共查询到20条相似文献，搜索用时 31 毫秒
1.
Fracture toughness of wood and wood composites has traditionally been characterized by a stress intensity factor, an initiation strain energy release rate (G init) or a total energy to fracture (G f). These parameters provide incomplete fracture characterization for these materials because the toughness changes as the crack propagates. Thus, for materials such as wood, oriented strand board (OSB), plywood and laminated veneer lumber (LVL), it is essential to characterize the fracture properties during crack propagation by measuring a full crack resistant or R curve. This study used energy methods during crack propagation to measure full R curves and then compared the fracture properties of wood and various wood-based composites such as, OSB, LVL and plywood. The effect of exposure to elevated temperature on fracture properties of these materials was also studied. The steady-state energy release rate (G SS) of wood was lower than that of wood composites such as LVL, plywood and OSB. The resin in wood composites provides them with a higher fracture toughness compared to solid lumber. Depending upon the internal structure of the material, the mode of failure also varied. With exposure to elevated temperatures, G SS for all materials decreased while the failure mode remained the same. The scatter associated with conventional bond strength tests, such as internal bond and bond classification tests, renders any statistical comparison using those tests difficult. In contrast, fracture tests with R curve analysis may provide an improved tool for characterization of bond quality in wood composites. 相似文献
2.
A simple experimental setup for mode III and mixed mode (I?+?III) fracture tests with anisotropic materials under steady state crack propagation has been developed. Load-displacement curves can be recorded up to the complete separation of the specimen. From the load-displacement curves several mechanical material parameters can be derived. The tests have been performed for solid wood and different wood composites, being PARALLAM® PSL in different orientations, particleboard and INTRALLAM® LSL, and the fracture behaviour is characterised by the specific fracture energy. 相似文献
3.
Bamboo is a unidirectional fiber-reinforced bio-composite. Once having cracks, the delaminating propagation is not controlled by the strength but by the interlaminar fracture toughness. In this paper, the behaviors of Mode I (crack opening mode) interlaminar fracture parallel to grain of moso bamboo (Phyllostachys pubescens) were studied. Based on energy theory, the Mode I interlaminar fracture toughness, G IC, was measured using the double cantilever beam specimens, and the fracture surfaces were examined under scanning electron microscope. The results show that: (1) the interlaminar fracture toughness of Mode I is the basic characteristic of bamboo material. The mean value of G IC = 358 J/m2 (coefficient of variation = 16.88%) represents the resistance arresting crack propagation. No significant difference was found for G IC among the specimens located at different heights of the bamboo. (2) Due to the low G IC of bamboo, the crack propagation parallel to grain developed easily. The crack was a self-similar fracture without fiber-bridging. On the fracture surfaces, smooth fibers and plane ground tissue were found at the extended area of Mode I fracture along the longitudinal direction. Under scanning electron microscope, it could be seen that the crack propagation developed along the longitudinal interface between fibers or ground tissue. It indicates that the longitudinal interface strength was weak among bamboo cells. 相似文献
4.
Mode I fracture and acoustic emission of softwood and hardwood 总被引：7，自引：0，他引：7
The Mode I fracture behaviour of two softwoods (spruce and pine) and three hardwoods (alder, oak and ash) was studied in
the RL crack propagation system using the splitting test in combination with monitoring acoustic emission (AE) activity. Test
parameters measured included notch tensile strength, specific fracture energy, characteristic length and AE cumulative counts,
AE amplitudes as well as parameters characterizing the frequency spectra of the emitted acoustic emission events. The notch
tensile strength was found to correlate with density. The specific fracture energy and characteristic length showed the different
crack propagation process between the softwoods and hardwoods. The softwoods fractured in a more ductile way and the hardwoods
showed a more linear elastic behaviour. This finding was supported by the AE measurements showing much less cumulative counts
for the hardwoods indicating that less microcracks were formed and that processes like fiber bridging were not so effective.
Differences in the frequency domain of the AE signals between softwoods and hardwoods could not be detected.
Received 13 January 1999 相似文献
5.
