Tangential Radial Research Articles

Optimization of CZM parameters of Norway spruce tested in mode I using multistart simplex method

The paper addresses the advanced fracture analysis of the SEN-TPB test on Norway Spruce in three anatomical directions: tangential (T), radial (R) and tangential–radial (TR). It utilizes reverse engineering to obtain cohesive zone model (CZM) parameters, which are then compared to the experimental values obtained using the compliance-based beam method (CBBM) and additional standard material tensile tests perpendicular to the grain. The optimization employs force–displacement curves to find their optimal fit using the multistart simplex optimization method. Results indicate that CZM modeling can be relatively problematic when realistic values are used; however, the best agreement with experimental data is observed for the group of cracks in the TR direction. A fundamental issue with CZM modeling in the elastic part has been identified and discussed. The sensitivity of the problem to various parameters was assessed, and an energy balance in the CZM at Fmax was performed. This work contributes to knowledge about realistic modeling of the interface and a fundamental understanding of fracture in wood.

Evaluation of the Shear Performance of Douglas-Fir Wood at Elevated Temperatures

Shear fracture frequently occurs in timber beams and panels subjected to transverse loads. At elevated temperatures, wood will undergo complex physical and chemical processes which significantly affect the shear properties. In this paper, the v-notched Douglas-fir specimens with three different shear planes: (a) Radial-Tangential (RT); (b) Radial-Longitudinal (RL), and (c) Longitudinal-Radial (LR), were fabricated and tested under the elevated temperatures from 20 °C to 180 °C. The digital image correlation (DIC) technique was used to measure the shear strain. It was found that the shear plane had a significant effect on the failure modes, shear strength, and shear modulus. The shear strength and shear modulus generally decreased with the increase of temperature. However, the shear strength was significantly improved when the hardening of the dry lignin occurred between 100 °C and 140 °C. Moreover, the design curve for the shear strength in Eurocode 5 is conservative for all the specimens with different shear planes.

On the inverse identification of wood elastic properties using a DIC-based FEMU approach

PurposeThis work aims to identify the linear elastic orthotropic material paramters of Pinus pinaster Ait. wood, using full-field measurements and an inverse identification strategy based on the finite element (FE) method updating technique.Design/methodology/approachCompression tests are carried out under uniaxial and quasi-static loading conditions on wood specimens oriented on the radial-tangential (RT) plane, with different grain orientations. Full-field displacements and strains are measured using digital image correlation (DIC), which are then used as a reference in the identification procedure. A FE model is implemented assuming plane stress conditions, where wood is modelled as an orthotropic homogeneous material. Based on the numerical results, a synthetic image reconstruction scheme is implemented to synthetically deform the reference experimental image, which is then processed by DIC and further compared to the experimental results.FindingsThe results for both approaches were similar when both specimen configurations were used in a single run. However, when using the DIC-based FEMU approach with the on-axis configuration, the identified modulus of elasticity in the tangential direction and shear modulus are closer to the reference values.Originality/valueThis approach ensures a fair comparison between both sets of data since the full-field strain maps are obtained through the same filter and therefore have the same strain formulation, spatial resolution and data filtering. Firstly, the identification is performed using a single configuration, either the on-axis or the off-axis specimen. Secondly, the identification is carried out by merging data from both on-axis and off-axis configurations.

Comparative Study of Destructive, Nondestructive, and Numerical Methods on the Determination of Moisture-Dependent Shear Moduli of Calabrian Pine

In this study, the moisture-dependent shear moduli of Calabrian pine were determined by a 45° off-axis compression test, and ultrasonic measurement were performed to determine the effectiveness of the nondestructive method for shear modulus prediction. Also, finite element modeling and analysis was performed to compare the results with static stress-strain curves within the linear elastic region. Ultrasonic transverse wave velocities in longitudinal-radial (LR), longitudinal-tangential (LT), and radial-tangential (RT) planes decreased from 1447 to 1368, 1342 to 1264, and 682 to 642 m/s with an increase in relative humidity (RH) from 45% to 85%, respectively. Static and dynamic shear modulus in LR, LT, and RT planes decreased from 1054 to 933, 905 to 825, and 230 to 210 N/mm2, and 1141 to 1065, 982 to 909, and 254 to 235 N/mm2 when relative humidity increased from 45% to 85%, respectively. The influence of the moisture content on the transverse velocity and moduli was statistically significant. The coefficient of determination between the dynamic and static shear moduli ranged from 0.77 to 0.96.

