Nanoindentation of single pulp fibre cell walls

Unrefined chemical pulps of bleached and unbleached softwood (Scots pine, Pinus sylvestris) and hardwood (Eucalyptus, Eucalyptus globulus) were subjected to indentation tests using a nanoindenter equipped with an AFM scanner. Tests on unbleached pulps revealed no difference in hardness values between softwood and hardwood, but bleaching treatment decreased the hardness values of both pulps. Indentation modulus of 12 GPa was observed for unbleached softwood pulps, which is 25% higher than unbleached hardwood pulps. Bleaching treatment again decreased the indentation modulus of the softwood pulps, whereas it slightly increased the indentation modulus of the hardwood pulps. After bleaching and drying processes, only negligible difference was observed in cell wall mechanical properties (hardness and indentation modulus) between hardwood and softwood pulps. This study is based on latewood pulp fibres.     

Fourier transform infrared studies of heterogeneity, photodegradation, and lignin/hemicellulose ratios within hardwoods and softwoods

There is little information available on the variation in lignin content of growth rings in hardwoods. This study examines whether infrared microscopy can detect intra-incremental differences in the chemical composition of three hardwoods (R. pseudoacacia, P. Americana, and G. triacanthos) and the effect of such differences on the delignification of the hardwoods during weathering. Earlywood has higher lignin content than latewood in R. pseudoacacia and P. americana, but the opposite was found for G. triacanthos. The delignification of the earlywood and latewood during weathering varied for the three species. It was greater in the earlywood of R. pseuoacacia, whereas in P. americana and G. triacanthus it was more pronounced in latewood. Differences in density and lignin content of earlywood and latewood help explain these differences. In addition, a deconvoluting software package was used to determine whether it is possible to estimate the lignin/hemicellulose ratio in softwoods and hardwoods. Results from the 1760-1580 cm(-1) region provided data that can be used to estimate the lignin/hemicellulose ratio of softwoods and hardwoods. This information can be obtained far more easily using infrared microscopy than with conventional wet chemical techniques, potentially allowing characterization of greater numbers of species than has hitherto been possible.     

Measurements of crack tip strain field in wood at the scale of growth rings

The fracture mechanisms of wood have often been interpreted on the scale of cell walls. Although this scale is important, the scale of growth rings needs to be considered in the same context. In the present study, the crack tip strain field of radial TR cracks at the scale of growth rings is measured by electronic speckle photography. The methodology is discussed in detail as well as the data reduction scheme. The tip is in the earlywood layer and the crack plane of the TR crack is perpendicular to the stiffer latewood layer. Increasing opening mode load is applied in-situ as the crack is observed by reflected light optical microscopy. Strains are measured on direct images of the microstucture. In contrast to some other methodologies, this allows direct correlation between strain field and microstructure. In the softer earlywood, tangential strains extend considerable distances in the tangential direction. Due to the stiff latewood, the strain is heavily constrained in the radial direction. This nature of the local strain field has been largely neglected, despite its obuius significance to TR crack growth mechanisms.     

Influence of repetitive stiffness variation on crack growth behaviour in wood

Softwoods have a repetitive variation in stiffness over their growth rings, which is due to the difference in cellular structure between the latewood and earlywood. In this paper, the influence of the repetitive stiffness variation on radially growing cracks is studied by detailed finite element analyses, in which the wood material is represented by a layered orthotropic continuum. The distribution of stress around the crack is found to be very different from crack tip stress fields in homogenous isotropic materials. The latewood layer ahead of the crack experiences a significant tensile stress, which indicates that formation of new secondary cracks ahead of the primary crack front is a likely mechanism for crack propagation. This mechanism is also favoured by the fact that the primary crack is subjected to a significant shielding from the stiff latewood, which tends to arrest the primary crack in the soft earlywood layer. Analyses are performed for materials with various growth ring widths, and the calculated results are compared with reported experimental observations.     

Mechanical Properties of Wood from Transgenic Poplar Trees with Modified Lignification

Double cantilever beam (DCB) and instrumented microtome cutting (IMC) tests were carried out in testing wood fracture properties of control and various transgenic poplar plants. The results show that control plants have higher work of fracture (WF) than most transgenic plants. The air-dry materials have higher WF than wet ones and latewood is higher than earlywood. Also, samples from parts of the trunk above stye points usually have higher WF than those from below. The comparison between DCB tests and IMC studies show that, the WF obtained from DCB is higher than the cutting work measured by IMC. Scanning electronic microscopy (SEM) and confocal laser scanning microscopy (CLSM) were used to study specimens'fracture surfaces and structure. Though in all samples fracture occurred between the middle lamella and S1layers, between S1 and S2 layers, as well as within S2 layers, there are more transwall fractures in control plants.There are also differences in cell wall area fraction among these materials.     

