Tensile strength of thermally modified laminated strand lumber and laminated veneer lumber

Laminated strand lumber (LSL) and laminated veneer lumber (LVL) were thermally modified as a post-treatment at 140°C, 150°C, 160°C, 170°C, and 180°C. The tension modulus of elasticity (MOE) of LSL was not significantly impacted by the treatments, with the 180°C treatment group exhibiting the highest tension MOE (11.8?GPa). The LVL also experienced minimal impacts, with the 150°C treatment group having the highest tension MOE (19.4?GPa) and the 160°C treatment group exhibiting the lowest (17.1?GPa). The maximum tensile strength (MTS) of the LSL and LVL significantly decreased with increasing temperatures, with the control and 180°C treatment groups experiencing the highest and lowest MTS, respectively. The lowest MTS for LSL was 10.8?MPa (180°C treatment), which was 70% lower than the controls. The lowest MTS of the LVL was 24.4?MPa (also at the 180°C treatment), which was a 49% decrease compared to the controls. These results suggest that thermal-modification post-treatments minimally impact tension MOE, but can significantly reduce MTS at higher treatment temperatures. Combined with previous work improving the moisture properties and equilibrium moisture content of thermally modified LSL and LVL, it may be possible to optimize the treatment technique(s) to yield products with desirable properties.     

Variation of tensile strength along the length of lumber

Summary The within member variation of tensile strength parallel to grain in nominal 38×89 mm No. 2 Spruce-Pine-Fir lumber has been experimentally evaluated. Window analyses yielded the within member tensile strength cumulative probability distributions of lumber segments with various lengths. Semivariogram and regression analyses have been performed to characterize the spatial correlation of the tensile strength of lumber. The results indicate that the tensile strength values within a piece of lumber separated by a distance greater than 1.83 m can be considered statistically independent.The Authors gratefully acknowledge the support of the Forestry Canada and the technical assistance of Mr. H. Sue, Mr. B. Deacon and Dr. J. Cook for this study. The research was performed while the principal author was working at Forintek Canada Corp.     

Variation of tensile strength along the length of lumber

Summary A model has been developed for the within member variation of tensile strengths parallel to grain in nominal 38 × 89 mm No. 2 Spruce-Pine-Fir lumber. Tensile strength data from two modulus of elasticity matched groups were considered. Model parameters obtained from data group 1 were used to generate tensile strength profiles of lumber 6.10 m long. Within member tensile strength cumulative probability distributions of the simulated data were evaluated by window analyses. The spatial correlation of the simulated data were evaluated by semivariogram and regression analyses. Good agreement was obtained between model predictions and test results of both groups.The authors gratefully acknowledge the support of the Canadian Forest Service and the technical assistance of Mr. H. Sue, Mr. B. Deacon and Dr. J. Cook for this study. The research was performed while the principal author was working at Forintek Canada Corp.     

Differences of tensile strength distribution between mechanically high-grade and low-grade Japanese larch lumber I: Effect of length on the strength of lumber

An experimental study was conducted to evaluate the effect of length on the parallel-to-grain tensile strength of Japanese larch (Larix kaempferi, Carriere) lumber. Six hundred pieces of mechanically graded lumber were tested at gauge lengths of 60, 100, and 180 cm. The lumber was sorted into matched groups according to the dynamic Young's modulus measured by the longitudinal vibration method before the lumber was cut to the particular length. The averages of the dynamic Young's modulus of high-grade (H) and low-grade (L) specimens were 12.8 and 7.5 GPa, respectively. Using nonparametric estimates, the estimated length effect parameters of H and L were 0.268 and 0.304 for the 50th percentile and 0.121 and 0.256 for the 5th percentile, respectively. We then concluded that the different length effect factors between H and L could be used when using the lumber for practical purposes. The parameters of L were larger than those for H, and the parameters for 5th percentiles were smaller than the parameters for 50th percentiles. When two-parameter Weibull distribution functions were fitted to the strength data, the estimated shape parameters of the Weibull distribution by the parametric method were almost identifical to the inverse of nonparametric parameters except the 5th percentiles for H. The influence of defects such as knots on the lower tail of the strength distribution in H may be different from that in L.Part of this paper was presented at the 48th annual meeting of the Japan Wood Research Society, Shizuoka, April 1998     

Differences of tensile strength distributions between mechanically high-grade and low-grade Japanese larch lumber III: effect of knot restriction on the strength of lumber

