Preparation And Mechanical Properties Of Chopped Fiber Interlaminar Toughened CF/EP Composites

Carbon fiber reinforced epoxy resin(CF/EP)composites are widely used in aerospace,automotive,sports and other fields for their light weight,high strength,corrosion resistance and design strength.The laminate structure,as the most common structure of composite materials,relies only on the intrinsically brittle epoxy resin bonding without fiber reinforcement between the layers,which makes the laminate present low fracture toughness and is very easy to cause delamination damage after external impact eventually leading to the failure of the composite material,which seriously affects the development and application of CF/EP composites.At present,the methods used to improve the interlaminar fracture toughness of composite materials mainly include resin toughening,Z-directional toughening,and interlaminar toughening.Among them,interlaminar toughening is the addition of tough materials(usually particles,films,fibers,etc.)to the plies to improve the interlaminar fracture toughness of composites without changing the original process.Chopped fibers are ideal for interlaminar toughening because of their easy to get,low cost,and can solve the problem of resource waste in the production of long fibers and composites.Therefore,investigating the process and mechanism of interlaminar toughening of composite materials using chopped fibers is of great research significance and practical value.In this paper,two different chopped fibers(aramid fiber and carbon fiber)were selected.The chopped fibers were mixed with epoxy resin to make toughened materials,and the interlaminar toughened CF/EP(T700/MT3)composite laminates were prepared by the hot pressing molding process.The effects of chopped fiber density on the mode II interlaminar fracture toughness,flexural properties,and impact properties of the composite laminates were investigated by end notch flexural test,three-point flexural test,and pendulum impact test.The main conclusions of the tests were follows:(1)Chopped aramid fibers can effectively improve the mode II interlaminar fracture toughness of the composites but had a mediocre effect on composite flexural and impact properties.The best interlaminar toughening effect is achieved at a chopped fiber content of 10 g/m~2,and the interlaminar fracture toughness could be increased up to nearly 61.8%.The fracture surface photos and SEM image showed that chopped aramid fibers deflected the cracks,and the cracks developed across layers as the content of chopped aramid fibers increased.The toughening mechanism of chopped aramid fibers was mainly the debonding and pulling out of chopped fibers from resin,transverse fracture and longitudinal tearing of fibers.(2)Chopped carbon fiber can improve the mode II interlaminar fracture toughness and flexural properties of composite.The interlaminar fracture toughness was greatest at the chopped carbon fiber content of 10 g/m~2,with an increase of about 36.92%compared with the control sample.The fracture surface photos and SEM images showed that chopped carbon fibers deflected the cracks.The failure area across the interface increased with increased chopped carbon fiber content.The toughening mechanism of chopped carbon fibers was mainly the debonding and pulling out of chopped fibers from resin and the transverse fracture of fibers.(3)This thesis compared and analyzed the property data,damage modes and toughening mechanisms of two chopped fiber interlaminar toughened composites.It was concluded that the type of chopped fiber is the main factor affecting the toughening effect.