Determine the design stress for bolts in a cylinder cover where the load is fluctuating due to gas pressure. The maximum load on the bolt is 50 kN and the minimum is 30 kN. The load is unpredictable and the factor of safety is 3. The surface of the bolt is hot rolled and the surface finish factor is 0.9. During a simple tension test and rotating beam test on ductile materials (40 C 8 steel annealed), the following results were obtained: 

Diameter of specimen = 12.5 mm; 

Yield strength = 240 MPa; 

Ultimate strength = 450 MPa; 

Endurance limit = 180 MPa.