The intergranular crack propagation of the lamellar structure $\beta$ titanium alloys is investigated by using a modified Gurson-type damage model. The representative microstructure of the lamellar alloy, which consists of the soft $\alpha$ phase layer surrounding the hard grain interiors, is generated based on an advanced Voronoi algorithm. Both the normal fracture due to void growth and the shear fracture associated with void shearing are considered for the grain boundary $\alpha$ layer. The individual phase properties are determined according to the experimental nanoindentation result and the macroscopic stress–strain curve from a uni-axial tensile test. The effects of the strain hardening exponent of the grain interiors and the void shearing mechanism of the grain boundary $\alpha$ layer on fracture toughness and the intergranular crack growth behavior are emphatically studied. The computational predictions indicate that fracture toughness can be increased with increasing the strain hardening ability of the grain interiors and void shearing can be deleterious to fracture toughness. Based on the current simulation technique, qualitative understanding of relationships between the individual phase features and the fracture toughness of the lamellar alloys can be obtained, which provides useful suggestions to the heat treatment process of the $\beta$ titanium alloys.

A cyclic cohesive zone model is applied to characterize the fatigue crack growth behavior of a IN718 superalloy which is frequently used in aerospace components. In order to improve the limitation of fracture mechanics-based models, besides the predictions of the moderate fatigue crack growth rates at the Paris’ regime and the high fatigue crack growth rates at the high stress intensity factor ΔK levels, the present work is also aimed at simulating the material damage uniformly and examining the influence of the cohesive model parameters on fatigue crack growth systematically. The gradual loss of the stress-bearing ability of the material is considered through the degradation of a novel cohesive envelope. The experimental data of cracked specimens are used to validate the simulation result. Based on the reasonable estimation for the model parameters, the fatigue crack growth from moderate to high ΔK levels can be reproduced under the small-scale yielding condition, which is in fair agreement with the experimental results.

Aiming at the valve operating parameters optimization of electro-hydraulic variable valve engine(EHVV), a one-dimensional simulation model based on WAVE was established, and the effects of EHVV system on the engine performance were studied. Then, a variable valve operating parameters optimization algorithm based on genetic algorithm was proposed. The HC emission indexed penalty function was designed to ensure the optimal power and economy performances under the proper emission constraints. Based on Matlab/Simulink and Ricardo WAVE co-simulation platform, detailed algorithm implementation was carried out. The variable valve operating parameters optimization under full load under at different engine speeds were carried out, and the effects of different penalty functions on optimization results were studied. The simulation results show that, using the optimal parameters, under the same emission performance as the original engine, the overall average fuel consumption decreased by 2.1% and the torque increased by 8.5%. Especially at low and medium speeds, the improvement of power performance is more obvious.