Study On Impact Resistance And Damage Mechanism Of Fiber Metal Laminates Under High Velocity Impact

In order to ensure the safety performance requirements of aircraft in structural design,impact resistance evaluation has become one of the key steps in structural design.Any impact may pose a great threat to aviation safety for aircraft which flies at high velocity.Therefore,it is necessary to analyze the impact resistance of aerospace materials.Fiber Metal Laminates(FMLs)have outstanding comprehensive mechanical properties,which have been widely used in the field of aerospace.The research on the impact resistance of FMLs has become one of the hot topics in the field of composite structures.In this paper,FMLs model for high velocity impact problems is established based on the nonlinear finite element method.Johnson-Cook material model and a 3D rate-dependent constitutive model as well as Hashin failure criterion are applied to identify the in-plane damage in aluminum and fiber composite layers respectively.Cohesive elements governed by bilinear traction-separation constitutive model are implemented to simulate the inter-laminar delamination induced by impact.Firstly,based on the damage constitutive model,the FMLs under high velocity impact by projectiles with three different shapes(flat,hemisphere and sharp noses)are numerically simulated.The residual velocity and energy absorption of three shapes projectiles impacting FMLs at different initial velocities are analyzed respectively.The progressive damage process of fiber reinforced composite layer,metal layer and interface layer is also analyzed.The failure mechanism of the FMLs under high velocity impact is studied.The damage evolution of composite layer,metal layer and interface during impact is studied in detail.Analysis shows that the impact resistance of the laminate is related to the shape of the projectile.However,with the increase of impact velocity,the impact will be weakened.Secondly,there are many researches on the normal impact behavior of FMLs at present.However,ideal normal impact is relatively rare due to the change of target plate installation position and projectile track in real experiment while more general problem is oblique impact with a certain angle.In this paper,we develop a nonlinear dynamic finite element model in terms of CDM to predict the ballistic performance and characterize the damage mechanism of the FMLs subjected to general oblique impact loadings.The ballistic oblique impact process of two different structural FMLs at impact angles of 0o,30o,45o and 60o are simulated in detail.The ballistic performance and damage characteristics in this impact process are analyzed thoroughly.It is found that the residual velocity of the projectile and the energy absorption of the laminate are related to the initial impact velocity and impact angle of the projectile.Finally,current research on FMLs high velocity impact performance mainly focuses on non-preload condition which cannot reflect the actual loading state of FMLs in actual application.In this paper,three stress cases,including non-preload,uniaxial tensile preload and biaxial tensile preload,are considered to explore the effect of preloads on the high velocity impact behavior of FMLs.The velocity-time curve,ballistic limit,damage process and delamination area of representative FMLs with different preloads and impact velocities are thoroughly examined and analyzed.The ballistic performance and damage response of representative FMLs with different preloads and impact velocities are discussed and analyzed in detail.Tensile preload has little influence on the residual velocity of the projectile and the impact resistance of the laminate.But it can affect the area of delamination damage.In summary,in this paper,an effective numerical model is established to analyze the impact resistance and damage mechanism of FMLs,which provides theoretical basis and technical reference for the design and impact issues in other FMLs.