By Objective Speckle Field Measurements Of Young's Modulus Of Solid Materials

Initially, speckles were first recognized as a kind of noise by people. Until the 70's in 20th century, the speckles were used for a measurement as a useful method. The common measurement with speckles contain the tradition speckle photography,the speckle interferometry and the speckle correlation technique. With the development of the computer science and CCD technology, the digital speckles technology have been widely used in nondestructive testing, measurements of deformation, displacement measuring, surface roughness measurement and vibration analysis etc. Compared with the advantages and disadvantages of these methods above, in this paper, an approach based on the objective speckle field to measure the elastic modulus of solid materials with three-points bending method is presented.According to the characteristics of speckle field, if the translation of scatter specimen is small before and after the translation, we can record two speckles fields with same displacement and distribution. These speckles pairs on the image of speckles field are called "Young double pinhole". The images of objective speckle field on the force rod loading are recorded on CCD, adding the each two images and performed fast Fourier transformation(FFT), the parallel straight and even separated fringes appear in the FFT frequency spectrum, it is similar to the image of Fraunhofer diffraction. The displacement of the speckles field can be obtained from the number of the fringes and pixel size. This is the first innovation of this paper. The measurement of deflection of the specimen is the key step to measure Young Modulus of material using three points bending method. After Analysising this problem carefully, we find out that the displacement of the each spot on bending specimen is not equal, so the deflection of the specimen can't be measured with the speckles field of the specimen. However, the displacement of loading force rod is equal to the deflection of specimen, so the objective speckle field of the loading force rod can be used to measure the deflection of specimen. This is the second innovation in this paper. On the basis of studying the Characteristic of speckle field and the digital image processing. First, the relation between the displacement of objective speckels image and fringe number of the FFT frequency spectrum was verified by theory simulation. Then, an experiment to verify the feasibility of this method was performed, meanwhile, the size of the CCD pixel was measured. Finally, the Young Modulus of the solid material is measured with three-points bending method, the experiment result demonstrated the method has very high accuracy and reliability, and has provided a new approach to measure Young Modulus of solid material with three-points bending method.