Low-speed camera for high frequency dynamic behavior characterization with DIC

In the aerospace, automotive, and other mechanical industries, it is crucial to assess the structure's dynamic behavior in order to validate the products. Traditional methods of performing vibration measurement and Experimental Modal Analysis (EMA), such as using accelerometers, can provide accurate measurements but are invasive and may not be able to comprehensively measure the dynamic response of the test structure, as they can provide information only at discrete points. An alternative approach for performing EMA is to use camera-based methods, such as Digital Image Correlation (DIC), which can provide full-field displacement and strain measurements without physical contact with the object being studied. However, this method is limited by the frame rate of the cameras, which is lower with respect to contact transducers. In this framework, high-speed cameras may initially seem like a more suitable solution for reaching higher frequencies, but they can result in lower resolution and higher testing costs due to the expense of acquiring such cameras. Therefore, this thesis proposes two alternative methods for using DIC with low-speed cameras with the aim of characterizing the dynamics of structures at frequencies higher than the Nyquist-Shannon limit. The first method involves a random sampling scheme in time, which is coupled with a nonlinear optimization problem that seeks to reconstruct the initial randomly sampled signal with higher resolution. The second exploits the periodicity of the structure’s dynamics excited by a shaker to under-sample the specimen over numerous excitation cycles and then remap the time history that characterizes the analyzed object. The proposed approaches are validated through numerical validation and experimental tests on a cantilever beam. The results demonstrate the effectiveness of the proposed method of capturing the mode shapes of the structure at a frequency over the Nyquist-Shannon limit, focusing attention on the benefits and drawbacks of the proposed approach.