Influence Of Knots Research Articles

Estimation of the static bending modulus of elasticity in glulam elements by ultrasound and modal-updating NDT techniques

Glulam timber is gaining popularity as a structural element due to its cost-effectiveness, strength, and environmental benefits. However, being an organic material, it is susceptible to degradation. Therefore, nondestructive testing (NDT) methods are essential for assessing its structural health. In this study, two NDT approaches, ultrasound-based and modal updating, were used to predict the static bending modulus of elasticity (MOE) in Pinus Sylvestris and Spruce species. The results demonstrated that the modal updating and linear regression models outperformed the ultrasound technique in accurately determining the static MOE. Additionally, considering the influence of knots improved the accuracy of ultrasound predictions.

Influence of knots on the adhesion of wood from young Eucalyptus grandis plantations

ABSTRACT The structural use of wood from young trees requires gluing smaller timber pieces to form large elements. In young Eucalyptus timber of structural dimensions, knots do not significantly diminish the mechanical properties, yet their influence on wood adhesion has not been widely studied. Therefore, the aim of the study was to assess the influence of knots on the bond quality of face-glued wood from young E. grandis. Wood from seven-year-old E. grandis trees was tested in shear on the bond-line (polyurethane adhesive) using paired samples of knot-containing and clear wood samples. Knot features did not correlate with shear strength or wood failure percentage. Knot-containing samples had a lower percentage of wood failure (5.8%), but the same shear strength as clear wood samples. Below a wood density of 650 kg m−3, clear and knot-containing wood behaved similarly, but above this value, knots influenced wood adhesion negatively. By simulating glulam beams with high-density wood on the outer lamellas, it was possible to show that shear stress on those bond-lines is lower than the minimum shear strength observed on dense knot samples. Hence, lower adhesion caused by knots on denser wood probably does not represent a significant problem for glued laminated products from young Eucalyptus grandis trees.

Modeling knot features using branch scars from Mongolian oak (Quercus mongolica).

Wood quality is an important indicator for modern sawmills. Internal wood characteristics can be derived from their correlations with external appearances. In this study, we developed linear regression models to predict knot size from surface features of Mongolian oak (Quercus mongolica) using data collected from 53 trees. For this, manual measurements and X-ray computed tomography scanning technology was respectively used to obtain internal and external features of 1,297 knots. Our results showed that Mongolian oak knots were generally concentrated in the middle part of oak stems, with fewer knots observed at the top and base. The parameters of knot and scar showed significant correlations (P<0.01), where length and diameter of the corresponding external scar increase with increasing the length and diameter of a knot. The corresponding external scar can be used as an effective indicator to predict the internal value of oak logs. The accuracy of our constructed model is more than 95% when assessed against independent test samples. These models thus can be applied to improve the practical production of oak timber and reduce commercial loss caused by knots. These additional data can improve the estimation of the influence of knots on wood quality and provide a theoretical foundation for investigating the characteristics of hardwood knots.

A Parametric Investigation of the Influence of Knots on the Flexural Behavior of Timber Beams

The presence of knots significantly influences the mechanical behavior of timber. This research presents a parametric finite element analysis of the effect of knot characteristics on the flexural capacity of timber beams. Parameters include knot radius, longitudinal and vertical positions, length, and diving angle. The generation of the FE model is based on a technique developed in a previous research stage. The validation of the model is done via bending tests on a set of beams as well as a number of independent research studies. The numerical model accurately recreates the three-dimensional geometry of the knot and related fiber deviations. The capacity of the beams under bending is evaluated via the Tsai–Wu failure criterion. Findings reveal that the presence of knots in the tension zone leads to an early tensile failure and insufficient utilization of compression capacity, the decrease of bending capacity may rise significantly (up to 39% in this investigation), the knot inclination may positively or negatively influence the behavior depending on the diving angle, and moderate knot length can be detrimental to flexural capacity.

