An ultrasonic method for dynamic monitoring of fatigue crack initiation and growth

Abstract

Attached ultrasonic sensors can detect changes caused by crack initiation and growth if the wave path is directed through the area of critical crack formation. Dynamics of cracks opening and closing under load cause nonlinear modulation of received ultrasonic signals, enabling small cracks to be detected by stationary sensors. A methodology is presented based upon the behavior of ultrasonic signals versus applied load to detect and monitor formation and growth of cracks originating from fastener holes. Shear wave angle beam transducers operating in through transmission mode are mounted on either side of the hole such that the transmitted wave travels through the area of expected cracking. Time shift is linear with respect to load, and is well explained by path changes due to strain combined with wave speed changes due to acoustoelasticity. During subsequent in situ monitoring with unknown loads, the measured time of flight is used to estimate the load, and behavior of the received energy as a function of load is the basis for crack detection. Results are presented from low cycle fatigue tests of several aluminum specimens and illustrate the efficacy of the method in both determining the applied load and monitoring crack initiation and growth.

Size Estimation of Fatigue Crack Appearing at Bolt Joints of Aluminum Alloy Plates by Synchronized SAW Measurement

“…The ultrasonic monitoring technique has been developed to detect the initiation and expansion of the fatigue crack in the fatigue testing [4][5][6][7][8][9][10]. In most of these monitoring techniques, data were recorded under pausing the fatigue tester [4,7,8,10,11], and these measurements can be called as stationary ultrasonic measurements.…”

“…The ultrasonic data obtained in the previous studies with the stationary measurements were either at the maximum loads [5,10], the mean load [4,11], the minimum load [5] or the random loads [8] in the fatigue testing. In these studies, however, behaviors of the ultrasonic wave at any stress levels in a fatigue cycle were not examined precisely.…”

“…In most of these monitoring techniques, data were recorded under pausing the fatigue tester [4,7,8,10,11], and these measurements can be called as stationary ultrasonic measurements. Authors [11] also carried out the stationary ultrasonic measurement of the bolted specimen during the fatigue testing with the surface acoustic wave (SAW), and they showed that the SAW scattered in the fretted region ahead of the bolt hole gradually increased with increasing number of fatigue cycles, and then steeply increased at a cycle when the fatigue crack might appear.…”

Nonlinear acoustic resonances to probe a threaded interface

“…More generally, nonlinear acoustics offers sensitive techniques to detect an isolated and localized microcrack, 20 as well as to evaluate the global quantity of microdamage in materials such as rock, 21,22 nickel, 23 concrete, 24-26 wood, 27 bone, 28,29 etc. These techniques are primarily based on harmonic generation, [30][31][32] frequency mixing, 33-35 acoustoelasticity, 36,37 or shift in the resonance frequency. 26,28,38,39 The latter provides the means to extract nonlinear elastic and dissipative parameters, associated to changes in the resonance frequency and damping with level of excitation, respectively.…”

Influence of nonlinear effects on the acoustic levitation of small spheres

“…The use of this approach is promising to measure and to monitor the strain or stress states in plates. Single mode guided waves [5,6] and simple angled beam reflection in plates [7] are used for this application.…”