Study On The Mechanical Properties Of Wood Friction Welded Joints

Friction welding of wood is a connection technology that does not rely on any connectors or additives.It avoids the corrosion problems caused by metal connectors.Without using adhesives,green and environmentally friendly glueless bonding is realized,which is of great significance for environmental protection,human health and sustainable economic development.In recent years,most of the research on wood friction welding is to explore the change of welding performance when the welding parameters are changed by means of experiments,or to study the stress characteristics of the welded member by establishing a simplified finite element model.There is a lack of research on the failure mechanism of the connection surface.In this thesis the mechanical properties of friction welded joints of beech wood,one of the industrial woods,are studied through the combination of experiments and finite element simulation.Firstly,the optimal combination of welding parameters was determined by the hybrid orthogonal test,the shear strength of the welded joint was tested by tensile shear test,and the effects of welding pressure,welding amplitude,welding depth and wood type on the shear strength were studied and analyzed.Then,based on the cohesion theory,the finite element model of the welding layer of the welded joint is established,and the parametric research and analysis of the cohesion model are carried out.The main work and research conclusions of the thesis are as follows:Through friction welding,wood friction welding joint specimens were prepared,tensile shear tests were carried out according to the specifications,and the optimal combination of welding pressure,welding amplitude and welding depth were determined by means of hybrid orthogonal test.Studies have shown that when the welding pressure is 8 MPa,the welding amplitude is 1.7 mm,and the welding depth is 0.8 mm,the welded joint can obtain the best shear strength.Based on the optimal combination parameters,the influence of welding pressure,welding amplitude,welding depth and wood type on the welding strength was tested by mechanical testing,regression analysis and paired sample test.The study found that within a certain range,the shear strength of beech welded joints increases with increasing welding pressure and decreases with increasing welding amplitude and welding depth.There is a difference in the shear strength of different wood welded joints.As the pressure increases,the shear strength of poplar first increases and then decreases,but the shear strength of beech continues to increase.The shear failure characteristics of beech welded joints were observed,and the wood breakage rate of the welded joints was calculated.The study found that beech wood friction welded joints have a wood breakage rate of 0% for 89%,a wood breakage rate of 100% for 4%,a wood breakage rate of 0-100% for 7%,and the main damage form of welded wood joints is the welded layer cohesive destruction.The cohesion model of wood friction welded joints was established,and the destruction rule of the welded layer was explored.The study found that in the process of tensile shear failure of welded joints,the stress of the welded layer increases with the increase of the displacement load,and suddenly decreases to 0 after reaching the critical stress.The initial damage variable increases from 0 in the elastic phase and increases to 1 when the joint stress reaches the critical stress.At this time,the initial damage of the weld layer begins.The damage expansion variable remains at 0 in the elastic phase until the material stiffness deteriorates.It starts to increase from0 and increases to 1 when the material fails completely.The simulated stress-displacement curve shows that the damage evolution process of the friction welded joint is very short,which further indicates that the fracture of the welded joint is a brittle fracture.A parametric study of the cohesive force model of welded joints was carried out.It was found that with the increase of the cohesive strength,the brittleness of the welding layer during the failure process increased,and the ultimate load that the welded joint can bear gradually increased.With the increase of the elastic modulus,the ultimate displacement when the welding layer failed gradually increased.The crack resistance of the weld layer increases with the increase of the fracture energy,and the damage evolution process slows down with the increase of the fracture energy.With the increase of the lap length,the shear strength of the weld layer increases first and then decreases.The joint form has a greater impact on the connection strength of the welded joint.The connection strength of the butt joint is 74.6% lower than that of the single lap joint.Therefore,in actual engineering,the reasonable joint form should be selected according to the mechanical characteristics of the structure to improve the structure overall carrying capacity.