Drive-in torque for self-tapping screws into timber

Self-tapping screws have been widely used in timber constructions. The current practice considers self-tapping screws perform best in connecting two members when they are fully threaded; however, the drive-in resistance caused by the friction between the wood and the screw can potentially damage the screw and reduce the effectiveness of its applications. The relationship between their thread configuration and the drive-in torque force has not been investigated, and the question as to how would knots in the member affect the drive-in force, remains a unanswered. In this study a series of tests was conducted, aiming to demonstrate the influence of thread configuration on the drive-in torque of screws. Two types of self-tapping screws and three different thread configurations were studied. The drive-in torque for partially threaded screws was found to be significantly slower than that of fully threaded ones. The results showed that knots can significantly influence the positioning of the screw and increase the drive-in torque. The application of a pre-drilled hole was found to be an effective way to minimise the influence of knots. This paper points out that with appropriate consideration of thread configuration, partially threaded self-tapping screws can not only achieve the same efficiency as fully threaded ones, they will also benefit from reduced drive-in torque force.

Modeling the Influence of Knots on Douglas-Fir Veneer Fiber Orientation

This study presents a method for predicting the local fiber orientation of veneers made from peeled Douglas-fir logs based on the knowledge of the tree branch characteristics (location, radius, insertion angle, azimuth angle, and living branch ratio). This model is based on the Rankine oval theory approach and focuses on the local deviation of the fiber orientation in the vicinity of knots. The local fiber orientation was measured online during the peeling process with an in-house-developed scanner based on the tracheid effect. Two logs from the same tree were peeled, and their ribbons were scanned. The knot locations and fiber orientation were deduced from the scanner data. The first objective was to compare the fiber orientation model with measurements to enhance and validate the model for French Douglas-fir. The second objective was to link data measurable on logs to veneer quality.

Mechanical performance curves for the strength grading of maritime pine

Finite element models (FEM) were used to simulate beam sections in bending with a knot. They aim at evaluating the mechanical behaviour of timber considering the effect of this singularity on resulting bending strength. By using linear elastic theory and an adapted failure criterion (Tsai–Hill), an equivalent state can be deduced, giving information about the influence of knot on the load bearing capacity of the element. Several 2D-FEM were then developed using the free software Castem® by considering plane strain. The beam section was considered like a composite material with two materials; the wood and the knot. The slope of grain around the defect was implemented by using a simple exponential equation parameterized as a function of the radius of the singularity. The location and the size of the defect and the longitudinal modulus of elasticity of wood were investigated in this study. Results exhibit performance curves specific to bending strength. These numerical performance curves were not used to predict the bending strength but to identify some thresholds, function of the size and the location of a critical knot and the stiffness of structural beams. 188 beams with a length of 4.5 m and a cross-section of 200 × 100 mm² were tested to study the efficiency of numerical thresholds. This study shows that it is possible, by identifying only the geometrical properties of the critical defect and the stiffness of the structural beam, to improve the prediction of the bending strength of beams with a large cross-section in Maritime pine up to C40.

Impact of knots on the fracture propagating along grain in timber beams

Fracture propagating along the grain of timber is one of the most relevant failure mechanisms due to the wood’s low strength and its brittle failure behaviour when subjected to excessive shear and tensile stresses perpendicular to the grain. According to the current procedures, the fracture energy of wood is determined on small clear specimens. However, for the prediction of the structural behaviour of full-scale structural timber members, the influence of knots has to be accounted for. The discrepancy between the fracture behaviour of small specimens and members of structural size has already been observed in several studies. The aim of this paper is to provide a review on a selection of these studies and to discuss their relevancy for the evaluation of the fracture properties and the structural behaviour of full-scale structural timber.

Влияние сучков на предел прочности при сжатии вдоль волокон круглых сортиментов сосны малых диаметров

Влияние сучков на предел прочности при сжатии вдоль волокон круглых сортиментов сосны малых диаметров

Effect of Distribution of Knots on the Strength of the Glued Laminated Timber Beams

This article will examine the effect of the knots distribution on strength of glued laminated timber beams. A finite element model was made to simulate glued laminated timber beams with defects (knots) to simulate and predict behavior in areas containing knots while bending. This model was then compared with results from static bending test of glued laminated timber beam. The initial position of the rupture was then compared with FEM model [6], so the influence of knots on strength could be determined.

