The Research Of Mechano-sorptive Creep Of Birch Under Wide Temperature Field

Wood is viscoelastic material with hygroscopic properties.Mechano-sorptive creep occurs in hot pressing of wood-based panels,wood drying,and wood structure building,etc.In this paper,the properties of the mechano-sorptive creep during birch drying at three moisture content levels 12%,18% and 24%,and at three temperature levels 25?,55? and 85? were studied by using DMA with humidity accessory.The properties of the mechano-sorptive creep with cycling of relative humidity at 12% MC and at three temperature levels 25?,55? and 85? were also studied.The experimental results show that: Mechano-sorptive creep during wood dryingWhen the specimen is subjected to external force,it will produce an instantaneous deformation independent of time.With regard to this,mechano-sorptive creep of birch drying is the same as that of conventional creep.The moisture content of the specimen decreases gradually to 6% of the final moisture content,during which mechano-sorptive creep produces great creep.Creep continues for a period of time at the final moisture content of 6%.Mechano-sorptive creep completely change into conventional creep.Finally,the creep value of mechano-sorptive creep is several times that of the conventional creepThe total compliance and creep compliance of mechano-sorptive creep during birch drying increases with the increase of temperature and moisture content.When the temperature and initial moisture content are the same,the lower the final moisture content is,the greater the creep compliance of the mechano-sorptive creep during birch drying is.Mechano-sorptive creep with cycling of relative humidityThe total compliance of the mechano-sorptive creep of birch with cycling of relative humidity increased with the number of cycles.Creep recovers(ie creep reduces)when the specimen absorbs moisture,and creep increases when the specimen desorbs moisture.The desorption creep compliance increment is greater than the absorption creep compliance recovery,so each cycle will produce a certain degree of net increment of creep(cyclic creep increment).With the increase of the number of cycles,cyclic creep increment gradually decreases,and eventually the mechano-sorptive creep of birch with cycling of relative humidity tends to be stable.The higher the temperature and the greater the number of cycles are,the greater the total compliance at the end point of the desorption,the total compliance at the end point of the absorption creep,the desorption creep increment and the increase of the absorption creep are.The creep compliance increases at low temperatures,and the creep compliance decreases at high temperatures when birch first absorbs moisture.The mechano-sorptive creep of birch with cycling of relative humidity lags behind the change of humidity,and there is a phase difference between them.The lower the temperature is,the greater the phase difference is.