Effect Of Plastic Deformation On Fatigue Crack Growth Rate

Because of plastic deformation the crack may close at a stress level s_cl which may be higher than s_min. This will effectively reduce the magnitude of ΔK_I. Effective value of the differential stress intensity factor may be written as

                                                           Δ K _{I eff} =  K _{I max}  - K _{I cl}

where, K_{I cl} corresponds to s_cl in the sense that

and hence

This effect has been investigated experimentally for the regime of σ_min > 0 and the experimental results suggest that

                                                        Δ K _{I eff} / ΔK _I =  0.5+ 0.4 R

where, R is the ratio  s_min  /s_max. Apparently if Eq. (30) is extended to pulsating stress cycle withsmin = 0, Δ K _{I eff}    will be only half of Δ K _I , indicating that fatigue crack growth rate is reduced in fluctuating and pulsating stress cycle. This conclusion was also drawn from crack growth model in previous section.