Effective stress intensities in stress corrosion cracking

The phenomena of branching and blunting of stress corrosion cracks are reviewed and their effects demonstrated for a martensitic steel. The stress intensity that a crack can sustain is proportional to the square root of its tip radius, so that blunt cracks require a higher apparent stress intensity. A simple procedure is outlined for converting apparent stress intensities to effective stress intensities, so eliminating anomalous effects due to crack branching and blunting.

---

Résumé On passe en revue les phénomènes de ramification et d'arrondissement de l'extrémité des fissures de corrosion sous tension, et on examine leurs effets dans le cas d'un acier martensitique.L'intensité de contrainte qu'une fissure peut supporter est proportionnelle à la racine carrée de son rayon d'entaille, en sorte qu'à des fissures émoussées correspondent une intensité apparente de contrainte plus élevée. On propose une procédure simple permettant de convertir les intensités apparentes de contraintes ou intensité effective, ce qui permet d'éliminer les effets parasites associés à la ramification et à l'arrondissement des fissures.

     

Intergranular corrosion of a stud used in safety relief valve

In the present paper, a failure case study on intergranular corrosion of stainless steel stud is discussed. The stud was in the assembly under marine environment, for a period of 11 years. Detailed investigation revealed that the crack had propagated in an intergranular manner, after initiation at threaded region of the stud, leaving behind widespread grain dissolution. The presence of chloride ions in deposits on central region of stud, exposed to marine environment was also established. The high carbon content, coupled with exposure to sensitization range of stainless steel during processing caused corrosion along intergranular corrosion.This paper brings out the details of investigation carried out on the failed stud.     

The significance of stress corrosion cracking in corrosion fatigue crack growth studies

A model is proposed to account for interactions between fatigue and stress corrosion crack propagation mechanisms in appropriate corrosion fatigue conditions. Tests on an alloy steel, and both wrought and cast aluminium alloys, are reported. Despite the use of very simple coefficients in the equations derived, encouraging results are obtained.     

Intergranular corrosion in electrochemical machining

In a study of electrochemical drilling, the influence of the electrolyte composition on intergranular corrosion of various nickel alloys and of hardenable stainless steel was investigated. The nickel-base alloys remained unaffected in a solution of 15 % NaNO₃ + 20 % NaClO₃ + 65 % H₂O but suffered intergranular corrosion in solutions of NaNO₃ and NaNO₃ + NaCl. Stainless steel showed no signs of intergranular attack in any of the electrolyte solutions used.     

Failure analysis of stress corrosion cracking in aircraft bolts

This research was conducted on the failure analysis of the failed clamp bolt from a helicopter engine in the RoKAF. Through the fractography, metallography, and stress analysis of the failed part, it was found that the clamp bolt was fractured by stress corrosion cracking due to the interaction of tensile residual stress and corrosive environment. The stress corrosion crack is a phenomenon that occurs in susceptible alloys and is caused by the conjoint action of a surface tensile stress and the presence of a specific corrosive environment. Therefore, it is recommended that the material of the clamp bolt should be changed to prevent similar failures.     

Chloride-induced stress corrosion cracking of furnace burner tubes

The burner tubes (316SS) of an ethylene cracking furnace in a Saudi petrochemical plant experienced repeated premature failures. One failed sample was investigated by chemical and microstructural analytical techniques to find out the failure cause and provide preventive measures. The results indicate that the burner tube failed due to chloride-induced stress corrosion cracking (SCC). Chloride could be due to the contamination coming from the ambient industrial environment. The forming process (bending and drawing) of the tubes prior to installation introduced residual tensile stresses necessary for SCC. It is recommended to stress relieve (or shot peen) the tubes following the forming process, or alternatively apply protective coatings (chloride-free) to prevent contamination. Periodic cleaning of the tube exterior is necessary.     

Nucleation mechanism of stress corrosion cracking from notches

Crack nucleation mechanism of hydrogen assisted cracking at notched cracks in aqueous solutions is investigated, using the compact type specimens with various notch radius in low-tempered 4340 steel. A detached crack initiates at some distance ahead of the notch root. The crack nucleation at the notched root is determined by the electrical potential method. When the crack initiates, the voltage difference starts to increase. The crack nucleation site is examined by SEM. The time for crack nucleation increases with the notch root radius, ρ, and decreases with the apparent stress intensity factor K_ρ. A linear relationship between the crack nucleation time, t_n, and the parameter ${}^2\text{K}{}_{\mathrm{\rho }}\text{/(πρ)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{-(2K}{}_{\mathrm{\rho }}\text{/(πρ)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}{}_{\mathrm{th}}\text{}}$ is seen in semi-log diagram, where $\text{(2K}{}_{\mathrm{\rho }}\text{/(πρ)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}{}_{\mathrm{th}}$ is almost equal to the yield shear strength.In order to explain these experimental results, a new model of micromechanics is proposed on the basis of stress induced diffusion of hydrogen in the high stress region ahead of the notch root. This model suggests that the detached crack initiates at the elasto-plastic boundary where the hydrogen concentration is from 2 to 5 times higher than that of the notch root surface. The theory agrees with experiments with respect to $\text{{2K}{}_{\mathrm{\rho }}\text{/(πρ)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{-(2K}{}_{\mathrm{\rho }}\text{/(πρ)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}{}_{\mathrm{th}}\text{}}$ vs t_n and t_n vs ρ.The empirical equation holds under constant t_n, K_ρ = K_o(ρ/ρ_eff)^m where K₀ is the stress intensity factor with ρ ≈ 0 under the present environment, ρeff is the effective notch radius and m is constant. The value of m is 0.25 for the crack nucleation time (t_n)_th corresponding to the threshold stress intensity factor (K_ρ)_th, 0.5 for t_n < (t_n)_th and 0 for ρ ≦ ρ_eff. The above equation agrees with the theoretical equation proposed by Tanaka and Mura for any t_n and ρ_eff.     