Biomechanical evaluation of double-strand (looped) and single-strand polyamide multifilament suture: influence of knot and suture size.

Flexor tendon repair in zone II remains a vexing problem. Repair techniques have been developed to strengthen and optimize the number of core strands crossing a repair. A polyamide looped suture doubles the number of core strands for every needle path. This simplifies repairs, but the knot remains a potential weakness. The purpose of our study was to create a biomechanical model used to evaluate the bulky knot of a looped suture as it may be weaker, resulting in greater deformation. Using machined steel rods to hold our suture constructs, we compared four different knot configurations using looped and non-looped sutures in 3-0 and 4-0 varieties using a four-core strand technique. The constructs were tested under increased cyclic loading recording both forces applied and suture construct lengthening ("clinical gapping") and ultimate breaking strength. During continuous periods of cyclic loading, we measured permanent deformation and ultimate breaking strength. Permanent deformation results when there is no recoverable change after force removal defined as a permanent rod separation (or gapping) of 2mm. Four-strand 3-0 and 4-0 looped sutures failed at 39.9 and 27.1N faring worse than a four-strand non-looped suture which reached a rod separation of 2mm at 60.7 and 41.3N. The ultimate breaking strength demonstrated absolute failure (construct rupture) with the 3-0 looped suture breaking at the knot at 50.3N and the non-looped suture at 61.5N. For the 4-0 suture, these values were 32.4 and 41.76N. Within the constraints of this model, a looped suture fared worse than a non-looped suture especially when comparing 4-0 and 3-0 sutures. However, two-knot 3-0 looped suture constructs did resist the force generally accepted as occurring with early non-resistive tendon motion protocols, while two-knot 4-0 looped suture constructs did not. This paper provides a description of a model to evaluate various suture materials and knot strengths in isolation of the tendon itself. This allowed us to evaluate mechanical differences between looped and non-looped sutures for polyamide, which are commonly used in flexor tendon repair. These differences between sutures may impact choices for a suture type selected for these repairs.

Finite element analysis of timber containing branches – An approach to model the grain course and the influence on the structural behaviour

Wood, as a naturally grown material, is affected by growth inhomogeneities like branches. Especially in solid timber or glulam beams, knots lead to a decrease in stiffness and strength. In this contribution, approaches to model knots and the grain course in an FE-simulation are presented. Based on these methods, the influence of knots is evaluated by structural analyses. For validation, the numerical results are compared to experimental data. Since knots often initiate damage in structural timber parts, a failure criterion is applied in the area of the knots.

Determination of the influence of size and position of knots on load capacity and stress distribution in timber beams of Pinus sylvestris using finite element model

The finite element method was used to investigate the influence of size and position of cylindrical knots on load capacity considering the elastic–plastic constitutive law of Scots pine timber. A finite element model for a four points bending test was generated considering four different knot conditions in the beams: without knot; knot as a hole; live knot and spring contact between the knot and the beam. For knots placed in the compression side, the live-knot-model best simulates real behaviour; however, when located in the tension side, the hole-model was most reliable. The bending strength of the beam, including different sizes and positions of knots, were presented in simplified diagrams and compared with clear timber strength. The results showed the influence of knots and their local grain deviation on stress distribution. The model allowed the ranking of bending strength of the beams caused by knots as a combination of three quantified indexes: tension parallel and perpendicular to the grain and shear.

