Repeated Low-velocity Impact Behaviour And The Residual Compressive Strength After Impact For CFRP Laminates

Fiber reinforced polymer(FRP)has the excellent characteristics of low density,high specific strength,high specific modulus and easy processing and molding.It is widely used in aerospace,automotive engineering,marine engineering,civil engineering and many other fields.The commonly used fiber-reinforced composites are laminated structures,whose interfacial strength is much lower than the in-plane strength,and they are more sensitive to impact load.Composite laminated structures may be exposed to complex load environment during manufacturing,maintenance and service,and local parts of the structure may be subjected to repeated impact.Each impact may bring new damage to the composites,resulting in the reduction of the residual compressive strength and affecting the service life of the structure.Therefore,the research on the repeated impact damage behavior and residual compressive strength of composite laminated structures is of great significance for the design,use and maintenance of composite laminated structures.Taking carbon fiber reinforced composite laminates(Carbon Fiber Reinforced Polymer,CFRP)as the research object,this paper studies its single impact,repeated impact and compression after impact(CAI).The main work includes the following aspects:(1)The single impact and repeated impact behavior of CFRP laminates are experimentally studied.Under different single impact energy and different combination of repeated impact energy,the single impact and repeated impact tests of CFRP specimens are carried out by drop hammer method,and the corresponding structural dynamic response and energy absorption are obtained.Moreover,the modal frequency variation of CFRP laminates before and after impact are tracked throughing modal test,and the vibration mode and frequency changes of specimens before and after impact are obtained.Then,ultrasonic C-scan was performed on the impacted laminate to detect the internal damage of the laminate.Finally,the residual compressive strength test of laminates is carried out.(2)The effects of single impact and repeated impact on the structural dynamic response,energy absorption,damage condition,frequency change and residual strength of laminates are systematically studied and compared.The experimental results show that during the single impact,with the increase of impact energy,the damage area and energy absorption also increase,and the residual compressive strength of laminates decreases.When the total impact energy is the same,the damage caused by repeated impact is less than that of single impact,and the residual compressive strength of single impact is smaller.In addition,the curves of CAI-impact energy,CAI-absorbed energy,CAI-damaged area and impact energy-frequency change rate all appear inflection points under the impact energy of 20 J.Among them,the structural frequency can be easily measured online and in situ without damaging the specimen.Therefore,the impact energy inflection point of the laminate can be easily determined through the frequency index,and the residual compressive strength at the inflection point can be used to characterize the impact damage tolerance of the composite laminates.(3)The progressive damage model and cohesive zone model of low-velocity impact are established,and the whole process finite element simulation of low-velocity impact and residual compressive strength after impact of composite laminates is realized through the predefined field setting built-in in ABAQUS software.Firstly,the single and repeated low-velocity impact of composite laminates are simulated by finite element method,and the dynamic response and damage evolution behavior of CFRP laminates under two kinds of low-velocity impact are studied and compared with the experimental results.The results show that the simulation results are in good agreement with the experimental data in terms of impact force-time curve,impact force-displacement curve,energy time-curve,impact damage area and morphology.Among them,the measured compressive strength of non-destructive CFRP specimen,the CFRP specimen after 20 J single impact and 10J+10J repeated impact are 226.77 MPa,140.01 MPa and169.29 MPa respectively.The residual compressive strength in the simulation are 219.10 MPa,131.23 MPa and 158.62 MPa respectively,with an error of no more than 8%,indicating that the test is in good agreement with the simulation results...