Fan beam and double crosshole Lamb wave tomography for mapping flaws in aging aircraft structures

As the worldwide aviation fleet continues to age, methods for accurately pr edicting the presence of structural flaws-such as hidden corrosion and disb onds-that compromise airworthiness become increasingly necessary. Ultrasoni c guided waves, Lamb waves, allow large sections of aircraft structures to be rapidly inspected. However, extracting quantitative information from Lam b wave data has always involved highly trained personnel with a detailed kn owledge of mechanical waveguide physics. The work summarized here focuses o n a variety of different tomographic reconstruction techniques to graphical ly represent the Lamb wave data in quantitative maps that can be easily int erpreted by technicians. Because the velocity of Lamb waves depends on thic kness, for example, the traveltimes of the fundamental Lamb modes can be co nverted into a thickness map of the inspection region. This article describ es two potentially practical implementations of Lamb wave tomographic imagi ng techniques that can be optimized for in-the-field testing of large-area aircraft structures. Laboratory measurements discussed here demonstrate tha t Lamb wave tomography using either a ring of transducers with fan beam rec onstructions, or a square array of transducers with algebraic reconstructio n tomography, is appropriate for detecting flaws in multilayer aircraft mat erials. The speed and fidelity of the reconstruction algorithms as well as practical considerations for person-portable array-based systems are discus sed in this article. (C) 2000 Acoustical Society of America. [S0001-4966(00 )02110-X].