Inverse thermographic characterization of optically unresolvable through cracks in thin metal plates W.Y. Jeong a , C.J. Earls a,n , W.D. Philpot a , A.T. Zehnder b a School of Civil and Environmental Engineering, 220 Hollister Hall, Ithaca, NY 14853, United States b Sibley School of Mechanical and Aerospace Engineering, 210 Kimball Hall, Ithaca, NY 14853, United States article info Article history: Received 22 November 2010 Received in revised form 21 September 2011 Accepted 24 September 2011 Available online 13 October 2011 Keywords: Inverse problems Crack detection Thermography Crack imaging abstract The present research explores the feasibility of a novel non-contact means for the detection and characterization of small scale, open crack-like defects within thin metal plate components. The approach employs inexpensive infrared (IR) digital camera technology (e.g. E$5000US) and sophisticated numerical and algorithmic means as part of an inverse solution framework aimed at inferring the nature (i.e. presence, size, orientation, etc.) of micro-scale defects that are below the optical resolution capability of the IR imaging system. The current work explores a technique for inferring the presence and character of optically unresolvable flaws penetrating metallic plate components (i.e. through cracks). & 2011 Elsevier Ltd. All rights reserved. 1. Introduction The identification of improved strategies for the nondestructive evaluation (NDE) of engineered artifacts continues to be an important line of inquiry, as the various elements within the built environment are called to ever increasing service lives (due to economic influences, etc.) One important application of NDE emanates from a need to identify the presence of critical flaws in engineered components within a larger system (e.g. composite delaminations, cracking in ceramics and metallics, etc.) A class of structures where such flaw identification is of critical importance is within the commercial air fleets. Specifically, the occurrence of small cracks within an aircraft fuselage has been known to precipitate unfortunate consequences [8], and thus an ability to detect such defects at a nascent, non-critical stage, becomes important. The present research explores the feasibility of a novel non-contact means for the detection and characterization of small scale, open crack-like defects within thin metal plate components. The approach employs inexpensive infrared (IR) digital camera technology (e.g.  $5000 US) and sophisticated numerical and algorithmic means as part of an inverse solution framework aimed at inferring the nature (i.e. presence, size, orientation, etc.) of micro-scale defects that are below the optical resolution capability of the IR imaging system. The current work explores a technique for inferring the presence and character of optically unresolvable flaws penetrating metallic plate components (i.e. through cracks). 1.1. Background When considering the detection of cracks in metal components, various non-destructive testing and evaluation (NDT/ NDE) techniques have been in use for many decades. Techniques such as dye penetrant testing and magnetic particle testing have been in wide use before the 1940s; while more recent means involving radiographic, acoustic emission, Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ymssp Mechanical Systems and Signal Processing 0888-3270/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ymssp.2011.09.021 n Corresponding author. Tel.: þ1 607 255 1652. E-mail address: cje23@cornell.edu (C.J. Earls). Mechanical Systems and Signal Processing 27 (2012) 634–650