Determine the elastic load-bearing capacity \( M = M_s \) and the limit \( M = M_n \) of the given twisted rod below. Determine the ratio \( M_n / M_s \). For the determined values \( M_s \) and \( M_n \), create graphs of the twisting moments. In the calculations, assume \( G = 26 \, \text{GPa} \), the Tresca hypothesis, and the above-given values \( a \) and \( R_{pl} \) (under tension).