Craze initiation during the environmental stress cracking of polymers

The environmental stress cracking behaviour of polycarbonate in ethanol was studied with the aim of critically evaluating craze initiation criteria. A combination of constant strain-rate tensile tests and creep tests were conducted in air and ethanol. The onset of crazing was determined by the use of departure points, which were shown to correlate well with the formation of optically visible crazes. Samples with different states of physical ageing and hence different relaxational behaviour were used to test out various initiation criteria. It was found that crazing occurred when the slower relaxations had accumulated a certain amount of strain, in this case, 0.1%, and that a criterion based on critical inelastic strain was the most appropriate. Post-immersion tests showed that the development of craze precursors occurs independently of the environment.     

A mechanism for transgranular stress-corrosion cracking

Analysis of load and current transients accompanying discontinuous transgranular stress corrosion cracking (T-SSC) are analyzed in light of fractographic evidence, resulting in a model for T-SCC which involves corrosion-enhanced cleavage. The model accounts for the crystallography and microstructural features of the resulting fracture, the orientation dependence of the fracture process, and the role of the electrochemical/alloy environment.Presented at Fourth Greek National Congress on Mechanics, 26–29 June 1995, held at Xanthi, Greece.     

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.

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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.

     

A model for prediction of time from corrosion initiation to corrosion cracking

Prediction of time to corrosion cracking is a key element in evaluating the service life of corroded reinforced concrete (RC) structures. This paper presents a mathematical model that predicts the time from corrosion initiation to corrosion cracking. In the present model a relationship between the steel mass loss and the internal radial pressure caused by the expansion of corrosion products is developed. The concrete around a corroding steel reinforcing bar is modeled as a thick-walled cylinder with a wall thickness equal to the thinnest concrete cover. The concrete ring is assumed to crack when the tensile stresses in the circumferential direction at every part of the ring have reached the tensile strength of concrete. The internal radial pressure at cracking is then determined and related to the steel mass loss. Faraday’s law is then utilized to predict the time from corrosion initiation to corrosion cracking. The model accounts for the time required for corrosion products to fill a porous zone before they start inducing expansive pressure on the concrete surrounding the steel reinforcing bar. The accuracy of the model is demonstrated by comparing the model’s predictions with experimental data published in the literature.     

Intergranular and transgranular cracking of Cu–30Zn brass during fatigue loading

Abstract

Fatigue tests have been performed on annealed α–brass to examine the dependence of fracture morphology on ?K. The cracking was predominantly intergranular at low values of ?K_Q (the stress intensity factor range when the specimen does not comply with plane strain conditions) and changed progressively to transgranular cracking at high ?K_Q values. Detailed scanning electron microscopy has been performed on the fracture surfaces of the specimens, especially from matching areas on opposite faces. It has been shown that matching extrusion and intrusion pairs as well as one–to–one matching of fine slip lines occurred on the intergranular facets indicating that plastic deformation causes the intergranular cracking. Intergranular cracking persists at low ?K_Q values even though the crack growth rate is smaller than for transgranular cracking because the latter is difficult to initiate. Transgranular cracks form only at regions of localized strain, e.g. coarse slip bands, or at cold–worked surfaces but such transgranular cracking cannot be maintained at low ?KQ values.

MST/209     

Failure analysis of stress corrosion cracking in heat exchanger tubes during start-up operation

The cracking failure of a new heat exchanger during first start-up operation has been analyzed. Through the investigation of the operating history of the equipment, analysis of the chemical compositions of tube material and corrosion products, metallographic test of specimens with cracks, the cracking mode can be described as the Stress Corrosion Cracking (SCC) of austenitic stainless steel. This kind of cracking was induced by the chloride in high temperature steam and tensile stress. The residual tensile stress due to seal expansion has been proved by numerical calculation. The pre-heating steam which was polluted by the catalyst with chloride is the main reason for the tube cracking in this case.     

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.     

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.     

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.     

Effect of hydrogen on stress corrosion cracking of copper

The effects of hydrogen on electrochemical behavior and susceptibility of stress corrosion cracking (SCC) of pure copper were studied. SCC susceptibility of pure copper in a 1 M NaNO₂ solution was increased by pre-charged hydrogen. The effect of hydrogen on the susceptibility is more obvious in the low stress region due to the longer fracture time, which resulted in a longer time for more hydrogen to diffuse toward the crack tip. Synergistic effects of hydrogen and stress on corrosion and SCC processes were discussed. The results showed that an interaction between stress and hydrogen at the crack tip could increase the anodic dissolution rate remarkably.     

Effect of hydrogen on stress corrosion cracking of copper

The effects of hydrogen on electrochemical behavior and susceptibility of stress corrosion cracking (SCC) of pure copper were studied. SCC susceptibility of pure copper in a 1 M NaNO₂ solution was increased by pre-charged hydrogen. The effect of hydrogen on the susceptibility is more obvious in the low stress region due to the longer fracture time, which resulted in a longer time for more hydrogen to diffuse toward the crack tip. Synergistic effects of hydrogen and stress on corrosion and SCC processes were discussed. The results showed that an interaction between stress and hydrogen at the crack tip could increase the anodic dissolution rate remarkably.     

Some characteristics of early stages of stress corrosion cracking

Environment-induced crack growth generally progresses through several stages prior to component failure. Crack initiation, short crack growth, and stage I growth are early stages in crack development that are summarized in this paper. The implications of these stages on component reliability derive from the extended time that the crack exists in the early stages because crack velocity is low. The duration of the early stages provides a greater opportunity for corrective action if the short cracks can be detected. Several important factors about the value of understanding short crack behavior include: (1) component life prediction requires a knowledge of the total life cycle of the crack including the early stages, (2) greater reliability is possible if the transition between short and long crack behavior is known since component life after this transition is short, and (3) remedial actions are often more effective for short than long cracks.     

The control of catalytic poisoning and stress corrosion cracking

When a structural metal is stressed in a hydrogen environment, the metal may crack at stress levels much lower than its normal strength. This embrittlement is caused by hydrogen atoms or ions rather than hydrogen molecules. The catalytic dissociation of hydrogen molecules into atoms or ions takes place at dislocation sites on a metal surface. The termini of dislocations on a metal surface are sites of localized high energy. There are “poisons” which will retard or even stop catalytic processes. The toxicity of a poison depends on the binding energy of the foreign atom to a dislocation terminus.Cracked specimens made of 4340 steel were tested in hydrogen gas and its mixture with SO₂, CS₂, CO₂, N₂ and Ar. Both SO₂ and CS₂ are very “toxic” and can stop a running crack. The toxicity of CO₂ is moderate. n₂ and Ar have no noticeable effect on a running crack.     

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.