High-precision large deflection measurements of thin films using time sequence speckle pattern interferometry

The rapid development of microelectromechanical systems, thin-film technology and material research in mechanical behaviours has provided new impetus to handle high-precision and large-amplitude deformation measurements. Optical methods are very useful in making non-contact and full field measurements of the deformation of an object. However, there is a gap between the sensitivity and the measurement range in the existing coherent and incoherent optical methods. In this paper, we propose some approaches to employ time sequence speckle pattern interferometry to measure large deformations of pure Ni thin films subjected to bending with high sensitivity. We introduce a new large deformation measurement system, which includes a digital speckle interferometry and a micro loading system, and we propose two methods of phase demodulation in this paper. The new system is capable of automatically applying micro loading and continuously capturing the temporal speckle patterns imaged from the flexural surfaces of the thin film. Forty thousand frames of the time sequence speckle patterns have been recorded and the maximum deflection of about 1400 µm has been measured with sensitivities of λ/2 and λ/4 by the different proposed methods, respectively.