An ultrasonic technique to monitor fatigue damage in aircraft structures.

Abstract

A procedure is proposed for the development of a practical and reliable nondestructive technique toward the in situ monitoring within aircraft structures of fatigue damage, prior to and during propagation of running fatigue cracks. The technique utilizes precalibrated ultrasonic surface wave signal strength changes and can monitor both areas of uniform stress and points of stress concentration. Multiple sensor installations can be effectively monitored by a single portable instrument obtaining both manual readout and permanent oscillograph records, easily interpretable. The procedure is applicable to either continuous in-flight readout or intermittent readout during periodic ground maintenance checkouts. The requirements for establishing calibration data for both laboratory and field testing are presented along with a discussion of limitations in the technique. A proposed basis for interpretation of the ultrasonic data using a simple analytical cumulative damage model also is presented.