PSI - Issue 77

H. Lopes et al. / Procedia Structural Integrity 77 (2026) 673–680 H. Lopes/ Structural Integrity Procedia 00 (2026) 000 – 000

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an interference pattern, which is recorded by a digital camera. The second mirror is attached to a piezoelectric actuator, allowing for precise position control. This control is essential for implementing the temporal phase modulation technique used to extract the interference pattern's phase.

Fig. 1. Surface of the plate showing two impact locations and the clamped regions.

The entire experimental setup, shown in Figure 2, including the optical components, the DS system, and the inspected plate, was mounted on a Newport® optical table to guarantee stability and isolation from external vibrations during the measurements. The CFRP plate was fixed in an upright position by its two lower corners, with the impact surface facing the DS system. For illumination, a Coherent Verdi continuous wave (CW) laser (532 nm wavelength and 300 mW output power) was used. The beam was expanded via a circular flat-hat diffuse projector (ED1-C20 MD) from Thorlabs® to uniformly illuminate the plate surface. Before the experimental measurements, the plate surface, which retained marks from the impact procedure, was cleaned and then coated with a thin layer of white powder to ensure uniform reflection of the incident laser light. The intensity of the resulting interference pattern was captured by the DS system's 12-megapixel digital camera (Basler® ace acA4112-20um). The high resolution of this camera pr ovides detailed spatial resolution of the plate’s strain field, enabling comprehensive subsequent analysis. The experimental procedure began by applying a pulse thermal excitation to the inspected surface using a 500 W halogen lamp. After the excitation, the system acquired the interference pattern's phase at 8-second intervals over a total period of 96 seconds. To extract the phase at each interval, the temporal phase modulation technique was applied. This phase-shifting technique required the acquisition of four intensity images and the precise translation of one mirror in the Michelson interferometer (Kreis, 2006). This translation was performed with increments of ⁄2 of the laser wavelength, achieved by controlling an attached piezoelectric transducer (PZT) via a piezo controller. This process takes approximately 0.6 seconds. Finally, the four acquired intensity images are processed using the temporal phase modulation technique to retrieve the interference phase ( , ) at time . Subsequently, the raw phase map Δ +1 ( , ) at time +1 , which corresponds to the strain field, is evaluated by subtracting two consecutive interference phases: Δ +1 ( , ) = +1 ( , ) − ( , ) . These maps capture the strain developed between successive 8-second measurements throughout the plate's 96-second cooling stage.

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