A coupled experimental and numerical modelling approach was used to investigate the mechanism of softwood fracture at the fibre level. First, a three-dimensional mixed lattice-continuum fracture model was developed to investigate the mechanism of wood fracture, taking into account the porosity of its structure and its heterogeneities at the fibre level. The critical volumes in the specimen where crack propagation was more probable were modelled by a lattice that could show the alternation of earlywood and latewood fibres, and the other regions were considered as the continuum medium. The proposed model was used to investigate the mode I fracture of a small softwood sample in RL orientation. Secondly, a method was developed for microscopic observation of the crack trajectory and investigating the mechanism of initiation and propagation of cracks. This approach was used for microscopic investigation of the fracture behaviour of spruce specimens in mode I and RL orientation. The results of the numerical study were compared with the experimental results. The prepeak and postpeak behaviour of the obtained stress-displacement curve and also the crack opening trajectory in cross-section and longitudinal section in the model and experiments were in good agreement. Both the model and the microscopic observation showed that in mode I fracture and RL orientation, the main trajectory of the crack propagates in the earlywood ring. 相似文献
6.
《Wood material science & engineering》2013,8(2):52-58
Abstract A coupled experimental and numerical modelling approach was used to investigate the mechanism of softwood fracture at the fibre level. First, a three-dimensional mixed lattice–continuum fracture model was developed to investigate the mechanism of wood fracture, taking into account the porosity of its structure and its heterogeneities at the fibre level. The critical volumes in the specimen where crack propagation was more probable were modelled by a lattice that could show the alternation of earlywood and latewood fibres, and the other regions were considered as the continuum medium. The proposed model was used to investigate the mode I fracture of a small softwood sample in RL orientation. Secondly, a method was developed for microscopic observation of the crack trajectory and investigating the mechanism of initiation and propagation of cracks. This approach was used for microscopic investigation of the fracture behaviour of spruce specimens in mode I and RL orientation. The results of the numerical study were compared with the experimental results. The prepeak and postpeak behaviour of the obtained stress–displacement curve and also the crack opening trajectory in cross-section and longitudinal section in the model and experiments were in good agreement. Both the model and the microscopic observation showed that in mode I fracture and RL orientation, the main trajectory of the crack propagates in the earlywood ring. 相似文献
7.
Fracturing of wood under superimposed tension and torsion 总被引：1，自引：1，他引：0
A testing method using circumferentially notched round bars for investigating mixed mode behaviour under loading in tension
and torsion is applied to wood. The applicability of the method to anisotropic materials is investigated for two types of
wood, beech and spruce, considering the longitudinal and radial orientation with respect to the stem axis of the tree. The
strong anisotropy of wood requires different evaluation procedures for radial and longitudinal sample orientation. The K-concept
of linear elastic fracture mechanics (LEFM) and concepts of non-linear elastic fracture mechanics (NLEFM) were used for the
evaluation of radial and longitudinal samples, respectively. Differences between the investigated wood types under radial
orientation, in their durability to withstand torsional loads, could be observed by examining ratios of the values of the
fracture toughness in mode III against mode I. Micrographs of the fracture surfaces support the assumption that the higher
amount of wood rays in beech is responsible for the higher toughness under torsion. In case of longitudinal specimen geometry
it was found that at very high levels of torsional deformation beech and spruce reach similar values in their specific fracture
energy in mode I. 相似文献
8.
《Wood material science & engineering》2013,8(4):176-185
Abstract A new testing method measuring the specific fracture energy of wood-based panels in Mode I is proposed. Three types of wood-based panels, i.e. oriented strand board (OSB), particleboard (PB) and medium density fibreboard (MDF) are investigated, using fracture energy and the industrial European standard method of internal bond strength according to EN 319. Double cantilever beam specimens are notched in the middle layer to introduce an initial crack. To apply tensile load perpendicular to the surface of the panels to open the crack in Mode I specimens were adhesively bonded to steel braces. Besides the calculation of the total fracture energy an advanced analysis of the load–displacement curve was also performed. Results of the fracture energy method were compared to internal bond strength (IB). Specimen shape is optimized for industrial purposes using double cantilever beams, while the determination of the fracture energy is performed by simple integration of the load–displacement curve. While IB showed a large scattering of data, the fracture energy test yielded statistically significant differences between the board types. 相似文献
9.
