Investigation On Low-cycle Fatigue Failure Behaviors And Life Prediction Of Adhesively Bonded Butt-joints

Adhesively bonded joints have been extensively used in many industrial applications such as aerospace,automotive,building structure,microelectronic packaging,etc.Fatigue performance and service life of the adhesively bonded joints are necessary data for the engineering design and applications.However,the use specification,test standards and test methods of adhesively bonded joints have not been established in our country.The fundamental research in this field is still scarce at present.Uniaxial and multiaxial fatigue experiments were carried out on the hollow cylindrical adhesively bonded butt-joints specimens bonded by automotive silicone sealant SY-227,and Stress control mode was used in the test.Based on the analysis of ratchetting strain,ratchetting strain rate,fatigue damage,stress-strain component and multiaxial fatigue life,the evolution of fatigue damage of the adhesively bonded butt-joints was studied.According to the characteristics of fatigue failure of the adhesively bonded joints,a fatigue life prediction model for the adhesively bonded joints was proposed.The main research contents and conclusions are as follows:On account of the difficulty of manufacturing the hollow cylindrical adhesively bonded butt-joints specimens and the difficulty of ensuring the quality of the specimens,a mold was designed for the preparation of the specimens.The mold could produce the adhesively bonded butt-joints specimens of high coaxiality,the same bonding thickness and similar bonding strength to perform the uniaxial and multiaxial fatigue experiments and it could guarantee the stability of experimental data and reliability of results.A series of uniaxial tension and uniaxial torsion fatigue performance experiments were conducted on the adhesively bonded butt-joints.The effects of stress amplitude,mean stress,cycle time and loading history on the uniaxial rattcheting strain,uniaxial ratchetting strain rate,fatigue damage and fatigue life of the adhesively bonded butt-joints were analyzed.The results showed that the ratcheting strain and ratcheting strain rate both increased with the increase of stress amplitude and mean stress,and the shorter cycle time,which also reduced the low-cycle fatigue life.Furthermore,the stress amplitude loading history could affect subsequent ratchetting strain response,and the influence of stress amplitude loading history on rattcheting strain response was greater than that of mean stress loading history.In addition,different from the uniaxial tensile cycle,the evolution of rattcheting strain under uniaxial torsion cycle has two stages: transient and steady-state.The fatigue life of torsion fatigue is generally higher than that of tensile fatigue.The multiaxial fatigue experiments were conducted on the adhesively bonded butt-joints in order to study their multiaxial fatigue performance.Combined with the stress-strain response curve,the tensile and torsion component analysis and ratchet strain evolution analysis,the variation rule of multiaxial fatigue life is summarized.The experimental results showed that the effects of non-proportional loading path on the rattchetting strain were greater than that of the proportional loading path.Moreover,the effects of the normal stress on the ratchetting strain were relatively large than that of the equivalent shear stress under the multiaxial loading paths.In addition,the ratchetting strain increases and fatigue life decreased with the increase of equivalent stress amplitude and mean stress,and the decreasing cycle time.The effects of cycle time on the multiaxial ratchetting strain and fatigue life under the elliptic loading path were slightly and the fatigue life of the adhesively bonded butt-joints under elliptic loading path was the shortest in all loading paths.The critical surface method and energy density fatigue life prediction models were employed to predict the multiaxial fatigue life of the adhesively bonded butt-joints.The prediction results showed that tensile strain amplitude model prediction for the fatigue life of adhesive specimens was conservative,and the life prediction by the shear strain amplitude model was dangerous.Although,the predicted results of the energy density model were conservative,compared with the predicted results of the previous two models,the predicted accuracy was better.Finally,combined with the fracture morphology and failure characteristics of adhesively bonded butt-joints under multiaxial cyclic loading,the energy density prediction model was modified by using the proportional coefficient of mixed damage.It was found that the modified model greatly improved the multiaxial fatigue life prediction accuracy for the adhesively bonded butt-joints.Therefore,the modified energy density model can be used to predict the multiaxial fatigue life of the adhesively bonded butt-joints.