A type of colddrawn AISI steel bars shown below is subjected

A type of cold-drawn AISI steel bars shown below is subjected to an axial load fluctuating between 40kN in compression to 40kN in tension. The stress concentration factor of the hole is K_t = 2.44, and the stress concentration factor for fatigue is K_f = 2.20. The selected steel material has mechanical properties as shown in the following S-N fatigue diagram. Note that 1 kpsi = 6.895 MPa. Roughly estimate the fatigue life cycle of the rod based on the S-N diagram. Exactly calculate the fatigue life cycle by assuming S_f = aN^b. Instead of completely reversal, if the axial load fluctuating between 15kN in compression to 40kN in tension, using the modified Goodman criterion to check if the part has infinite life or not.

Solution

The cross section of the bar is 25*10 mm²

The stress produced by the force is 40000N/250 MPa =160 MPa alternating tension and compression

Due to the stress conc factor of the hole, the resulting stress is 2.44*160 =390MPa = 56.6 kpsi

if the stress conc factor for fatigue is applied, the effective stress is 56.6*2.3 =124.56 kpsi

from the graph, the bar is likely to break or fail after 1 cycle.

forget the rest.