金属管线高温环境下的断裂失效分析

FRACTURE AND FAILURE ANALYSIS OF METALLIC PIPES AT HIGH TEMPERATURE

超燃冲压发动机燃烧室试验台配置了大量金属管线,煤油作为燃料和冷却介质在其中输运,在实验过程中发生管线早期断裂失效,对此开展了分析。首先利用三维体视显微镜与扫描电镜进行了断口观察与分析,表明管线的开裂起源于焊缝根部,具有典型的多源起源特征;裂纹源区具有蓝色半圆形特征,表明存在焊接裂纹。同时,断口呈现明显的疲劳辉纹,间距为亚微米量级。根据辉纹间距与管材壁厚以及使用寿命分析,确定了疲劳载荷的频率范围;根据理论公式与有限元模态分析对管线的自振频率进行了估算,其结果与断口分析的结果吻合良好。以上研究结果表明管线断裂是典型的振动疲劳失效导致的。为此,建议在管线中部增加固定点约束,改变管线的自振频率以降低振幅,并改进焊接工艺,提高管线焊接质量。基于以上措施,有效解决了管线的早期断裂失效问题。

During the combustion testing of the scramjet engine, early failure of pipes may occur, due to the presence of fractures followed by catching fire 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 analysismentioned above, a few fix 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 effectively avoided.