Neutron radiography of aerospace structure hidden corrosion

Abstract

The detection of corrosion in operational aircraft is vital for an effective repair and maintenance program intended to extend the service lifetimes of these aircraft. Neutron radiography is an established non-destructive inspection technique that can detect early-stage corrosion, even within retatively thick aluminum structures. This report presents the results of a study to measure experimentally the detection sensitivity of ncutroR raciiography to aircrait corrosion, using both laboratory-prepared and environmentally produced aircratt corrosion specirnens. Thermal neutrons were obtained from the medical research reactor at the DOE's Brookhaven National Laboratory (BNL) as part of a Cooperative Research and Development Agreement (CRADA) between Northrop Grumman Corporation and BNL. Initial work at the reactor consisted of enhancing the beam collimation for high-resolution radiography. The neutron beam conditions were then measured using activation foils and several commercial beam-quality indicators. Radiography of the corrosion specimens was carried out with a vacuum cassette employing a gadoliniurn converter screen and finegrain X-ray film. Measurement of the optical film density of the corrosion images allowed an evaluation of the etfective thermal neutron anenuation coetficient for corrosion products. This evaluation was facilitated by the use of water specirnens to calibrate the measurement procedure, since water has a known thermal neutron attenuation coefficient. An assessment of the thickness threshold for visually distinguishing corrosion by-products in neutron radiographs indicated a threshòU ot about 2 mils (0.002 in.) for the laboratory-prepared corrosion and less than that for the aircraft corrosion samples examined. The experimental results obtained with the BNL reactor thermal-neutron beam were used to help predict the potential capability of an accelerator-based neutron radiography system that could be deployed in a hangar type of environment to inspect either intact aircraft or subassemblies from such aircraft.