Fatigue remaining life prediction of high pressure turbine casing with unacceptable defects

Abstract

Remaining life of a high-pressure turbine (HPT) casing of aircraft turbofan engine with cracks was determined. Cracks were detected during regular inspection and found out to be initiated by scratches made by non-skilled use of Electrical Discharge Machining (EDM). In total, 9 visible surface marks were made. Keeping in mind that this component is not repairable, and its replacement might be time-consuming and/or too expensive, it was essential to evaluate the remaining life and prevent failure if this component was to be used with cracks. Toward this aim, numerical methods had to be used, since testing of samples or component itself was not possible due to geometry restrictions. To get a reliable estimation of the remaining life, it was necessary to evaluate Paris coefficients C and m. Use of available data for fatigue crack growth was not an option, since coefficients are determined for virgin material and could not provide acceptable predictions for the used material. To overcome these difficulties, an innovative technique in two steps, based on finite element method, has been introduced. First step was to simulate fatigue crack growth. Then, response surface optimization (RSO) was used to obtain Paris’ coefficients that predict accurately the number of loading cycles to reach detected crack lengths, provided by the maintenance, repair, and operations (MRO) company’s workshop. Coefficients C and m, determined in this way, were then used to evaluate remaining life of the HPT casing and estimate the risk of its further use.