Crack Initiation and Propagation Behaviour of a Heat‐treated Carbon Steel in Corrosion Fatigue

Abstract

When estimating fatigue damage quantitatively it is important to clarify its physical basis. In this study, rotating bending fatigue tests of a heat-treated 0.45% carbon steel were carried out in 3% NaCl solution, in order to clarify the physical basis of corrosion fatigue damage from successive observations of plastic replicas. The results show that corrosion pits are generated during the early stages of cycling, then most of them grow with further cycling until a crack is initiated from each corrosion pit. The initiation of corrosion pits from slip bands is observed only in the case when the stress range is relatively large, and in the range of stress for which slip bands are produced in air. After initiation of a crack, the crack propagates by frequent interactions and coalescence with other cracks. The growth rate of an especially small crack in NaCl solution is larger than that in air. However, the growth rate of a comparatively large crack is smaller in NaCl solution than in air.

Influence of Pitting Corrosion on Fatigue and Corrosion Fatigue of Ship and Offshore Structures, Part II: Load - Pit - Crack Interaction

“…At low stress amplitudes a small number of relatively large pits is generated as a result of their nucleation and growth , and fatigue cracks are always initiated in the pits, whereas at higher stress amplitudes a large number of small pits is generated in slip bands, and the test duration time is probably too short to cause pits to reach their threshold size as a result of their growth process, therefore cracks start in slip bands and only from time to time in small pits. The above described behaviour cases were the same for the steel [11] and Al-alloy [2]. Some authors have introduced the notion of the so called non-damaging depth of pit which never leads to fatigue crack initiation.…”

“…In another corrosion fatigue tests [11] on a medium-carbon steel, pits were initiated in the very early stage of the tests , i.e. approximately after 5000 load cycles (the total corrosion fatigue life was in the range from 20000 to 500000 cycles), almost independently of stress level.…”

“…Some authors have introduced the notion of the so called non-damaging depth of pit which never leads to fatigue crack initiation. Goto and Nisitani [11] considered pit diameter to be pit threshold size , but noted that some pits whose diameter values were greater than threshold ones, did not undergo cracks because pit depth is a more important parameter than pit diameter. Most authors mainly consider pit depth despite difficulties met in measuring pit depth in real structures.…”

“…Almost all researchers are unanimous that pit's critical size is decreasing along with increasing cyclic stress level, see e.g. [5,6,10,11]. At low stress amplitudes a small number of relatively large pits is generated as a result of their nucleation and growth , and fatigue cracks are always initiated in the pits, whereas at higher stress amplitudes a large number of small pits is generated in slip bands, and the test duration time is probably too short to cause pits to reach their threshold size as a result of their growth process, therefore cracks start in slip bands and only from time to time in small pits.…”

“…As far as steels are concerned, majority of research papers deals with steels applied in machinery industry, i.e. medium-carbon steels after different heat treatment [11][12][13][14][15], or even high -carbon steels [16,17] or a martensitic stainless steel [18]. Only a few papers concern weldable low-alloy, low-carbon steels applicable to ship and offshore structures [4,[19][20][21][22].…”