8. 一元應力承受平面應力如圖(六)所示,則其主應力 $\sigma_1$、$\sigma_2$ 與最大剪應力 $\tau_{max}$ 之值為何?
(A) $\sigma_1 = 20 \text{ psi} \cdot \sigma_2 = -5 \text{ psi} \cdot \tau_{max} = 10 \text{ psi}$
(B) $\sigma_1 = 29 \text{ psi} \cdot \sigma_2 = -5 \text{ psi} \cdot \tau_{max} = 17 \text{ psi}$
(C) $\sigma_1 = 29 \text{ psi} \cdot \sigma_2 = 15 \text{ psi} \cdot \tau_{max} = 17 \text{ psi}$
(D) $\sigma_1 = 29 \text{ psi} \cdot \sigma_2 = 15 \text{ psi} \cdot \tau_{max} = 10 \text{ psi}$