The Influence of Salt Water on Fatigue-Crack Growth in High-Strength Structural Steels

Abstract

Fatigue-crack growth is a potential failure mechanism that can be strongly influenced by a salt-water environment. This paper discusses fatigue-crack-propagation studies performed on three high-strength structural steels: 9Ni-4Co-0.25C, 12Ni 180-grade maraging, and 18Ni 200-grade maraging. Each steel underwent a program of fatigue cycling in two environments, a “dry” environment consisting of ambient room air and a “wet” environment consisting of 3.5 percent NaCl salt water. Relationships for fatigue-crack-growth rates as a function of the fracture mechanics stress-intensity factor (K) were obtained experimentally for each steel in both environments. The engineering significance of these relationships is presented in terms of stress levels and flaw sizes relevant to the steels under investigation. The results of this work indicate that, in an air environment, the fatigue-crack-growth rate correlates with the stress-intensity-factor range according to the power-law relationship, da/dN = C (ΔK)m. Fatigue-crack growth in each of the steels was accelerated by the salt-water environment. However, the environmental response varied significantly, depending upon the material and the ΔK level. The lower-toughness steel was less affected by environment, and environmental effects in all the steels diminished with increasing ΔK levels. No correlation was observed between fatigue-crack-growth behavior in the salt-water environment and the stress-corrosion-cracking parameter (K Iscc) obtained on the same materials.

Incomplete self-similarity and fatigue-crack growth

“…To facilitate this approach, we use the compilation of ambient-temperature data from the work of Ritchie and Knott (1973), based on experimental fatigue-crack propagation studies at ambient temperatures on a wide range of steels from (Carman and Katlin, 1966;Wei et al, 1967;Crooker et al, 1968;Miller, 1968;Bates et al, 1969;Crooker and Lange, 1970;Clark and Wessel, 1970;Evans et al, 1971;Ritchie and Knott, 1973). This collection of data represents steels with almost a fivefold variation in yield strength, from 433 to 2035 MPa, and includes several Ni-Cr low alloy steels (4340, En24, En30A, D6Ac), a 9Ni-4Co steel, NiMo-V rotor steel, an H-11 tool steel, and a series of ultrahigh strength Ni-maraging steels.…”

“…In the diagram, h is identified both with the in-plane dimension of specimen width W (closed symbols) and with the out-of-plane specimen thickness B (open symbols). Data are taken from the compilation in Ritchie and Knott (1973), derived from the experimental fatigue-crack growth results for low to ultrahigh strength steels at ambient temperatures at R ∼ 0 in Carman and Katlin (1966), Wei et al (1967), Crooker et al (1968), Miller (1968, Bates et al (1969), Crooker and Lange (1970), Clark and Wessel (1970), Evans et al (1971), Ritchie and Knott (1973).…”

Long fatigue crack growth behaviour of ferritic steel weld metal

“…26, the effective stress range ratio was calculated using equation (13). Substituting equation (13) into equation (12), DK eff values were obtained for use in the present study.…”

“…It was shown in steels that, for a given value of K max , the rate of crack propagation at R521 was approximately 1?5 times higher than that at R50 . 12 Taylor 13 suggested that the compressive part of the cycle must play a role in crack sharpening, which may cause further growth in the next tensile part of the cycle. In support of the above view, Fonte et al 14 explained that the controlling mechanism switches from a K max controlled behaviour for positive R values to a DK controlled mechanism for negative R values in aluminium alloys.…”