Shear moduli in the longitudinal-radial and radial-tangential planes of Sitka spruce measured by torsional vibration tests

Abstract The shear moduli (SM) of solid wood in the longitudinal-radial (LR) and radial-tangential (RT) planes of Sitka spruce are extremely different from each other and were obtained via a torsional vibration (TV) test by varying the aspect ratios and conducting subsequent numerical analyses. The SM in both planes were accurate, when the aspect ratios of the specimens were appropriately selected, even though the SM around the torsional axis were extremely different from each other.

Practical techniques for the vibration method with additional mass: bending vibration generated by tapping cross section

This work examines the effect of a method for generating bending vibration on the accuracy of a nondestructive and simple estimation of weight, density, and Young’s modulus through a vibration test without measuring specimen weight. The resonance frequencies with and without the concentrated mass generated by tapping the RT (radial tangential)-plane under the free-free condition were compared with those generated by the normal free-free bending vibration. The air-dried specimens and wet specimens in a drying process at 20 °C and 65% relative humidity were used and then their weight, density, and Young’s modulus were estimated by the vibration test. The appropriate resonance frequency of the bending vibration could be obtained by tapping the RT-plane. Generating bending vibration by tapping the RT-plane is effective for the application of the vibration method with additional mass to a drying process.

Determination of mode I and mode II fracture toughness of walnut and cherry in TR and RT crack propagation system by the Arcan test

Abstract Two specimen types, each from walnut and cherry wood, were prepared for tangential-radial (TR) and radial-tangential (RT) crack propagation systems at 65% of RH and 20°C before mode I and mode II fracture toughness was determined through Arcan tests. It was found that fracture toughness in mode I is in agreement with literature data. In the mode II test, however, the crack propagated in the direction normal to the shear plane and not parallel to it. The release rate of strain energy in terms of the opening failure in mode II was lower than that in mode I. It can be concluded that it is difficult to determine the fracture toughness of RT or TR propagation in hardwood specimens in mode II.

The Effect of the Fiber of Wood and Connection Tools on the Vibration Characteristics of Gofasa Wood (<em>Vitex cofassus</em>)

The aim of this study was to determine the characteristics of vibration transmission of Gofasa wood (vitex cofassus), which the primary material used in wooden shipbuilding in Maluku. The improvement of the connections of wood has an influence on the stiffness values of the binding system because of its mechanical properties. In this research, the excitation test was carried out on the connection with Radial-Radial (RR), Tangential-Tangential (TT) and Radial-Tangential (RT) fiber direction to see the effect of the orientation of the wood fiber on the vibration characteristics. The excitation test to the specimens using bolt, nail and peg connections was conducted to see the effect of the fastening tool on vibration characteristics. The results of the tests showed that the dynamic stiffness of the RR connection using a bolt had the highest value at the four measurement points exceeding the dynamic stiffness of the material without connections. Dynamic stiffness k of the TT connections using a nail had the highest value and k with the RT joint recorded the highest value using the peg connection tools. This study shows that the determination of the direction of the fiber and the type of connecting devices have an effect on the magnitude of the vibration transmission. This research indicates that vibration characteristic can be using as a reference in determining the appropriate connection tool to be used on wood material based on the fiber direction.

Fracture properties of green wood formed within the forks of hazel (Corylus avellana L.)

Key messageCentral apex of bifurcations has higher specific fracture energy in TR fracture system than that of four sampling locations. This could be due to higher density and interlocked grain formation.Forks are one of the important biomechanical structures in trees because of their potential vulnerability to splitting. Many researchers have investigated the strength and stiffness properties of tree forks, but very little is known about the toughening mechanism within tree forks. In this study, the specific fracture energy (Gf, Jm−2) of forks of hazel (Corylus avellana L.) was investigated in the RT (Radial-Tangential) and TR (Tangential-Radial) fracture systems using double-edge-notched tensile tests. Sample Gf values were measured at between the central apex of bifurcations, at the side apices of bifurcations, in the parent stems and in the two branches of forks. The fracture surfaces were analysed by Scanning Electron Microscopy (SEM), and the wood density was determined. Gf was found to be considerably greater at the central apex of a bifurcation than in other sampling locations. Surprisingly, Gf of TR was greater than Gf of RT at the central apex, while the other four locations showed greater Gf values in their RT fracture systems. The density of the central apex of bifurcations was found to be around 22% greater than elsewhere. In addition, it was shown that there was a more tortuous and interlocked wood grain formation at the central apex of bifurcations. The combination of higher density and tortuous grain structure provides reinforcement at the central apex.