Almudena Majano-Majano Mark Hughes Jose L. Fernandez-Cabo 《Wood Science and Technology》2012,46(1-3):5-21
The fracture toughness of thermally modified beech (Fagus sylvatica L) and ash (Fraxinus excelsior L) wood under Mode I loading was quantified using Compact Tension (CT) specimens, loaded under steady-state crack propagation conditions. The influence of three heat-treatment levels and three moisture contents, as well as two crack propagation systems (RL and TL) was studied. Complete load–displacement records were analysed, and the initial slope, k init, critical stress intensity factor, K Ic, and specific fracture energy, G f, evaluated. In the case of both species, thermal modification was found to be significantly affect the material behaviour; the more severe the thermal treatment, the lower the values of K Ic and G f, with less difference being observed between the most severe treatments. Moisture content was also found to influence fracture toughness, but had a much less significant effect than the heat treatment. 相似文献
10.
To investigate the fracture behavior of wood, the porosity and heterogeneities of its microstructure should be taken into
account. Considering these features of wood microstructure in a continuum-based model is still a difficult problem and the
lattice model might be an alternative. In the developed mixed lattice-continuum model, the probable crack propagation volume
was modeled by defining a three-dimensional lattice of different beam elements and the other regions were considered as continuum
medium. Different beam elements of lattice represented the earlywood fibers, latewood fibers, ray cells and bonding medium
between the fibers. The proposed model was used to investigate the mechanism of mode I fracture in a small notched wood specimen
in RL orientation. The resulting pre-peak and softening curve and also the crack opening trajectory in both cross-section
and longitudinal-section in model were in good agreement with the experimental observations. This model shows the importance
of considering the three-dimensional and distributed propagation of microcracks and main cracks in fracture stability. It
was also shown that in mode I fracture, RL orientation, the main crack propagates in the earlywood ring. 相似文献
11.
This experiment explored the technical feasibility of using bamboo zephyr mat with pre-hot-pressed treatment for the manufacture of laminated bamboo lumber (LBL), which is similar in construction to that of laminated veneer lumber (LVL). Six LBL boards (made from four-ply bamboo zephyr mats) with approximate dimensions of 2×42× 42cm were fabricated using resorcinol-based adhesive. The experimental design involved three combinations of layered structures (types I, II, and III) and two LBL loading positions (H-beam and V-beam) during the bending test. These materials were then compared to ordinary LVL. Results indicated that the bending properties (moduli of rupture and elasticity) of LBL were comparable to those of LVL, but there was no significant effect on the physical and mechanical properties among the three types of LBL beam. Interestingly, orienting the glue line to the vertical direction (V-beam) could maximize the ultimate strength of the LBL.Part of this research was presented at the 49th Annual Meeting of the Japan Wood Research Society, Tokyo. April 1999, and at the Pacific Timber Engineering Conference, Rotorua, New Zealand, March 1999 相似文献
12.
Pierre Berard Ping Yang Hidefumi Yamauchi Kenji Umemura Shuichi Kawai 《Journal of Wood Science》2011,57(2):100-106
The use of low-grade logs to build spirally wound laminated veneer lumber (LVL) has been studied and improved from the point
of view of the gluing process, fiber orientation angle, and end joint of the LVL. The butt joint appears to be the fracture
point when the column is submitted to a compressive or bending load. Owing to the complexity of cylindrical LVL, we used a
finite element method to simulate the mechanical behavior of part of its wall. This part was small enough to be considered
flat but was representative of the structure, especially in the area of the butt joint. This allowed us to test the validity
of different settings of the parameters involved in the manufacturing process. To feed data for this model, we used the results
established for the linear and nonlinear behavior of raw hinoki in Part I of this series of articles. We then used this numerical
model to improve the quality of the butt joint by testing different settings of the joint. We show that reducing the butt
joint gap under 0.5 mm, which requires only a few changes in the production line, provides an important increase in the modulus
of upture and nonnegligible improvement of the modulus of elasticity compared to that for a ≥ 1 mm butt joint gap. 相似文献
13.
