$(15$ points$)$ A stepped bar with fillets is loaded in pure tension as shown below. Use

$D=39mm,d=30mm,r=1\mathrm{.}6mm,h=7mm$ and $P=6kN$

$K_t$~$=A(\frac{r}{d}{)}^b$

where:

a$)(8$ points$)$ Determine the nominal stress at the fillets in MPa $.$

b$)(7$ points$)$ Determine the actual stress at the fillets in MPa.