气缸盖蠕变-疲劳寿命预测

Creep-Thermal Fatigue Life Prediction of Cylinder Head

气缸盖在工作中受到低周热疲劳损伤、高周热疲劳损伤和蠕变损伤,其寿命和可靠性是发动机的重要指标。用热-结构顺序耦合分析方法计算了气缸盖的温度场和应力场,分析了气缸盖上危险点在10个启动-工作-停车循环的弹性应变、塑性应变和蠕变应变,从理论上证明了影响气缸盖寿命的主要因素是低周热疲劳损伤,启动次数是其最主要的寿命指标。蠕变不影响低周热疲劳的应力幅,只是使循环中的平均应力增加。高温下松弛与蠕变同时发生,降低了平均应力的增长速度。经历有限个循环,平均应力基本稳定。因此,把发动机气缸盖的蠕变-低周热疲劳等效为一定应力幅和平均应力的热-机械疲劳,并用标准试样的热-机械疲劳试验预测了发动机气缸盖的使用寿命(可承受的最大启动次数)。

Cylinder head is damaged by low cycle thermal fatigue, high cycle thermal fatigue and creep in operation. Thus, the life and reliability of the cylinder head are important criteria for engines. The temperature field and stress field of the cylinder head are computed by the sequentially coupled analysis method ： the elastic strain, plastic strain and creep strain of the dangerous point on the cylinder head are analyzed in ten startup-working-shut down cycles; and it is proved in theory that low cycle thermal fatigue is the primary factor affecting the service life of the cylinder head and the startup time of the engine is an appropriate parameter to estimate the life of the cylinder head. The creep has little influence on the stress amplitude, and makes the mean stress increased. The relaxation is coexisting with creep in high temperature; it reduces the increase speed of mean stress. After finite cycles, the mean stress tends to constant gradually. Therefore, the creep-fatigue interaction can equal to thermo-mechanical fatigue whose stress amplitude and mean stress are constant. At last, the mean service life of cylinder is predicted