Spatial Phase-Shift Digital Shearography for Out-of-Plane Deformation Measurement

Measuring deformation under dynamic loading is still a key problem in the automobile industry. The first spatial phase-shift shearography system for relative deformation measurement is reported. Traditional temporal phase-shift technique-based shearography systems are capable of measuring relative deformation by using a reference object. However, due to its low acquisition rate, the existing temporal phase-shift shearography system can be only used under static loading situations. This paper introduces a digital shearography system which utilizes the spatial phase-shift technique to obtain an extremely high acquisition rate. The newly developed spatial phase-shift shearography system uses a Michelson-Interferometer as the shearing device. A high power laser at 532nm wavelength is used as the light source. A one mega pixels high speed CCD camera is used to record the speckle pattern interference. The spatial frequency carrier is introduced by tilting one of the mirrors in the Michelson-Interferometer. The Fourier Transform method (FT) is used to separate the spectra on spatial frequency domain. The Windowed Inverse Fourier Transform (WIFT) method is then used to evaluate the phase information of the recorded speckle pattern interference. The newly developed spatial phase-shift shearography system is used to measure an impact on a square metal plate clamped along its entire perimeter. The theory and experiment are shown in detail in this paper.