Primary and secondary galvanic cells in stress corrosion cracking

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 27, No. 4, pp. 18–22, July–August, 1991.     

Investigation of stress corrosion cracking of low-alloy steel in water

For four low-alloy steels with a wide range of tensile strength, the dynamical processes of the nucleation and propagation of stress corrosion cracking (SCC) in water with various polarization conditions and in a 0.1 N K₂Cr₂O₇ solution were traced with an optical microscope. The results show that if the tensile strength of the steel is higher than a critical value that is different in different polarization conditions and K_IK_ISCC, the plastic zone in front of a loaded crack tip is enlarged with time i.e. the delayed plastic deformation occurs in all the environments used. The nucleation and propagation of SCR will proceed when this delayed plastic deformation develops to a critical condition. Neither anodic and cathodic polarization nor the inhibitor can change the feature of the delayed plasticity and the nucleation and propagation of SCC in water.In all the environments used, K_ISCC is increased and $\text{da}\text{dt}$ is decreased with the decrease in the strength of the steel. K_ISCC is increased and da/dt is decreased with the anodic polarization and the addition of the inhibitor, but the cathodic polarization has the opposite effect.     

DFT studies of sulfur induced stress corrosion cracking in nickel

The sulfur induced embrittlement of nickel Σ 5 (0 1 2), a symmetrical tilt grain boundary, has been studied to understand why and how impurity atoms weaken the grain boundaries of nickel. By using first-principles calculations, the effect of concentration of sulfur has been investigated to comprehend the basic rules on the deterioration of a nickel grain boundary. We have found that the extension of the nickel grain boundary is a result of the repulsion of the segregated and neighboring sulfur atoms.     

Evolution of anodic stress corrosion cracking in a coated material

In the present paper, we investigate the influence of corrosion driving forces and interfacial toughness for a coated material subjected to mechanical loading. If the protective coating is cracked, the substrate material may become exposed to a corrosive media. For a stress corrosion sensitive substrate material, this may lead to detrimental crack growth. A crack is assumed to grow by anodic dissolution, inherently leading to a blunt crack tip. The evolution of the crack surface is modelled as a moving boundary problem using an adaptive finite element method. The rate of dissolution along the crack surface in the substrate is assumed to be proportional to the chemical potential, which is function of the local surface energy density and elastic strain energy density. The surface energy tends to flatten the surface, whereas the strain energy due to stress concentration promotes material dissolution. The influence of the interface energy density parameter for the solid–fluid combination, interface corrosion resistance and stiffness ratios between coating and substrate is investigated. Three characteristic crack shapes are obtained; deepening and narrowing single cracks, branched cracks and sharp interface cracks. The crack shapes obtained by our simulations are similar to real sub-coating cracks reported in the literature.     

Initiation of stress corrosion cracking and fatigue crack under compressive stress

Macroscopic compressive stress was able to induce stress corrosion cracking (SSC) of type 304 stainless steel in a boiling 42% MgCl₂ solution and of mild steel in a boiling 60% Ca(NO₃)₂ + 3% NH₄NO₃. The incubation period of SCC under the compressive stress was ten to hundred times longer than that under the tensile stress.For ultra-high strength steel and aluminum alloy, a fatigue crack could initiate from a notch tip under a cyclic compressive load. The threshold value Δσ_th or Δ$\text{K}{}_{\mathrm{th(\rho )}}$ for fatigue crack initiation under the compressive load was four times as high as that under tensile load. The crack grew at a decreasing rate until eventually it stopped growing altogether under cyclic compressive load. The crack nucleation under compressive stress became easier and the propagation distance of the fatigue crack was longer if the minimum cyclic compressive load was near zero.     

Growth mechanism of stress corrosion cracking in high strength steel

Stress corrosion crack growth rates are measured at sveral stress intensity levels for low-tempered 4340 steel in 0.1N H₂SO₄ solution. The characteristics of the growth rates are divided into three regions of stress intensity factors: Region I near K_1SCC; Region III near unstable fracture toughness, K_1SC; and Region II, which lies between the two. K_1SCC is the value of K at which no crack growth can be detected after 240 hr.

In order to explain these experimental results, the crack initiation analysis reported in a previous paper is extended to the growth rates. A detached crack initiates and grows at the tip of an already existing crack. When the detached crack reaches the tip of the main crack, the process repeats as a new existing crack.

A relationship between crack growth rate, v, and stress intensity factor, K, is obtained as a function of b/a and a = b + d, where b is the distance from the tip of the main crack to the detached crack, and d is the ydrogen atom saturated domain.

The experimental data are in good agreement with the theoretical values in Region II when a = 0.02 mm, b/a = 0.8, c₁/c₀ = 2.8 for 200°C tempered specimens and a = 0.015 mm, b/a = 0.7, c₁/c₀ = 3.0, ρ_b = 0.055 mm for 400°C tempered specimens, where ρ_b is a fictitious notch radius. The plateau part in Region II for 400°C tempered specimens is also successfully explained by the present theory. For Region III, the value of b/a will be almost equal to 1 because v → ∞ for b/a → 1. On the other hand, for Region I, b/a will be zero, since the value of v becomes negligibly small and no crack growth is observable.