Evaluation, modification and damage tolerance of an inservice aircraft critical area

Abstract

In-service component damage on the wing-fold sheartie of a Canadian Forces (CF) fighter aircraft, led to a full scale localized fatigue test of a critical area using loads derived from inservice usage. The aim was to generate a crack in the test article, at the correct location and in a similar manner to that found on inservice aircraft inner wings. Then the structure was modified to the repair configuration and fatigue testing of the modified component was carried out to define the life of the modification. The project scope required: generation of nucleated cracking damage to a size typically found in the field with substantiation via non-destructive inspection; excision of the in-service premodification crack; introduction of the planned in-service hand blended repair to the test article; measurement of both surface strain and the altered geometry in the blend region; and use of quantitative fractography to develop the test crack growth curves for pre-modification and post-modification. Testing applied the equivalent of seven life times of simulated usage in the two phases of the fatigue testing. This included the required factor of five lifetimes, post-modification, for an unmonitored dynamically loaded component. The applied dynamic spectrum alternated between blocks of loads for a tip mounted store with fins-off and fins-on. By the end of the test a significant size crack had developed. To complete the test, a residual strength test was carried out to failure. Post-test quantitative fractography was used to obtain the crack growth rate information. This data and the residual strength test failure load were then used to generate a residual strength curve. Based on the loading spectrum, a Gumble extrapolation was carried out to establish the once in a lifetime design limit load. This was then scaled to obtain the residual strength, end of life, 120% load and an end of life critical crack size was estimated. The client and the repair and overhaul contractor utilized this test and the derived data to establish fleet repairs, scheduling and inspection requirements for the modified structure. © National Research Council Canada 2009.