The Simulation Of Crack Tip Damage Evolution And Crack Propagation Under The Three-dimensional Constraint

Fatigue and fracture issues are critical to the development of lighter, safer and more efficient aerospace vehicles. In aerospace engineering, structure is considered to be damage tolerant which reduces the residual strength of the structure below an acceptable limit. Damage tolerance is a property of a structure relating to its ability to sustain defects safely until repair can be effected. By assuming the damaged condition of the structure, durability and damage tolerance analysis techniques ensure the safety of the aircraft. At present, damage tolerance evaluations are still based on two-dimensional fracture theory in engineering. However, there are evident three-dimensional effects in real structures due to the influence from complex structure and loading conditions. In this thesis, a series of studies are carried out to analyze the effect of constraint on the fracture behavior and fracture toughness of materials and structures. The main work and results are as following:1) For elastic material, the distribution of Tz in front of crack tip is only relevant to Poisson ratio and thickness size, it has nothing to do with load and geometry size, Tz-r/B curve has a high degree of unity. For elastic-plastic materials, the distribution of Tz is dependent on the crack tip yield condition. When the thickness and geometry size is different and the constraint of crack tip is different, the deformation of crack tip is different. So the constraint of crack tip is not only related to the thickness size right now, but also associated with distal load and geometry size. Tz is sensitive for crack tip deformation.2) In order to further analysis of crack tip of three dimensional constraint function, this paper quantitatively analyzed the out-plane constraint factors of Tz distribution, a revised forecast formula can better describe the Tz distribution of the elastic deformation.3) The different thickness of single edge crack sample under three point bending deformation have been simulated to analyse the constraints effect on the process of crack propagation. The results indicated that the deformation of crack tip is intensively depended on the geometric constraints and this constraint could be characterized by Tz. The double parameters J-Tz theory can be a very good description of the stress field of wide scale yield after crack tip extented.