Defect sizing by local excitation thermography

Abstract

In this article, we present a measurement procedure to gain information about depth and angle of open surface cracks. The method is based on a local excitation with, e.g., a laser. The resulting surface temperature is recorded with an infrared camera. Based on this data, crack-caused anisotropies in the lateral heat flow can be detected and exploited to characterise the cracks.The experimental set-up is based on a Nd:YAG laser. The beam is focused on the test sample by using an optical scanner to generate the required lateral heat flow. The time resolved temperature distribution is recorded with a high-speed infrared camera (InSb FPA, 3 to 5 µm) providing a frame rate of up to 500 Hz.Up to now, only qualitative information was gained from measurements of this type. Whereas the local transient behaviour of temperature distribution provides also quantitative information of the crack parameters. The general concept of the method presented herein has already been published [1], but the mentioned publication is focused on the crack depth only.In this paper, we can show that it is possible to simultaneously resolve the angle and depth and, in particular, the depth of non-perpendicular cracks.

Depth Retrieval Procedures in Pulsed Thermography: Remarks in Time and Frequency Domain Analyses

“…Usually, only one parameter is of interest (e.g., depth) which can be estimated by correlating the numerical and the experimental results. Quantitative characterisation by means of pulsed, and more generally speaking of active thermal imaging, includes mainly depth information retrieval [14][15][16][17][18] and the estimation of defect's lateral dimension [19][20][21], while several studies have also investigated the determination of the thermal properties either of the material being inspected [22,23] or of the hidden features [24].…”

Efficient data evaluation for thermographic crack detection

“…[2]), which can be used to characterize its geometries -after calibration -by a more sophisticated approach (Schlichting et al, 2011).…”

Review of pulsed thermal NDT: Physical principles, theory and data processing

“…Schlichting et al proposed a phenomenological procedure for the determination of open surface crack depth and angle when using a laser for heating a specimen at a fixed position in proximity to the crack [80]. The procedure is based on calculating a crack depth value (cdv):…”