| Small hole structure is a typical stress concentration structure.The fastening holes with a number of millions on the aircraft belong to this structure,and the holes are prone to fatigue failure,which ultimately leads to the failure of parts.Traditional reinforcement methods have limitations in applications,and most of them cannot meet the current requirements.In order to make the small hole fully strengthened,the ultrasonic extrusion technology can compensate for the limitation of the laser shock processing(LSP),and the two processes are combined to form a new composite strengthening process.At present,the anti-fatigue mechanism of LSP and ultrasonic extrusion composite strengthening is still unclear.Therefore,it is important to study the influence of composite strengthening and related process parameters on the fatigue performance of small hole structure to improve the fatigue life of the hole.Based on the combination of theoretical analysis,numerical simulation and experimental research,the influence of LSP and ultrasonic extrusion composite strengthening on fatigue life of 7050-T7451 aluminum alloy small hole structure was studied in this paper.And explored a new method for synergistic strengthening of high-frequency ultrasonic vibration and LSP of small hole structure.The main research contents and conclusions are as follows:(1)Based on the ABAQUS finite element analysis software,numerical simulation model of LSP and ultrasonic extrusion composite strengthening of small hole structure was established.The three-dimensional stress distribution after composite strengthening was analyzed,and the effect of process parameters on the composite strengthening effect was studied.The results show that the residual stress of the hole wall and angle are better after composite strengthening.The stress distribution after composite strengthening is better than single LSP and ultrasonic extrusion.The change of power density and pulse width affect the stress distribution ofsmall hole structure after composite strengthening.The amplitude has a great influence on the mechanical properties during the composite strengthening process.With the increase of the amplitude,the friction force decreases,and the change of the amplitude has little effect on the residual stress distribution of small hole structure after the composite strengthening.The change of the extrusion amount will affect the distribution of the residual stress after the composite strengthening.(2)Through ANSYS nCode Design Life fatigue analysis software,fatigue life analysis of aluminum small hole structure was carried out.The results show that from the fatigue life cloud diagram,the fatigue damage areas of LSP,ultrasonic extrusion and composite strengthening occur in the middle of the hole wall,at the corner of the extrusion end and near the middle of the hole wall,respectively.Under the same stress level,the fatigue life of the small hole structure after composite strengthening is the highest.At high stress level,the fatigue life gain of small hole structure is less affected by different strengthening methods.(3)According to the simulation results,reasonable parameters were selected to carry out the strengthening test on the double-joint specimens of 6mm aluminum alloy,and fatigue test of the strengthened specimens was carried out.The results are as follows: With the increase of amplitude,the drawing force decreases in the process of composite strengthening,and there is no significant effect of different amplitude on the fatigue properties of the specimens after composite strengthening.The fatigue gain of the composite strengthened specimens is much larger than that of LSP and ultrasonic extrusion strengthened specimens,which is consistent with the simulation results.After composite strengthening,the main fatigue source originates in the middle of the hole wall,and the fatigue crack propagates fan-shaped to the interior,and the fatigue crack growth rate is lower than that of LSP and ultrasonic extrusion specimens.(4)Through the above simulation and experimental research,it is found that the effect of composite strengthening is better but the technological process is tedious.In order to give full play to the ultrasonic characteristics and improve the strengthening effect,a new method of synergistic strengthening with high frequency ultrasonic vibration and LSP was proposed,and ABAQUS software was used to explore three-dimensional stress distribution after synergistic strengthening.The results show that the synergistic strengthening of high frequency ultrasonic vibration and LSP has better residual stress distribution,which makes the stress distribution uniform and the strengthening area large,and the process simple.The variation of pulse width and frequency will affect the three-dimensional stress distribution of small hole structure after synergistic strengthening. |
PDF Full Text Request