Influence of frequency and exposure to a saline solution on the corrosion fatigue crack growth behavior of the aluminum alloy 2024

The aim of this study is to identify domains where the interactions between mechanical, environmental and microstructural parameters may occur during corrosion fatigue crack growth in the aluminum alloy 2024. The scope considered encompasses the influence of frequency and of alternate immersion in saline solution. Corrosion fatigue crack propagation tests have been carried out under sinusoidal and saw-tooth waveforms, and at different frequencies, load ratios, grain orientations and tempers, in air, distilled water and 3.5% NaCl in permanent and alternate immersion. The stress corrosion cracking behavior of the alloy 2024 has also been considered in order to evaluate the possible contribution of this type of damage during corrosion fatigue crack growth. In 3.5% NaCl, growth rates were found to decrease with decreasing frequency. In alternate immersion, growth rates were increased by up to an order of magnitude for the ΔK values considered compared to permanent immersion and air. The possible mechanisms that govern the corrosion fatigue behavior of the 2024 alloy are discussed in terms of a competition between passivation and anodic dissolution and/or hydrogen embrittlement. Finally, it is proposed that the fatigue crack growth enhancement observed during permanent immersion is related to a crack-tip hydrogen embrittlement mechanism. Hydrogen would be produced by anodic dissolution in relation with film rupture periodicity and then be dragged into the process zone. In alternate immersion, precipitate-free zone dissolution would govern crack advance, as during stress corrosion cracking.     

The interaction between pitting corrosion,grain boundaries,and constituent particles during corrosion fatigue of 7075-T6 aluminum alloy

Concomitant corrosion fatigue research was performed on 7075-T6 aluminum alloy to gain an increased understanding of how microstructure influences pit growth, pit-to-crack transition, and critical crack propagation to fracture. Two thicknesses of rolled sheet and an extrusion of 7075-T6 aluminum alloy were etched and subjected to concomitant corrosion fatigue in a 3.5% sodium chloride solution. Testing was interrupted at various intervals to obtain information on pit generation, growth, and potential cracking. Results indicated that microstructure has a significant influence on pit-to-crack transition and fatigue crack propagation. Constituent particles competed with corrosion pits as critical crack nucleation sites, with some affecting the critical crack by either nucleation of additional cracking or linkage with the main crack. Post-fracture analysis confirmed the presence of noncritical cracks within the corroded region, related to pitting and constituent particles.     

Crack propagation in notched specimens of porous silicon carbide under cyclic loading

Notched specimens of porous silicon carbide with porosity 37% were fatigued under four‐point bending at frequencies of 30 and 0.3 Hz. The fatigue life expressed in terms of time was rather insensitive to the test frequency, while that expressed in terms of cycles was much shorter for the case of 0.3 Hz than for 30 Hz. A time‐dependent mechanism of stress corrosion cracking was mainly responsible for crack propagation, and stress cycling enhanced the crack‐propagation mechanism. The crack length was estimated from the change in compliance of the specimen. The crack‐propagation curve was divided into stages I and II. In stage I, the crack‐propagation rate decreased even though the applied stress intensity factor became larger with crack extension, and then turned to increase in stage II. The transition from stage I to II took place at a crack extension of around 0.8 mm. This anomalous behaviour is caused by crack‐tip shielding due to microcracking and asperity contact. Fractographic observations showed that the fracture path was along the binder phase between silicon carbide particles, or more precisely along the interface between particles and binders.     

Influence of stress state on high temperature fatigue crack growth in Inconel 718

The influence of stress state on fatigue crack growth in nickel-base superalloys at high temperature is considered, based on studies in corner crack specimens of Inconel 718 at 600 °C. At high frequency and low R , cycle-dependent trans-granular crack growth occurs along the whole crack front, and growth rates are similar at the surface and within the interior of specimens, maintaining the original quarter-circular shape. For conditions of low frequency and high R , increased crack growth rate per cycle is observed with the crack tunnelling ahead at the centre. A time-dependent intergranular crack propagation mode occurs in the plane strain interior, attributed to an oxidation mechanism, whereas near the surfaces under plane stress, a trans-granular cyclic plasticity mechanism is observed. It is proposed that in addition to frequency and R , that stress state influences the competition between the mechanisms controlling crack growth and the transition between them: plane strain in the interior favouring an oxidation-controlled intergranular cracking mechanism as compared with the plane stress surfaces where cyclic plasticity dominates. An FEM study suggests that this influence of stress state is not associated with variation of Δ K along the crack front.     

Cyclic corrosion crack-growth resistance of 26KhN3M2FA steel

We present results of an investigation of the cyclic corrosion crack-growth resistance of 26KhN3M2FA steel under conditions of the water-vapor phase transition and plot invariant dependences (pH _t = const) of the corrosion-fatigue crack-growth rate on the stress intensity factors for various stress ratios and temperatures of the medium under extreme electrochemical conditions at the tip of a corrosion-fatigue crack and taking the influence of an inhibitor (NaH₂PO₄ · 12H₂O) into account. We also study the behavior of electrochemical characteristics on the specimen surface and at the tip of a propagating corrosion-fatigue crack, quantitatively determine the intensity of local anode dissolution and hydrogen embrittlement in the course of crack propagation, and compare cyclic corrosion cracking resistances of 26KhN3M2FA and 34KhN1M steels.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 5, pp. 19–27, September – October, 1994.     

