ISOCHROMATIC FRINGES

The following table is taken from the User's Manual for the Overhead Polariscope Model 081 sold by the Measurements Group, Raleigh, NC 27611, (919) 365-3800.

The Tint-of-Passage is the purple color with wavelength 575 nm.

To obtain the actual magnitude of the maximum shear stress at some point in the model, first calibrate the model material to get the stress-optic coefficient C and measure the thickness t of the model.

Determine the fractional fringe number N of the isochromatic fringe pattern at the point in question and use the equation:

t _max = CN/(2t)

The direction of the shear stress at the point is found by determining the analyzer angle at which an isoclinic line passes through the point.

Understanding the sequence of colors as the stress in a model increases takes considerable practice. An excellent description of the color changes is found in the User's Manual mentioned earlier:

The Polariscope is normally used as a dark-field instrument, which means that with no stress in the model, and therefore no stress-induced birefringence in the model, all light is extinguished and the model appears uniformly black. As load is gradually applied to the model, the most highly stressed region begins to take on color -- first gray, then white; and, when the violet is extinguished, yellow. With further load, the blue is extinguished to produce orange; and then green to give red. The next color to vanish with increasing load is yellow, leaving a purple color; and this is followed by the extinction of orange, producing a deep blue fringe. ... Continuing to increase the load on the model and producing additional relative retardation, the red is extinguished from the white light spectrum, and the fringe color is blue-green. With still greater load, the relative retardation reaches the point where it corresponds to twice the wavelength of violet, extinguishing this color for the second time and starting the fringe cycle over again. However, the deep red color at the far end of the white light spectrum also has twice the wavelength of violet, and thus undergoes its first extinction simultaneously with the second extinction of violet. The result is that the fringe color is the combination of two complementary colors, yellow and green. As the load and relative retardation continue to increase, the fringe color cycle is repeated, but the colors are not exactly the same as in the first cycle because of the simultaneous extinction of two or more colors. With each successive complete color cycle, the effect of increasingly complex simultaneous extinctions is to cause the fringe colors to become paler and less distinct. Because of this effect, fringe orders above 4 or 5 are not distinguishable by color in white light. Although fringe orders higher than 3 are rarely encountered (or needed) in stress analysis, fringes of very high order can always be detected with monochromatic light.

 

 

Last Modified on April 20, 1997