[PDF] A Fundamental Study Of The Fracture And Fatigue Characteristics Of Single Wood Pulp Fibres Microform Application To Mechanical Refiners eBook
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"The engendered conclusions may be summed up as follows. From a litany of tenuously-oriented microcracks, dominant macrocracks propagate along the axis of the fibre which may sharply deflect in the presence of natural bias (e.g. pits in the fibre wall). The material property degradation characteristics further include: volumetric expansion due to internal fibrillation in the cell wall, extensive external fibrillation, gradual delamination of the layers and partial peeling-off of the cell wall material. Cumulative damage due to cyclic shear is shown to be the most significant, further supported by the high structural collapsibility of the fibre wall layers. The mechanisms of fracture are either due to the development of transverse cracks at regions of high stress concentration (such as bordered pits), or owing to the gradual slippage of the fibre wall layers." --
"The engendered conclusions may be summed up as follows. From a litany of tenuously-oriented microcracks, dominant macrocracks propagate along the axis of the fibre which may sharply deflect in the presence of natural bias (e.g. pits in the fibre wall). The material property degradation characteristics further include: volumetric expansion due to internal fibrillation in the cell wall, extensive external fibrillation, gradual delamination of the layers and partial peeling-off of the cell wall material. Cumulative damage due to cyclic shear is shown to be the most significant, further supported by the high structural collapsibility of the fibre wall layers. The mechanisms of fracture are either due to the development of transverse cracks at regions of high stress concentration (such as bordered pits), or owing to the gradual slippage of the fibre wall layers." --
Damage in wood is principally the result of fatigue. Fatigue is the process of progressive localised irreversible change in a material, and may culminate in cracks or complete fracture if conditions that initiated or propagated the process persist. Comprehensive understanding of fatigue and fracture in engineered wood components must be founded on a proper understanding of the damage processes. Although wood is the world's most widely used structural material, whether measured by volume consumed or value of finished construction, its behaviour is not well understood even by people who have spent their careers studying it. * What is known about failure processes comes almost entirely from empirical evidence collected for engineering purposes. * Hypotheses about behaviour of wood are based on macroscopic observation of specimens during and following tests. * With only limited resources and the need to obtain practical results quickly, the timber engineering research community has steered away from the scientific approach. * Forestry practices are changing and are known to influence characteristics of wood cells therefore there is a need to periodically reassess the mechanical properties of visually graded lumber the blackbox approach. Fatigue and Fracture of Wood examines the above issues from a scientific point of view by drawing on the authors' own research as well as previously published material. Unlike the empirical research, the book begins by examining growth of wood. It briefly examines its structure in relation to how trees grow, before assessing the fatigue and fracture of wood and discussing the scientific methods of modelling fatigue. * Covers from macro to micro behaviour of wood * Presents direct evidence of how wood fractures using Scanning Electron Microscopy * The first book to present a physically correct model for fracture in wood * Provides experimental proof of so-called memory in wood (i.e. dependence of fatigue behaviour on the loading sequence) * Givse practical illustrations of how theories and models can be applied in practice An essential resource for wood scientists/engineers, timber-engineering practitioners, and graduate students studying wood and solid mechanics.
To achieve full utilization of each harvested tree, interest centers on use of all of the tree components. The objective of the study is to determine the influence of the morphological characteristics of pulp fiber from 12 western U.S. softwood species, representative of a wide range of fiber. The effect of the morphology of the wood pulp fiber as distinguished from that of the wood fiber is emphasized because differences in performance of fiber-based products are traced to the pulp fiber. Interrelationships of fiber morphology and pulp sheet properties are discussed. The results indicate that it is not possible to fully characterize the performance of a pulp by a single morphological characteristic and that the morphological characteristics significantly influence strength properties of sheets.
The thesis comprises three parts: Existing methods for characterisation of fibre crosssections have been improved, and new methods have been developed. These methods have then been applied to study the effects of wood characteristics and the pulping process on mechanical pulp fibres. Links have been established between fibre structure and paper properties such as surface smoothness and light scattering coefficient. New methods, based on SEM-images and image analysis, are described for providing cross-sectional fibre dimensions for large fibre populations, for wood tracheids (app. 60 000 tracheids in a wood trunk) and for processed pulp fibres (app. 1000 fibres per sample). The methods are suited e.g. for evaluation of changes in the fibre cross-sections from wood to the finished paper, or for mapping of fibre parameters within and between growth rings in a wood trunk. The treatment of data is discussed, showing how one may examine the changes in different groups of fibres (earlywood fibres, latewood fibres, split fibres) throughout a process. It is known from the literature that groundwood-based paper is superior to TMP-based paper with respect to printability. Fibres from SGW and PGW-pulp were found to be much more split in the longitudinal direction than TMP-fibres at comparable freeness. Intact groundwood fibres had thicker walls than intact TMP-fibres, but nevertheless super calendered hand sheets made from groundwood fibres were less roughened by moistening than were TMP-based sheets. Both for groundwood pulps and for TMPpulps, it was shown that reduced fibre wall thickness and increased fibre splitting was beneficial for improved surface smoothness and opacity. Latewood defibrate easier than earlywood during refining. In the case of grinding, there was no particular preference for earlywood or latewood to be defibrated. Reject refining of groundwood reject was, however, found to be very important for defibration of latewood-containing shives. Pulps made from a raw material with more compact fibres (high wall area to lumen area ratio) were found to defibrate easier, and contain less shives. It was found that refining tends to reduce wall thickness most on thickwalled parts of the fibre, thus causing a reduction of the wall thickness variation around the perimeter. Earlywood fibres were found to be preferentially split during refining. Most fibre splitting occurs during the primary stage, while the fibres are firmly attached to chips or fibre bundles. Latewood fibre wall thickness decreases considerably more than earlywood fibre wall thickness during refining. It seems that choosing an appropriate raw material is more effective than using excessive energy on reducing the wall thickness of thickwalled fibres. Earlywood fibres became more flattened during refining compared to latewood fibres, possibly due to repeated compressions and relaxations in the refiner. The energy consumption to a given freeness was found to be considerably larger for Scots Pine than for Norway Spruce. However, the fibre transverse dimensions did not differ much between Norway Spruce and Scots Pine. Pine pulps were far less developed than spruce pulps at similar energy level. A possible explanation for the large energy consumption may be that redistribution of extractives at the fibre surface could reduce friction in the refiner. This hypothesis should be further explored. The results in this study improve the knowledge of which fibre parameters that matter for surface smoothness and opacity of wood-containing publication paper. Further, this study elucidates how important fibre parameters such as wall thickness and fibre splitting are altered during a refining process. The results may be utilized to identify possible ways of modifying the TMP-process in order to produce paper with improved surface smoothness and opacity.