RELATIONSHIP BETWEEN CHARPY V-NOTCH IMPACT VALUE AND FRACTURE MECHANICS TOUGHNESS: ADDED VALUE OF FINITE ELEMENT ANALYSIS AND INSTRUMENTED IMPACT TESTS

摘要

Each structure is designed with resistance versus the fracture, which requires the knowledge of the fracture resistance of the material. If no fracture mechanics data of the material is available, a K_(JC) can be inferred from the master curve approach. The master curve approach relates a fracture toughness of 100 MPAm~(1/2) to the impact transition temperature T_(27J) with a shift of 18°C. Although this relationship was successfully applied to a large number of experiments, some steels deviate significantly from the previous relationship, which can even lead to non-conservative design. In the present paper, instrumented impact tests (Charpy V-Notch CVN) and compact tensile (CT) tests were performed on two materials, one thermomechanically (TM-) rolled and one normalized steel. The difference between T_0 and T_(27J) was found to be different for these materials. Furthermore, the normalized steel exhibits a smooth transition from brittle to ductile behaviour, while the TM-rolled material shows a very steep transition. Extra information is gained by combining the instrumentation of the impact test and the finite element simulations of both the CT and impact tests. From the instrumented tests, it is also possible to determine the load at unstable crack propagation, the amount of energy dissipated at that moment, the load at crack arrest and the energy dissipated after crack arrest. From the finite element simulation, one learns about the constraints ahead of the crack tip for both configurations. The investigation teaches us that the smooth transition of the normalized material is related to a high energy dissipated after crack arrest, while the TM-rolled material has a much lower crack arrest load. The difference between TO and T27J is then discussed by decomposing the total energy in the impact test between crack initiation, propagation and arrest. It is compared with K_(JC), which determines the toughness at unstable crack propagation, by reviewing the literature and local stress states computed from finite element.