Maximum stress at the hole:
σ=P/A
Then A = (D-dh)*t   A = (9.00-1.50)cm*1.00cm = 7.50cm².
σ = 12,000 N / 7.50cm² = 1,600 N/cm²
We calculate dh / D = 15mm / 90mm = 0.166
Using the diagrams of R.E. Peterson, “Design Factors for Stress Concentration”. Machine Design.
1953 for obtain Kt with (dh / D):
Kt  2.55
Then σmax = Kt*σ  σmax = 2.55*1,600 N/cm²  4,080 N/cm²
Maximum stress at the fillet:
σ=P/A
Then A = d*t   A = 1.50cm*1.00cm = 1.50cm².
σ = 12,000 N / 1.50cm² = 8,000 N/cm²
We calculate r/d and D/d:
r/d = 7.50mm / 15mm  r/d = 0.50
D/d = 90mm / 15mm  D/d = 6
Using the diagrams of R.E. Peterson, “Design Factors for Stress Concentration”. Machine Design.
1953 for obtain Kt with (r/d and D/d):
Kt  1.60
Then σmax = Kt*σ  σmax = 1.60*8,000 N/cm²  12,800 N/cm²