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Thermographic Inspection of Metal Under Coating

Objective

To conduct a feasibility study on detecting surface breaking defects in metallic structures under coating using active thermographic inspection.

Background

TWI delivers various non-destructive testing (NDT) services to Member companies. When an inspection is carried out, it is vital that the downtime of the system is kept to a minimum. In situations where the part or the surface to be inspected is covered with a layer of protective coating (for instance the inspection of a storage tank floor), the coating has to be removed prior to the inspection for accurate sizing of the defect.  This increases both the duration of the downtime and the inspection related costs. As such, there is a need for an inspection technique that is able to perform the same task without the need to remove the coating.

Traditionally, magnetic particle inspection (MPI) and liquid penetrant inspection (LPI) have been employed for surface inspection. LPI can detect fine surface breaking cracks that are not visible to the naked eye, whereas MPI can detect both surface and near-surface defects. As with other industries, there is an ongoing drive to automate NDT techniques. Both LPI and MPI are not completely automated – some aspects of the inspection techniques have been successfully automated while other aspects still rely on skilled operators. An easier route to full automation is potentially adapting an automation-friendly inspection technique for this purpose.

One possible technique is thermography. It is particularly advantageous since it’s non-contact and full-field in nature, enabling rapid inspection of a part. Current commercially available infrared detectors are also capable of achieving sub mm spatial resolution with ease.

Figure 1. Pulsed Thermography Experimental Setup
Figure 1. Pulsed Thermography Experimental Setup
Figure 2. Sample with Defects
Figure 2. Sample with Defects

Active Thermography

Thermographic cameras use infrared sensors that detect incident infrared radiation that can then be converted to temperature values. This results in a map of temperature values, also known as thermograms. Inspected parts are usually in thermal equilibrium and therefore require the use of a heat source to produce a thermal contrast between the defective region of interest and the background material. This is known as active thermography. Several options exist in delivering the required heat source including laser heating, light sources (halogen light, infrared light, etc.), flash, and induction heating (eddy current thermography). It is noteworthy that commercial systems exist for eddy current, pulsed and lock-in thermography.  

In this case study, pulsed thermographic inspection was performed using a 1 kW photographic flash. This is mainly due to the versatility of the flash – it is portable and easy to use yet can heat up a relatively large area. The infrared camera used was an FLIR A6751 SC and the data was recorded at 125 Hz at full window (640 x 512 pixels) from just before the flash to a few seconds after the flash. In pulsed thermography, the infrared camera will observe the variation in surface temperature of the material as the thermal wave from the heat source propagates through the part. The presence of defects will alter the heat transfer local to the region and hence appear as a region with different temperature distribution, thus allowing sub-surface defects to be detected.

A sample containing realistic defects was obtained from a previous TWI collaborative research project X-Scan (x-scan.eu). The 6 mm thick butt-welded carbon steel plate contained various types of surface and sub-surface defects. The plate was coated with a layer of paint.

Results and Validation

It was found from experimentation that the highest thermal contrast exists a few frames after the flash. However, this is expected to vary with the depth of the defect or coating thickness. For one of the defects in the plate – a toe crack that is sub mm in width and approximately 40mm in length – the defect was visible with sufficient contrast even without post-processing. The results were validated with the radiograph of the plate, which clearly showed the presence of the toe crack at the same location.

Figure 3. (a) Pulsed Thermography result (b) Radiography result
Figure 3. (a) Pulsed Thermography result (b) Radiography result
Figure 4
Figure 4
Avatar Vishnu Seelan Project Leader

Vishnu joined TWI in August 2018 after graduating with a PhD in experimental mechanics from the University of Southampton. Working in collaboration with TWI, EDF, NPL and Amec Foster Wheeler for the RESIST project, Vishnu carried out exploratory research investigating the possibility of using infrared thermography for microstructural assessment of welds. Vishnu is also experienced in other experimental mechanics techniques such as Thermoelastic Stress Analysis (TSA) and Digital Image Correlation (DIC). His research interests are focused on infrared thermography and image processing. Currently, Vishnu is gaining exposure to ultrasonic techniques at TWI.

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