Angle α₊ Between Local Axis x (or y) and Principal Axis 1 (or 2) for Strains on Positive Surface Side

α_{+} = \frac{1}{2} [\arctan (\frac{γ_{x y, +}}{ε_{x, +} - ε_{y, +}}))]

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Angle α+ Between Local Axis x (or y) and Principal Axis 1 (or 2) for Strains on Positive Surface Side