From assumption no. (3) in the previous page: The strains of any two adjacent materials at their junction point are equal. $\epsilon_s = \epsilon_w$
$\dfrac{f_{bs}}{E_s} = \dfrac{f_{bw}}{E_w}$
$\dfrac{f_{bs}}{f_{bw}} = \dfrac{E_s}{E_w}$
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