Time-domain image filtering for DIC vibration measurements

A time-domain filter is presented in this paper for the noise reduction of the camera images, then elaborated for the full-field 3D-DIC vibration analysis of mechanical components. The basic idea behind this filtering is to initially decompose the light signal of each pixel of the raw images, both for the left and the right cameras, with the Fast Fourier Transform (FFT). The signal is then reconstructed with the Inverse FFT by keeping the zero-order harmonic and the first harmonic and just discarding all the others, which only introduce noise contributions. The down-sampling approach is also used in combination with this filtering to reach high loading frequencies, even without (expensive) high-speed cameras, provided that a quite short exposure time is available. A cantilever aluminum plate was tested at 591 Hz, and a similar experiment was repeated on a bladed disk (or blisk) excited at high frequency, 6458 Hz, obtaining clear and smooth displacement maps even in the range of 10 microns. In the blisk application, a single blade with a significantly larger displacement amplitude was clearly observed, which could be interpreted as mistuning evidence.