The utility of low-cost photogrammetry for stiffness analysis and finite-element validation of wood with knots in bending

This article presents the capabilities of low-cost photogrammetry as a tool for stiffness analysis and finite-element (FE) validation in conventional bending tests of structural timber that contains knots. The accuracy offered by three consumer-grade digital cameras with a resolution of 10 megapixels was 1/6200, which did not allow for the evaluation of the stresses or strains in a multiaxial stress field, but provided a thorough contrast of the 3D nodal displacements, showing the distortion of knots in the modulus of elasticity (MOE), so that the heterogeneous stiffness of this natural material could be measured and accounted for. This allowed a more precise determination of the wood stiffness in bending, and a novel procedure for the estimation of a Clear MOE related to the defect-free areas, an MOE in Knots related to the distorted regions, and a Global MOE which encompasses both was developed. The proposed method can be applied to conventional mechanical tests. In addition, other interesting possibilities for FE validation and monitoring were obtained. The case study demonstrates how, with a very small investment, an FE model that simulates the influence of knots in bending could be accurately validated with many fewer specimens, and how the predictions of displacements could be improved by up to 49% using the Clear MOE measured by means of the photogrammetry rather than the conventional MOE without shearing strain. ► Low-cost photogrammetry used to determine mechanical properties of timber pieces. ► Novel procedure for obtaining more precise wood stiffness measurement is proposed. ► Predicted stiffness using proposed numerical simulations improved by up to 49%

Acoustic and bending properties in Pinus elliottii beams obtained from trees of different ages

In contrast to wood from native forests, the lumber harvested from planted forests is primarily composed of boards from younger trees. Given the possibility of using wave propagation methods to classify wood for structural purposes, it is important to evaluate if the ultrasonic parameters tend to vary with the age of the tree in a similar manner as static bending parameters. Experiments were conducted using structural beams taken from Pinus elliottii trees that were 8, 9, 13, 15, 22 and 23 years old. The beams were tested by ultrasound in the longitudinal direction and by static bending. The results showed that age of the tree influences the wave propagation velocity, stiffness constant and modulus of elasticity. The results were sorted into two groups based on the variation of these parameters: wood pieces from trees up to 15 years old and wood pieces from trees older than 15 years of age. Statistically, the flexural strength did not change with age due to the significant influence of knots on this parameter, which overcomes the influence of age. The data obtained by ultrasound follow the same trends with age as the elastic modulus in static bending.

The Effect of Wood Knot as a Defect on Modulus of Elasticity (MOE) and Damping Correlation

This research was carried out to investigate the influence of knots on modulus of elasticity and on damping factor of vibration. Rectangular bars from Cupressus arizonica wood were used. Five individual trees were selected, and samples of 2*2*36 cm were cut and conditioned at 21°C and 65% relative humidity. Experiments based on free vibration and free-free bar method were performed and controlled in static bending test due to their modulus of elasticity. As expected, this kind of defect showed significant effects on modulus of elasticity, damping factor of vibration and FFT diagrams. A significant relation was observed between modulus of elasticity and damping factor between the two dynamic and static methods, but this relation decrease about 60% in knot-containing specimens. Modes of FFT spectrums of defect-free specimens were pick-like, symmetric specimens without any breaks, but the spectra of knotcontaining samples spectrums were entirely asymmetric. Applying these diagrams in order to recognize the suitability or unsuitability of a piece of wood for specific applications, especially due to internal defects that are not visible, could be very important, and must be considered in future research.

An analysis of sensitivities contributing measurement error to Resistograph values

As a hand-held, portable tool capable of assessing wood density of standing trees quickly and inexpensively, the Resistograph holds considerable potential application for evaluating forest genetics field tests. However, phenotypic correlations between Resistograph density index values and values from conventional wood density assessment techniques are weak. In an effort to investigate the extent to which environmental, operator, or instrument factors may affect density index values, we evaluated the sensitivity of Resistograph measurements to seven experimental factors. Drill bit flexion (a measure of operator steadiness), moisture content of wood, and air temperature significantly affected Resistograph density index values, while the influence of knots is minimized at a vertical distance of 3 cm. Battery type, sharpness of the drill bit (at least up to 350 uses), and battery charge (at least up to 310 uses with a 12 V motorcycle battery) had no significant effect on density index. By ensuring that the operator remains steady while drilling, sampling only live trees, only when air temperatures are above freezing, and by drilling at least 3 cm vertical distance from knots, measurement error should be minimized. Three measurements per tree are required to estimate density index to within 3 units 19 times out of 20.