Structural changes in primed Scots pine and Norway spruce during weathering

The effect of weathering on Scots pine (Pinus sylvestris) and Norway spurce (Picea abies) treated with unpigmented commercial primers was studied. Structural changes occurring in radial longitudinal surfaces during weathering over a period of 24 weeks were examined using a reflected light microscope and a scanning electron microscope. Micro-checks were aligned parallel to microfibrils of the cell wall or parallel to the axis of the tracheids, and occurred first in latewood. The first larger checks appeared in thin-walled earlywood tracheids near thick-walled latewood tracheids. After 12 weeks exposure, delamination of the secondary wall occurred. After 24 weeks there were some visible macroiscopic checks on the surfaces and very little separation of individual cells occurred. There were no notable differences in checking and surface erosion between pine and spruce. Blue stain colonization was found first in the cracks between tracheids and it was more common on pine than on spruce. The priming of wood did not prevent but retarded structural changes occurring in the underlying wood. The higher the level of the solid contents in the primers, the more likely the occurrence of structural changes was retarded.     

A thermal imaging technique for studying crack development in wood under torsional loading

A thermal imaging system has been used for monitoring fracture in wood under both static and fatigue torsional loading. The thermal images of softwood test-pieces containing a knot under torsional loading predicted the cracking time and crack position that agreed well with visual observation. The thermal images obtained under torsional fatigue loading indicated a temperature increase during the unloading part of a loading cycle, which meant that thermal energy was dissipated during the relaxation stage of the loading cycle. The maximum temperature reached also increased as the loading cycles increased. Results from thermal images of a softwood indicated that the earlywood exchanged more thermal energy than latewood. Optical microscopy and SEM confirmed that in earlywood the region near a growth ring is the weaker area. For all the test pieces, whether softwood or hardwood, with or without a knot, the hotspots revealed during thermal imaging appeared before the load dropped sharply and these were confirmed to be the positions for crack initiation. This shows that it is possible to predict and depict failure and its progress using thermal imaging techniques.     

Wet fiber shear flexibility and its contribution to the overall transverse deformation of fibers

A new method has been developed for measuring not only bending but also shear flexibility of pulp fibers by using confocal laser scanning microscopy. Based on the Steadman and Luner method, a two-stage wet pressing process was used which enabled both bending and shear flexibility and both bending and shear modulii of fibers to be determined with a single test. Three types of fibers, i.e., bleached spruce Kraft Pulp (BKP), aspen bleached chemi-themomechanical pulp (BCTMP), and aspen themomechanical pulp (CTMP), were tested. Results show that the longitudinal elastic modulii of the fibers are in a range of 3–37 GPa, and the transverse shear modulii of them are in a range of 27–103 MPa. It was also found that the shear contribution to the overall fiber deformation ranged from 60% to 90% for the fibers measured. This substantiates the concept of shear contribution to measured fiber flexibility as proposed by Waterhouse and Page.     

Efficient evaluation of the flexibility of tapered I-beams accounting for shear deformations

The principle of complementary virtual work is used to evaluate numerically the flexibility matrix of tapered I-beams accounting for shear deformations. Equilibrium considerations of the top and bottom fibres reveal that the shear stress is not equal to zero at these locations. To correct for this non-vanishing shear, a statically admissible shear stress field is considered by assuming a parabolic distribution of shear stress which takes non-zero values at the top and bottom fibres such that the global equilibrium is satisfied within the assumed stress profile. The flexibility matrices of the proposed tapered I-beam finite element with different slopes are generated using numerical integration based on Gauss quadrature. The results are compared to full-blown shell finite element models, and stepped beam models constituted by a series of uniform beam elements, to illustrate the effectiveness of the proposed method.     

