Determination of isoclinic and isochromatic parameters using the three-load method

Ekman and Nurse's load-stepping approach to phase-shifting photoelasticity (1998) is able to determine automatically the isoclinic and isochromatic parameters. A problem associated with this method is that, because of the small load increments used, it is difficult for the user to obtain three accurately equal-stepped loads even with a rather precise loading rig. This deviation between two load increments then has a direct effect on the accuracy of the results. Also, experiment shows that this method does not work well for low stressed regions because the phase differences between loads are too small. In view of this, a modified approach to load stepping is proposed and is called the three-load method. This new method uses the relationships of phase retardations at each point for three different loads to determine the isoclinic angle theta in the range -pi/2 < theta < pi/2. Then the isochromatic parameter can be determined unambiguously. This method is easy to carry out and basically retains the accuracy of the phase shifting technique except for the change in sign of the parameters in the ambiguous regions.