FRACTURE AND FAILURE ANALYSIS OF METALLIC PIPES AT HIGH TEMPERATURE

During the combustion testing of the scramjet engine, early failure of pipes may occur, due to the presence of fractures followed by catching flre of aviation kerosene in the pipes as the fuel and coolant. This paper investigates the failure mechanism of the pipes. First, the fractography is analyzed by using both the stereomicroscopy and the scanning electron microscopy. It is indicated that the cracks are initiated exactly at the root segment of the welding line of the pipe connections, as a classical multi-source origin type. Besides, a half-circle shape with blue color is visible in the initiation area, indicative of the welding cracks. Second, fatigue striations are observed on the fracture surface with the sub micron-sized interval. According to the striation interval, the pipe thickness and the experiment period, the frequency of the fatigue loading can be estimated, which is well in agreement with the natural vibration frequency estimated based on the elastic theory and the modal analysis. Finally, it is concluded that the failure is due to the vibration fatigue. Based on the analysis mentioned above, a few flx points are added in order to change the natural vibration frequency of the pipe and to reduce the vibration amplitude. The welding process is also improved to avoid initial damages. As a result, the early failure is efiectively avoided.