This paper was produced for the 2019 NAFEMS World Congress in Quebec Canada

Resource Abstract

Inspection, modification, overhaul and replacement times for critical structural parts are adapted according to operational usage of each aircraft within the scope of Individual Aircraft Tracking (IAT) Program. It is performed to estimate crack propagation in appropriate environment as a function of the measured usage and to determine variations in actual usage. The effective flight hours are estimated and the required maintenance schedule for the critical locations on each individual aircraft is adjusted. The IAT Program predicts when aircraft structural part life limits will be reached.



Reference points are the locations typically at wing, fuselage, vertical stabilizer and horizontal stabilizer where strain gages are installed. Moreover, airframe structural control points are the critical, life-limited points specified for tracking potential crack growth. Flight data are processed to generate data output for implementing fatigue monitoring at the control points within the scope of IAT activities.



In this study, approach for derivation of transfer function to relate strain data to control point stresses are presented. In order to implement fatigue monitoring, stress spectra at each control point are predicted by utilizing strain survey data of reference points. For this purpose, correlation between the reference point and control point stresses are expressed as transfer functions to obtain stress history at the control point.



Depending on location of the control point, related reference points are taken into account. Strain data at the reference points are evaluated to clarify if there are reasonable relations to control point stress values. Corresponding transfer function alternatives are derived at each control point for the reference points. Correlation coefficients obtained for the transfer function alternatives are compared and the one having the least scatter is selected.



Fatigue crack initiation life at each control point is estimated using the local strain-based approach for selected transfer function and strain data by using baseline design spectrum. Notch stress value to be used for the transfer function is scaled to obtain the same crack initiation life and spectrum severity at the control point with the one obtained from strain survey. Knowing the crack initiation life, hence the fatigue damage, unique fatigue index value at the control points is estimated for each aircraft. Effective flight hour (EFH) is then determined by using the ratio of fatigue index values for design and actual usage data so that crack length at the EFH is predicted by utilizing crack growth analysis results of the control point. Knowing the predicted fatigue life expended per service usage of individual aircraft, inspection threshold and intervals for the control points are adjusted accordingly.