We report on recent developments in the detection of surface breaking cracks using
flying laser spot thermography. Application of an infrared camera for mapping the
thermal radiation after excitation with a diode laser equipped with an optical scanner
allows us to examine a surface containing cracks in an entirely non-destructive,
contactless and fast way, without even moving the camera. We developed an efficient
and robust algorithm that can be applied directly to the recorded thermal sequences,
and that derives a single image containing all crack signatures. For this crack detection
technique, no specific synchronisation between laser and camera is required. Hence,
our approach is suitable for an upgrade of existing thermographic systems. The
feasibility of the proposed procedure is proven by testing an artificial test sample and a
piece of rail that comprises roll contact fatigue cracks and by comparing the results
with magnetic particle testing.
Active thermography is a nowadays widely used NDT method making use of thermal material properties for defect detection. Basically, the sample is heated and the resulting surface temperature is recorded by an IR camera. For laser thermography a laser is used to heat the sample locally. The resulting spherical heat flow allows the detection of voids in arbitrary orientation. In this work, a method is presented which is suitable for the quantitative characterization of depth and angle of surface cracks. The main idea is to evaluate the crack-caused asymmetries of the laser's thermal footprint. The heat is introduced at fixed reference positions relative to the crack. In this paper a data analysis procedure is presented which allows the crack depth and angle to be described by only two characteristic scalar parameters. By investigating artificial test specimens with spark eroded notches, the feasibility of this method is validated. Furthermore, the behavior of the characteristic parameters with variations of crack angle, depth and experimental conditions is studied systematically by FEM simulations, showing that these parameters are well behaved.
Spot welding is one of the most important technologies for joining sheet metal. While there are lot of approaches to non-destructive testing, quality assurance still mainly relies on welding parameter monitoring and destructive testing, leading to significant failure rates. In this paper an approach to spot weld testing using flash thermography is presented. The main focus of attention is on the identification of two typical error classes: stick welds and welds at the splash limit. Besides investigating the principal feasibility of thermography for zinc plated samples the results of a series test of spot welds joining 1 mm thick TRIP steel are shown. Based upon these results a statistical criterion is developed which allows a reliable classification of the named error classes.
We present a systematic study on the performance of laser-thermography for the detection of surface cracks in metallic components. Scanning a metallic surface with laser causes local heating that is mapped simultaneously by an IR-camera and allows identifying cracks with sub-µm openings. The detectability, however, depends on a number of acquisition parameters (e.g. scanning speed, laser power, IR-camera resolution) that typically relate on each other. Most importantly, the detection-sensitivity of surface breaking cracks is given by a particular combination for the acquisition parameter values. As a result, this sensitivity is adaptable within wide ranges allowing the detection of cracks with openings ranging from 200 to 0.1 µm at testing speeds of 100 to 0.05 cm²/s. By examining artificial as well as fatigue cracks, we demonstrate that the method can be even applied to shiny surfaces with no need of pretreatments, which makes it an entirely contactless, remote and automatable NDT technique. A comparison with magnetic particle testing shows that laser-thermography has the potential to become a strong competitor to conventional surface inspection methods in the future.
Heat transfer modeling of local thermal excitation for surface crack detection and characterization
(2010)
In the presented paper the use of local excitation (laser beam) for crack detection and
characterization is simulated. As we could see, using the temperature differences between two
reference points was large enough to detect and characterize the crack, even for such small value of
laser power as 2 W. The influence of radiation and convection on the temperature difference was
negligible (about 0.1%). The influence of heat conduction, in case of crack with air, was very small
(about 3%). The main mechanism which influences the temperature differences between the
reference points is the heat conduction in steel. From the presented results one can see that
temperature differences are increasing with an increase of the crack depth or the crack length. Better
resolution and characterization of the crack depths and lengths should be achieved with increased
duration of heating time or the laser power. We could also see that the best position for the laser
spot to characterize the crack is the closest one. Also, comparison of 2D and 3D simulation results
for circular heat source used as local excitation mechanism had showed that only full 3D modelling
will allow accomplishing a quantitative level of comparison between experimental and simulation
results.