Critical section effect during derivation of settings for grading machines based on dynamic modulus of elasticity

Testing of the critical section between the load points in four-point bending testing is required according to [DIN EN 384:2010–08, Structural Timber – Determination of Characteristic Values of Mechanical Properties and Density (Brussels European Committee for Standardization)]. However, strength grading machines based on measurements of the dynamic modulus of elasticity (MOEdyn) benefit as the strength of the weakest section is not always measured when deriving settings. According to the current procedure for derivation of settings, violation of the critical section requirement is accepted. The extent of such violation and its impact on grading settings of strength classes are quantified in two ways using about 3500 recently tested specimens of Norway spruce: empirically and modelled. The results show that for strength grading machines based on MOEdyn, the settings would be higher if the weakest section was always placed between the inner load points. The current procedure, however, leads to lower settings and hence better yields in practice. The settings for machines based on MOEdyn are up to more than 20% higher if the lowest bending strength of the entire board is applied. To treat different grading principles equally, a correction factor for settings of MOEdyn-based grading machines should be introduced – especially for lower strength classes. A simplified factor between 1.00 and 1.20 for standardization is recommended.     

Strength grading of Norway spruce structural timber: revisiting property relationships used in EN 338 classification system

Solid timber for structural applications has to be strength graded prior to its use. In order to remain economic the grading process usually focuses on the most important physical and mechanical properties: density, modulus of elasticity (MOE) and bending strength. Based on respective limits given in standards, the timber is assigned to strength classes. Additional mechanical properties such as tensile and compression strength parallel to the grain are derived from the basic property values by empirical relationships. The objective of this study was to review some of these property relationships based on recently compiled large data sets as a contribution for a future revision of the grading standards. Based on mechanical tests of Norway spruce structural timber with different cross-sections, the following characteristic values and property relationships were evaluated: (a) strength and MOE in bending, (b) in-grade characteristic values of bending strength, bending MOE and density, (c) relationship of characteristic values of tension and compression strength parallel to the grain with respect to the corresponding characteristic value of bending strength, (d) ratio of fifth percentiles and mean values of density and MOE, as well as (e) the ratio of MOE in bending, tension and compression. Mechanical tests were accompanied by measurements of density and ultrasonic wave speed. Resulting dynamic MOE was partly used as an indicator of timber quality.     

Axially loaded glued-in hardwood dowels

The failure load of axially loaded hardwood dowels glued-in parallel to the grain direction of the jointed timber parts is considered. Two simple theoretical solutions using linear elastic fracture mechanics/ideal plasticity and linear elastic stress analysis, taking into account the finite shear stiffness of the bond line are, presented and compared with experimental results. Theory shows that bond line shear strength is the governing strength property for ductile joints and fracture energy is the governing strength property for brittle joints. Bond line shear strength and fracture energy are determined by means of curve-fitting. Received 24 April 1997     

Effective sampling method for estimating bending strength distribution of Japanese larch square-sawn timber

Information on the strength distribution of timbers and other wood products seems to have become more important for users and producers after revision of the Japan architectural standard in 1998, which emphasizes the performance requirements of structures. Because there is no way other than expensive destructive tests to collect strength data, many researchers have proposed many inspecting methods for predicting strength by nondestructive evaluation. The most popular method for structural timber is the mechanical grading method based on the relation between Young's modulus (E) and strength () with some linear regression models. On the other hand, it is well known that the proof loading test is superior for obtaining information on the lower tail of distribution. If the E distribution of the objective timbers is known approximately, selecting timbers nearest to the projected E values saves timbers for destructive tests. We examined the alternative sampling method using the reported e- data sets of Japanese larch square-sawn timber. The simulated results showed that the estimated lower tail of the bending strength distribution by the alternative method was a better approximation of the experimental distribution than that derived from the conventional linear regression model.Part of this paper was presented at the 47th Annual Meeting of the Japan Wood Research Society, Kochi, April 1997     

Prediction of oriented strand board properties from mat formation and compression operating conditions. Part 1. Horizontal density distribution and vertical density profile

