Study On Failure Of Composite Foams Reinforced By Carbon Nanotubes Under Fatigue Load

Aluminum foams possess lots of attractive properties such as energy absorption,damping,sound insulation and fire retardant properties.Possessing these properties makes Al foams widely used in the aerospace,rail transportation and defense military field.Due to their excellent comprehensive properties,carbon nanotubes(CNTs)are considered as the ideal reinforcement for Al matrix composite foams,whose addition significantly enhances the strength of Al foams.The current studies on the properties of Al foams concentrate on the quasi-static and high strain rate compression tests.However,in practical engineering applications,structural components are also subjected to dynamic cyclic(fatigue)loading.Therefore,it is a great significance to evaluate the mechanical properties and failure behavior of Al foams under cyclic loading for its application in more complex environments.The quasi-static compression and compression-compression fatigue tests of pure Al foams are conducted,combined with synchronous photographing and failure fracture analysis using SEM.Based on the study of compressive properties of pure Al foams,the effects of specimen size and shape,stress level,porosity and stress ratio on the fatigue properties of pure Al foams are systematically analyzed.The results show that the ε–N curve shows a three-stages for k<0.9: elastic deformation stage,strain accumulation stage and strain steep stage.The fatigue strength decreases linearly with the increment of porosity,which shows the decrease of fatigue properties.The deformation of pure Al foams under quasi-static compression loading is mainly the plastic deformation of the pore edges and walls;While,in addition to the above deformation modes,the initiation and propagation of cracks are also important factors leading to the failure of the specimen under compression fatigue loading.The effects of different CNT contents and failure mechanism on the compressive and fatigue properties of CNT/Al composite foams are explored.The results show that the compressive property of CNT/Al composite foams is gradually improved but the brittleness increases with the increase of CNT content.The fatigue strength of CNT/Al composite foams with CNT contents 2.0 wt%,2.5 wt% and 3.0 wt% increase 6%,44% and 102% than that of pure Al foam,respectively.The fatigue properties enhancement of CNT/Al composite foams are mainly caused by the following factors: the CNT/Al composite foams has good interfacial bonding between CNTs and Al matrix;CNTs are evenly dispersed to exert dispersion strengthening;CNTs make full use of their excellent performance.