In order to predict more accurately the cracking failure of cylinder head during the durability test of turbocharged engine in the development, a comprehensive evaluation method of cylinder head durability is established. In this method, both high cycle and low cycle fatigue performance are calculated to provide failure assessment. The method is then applied to investigate the root cause of cracking of cylinder head and assess design optimizations. Multidisciplinary approach is adopted to optimize high cycle fatigue and low cycle fatigue performance simultaneously to achieve the best comprehensive performance.
In this paper, the details of the method development are described. First, the high cycle and low cycle fatigue properties of cylinder head material were measured at different temperature condition, and the fatigue life and high temperature creep properties of materials under thermo-mechanical fatigue cycle were also tested. These material properties provide basis for accurate simulations. For the low cycle fatigue analysis model, a thermal shock cycle same as bench test is simulated using transient method, which accurately reflects the stress and strain of the cylinder head during the alternating process of cooling and heating. The effect of creep, cyclic hardening and softening of the material properties at high temperature are included in the model due to using real material properties.
The simulation results show that the high cycle fatigue safety factor and thermo-mechanical fatigue life are lower than the guideline at the exhaust side of the water jacket. This high-risk location coincides with the test failure location. Through local structural shape optimization of the water jacket, all assessment indexes meet the design requirements. The cylinder head with optimized design passed all durability test evaluation.