Research On Crack Growth Mechanism Of SiCp/A356 Composites

This paper takes SiCp/A356 composite material(short for composite)brake disc applied in urban rail train as engineering background.The thermal stress characteristics during the servie process of aluminum matrix composites brake disc were studied basing on finite element simulation technology.And the fatigue crack initiation and propagation mechanism of aluminum matrix composites in particular working conditions were studied.The effects of different particle content and particle distribution on the thermal fatigue crack initiation and propagation of composite materials were also compared and analyzed in this paper.ANSYS brake-thermal analysis shows that the friction surface of aluminum matrix composites brake disc applied in urban rail train has different temperature ranges in different working conditions.The highest temperature is about 276℃ in the working condition of two consecutive emergency braking,while the highest temperature is about 343 ℃ in the working condition of full time trip braking.The cold and hot fatigue test of composite materials show that the thermal fatigue crack propagation is always significantly affected by the inner cavity and microcrack connectivity effect of the material at different cycle temperatures,but present different propagation patterns.When the cycle temperature is lower than 250 ℃,the crack propagation of the composite is dominated by particle fracture with small amount of matrix tear,and there rarely has pores and microcracks caused by particle breakaway.In this condition,the thermal fatigue crack propagation shows obvious characteristics of"flat like" slow growth and "step like" mutation with slow propagation rate and small step width When the cycle temperature is higher than 250 ℃,the crack propagation of composites is dominated by particle shedding with large matrix tearing.The defects of pores and microcracks in the composites increase obviously,and the "step like"mutation of the thermal fatigue crack propagation becomes more obvious.The propagation rate is higher and the width is larger.With the increase of the content of SiC particles reinforced phase,the overall crack propagation rate of the composites shows a trend of increase,meanwhile,the inhibition effect of SiC particles on the thermal fatigue crack propagation turns obvious gradually.When the reinforced phase particles distributes evenly,the distribution of material internal thermal stress is more uniform;obvious thermal stress concentration will occur at the agglomeration of the particles,and can cause local micro damages agglomerate,which provides the source and path for thermal fatigue crack initiation and propagation.The study in this paper has clear engineering application background.The research results not only enrich the thermal fatigue crack propagation of the particle reinforced aluminum matrix composites,but also provide a scientific basis for the design and quality control of composite materials,and can also provide technical support for the engineering application and service safety of aluminum alloy brake discs.