Nondestructive detecting for internal defects of specular surface materials

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Shearography has the advantage of being a full-field, non-contact method with strong anti-interference. Speckles are used to carry the surface information of the specimen when they only occur on rough surfaces; thus, shearography can only be used to detect internal defects with rough surfaces. With the development of technology, more and more specular surface composite material applications are being found. When using shearography to detect the internal defects of specular surface specimens, the most common method is to spray developer on the specular surface. The specular surface is turned into a rough surface and speckles are generated. Using a rough surface to generate and reflect speckles on a specular surface is another common method. However, the developer will have some influence on the properties of the specular surfaces of the specimen while being cleaned. Laser utilization efficiency is low when using a rough surface to generate speckles and reflect them onto a specular surface. This paper introduces a method to detect the internal defects of the specular surface material. The detecting method combines the method of producing speckles by employing several pieces of frosted glass with that of using a spatial light modulator (SLM) to control the shearing amount and phase shift. By placing frosted glass behind the beam expander, speckles of a suitable size can be produced. Using a piece of frosted glass of the same size, a single detection area can be enlarged. SLM is used to produce an appropriate shear amount and phase shift. By using this method, shearography can be used to detect the internal defects of the material with a specular surface. The experimental results of standard defect-free specimens, as well as those with internal defects, show that this method is suitable for detecting the internal defects of specular surface specimens.