Fatigue Crack Growth in the Base Metal and Weld of the Combustion Chamber Casing of an Aircraft Gas-Turbine Engine

Abstract

To substantiate the serviceability of aircraft gas turbine engines on the basis of technical condition, it is necessary to have information on the rate of crack growth in the material of engine components from the initial to critical dimensions. According to normative documents, this information is used in fixing the time limits for and amount of scheduled maintenance. The rate of fatigue crack growth in the high-temperature alloy of the combustion chamber of an aircraft gas turbine engine was studied taking into account operational (temperature) and constructional (weld, thickness of the product), factors. The experiments were performed on flat rectangular specimens with an edge and a central crack by the standard procedure at test temperatures of 500 and 600°C. The fatigue crack growth rate was studied in the base metal, weld and in the heat-affected zone at a distance of 2–3 mm from the weld. To do this, a fatigue crack was initiated from a mechanically cut notch at the appropriate site relative to the weld: in the base metal, weld and in the heat-affected zone. A linear section of a fatigue fracture diagram has been constructed, and Paris equation coefficients have been obtained. Confidence intervals are given, which illustrate the area within which the experimental results fall with a probability of 95%. A statistical treatment of experimental data in terms of the kinetics of fatigue crack growth in the heat-affected zone and in the base metal showed them to differ only slightly, whereas the rate of fatigue crack growth in the weld increases by a factor of two or three. To estimate the change in the mechanical properties of the alloy under investigation on transition from the base metal through the heat-affected zone to the weld, Rockwell tests were carried out. The results showed a small change in hardness, which indirectly accounts for the small discrepancy (within the statistical error) between the rate of crack growth in the base metal and that in the heat-affected zone.