Weld Crack Propagation Model Considering The Three-dimensional Constraints And Transition Of Stress/Strain State For TBM Cutter Mousing

In the application of TBM to tunnel excavation, Cutter Mousing is the most vulnerable structure on TBM Cutter-head. The most common failure modes of Cutter Mousing are cracks along the weld toe and deformation due to overload, and these destructions will finally lead to Cutter Mousing failure. On the other hand, the failure of Cutter Mousing would have impact on the mounting accuracy and work condition of Cutter, thus affect the mechanical characteristics of Cutter and accelerate its failure process and finally the excavation efficiency would be influenced. Therefore, the research on the weld fatigue life prediction and structure optimization of Cutter Mousing is of great significance.This paper conducted a series of study in terms of structure optimization and weld fatigue life prediction of Cutter Mousing on TBM Cutter-head.Firstly, due to the super thickness of TBM Cutter-head plate, the plain stress-based small time scale fatigue crack growth model was no more applicable, so a plastic constraint factor a is introduced to change the yield stress value of material to simulate the transition of the stress state at the crack tip from plain stress to plain strain. According to fatigue tests of two test specimens with different plate thicknesses, the relation equation between a and plate thickness t is acquired as:?=0.73t-0.943. Therefore, weld crack growth model considering the three-dimensional constraints and the transition of stress/strain state that can be applied to thick plate is proposed. What's more, the influence of residual stress on stress characteristic in weld area is also considered, so the model was further modified and can be applied to weld crack propagation life prediction.Secondly, conducted several fatigue crack propagation tests on C(T) weld test pieces and recorded the data of crack length a0-load cycle N. The weld fatigue life of the C(T) test piece was also predicted by using the modified model. The comparison of the test data and the theoretical prediction showed that the test data was within 0.5-2 times of the prediction result. The result was acceptable and thus demonstrated that the modified model is effective.Thirdly, according to the structure and load of TBM Cutter Mousing in real project and the common occurrence of crack in weld, the structure of current Cutter Mousing was improved by changing the face fillet weld into butt weld. In order to gain the simulation time, simplified models representing the structure of two types of Cutter Mousing were designed and their residual stress was simulated and compared with each other. The result showed that the new structure is more beneficial for the fatigue performance.At last, predicted the weld crack propagation life of reduced scale TBM Cutter Mousing by applying the proposed model. Then the life of TBM Cutter Mousing in real project was deduced according to the theory of similarity. The result showed that the Cutter Mousing life of present welding structure is 6.04 km and that of the new welding structure is 7.74km, thus it's 28% longer compared to the life of current structure. In general, fatigue property of the new welded structure is greatly improved.