某直升机的发动机主喷管在例行检查中发现多件主喷管出现裂纹，裂纹位置主要集中在主喷管加强边处和法兰边螺钉孔边缘。通过宏观观察、断口宏观分析、断口微观分析、金相检验、法兰边裂纹源周围特征观察、法兰边平面度核查及加强边焊接质量检验等方法对主喷管开裂原因进行了分析。结果表明：主喷管上的裂纹均为疲劳裂纹。法兰边裂纹起源于螺钉孔正面挤压痕迹处，该部位由于使用环境恶劣（高温、气流颤震），法兰边连接孔单面安装垫片无法承受震动载荷而形成疲劳裂纹。加强边裂纹起源于端头对接熔合点处及附近，该部位电阻点焊与熔焊并存，热影响区材料性能下降，加强边端头对接焊缝位于震动最大的长边，最终形成疲劳裂纹。

During the routine inspection of the main nozzle of a helicopter engine, cracks were found in many parts of the main nozzle. The cracks were mainly located at the reinforced edge and the edge of the screw hole on the flange edge of the main nozzle. The cracking causes of the main nozzle were analyzed by means of macro observation, macro analysis of fracture, micro analysis of fracture, metallographic examination, observation the characteristics around the crack source at flange edge, flatness verification of the flange edge and welding quality test of the reinforced edge. The results show that the cracks on the main nozzle were fatigue cracks. The flange edge cracks originated from the extrusion trace on the front of the screw hole. Due to the poor service environment (high temperature and air flow vibration), the single-sided gasket of the flange edge connection holes could not bear the vibration load and formed the fatigue cracks. The cracks in the reinforced edge originated from the fusion point of the end butt joint. The resistance spot welding and fusion welding coexist in this part, and the material properties of the heat affected zone decreased. The butt weld at the end of the reinforced edge was located in the long side with the largest vibration, and the fatigue cracks were finally formed.