A model is presented to determine the horizontal density distribution (HDD) and vertical density profile (VDP) of oriented strand board (OSB) panels produced by batch pressing. The HDD is simulated using input distributions of flake dimensions and orientation from plant measurements. Many previous HDD models rely on assumed distributions, which may not accurately characterize current manufacturing processes. The model predicts the VDP based on the compression behaviour of cellular materials in combination with temperature and moisture profiles calculated using a previously published heat and mass transport model. A novel empirical approach is applied rather than the time–temperature–moisture superposition method commonly used. The model predictions compare favourably with plant data and exhibit trends similar to previously reported experimental results. This work is the first of a two-part publication. The second part is concerned with stiffness property prediction and an optimization of the OSB manufacturing process. This work is novel in that no comprehensive model including HDD, VDP, stiffness property prediction and optimization has been reported in the literature.     

A multivariate approach to automatic grading of Pinus sylvestris sawn timber

Abstract

The objective of this study was to create an easier way to handle the often complicated and intricate situations with which the operator of an automatic grading system is faced each time a change to the grading rules is proposed. The scope of the study was the possibility of a holistic method of automatic appearance grading of sawn wood similar to manual grading and based on multivariate statistics. The study was based on 90 Scots pine (Pinus sylvestris L.) sawlogs. The logs were sawn and the boards were scanned and manually graded. The result of the manual grading was defined as the true grade. Models for prediction of board grade based on aggregated defect variables were calibrated using partial least squares regression. The classification based on the multivariate models resulted in 80–85% of the boards being correctly graded according to the manual grading. In conclusion, this paper shows that a multivariate statistical approach for grading timber is a possible way to simplify the process of grading and to customize the grading rules when using an automatic grading system.     

Bending strength and modulus of elasticity of Pinus sylvestris round timber from southern Norway

Abstract

To enable use of round Scots pine timber in structural frameworks it is necessary to estimate the mechanical properties of the material. This paper presents data on density, bending strength and modulus of elasticity (MOE) of 533 debarked Scots pine logs with diameter from 75 mm to 250 mm sampled from 10 sites in southern Norway. The results show that round timber can have high values of bending strength and MOE, depending on the sites from which the trees have been collected. Some of the variation in bending properties can be explained by visual characteristics, but since a significant proportion of the residual variance is related to sites, criteria for visual strength grading have to be conservative to be valid across all sites. The potential for machine grading based on measuring MOE is better since this model is more accurate and the random effect of site is smaller.     

Customer adapted grading of Scots pine sawn timber using a multivariate method

To define new grading rules, or to customize the ones in use in a rule-based automatic grading (RBAG) system of boards, is a time-consuming job for a sawmill engineer. This has the effect that changes are rarely made. The objective of this study was to continue the development of a method that replaces the calibration of grading rule settings by a holistic-subjective automatic grading, using multivariate models. The objective was also to investigate if this approach can improve sawmill profitability and at the same time have a satisfied customer. For the study, 323 Scots pine (Pinus sylvestris L.) boards were manually graded according to the preferences of an important customer. That is, a customer that regularly purchases significant volumes of sawn timber. This manual grading was seen as reference grading in this work. The same boards were also scanned and graded by a RBAG system, calibrated for the same customer. Multivariate models for prediction of board grade based on aggregated knot variables, obtained from the scanning, were calibrated using partial least squares regression. The results show that prediction of board grades by the multivariate models were more correct, with respect to the manual grading, than the grading by the RBAG system. The prediction of board grades based on multivariate models resulted in 76–87% of the boards graded correctly, according to the manual grading, while the corresponding number was 63% for the RBAG system.     

Analysis of strength grading of sawn timber based on numerical simulation

The main problem in machine strength grading of wood lies in the grade determining parameters and machine grading parameters, which are all stochastic, and the correlation between them is of critical importance. Therefore it is difficult to reliably determine the grade of a particular piece. In order to understand the effectiveness and accuracy of machine grading a large number of experiments should be performed in which the non-destructive machine grading parameters as well as destructive grade determining parameters should be determined. So far we have had access to only a limited number of such experiments. The basic idea of this paper is to use experimental data only for the determination of statistical parameters—mean vector and variance—covariance matrix—and afterwards use these statistical parameters in numerical simulation of machine grading. This numerical procedure gave some valuable data on the average values and the variability of grading results which now make it easier to decide what sample size should be used in determining machine settings.     

