engleski

Experimental Caustics Analysis in Fracture Mechanics of Anisotropic Materials

The optical method of caustics is employed in fracture mechanics for determining the stress-intensity factors of cracks in deformed structures. The originally developed reflection method of caustics is presented for application to cracks in mechanically anisotropic materials such as fiber reinforced composites. The derived solutions for general mixed mode (combination of mode I and mode II crack opening) are based on the geometrical principles of the reflected light beams deflecting from the surface surrounding the crack tip. The size of the dark spot depends on the load intensity, while its shape depends strongly on the mechanical properties of the material, the orientation of the crack-tip and the mixed mode mixture. The solution for the singular curve that surrounds caustics cannot be given in the general form; therefore the optical effect has to be simulated separately for each problem. The evaluation of optical effects is possible by using the diameter measuring method or advanced multi-point over-deterministic data reduction method. In order to find the exact position of caustics on the experimental images, the image analysis principles are applied through different methods based on the light intensity measurements along the radials. Two methods are considered: the simple boundary value method and a more sophisticated differential method, which is accomplished by shifting the real image into the gradient image, and collecting the experimental points that are located on the position of the maximum light intensity gradient. Computer programs are added to the image processing system and the optical method of caustics presented is improved through standard experimental testing procedure for cracks oriented 0, 45, 90 to the material axes in carbon fiber-reinforced polymer composites.

method of caustics; fracture mechanics; mechanical anisotropy; stress-intensity factor

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