Transient deformation measurement using dual-pulse addition ESPI

An electronic speckle pattern interferometry system based on a frequency-doubled twin Nd:YAG laser emitting dual pulses at a TV camera field rate (50 Hz) was developed to analyse addition fringes generated by transient deformation of a test object. The main advance has been the automatic, quantitative analysis of dual-pulsed addition ESPI data by the introduction of carrier fringes and the application of Fourier methods. The carrier fringes are introduced between dual pulses by tilting the reference beam with a mirror driven by a galvanometer. The resulting deformation-modulate addition fringes are enhanced with a variance filter before evaluation of the phase distribution using a Fourier transform method with bandpass filtering. From the wrapped phase distribution, a continuous phase map is reconstructed using an iterative weighted least-squares unwrapper. The linear phase distribution associated with the carrier fringes is removed by evaluating it with a least-squares fitting algorithm. preliminary results obtained for a thin plate submitted to an acoustic shock show the suitability of the system for the quantitative: evaluation of transient deformation fields.