In situ observations of fracture mechanisms for radial cracks in wood

This paper presents the findings of work carried out to describe the micromechanisms of radial crack growth in wood. TR and TL cracks are both radial cracks but TR grows radially and TL longitudinally. TR cracks are known to show higher fracture toughness than TL cracks. The TR fracture surfaces also indicate a more tortuous crack path. Since the reason for this is unclear, details of the TR crack growth mechanisms in green Pinus sylvestris L were studied. This was done by in-situ optical microscopy as the crack was cutting through alternating layers of soft earlywood and stiff latewood. At the scale of individual cells, the crack tip advanced by separating cell walls at the middle lamella in a splitting or peeling mode. At the scale of growth rings, stick-slip type of crack growth was observed and new crack planes were often formed. The stress distribution in a material with alternating stiff and soft layers is causing this. This stress distribution also contributes to the tendency for inclined cracks to deviate in the radial direction. For interpretation of fracture mechanisms, the importance of scale interaction and the combined influences of microstructure and stress state are emphasized.     

Computational up-scaling of anisotropic swelling and mechanical behavior of hierarchical cellular materials

The hygro-mechanical behavior of a hierarchical cellular material, i.e. growth rings of softwood is investigated using a two-scale micro-mechanics model based on a computational homogenization technique. The lower scale considers the individual wood cells of varying geometry and dimensions. Honeycomb unit cells with periodic boundary conditions are utilized to calculate the mechanical properties and swelling coefficients of wood cells. Using the cellular scale results, the anisotropy in mechanical and swelling behavior of a growth ring in transverse directions is investigated. Predicted results are found to be comparable to experimental data. It is found that the orthotropic swelling properties of the cell wall in thin-walled earlywood cells produce anisotropic swelling behavior while, in thick latewood cells, this anisotropy vanishes. The proposed approach provides the ability to consider the complex microstructure when predicting the effective mechanical and swelling properties of softwood.     

Mechanical properties of sisal fibre at elevated temperatures

Sisal fibres extracted from the leaves of Agava sisalana plants 3, 5, 7 and 9 years old were tested at different temperatures for tensile strength, elongation, toughness and modulus. The tensile strength, modulus and toughness values of sisal fibre decreased with increase in temperature. The effect of plant age on tensile strength, tensile modulus and toughness of sisal fibre became very much less at 100 °C as compared to 30 °C. Fractured fibres were observed by using a scanning electron microscope. The ends of fibres fractured at elevated temperature showed a failure similar to that of inorganic fibres. Elongation values at all temperatures increased with age. Elongated capillaries were observed in fibres fractured at 80 and 100 °C, due to the removal of moisture and volatiles originally present in the fibres. The fibrils are clearly observed in the form of hollow cylinders. Fractured surfaces are composed of brittle as well as ductile phases. The ductile portion increased with the increase of temperature.     

High modulus/high strength poly-(p-phenylene benzobisthiazole) fibres

Wide-angle X-ray diffraction studies of poly-(p-phenylene benzobisthiazole) fibres from Part 1 of this work were undertaken to examine fibre structural changes associated with the heat treatment process and which contribute to the observed significant enhancement of mechanical properties. Crystallite size perpendicular to the fibre axis increases from approximately 2 nm in as-spun fibres to 10 to 12 nm in fibres heat treated at temperatures above 600 C. Fibre tensile strength was found to increase with this increase in the extent of the lateral molecular order. However, tensile modulus and tensile strength did not depend directly on heat treatment parameters but rather indirectly through the effect of applied tension during heat treatment on the overall axial orientation. Higher values of fibre tensile modulus and tensile strength were exhibited by the more highly oriented fibres.     

Brazilian waste fibres as reinforcement for cement-based composites

Fibre reinforced cement-based composites were prepared using kraft pulps from sisal and banana waste and from Eucalyptus grandis pulp mill residues. The study adapted conventional chemical pulping conditions for the non-wood strands and a slurry vacuum de-watering method for composite preparation followed by air-curing. Plain cement paste and Pinus radiata kraft reinforced cement composites were used as reference materials. Mechanical testing showed that optimum performance of the various waste fibre reinforced composites was obtained at a fibre content of around 12% by mass, with flexural strength values of about 20 MPa and fracture toughness values in the range of 1.0–1.5 kJ m⁻². Experimental results showed that, of the waste fibres studied, E. grandis is the preferred reinforcement for low-cost fibre-cement.     

Optimum stacking sequence design of laminated composite circular plates with curvilinear fibres by a layer-wise optimization method

The optimum stacking sequence design for the maximum fundamental frequency of symmetrically laminated composite circular plates with curvilinear fibres is investigated for the first time using a layer-wise optimization method. The design variables are two fibre orientation angles per layer. The fibre paths are constructed using the method of shifted paths. The first-order shear deformation plate theory and a curved square p-element are used to calculate the objective function. The blending function method is used to model accurately the geometry of the circular plate. The equations of motion are derived using Lagrange’s method. The numerical results are validated by means of a convergence test and comparison with published values for symmetrically laminated composite circular plates with rectilinear fibres. The material parameters, boundary conditions, number of layers and thickness are shown to influence the optimum solutions to different extents. The results should serve as a benchmark for optimum stacking sequences of symmetrically laminated composite circular plates with curvilinear fibres.     

