The Influence Of Loading Rate On The Fracture Behavior Of Different Gap Size And Loading Mode Sample

In this paper, the tension test is carried out at various loading rates of 1-500mm/min at a temperature of-110℃ for a pressure vessel steel (16MnR) , and the mechanical properties at different strain rates are measured. The three-point bending (3PB) tests and the four-point bending (4PB) tests are carried out at -110℃ and various load rates of l-500mm/min for two types of notched specimens (V, I) with different notch sizes and a kind of cracked specimens (C). By experimental observation, parameter measurement and the elastic-plastic finite element method (FEM) calculations of the distribution of stress, strain and strain rate ahead of the notches at different loading rates, the effect of loading rate on fracture behavior of the notched specimens with different notch sizes under different loading methods (4PB and 3PB) is investigated. The main conclusions obtained are as follows:(1) With increasing stress concentration and plastic constraint for the different notched size specimens V, I and C, the sensitive loading rate range of fracture behavior is decreased from l-120mm/min (V), through l-60mm/min (I) to 1-30mm/min (C), and in the sensitive loading rate range, the fracture model is rapidly changed from the ductile-brittle transition fracture to brittle cleavage fracture and the toughness (E) is rapidly decreased. In the higher loading rate range over the sensitive range, the fracture behavior of the three specimens is not sensitive to loading rate; and the brittle cleavage fracture model remains; and the toughness is lower and does not essentially change, with loading rate.(2) The FEM calculation shows that with increasing loading rate, general yield load Pgy, stress and strain rate ahead of notches for the notched specimens increase. The effect of loading methods (3PB, 4PB) on the distribution of stress, strain and strain rate ahead of notches could not be found essentially. The general yield load, stress and strain rate ahead of notches of I specimens with larger plastic constraint is larger than that of V specimens with small plastic constraint. The distribution of stress ahead of I notch is more sensitive to the loading rate and applied load. That means that with increasing loading rate and applied load, the stress ahead of acuite notch is more rapidly increased. This is one of the reasons that the fracture behavior of the acuite notched specimens is more sensitive to loading rate.(3) The local cleavage fracture stress Of of the 16MnR steel is essentially independent on the loading rates (1-500mm/min), notched sizes (I, V) and loading methods (3PB, 4PB). The reason for this is that the critical event for cleavage fracture for 16MnR steel is not changed in specimens with different notch sizes at various loading rates and loading methods. The critical event for cleavage fracture is a mixture of aferrite grain-sized crack propagation controlled and the crack nucleation-controlled. Therefore,of is an intrinsic toughness parameter of steel, and it does not essentially change with specimen sizes and loading conditions.(4) The macroscopic cleavage fracture stress F is the maximum normal stress ahead of a notch when the specimen is fractured, its measured value is determined by the local cleavage fracture stress of, and it is also independent on notch sizes, loading rate and loading methods. The value of oF is slightly higher than that of Of, and its scatter is small, and its measured value is more stable. The measurement of OF is more simple and convenient, and its value is closely related to the applicated load P/Pgy. So the macroscopic cleavage fracture stress OF is suggested to be a potential engineering parameter, which can be used to assess fracture toughness and safety of materials and structure with different notch sizes at various loading rate and loading methods.(5) Using the criterion o-yymax>F and combining finite element method calculations of the distribution of stress ahead of the notches, the ratio of fracture load Pf to general yield Pgy for the specimens with different no...