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An experimental set-up of the method of caustics was proposed for the accurate evaluation of stress intensity
factor. The stress intensity factor evaluation was based on a new formula which was based on the caustic shape area. The
new formula was based on the shape area of the caustics. The stress optical constants were evaluated according to the new
formula of the stress intensity factor. The stress optical constants can be calculated by the shape area of the caustics. For
simple and accurate evaluation of the stress intensity factor, the caustic formed by the convergent light beams (caustic (f))
was used. For this caustic, the stress optical constant is cf = ν/E, where ν is the Poisson’s ratio and E is the modulus of
elasticity of the material. The Poisson’s ratio and the modulus of elasticity were evaluated by the method of strain-gauges.
The stress intensity factor must be independent on the z0 (z0 is the distance between reference plane and specimen) and the
magnification ratio λm (λm is dependent on the z0 and zi). Experiments have shown that the stress intensity factor was
changed for deference z0 and λm (deference zi). The accurate value of the stress intensity factor is that which is closed to
theoretical one. So, for a proper experimental set-up (combination of the z0 and zi) the evaluating stress intensity factor
values are accurate and independent of the z0 and zi..