Question $1(0\mathrm{.}4$ points$)$

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A steel tension bar $8$ mm thick and $90$ mm wide with double edge cracks each $8$ mm

long is subjected to a uniaxial stress ${}_{-}0=120$MPa. Determine the stress intensity

factor $K_t.$ If $K_{lc}$ for this steel is $125$MPa$-\sqrt[\phantom{2}]{?}m,$ is the crack stable?

$K_{\text{I}}=200$MPa$-\sqrt[\phantom{2}]{m},$ the crack is unstable.

$K_t=20$MPa$-\sqrt[\phantom{2}]{?}m,$ the crack is stable.

$K_t=630$MPa$-\sqrt[\phantom{2}]{?}m,$ the crack is unstable.

$K_{\text{I}}=20Pa-\sqrt[\phantom{2}]{?}m,$ the crack is stable.