Dynamic Fracture Behavior Of Pressure Vessel Metal Materials Under Impact Loads

Performance assessments and disaster countermeasures of dynamic fracture of structural materials are involved in many engineering fields such as pressure vessels and pipelines burst, crack arrest, the impact of the nuclear power plant protection security, buildings and structures, seismic design etc.. In order to ensure the safety and reliability of the metal components under dynamic loads, crack initiation and propagation in the componemts with prefabricated crack should be prevend in impact loading design. Therefore, the importance of study on dynamic fracture behavior of metallic materials is self-evident. The present dissertation investigates the dynamic fracture behavior of metallic materials under impact loads via theoretical analyses, experimental test and numerical simulation. The main contents in the dissertation are listed as following.The effects of variety factors on the dynamic stress intensity factor under ideal impact loads are studied for plate with centeral prefabricated crack, twin cantilever beam and three point bend specimens. The calculated results of the plate with centeral prefabricated crack show that the maximum value of dynamic stress intensity factor generally occurs at the moment of intersection of the stress wave with crack tip. The stress intensity factor of twin cantilever beam can be approximated by a periodic function under tensile step load. The change cycle is as the same as the displacement cycle of loading point, and is a function of prefabricated crack. Spring-mass model is used to solve the dynamic stress intensity factor of the three-point bend specimens, and also be used to calculate the values of dynamic stress intensity factors under three ideal impact loads. The calculation results of completely dynamic finite element analysis are compared with the spring-mass model results.Charpy pendulum impact tests are carried out on S30408austenitic stainless steel base metal and weld metal under-196℃. Modified compliance changing rate method is used to obtain the crack initiation point of stainless steel base metal charpy specimen. The analysis show that results obtained by the improved method is more accurate than that by traditional compliance changing rate method. Schindler procedure and key curve method are used in conjuction to get approaching real dynamic crack growth resistance curve in terms of load-displacement curves by tests. According to characteristics of fracture mechanism and load-displacement curves, linear elastic fracture mechanics approach is used to obtain the dynamic fracture toughness of weld metal.Oscillographic impact tests are carried out using precracked charpy speciments of typical pressure vessel steel Q345R, the load-displacement curves are obtained. J-integral incremental equation procedure and Schindler method are used to estimate dynamic crack resistance curve of Q345R under impact loading rate respectively. The obtained results are consistent each other. By comparing the dynamic J-R curve and qusi-static J-R curve, it is found that the J-R curve under dynamic loading conditions is higher than that obtained under quasi-static loadings. A large number of experimental results show that there is a certain quantitative relationship between them. A calculation equation is established between Q345R quasi-static and dynamic crack growth resistance curve according to the obtained test data.Oscillographic impact test of material Q345R are carried out under temperature-40℃. It is found that prefabricated fatigue crack charpy specimens show completely cleavage fracture performance. Standard charpy impact test specimen have small amount of plastic deformation in the gap. Fracture surfaces of specimens without inclusions are flatter and have lower impact energy value. The quasi static formula in standard ASME E399are used to calculate dynamic fracture toughness for samples of two types, the results are compared and discussed. The rupture time is compared with specimen vibration cycle to verify the validity of the test results. Numerical simulation of these tests is performed with the well known general purpose finite element software ABAQUS. Dynamic stress intensity factor at crack initiation is calculated based on the nodal displacements extrapolation. It is found that the calculation accuracy can be increased if excluding displacement data in the vicinity of crack tip. 3D and2D numerical simulations are also conducted for Dynamic fracture toughness test using TPB specimen in SHPB by the ABAQUS6.8code and3D fracture mechanics ZENCRACK7.7code.The finite element model includes the whole experimental system (the projectile, the input bar, the specimen and the supporting device). Three specimens with different initial crack length impacted by projectiles are calculated. Dynamic stress intensity factor impulse response curves are obtained. The dynamic initial fracture time, tf, can be received by RKR local stress fracture criterion. The material dynamic fracture toughness is successfully obtained from numerical simulation. The3D finite element results and2D results are compared with test results benchmark, it is found that3D finite element results are better than2D results.