致的灾难性事故.在本研究中,通过振动测试获取前两阶模态振型,并利用锯材梁损伤前后的模态柔度矩阵差值曲率提出损伤识别指标.为了验证提案损伤指标的有效性,通过人工切除截面质量来模拟不同损伤程度、不同损伤位置和双数个损伤的情景,试验结果表明损伤指标对不同程度、不同位置和双数个损伤均能准确定位,并可对损伤轻重程度进行定性评价.     

Damage Detection of Picea asperata Sawn Timber Beams Using Modal Flexibility Curvature

The local damages such as knots,decay, and cracks can be translated into a reduction of service life due to mechanical and environmental loadings.In wood construction,it is very important to evaluate the weakest location and detect damage at the earliest possible stage to avoid the future catastrophic failure.In this study,the modal testing was operated on wood beams to generate the first two mode shapes.A novel statistical algorithm was proposed to extract the damage indicator by computing the modal flexibility curvature before and after damage in timbers.The different damage severities,damage locations,and damage counts were simulated by removing mass from intact beams to verify the algorithm.The results showed that the proposed statistical algorithm was effective and suitable to the designed damage scenarios.It was reliable to detect and locate local damages under different severities,locations,and counts.The peak values of the damage indicators computed from the first two mode shapes were sensitive to different damage severities and locations.They were also reliable to detect the multiple damages.     

仿水曲柳玻镁平板的研制

本研究以锯屑或农作物秸秆为填料，以氧化镁及氯等无机原料为胶凝剂，以中碱玻纤布产稆在特制模中成型，晟仿水曲柳玻平板。平板的物理力学性能与外观近似木质水曲柳胶合板，且具有优良的防火性能。     

Examination of the failure behavior of wood with a short crack in the radial–longitudinal system by single-edge-notched bending test

The failure behavior of wood with a short crack was examined by conducting the single-edge-notched bending tests of a radial-longitudinal system on Agathis specimens. In the test, the mode I critical stress intensity factor was measured, and its validity was checked by the result from double cantilever beam testing method. The mode I critical stress intensity factor decreased when the crack length approached zero. With crack length correction, a constant critical stress intensity factor was obtained over a wide range of crack length including crack-free specimen.     

A statistical algorithm for comparing mode shapes of vibration testing before and after damage in timbers

Instances of local damage in timber such as knots, decay, and cracks can be translated into a reduction of service life due to mechanical and environmental loadings. In wood construction, it is very important to evaluate the weakest location and to detect damage at the earliest possible stage to avoid future catastrophic failure. In this study, modal testing was used on wood beams to generate the first two mode shapes. A novel statistical algorithm was proposed to extract a damage indicator by computing mode shapes of vibration testing before and after damage in timbers. The different damage severities, damage locations, and damage counts were simulated by removing mass from intact beams to verify the algorithm. The results showed that the proposed statistical algorithm is effective and suitable for the designed damage scenarios. It is reliable for the detection and location of local damage of different severities, location, and number. The peak values of the damage indicators computed from the first two mode shapes were sensitive to different damage severities and locations. They were also reliable for the detection of multiple cases of damage.     

Swelling of a fibre reinforced composite in which the matrix is water reactive

Summary The swelling behaviour of an elastic fibre reinforced composite consisting of an inert fibrous phase embedded in a water reactive matrix is described. It is supposed that the addition of water causes the matrix to swell and become more compliant while the fibres passively resist the swelling stresses exerted by the matrix. A concise relation for the shrinkage or swelling strain is derived in terms of moisture content, the volume proportions of the phases, the overall elastic constants of the composite, the elastic constants of the matrix, and the moisture content at which the reinforcing elements are stress free.