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.     

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.     

A wavelet analysis-based approach for damage localization in wood beams

Free vibration testing was conducted to generate the first two mode shapes for damage detection in timbers. A wavelet transform was proposed to postprocess the mode shapes for damage pattern recognition. The wavelet used here was “db3.” The different damage severities, damage locations, and number of damaged areas were simulated by removing mass from intact beams. The results showed that the chosen wavelet db3 is suitable and that the wavelet coefficients are sufficiently sensitive to identify the existence of damage and its location in cases of different damage location, severity, and number. An edge distortion effect was apparent at the two computing edges where the wavelet coefficients were abnormally high. The wavelet coefficients showed dominant spikes around the damage locations and were zero everywhere else except the two computing edges. The dominant spikes coincided well with the damage location.     

Application of modal-based damage-detection method to locate and evaluate damage in timber beams

Modal-based damage-detection algorithms were used to identify the location of defects commonly found in timber and to estimate their severities. In this study, the authors propose modifications to an existing damage-detection algorithm for locating and evaluating damage by comparing the modal strain energy before and after damage using the first two flexural modes of vibration. Experimental verification was performed on pin-pin supported timber beams by employing the algorithms with extracted modal parameters using experimental modal analysis. Single and multiple cases of damage used to simulate pocket(s) of rot with various severities were inflicted by removing sections of timber beam specimens. The proposed damage indicator, computed from the first two flexural modes, was capable of detecting all damage locations. It was also able to estimate, with reasonable accuracy, the severity of damage in term of loss of sectional moment of inertia. The modified damage index method is generally reliable in detecting the location and estimating the severity of simulated defects in timber beams.     

Application of modal analysis by transfer function to nondestructive testing of wood I: determination of localized defects in wood by the shape of the flexural vibration wave

This study was intended to detect nondestructively some defects such as knots and grain deviations in wood using modal analysis. The shapes of flexural vibration waves at the first mode generated by the tapping of wooden beams were determined using the transfer function. The wave shapes obtained were compared with the theoretical wave shape for a uniform material; and the possibility of detecting defects in wood was examined. The results are summarized as follows: (1) The shapes of flexural vibration waves at the first mode of wooden beams free of defects coincided almost completely with the theoretical wave shape. (2) The shapes of flexural vibration waves of wooden beams containing defects such as knots clearly differed from the theoretical wave shape, especially near the defect. (3) Based on these results, it should be possible to detect the presence of defects and to determine their location in wood.Part of this report was presented at the 49th Annual Meeting of the Japan Wood Research Society, Tokyo, April 1999. This article is translated from the Japanese edition published inMokuzai Gakkaishi Vol. 47, No. 4, 1988     

声发射技术在木材加工领域的应用

作为一种主动无损检测方式, 声发射技术通过分析被测物体内部因能量变化所引发的弹性波的特征, 判断物体内部损伤程度并确定损伤位置。声发射技术为木材加工过程应力监测提供了一种主动无损检测模式, 但受木材自身各向异性等特点的限制, 目前声发射技术在木材工业中的应用尚处于探索阶段。为此, 文中重点介绍了目前声发射技术在木材切削加工、木材及木质结构、木材力学性能、木材干燥过程等木材加工过程的应用现状, 并在此基础上根据木材声发射信号特点, 提出一种基于LabView及高速采集设备的木材干燥过程声发射监测系统设计方案。     

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。     

Relationship between near-infrared (NIR) spectra and the geographical provenance of timber

Since trees record all the environmental factors in the wood, many wood properties are related to the site characteristics. Despite of this fact, identifying the origin of a timber has always been considered a difficult task, and no effective tools are presently available for this purpose. The goal of this study was to verify whether significant differences among groups of the same wood species due to the provenance can be detected with Fourier transform near-infra-red spectroscopy (FT-NIR). Spruce (Picea abies L. Karst.) samples collected from stands in Finland, Northern and Southern Poland and Italy were analyzed using two different approaches: for the first approach samples were collected from four provenances scattered in a wide area throughout Europe, while for the second approach the samples were collected from provenances located in a narrow area within the same region in Italy. For the first approach, all the specimens were clearly divided into groups by using statistical methods. The separation among groups from the narrow local area was actual; even though less significant than in the previous case. It was concluded that trees growing in various locations have somewhat different chemical composition, and FT-NIR is sensitive enough to detect such differences. The presented method could be used for tracking wood provenances and as a technical tool for detecting logs harvested illegally from protected areas.     

