An Application Of Technique Of Surface Self-nanocrystallization In The Design Of Anti-fracturing And Anti-fatiguing Structures

In the engineering materials, the initiation and propagation of crack can cause structural fatigue under machining process and environmental conditions. The mechanical properties of the materials can be changed by technique of surface self-nanocrystallization. such as the increase of the elastic modulus, improving of the yield stress and the strength limit of metal materials and so on. In this paper,316L steel is taken as an example. Using the knowledge of fracture mechanics and the performance parameters of the new material obtained by the technology of surface self-nanocrystallization, the design of anti-fracturing and anti-fatiguing structures is presented.The following contents are included in this paper:(1) A design technology and a numerical analysis are proposed for anti-fracturing of two-dimensional plane strain cracks. Considering area's shape and size of the surface nanomaterial nearby the crack tip as the design variables, the J-integral of the crack tip as the objective functions, and the other factors as constraint conditions, including boundary conditions and loading modes. Based on the analysis results, the J-integral of the crack tip decreases with the increasing of the surface nanomaterial area, and verges to stabilization when the size of area reaches a certain degree. When the size of area maintains a certain value, four area shapes including heart-shaped, rectangle, elliptical and rhombus are studied. The results show that, the J-integral decrease degree varies from high to low is:heart-shaped, rectangle, elliptical, rhombus. This suggests that the surface nanomaterials can be used in designing of fracture resistance for cracked metal plate in plane strain state.(2) A new surface self-nanocrystallization method is proposed for anti-fracturing spatial plate structure with cracks. Based on the conclusions of two-dimensional plane strain problem, the 3-dimensional through-wall crack models are established by dividing the structure into three layers. By designing the area's shape and size of the surface nanomaterial on the two surface layers, J-integral of the tip is calculated. Results show that the conclusions are consistent with plane strain issue. In addition, it is also studied that the influence on the J integral caused by different surface nanomaterial positions on the surface layer, results indicate that the J-integral increases with a farther position. Therefore, the surface nanocrystallization area should be designed near the crack tip.(3) A method is proposed for improving the fatigue life of structures including U-notches. Based on the change of material fatigue performance parameters, a 3-d finite model including U-notches is established, and the influence of Surface nanomaterial on fatigue life of structures subjected to cyclic loading is studied. The result shows that surface self-nanocrystallization can obviously improve the fatigue life of the structures.