= Solution
For a coarse-grained, approximately untextured polycrystal undergoing general plastic flow, use the conventional orientation-and-compatibility estimate
$$
\sigma_y\simeq M\tau_c,\qquad M\simeq3.
$$
With the supplied $\tau_c=27.5\,\mathrm{MPa}$,
$$
\boxed{\sigma_y\simeq82.5\,\mathrm{MPa}\quad\text{for the }M=3\text{ estimate}.}
$$
The round $M=3$ result is a conventional estimate for an unspecified coarse polycrystal, not a material-independent exact conversion. The most favorably oriented individual grains can first slip at $2\tau_c=55\,\mathrm{MPa}$; that is not the same event as compatible macroscopic yielding throughout the aggregate.
There is no single crystallographic tensile axis for all grains. Individual grains rotate and can develop <crystallographic texture>, while the macroscopic bar remains loaded along its tensile axis. A specified initial texture or yield criterion would be needed for a more exact numerical prediction.
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