Ultrasonic Characterization of Thermal Gradients in a Homogeneous Plate

ABSTRACT. The effectiveness of structural health monitoring via ultrasonic methods is seriously compromised by changing environmental conditions such as temperature. Variations in temperature can be spuriously identified as damage owing to thermal expansion or contraction of the material as well as changing of the wave propagation speed. This problem becomes even more serious when temperature changes are inhomogeneous such as when the structure is in unevenly sunlit conditions. The focus of this paper is the detection and characterization of thermal gradients in a homogeneous plate, which is a necessary first step before considering the effect of thermal gradients on monitoring of damage. Experiments were performed by permanently mounting two piezoelectric disc transducers on a homogeneous aluminum plate. Long-time, reverberating signals were recorded from the plate under ambient conditions, homogeneous temperature changes, and thermal gradients. The gradients were generated by local heating of a small area on the plate, and signals were recorded during cooling until steady state conditions were reached. Signals from both homogeneous temperature changes and thermal gradients were compared with the baselines using several metric, and these metrics were evaluated in terms of their ability to detect and characterize thermal gradients as compared to homogeneous temperature changes. Results indicate that the peak coherence drop and the standard deviation of the time-dependent time delay were most effective.