Applying multifractal spectrum combined with fractal discrete Brownian motion model to wood defects recognition

Wood nondestructive testing technology is a new and multidisciplinary industry scientific research. It has attained fast development and achievements in recent years. X-ray computed tomography (CT) scanning technology is a kind of wood nondestructive testing technology in practice. CT scanning technology has been applied to the detection of internal defects in the logs for the purpose of obtaining prior information, which can be used to reach better wood sawing decision. Fractal geometry and its extension multifractal are used for describing, modeling, analyzing, and processing of different complex shapes and images. A method in CT image edge detection using multifractal theory combined with fractal Brownian motion is applied in the paper. First, its multifractal spectrum is estimated. Then, different types of pixels are classified by the spectrum; they are smoothing edge points and singular edge points. From the images processed by multifractal spectrum theory and compared with each image by different spectrum values, it can be seen that the larger the range of threshold is set, the more exact the edge can be detected. The paper provides a new method to recognize the defect information and to saw it in the condition of nondestructive wood.     

基于四元数矩阵奇异值分解的木材缺陷检测分析

当前,木材彩色图像的缺陷检测主要是通过分离彩色空间的3个分量分别进行灰度处理,然后再合成为缺陷的图像.将基于RGB彩色空间的木材图像作为一个整体,提出四元数矩阵奇异值分解（QSVD）的木材缺陷检测.把RGB的彩色空间图像转换为四元数矩阵,利用四元数奇异值分解得到不同奇异值的特征图像,通过对特征图像的分析,得到不同的木材缺陷图像,并通过对奇异值特征图像的分析得到木材彩色图像的缺陷检测,并做分析.     

Experimental evaluations of material damping in timber beams of structural dimensions

Understanding the inherent damping mechanisms of floor vibrations has become a matter of increasing importance following the development of new composite floor layouts and increased span. The present study focuses on the evaluation of material damping in timber beam specimens with dimensions that are typical of common timber floor structures. Using the impact test method, 11 solid wood beams and 11 glulam beams made out of Norway Spruce (Picea abies) were subjected to flexural vibrations. The tests involved different spans and orientations. A total of 420 material damping evaluations were performed, and the results are presented as mean values for each configuration along with important statistical indicators to quantify their reliability. The consistency of the experimental method was validated with respect to repeatability and reproducibility. General trends found an increasing damping ratio for higher modes, shorter spans, and edgewise orientations. It is concluded from the results that material damping of timber beams of structural dimensions is governed by shear deformation, which can be expressed more conveniently with respect to the specific mode shape and its derivatives.     

木质材料力学性能无损检测方法的研究现状与趋势

木质材料用途广泛,在建筑、家具和包装等领域发挥着重要作用。目前,木材无损检测技术发展很快,应用到木材性质检测的无损检测方法已达几十种,然而对木质材料力学性能的检测还处于相对初级阶段,具有较大的挑战性。本文对木质材料力学性能的无损检测方法和原理进行归纳与总结,阐述几种主要检测技术（如应力渡检测、超声渡检测、振动法检测）的国内外研究现状、检测特点及存在问题。在此基础上,提出木质材料力学性能无损检测技术的发展趋势。     

Microscopic observation of wood-based composites exposed to fungal deterioration

This study was conducted to investigate the susceptibility of various wood composite panels exposed to wood-deteriorating fungi. Five wood-attacking fungi (three mold fungi, one brown rot fungus, one white rot fungus) were inoculated into four types of commercial wood composite panels (plywood, oriented strand board, particleboard, and medium-density fiberboard). One solid wood sample was included as a control. The attacking patterns of the fungi in each panel was observed by scanning electron microscopy. The weight losses due to the exposure were compared. All wood composites were more or less susceptible to all fungi inoculated. The attacking mode of the fungi was highly dependent on the types of wood composite, which had inherently different shapes of voids owing to different shapes and characteristics of the raw furnish materials used. Plywood and medium-density fiberboard showed a large weight loss after an 8-week exposure to decay fungi. Plywood is the most susceptible to white and brown rot fungi. This study indicates that all wood composite panels should undergo careful consideration to prevent fungal deterioration when they are used for exterior and humid interior applications.     

Internal decay assessment in standing beech trees using ultrasonic velocity measurement

Ultrasonic wave timing inspection was used to detect the internal decay in standing Iranian beech trees (Fagus orientalis). To evaluate the influence of internal decay on ultrasonic velocity, healthy round sections of freshly cut fallen beech trees were selected. Holes [as heart or internal decay indicator and with different shapes (circular and slot) and locations] were manually created and progressively enlarged in the wood section disks, and ultrasonic wave velocity was measured by using a commercial ultrasonic tester (Sylvatest Duo). The results showed that ultrasonic wave velocity linearly and significantly decreased by increasing hole dimensions, and location of holes had no influence on the extent and trend of velocity decrease. Although slots covered a small volume fraction of disks, they had a greater effect on ultrasonic velocities reduction as compared with circular holes.     

