Fuselage inspection of Boeing-737 using lock-in thermography

Abstract

ABSTRACT Optically modulated elastic waves enter the surface of the inspected material by absorption of thermal radiation. An inhomogeneous disturbance in the material causes locally enhanced losses. Such material defects heat up at a different rate than the surrounding more homogenous material and therefore generate differences in thermal contrast. Modulating the amplitude of the optical stimulus turns defects into ther mal wave transmitters. The frequency of the stimulus signal must be matched to the specific thermal conductivity and mass density of the inspected material. It is possible to locate defects at different depths below the surface by varying the amplitude of the stimulus wave. Proper detection of thermal waves resulting from material defects near the surface requires a measurement setup that allows the recording of thermal images from an infrared camera. The re corded images can then be compared ag ainst the phase of the stimulus signal. This photo-thermal lock-in thermography method not only allows the evaluation of the amplitudal thermal wave information but more importantly it also allows for the extraction of the phase information. The phase shift between the stimulus signal and the captured thermal wave is dir ectly correlated to the thermal propagation time. Keywords: lock-in thermography, thermal waves, thermal radioscopy, delamination, rivets, water inclusion, disbonding