Open Access

ARTICLE

Damage Detection in a Geometrically Constrained Area¹

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1 The views expressed in this article are those of the authors and do not reflect the official policy or position of the Air Force, Department of Defense or the U.S. Government
2 Department of Aeronautics and Astronautics, U.S. Air Force Institute of Technology, WrightPatterson Air Force Base, OH, USA.
3 Department of Systems Engineering and Management, U.S. Air Force Institute of Technology, Wright-Patterson Air Force Base, OH, USA.

Structural Longevity 2009, 1(2), 95-110. https://doi.org/10.3970/sl.2009.001.095

Abstract

A "hot-spot" structural health monitoring (SHM) approach that uses lead zirconate titanate (PZT) sensor pairs to generate and sense Lamb waves is evaluated on a test article that accurately represents the complex geometry of a relatively inaccessible bulkhead section of an existing aircraft. This work is motivated by the fact that fatigue cracks have been known to propagate in this particular bulkhead in several aircraft. In order to simulate damage, electrical discharge machine (EDM) cuts are made to simulate real cracks in a test article. Because the damage occurs in a region of restricted geometry, PZT sensors must be placed in close proximity to each other, but on opposing sides of the expected crack. The close proximity of the piezoelectric sensors and restricted geometry create challenges in determining which portion of the collected response should be analyzed. The first approach evaluated is a tuning approach where specific excitation frequencies are chosen based on the relative S₀ and A₀ Lamb wave mode amplitudes. Theoretical time of arrival window lengths for the S₀ and A₀ Lamb wave modes are reduced in size to minimize the contribution of reflected wave energy. The second approach involves evaluating responses over a wide range of excitation frequencies in combination with only evaluating the responses in the dominant mode’s theoretical time of arrival window. A simple damage metric is applied to demonstrate that the presence of cracks can be detected as well as the length can be roughly approximated. This approach is further validated with laser vibrometery scans of the test article in both healthy and damaged states.

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