Fatigue treatment of wood by high-frequency cyclic loading S. Panula-Ontto 1 , M. Lucander 1 , T. Pöhler 1 , E. Saharinen 1 , T. Björkqvist 2 1 KCL 2 Tampere University of Technology ABSTRACT New scientific tools for investigating the qualitative and quantitative response of wood to mechanical loading under conditions close to those of mechanical pulping are needed to find ways of radically reducing energy consumption. At KCL a modulated loading device was developed to fulfill these requirements. The device allows the static load, the loading frequency and the amplitude to which wood is subjected to be varied over a wide range. Wood samples were cyclically stressed at large strain (1 mm) pulses and at frequencies in the range 100-1000 Hz. The temperature rise in the wood was measured with thermocouples inserted into the wood and on the wood surface using a thermographic camera. It was found that a high static load during cyclic loading increased heat generation in the wood. The cyclic loading frequency had less effect than the level of the static loading. However, low frequencies generated more heat at a specific number of impacts. The responses of heartwood and sapwood were significantly different in the fatigue treatment. The temperature rise was higher in heartwood than in sapwood. The wood underwent irreversible plastic deformations seen as heat generation and increased pore volume in the wood microstructure. INTRODUCTION In theory wood can be converted mechanically to fibers suitable for printing paper production at specific energy consumptions considerably lower than those achieved today. To accomplish this, more fundamental knowledge of wood rheology and fatigue behavior is needed. The structure of any polymeric material can be weakened or broken down by mechanical action. It is well known that existing mechanical pulping processes are energy inefficient. One reason for this is the composite structure of wood, including the ingenious multilayer structure of the wood fibers. The aim of mechanical pulping is to weaken the wood structure, to separate the fibers from each other, and mechanically to knead the fibers into flexible and well-bonding particles without major loss of fiber length. To achieve all this, the mechanical pulping processes used today have to be relatively gentle, and are therefore highly inefficient. Although it is over 150 years since the first mechanical pulping processes were developed, theoretical calculations show that in today’s processes 80% or even more of the specific energy input is used just to generate heat and only some 20% to fatigue the fibers. Despite intensive research efforts since the 1960s only relatively small stepwise reductions in energy consumption have been achieved. The technical and economic breakthroughs have not been achieved. The viscoelastic properties of wood need to be taken into consideration in optimizing the defibration process towards lower specific energy consumption. The goal should be for most of the mechanical energy to be converted into permanent deformation and to a lesser extent into heat [1, 2, 3]. The mechanical behavior of a viscoelastic material change as temperature and deformation rate of the material vary. Viscoelasticity means that: o the force needed to create a certain deformation increases with the strain rate. o the force needed to create a certain deformation decreases with increasing temperature. o stress-induced deformation is time dependent, i.e. there is a phase shift between applied load and arised deformation in cyclic loading. When a polymeric viscoelastic material is stressed mechanically, some of the loading energy is converted into heat due to the viscous behavior. Heat generation can be used as an indicator of strain variations. The generation of heat in the wood components depends on the amplitude and frequency of the loading and on the structure, elasticity and internal friction of the wood [4].
Fatigue treatment of wood by high-frequency cyclic loading
Jun 20, 2023
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