Investigation of fatigue life in superalloys structural components

The researches in the field of fatigue failure are mainly aimed at identifying the factors which determine the fatigue behavior of the structural components and their inter relations. It is known that the fatigue resistance of structures is influenced mostly by load sequences, combinations of load types, as well as by materials and their fatigue properties. Therefore, the fatigue life cannot be determined without a thorough analysis of these parameters. Materials known as superalloys exhibit excellent mechanical strength and considerable creep resistance, especially at high temperatures. However, a critical property of these alloys is their resistance to fatigue-crack propagation, particularly at service temperatures. Besides, their fatigue features are not as easily accessible as of other materials (aluminum or steel, for instance). Consequently, determining the fatigue life in superalloy structures is not an easy task to accomplish. Taking into consideration the superalloy properties set in the NASGRO database, this thesis has explored the fatigue life of the real structural components, considering – at the same time – the stresses of both constant and variable amplitudes. The structural components have been designed in the CATIA v5, whereas the propagations of the 2D and 3D fatigue cracks through the structure have been simulated afterwards by the application of the FEM in the FRANC2D/L, and the application of the extended finite element method (XFEM) in the Abaqus software. Special attention has been devoted to the fatigue analysis of the spar of a light aircraft. Numerical methods have been used to identify the weak points on the spar, along with the fatigue life of a spar crack and its direction of propagation. The results gained have corresponded well with the experimental values obtained on the spar of the alloy 2024-T3. Taking into account the good correlation between numerical and experimental values, the same finite element models have been used to estimate the crack life on the spars made of super alloys. The investigations have shown that the majority of the super alloys possess fatigue properties similar to aluminum, and in a few of them the life of fatigue crack has proved to be much longer than in the 2024-T3 alloy