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To find the critical resolved shear stress for a single crystal of a hypothetical alloy, you take the slip direction, the slip plane, and the direction of tensile stress.
You calculate the angle between the direction of tensile stress, and the slip direction, and also the angle between the direction of tensile stress and the direction perpendicular to the slip plane.
You take the cosines of both of those angles, multiply them together, and multiply them by the magnitude of the tensile stress, and boom, you have the critical resolved shear stress for the crystal of this hypothetical alloy.
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