Development of Digital Shearography for Complex Defects Inspection

Digital shearography measures the first derivative of the object surface deformation, which has the advantages of high sensitivity, full field, non-contact, realtime and anti-disturbance. It is widely used in materials inspection in industry. The information acquisition methods of the first derivative distribution are mainly divided into the intensity method and the phase shift method. The intensity method is to directly obtain the first derivative phase distribution by subtracting the light intensity map. The phase shift method to obtain phase information can be divided into temporal phase shift and spatial phase shift. Most digital shearography systems are single camera based and can only capture one image every shoot. However, unpredictable defects like the narrow crack and minor flaws could induce incomplete detection due to some limitations. There are two major issues on the measurement, one is the defect with the irregular shape which is not sensitive to the digital shearography, another is the deformation made of defects that is much smaller than the resolution of cameras. Digital Shearography measures the first derivative of deformation on the object surface because the shearing direction determines the derivative direction being measured, tests using multiple shearing directions are sometimes required to detect all kinds of defects. When the deformation is long and narrow as a crack while the shearing direction is perpendicular to the crack growing direction, digital shearography has the best sensitivity. In opposite, if the crack growing direction is parallel to the shearing direction, digital shearography is not able to find it out. Irregular shape defects detection is a tough challenge for digital shearography. Another challenge is the defects that are too small for the field of view. The limited pixels can miss the defects due to the low signal to noise ratio. To increase the sensitivity of detecting minor defects, a small field of view measurement is needed but it is time consuming in a large surface area inspection. The new development can be divided into three categories: 1) Modified Michelson interferometer based dual shearing digital shearography. 2) Spatial light modulator based dual shearing direction shearography. 3) Polarized digital shearography for simultaneous dual sensitive measurement. The basic theory, optical path analysis, preliminary studies, results analysis and research plan are shown in detail in this dissertation.