The Study On Decay And Shrinkage Cracks Of Timber Components

Durability is a main cause for the failure of timber structure,but research on the durability of timber structure is still rare in China."Unified standard of reliability design of building structures" updated in 2018 suggests that the durability of timber structure should be paid attention when the occurence of decay,insects,termite infestation,warping deformation and shrinkage cracks is notified.For the in-service timber structure,the most common damages are decay and shrinkage cracks.And these two damages can be regarded as two important indicators for the durability evaluation of timber structure.Therefore,this study utilized theoretical analysis,experiments and finite numerical simulation to do research on decay and shrinkage cracks in timber components.Currently,resistographic method is recognized as an effective way for inspecting decay in timber members.But existing code related resistographic method dose not give the method to determine decay dept.Due to different density of different wood quality,the corresponding resistance is highly related to wood density.Therefore,the second chapter proposed a waveform discrimination method to divide different part of wood when inspecting damage by resistographic method.In health wood,because the average resistance in heart wood is higher than the value of sapwood,the comparison of average resistance in different part can be used to make a preliminary judgment on whether decay occurs in heartwood part.Then,the method for discrimination of surface decay boundary based on resistance ratio was developed.When calculating resistance ratio,every other annual ring(crest or trough)is set as a temporary boundary from the surface to the center of timber components.And the part between surface and the boundary is considered as possible decayed region.If decay occurs only in the sapwood,the remaining area of the sapwood should be regarded as possible healthy area.If decay is deep into the heartwood,the remaining area of the heartwood should be considered as possible healthy area.In the process of calculating the resistance ratio,a minimum or abrupt value should exist at the real boundary on decayed/healthy wood.Finally,based on existing codes and methods described above,a detailed procedure of damage inspection by resistographic method was proposed.Existing accelarated decay test only offered results of mechanical properties for decayed wood,and there is no analysis of decay depth,which is an important parameter for the evaluation of bearing capacity of timber components.In order to provide a basis for the study on evolution of decay depth under natural environment,the third chapter designed and manufactured a batch of small specimens.Then the specimens were placed in a chamber with ideal temperature and relative humidity to realize single-side accelerated decay.The phenomenon of specimens subjected to different decay time was observed.Besides,the weight loss,nominal mechanical performance degradation rate and single-sided decay depth were tested as well.The results show that the weight loss was not sensitive to decay,while mechanical properties were more sensitive.At the 84 th day,the degradation of nominal flexural modulus,nominal bending strength and nominal compressive strength reached 20%,15% and 14%,respectively.Finally,based on the assumption which the compressive strength of decayed part is 10% of the corresponding value of healthy wood,and the singlesided decay extended uniformly,model of decay depth under ideal environment along time is established.The depth of cracks in cross section of timber can have significant impact on the bearing capacity and durability of the components.To investigate the phenomenon of development of moisture gradient that can lead to shrinkage crack formation and propagation in cross section of timber members,experiments for six cylindrical specimens made of Anhui fir were conducted in an environmental chamber.By adjusting the relative humidity in chamber,the specimens experienced absorbtion and desioption.Moisture distribution,depth,shape and location of shrinkage cracks on the cross section were measured at certain time.The results show that moisture content decreased exponentially with time,and expressed quadratic polynomial distribution in the radial direction during the desioption period.Shrinkage cracks occured were all in TR direction,and the initiation and propagation of cracks happened in the desorption period.The development of shrinkage cracks can be divided into a rapid developing stage and a stable stage with a boundary at 0.8 relative depth.In the rapid stage,the relative depth of cracks increased linearly with time.The speed of crack propagation in members without initial cracks was significantly higher than those specimens with initial cracks.However,the depth of initial crack did not show obvious effect on the speed.During the experiment,different cracks in the same specimen had almost the same crack developing speed,in spite of the time of crack's initiation being differentThe fifth chapter analysed the moisture transfer,initiation and propagation of cracks by theoretical method and finite element method.According to existing knowledge of heat transfer,the theoretical results of humidity field on cross section was discussed.Diffusion coefficient and surface emission coefficient of Anhui fir were determined based on experimental results and Fick's second law.The initiation of crack was analysed by elastic mechanics,and the crack propagation was determined by the fracture energy release rate criterion.Based on the parameters determined by experiments and theoretical analysis,moisture transfer was simulated by heat transfer part of ABAQUS firstly.Then the output file from moisture transfer was input to ABAQUS as predefined field,and extended finite element method(XFEM)based linear elastic fracture mechanics(LFEM)was used for energy calculation and crack propagation.Moreover,related parameters varying with moisture content were taken into account in this simulation.Good agreement of the experimental and the numerical results confirms the suitability and performance of this FEM based approach.Finally,the finite element method was used to study the crack propagation under different cyclic parameters.The results show that the lower the equilibrium moisture content corresponding to ambient environment was,the greater the decrease of moisture content will be,and the speed of crack propagation could be quicker as well.Besides,the longer the cycle was,the slower the crack propagated.Since the prediction of residual durability for timber components was determined based on damage evolution,which is highly related to environment.Therefore,the sixth chapter proposed the lag time of decay,decay degree factor,environmental factor,speed of decay,reducing coefficient of diffusion coefficient to establish evolution models of decay depth and shrinkage crack depth in timber components under natural environment.Then,utilising specific environmental data from five cities,Urumqi,Beijing,Nanjing,Mengla and Lhasa,the evolution models of decay and shrinkage cracks under the main environment in China were discussed.The prediction for year when the depth of decay and shrinkage cracks reach the failure value was analysed based on the models.The results show that the greater the fluctuation of daily temperature,the interval of environmental data has more obvious influence on the decay degree factor and environmental factor.Additionally,the developing speed of cracks obtained by the finite element method need to be corrected by the coefficient 0.4.The components of Beijing,Nanjing and Mengla can reach the value of decay failure within 123~293 years,22~45 years and 6~7 years respectively.The components in Urumqi,Beijing,Nanjing and Lhasa can reach the value of shrinkage cracks failure within 1~4 years,1~3 years,2~3 years and 1~2 years,respectively.