Laser Scanning Thermography for Coating Thickness Inspection

The paper deals with a new approach to laser thermography for the inspection of coating thickness. The approach is based on scanning the specimen surface point by point, using a low-power laser, and recording the temperature responses with an IR camera. A recorded sequence is then transformed into a sequence similar to a flash pulse thermography sequence. Fast Fourier transform was used as a processing technique. The results are compared with a flash pulse thermography measurement. It was shown that the laser thermography measurement provides a higher sensitivity to thickness changes than flash pulse thermography measurement.

ARTIFICIAL SPECIMENS MANUFACTURING OF GLASS FIBRE REINFORCED COMPOSITE FOR SUB-SURFACE DEFECTS INSPECTION BY ACTIVE LONG-PULSE THERMOGRAPHY

The detection and analysis of sub-surface defects in composite materials specimens have been problems of interest in recent years in different industries. For instance, in the wind industry, such materials are used for manufacturing wind turbine blades. A variety of defects can occur inside composite materials, e.g., porosities, fibre rupture, and inclusions. This work presents the design and manufacturing of artificial test specimens of composite material, to which specific defects were induced in a non-destructive way. The used manufacturing process was VARTM. In addition, a method to localise and characterise induced defects is proposed. Such method works by applying active long-pulse thermography to the test specimens. Further, we show that both the proposed low-cost optical system and the long-pulse active thermography technique allow the non-destructive thermal analysis of parts to detect sub-surface defects in composite materials based on fiberglass reinforced epoxy resin

Active Thermography for the Detection of Sub-Surface Defects on a Curved and Coated GFRP-Structure

Initial defects, for example, those occurring during the production of a rotor blade, encourage early damages such as rain erosion at the leading edge of wind turbine rotor blades. To investigate the potential that initial defects have for early damage, long-pulse thermography as a non-destructive and contactless measurement technique is applied to a strongly curved and coated test specimen for the first time. This specimen is similar in structural size and design to a rotor blade leading edge and introduced with sub-surface defects whose diameters range between 2mm and 3.5mm at depths between 1.5mm and 2.5mm below the surface. On the curved and coated test specimen, sub-surface defects with a depth-to-diameter ratio of up to 1.04 are successfully detected. In particular, defects are also detectable when being observed from a non-perpendicular viewing angle, where the intensity of the defects decreases with increasing viewing angle due to the strong surface curvature. In conclusion, long-pulse thermography is suitable for the detection of sub-surface defects on coated and curved components and is therefore a promising technique for the on-site application during inspection of rotor blade leading edges.

Damage Identification on Impact and Lightning Damage of Flax Composite Laminates (Linum usitatissimum) Using Long-Pulse Thermography of a Low-Resolution Infrared Camera

The demand for composite fiber material is significantly high due to its excellent mechanical properties and its use in various industries. Recently, with the increasing awareness of environmental issues, researchers are now focusing more on eco-friendly and green materials. A biocomposite offers a good balance of strength and stiffness ratio, bending and membrane mechanical properties, balanced thermal distortion stability, reduced weight and cost, improved fatigue resistance, reduced notch sensitivity, and, comparatively, better performance than synthetic composites. Yet, due to the complex anisotropy of the composite material, the inspection and detection of inner defects become a challenge. Long-pulse thermography is one of the non-destructive evaluations (NDEs) used to detect defects in composite materials. However, very limited research has been carried out on the usage of a low-resolution infrared camera to perform defect or damage inspection on flax composite laminates. In this paper, an experimental setup of a long pulse thermography system using low-resolution infrared camera was performed on flax bio-composite to identify impact and lightning damage. The result highlights that with control parameters, a low-resolution infrared camera has the capability to capture the lightning and impact defects of flax biocomposites using the long-pulse thermography system. An image processing method is then applied to the defect to improve the quality of defect detection and reduce background noise.