To show the feasibility of the modified optical detection setup for image processing data acquisition of crack tip growth, a reflected caustics experiment which can obtain two sets of images is adopted. A cracked plexiglas plate with a slit width smaller than 0.2 mm was submitted to a K_I mode of loading. The applied force system provides a constant strain rate of 3times 10^-5. As a result the variations of the caustic curve and interference fringes are slow . The thickness variation due to the Poisson's effect and the variation in the refractive index of the constrained material yields the fringes of the outer region in caustics. From the outer fringes of image # 1 we can monitor the distribution of stress under fracture condition. From the caustic curve of image # 2 we determine the stress intensity factor. The setting of two threshold values is beneficial to eliminate the effect of

noise. From the above algorithm we obtain G_high is 140 and G_low is 70. One example of the results of image # 1 processing is shown in Fig.5. The counter of fringe variation N(x,y) of the pixel (x,y) of the image is marked on the drawing. We can see the thickness variation near the crack tip zone is larger.

We use a Sobel mask operation to detect the edge of the caustic curves and by using the transverse diameter of the caustics, obtain the value of stress intensity factor. One example of the calculated data from image # 2 processing is presented in Fig.6.

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