Study On Visco-elasticity Of MDF

With rapid expanding, MDF has been taking up compelling status in manufacture and application of wood-based panels. However, the problems such as the blisters, delaminaion, stability in dimension, etc. have been occuring in the process of MDF repressing manufacture and application. These problems have inherent relationship with the visco-elasticity of MDF. In order to ensure the necessary property of the MDF, the visco-elasticity and internal structure damage mechanism of MDF are studied in the thesis. Two types of MDF's basic mechanical properties under press of monospindle are studied to test and evaluate MDF. The study result shows that the stress-strain curve of MDF is no yielding point between the linear and the non-linear elasticity part. Linear regression and minimax of differential-square is the best accurate method. The cycle of load is the important factor on the hardening experience of MDF, and the first load is the most important factor. The MDF was hardened at the expense of elastic modulus decrease. The slope of stress-strain curve, i.e., elastic modulus of MDF under pressing increases with the increase of the grade level of MDF. In order to study the MDF's Visco-elasticity, the creep-recovery and stress relaxation, internal energy property, creep compliance and relaxation modulus under compressive load are studied, by means of two types of MDF. MDF is elastic body within lower stress level. MDF is visco-elastic body within medium stress level. However, within high stress level, the deformations of MDF are composed of the above similar visco-elasticity and non-neglectable instantaneous plasticity, and MDF is non-linear visco-elasticity body. The deformations of creep and plasticity are analyzed by the internal energy method, which can be used to describe the coupled recoverable, unrecoverable and instantanesou plasticity deformation quantificationally. Internal energy is an effective parameter to study the relationship between microstrucure change and macrostructure deformation, and is a quantitive index which reflects the mechanical properties of MDF. The models of creep and stress relaxation of MDF within medium-lower stress level were deduced and improved based on visoco-elastic constitutive model applied. The improved new models of creep and stress relaxation of MDF separately have the character that assorts with time variable which contributes to simulate more kindly creep and stress relaxation of MDF, thus can achieve more accurate simulation results. The modulus of elasticity of MDF within lower stress level is 197.3MPa (SX),153.4MPa (SL) separately; The creep and stress relaxation of MDF within medium stress level can be better described by the improved new models of creep and stress relaxation; Within high stress level, a new physical model was founded by introducing the slip and switches based on the Burger constitutive model and it can achieve more accurate simulation results of the first creep of MDF and the damage of combintion of MDF. The visco-elasticity of MDF under repeated load is researched. The cycle of load is an evident influence factor on visco-elasticity within medium-high stress level under repeated load. The cyclic residual strain becomes smaller as the cycle of load increase under repeated load. The total strain under repeated load is sum of individual cyclic residual strain. The models of creep and stress relaxation of MDF under repeated load are founded separately based on linear addition, and they were verified by the experimental results separately. It can be concluded that the proceeded models are useful on the prediction of the creep-recovery and stress relaxation behavior of MDF under repeated load. The damage of MDF related to compressive creep was researched.The variation in thickness of creep of MDF does not occur within elasticity, however the variation in thickness and damage occur within visco-elasticity, and the creep damage of MDF becomes more serious with the increase of the stress. Compared with stress relaxation, Creep has more effect on MDF's variation in thickness and creep damage. With respect to the MDF's mechanical properties, the better the properties are, the larger the resistance to variation in thickness and creep damage, heat and water-airdry are. The press, press style and their interaction affect even the high-grade MDF's internal bond strengthremarkably, and the press has the most remarkable effect. Both the load level and the interaction between load and load style affect on the low-grade MDF's internal bond strength remarkably, whereas the load style has no appreciable effect. The different press is the remarkable effect on the thickness swelling and immersion water absorption of MDF with good properties. The microstructure of MDF and MDF tested were studied. The results showed that the fibers in MDF are arranged randomly. Some of them arranged separately, most of them arranged in bundles. There are many blind holes between the fiber cells in MDF. The damage of MDF's compressive creep is the result of new cranny occuring, spreading and the combination of old cranny. In the repressing process, besides the pressure, heat is also the main factor on the damage of MDF, which can accelerate the adhension layer to deteriorate and lead to the gule strength decreasing. The damage styles of MDF include pulling out of the fibers, the resin droping besides the fiber broken. The one-dimensional damage elastic constitutive equations of MDF under pull and press of monospindle were founded separately based on damage concept, and the formula of MDF's internal bond strength after repressing manufacture was put forward. The hot pressing curve of stress relaxation and pressing parameter were analysed and set down.