Comparison of step heating and modulated frequency thermography for detecting bubble defects in colored acrylic glass

Abstract

Step heating thermography and modulated frequency thermography are two commonly used inspection methods. This research compares active step heating thermography and modulated frequency thermography for use in detecting defects with diameters of 1.98 mm to 6 mm buried at depths of 1.5 to 4 mm beneath sample surfaces. Experiments were conducted using colored acrylic samples. Step heating thermography can detect shallowly buried defects as early as 4 seconds after heating. Also, lengthening heating time allows step heating thermography to reveal differences between defects of 4 to 6 mm in diameter buried 1.5 mm to 4 mm beneath glass surface. Modulated frequency thermography provides a higher signal-to-noise ratio of 1.3 for smaller defects. Thus it can be used to detect defects with diameters as small as 1.98 mm buried 4 mm beneath the glass surface. However, this level of detection requires phase frequency to be lowered by an order of 10⁻³. An open problem is to develop a systematic method and/or heuristics for determining the appropriate frequency for different defect types.