Pierre Berard Ping Yang Hidefumi Yamauchi Kenji Umemura Shuichi Kawai 《Journal of Wood Science》2011,57(2):107-113
The use of low-grade logs to build spirally wound laminated veneer lumber (LVL) has been studied and improved from the point of view of the gluing process, fiber orientation angle, and end joint of the LVL. The butt joint appears to be the fracture point when the column is submitted to a compressive or bending load. Owing to the complexity of cylindrical LVL, we used a finite element method to simulate the mechanical behavior of part of its wall. This part was small enough to be considered flat but was representative of the structure, especially in the area of the butt joint. This allowed us to test the validity of different settings of the parameters involved in the manufacturing process. To feed data for this model, we used the results established for the linear and nonlinear behavior of raw hinoki in Part I of this series of articles. We then used this numerical model to improve the quality of the butt joint by testing different settings of the joint. We show that reducing the butt joint gap under 0.5 mm, which requires only a few changes in the production line, provides an important increase in the modulus of upture and nonnegligible improvement of the modulus of elasticity compared to that for a ≥ 1 mm butt joint gap. 相似文献
14.
A fracture mechanics model for analysis of crack initiation and propagation in wood is defined and applied. The model has
the advantage of being simple, yet it enables reasonably general and accurate analysis commonly associated with more complex
models. The present applied calculations are made by means of the finite element method and relate to progressive cleavage
fracture along grain. The calculations concern a tapered double cantilever beam specimen and an end-notched beam. Comparisons
are made of experimental test results. The fracture properties of the wood are modelled by means of a very thin linear elastic
layer located along the crack propagation path. The properties of the layer are such that the strength and fracture energy
of the wood are represented correctly. This makes a single linear elastic calculation sufficient for strength prediction.
Both crack development and pre-existing cracks can be analyzed. Both material strength and fracture energy and stiffness are
taken into account, their relative influence on structural strength being different for different elements. The fracture layer
is in the finite element context represented by joint elements. Propagation of a crack can be analyzed either by a series
of elastic calculations corresponding to different crack lengths or by use of a finite element code for non-linear analysis.
The computational results include sensitivity analysis with respect to the influence of the various material parameters on
structural strength. 相似文献
15.
Hiroshi Yoshihara 《Journal of Wood Science》2003,49(3):210-215
The measurement method of mode II fracture toughness-crack propagation length relation (i.e., the resistance curve, or R-curve) was examined by end-notched flexure tests on sitka spruce (Picea sitchensis Carr.). The tests were conducted by varying the span/depth ratios under the constant loading point displacement condition.
The fracture toughness was measured from the load-crack shear displacement (CSD) and load-longitudinal strain relations. The
crack length was determined by a combination of load-CSD and load-strain compliances and Williams's end correction theory,
as well as the observation of crack propagation. When the specimen had an appropriate span/depth ratio, the fracture toughness
and crack propagation length were measured from the load-CSD compliance and combined load-CSD and load-strain compliances,
respectively, and the R-curve could be determined properly under the constant loading point displacement condition.
Received: March 15, 2002 / Accepted: July 25, 2002 相似文献
16.
Masumi Minagawa Yosuke Matsuda Yuko Fujiwara Yoshihisa Fujii 《Journal of Wood Science》2018,64(6):758-766
The method of digital image correlation (DIC) was applied to the digital image of orthogonal cutting parallel to the grain of hinoki, and the strain distribution near the cutting edge was evaluated. The wood fracture associated with chip generation was considered as mode I fracture, and the stress intensity factor KI for fracture mode I was calculated from the strain distribution according to the theory of linear elastic fracture mechanics for the anisotropic material. The calculated KI increased prior to crack propagation and decreased just after the crack propagation. The change in KI before and after crack propagation, ΔKI, decreased in accordance with the crack propagation length, although the variance in ΔKI should depend on the relationships between the resolution of DIC method and the dimensions of cellular structure. The calculated KI in this study was almost on the same order as reported in the literatures. It was also revealed, for the case of chip generation Type 0 or I, the stress intensity factor for fracture mode II could be negligible due to the higher longitudinal elastic properties of wood in the tool feed direction than the one radial ones, and the mode I fracture was dominant. 相似文献
17.
