Fatigue Of Wood Subjected To Cyclic Moisture-induced Stress Perpendicular To Grain

In actual engineering,the moisture and temperature of the surrounding environment will vary with the seasons changed.The wood will produce absorption and desorption process with the temperature and moisture vary because of its porous structure.The moisture gradient inside wood cause the moisture induced-stresses.The fatigue of wood subjected to moisture vary which induced recip rocating load.The wood will destoryed beacause of tensile stress which is approach to the lowed tensile strength.Two sets of fatigue tests under different cycles were carried out.The design tests included design of the specimen size,design of the loadi ng device,and design of the cycle and relative humidity amplitude.The period is one day and three days respectively.Now put the specimen into the standard environment to maintain moisture balance,and then into the artificial climate box for loading.Th en observe the cracking of wood.It is found that the lifetime of fatigue is different because of different tree species in the same period,and the lifetime of fatigue is different in different period.The elastic constants are chosen from the experiments done by the senior and the elastic constants of the two different species are determined respectively under the corresponding provisions.The constitutive model of wood was established by considering the effects of external temperature and moisture on str ain,viscoelastic creep,mechanical creep and elastic deformation.The simplified Luikov heat transfer model and Fick’s second law are used to establish the temperature and moisture transfer equations,respectively.The constitutive relations are written i n Fortran language and embedded into ABAQUS by UMAT subroutine to build the finite element model.And the finite element model under different size,period,amplitude used under the moisture content and moisture induced-stresses distribution.The simplified calculation of the temperature and moisture induced-stresses.The relationship between the simplified results and the simulation results can be used as a convenient and quick method in practical engineering.The damage of wood is a type Ⅰ failure mode,the fatigue model should be established by using the theory of viscoelasticity.The Fatigue Model of DVM is written and modified in R language.The fatigue life prediction results of the model are compared with those of the clean wood specimens tested in this paper.And considering the glued beam in the experiment,the results are compared with the DVM model.Considering the influence of different climatic types on the glued beam,the practicality of fatigue model is analyzed.