Experimental Study On Multi-level Variable Amplitude Fatigue Performance Of Glulam Beams

With the gradual restoration of forest resources in our country,the application of wooden bridges has become increasingly widespread.In order to ensure that the safety,strength,and rigidity of modern wooden bridges can meet the design requirements during normal service,it is particularly important to study the fatigue performance of glulam beams.This paper mainly studies the multi-level variable-amplitude fatigue performance of glulam beams and their fatigue life and stiffness degradation for theoretical model establishment and experimental analysis.The research work and results of this paper are as follows:The cumulative fatigue damage of the glulam beam is first developed from the compression side.As the height of the plastic zone on the compression side increases,the section height of the glulam beam decreases.Until the ultimate flexural bearing capacity of the glulam beam is reduced to the extent that it cannot bear the fatigue load,then fatigue fracture occurs.With increasing levels of stress and stress amplitude,the beam strength and stiffness test can be decreased.Therefore,whether it is two-level variable amplitude or multi-level variable amplitude,when the stress level and stress amplitude are loaded from low to high,the actual life will often be greater than the predicted life;On the contrary,when the stress level and stress amplitude are loaded from high to low,the actual life will be less than the predicted life.Based on the fatigue cumulative damage criterion,Miner theory,modified Miner theory,and Corten-Dolan theory are used to predict the fatigue life of glulam beams.Through comparative analysis,it is found that the fatigue life estimation value NMHL of the modified Miner theory has the highest estimation accuracy.Moreover,the estimated life value NMHL is close to the test value,and it is basically smaller than the test value.Therefore,it is recommended to use the modified Miner theory to predict the fatigue life of related components in actual engineering applications.The stiffness of the glulam beam is calculated by the deflection curve equation,and the stiffness degradation theory model is established by the damage variable characterized by the stiffness degradation.The rigidity degradation trend calculated by the mid-span deflection of the glulam beam shows an "S"-shaped distribution,which can be divided into three stages:the initial stage,the adaptation stage,and the failure stage.The nonlinear curve fitting of the stiffness degradation of the test beam was carried out by MATLAB,and the calculation formula for the stiffness degradation of the glulam beam was established.The finite element model of glulam beam was established by ANSYS,and the static and fatigue properties were analyzed by finite element.Based on the Corten-Dolan theory,the equivalent constant-amplitude fatigue stress amplitude method is derived.Using this method,the load spectrum of the multi-level variable-amplitude fatigue test is simplified to the equivalent constant-amplitude stress amplitude.Substitute the equivalent stress amplitude into the model to obtain the finite element simulation life.The finite element simulation life and the Miner theory calculated life and the actual test life are compared.Through comparison,it is found that the finite element analysis using the equivalent stress amplitude method has higher simulation accuracy,which makes the finite element model more reliable for structural life assessment and more suitable for engineering application.