Joachim Schlichting

Efficient data evaluation for thermographic crack detection

We present an all-purpose crack detection algorithm for flying spot thermography which is directly applicable to a thermogram sequence without the need of any additional information about the experimental setup. A single image containing distinct crack signatures is derived in two steps. Firstly, the spatial derivative is calculated for each frame of the sequence and, secondly, the resulting data set is sorted pixel wise along the time axis. The feasibility of the proposed procedure is proven by testing a piece of rail that comprises roll contact fatigue cracks and by comparing the results with magnetic particle testing.

DEFECT SIZING BY LOCAL EXCITATION THERMOGRAPHY

In this article, we present a measurement procedure to gain information about depth and angle of surface braking 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 for heating and 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. Using FEM simulation we can show herein that it is possible to simultaneously resolve the angle and depth and, in particular, the depth of non-perpendicular cracks.

Investigating historic masonry structures with a combination of active thermography and 3D laser scanner

Methods for the combination (i. e. comparison and overlay) and data fusion (i. e. integration of all data in one data set, replacement of data) of active thermography and 3D laser scanner (light section method) have been developed. Systematic investigations for quantification of damage in historic structures are presented using both techniques. A case study shows that reproducible investigations at regular time intervals are very well suited for structural monitoring.

IMAGING CRACKS BY LASER EXCITED THERMOGRAPHY

During the last years active thermography is increasingly used in a number of NDT problems in production and maintenance. In this work we focus on the detection of vertical cracks starting at the surface, which is an important indication of structural failure. By using local thermal excitation it is possible to image anisotropies in the lateral diffusivity by recording the temporal temperature data with an infrared camera. The regional transient behaviour of temperature distribution then can provide quantitative information of the crack parameter. In doing so, we present an advanced technique for the determination of the crack depth. The experimental set‐up is based on an Nd:YAG laser. The beam is focused on the test sample by using an optical scanner to create the required lateral heat flow. The time resolved temperature distribution is recorded with an infrared camera (InSb FPA, 3 to 5 μm) providing a frame rate of up to 500 Hz. In addition we report on numerical simulation to investigate the concept of local heat excitation for a quantitative estimation of crack parameters. The modeling also includes the influence of surface to surface radiation inside the crack. We obtained a good consistency between experimental and theoretical data.During the last years active thermography is increasingly used in a number of NDT problems in production and maintenance. In this work we focus on the detection of vertical cracks starting at the surface, which is an important indication of structural failure. By using local thermal excitation it is possible to image anisotropies in the lateral diffusivity by recording the temporal temperature data with an infrared camera. The regional transient behaviour of temperature distribution then can provide quantitative information of the crack parameter. In doing so, we present an advanced technique for the determination of the crack depth. The experimental set‐up is based on an Nd:YAG laser. The beam is focused on the test sample by using an optical scanner to create the required lateral heat flow. The time resolved temperature distribution is recorded with an infrared camera (InSb FPA, 3 to 5 μm) providing a frame rate of up to 500 Hz. In addition we report on numerical simulation to investigate the concept of...

Active thermography for evaluation of reinforced concrete structures

Abstract: Infrared (IR) thermography, which encompasses the determination of the surface temperature of an object using an IR camera, is an imaging technology that is contactless and completely non-destructive. Its applications are classified into passive and active methods. By using passive thermography, differences in emissivity and, if a temperature gradient is present, differences in temperatures can be related to subsurface structures. If additional energy is induced into the structure by heating or cooling, the procedure is called active thermography. Active thermography methods enable structural investigations of building elements taking into account many different testing problems. In this chapter, the physical background, the equipment used, and the influences from environment and material properties are discussed. Several results of applications concerning the detection of subsurface defects are presented.