The disk in Fig. 4.2-7 of Sec. 4.2.6 is sometimes used as a calibration specimen when using the experimental technique involving photoelastic fringes (see Chap. 8). The fringes depend on the difference in the principal stresses $\sigma_1-\sigma_2$. Owing to symmetry, $\tau_{x y}=0$ along the $x$ axis. Thus $\sigma_x$ and $\sigma_y$ are the principal stresses along the $x$ axis.
(a) Using Eqs. (4.2-37) with $y=0$, reduce $\sigma_x-\sigma_y$ to the simplest form.
(b) With $P=1.0 \mathrm{kN}, R=50 \mathrm{~mm}$, and $t=6 \mathrm{~mm}$ plot $\sigma_x-\sigma_y$, for $y=0$, as functions of $x$.