13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- , ) 2 2sin( d (5) where Θ – angle between the reference beam of a laser radiation and the focused scattered laser radiation from a research surface which forming image of this surface with speckle pattern in the plane of the photodetector. The kind of speckle pattern with the secondary interference pattern is presented in Fig. 2. Figure 2. Photography of the speckle pattern with the secondary interference pattern It is offered for effective registration of change of intensity of the single speckle to satisfy the following conditions: width of secondary interference fringes has to be agreed with the transversal size of the single speckle, and also with size of a sensitive cell of the photodetector. For agreement of the transversal size of the single speckle with size of a sensitive cell of the photodetector it is necessary that the following ratio was carried out: H h , (6) where h – size of a sensitive cell of the photodetector. Performance of this condition is achieved by sampling of the parameter α of a optical system, as appears from of Eq. (2). In order that the sensitive cell of the photodetector could measure change of a phase of the single speckle it is necessary that width of secondary interference fringes d was more or equally of the size of a sensitive cell of the photodetector that is the following ratio was carried out: d h , (7) Performance of this condition is achieved by sampling of a angle of a convergence Θ between the reference beam and the focused scattered laser radiation from a research surface which forming image of this surface with speckle pattern in the plane of the photodetector, as appears from Eq. (5). Performance of conditions – Eq. (6) and (7) – leads to that intensity of the single speckle, as whole, will change from minimum to the maximum value (or on the contrary) depending on displacements of a research surface. Thus, measurement of intensity of the single speckle allows to investigate dynamic processes in a solid mechanics. Application in the offered method of high-speed photodiodes expands the frequency range of measured dynamic displacements.
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