Investigation On The Dynamic Fracture Behavior Of Reactor Pressure Vessel Steel Considering The Effect Of Rate And Geometrical Constraint

Reactor pressure vessel（RPV）is the most important component of nuclear facilities,such as nuclear power plants and nuclear submarines.Because of its long-term service in harsh environment such as high temperature,high pressure,corrosion and irradiation,it may also be subjected to external impact load,so its structural safety plays a vital role in the service and safe operation of nuclear facilities.Once a nuclear leakage accident occurs in the reactor,it will cause serious disasters to human beings and the environment.The fracture failure evaluation of RPV material is directly related to the design and adjustment of key operating parameters such as the pressure-temperature curve of the reactor,and it is an important technical support to ensure the safe and reliable operation of the RPV.The fracture behavior of RPV material under dynamic loading conditions is significantly different from that under quasi-static loading.The change of loading rate and geometrical constraint will both make RPV material transform from ductile fracture to brittle fracture,which has an important impact on the structural safety of the reactor.Great progress has been made in the research on the fracture failure behavior of RPV material under a single influencing factor,but the research on the fracture failure behavior of RPV material considering both rate and geometrical constraint is very limited.Therefore,it is of great theoretical significance and engineering application value to carry out researches on the dynamic fracture behavior of RPV material under the influence of rate and geometrical constraint.In this work,the Chinese RPV steel A508-Ⅲis taken as the research object,and the dynamic fracture behavior of the material under the influence of rate and geometrical constraint has been studied by combining the experiment and finite element numerical simulation.The main work and conclusions are as follows:（1）The tensile test of Chinese A508-Ⅲsteel under different strain rates had been carried out,and the effect of strain rate on mechanical properties of Chinese A508-Ⅲsteel was investigated.The test results show that Chinese A508-Ⅲsteel has obvious strain rate sensitivity,and its yield strength and tensile strength increase with the increase of strain rate;based on the tensile test under different strain rates,the Johnson-Cook constitutive model of Chinese A508-Ⅲsteel was established.By comparing with the test results,the model was modified,and the Johnson-Cook constitutive correction model which could well describe the stress-strain relationship of Chinese A508-Ⅲsteel at low and medium strain rate was obtained;the quasi-static tensile test and finite element simulation of round bar specimens with different notch size were carried out,and the parameters of Johnson-Cook failure model of Chinese A508-Ⅲsteel were obtained.（2）The dynamic fracture test of Chinese A508-Ⅲsteel under different loading rates and specimen geometrical size was carried out.The impact absorbed energy and J-△a resistance curve were obtained from the load-displacement curve.The research results show that the Chinese A508-Ⅲsteel has good impact toughness.With the increase of loading rate,the total impact absorbed energy of the specimen remains basically constant,the crack initiation absorbed energy shows a slowly upward trend,and the crack propagation absorbed energy shows a downward trend.With the increase of loading rate,the fracture mode of the specimen changes from ductile fracture to ductile-brittle-ductile mixed fracture,and a typical rate-induced ductile-brittle transition phenomenon is exhibited.The loading rate and initial crack size have obvious effects on the J-△a resistance curve of Chinese A508-Ⅲsteel.Under the same initial crack size,the higher loading rate it has,the higher the J-△a resistance curve it will present.Under the same loading rate,the smaller the initial crack size it has,the higher J-△a resistance curve it will show.Compared with the effect of loading rate,the effect of initial crack size on the J-△a resistance curve is more significant.（3）Based on the Johnson-Cook constitutive correction model and failure model,the dynamic fracture behavior of SEB specimens of Chinese A508-Ⅲsteel was simulated by using the finite element software ABAQUS,and the dynamic fracture behavior of SEB specimens with the initial crack length a₀/W=0.4 under different loading rates was studied.（4）Based on the observation of macro and micro fracture,the dynamic fracture failure mechanism of Chinese A508-Ⅲsteel was analyzed.The results show that the brittle crack arrest in the ductile-brittle-ductile mixed fracture mode is not caused by the attenuation of loading rate,but is an inherent property of the material itself.Compared with the ductile-brittle-ductile mixed fracture mode,the fracture surface of the ductile fracture specimen contains more dimples,and the existence of dimples can absorb more energy and increase the impact absorbed energy of the material.（5）The conditional initiation toughness J_Q is affected by the loading rate and specimen geometry.The conditional initiation toughness J_Q of the same size specimen increases with the increase of loading rate.Under the same specimen thickness B,the larger width W the SEB specimen has,the higher value of J_Q it will present.When the width W is fixed,the larger thickness B the specimen has,the lower the conditional initiation toughness J_Q it will present.The effect of the initial crack size（in-plane geometrical constraint）on the value of the conditional initiation toughness J_Q is more obvious than that of specimen width W,thickness B（out-plane geometric constraint）and loading rate.The lower in-plane geometrical constraint it has,the higher value of the conditional initiation toughness J_Q value it will exhibit.For the failure specimen with ductile-brittle-ductile mixed fracture,the J_max value corresponding to the brittle crack initiation shows a good linear relationship with ln（△a_m）,and J_max increases linearly with the increase of ln（△a_m）.The lower in-plane geometrical constraint the specimen has,the larger J_max value it will present.The out-of-plane geometrical constraint has mixed effect on J_max value.Under the same in-plane geometrical constraint,J_max increases continuously with the decrease of out-of-plane geometrical constraint of the specimen.（6）In view of the diversity of fracture mode and the complexity of the variation of fracture toughness of Chinese A508-Ⅲsteel influnenced by the coupling of rate and geometrical constraint,the traditional evaluation method of fracture behavior based on J-integral is no longer applicable.Therefore,an evaluation method of dynamic fracture behavior of Chinese A508-Ⅲsteel based on impact absorbed energy is proposed.