Calculate number of cycles to fracture:

If the plastic deformation is taken into consideration calculate number of cycles to fracture in above Example.

Solution

Δ K I eff / Δ K I =  0.5 + 0.4 R

where, R =s_min/s_max = 30 /70 =0.43 and Δ K = 71√a

Δ KI eff = 71√a (0.5 + 0.4x0.43)

= 71 x 0.67√a 47.67 √a N/mm²

∴                      da /dN = 10^{- 8}  (47.67  )2.0 = 2.273 x 10^{- 5} a

∴                 

∴          N _f= 44003 x ln 146.14 /35= 44003 x 1.43 = 62890 cycles

Note that due to plastic deformation in crack tip zone there is more than 100% increase in fatigue life.