Instrumentation and Measurement Physics(DIMP)
Physique des instruments et mesures (DPIM)
Steven SHEPARD
Thermal Wave Imaging Inc.
Pulsed Thermography: Perspectives on the Evolution from Qualitative to Quantitative Application
Since infrared cameras were first offered commercially in the 1960's, the potential of thermography as a tool for NDE has been widely recognized and investigated. Motivated by the prospect of a safe, wide-area, non-contact inspection technology capable of imaging subsurface defects, researchers and practitioners in the field quickly determined that monitoring the surface temperature response of a sample to a thermal impulse was necessary to achieve a useful degree of sensitivity and repeatability. Although many stimulation methods have been successfully implemented, pulsed thermography, using optical flashlamp excitation, has gradually evolved as the most widely used configuration for manufacturing and maintenance applications in the aerospace and power generation industries. However, in most instances pulsed thermography has only been used as a qualitative adjunct to quantitative inspection technologies, such as ultrasound. For many years, quantitative interpretation of pulsed thermographic data was based on anaylysis of image contrast, which required a degree of calibration, preparation and inspector involvement that is rarely feasible in an industrial setting. Recently, new approaches to processing and interpreting pulsed thermographic data that do not rely on image contrast or visual analysis have been developed. In these approaches, the entire post-stimulation time evolution of each pixel.is analyzed as a separate entity, enabling sensitivity, resolution and measurement accuracy far beyond conventional image processing and previously accepted "rule-of-thumb" limits for thermography. The advent of these new methods, as well as complementary hardware developments, has led to and expansion of the field beyond NDE and into materials characterization. The evolution of pulsed thermography, from its contrast-based roots, to the current state of quantitative analysis will be discussed in detail, with examples spanning a broad range of materials and application.