Analysis of determination methods for characteristic timber properties as related to growth area and grade yield

The origin of the raw material is a key aspect for strength grading of timber. Large grading areas are favored by the sawmilling industry as they require less effort in handling and documentation during the production process. However, large growth areas can also cause problems, as too high mechanical properties can be declared or yields may become uneconomical. The presented study presents a method that should allow for timber from different countries to be combined into a single grading area. Additionally, the influence on the yield for guaranteeing timber properties for differently defined populations is analysed. In this process, a number of available calculation methods for characteristic values for modulus of rupture, modulus of elasticity, and density are considered as the determination method also influences the final yield. Non-destructive and destructive test data from 8487 spruce specimens from Europe tested in bending or tension are the basis for the presented study. Based on the grading results the presented method is able to simply identify countries that may be combined. The definition of pan-European grading areas seems problematic if characteristic timber properties need to be guaranteed separately for each individual country as it may result in a severe drop in yield. However, checking timber properties only for the European population is unsatisfying as calculated timber properties considerably vary depending on the origin. As for the calculation method, the preferred method itself seems to have less impact on bending class assignments then on tension class assignments.     

Mechanical properties and their interrelationships for medium-density European hardwoods,focusing on ash and beech

ABSTRACT

The usage of hardwoods for engineered wood products, such as glulam, requires defined mechanical properties reflecting the actual tensile strength of the material. Currently, the European strength class system EN 338 only covers profiles for hardwoods tested in bending. In this study, the material properties of medium-density hardwoods are analysed with the focus on a total of 3663 European ash (Fraxinus excelsior) and European beech (Fagus sylvatica) specimens tested in different loading modes (tension, compression, bending, and shear). The relationships between the material properties—tensile strength, stiffness, and density—are analysed on grouped data of both graded and ungraded specimens. As a result, a tailored ratio of tensile strength to tensile MOE and density is given, which allows to utilize a higher tensile strength of hardwoods (f_t,0,k over 30?N/mm²) compared to softwoods. Furthermore, the relationship of the test values and the derived values is checked. The equations for deriving the compression and bending strength from tensile strength are verified based on available data. For tensile and compression strength perpendicular to the grain and for shear strength of both beech and ash, higher strength values than the ones listed in EN 338 are possible. The relationship between the mechanical properties are combined to tensile strength profiles for hardwoods.     

A method for measuring vessel-free density distribution in hardwoods

A new method for rapid measurement of vessel-free density distribution in hardwood samples cut from increment cores is proposed. The method represents a specialised form of computer tomography which combines the data of X-ray microdensitometry with the information obtained from optical images of the transverse surface of the sample. The tests of the method using a Eucalyptus nitens sample show significant improvement in the accuracy of density determination compared with conventional X-ray densitometry. Received 30 June 1997     

基于磁弹性的扭矩测量方法及特性分析

基于磁弹性原理组建扭矩测量系统;归纳出"扭矩与磁感应强度之间存在线性正比例关系"的规律;提出以扭矩与磁感应强度之间的线性正比例关系为基础,利用具有巨磁阻效应的特斯拉计先测量磁感应强度值,再计算出相应扭矩值的扭矩测量新方法,为研发汽车扭矩传感器提供一种新的思路;利用SPSS软件进行数据分析,扭矩测量系统的线性度误差小于0.1%,重复性误差均小于0.1%,符合测量系统的误差要求,验证这种扭矩测量新方法的可行性;总结出扭矩测量系统的主要技术指标有:量程为0～100 N.m、线性度和重复性两个误差指标均为0.1%。     

基于TDC-GP1的波速法单板层积材分级系统设计

提出一种单板层积材力学强度分级系统的设计方案。该方案利用高精度数字时间转换芯片TDC-GP1测试超声波在单板层积材中的传播速度,从而评估单板层积材的力学强度并对其进行在线分级。试验表明,该系统工作稳定、抗干扰能力强,精度高,适合工业推广。     

Feasibility of near-infrared spectroscopy for online multiple trait assessment of sawn lumber

