Effect Of Cyclic Hygrothermal Aging On Mechanical Properties And Impact Resistance Of Three Dimensional Braided Composites

The three-dimensional braided composite material is an integral mesh braided composite structure,which can overcome the defects of traditional laminated composite such as low-interlaminar strength,delamination,and also has high specific strength,corrosion resistance,high damage tolerance and excellent impact resistance and so on.The three-dimensional braided composite has a great application prospect in aeronautics and astronautics.The mechanical properties and impact resistance of three-dimensional braided composite fan blades will change when they are exposed to cyclic hygrothermal aging environment during service,which will further affect the reliability of blades in service.Therefore,it is of great significance to evaluate and predict the ability of three-dimensional braided composite to resist external damage after cyclic hygrothermal aging in order to ensure the safety of aero-engines in service.In this paper,the three-dimensional carbon-fiberesin braided composite,which was treated after cyclic hygrothermal aging,was taken as the research object to carry out the following four aspects:(1)The cyclic hygrothermal aging test was carried out for three dimensional braided composites,then the moisture absorption curve and moisture absorption mechanism of three-dimensional braided composite was obtained and analyzed.By using the electron scanning microscopy,the internal changes of specimens through different hygrothermal aging days were observed,and the effects of the cyclic hygrothermal aging time on the internal components of three dimensional braided composites was discussed.The experimental results showed the three dimensional braided composites did not reach the saturated moisture absorption rate within 280 cyclic hygrothermal aging days and the hygroscopicity curve consistents wih the Fick law;The internal damage area of the three dimensional braided composites increases within the cyclic hygrothermal aging days and the internal morphological changes through four stages: resin moisture absorption swelling crack,crack area expansion,interface between fiber/resin debonding to form cracks,and the number of cracks increases and continues to expand.(2)The quasi-static tensile test of three dimensional braided composites under different cyclic hygrothermal aging days was carried out.The results shows: the stress-strain curve of three-dimensional braided composite shows a linear change within 210 cyclic hygrothermal aging days;The stress-strain curve of three-dimensional braided composite shows an obvious bilinear change;The elastic module of three-dimensional braided composite under 140 days cyclic hygrothermal aging days gradually decline,the largest decline is 15.2%,and then the elastic module starts to rise with cyclic hygrothermal aging days increasing;The failure strain within the whole cyclic hygrothermal aging days always rise;However,poisson's ratio decreases obviously in the early stage of cyclic hygrothermal aging,and recoveres in the later stage,and finally exceeded the poisson's ratio of three-dimensional braided composite without hygrothermal aging.(3)The Split Hopkinson Pressure Bar test of different cyclic hygrothermal aging days under different strain rate was carried out,and the effects of cyclic hygrothermal aging on dynamic mechanical properties of three-dimensional braided composite were analyzed.The experiment results show that the three dimensional braided compsites show some strain-rate related behavior at the early stage of cyclic hygrothermal aging,and there is a strain-rate strengthening effect;At the later stage of cyclic hygrothermal aging days,the effect of hygrothermaing aging plays a dominant role and the strain-rate related behavior is no longer obvious.The cyclic hygrothermal aging has a positive effect on the longitudinal dynamic compression strength of the three dimensional braided composite,but the cyclic hygrothermal aging for long time will obviously weaken the longitudinal bearing capacity of the three dimensional braided composites.(4)The high speed impact test,combined with the high speed camera,was carried out for different cyclic hygrothermal aging of three-dimensional braided composite,and the morphology of destruction after high speed impact was analyzed using industrial computed tomography scanning system.The results show the energy absorbed by the three-dimensional composite target plate at the same impact velocity increases with the increase of cyclic hygrothermal aging days,and the impact resistance of three-dimensional braided composite after hygrothermal agingis significantly improved;There is no significant difference in macroscopic damage of three dimensional braided composite target under high speed impact at different cyclic hygrothermal aging days;The main damage modes of three dimensional braided composites under high speed impact load including: matrix breakage,interface between fiber/matrix debonding,and fiber breakage and pull-out.The research work of this paper can provide certain reference significance for the evaluation and prediction about the mechanical properties and the design of resistance of external damage of three-dimensional braided carbon fiber composite blades of aero-engines after hygrothermal aging.