A component made out of $2024-$T$4$ Aluminum is repeatedly subjected to the above uniaxial stress history shown in the figure. Assume the Morrow equation is valid $($for $2024-$T$4)$ for computing ${}_{ar}($from ${}_m$ and ${}_a).$ Estimate the number of repetitions necessary to cause fatigue failure?

Morrow Equation:

$)=900$Mpa,$b=(-0\mathrm{.}102$

The # of repetitions $B_f$ is given by $($Palmgren$-$Minor Rule$)$

$B_f[{\displaystyle \underset{?}{\overset{?}{}}}\frac{N_j}{N_{fj}}{]}_{\text{o}\text{n}\text{e}\text{r}\text{e}\text{p}}=1$