Stop-hole improvement effect on fatigue life of Ti-6Al-4V and 7075-T6 Al alloys

When a crack is generated in structure during the service life, the crack can be arrested by some methods before final failure. So, crack retardation methods are so effective for structural components that cannot be replaced as soon as the cracks are observed and when the replacement of new parts is time consuming and costly. One of the crack retardation methods is the method of drilling holes close to the crack tip. This method is the easiest and most accessible crack arresting method. It can be divided into three main categories: The crack tip holes (CTH), the deflecting holes, drilling two crack flank holes (CFH) symmetric relative to the crack line. In this thesis, CTH method is investigated in terms of its effect on fatigue life for 7075-T6 Al and Ti-6Al-4V alloys. The effect of CTH method on fatigue life is simulated by tracking the procedure in chapter 3.1 for FCG life and in chapter 3.2 for crack initiation life in a detail. Firstly, total FCG life is calculated in a case of no stop-hole by extracting SIF at the crack tip with the help of fatigue code. Then, a stop hole is drilled at a certain crack length and crack initiation life until a new crack initiates at stop-hole is calculated by taking stress concentration factor from FEM. For calculating FCG life, a fatigue code is developed by using python programming language, SIF at crack tip taken by running fatigue code in Abaqus and SIF is used to calculate FCG life in Excel. After FCG life and crack initiation life are simulated, they are validated by using experimental data which is found from literature in terms of the direction of crack propagation, crack initiation life at the stop-hole and FCG life. The results found by code fit fairly well with the experimental data which is found from literature. After code is validated, the comparison about stop-hole effect on 7075-T6 Al and Ti-6Al-4V alloy is done. As a result, stop-hole improve greatly fatigue life and Ti-6Al-4V alloy is fairly better than 7075-T6 Al alloy in terms of fatigue life. The magnitude of stop-hole improvement on fatigue life for Ti-6Al-4V alloy is quite better than that for 7075-T6 Al alloy. Furthermore, the effect of cold expansion at stop-hole is examined for 7075-T6 Al and Ti-6Al-4V alloys. Cold expansion process creates residual compressive circumferential stress at stop-hole and this compressive stress diminishes the tensile stress due to operational load so it improves greatly crack initiation life. As a result, crack initiation life for 7075-T6 Al is improved greatly than that for Ti-6Al-4V since plastic deformation for 7075-T6 Al is greater than that for Ti-6Al-4V when stop-hole is expanded in a same ratio for both alloys.