| The purpose of this paper is to study the fracture properties of isotropic pyrocarbon used in artificial heart valve.Relevant research shows that the artificial mechanical heart valve is widely used in clinical operation because of its long service life in human body.At present,the most widely used artificial mechanical heart valve in the market is mainly made of pyrocarbon and its composite materials.Because of the long-term impact of blood circulation and the corrosion of blood in complex physiological environment,the pyrocarbon of artificial mechanical heart valve implanted in human heart is prone to cycle fatigue,which will cause cracks or even fractures and other dangerous situations,making the structure of the whole artificial heart valve component invalid,thus endangering the life safety of patients.Therefore,it is very important to study the fracture properties of pyrocarbon for artificial heart valve.In this paper,the fracture properties of pyrocarbon with artificial heart valve are studied in two parts: the fracture toughness of long crack of pyrocarbon is studied by MTS electrodynamics test system,and the fracture morphology of pyrocarbon is observed and analyzed by microscope;the microcracks on the surface of pyrocarbon were simulated and analyzed by ANSYS Workbench.The specific research contents and results are as follows:1 ? The fracture toughness of pyrocarbon and its coated graphite composite were measured by MTS electrodynamic test system.The results showed that the KIC of pyrocarbon was 1.691±0.098 MPa·m1/2,the KIC of two kinds of pyrocarbon coated graphite composites were 2.224±0.099 MPa·m1/2 and 2.397±0.035 MPa·m1/2,the KIC of graphite was 1.801±0.073MPa·m1/2.The fracture toughness values of two kinds of pyrocarbon coated graphite composites are greater than that of graphite and pure pyrocarbon,and the fracture toughness values of graphite are greater than that of pure pyrocarbon.The fracture toughness values of pure pyrocarbon,two kinds of pyrocarbon coated graphite composites,and graphite samples are all larger.When the thickness ratio of the coating to the substrate is different,the variation trend of KIC is the same as that of the finite element method,which indicates the reliability of the experimental results.2?The fracture surface of pyrolytic carbon and its coated graphite composite was made by MTS electrodynamic test system.The macrostructure and microstructure of the fracture surface were observed and analyzed by optical microscope and scanning electron microscope.The conclusion is as follows: the fracture surface of pure pyrocarbon and pyrocarbon coatingis relatively flat in macro structure,and the fracture surface of pure graphite and graphite matrix is relatively rough,and different from that of metal,pyrocarbon,as a ceramic like material,has almost no difference in fatigue growth zone and overload fracture zone.In micro fracture mechanism,the fracture morphology of pure pyrocarbon and pyrocarbon coating is mainly intergranular fracture,while that of pure graphite and graphite matrix is mainly transgranular fracture.3?The stress intensity factors and J-integral of the front edge of the compound crack on the surface of pyrocarbon for artificial heart valve were calculated by ANSYS Workbench.Comparing the results of finite element simulation with those of calculation,the relative error is less than 5%,which shows that the simulation based on ANSYS Workbench is reliable.At the same crack depth,with the increase of inclination angle,the I-type stress intensity factor and J-integral increase,while the II-type stress intensity factor and III-type stress intensity factor increase first and then decrease.At the same inclined angle,with the increase of crack depth,the I-type stress intensity factor,III-type stress intensity factor and J-integral decrease,and the II-type stress intensity factor increases,in which the I-type stress intensity factor and J-integral gradually change from the maximum value to the minimum value at the deepest crack. |
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