Investigation Of Laser Shock Peening And Antifatigue Of Aluminum Alloy Fastener Hole On Shock Number

A large number of small holes in aircraft structures are fatigue weakness of aircrafts,laser shock peening(LSP) can effectively prolong service life of these holes.However,stress distribution is unreasonable and fatigue gain is unstable for single impact.To solve these problems,shot number is increased to regulate stress field around fastener holes to effectively extend fatigue life of fastener holes.Under the premise to meet fatigue gain stability,optimizing the impact mumber can simplify the process and reduce cost.Firstly,research status of laser shock number is analysised,then the influence of parameters such as shock number on residual stress of fastener holes are simulated by the software of ABAQUS.Secondly,parameters such as shock number are optimized.According to the optimized results,LSP experiment of different shot number is designed.Finally,fatigue verification test is performed.The main contents and results are as follows:(1)The deformation and estimation of residual stress induced by one shot LSP are analysised from the point of shock wave,and the superposition of residual stress field with multiple shot is explained from the point of elastic limit and material work harding.Finally,according to the influence of residual stress on fastener hole’s stress intensity factor(SIF),the reason for LSP extending the service period of fastener holes has been revealed.Results show that:increasing shot number can improve residual stress,but the increasing rate decrease for each impact.Compressive stresss induced by LSP will effectively reduce SIF amplitude of fastener holes.(2)Influence of parameters such as shot number on residual stress of fastener holes are performed by the software of ABAQUS.Results are as follows: the accumulation of shot number leads to the increasing of residual stress and reduction of tensile stress zone around middle hole wall,but the increasing proportion reduces for each impact,and there is little difference between two impact and three impact.Compared with shock number and FWHM,peak pressure has greater effect on residual stress.The superposition of shock number and FWHM will compensate for the lack of peak pressure when peak pressure is small.But when peak pressure is large engough,this remedy is not obvious.(3) SIF of fastener hole under uniform tension is calculated.On the basis of LSP simulation,SIF of fastener holes under uniform tension and residual stress is calculated.And the modification of LSP to SIF is performed.Results show that: LSP can effectively reduce SIF amplitude around fastener holes.On the condition of uniform tension,3-D dimensionless and dimensional SIF have consistent trend,with the increasing of angle θ, SIF firstly increases and then decreases. SIF has the maximum value at the position of θ=0 and θ=90. SIF increases in any crack propogation directions.(4)Combined residual stress data with fracture microscopic characteristics, impact number on fatigue crack initiation and growth rate of fastener holes is revealed.By analyzing the effect of shock layers on microscopic characteristics of dimple,the influence of LSP on aluminum alloy plasticity is revealed.Results show that: fatigue gain of single impact is small, fatigue gain of double and triple impacts is twice as much as one shot,and fatigue life of double and triple impacts are the same.Unreinforced specimen is a single point of corner cracks, and multi-point fatigue source exist in the hole wall for reinforced specimen.With the increasing of impact number,fatigue source transfers along thickness direction.Compared with un-treated specimen,fatigue striation spacing of strenghened specimen decreases significantly.The accumulation of impact number makes fatigue striation spacing become narrower,and there is little difference between double impact and triple impact.The more the impact number,the larger and deeper the dimple,indicating LSP can improve the plastic deformation of the specimen.The relationships of impact number,residual stress,fatigue life are established,the modification of SIF amplitude by LSP is the reason for the increasing of crack growth life.The compressive effect of the accumulation of impact number on tensile stress zone leades to the transfer of fatigue source.Researching the impact number on fatigue life of fastener hole will provide reference for engineering application.