Strength Ratios of Knots in Bending for Two Alaskan Softwood Species

The effects of knots on the bending strength of dimension softwood lumber were investigated for two Alaska-grown species, yellow cedar (Chamaecyparis nootkatensis) and Sitka spruce (Picea sitchensis). An empirical measure of the bending strength ratio for knots was determined by comparison of full-size in-grade test results to corresponding test results from matching small clear specimens. The empirical strength ratios were compared with theoretical strength ratios calculated using existing formulas given in ASTM D245 (ASTM International 2005). It was determined that existing formulas do not provide accurate estimates of bending strength ratio for the two Alaska-grown species under study. Additional predictor variables were used to develop a new model to estimate the influence of knots on the bending strength ratio. Comparison of new formulas to existing formulas demonstrates that this new model is more effective at accounting for variations in bending strength ratio caused by the presence of a knot. It was concluded that the use of the models presented in the 2005 version of the ASTM standards for knot bending strength ratios should be reevaluated. This is especially important for any new timber species being added to the design tables. Species-specific models for estimated bending strength ratios, utilizing additional predictor variables, would help to better predict the effect of knots on bending strength of structural lumber.

Small-Diameter Scots Pine and Birch Timber as Raw Materials for Engineered Wood Products

In the future, the building industries will need predictable, homogeneous and cost-competitive wood products with structural safety in increasing quantity and quality. This can be provided by, e.g., breaking solid wood and reconstructing the structure in a way that the degrading influence of knots, cracks, decay and other natural irregularities in wood will be eliminated. Beams, panels or boards made by this principle are called the engineered wood products (EWP). The purpose of this study was to investigate the possibilities to utilize small-diameter Scots pine and birch timber for production of EWPs that are reconstituted of strands. The wood technological characteristics of the tree species used in these products worldwide were studied based on the literature, and the findings were compared to the characteristics of domestic woods. In addition, test specimens were manufactured from domestic raw materials of Scots pine and birch species, and tested in order to examine the differences between woods from young trees from the first commercial thinnings and top sections of mature trees from final cuttings as a raw material. According to the literature review, the average basic density and, consequently, many mechanical properties of pine and birch grown in Finland do not markedly differ from those of the numerous foreign species used for EWPs. The empirical tests indicated that beams (air-dry density ca. 620–800 kgm-3) with relatively auspicious static stiffness (ca. 6000–8500 MPa) and bending strength (ca. 32–42 MPa) could be manufactured from timber equal to or smaller than pulpwood in diameter.

ESTIMATION OF STRENGTH OF GLULAM BEAMS DEPENDING ON DEFECTS OF LAYERS/MEDINIŲ KLIJUOTŲ LENKIAMŲJŲ ELEMENTŲ STIPRUMO VERTINIMAS PAGAL SLUOKSNIŲ DEFEKTUS

Timber, as structural material, has various defects, especially knots. They influence the strength of timber members. The influence of knots is lower for glulam members when comparing with solid wood. The article is the theoretical and experimental investigation of influence of knots on the strength of glulam beams. Theoretical investigation deals with probability of coinciding of knots in the same cross-section of member. The factors of influence of knots on glulam beam strength are the results of theoretical investigations. The short-term tests on 12 model beams with measurements 2140×120×152(h) mm were carried out in a testing machine. Results of these tests are in line with the conclusions of theoretical investigations and show the higher strength of glulam beams when comparing with solid members of the same quality. The long-term tests were carried out on model beams when temperature and moisture were cyclically changed. Wetting cycle—14 days long, drying cycle—7 days. Two series, 12 model beams each, were tested. The moisture of outer layer 20 mm depth changed from 6–8% to 20–22%, while the moisture in the rest area changed from 9% to 13%. The load of the first group of beams was 40% of short-term strength, and duration of tests 147 days. For the second group the normal stresses were σ = 18.2 MPa for each beam, and the duration of tests—210 days. Deflection of beams was observed and moments of beams rupture were registered. The relationships between long-term strength and logarithm of time to failure for two types of model beams were the results of long-term tests. Long-term tests show the dependence upon the size of knots and long-term strength of glulam beams.