Bamboo is a kind of biological composite reinforced by unidirectional long fibers. The cleavage strength along grain of bamboo internode is low; however, the existence of bamboo node can hinder the propagation of interlaminar crack to make up for the defect of weak opening mode fracture toughness along interlamination. In this article, the double cantilever beam method was applied to test the Mode I interlaminar fracture toughness of Moso bamboo internode specimens and specimens with node to study the difference of the Mode I interlaminar fracture toughness between Moso bamboo internode specimens and specimens with node. The results are shown as follows: the Mode I interlaminar fracture toughness of Moso bamboo internode specimens was \( G_{{{\text{I}}C}}^{\text{Internode}} \) = 498 J/m2 (SD = 65 J/m2); the Mode I interlaminar fracture toughness of Moso bamboo specimens with node was \( G_{{{\text{I}}C}}^{\text{Node}}\) = 1,431 J/m2 (SD = 198 J/m2). It can be seen that the Mode I interlaminar fracture toughness of bamboo specimens with node was higher than that of bamboo internode specimens, and the toughness contribution of node to bamboo Mode I interlaminar fracture toughness was 1.87 times. The conclusion was drawn that bamboo node can contribute a lot to hinder the interlaminar fracture of bamboo. 相似文献
18.
The relation between crack propagation based on fracture mechanics and end-check propagation during drying was evaluated in
this study. Corresponding to the direction of end-check propagation, the mode I fracture toughness of air-dried sugi specimens
in TR, TL, and intermediate systems was examined by single-edge-notched bending tests. The occurrence and propagation of end
checks on sugi (Cryptomeria japonica D. Don) blocks during drying were observed at the scale of the annual rings. It was found that the critical stress intensity
factor (K
IC) decreased as the crack propagation changed from TL to TR. The value of K
IC in the TR system was significantly lower than that in the TL system. As a measure of fracture energy, the area under the
load-crack opening displacement curve in the TR system was more than twice that in the TL and intermediate systems. These
results indicate that cracks perpendicular to the tangential direction initiate radially with ease, and then crack arrest
occurs to prevent growing. This finding provides a consistent interpretation of the end-check propagation observed during
drying as follows: tiny end checks, as an analog of TR cracks, occur easily and selectively in latewood or transition wood
and propagate toward the pith during drying. When there is no corresponding secondary check in the forward latewood, the checks
are arrested and do not propagate further. 相似文献
19.
Summary Mode I fracture energy of premature plantation-grown red pine is discussed, for crack growth in the longitudinal direction. It is demonstrated that fracture energy is influenced by moisture content at test and the direction that stress is applied in the radial-tangential plane. Secondary influences of moisture conditioning and density on fracture energy were observed, with the severity related to the moisture content of the material at test. Discrepancies with findings in the literature are identified and discussed. It is likely that results of this study apply to other conifer species with low extractive contents.The work was undertaken with financial support from the Natural Sciences and Engineering Research Council of Canada under operating grants OGP 0004417 and OGP 0109473 相似文献
20.
The crack initiation and propagation of multiplybolted joints subjected to lateral forces perpendicular to the grain were analyzed. Two types of bolted joint were subjected to lateral loads perpendicular to the grain. One had joints of two bolts aligned with the wood grain (type H), and the other had joints of two or three bolts aligned perpendicular to the grain (type V). The crack initiation and propagation were analyzed by means of the average stress method (ASM) and linear elastic fracture mechanics (LEFM), respectively. The maximum loads calculated by LEFM agreed comparatively well with the experimental results, and it was proved that the LEFM was an appropriate tool to analyze the fracture of multiply-bolted joints subjected to a force perpendicular to the grain. It was also found that the multiply-bolted joints failed with the fracture of the wood before the joints yielded, and that it caused a considerable decrease of the maximum loads. The reduction of strength should be considered in the design of multiply-bolted joints subjected to lateral forces perpendicular to the grain.Part of this work was presented at the annual meeting of the Architectural Institute of Japan, Hikone, September 1996 相似文献