Guided waves (e.g., Lamb waves) have been considered for many structural health monitoring (SHM) applications because of their ability to travel long distances and maintain sensitivity to damage. One conventional approach to detect damage is to compare in situ signals to baselines recorded from the undamaged structure. By comparing current signals to damage-free baselines, signal changes caused by structural damage can be tracked. Such methods can handle some structural complexity, but have unwanted sensitivity to variations in environmental and operational conditions (e.g., temperatures and loads).

The Aluminum Plate Specimen with attached PZT transducers. Note that the uniaxial loads were applied in the vertical direction.
This work considers varying applied tensile static loads such as those that arise during normal operation of a structure. The effects of such loads on propagation of both bulk and guided ultrasonic waves in homogeneous media are generally well understood. Even though the effects of applied loads may be unavoidable in the in situ environment, and significantly affect the ultrasonic signals by changing both structural dimensions and wave speeds, applied loads can also improve damage detectability when the tensile load is large enough to open a tight crack.

Load-differential imaging generates a series of images from differences in sparse array signals caused by small static loading variations. The efficacy of the proposed method in detecting and locating fatigue cracks is demonstrated from fatigue tests of an aluminum plate having six surface-bonded piezoelectric discs.

Data analysis is a two-step process consisting of chirp filtering followed by imaging. Lamb waves in a plate may be generated by a linear chirp source, where the frequency is swept from a minimum value to a maximum value over a fixed time interval. The imaging method used here is based upon the signal changes between two measurements, and is thus a differential method.

An aluminum plate specimen was instrumented with an array of six piezoelectric discs and subjected to cyclic loading to investigate loading effects on guided wave propagation. A 6061 aluminum plate of 305 × 610 × 3.18 mm was machined to enable mounting in an MTS machine. The transducers were fabricated from 7-mm-diameter, 300-kHz, radialmode PZT discs that were attached to the plate with epoxy and further protected with a backing of bubble-filled epoxy.

An arbitrary waveform generator applied a ±10V, 50−500 kHz linear chirp excitation to the transducers, and signals were digitized with a 20-MHz sampling rate and a 14-bit resolution. Twenty waveforms were averaged for each acquisition to improve the signal-to-noise ratio. The specimen was fatigued with a 3-Hz sinusoidal tension-tension loading profile from 16.5 to 165 MPa. Fatiguing was periodically paused to record ultrasonic data as a function of applied tensile load from 0 MPa to 115 MPa in steps of 11.5 MPa, resulting in a total of 11 static loading conditions for each data set. After the first dataset was recorded from the pristine plate (i.e. before fatiguing), a 5.1- mm-diameter through hole was drilled in the center of the specimen. A small starter notch was subsequently made on one side of the hole to act as a site for initiation of a fatigue crack. Data were recorded where crack lengths were measured with a scale under an applied load. Fatiguing was continued until the largest crack reached about 25 mm in length.

As expected, when there is no damage, all the signals are similar, and the corresponding differential signals are similar as well even though the applied loads increase. Even when there are fatigue cracks, the raw signals look similar. However, there is an initial large change in the first arrival of the first differential signal as the crack on one side of the hole opens up and blocks the direct wave. At about 70% load, it appears that the crack on the other side of the hole opens up.

The results of the tests motivate a new approach to imaging of fatigue cracks referred to as the load-differential imaging method. In this method, the “baseline signals” are recorded at one load, and the “current signals” are recorded at the same damage state, but at a slightly increased tensile load (10% increment of the maximum load in this study). The difference between the signals is thus caused by a combination of crack opening effects and loading effects for certain loading combinations. The proposed load-differential imaging method has the potential to detect multiple cracks from the loaddependent behavior of crack opening.

This study has proposed and demonstrated a load-differential imaging method for detecting and locating fatigue cracks via guided waves. The measured ultrasonic signals show that an applied tensile load can open a crack, where the loading amplitude depends on its size. The images generated from the load-differential signals clearly show the initiation and progression of fatigue crack growth without using any previously obtained damagefree baseline signals. Also, load-differential imaging has the potential for imaging multiple cracks that have different load responses.

This work was done by Sang Jun Lee, Jennifer E. Michaels, Xin Chen, and Thomas E. Michaels of the Georgia Institute of Technology for the Air Force Research Laboratory. AFRL-0210

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Fatigue Crack Detection Via Load-Differential Guided Wave Methods

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