Potential of alternative fibre cements as building materials for developing areas

This project evaluated the performance of thin fibre-cement elements produced from alternative raw materials using the Hatschek process, with a view to their use in low-cost housing. Sisal and banana fibres were prepared using mechanical and kraft pulping procedures while residual Eucalyptus grandis pulp was obtained from a commercial pulp mill. Granulated blast furnace slag (BFS) was used as the major component of an alternative hydraulic binder and ordinary Portland cement as a control. Composites were prepared using a slurry vacuum de-watering process, pressing and air-curing. At fibre contents of 8–12% by mass, moduli of rupture (MOR) up to 23 MPa and fracture toughness (FT) values in the range of 0.6–1.7 kJ/m² were obtained at 28 days. After 12 months of exposure under temperate and tropical conditions, the MOR of the BFS-based composites had decreased to values in the range of 6.6–10.1 MPa. FT values remained stable or even increased with the weathering exposure. The results indicate that the mechanical performance of the composites being studied is currently satisfactory, but further optimisation of formulation and processing parameters should be investigated.     

A 3D multilevel model of damage and strength of wood: Analysis of microstructural effects

A 3D hierarchical computational model of damage and strength of wood is developed. The model takes into account the four scale microstructures of wood, including the microfibril reinforced structure at nanoscale, multilayered cell walls at microscale, hexagon-shape-tube cellular structure at mesoscale and annual rings at the macroscale. With the use of the developed hierarchical model, the influence of the microstructure, including microfibril angle (MFA), the cell shape and the wood density (annual ring structure), differences between earlywood and latewood as well as microstructural arrangements and cellulose strength distributions on the tensile strength of wood is studied numerically. Good agreement of the theoretical results with experimental data has been obtained.     

含圆周非贯穿裂纹悬臂管道的振动分析与裂纹识别

摘 要：根据线性断裂力学理论和应变能释放原理,推导了含圆周非贯穿裂纹管道在轴力、剪力和弯矩等荷载作用下的局部柔度系数方程,利用适应性Simpson方法编写了数值积分程序进行局部柔度系数求解,建立了含裂纹管道的二维有限元模型进行含裂纹悬臂管道的振动特性分析,应用等值线图原理进行了悬臂管道的裂纹识别。研究结果表明：本文裂纹模型克服了当前裂纹模型仅针对特定的荷载模式或非空心截面的缺陷,基于等值线图法能有效识别含裂纹悬臂管道的裂纹位置、深度。     

INTERLAMINAR FRACTURE ENERGY OF UHMPE/EPOXY COMPOSITES BY DOUBLE CANTILEVER BEAM AND PEEL TESTS

Abstract— Interlaminar mechanical properties of composite materials such as shear strength and fracture toughness depend on the level of fibre-matrix adhesion which has to be optimised according to the end-use of the composite. Various surface treatments of fibres are used for this purpose. Double cantilever beam (DCB) tests are commonly used for estimating the interlaminar fracture toughness in Mode I. It is shown in this work that this parameter can be conveniently determined using a simpler technique involving a 90° flexible-to-rigid substrate peel test. The values of G_Ic determined by DCB and 90° peel tests are comparable within acceptable experimental error margins. These two alternative techniques are used for assessing the effectiveness of a novel surface engineering process for enhanced adhesion of ultra-high modulus polyethylene (UHMPE) fibres to an epoxy matrix.     

The importance of seasonal differences in the cellulose microfibril angle in softwoods in determining acoustic properties

The influence of the micro- and mesoscopic structure of wood cell walls on the acoustic properties of softwood was investigated in a synchrotron X-ray microbeam diffraction experiment with particular attention to the seasonal differences in crystallographic features. A multiple regression analysis was performed for data from 12 different softwood species in order to determine the dependence of longitudinal relative Young's modulus (E/) and loss tangent (tan) on seasonal cellulose microfibril angles (MFAs), crystal width of cellulose microfibrils etc. We conclude that a low MFA in both latewood and earlywood yields high E/ and low tan, which is an attribute of wood used as violin or piano soundboards. Among the softwood species we characterized Sitka spruce best fits this criterion.