Fatigue Damage Mechanism And Behavior Of Al-Li Alloy Based On DIC Technology

During the service process of aircraft,the structural damage has a significant influence on its operational life and service performance.Common structural damage includes Foreign Object Damage,Multi-site Damage,etc.In severe cases,the tensile and fatigue resistance of the material will be significantly weakened,which will further reduce the service life of the aircraft structure.This article mainly focuses on the fatigue performance of a series of new generations represented by domestic aircraft C919 after certain flight cycles,which use 2198 Aluminum-Lithium alloy as the main choice of the skin.Based on Digital Image Correlation(DIC)technology,we concentrate on the fatigue damage behavior and mechanism of 2198Aluminum-Lithium thin sheet under coupling damage conditions.In terms of Foreign Object Damage: first of all,the effects of different impactor types and impact energy on the fatigue performance of 2198-T8 thin sheet with pre-cracked specimen were studied;secondly,the influence of impactor size,impact energy,and rivet hole size on the fatigue performance of coupling damage specimen was explored.Finally,the DIC technology was used to characterize the axial strain distribution and crack growth of the specimen surface under fatigue cyclic loading,the prediction of fatigue life was performed by the combination of ABAQUS/Explicit and Fe-Safe.The results are as follows:(1)the impact damage caused by the conical impactor will greatly reduce the fatigue life of the specimen,and the relationship between the residual fatigue life under the same impact energy is U-shaped >hemispherical > conical impactor;(2)the impact damage caused by the hemispherical and U-shaped impactor under 5J will cause local hardening of the plastic deformation area near the impact dent,forming a plastic zone inhibitory effect and slightly extending the fatigue life of the specimen by 12.09%.(3)For the specimen with coupling damage,the structural discontinuity caused by the rivet hole and the damage caused by impact have a competitive relationship in the fatigue failure process,and for the specimen impacted by the same impactor,the critical local plastic deformation ratio of the fatigue fracture position transition point is related to the rivet hole’s diameter,and this critical value increases with the hole’s diameter.The fatigue life prediction results are conservative and distributed in the double dispersion zone,and the errors are all less than 14%,which proves that the prediction results by the combination of ABAQUS and Fe-Safe are reliable.In terms of Multi-Site Damage: the mechanical properties of the 2198-T8 thin sheet with rivet holes under scratch damage,pre-fatigue damage,and hole arrangement with multi-size rivet hole were mainly studied.By the combination of fatigue test,simulation,and numerical calculation: firstly,the influence of the scratch tip angle,scratch length,and depth on the fatigue performance of the thin sheet was explored;secondly,the effect of the pre-fatigue loading ratio on the residual strength was studied;then the influence of multi-size rivet hole arrangement on the fatigue life of the specimen was analyzed.The results are as follows:(1)based on the simulation results,the order of the influence of the scratch size parameters on the standardized relative stress concentration factors is: scratch length > scratch depth > scratch tip angle;according to the fatigue results,it could be seen that when the scratch depth exceeds a specified range,that is,after 0.15 mm,the fatigue fracture position of the specimen is transformed from the rivet hole section to the scratch section,and the fatigue life is significantly reduced;on the other hand,when the depth of the scratch is constant,the fatigue life of the specimen decreases with the increase of the scratch length.(2)When the pre-fatigue loading ratio reaches 80%,the residual strength has decreased by 55.2% compared to the specimen without pre-fatigue damage,after a comprehensive consideration of residual strength,residual strain,and material parameter constraints,the residual strength degradation formula with fitting goodness of 99.9% is obtained.(3)The fatigue crack propagation of single-hole specimen with double pre-cracks under uniaxial fatigue load is gradually transformed from mode I to mode I+II,and the modified stress intensity factor formula is used to predict the fatigue life of the specimen with multi-diameter rivet holes,the result tends to be conservative and the maximum error is within 7%.