Fatigue Crack Propagation in Austenitic Stainless Steel Under Low Frequency Loading and Salt Water Conditions

Abstract

Fatigue crack growth rates (FCGR) for a steel similar to AISI 316LN have been evaluated in air and salt water. At free corrosion potential, a short crack effect appeared for crack lengths shorter than 4 mm. For longer cracks, a segment (called region I) of the corrosion FCGR curve can be described by an equation with the same coefficients as for steels of ordinary resistance to corrosion. Thus, the process controlling crack propagation is presumably the same for both steels. Corrosion FCGR in region II (range of longer crack lengths and higher ΔK values, where the curve cannot be described by the previously mentioned equation) are slower than for ordinary steels. The plateau in FCGR observed at a cathodic potential corresponds to the higher plateau at the free corrosion potential. An interpretation is presented for the shape of the corrosion FCGR curves at both potentials.

A model of corrosion fatigue crack growth in ship and offshore steels

“…4 In bcc materials, as structural steels, the hydrogen diffusion is relatively too fast to be the rate-determining step. The assumption given by (6) seems to be applicable in almost every case of possible micro-mechanisms of fracture and it is chosen as the basic assumption of the present model.…”

“…The assumption given by (5) has been successfully applied to corrosion fatigue cracking of fcc materials: an aluminium alloy 12 and an austenitic stainless steel. 4 In bcc materials, as structural steels, the hydrogen diffusion is relatively too fast to be the rate-determining step. The assumption given by (6) seems to be applicable in almost every case of possible micro-mechanisms of fracture and it is chosen as the basic assumption of the present model.…”

“…increases as the crack length is increased. In his previous papers, 3,4 , the author proposed for low carbon and low alloy steels in salt water environments the following empirical functions to describe the CFCGR characteristics more completely (a n in millimetres): da dN e = da dN air · β = da dN air 3.8 log(a n ) for region I (1a) or da dN e = da dN air · β = da dN air 1.15(a n ) 0.5 for region I (1b) and da dN e = da dN air C II · ( K ) −n for region II,…”

“…increases as the crack length is increased. In his previous papers, 3,4 , the author proposed for low carbon and low alloy steels in salt water environments the following empirical functions to describe the CFCGR characteristics more completely (a n in millimetres):…”

“…The same Eqs (1a) or (1b) can describe region I of the characteristics for high alloy stainless steels too, but a different equation describes region II in this case. 4 Previously, 3,4 the above empirical equations were explained qualitatively only. The main aim of the present paper is to elaborate a model that rationalizes the above empirical equations.…”