A $65\mathrm{.}4$mm wide air craft inspection panel ismadeof$7074-$T$651$ aluminum alloy. The material properties are :$$Fracture toughness, Kc$=25\mathrm{.}8$MNm$-3/2,$ Yield stress, $\backslash$sigma y$=505$MPa.

$$Following inspection, an edge through$-$crack of length $6\mathrm{.}4$mm is found. During flight, this plate is subjected to acyclic stress of $90\backslash$pm $30$MPa. $1.$ Ignoring the effect of the mean stress, calculate the number of cycles to failure $($Nf$)$ using Paris$$ Lawand the material constants$($in their standard units$)$:A$=1\mathrm{.}2$x$10-12$ and m$=2\mathrm{.}8$