応力拡大係数
δ = { 4 π K I 2 E σ Y (plane stress) 4 ( 1 − ν 2 ) π K I 2 λ E σ Y (plane strain) {\displaystyle \delta ={\begin{cases}{\frac {4}{\pi }}{\frac {K_{\rm {I}}^{2}}{E\sigma _{Y}}}\qquad {\mbox{(plane stress)}}\\{\frac {4(1-\nu ^{2})}{\pi }}{\frac {K_{\rm {I}}^{2}}{\lambda E\sigma _{Y}}}\qquad {\mbox{(plane strain)}}\end{cases}}} ... (21)