PSI - Issue 77

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

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excitation, a short burst of energy applies a wide range of frequencies at once. In contrast, sinusoidal excitation applies energy at a single, specific frequency. Thicker structures require longer pulse durations or lower excitation frequencies because lower frequencies penetrate deeper (Ibarra-Castanedo and Maldague, 2013). An infrared camera records the surface temperature over time, indirectly evaluating the thermal wave's propagation. When a thermal wave encounters an internal flaw, it causes a localized change in the temperature amplitude or phase at the surface. By analyzing these variations, it is possible to identify and map out internal damage. The experimental setup used in this study is shown in Figure 2.2. The CFRP plate specimen, supported on its lower corners, was mounted with the impacted surface facing the camera. Thermograms were recorded using a FLIR® A6751sc infrared camera, which provides a 640 × 512 pixel resolution and operates in the 3 – 5 μm spectral band. Thermal excitation was applied using two 2 kW Automation Technologies® halogen lamps, powered by an IRX-box, capable of both pulse and sinusoidal modulation. These were positioned symmetrically relative to the specimen, 50 cm away from it, and at a 45-degree angle relative to the symmetrical axis. In addition, the inspected surface was coated with a thin layer of matte black paint to reduce surface emissivity variations and its reflectivity. Synchronization between thermogram acquisition and thermal excitation was achieved using a National Instruments® NI USB-6251 module.

Fig. 2. Experimental setup for thermography tests.

3.2 Digital Shearography DS is an optical, NDT technique that utilizes laser illumination and a Michelson interferometer to directly measure displacement gradients, or the strain field, on a structure’s surface. The process involves correlating two speckle interference patterns: one taken from the object in an undeformed state and a second one after a small external load has been applied. Because DS is a highly sensitive optical technique, a very small mechanical or thermal load is sufficient to produce a measurable strain field. For effective damage identification, it is crucial that the applied load uniformly excites the entire inspection area. This ensures consistent sensitivity to potential flaws. However, achieving this can be challenging due to boundary conditions and other physical phenomena. Generally, defects and damage cause a local reduction in stiffness, which is revealed as a small and localized variation in the strain field. The measurements were performed on top of a Newport® optical table to ensure its isolation from undesired external mechanical vibrations. The CFRP plate was placed in an upright position and secured by its lower corners to the table. The inspected surface was illuminated by a Coherent® Verdi continuous wave (CW) laser with 532 nm wavelength and 300 mW output power. The intensity of laser light reflected by the specimen surface is captured by a 4000 × 3000-pixel digital camera (Basler® ace acA4112-20um) that is part of the in-house DS system. To improve the reflection of the laser light, the surface was coated with a thin layer of white powder.