The effect of elevated temperature and high moisture content on the fracture behaviour of thermally modified spruce

The aim of the present study was to investigate the effect of moisture content and temperature on the fracture behaviour of thermally modified Norway spruce (Picea abies [L.] Karst.) in the transverse plane. Spruce was thermally modified at two heat treatment temperatures, 190 and 210 °C. Mode I fracture tests were carried out at temperatures of 22 and 50 °C on air-dried and fully water-saturated (>fibre saturation point) material. Small CT specimens were used, and load–CMOD curves were recorded. Characteristic fracture parameters were calculated. The fracture surfaces were subsequently examined using optical microscopy. Thermal modification altered all the calculated fracture parameter values and the changes increased as the severity of heat treatment increased. The parameters were altered more in the radial-tangential (RT) than in tangential-radial (TR) orientation. The failure mode changed due to heat treatment in the TR orientation but in the RT orientation the failure mode of unmodified and heat treated material differs only at high moisture content and elevated temperature.

The effect of temperature and moisture content on the fracture behaviour of spruce and birch

Abstract The fracture behaviour of wood is affected by, amongst other factors, its microscopic structure, moisture content and temperature, as well as the direction of crack propagation. Spruce (Picea abies [L.] Karst.) and birch (Betula pendula Roth.) compact tension specimens were subjected to pure mode I loading in both the radial-tangential (RT) and tangential-radial (TR) orientations under four climatic conditions. The effect of microscopic structure on crack propagation was observed, and the “crack mouth opening displacement vs. load” histories were recorded, from which characteristic fracture parameters were calculated. The fracture surfaces were subsequently examined by optical microscopy. In both species, distinct changes in the fracture behaviour were observed as moisture content (MC) and temperature (T) were varied. The results were specific for the wood species, but MC and T influenced fracture behaviour in a similar way. The effects were more pronounced in birch than in spruce. Elevated T and MC did not affect the failure mode, except in the case of spruce in the RT orientation.

Shear stiffness and its relation to the microstructure of 10 European and tropical hardwood species

In this study, shear stiffness properties of 10 different hardwood species and their relation to the corresponding species-specific microstructure are investigated. For this purpose, shear stiffness of 10 different hardwood species is experimentally measured by means of ultrasonic testing. In addition, a micromechanical model for hardwood is applied in order to illustrate the influence of certain microstructural characteristics such as mass density and volume fractions of vessels and ray cells on the shear stiffness. Comprehensive microstructural and mechanical data from previous investigations of the same hardwood material support the interpretation of the microstructure–shear stiffness relationships. Mass density was confirmed to be the dominant microstructural characteristic for shear stiffness. Also, ultrasound shear wave propagation velocity increases with density, particularly in the radial-tangential (RT) plane. In addition to density, comparably higher shear stiffness GLR can be explained by comparably higher ray content and lower vessel content. As for GLT, a ring porous structure seems to lead to higher shear stiffness as compared to a diffuse porous structure. For this shear stiffness, vessel and ray content were found to have a less impact. Also, the rolling shear stiffness GRT was found to be higher for a diffuse porous structure than for a ring porous one. Moreover, the data supports that ray cells act as reinforcements in the RT plane and lead to higher GRT.

The cellular level mode I fracture behaviour of spruce and birch in the RT crack propagation system

Abstract The effect of the microscopic structure and the moisture content (MC) of wood on its fracture behaviour has been investigated. Green and air-dried spruce (Picea abies [L.] Karst.) and birch (Betula pendula Roth.) wood were subjected to pure mode I loading in the radial- tangential (RT) crack propagation system. Tests were carried out in situ in an environmental scanning electron microscope to observe crack propagation at the cellular level. Crack-tip displacement fields were computed by digital image correlation, and crack propagation was observed from the images captured during testing. Both the MC and the microscopic structure were found to affect the fracture process. In the air-dried birch and spruce, only microcracking caused large displacements ahead of the crack-tip. In spruce, the microcracking zone was larger than in birch. In green birch and spruce, microcracking was less evident than in the air-dried specimens, and in some cases, there were notable deformations in a few cells ahead of the crack-tip before crack extension. Microcracking is considered to be the main toughening mechanism in spruce and birch in the RT crack propagation system.

Modeling of damage evolution in soft-wood perpendicular to grain by means of a discrete element approach

The anisotropy of wood within the radial–tangential (RT) growth plane has a major influence on the cracking behavior perpendicular to grain. Within the scope of this work, a two-dimensional discrete element model is developed, consisting of beam elements for the representation of the microstructure of wood. Molecular dynamics simulation is used to follow the time evolution of the model system during the damage evolution in the RT plane under various loading conditions. It is shown that the results are in good agreement with experiments on spruce wood, and that the presented discrete element approach is applicable for detailed studies of the dependence of the microstructure on mesoscopic damage mechanism and dynamics of crack propagation in microstructured and cellular materials like wood.