Self-calibrating Ultrasonic Methods for In-situ Monitoring of Fatigue Crack Progression

Ultrasonic sensors permanently affixed to aluminum coupons are used to monitor progression of damage during fatigue testing with the long term goal of structural health monitoring for diagnostics and prognostics. Necessary for success are proper design of the ultrasonic testing methods, robust transducer mounting techniques, and real-time signal processing for determining the state of the structure. It is also highly desirable for the overall system to be self-calibrating with built-in diagnostics in order to detect and compensate for sensor degradation or failure. Selfcalibrating ultrasonic techniques are applied for monitoring of cracks initiating and propagating from the inaccessible inner diameters of rivet holes where the transducers are mounted on the accessible specimen surface. Angle beam ultrasonic methods are utilized that are suitable for detecting small defects in critical local regions of high stress. Results are presented for aluminum coupons subjected to low cycle fatigue and demonstrate ultrasonic tracking of crack growth. INTRODUCTION Techniques for in-situ monitoring of structural integrity are of considerable interest for critical aerospace and civil structures. Many methods have been proposed for structural health monitoring and can be grouped into two categories: global and local. A global test is one that is sensitive to a large area of the structure, whereas a local test is sensitive only to a specific localized region. Vibration modal analysis [1] [2], diffuse ultrasonic waves [3] and guided ultrasonic waves [4] are examples of global methods, whereas strain gages and conventional ultrasonic inspection techniques are examples of local methods. The protocol for many critical components is to retire the component after its calculated service life is exceeded, which is usually overly conservative in that many good parts are prematurely retired. Condition based maintenance in which components are assessed and repaired or retired only when necessary is an alternative approach with the benefit of keeping good parts in service until their true useful life is over. For many structures the critical “hot spots” are known; that is, the high stress areas which are suspectable to cracking during cyclic fatigue loading. There are two distinct but complementary scenarios for assessing these critical areas of a structural component: (a) perform visual or other nondestructive inspections to certify the structure for continued service, or (b) use in-situ measurements to continually monitor these areas in order to reduce the need for periodic inspections or maintenance actions. This paper addresses the latter case in which in-situ measurements are made using permanently attached ultrasonic transducers in order to detect the initiation and growth of fatigue cracks during cyclic loading.