基于Faster R-CNN的实木板材缺陷检测识别系统

我国木材资源有限,为了提高木材的利用率,采用机器视觉来实现木材缺陷快速而稳定的检测,不仅可以克服人工检测的低效率和木材缺陷识别的低准确率,而且对提高木材加工企业的智能化水平具有重要意义。为了高效、快速、准确地进行无损检测,采用深度学习方法,建立了一种基于快速深度神经网络的实木板材缺陷识别模型。首先采用Resnet V2结构对采集到的实木板材缺陷图像进行特征提取,然后应用该模型对节子、孔洞等实木板材缺陷进行训练学习,最后构建了Faster R-CNN检测框架,并使用tensorflow开发平台对节子、孔洞等实木板材缺陷进行预测输出。具体选取了2 000块杉木样本,通过旋转对原始的实木板材图像进行数据扩充,扩充后图像的80%作为训练集,20%作为验证集来进行仿真。仿真结果表明,该模型对实木板材节子缺陷检测正确率为98%,对实木板材孔洞缺陷检测正确率为95%,验证了将深度学习算法应用于实木板材缺陷检测中的有效性。     

木材材色的定量表征

本文对木材材色的定量表征方法作了简要的评述,并就红种国产针、阔叶树材按CIE(1931)和CIE(1976)(L~*a~*b~*)做了详细的测定和分析。测定结果表明,表色参数(主波长、色纯度、亮度等)要具有可比性,所用的方法和测定条件必须相同。树种之间的色度坐标值的差异大于树种之内的差异。分析木材表面的反射率曲线,不同树种间的材色差异既反映在亮度上,也反映在色调上;但同一树种内的差异主要反映在亮度上。另外,树种之间与树种之内木材表面反射率与波长关系的曲线形状相似,趋向也一致,说明尽管材色的变异性很大,但有一定的规律性。因此采用了主成分分析法,简化表色空间,获得了综合材色参数的实验式;初步测试,实验式具有一定的可靠性与实用意义。为了能直观地表达材色,除了使用综合材色参数外,还给出了孟塞尔标号。     

木材及木基材料吸湿尺寸稳定性检测方法研究

为实现木材及木基材料吸湿尺寸稳定性横向比较,规范其检测方法,依据现有相关检测标准,借鉴日本工业木材吸湿尺寸稳定性检测方法,结合木质材料特性以及我国的具体情况,提出一种木材及木基材料吸湿尺寸稳定性的检测方法,即以温度20℃、相对湿度65%条件下的材料尺寸为基准,测定在温度40℃、相对湿度为75%和90%的两种吸湿环境条件下的材料尺寸变化。通过该方法对柚木、印茄木、朴木3种不同尺寸的木材和对多层材料、高密度纤维板、普通刨花板3种木基材料的吸湿尺寸稳定性进行测定,以评价该方法的适用性和可行性。结果表明:木材与木基材料试样不超过12 d即可达到吸湿平衡,不同材料尺寸变化率、湿胀系数的大小关系也与实际情况一致。因此木材与木基材料试样均适于用该方法。     

Location of the neutral axis in wood beams: A preliminary study

Abstract

It is commonly accepted in the analysis of wood beams that the neutral axis coincides with the beam's centroid. However, wood is not an isotropic material, has different elastic properties in the tangential, radial and longitudinal directions, and is non-homogeneous, as it contains characteristics such as knots. Therefore, there is need for an analysis of the neutral axis for anisotropic, non-homogeneous materials, such as wood, to predict deformations and strains. Specifically, digital imaging correlation, a non-contact technique to measure deformation of an object's surface, is used to examine how knots influence neutral axis location. Output from digital imaging correlation software provides a clear image of the location of the neutral axis. The neutral axis in a clear wood beam remains close to the centroidal axis throughout loading, while the location of a knot determines the size of the compression and tension zones as well as the location of the neutral axis.     

Mechano-sorptive effects in wooden material

Summary The effects on wood of simultaneous mechanical and moisture loading are studied. In order to clarify the mechano-sorptive behaviour of wood, a review of different phenomena presented in the literature is included. Based on this review a constitutive model is proposed for the case of uniaxial stress in the longitudinal direction. The validity of the model is checked independently against test results. The calculations show that the model is capable of describing the response of wood with reasonable accuracy. Simulations indicate that the response of small test specimens is more difficult to describe than that of larger beams. Some differences in behaviour are found to depend on loading mode and nature of moisture cycling. Very large and fast moisture cycles seem to give larger mechano-sorption than smaller variations. The results of the simulations show that there is a significant influence of strain on the shrinkage and swelling response.