Instable crack propagation at high temperature
Congress title :13th International Conference on Experimental Mechanics
Congress location :Alexandroupolis
Congress date :01/07/2007
This paper presents high temperature (900°C or 1000°C) tensile tests on precracked flat 16MND5 test pieces with a rectangular section. Cohesive zone model was used to simulate the crack propagation in a very ductile material. The context of these tests is the study of the rupture of the nuclear reactor bottom head submitted to an accidental mechanical and thermal loading due to the reactor core melting during an uncontrolled nuclear reaction scenario. The test purpose is the crack initiation threshold determination and the propagation crack speed characterization. The 16MND5 test piece had a 4 x 25 mm2 effective area at the 5 mm length notch. The mechanical loading was applied by a tensile testing machine which was controlled in force. The thermal loading was applied by an inductor heating. We first imposed a slope in temperature until the aimed one (900°C or 1000°C) and then a slope in stress until rupture. Force and displacement were recorded. An infrared camera and thermocouples were used to measure the temperature distribution. Finally, two methods were used to obtain the crack propagation speed. A high-speed digital camera (1000 f/s) enabled us to find the position of the crack tip. Moreover, we applied a constant electrical current in the test piece and we measured the voltage on both side of the crack during the propagation. The voltage evolution was correlated to the crack tip position. To simulate the test, we modelled the crack by a cohesive zone. An elastic damageable behaviour law from Hinte has been used for the cohesive elements. The 16MND5 behaviour was modelled by an elasto-plastic with kinematic hardening behaviour. First, we were interested in the Hinte parameters adjustment and then, in the plastic parameters influence on the crack propagation. The propagation speed resulted from the competition of two phenomena. The snap-back instability, governed by the ratio of the material stiffness and the interface one, tends to accelerate the propagation. On the contrary, plastic dissipation in the material tends to reduce the propagation speed.