It is well known that the presence of knots in structural lumber is one of the most important strengthreducing factors. For practical purposes, visual grading including knot restriction is an effective method for nondestructive evaluation of strength. Edge knot restriction for not only visually graded lumbers but also mechanically graded lumbers is specified in the Japanese agricultural standards for glued laminated lumber. We conducted experimental studies on differences of tensile strength distributions between mechanically high-grade and low-grade Japanese larch (Larix kaempferi, carriere) lumbers daily used for manufacturing glued laminated timbers in Nagano, Japan. We then examined the additional visual grading of mechanically graded lumbers for nondestructive evaluation. We visually graded the prepared mechanically graded lumber by focusing on the knots' area ratio of grouped knots. We confirmed that the higher visual grade related to the stronger tensile strength, similar to our present knowledge; but the effects of knot restriction were reduced when the length of the lumber increased in view of nonparametric 5th percentiles of tensile strength. The differences in the strength/elasticity ratio between mechanically high-grade and low-grade lumber were negligible. It was clear that the length effect on the ratio in visually graded high-grade lumber was smaller than that of visually graded low-grade lumber. It was thus concluded that knot restriction should have little effect on the tensile strength of mechanically graded lumber.     

Effect of size on the bending strength of laminated veneer lumber

Summary The effect of size on bending strength has been experimentally determined for laminated veneer lumber. Width was found to have no effect on bending strength. The effect of depth times length on bending strength obtained by the slope method was about 0.075, which is in good agreement with the results obtained by the shape parameter method. The effect of length is somewhat more severe than the effect of depth. Size was found to have no effect on modulus of elasticity or modulus of rigidity.In addition, the relationships between bending strength, modulus of elasticity and density of laminated veneer lumber were experimentally modelled.The author is pleased to acknowledge Research and Development Manager Matti Kairi, who represents the Kerto laminated veneer lumber manufacturer of Finnforest Oy, for fruitful co-operation over several years. The patience of senior research scientist Markku Kortesmaa, who calculated and several times explained the details of Appendix A for the author, is also acknowledged     

Bending strength adjustments for moisture content for structural lumber

Summary Effects of moisture content on bending strength and section capacity have been studied assuming that strength properties are linearly related to moisture content below the fiber saturation point. The models developed using the linear strength vs. moisture content relationships are compared with quadratic models developed earlier. The linear representation of strength variation with moisture content leads to predicted member bending capacity relationships which show that increases in member bending strength with moisture content will compensate for section property decreases at all bending strength property levels. Comparisons of bending strength and bending capacity models show that the two modelling approaches lead to similar predicted member performance. Simplified procedures are provided for adjusting member bending strength for moisture content which are suitable for materials properties codes or standards.     

Evaluating the strength of finger-jointed lumber of Gmelina arborea in Costa Rica

The use of finger-jointed lumber of Gmelina arborea (gmelina) for the production of glue-laminated beams is becoming more important to the wood industry in Costa Rica. At the request of the local wood industry, two different kinds of adhesives, thermoplastic polyvinyl emulsion (catalyzed PVA) and 1-part polyurethane adhesive (PU) were tested on finger joints of gmelina wood and their strength properties compared under dry, boiled, and saturated conditions. The results showed that under the boiled and saturated conditions, the PVA adhesive exhibited low tensile strength and poor distribution of failures while PU showed much more promise. Based on our preliminary results, the PU adhesive can be successfully used for finger-joint boards of gmelina. Palabras clave: Adhesivos, Melina, Modos de falla, Vigas laminadas Resumen. El uso de la madera unida con juntas de dedo (finger-jointed) para la producción de vigas laminadas-encoladas está tomando mayor importancia en la industria maderera de Costa Rica. Atendiendo la solicitud de la industria, sobre la calidad de los finger joints de la madera de melina, se ensayaron dos tipos diferentes de adhesivos, una emulsión de polivinilo catalizado (thermoplastic polyvinyl emulsion, catalizado, PVA) y un adhesivo de poliuretano (polyurethane adhesive, PU) de un componente, para comparar sus características de resistencia bajo condiciones secas, de ebullición y de saturación. Los resultados muestran que bajo condiciones de ebullición y saturación, el adhesivo PVA tiene una baja resistencia a la tensión y una deficiente distribución de fallas, mientras que el PU mostró resultados mucho más prometedores. Basados en los resultados preliminares, el adhesivo PU puede ser utilizado exitosamente para unir las tablas de melina con juntas de dedo.     

