Surface Integrity And Fatigue Properties Of Fibre Metal Laminates Strengthened By Laser Shock Peening

Fibre metal laminates(FMLs)are interlaminar hybrid materials that are cured under a certain temperature and pressure after being laid alternately by metal sheets and fibre reinforced composites.FMLs have excellent impact resistance and high damage tolerance,which makes them have broad application prospects in aerospace,rail transit,special equipment and other fields.During the service of FMLs,fatigue cracks first initiate on the surface of the outer metal layer,and the lack of integrity of the outer metal layer will cause the inner fibre reinforced composites to be exposed to a severe environment.In addition,fretting fatigue in the mechanical connections of FMLs will accelerate the crack growth of the outer metal layer,leading to a significant reduction in the fatigue life of the connections.FMLs have a unique multi-interface structure,and the traditional surface plastic deformation modification methods prefer to cause interface damage.Taking titanium-based carbon-fibre/epoxy laminates(Ti-CF FMLs)as the object,this doctoral dissertation systematically investigated the interface damage behavior and surface integrity of the laminates under laser-induced shock wave.The fatigue behaviors of open-hole Ti-CF FMLs and single-lap bolted connections were analyzed.The mechanism of laser shock peening(LSP)on fatigue life extension of TiCF FMLs was revealed.The main research contents and results are as follows:Interface damage of Ti-CF FMLs under laser-induced shock wave: The damage behavior of the interface between metal and fibre composites in Ti-CF FMLs under laser-induced shock wave was studied.The results show that the laser-induced shock wave forms unloading wave after reflecting from the free surface of Ti-CF FMLs.The unloading wave interacts with the incident wave,forming tensile stress at the interface of metal/fibre composite in Ti-CF FMLs,which results in interface damage.The extent of interface damage is positively related to the laser pulse width,pulse energy and spot size.After LSP with large-area,the compressive residual stress formed in the metal layer causes the reverse bending of Ti-CF FMLs,resulting in delamination failure of the damaged interface.The interface damage of FMLs can be prevented by using small size laser spot during LSP.Surface integrity and microstructure of Ti-CF FMLs after LSP: The surface integrity of Ti-CF FMLs after LSP without protective layer was analyzed,and the microstructure evolution mechanism of the titanium layer in Ti-CF FMLs after LSP without protective layer was proposed.The results show that during LSP,the synergistic effect of laser-induced heat and shock wave contributes to the increase of surface roughness.A work-hardened layer and compressive residual stress layer are formed in the surface layer of Ti-CF FMLs after LSP.Under the action of laser-induced shock wave on the titanium layer of Ti-CF FMLs,dislocations undergo multiplication in the grains that are prone to experiencing dislocation slip.Meanwhile,twins are created in the grains that are less susceptible to slip,serving to adjust the grain orientation and facilitate dislocation slip.With the increase in plastic deformation in the titanium layer,dislocation cells are formed due to the interaction between dislocation.In order to coordinate the deformation of adjacent dislocation cells,the geometrically necessary dislocation interface structures are formed.Dislocations continue to gather at twin boundaries and geometrically necessary dislocation interface structures to form smallangle grain boundaries.The surface of the titanium layer undergoes rapid melting and solidification due to the heat generated by laser,resulting in the formation of martensitic lamellar grains.However,laser-induced heat has little influence on the sub-surface layer,and the microstructure is generated by the action of laser-induced shock wave.Process optimization of LSP on open-hole Ti-CF FMLs: A finite element model of multi-point LSP was established for open-hole Ti-CF FMLs,and the effects of laser spot scanning path,lapping mode between laser spot and hole edge on residual stress distribution and hole edge displacement of Ti-CF FMLs were investigated.The results show that compressive residual stress is introduced into the impact area within the entire thickness range of the metal layer in Ti-CF FMLs after LSP.The surface residual stress fields introduced by the ‘回’ shaped and ‘S’ shaped strategies are axisymmetric,while the surface stress field introduced by the ‘concentric circle’ shaped strategy is centrosymmetric.The ‘S’ shaped strategy causes the smallest edge displacement of the hole,while the ‘concentric circle’ type causes the largest edge displacement.For the‘concentric circle’ shaped strategy,when the edge of the last circle of laser spot coincides with the edge of the hole,the displacement of the hole edge of Ti-CF FMLs after LSP is significantly reduced,and the compressive residual stress distribution near the hole can be controlled by increasing the laser pulse energy.For multidirectional loading,the optimized ‘concentric circle’ shaped strategy can obtain better residual stress distribution,while for unidirectional loading,the ‘回’ shaped strategy can be used.Fatigue behavior of open-hole Ti-CF FMLs after LSP: The effect of LSP without protective layer on the fatigue crack initiation and propagation behavior of open-hole Ti-CF FMLs was investigated.The results show that the fatigue crack initiation lives of open-hole Ti-CF FMLs after LSP are significantly increased,but the fatigue crack propagation lives are decreased.When the net section stress is 160 MPa,the fatigue crack initiation lives of 2/1 and 3/2 open-hole Ti-CF FMLs are increased by68.4% and 81.0%,respectively.After LSP,compressive residual stress is produced in the whole depth range in LSPed area,so the fatigue crack initiation and short crack propagation lives are both improved compared with the untreated Ti-CF FMLs.However,the balanced tensile stress causes fast fatigue crack propagation outside the LSPed area.By properly defining the LSP area,the fatigue lives of open-hole Ti-CF FMLs can be effectively improved.Fatigue behavior of Ti-CF FMLs bolted connections after LSP: The effect of LSP without protective layer on fatigue behavior of single-lap bolted connections of TiCF FMLs was investigated.The results show that when the fatigue peak pressure is3000 N and the stress ratio is 0.1,the fatigue failure of Ti-CF FMLs bolted connections starts from the titanium layer of the laminates that are in contact with each other,and the fatigue crack propagation direction is perpendicular to the load direction.The fatigue fracture lives of the titanium layer in 2/1 Ti-CF FMLs bolted connections before and after LSP are similar,while the fatigue fracture life of 3/2 Ti-CF FMLs bolted connections is increased by 36.6% after LSP.The fatigue crack of Ti-CF FMLs bolted connections originates from the edge of the fastener hole near the loading end,and with the increase in the fatigue load,the fatigue initiation position is closer to the edge of the fastener hole.However,LSP has little effect on the fatigue crack initiation position.The compressive residual stress introduced by LSP significantly reduces the normal stress on the critical plane,hence prolonging the fatigue life of the lower titanium layer in the Ti-CF FMLs bolted connections.