Research On Residual Strength And Fatigue Performance Of Composite Laminates With Low-velocity Impact Damage

Composite laminates have been widely applied to the aeronautics and astronautics industrybecause of their high specific strength, high specific stiffness and advantages of designability.However, laminates are susceptible to low-velocity impact which can lead to significant damageincluding matrix cracks, fiber fractures and internal delaminations etc. The internal damage canconsiderably reduce the residual strength and safely service life of composite structures. Theallowable strain of composite laminates was restricted to a low level according to traditional designphilosophy of composite structures based on the concept of “static covering fatigue”, which partlyoffset the advantages of composite laminates. To enhance the application level of composites, researchon the residual strength and fatigue performance of composite laminates after low-velocity impactshould be carried out. On this thesis, the main contents are as follow:(1) Impact damage and residual compressive strength after impact tests of T300/QY8911laminates were conducted with eight impact energy levels. The damage characteristic of impactedlaminates was obtained. The relationship between damage area, compressive strength after impact andimpact energy was discussed and the dominant damage mechanism of damaged composites undercompressive loads was also analyzed. The results show that low velocity impact damage has a greateffect on compressive strength; residual compressive strength was reduced by65%when thelaminates were impacted with energy level of3.75J/mm. The damage mechanism of impactedlaminates under compressive load is similar to notched laminates and fiber bending around the hole(or damage region) is the main failure mode.(2) Compress-compress fatigue test of T300/QY8911laminates after13.75J impact was carriedout. Damage propagation in compression-compression fatigue test was measured in detail and thedamage propagation law was obtained. It shows the damage propagation under fatigue load can bedivided into two stages: during the initial60%of fatigue life, damage grows slowly; during the rest offatigue life, damage experiences an accelerated propagation; and the stiffness of laminate willdramatically decline at the stage near final failure.(3) A finite element model was established for predicting the damage evolution of compositelaminates under transverse low-velocity impact. The model included cohesive elements whichallowed simulating delaminations between layers and three-dimensional solid elements which couldmodel fiber tensile failure, fiber compress failure, matrix crushing, and matrix cracking etc. inside layers. This model also considered the effects of in-plane damage on interlaminar strength. Model wasexecuted by ABAQUS/Explicit module with material subroutine VUMAT, the mechanism of damageand the main factor of delaminations under low-velocity impact were also discussed.(4) The damage data gathered by the impact model above was imported to a compressive finiteelement model using an import process and a finite element analytic method was established topredict residual compressive strength and damage evolution of composite laminates after low-velocityimpact. It is achieved to simulate the whole process from initial low-velocity impact damage to finalcompressive failure of composite laminates.(5) The impact damage was considered as a circle hole, and a special finite element including anelliptic inclusion was applied to analyze the stress distribution. Base on the damage propagation rule,characteristic length and point stress criterion were used for predicting the residual strength and thecompression-compression fatigue life of post-impacted laminate. The effects of some parameters onresidual strength and fatigue life were discussed.