<title>Dynamic thermal tomography for nondestructive inspection of aging aircraft</title>

Abstract

We apply dual-band infrared (DBIR) imaging as a dynamic thermal tomography tool for wide area inspection of a Boeing 737 aircraft (owned by the FAA/AANC at the Sandia hangar in Albuquerque, NM) and several Boeing KC-135 aircraft panels (used for the round robin experiment conducted at Tinker AFB, OK). Our analyses are discussed in this report. After flash-heating the aircraft skin, we record synchronized DBIR images every 40 ms, from onset to 8 seconds after the heat flash. We analyze selective DBIR image ratios which enhance surface temperature contrast and remove surface-emissivity clutter (from dirt, dents, tape, markings, ink, sealants, uneven paint, paint stripper, exposed metal and roughness variations). The Boeing 737 and KC-135 aircraft fuselage panels have varying percent thickness losses from corrosion. We established the correlation of percent thickness loss with surface temperature rise (above ambient) for a partially corroded F-18 wing box structure (with a 2.9 mm uncorroded thickness) and several aluminum plates (with 1.0, 1.1, 2.3 and 3.9 mm thicknesses) which had 6 to 60 % thickness losses at milled flat-bottom hole sites. Based on this correlation, lap splice temperatures rise 1 °C per 24 5 % material loss at 0.4 s after the heat flash. We tabulate and map corrosion-related percent thickness loss effects (related to the corresponding surface temperature rise at 0.4 s after the heat flash) for the riveted (and bonded) Boeing 737, and the riveted (but unbonded) Boeing KC-135. We map the fuselage composite thermal inertia, (kpc)"2, based on the (inverse) slope of the surface temperature versus inverse square root of time. Composite thermal inertia maps characterize shallow skin defects within the lap splice at early times (<0.3 s) and deeper skin defects within the lap splice at late times (>0.4 s). Late time composite thermal inertia maps depict where corrosion-related thickness losses occur (e.g., on the inside of the Boeing 737 lap splice, beneath the galley and the latrine). Lap splice sites on a typical Boeing KC-135 panel with low composite thermal inertia values had high skin-thickness losses from corrosion. 0-8194-1250-3/93/$6.00 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/20/2015 Terms of Use: http://spiedl.org/terms SPIE Vol. 2001 Nondestructive Inspection of Aging Aircraft (1993) / 67 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/20/2015 Terms of Use: http://spiedl.org/terms