Evaluation of the Crack Growth Gage Concept as an Individual Aircraft Tracking Device. Volume 2

This report summarizes research conducted to develop the crack growth gage technique for individual aircraft tracking. The crack growth gage concept consists of mounting precracked coupons on a structural component so that service loads cause measurable crack extension in the coupon or gage. Fracture mechanics techniques are then used to relate the gage crack growth with extension of an assumed structural flaw. Thus, the precracked coupon serves as a sensor which records service loads and responds with an output which may be related to the service life of the parent structural member. The effort described here involved two primary goals development of a thin crack gage coupon which simulates thick section flaw growth, and development of an improved analysis method for relating gage and assumed structural crack growth. The approach employed for the first task was to design a crack gage which contained side-grooves along the crack plane. The side-grooves forced plane strain conditions in the thin coupon, and provided fatigue crack retardation behavior which more nearly matched that seen in the thicker structural member. A series of experiments conducted with the side-grooved gage geometry compared favorably with numerical predictions for the gage response.