Measurement of Principal Strains Directions
The principal strains directions refer to a Cartesian coordinate system, consisting of the symmetry axes of the body or, in their absence, of horizontal and vertical directions.
In the same way as the observation in a transmission polariscope, the isoclines are observed in white light because here they appear black on a colored background of isochromatics.
At every point of an isocline, the principal strains directions are parallel to the crossed axes of the polarizer and analyzer P and A. The isocline value at a point of the coating is obtained by the joint rotation of both polarizer and analyzer, until the isocline is brought to that point. The value of the isocline is given by the angle of rotation of the filters with respect to the predetermined reference position. From the isoclines obtained for a number of selected directions, the isostatics family can be deduced as previously described.
If the isoclines appear sharp, the directions of the principal strains vary greatly from one point to another; if the isoclines are large and shaded, the directions vary little, as happens, e. g. in a rectangular specimen loaded along its axis, with small geometrical discontinuities.