Damage detection in rotating objects using position-triggered thermography
Introduction
Detecting damage in its early stages is important to prevent failure of structures with long operating times. This should be done using non-destructive testing techniques, as the intention is for them to keep working as long as possible. There are several methods that can be successfully used for these procedures [1], [2]. Among them, Thermography is a widely used non-contact technique with usage in multiple fields, from aeronautics and energy generation to electronics [3], [4], [5], [6], and it is well suited for non-destructive qualitative detection with few, although costly [2], requirements. The basic approach to damage detection using thermography involves obtaining two thermograms in differing stages of cooling/heating and analysing their subtraction. Thus, it only requires the use of a thermographic camera and the presence of a thermal load that can be either artificial or natural.
This method is commonly used in composites to detect the presence of voids and delamination [7]. These are characterized by an enclosed air pocket, which has a thermal conductivity that is significantly different from the surrounding regions. This will be noticeable in the thermogram as a region that stays hot for longer, while the surrounding region cools down [4]. It should be noted, however, that similar behaviour could also be caused by incoming heat being concentrated in a region. Thus, multiple evaluations should be made before concluding about the presence of a defect, if possible in different heating situations.
In the case of structures with minimal downtime, such as wind turbines, it is advantageous to be able to detect these damages during operation. However, if the inspected components are moving, this will cause difficulties in obtaining the difference thermograms, as the two thermal images that generate them should be well aligned.
This problem can be approached using digital image processing, by registering two images in different points in time and different angular positions and afterwards subtracting them. For example, M. Doroshtnasir et al. [8] use this approach successfully to analyse wind turbine blades on-site.
Another possible approach is to synchronize the image acquisition with the rotation of the blades. This way, it is possible to obtain sequential images that are already aligned and, as such, do not require any image processing except subtraction. This is a natural extension towards damage detection of the approach previously undertaken for digital image correlation in [9], [10], [11], [12], [13]. It uses similar triggering procedures, including an upgraded version of the trigger controller and a custom photodetector, focused on improving the versatility of the system by sparing the need for a laser beam. By not requiring this laser, it is possible to place the whole acquisition system on the same side of the target object and thus improve its usability for large-scale operations, as everything can be placed on the ground and close to each other.
For the present work, this method was implemented in a laboratorial test, using only external non-contact devices to trigger the thermographic camera, and with the complete setup positioned on the same side of the specimen.
Section snippets
Methodology
The main goal of this work is to develop a method for defect detection in rotating parts using position-triggered thermography. In order to assess its feasibility, a blade was analysed in both static and dynamic situations, with the aim of finding the same potential defects in both. The same setup was used in both situations, using position-triggering in the dynamic one and time-based triggering in the static.
Both analyses were performed using simple thermogram subtraction, where a colder
Results and discussion
During the experiment, the first stage was the static one, as it was important to find if there were any defects on the target blade and, if so, where. Thus, similar to the dynamic stage, the blade was heated with incandescent lighting and both the heating and cooling periods were recorded. From this set, two images were selected to calculate the difference thermogram, one hotter (Fig. 7a) that the other colder (Fig. 7b). The static difference thermogram of Fig. 8a, clearly shows the presence
Conclusions
The presented results demonstrate that it is possible to detect potential defects in rotating objects using position-based triggering.
The developed methodology employs a custom photodetector, with a working principle similar to a single pixel camera, where its high amplification provides a grey level of a particular region. This level was processed with the developed controller to trigger the acquisition of thermal images in the same position in every rotation. And consequently enable a direct
Declarations of interest
none
Acknowledgements
Pedro J. Sousa gratefully acknowledges the FCT (Fundação para a Ciência e a Tecnologia) for the funding of the PhD scholarship SFRH/BD/129398/2017.
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