Fatigue Crack Detection Via Load-Differential Guided Wave Methods

Load-differential imaging detects multiple fatigue cracks that have different load responses.

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).

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.