Direct strain measurement method based on the correlation of defocused laser speckle pattern

The structural strain field corresponds to the stress of each point and is an important unavoidable parameter in structural analysis or structural testing. The noncontact optical strain measurement methods are mainly divided into indirect strain measurement methods derived from displacement field differentiation and direct strain measurement methods represented by the digital shearing speckle pattern interferometry. The full-field displacement in the indirect measurement technique is commonly obtained using the electronic speckle pattern interferometry method, the digital image correlation method based on binocular camera or scanning laser doppler vibrometer method, and the differential operation further amplifies the measurement error. This study proposed a strain measurement method based on the defocused laser speckle pattern. In the case of defocus, the laser speckle field is sensitive to only the tilt angle, and the motion mode is the lateral translation of the same speckle field. The strain information can be obtained directly by the correlation calculation of laser speckle patterns before and after deformation. Experiments of the cantilever beam and bilateral clamped plate verified the effectiveness of the proposed method and proved that the proposed method made up for the limitations of the digital image correlation method and digital shearing speckle pattern interferometry method to a certain extent. Appropriate optical parameters and imaging parameters should be selected to ensure the quality of the laser speckle pattern and measurement accuracy.