Outdoor exposure tests of structural laminated veneer lumber (II): evaluation of the strength properties after nine years

To investigate the durability of structural laminated veneer lumber (LVL), outdoor exposure tests have been conducted since 1990 at a field-testing site at the Forestry and Forest Products Research Institute. This article is the second interim report on the results after 9 years of exposure. Seven kinds of structural LVL with no preservative treatment were subjected to the tests. Almost all the exposed specimens were decayed by a kind of brown rot fungi (Pseudomerulius aureus (Fr.) Julich). The degree of decay varied with wood species; grand fir and western hemlock LVL in particular showed weak resistance against the decay. All the specimens were stored for more than 1 year in a testing room conditioned at 20°C and 65% relative humidity. We then measured the ultrasonic velocity of the specimens by the Pundit method, penetration depth by the Pilodyn method, and bending strength by a conventional bending test. Correlation between nondestructive measurement factors and the density was strong even on LVL with many adhesive layers. The nondestructive testing method was found to be applicable to LVL as well as solid lumber. After the nondestructive measurements, each LVL was cut into three types of specimen (top: T, middle: M, and bottom: B) for the bending tests. The bending strength varied with the type of specimens. Correlation between modulus of elasticity and modulus of rupture was strong even in the decayed specimens.     

Variation of tensile strength with annual rings for lumber from the Japanese larch

To examine the effectiveness of long rotation forestry and the potential of complete utilization of Japanese larch (Larix kaempferi Carriere), we designed a tensile test using the lumber from six 87-year-old sample trees. Results showed that strength properties of lumber varied greatly in the radial direction within trees, but all sample trees showed a similar trend. There was little difference in dynamic Young's modulus but a large difference in tensile strength (TS) between the lumber and small clear specimens from undestroyed parts of the lumber. These differences decreased with an increase in ring number and became constant after 30 years. The presence and distribution of knots markedly affected the TS; and among the knot indices, the knot number (Kn) and knot area ratio of a maximum single knot (Km) proved to be effective for explaining the effect of knots. The distribution of Kn and Km in the radial direction agreed with the variation of TS in the radial direction. By investigating the variation patterns of lumber and small clear specimens in the radial direction, it was found that the strength properties of both required a long time, about 30 years, to reach a relatively constant state.Part of this report was presented at the 49th annual meeting of the Japan Wood Research Society, Tokyo, April 1999     

A fracture mechanics approach to the tensile strength perpendicular to grain of dimension lumber

Summary Six hundred short sections taken from 23 flatsawn 2x12-inch planks of construction grade Doughas-fir were tested in tension perpendicular to grain. Most of the specimens contained defects in the form of checks, knots, resin streaks, pitch pockets, and pith. Checks were found to be the major strength-reducing defect. The effect of checks could be analyzed effectively by using the concepts of fracture mechanics.     

Modeling lumber strength spatial variation using trend removal and kriging analyses

Summary The techniques of kriging and trend removal analysis for modeling the strength properties of lumber have been discussed. Using these techniques, the within member compressive strengths of 38 mm × 89 mm 2100f-1.8E and the within member tensile strengths of 38 mm × 89 mm No. 2 spruce-pine-fir lumber have been modeled. The suitability of these techniques to model the strength properties of lumber has been evaluated by comparing the kriged compressive and tensile strength data to baseline experimentally collected compressive strength data and simulated tensile strength data, respectively. Comparisons of the results of statistical analyses of the baseline data and the kriged data show good agreement. The use of kriging and trend removal techniques to generate strength values in simulation studies for finite element analyses of wood structures is judged to be effective.     

Application of the SB distribution to the simulation of correlated lumber properties data

Summary With the emergence of probabilistic design procedures, the need for precise knowledge of the entire probability distributions of load effects and material resistance has never been greater. In order to evaluate these distributions, simulation techniques have provided a reliable and cost and time effective alternative to large scale destructive testing. With the use of the Johnson's S_B probability distribution, a closed-form, analytic procedure has been developed to model the inherent variability in strength, given some nondestructively evaluated parameter. This modeling procedure serves as the basis of a verified simulation process to predict a strength distribution, given a probability distribution of the NDE parameter. The approach presented here, represents a closed-form, analytic solution to a problem which has heretofore been treated in a more subjective fashion. This simulation procedure is complemented by a stratified sampling scheme.The author wishes to recognize Engineering Data Management, Inc. for the use of their computer software, STADMAN, and the Mc Intire-Stennis Research Program for the financial support of this study     

竹重组材微晶结构的X射线衍射分析

运用X射线衍射分析(XRD)的检测手段,对冷压热固化与热压法两种生产模式的竹重组材试样进行分析,结果表明竹材纤维素是以微晶结构为基础的无定型结构,仅在衍射角2θ为22.5°附近出现较突出的晶体衍射峰,在衍射角16.7°、35°附近出现对应强度较弱的晶体衍射峰。冷压热固化与热压过程中,竹材纤维素结晶区主体结构未见明显变化,但相对结晶度有不同程度下降,对竹重组材的物理力学性能有所影响。     

