The Research On Mechanical Properties Of Larch Glulam Beams

Glulam is not only an engineering wood composite which is added into modern processing technology and high in strength mass ratio,environmental-friendly,beautiful,but also a sustainable green building material.Today,glulam is widely used in the engineering field due to its superior performance.As people's living standard improving and environment worsening increasing,China is implementing the policy of low carbon and environmental protection,energy conservation and emissions reduction,and advocates strongly development of green building materials,and resolves excess productivity,and promotes the optimization and upgrading of timber structure industry.Thus,the manufacture and application of glulam will be helpful to resolve the present situation.However,the research and application of glulam in China start relatively late.Mechanical properties of larch glulam timber member are still a blind spot.Relevant manufacture and design specifications are still not perfect.In order to promote the application of the larch glulam in our country,this paper studied the glulam of manufacturing process and the elastic modulus of the evaluation method.Then,flexural behaviors,fatigue properties,shear properties and creep performances researches of glulam based on the manufacturing process were carried out.The following are summarizes of the main research work and results1.According to the status of manufacturing and application of the domestic larch glulam member,the key techniques of glulam manufacturing were discussed from four aspects:selection of larch laminate,finger joint process,assembly process and gluing process.Manufacturing process parameters of each process were determined,and quality control and inspection points were introduced.The basis are provided for the industrial production.Compression tests,bending tests,shear tests,tension tests of larch small clear specimens were conducted.Failure modes of clear wood specimens were analyzed,and compressive strength parallel to grain,bending strength,shear strength parallel to grain,tensile strength parallel to grain were obtained.2.In order to make sure the quality of glulam fabrication,Three nondestruction methods including ultrasonic,stress wave,transverse vibration were adopted to detect the dynamic modulus of elasticity of glulam beams,which was compared with the static modulus of elasticity.the results indicates that the DMOE by three nondestruction method is higher than MOE,and its order is Eu>Esw>Et>Est.Through linear regression analysis to the static and dynamic modulus of elasticity,the linear correlation between DMOE and MOE is relatively notable.The results tested by transverse vibration method show the high correlation and have minimum errors,so transverse vibration method is the most accuracy and best,and could be evaluated MOE quickly in engineering.3.The flexural behavior of four groups of different size glulam was carried out,and the failure patterns and failure mechanisms of glulam beams were discussed,and the whole loading progress of glulam beams was analyzed systematically.Size effect on the bending strength,ductility,deflection and bending strength of glulam beams are studied.The experimental results show that with the size of glulam increasing,the bending strength of glulam gradually decreases and presents remarkable size effect.Based on Weibull brittle fracture theory,two parameter Weibull model was established,and the size effect coefficient of the length times height of the Larch Glulam beam is obtained for 0.275 by the method of slope;Calculation formula of ductility coefficient based on strain of glulam was proposed,and on this basis,deflection calculation formula was improved;Combining with the compression test of glulam column,elastic-plastic constitutive relation model of tension and compression was put forward;Based on the constitutive model,The calculation method of flexural capacity of Glulam beam is presented.The calculation results agree well with the experiment results.4.Through constant amplitude bending fatigue test and static test of larch glulam beams,the fatigue failure patterns and failure mechanisms of glulam beams were analyzed,and the fatigue properties and the law of stiffness degradation were studied.The research results indicate that the compression side of larch glulam beams gradually damages to the flexural bearing capacity decreasing until tensile failure and shear failure parallel to grain occur;The flexural rigidity of larch glulam beams presents a linear degradation with fatigue cycle number.The greater the stress level is,the more obviously the stiffness reduces,moreover,the stiffness degradation equations of larch glulam beams under different stress level are given.With the increase of stress level,The fatigue life of larch glulam beams is reduced gradually;When the stress ratio is 0.2 and stress level is 0.55 times ultimate flexural strength,the fatigue cycles exceeds two million,which meets requirements of fatigue design of finite lifetime and indicates that the glulam has good fatigue properties.By regression analysis of experiment data,the mathematical expressions of S-N curves of larch glulam beam are obtained preliminarily.5.Based on the fatigue modulus,linear damage theory,nonlinear damage theory and thermodynamics damage theory are applied to deduce damage evolution equation respectively.The fitting curves are in good agreement with experiment results.Based on macroscopic phenomenological damage theory and damage evolution equation,the expressions of the residual fatigue life and the residual bending strength of glulam beams are deduced,therefore,the residual fatigue life and the residual bending strength of glulam beams could be obtained at the given fatigue stress level and damage variable values.References are provided for safety assessment of glulam beams.The GM(1,1)model of single variable for predicting fatigue life model is established by applying the grey system theory.In addition,the method for predicting fatigue life based on the stiffness degradation equation and cumulative damage theory is got.The predictions for two methods agree well with the experimental results,so the fatigue experiment work and economic loss will be reduced greatly.6.Shear experiments using different loading ways for larch glulam of different size were carried out,the failure patterns and failure mechanisms of glulam beams were analyzed,the effects of loading way,sectional area and shear span ratio on the shear strength were studied.the experimental results shows that shear failure patterns of glulam beams are all dislocation failure;Load-displacement curves have obviously two phase nonlinear characteristic;Shear strength decreases approximate linearly with shear span ratio increasing;With the increase of sectional area,shear strength reduces gradually and is the power exponential function of sectional area.Empirical expression between shear strength of larch glulam and shear strength of small clear specimen is obtained by comparative analysis.7.Three groups Creep experiments of glulam beams were Carried out under different stress levels.Creep deformation mechanism of glulam beams was analyzed,and the effect of creep history on the static performance of glulam was discussed.The law of creep deformation was studied.The results indicate that creep history makes modulus of elasticity of glulam beams reducing;With the stress level increasing,transient creep deformation,deformation velocity in steady phase and the total creep deformation all increase.Based on the Burgers creep model,the experimental data are fitted by origin software.The model which has better fitting precision can be used to predict short-term creep deformation.Creep constitutive model and creep characteristics constant of larch glulam under different stress levels are obtained.In order to predict the long-term creep,a parameter k is introduced to improve the Burgers model.long-term creep of glulam beams was predicted by improved model,and prediction agrees well with the actual results.