Mass transfer, creep and stress development during the drying of red oak

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Virginia Polytechnic Institute and State University


The purpose of this research was to measure and simulate the major perpendicular to gain strain components and associated stresses which develop as the result of mass transfer and restraint of shrinkage in red oak. Particular emphasis was placed on the rheological or creep components of strain.

Mass transfer was measured during the first four days of drying under increasingly severe conditions. The resulting moisture gradient profiles were parabolic in shape under nearly all drying conditions. The pattern developed quickly and was modeled with reasonable accuracy using Fick's second law.

Three major strain components occur in drying. These are termed elastic strain, visco-elastic creep and "set" or mechano-sorptive creep. The magnitude and variation of each of these components was measured during the first four days of drying under increasingly severe conditions.

Using a slicing technique to cut very thin wafers of wood parallel to the surface, the elastic strain was shown to be quite small. The experiments led to the conclusion that the maximum stress develops within a few cell thicknesses of the surface.

Much of the experimental work centered on the t of the rheological or creep components of strain, The magnitude of the visco-elastic creep was found to be about the same order of magnitude as the elastic strain and was clearly a function of the applied stress. The major strain component during drying was shown to be mechano-sorptive creep. This type of creep occurs in a number of polymeric materials undergoing diluent movement or temperature change. The maximum magnitude of the mechano-sorptive component was about 30 times larger than the maximum elastic strain. Mechno-sorptive creep was shown to be directly related to moisture loss and, to a lesser extent, a function of applied load.

Using relationships derived from the experimental data, a computerized simulation was developed to predict the development of stress and the probability of checking early in drying. The simulation makes use of data on elastic strain, mechano-sorptive creep and the elastic modulus. The concept worked well in this study, but its application is limited by the lack of strain data for the surface layers representing the outer few cell thicknesses where stress is greatest early in drying.

As an adjunct, acoustic response tests were performed on green samples stressed perpendicular to grain in flexure under third point loading conditions. The evidence indicates that the onset of surface failure can be predicted prior to the appearance of macroscopic checking. The acoustic response pattern is typical of that which occurs in brittle glassy polymers such as polystyrene.