PSI - Issue 77

João Queirós et al. / Procedia Structural Integrity 77 (2026) 475–483

477

in the original image, enhancing their visibility in the less significant orthogonal features. For DS phase maps, while detection is possible from their raw form, more advanced methods are used, such as unwrapping and computing their spatial derivatives, which improve identification (dos Santos and Lopes, 2024). However, in this work, a recent novel approach is employed (Queirós et al., 2025), which is based on a band-pass filter and summation processing of the phase maps. This novel method has been shown to significantly improve results by not only removing speckle noise but also eliminating the global strain field, which makes its interpretation more difficult. 2. Materials A rectangular plate made of carbon fiber-reinforced polymer (CFRP) was used as the test sample. Its dimensions were 0.2765 m (length), 0.198 m (width), and 1.825 mm (thickness). To create two distinct internal damages, the plate was subjected to two separate low-energy impacts. These impacts were generated by dropping a 0.988 kg steel sphere from two different heights, which varied the impact energy. To minimize air resistance and ensure an accurate drop, the sphere was guided by an acrylic tube with holes along its length. The plate itself was secured along its two shorter edges against a supporting structure. The first impact of 13.5 J of energy occurred at coordinates (74 mm, 147 mm). The second impact, with an energy of 26.2 J, was located at (215 mm, 38 mm). To identify the impacts, the plate's surface was coated with thin layer of white powder. Figure 1 shows both the impacted locations and the plate's clamped regions. After the impacts, a visual inspection of the surface did not reveal any visible sign of damage.

Fig. 1. Plate surface showing two impact locations and the clamped regions.

3. Non-destructive Testing Techniques Employed for Inspection The subsequent two sections describe both techniques and the experimental setups employed for the inspection. In AT, the plate's surface thermal response is measured by applying either pulsed or sinusoidal thermal excitation. On the other hand, DS utilizes laser illumination and an optical interferometer to acquire the surface strain field during the plate’s cooling stage, after a pulsed thermal excitation. 3.1 Active Thermography AT is an NDT technique that uses controlled thermal excitation and measures the resulting surface temperature response. The core principle involves applying a controlled heat source, typically a pulsed or sinusoidal one, to the surface of a material. This creates thermal waves that propagate through the structure. The way these waves travel depends on several factors, such as the material's thermal properties (e.g., conductivity, specific heat), the geometry of the structure, and the presence of internal defects or damage (Ibarra-Castanedo and Maldague, 2013). For pulsed