Experimental Research On Shrinkage Crack Development Of Timber Structural Members

Because of its unique hygroscopicity and anisotropic characteristics,wood not only makes the timber structure sensitive to the temperature and humidity of the environment,but also leads to the deference of tangential shrinkage and radial shrinkage,and it is easy to produce shrinkage cracks.Cracks have great influence on the safety and durability of the timber structure.Therefor,this paper makes a deeper study on the development law of shrinkage cracks on the basis of summarizing the existing research results.In this paper,two sizes of log components were tested for shrinkage cracks.The test environment was set to constant temperature and humidity,constant temperature and humidity cycle.The results showed that: the moisture content(MC)of the timber components in the constant temperature and humidity phase was a quadratic function distribution along the radial and decreased exponentially with time.During the cyclical humidity,the MC periodically changed with humidity,the outer layer responded more promptly to the environment,and the inner layer had hysteresis.The cracks of timber components change were linear with the time and were almost TR cracks.After extending to the vicinity of the pith,it slowly expanded or stopped expanding.For multiple cracks of the same specimens,the rate of crack propagation was nearly the same.The larger the size,the more cracks on specimen,the slower the expansion rate,and the smaller the final crack length.Based on the experimental results and the discussion of Fick's second law,the spread flow equation on wood was got.Using the similarity between the humidity field and the temperature field control equations,a heat conduction module was used to simulate the moisture transfer of the timber component.The finite element analysis software ABAQUS was used to establish the humidity field model.The finite element calculation results had good agreement with the test.In the finite element analysis,the non-idealized initial MC and humidity were modified.The results showed that the MC of each layer of the timber component decreased exponentially with time in the drying stage,and the surface layer change rate was greater in the early stage than in the inner layer.The size had a great influence on the rate of decline of the MC.The larger the size,the faster the change of the surface,the inner layer was the opposite.The time for the timber components to reach the equilibrium MC was also related to the size.Under the same conditions,the small-size components first reach the equilibrium MC.The humidity cycle parameters(frequency,period,and amplitude)were analyzed by the finite element software and the following conclusions were reached: when the temperature,initial MC,and size were same,humidity cycling times,period,and amplitudes has no influence of the reaching the equilibrium MC time.However,there had difference between the gradient of MC and the lag time.As the number of cycles increased,the gradient of MC at the end of desorption decreased,the gradient of MC at the end of moisture absorption increased.And the larger the humidity amplitude was,the greater the gradient of the maximum MC.The larger the period was,the greater the gradient of the MC,and the shorter the MC lag time.On the basis of this humidity field model,humidity stress could be calculated.The cracking criterion and expansion criterion were obtained through theoretical analysis,a crack propagation model was established for the first time based on linear elastic fracture mechanics by sequential coupling,combined with extended finite element.This model can fully reflect the development of cracks in timber components under humidity changes,which is in good agreement with the test.The calculation results of this model indicate that the crack grew linearly before reaching the pith;for specimens had more than one crack,all cracks propagation rates were nearly consistent.The number of cracks has an effect on the rate of expansion.The more cracks,the slower the expansion,and the smaller the final depth.Under cyclic humidity,the cracks grew linearly,and the expansion of cracks always occurs during desorption,when the tensile stress reaches near the peak.Finally,using the mean monthly climate in Nanjing over the past three decades,the finite element method was used to successfully simulate the crack propagation of wood components with different initial MCs in the air-dry state.The results showed that the cracks linearly expanded with time.The initial MC had a great influence on the crack propagation rate: for wood components with an initial MC of 25%,the relative depth of crack propagation rate was 0.0622/month,and the crack had reached 1/3 of the diameter after eight and a half months air-drying;the component with an initial MC of 15% would not crack under the same conditions.Using the same method,the rate of development of shrinkage cracks under other climatic conditions can be obtained.This will provide a basis for the future study of the stress conditions and crack propagation of timber components in the coupling of temperature and humidity and load,as well as the corresponding durability evaluation.