Development of constitutive model for laminated veneer lumber using digital image correlation technique

This paper describes the development of a three-dimensional constitutive model for laminated veneer lumber (LVL) needed for new developments using this material. The LVL was manufactured in New Zealand from Radiata Pine. Experimental testing has been performed according to European timber testing standards. Block compression testing has resulted in modulus of elasticity values in the three material directions. Digital image correlation (DIC) technique has been used to determine the six Poisson’s ratios. Shear testing, whereby timber specimens were glued between two steel plates, has given stiffness values using DIC measurements. Experimental testing results have been compared with values found in literature. Results from this experimental testing programme have made it possible to create a three-dimensional elastic material model of LVL for the use in finite element analysis programmes. Although the material properties do not result in a symmetrical constitutive matrix, only minor adjustments are needed to gain the benefits of a symmetrical matrix.     

Characterization of strength properties of branchwood and stemwood of some tropical hardwood species

The mechanical strength properties of the branchwood of Aningeria robusta and Terminalia ivorensis with diameters ranging from 10 to 25 cm were examined to determine the suitability of branchwood as raw material for downstream processing. The study precisely assessed the static bending strengths, compression strengths and shear strengths parallel to the grain of the branchwood and stemwood of T. ivorensis and A. robusta. It was observed that under static bending, the overall (sapwood and heartwood combined) modulus of rupture and modulus of elasticity of the branchwood of A. robusta and T. ivorensis were lower than that of their corresponding stemwood. The results further show that the overall compression and shear strengths parallel to the grain of the branchwood of A. robusta and T. ivorensis were higher than that of their corresponding stemwood. Under static bending, compression parallel to the grain, and shear parallel to the grain, the experimental results indicate that the sapwood of the branchwood from both T. ivorensis and A. robusta had lower strength values than that of their corresponding heartwood. Similar results were recorded for the stemwood of T. ivorensis where the heartwood had higher strength values than the sapwood. However, in the case of the stemwood of A. robusta, the sapwood had higher strength values than the corresponding heartwood.     

Estimation of the internal shear strength distribution of the element for laminated veneer lumber by nonlinear least-squares method

Until now we developed an estimation method for strength distributions of laminated veneer lumber (LVL) element by nonlinear least-squares method (NLM). Estimated strengths by this method were modulus of elasticity (MOE) and modulus of rupture (MOR) in the horizontal use direction and the vertical use direction, tensile strength and compression strength. But to use LVL for structural members, shear strength was also needed. Therefore, we tried to estimate the shear strength distribution of LVL element by NLM same as MOE and MOR in the horizontal use direction and the vertical use direction, the tensile strength of LVL and the compression strength of LVL in the previous reports. We conducted shear strength test for LVL and estimated element shear strength distribution by LVL strength data in the horizontal and vertical use direction. Next, we simulated LVL shear strength distribution using element shear strength distribution and compared with experimental ones in each use direction. They were overlapped in both use direction. Therefore, we could validate NLM for estimating element shear strength distribution.     

Relationship between apparent modulus of elasticity,gage length,and tensile strength of lumber

The percent explained variation (r²) of tensile strength (T) of dimension lumber can be accounted for primarily by apparent modulus of elasticity (E_a).Tensile strength ratio, a good index of T of structural lumber, and E_a are both dependent on relative knot size, making T a function of E_a. This theoretical relationship helps to increase r² when T is regressed on E_a and gives the cause and effect for some multiple regression analyses. An independent variable was identified on the basis of this theory.Shortening the gage length enhances ability to predict T when flatwise bending E_a or tension E_a is used as an independent variable, but not when edgewise bending E_a is used.Two single variables not previously reported, E_a measured in flatwise bending on 16-inch gage length (EF 16) and E_a measured in tension on 6-inch gage length (ET 6) (based solely on deformation measurements), are individually able to explain approximately as much variation of T (r²=0.75) as the combination of E_a measured flatwise on 48-inch gage length (span length for the existing American grading machines), and ASTM bending strength ratio. The combination of EF 16 and ET6 can explain 85% of the variation of T. The highest r² (0.87) was obtained when a modified bending strength ratio was added to these new variables.This research was done in partial fulfillment for the degree of Master of Science, Department of Civil Engineering, University of Wisconsin, Madison, Wis., May 1971.