Near-infrared (NIR) spectroscopy coupled with multivariate analysis was applied to estimate multiple traits of sawn lumber. The effects of the lumber conveying speed (LCS) and measurement resolution of spectra (MRS) on the calibrations were examined. NIR spectra ranging from 1300 to 2300 nm were acquired at LCSs of 10, 20, and 30 m/min and at MRSs of 2, 4, and 16 nm. Prediction models of bending strength (F _b), modulus of elasticity in bending tests (E _b), dynamic modulus of elasticity (E _fr), and wood density (DEN) were developed using partial least-squares (PLS) analysis. LCS and MRS did not significantly influence the calibration performance for any wood property. The regression coefficients also showed no clear differences for any of the conditions. This indicates that the important explanatory variables included in the models are not greatly influenced by these measurement conditions. PLS2 analysis results, when presented graphically, allowed easy interpretation of the relationships between wood mechanical properties and chemical components, e.g., bending strength and stiffness were mainly related to polysaccharides cellulose and hemicellulose. NIR spectroscopy has considerable potential for online grading of sawn lumber, despite the harsh measurement conditions.     

Prediction of bending stiffness and moment carrying capacity of sugi cross-laminated timber

Cross-laminated timber (CLT) panels consist of several layers of lumber stacked crosswise and glued together on their faces. Prototype sugi CLT floor panels were manufactured and bending tests were carried out under the different parameters of lumber modulus of elasticity (MOE), number of layers, thickness of lumber and thickness of CLT panels. On the basis of above tests, bending stiffness and moment carrying capacity were predicted by Monte Carlo method. MOE of lumber was measured by using grading machine and tensile strength of lumber was assumed to be 60 % of bending strength based on the obtained bending test. Bending stiffness EI of CLT panels could be estimated by adopting composite theory and equivalent section area. Experimental moment carrying capacity showed 12 % higher value than the calculated moment carrying capacity by average lumber failure method, and also showed 45 % higher value than the calculated moment carrying capacity by minimum lumber failure method due to the reinforcement of the outer layer by the neighboring cross layer.     

Size effect on longitudinal MOE of microtomed wood sections and relevant theoretical explanation

It has been repeatedly observed that the mechanical properties of microtomed wood sections are significantly lower than those of samples of normal size, but few investigations have been conducted to deal with this topic, especially based on theoretical approaches. We measured the longitudinal MOE of Chinese fir on microtomed sections ranging in thickness from 70 to 200 μm and compared these with the values of samples of normal size. The results indicate that the MOE of microtomed wood sections increases with thickness from 70 to 200 μm, but is significantly less than that of normal samples. A size effect coefficient of 2.63 is inferred based on statistical data for samples of normal size and 200 μm thick microtomed sections. Finally, an explanation based on a complete shear restraint model of cell walls and a single fiber multi-ply model is proposed for the size effect on stiffness of microtomed wood sections.     

The analytical influence of all types of knots on bending

The influence of different types of knots and fiber deviations on the bending of wood, as managed using visual grading standards, was analytically discussed in this research by means of the finite element method. The effect of each geometrical variable on the strength and stress distribution has been studied individually. This was accomplished after validating a previously presented finite element model that accounted for the three-dimensional shape of the knots and fiber deviations. The simulated sizes and positions were those commonly managed by the standards for strength grading. The research presents the effect of: (1) the size and position of cylindrical face knots; (2) size and position of truncated conical face knots; (3) size and position of shallow conical face knots; (4) size and position of conical edge knots; (5) inclination of face knots. In each case, the analytical predictions of the modulus of rupture and stress distribution are detailed. Seven characteristic positions with distinct mechanical behavior were found, which should be accounted for in order to improve the wood grading efficiency.     

基于虚拟仪器技术的水泵性能测试系统设计

将现代虚拟仪器技术应用于水泵性能测试系统,充分发挥了虚拟仪器技术开发效率高、灵活性兼容性强和可重用度高的特点,设计与实现了水泵性能的在线测试,详述了系统的工作原理和主要功能,说明了系统硬件和软件的设计方法,重点介绍了水泵性能数据处理的数学模型推导.最后,对系统进行了测试,实验证明系统性能稳定、操作灵活,能可靠实现水泵性能测试.