Experimental Study On Fatigue Performance Under Loading Of Variable Amplitude Glue Wood Beam

The research of modern wood structure is at the beginning stage in our country.At present,the experimental research of modern wood structure mainly focuses on static test,but the research of dynamic performance is not enough.In recent years,glulam has been gradually applied to buildings,Bridges and other engineering structures.With the increase of structural service life,the fatigue problem of glulam has attracted the attention of researchers.It is of theoretical and practical significance to carry out the research on the amplitude fatigue performance of glulam beams.The research work and results of this paper are as follows:(1)Through the static test analysis of the glulam beam,the failure rule and failure form of glulam beam are obtained.Before the glue-wood beam is about to break,there are no obvious features other than the slight tearing sound of wood fibers.It can be seen that the static failure type of glulam beam belongs to brittle failure.The failure of the test beam is located in the pure bending section at the bottom of the beam,which is a typical tensile failure.The test results show that there are two stages:elastic stage and very short plastic stage.In the process of stress,the test beam is basically in the elastic stage.Once the plastic stage is entered and the load continues to be applied,the glulam beam will be destroyed rapidly.(2)Control key parameters of glued wood beam:preloading times;The variation fatigue test of three test beams was carried out,and the influence rule of preloading stress level on fatigue life of test beams was obtained.Under low stress amplitude loading,the stiffness degradation of glulam beams appears with the increase of loading times.Although the test beam f-1 was loaded 2 million times under low stress amplitude preloading,its ultimate bearing capacity was 15.9%lower than the bending strength under static load.The test results show that the structure will also suffer cumulative damage under low stress amplitude loading,and with the increase of preloading stress level,the fatigue life of glulam beams gradually decreases,and the greater the stress level under secondary loading,the more obvious the fatigue life reduction.(3)The stiffness degradation rule and failure pattern of glulam beams with different preloading times under amplitude fatigue are studied.The stiffness degradation of the test beam increases with the stress level of the second stage loading.At the beginning of the secondary variable amplitude cyclic loading(two hundred thousand times)after times,there is an obvious stiffness degradation of experimental beams are based on amplitude from small to large load after two hundred thousand times,the damage than were 3.7%,5.7%,6.4%,along with the increase in loading times,glulam beams after secondary load stiffness degradation trend gradually smooth,F-2,destroyed F-3 test beam stiffness degradation ratio of 10.7%,9.8%.This shows that the glued wood beam has a great damage at the initial stage of amplitude variation loading,and the greater the stress level of the secondary loading,the greater the damage.When the total stiffness damage of the glued wood beam reaches about 10%,the fatigue damage will occur.The failure forms of glulam beams are local shear failure and tensile failure in the pure bending section.As the stress level of secondary loading increases,the location of local shear failure is shifted from the neutral axial floor.(4)Based on the test results and the calculation method of fatigue linear accumulation,the modified Miner theoretical coefficient a was modified under the second-order amplitude fatigue.The results show that the modified Miner theory coefficient a ranges from 0.59 to 0.73.(5)The finite element model of glued wood beams is established.Through comparison between numerical simulation and experimental data,the results show that the fatigue life of numerical simulation is larger than that of experimental results,but the error is within the allowable range,which can provide a reference for the subsequent analysis and research of the fatigue life of glued wood beams.