Method Study On Quantitative Calculation Of Out-of-plane Displacement Based On Digital Shearography

Digital shearography possess advantages of high precision, full-filled measurement, non-contact testing and high interference immunity. It not only can solve the problem of small vibration and transient fluctuation measurement, but also can be used for measuring in-plane displacement, out-of-plane displacement and the strain of the object’s surface or internal etc. So, Digital shearography has been widely used in strain measurement, vibration analysis and non-destructive testing.In the paper, the theory concerned with digital shearography is represented firstly. Then the method of measuring out-of-plane displacement quantificationally based on digital shearography is simulated on a computer through the programming technology of VC++. A shearography fringe patterns caused by object deformation is demodulated by four-step phase shifting algorithm in advance. By using the technologies, such as median filtering, binarization and fringe thinning, the demodulated result is processed so as to obtain the skeleton of strips. Moreover, the fitting phase curve along the shear direction can be gotten through stripe skeleton. It is proportional to the differential of out-of-plane displacement. Thus, the out-of-plane displacement distribution curve can be calculated quantificationally using Gaussian-type function to interpolation.In order to verify calculation method of theoretical simulation, an experiment is conducted. In the experiment, shearography images are gotten through Michelson shearing imaging manner. The images are handled in proposed method mention at previous paragraph. A displacement is worked out quantificationally by using differential of out-of-plane displacement.Finally, a digital holographic experiment is made under the same condition with digital shearography measurement to get a digital hologram. The object’s out-of-plane displacement can be got through the phase demodulation of the digital hologram. The results are compared with the out-of-plane displacement obtained by the digital shearography method. The measured results of two methods are in a good agreement, i.e. the relative error is very small. The experimental results show that it could provide an effective calculation method for measuring out-of-plane displacement based on the digital shearography.