CRACK NUCLEATION AND PROPAGATION IN BLADE STEEL MATERIAL

Stress corrosion cracking and corrosion fatigue of 12 Cr steel in sodium chloride solution has been investigated. Tests have been performed in air at room temperature and in aqueous solution with 22% NaCl at 80°C. The influence of corrosion pits on crack nucleation has been investigated. On fracture surfaces tested in environment (22% NaCl solution), crack initiation was observed in correspondence with corrosion pits; in this case fatigue life can be described using a fracture mechanics approach. The ΔK value for crack nucleation from a pit in rotating bending fatigue tests is very low in air (about 3 MPa√m). The results of slow strain rate tests on smooth specimens show that there is a threshold stress intensity, K_ISCC, of about 15 MPa√m and a plateau in stress corrosion crack growth rate of about 10^-5mm/s.     

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.     

Spannungsrißkorrosion in hochreinem Wasser von 3½ % NiCrMoV-Vergütungsstählen für Dampfturbinen-Scheiben und -Wellen

Stress Corrosion Cracking in High Purity Water of 3½ % NiCrMoV – Quenched and Tempered Steel for Steam Turbine Discs and Shafts In recent years intergranular stress corrosion cracking has occured world-wide in the shrink-fitted discs of low pressure turbine rotors made of low alloy steels. Only in a few cases steam impurities such as NaOH, Na₂CO₃, Na₂SO₄, H₂S, or NaCl, which initiate SCC, could be found. The stress corrosion cracking behaviour of the turbine disc steel 26 NiCrMoV 14 5 with a yield strength of approx. 850 N/mm² was examined under special corrosion conditions. Gaseous and other impurities of the water, which lead to higher conductivity can initiate stress corosion cracks and increase the stress corrosion crack velocity insignificant. Stress corrosion crack initiation can be prevented by shifting the pH-value and the free corrosion potential in the region of passivity. Unfavourable crevice conditions must be avoided. Solutions are shown, how to prevent stress corrosion cracking of steam turbine discs.     

Spannungsrißkorrosion in hochreinem Wasser von 3–3 ½% NiCrMoV-Vergütungsstählen für Dampfturbinen-Scheiben und -Wellen. Teil I

SCC in High Parity Water In recent years intergranular stress corrosion cracking has occured world-wide in the shrink-fitted discs of low pressure turbine rotors made of low alloy steels. Only in a few cases steam impurities such as NaOH, Na₂CO₃, Na₂SO₄, H₂S or Nacl, which initiate SCC, could be found. To clarify the SCC-behaviour experiments on turbine disc steels with different chemical compositions an yield strength were performed in high purity water. The results show, that chemical composition has no effect on the crack initiation. Under high purity water conditions no crack initiation due to stress corrosion cracking is observed on the steel with a yield strength of 850 N/mm². On the steel with a yield strength of 1250 N/mm² which is not used in service, crack initiation occurs in pure water. But if sharp cracks already exist, crack propagation occurs in both cases. The investigations showed, that stress corrosion cracking of turbine discs can be prevented by a good water chemistry with a cation conductivity less than 0.2 μS/cm (μmho/cm).     

ENVIRONMENTAL EFFECTS ON FATIGUE FRACTURE MODE TRANSITIONS OBSERVED IN ALUMINIUM ALLOYS

Abstract— Fatigue crack propagation tests were carried out in different environments on 7075–T6 and 2024–T3 centre-cracked sheet specimens. Observations were made on the macroscopic transition from tensile mode to shear mode. The transition is suppressed by an aggressive environment, whereas it is promoted by an inert environment. As a consequence there is no unique correlation between the state of stress and the mode of cracking. Both the state of stress and the environment have a significant effect on the mode of cracking. A simple model for the effect of environment on fatigue crack growth is presented. The implications for crack growth under corrosion fatigue conditions are discussed.     

Review of stress corrosion cracking of pipeline steels in “low” and “high” pH solutions

This paper reviews the current understanding of the mechanisms of stress corrosion cracking of pipeline steels. The similarities, the differences and the influencing factors are considered for the high pH stress corrosion cracking caused by a concentrated bicarbonate-carbonate solution, and for the low pH stress corrosion cracking due to a diluter solution. For high pH stress corrosion cracking, it is well accepted that the mechanism involves anodic dissolution for crack initiation and propagation. In contrast, it has been suggested that the low pH stress corrosion cracking is associated with the dissolution of the crack tip and sides, accompanied by the ingress of hydrogen into the pipeline steel. But the precise influence of hydrogen on the mechanism needs to be further studied.     

The theory of non-steady state fracture kinetics analysis—2: Non-steady state crack propagation in stress corrosion cracking

The extension of the kinetics theory of thermally activated crack propagation to the analysis of non-steady state conditions leads to a series of n homogeneous linear first order differential equations. The theory is applied to Regions I and II of stress corrosion cracking, controlled by the cyclically repeating two-step consecutive energy barrier system of corrosion reaction and bond breaking. For the model the system of n differential equations representing the crack tip concentration distribution is reduced to three expressions. The analysis provides a means by which the existence and the duration of the transition state can be determined, a necessary information for both steady state and for high frequency stress corrosion fatigue tests.     

ENVIRONMENTAL EFFECTS ON CRACK PROPAGATION IN ALUMINIUM ALLOYS

Abstract Crack propagation rates have been measured in two aluminium alloys under cyclic and static loading, in air, and in salt solution. On the basis of these results, a model is proposed, whereby corrosion fatigue crack propagation may be interpreted in terms of fatigue and static stress corrosion characteristics. Two interacting processes are operative; one is "stress assisted dissolution", which tends to inhibit mechanical failure by crack blunting and microbranching. The other is "environment assisted fracture" which occurs too rapidly for dissolution to occur. One or other of these processes is always observed to be dominant. This proposal is discussed in relation to other recent models for corrosion fatigue cracking. The effects of frequency, waveform and mean stress variations are also considered.     

Zusammenwirken von Spannungsrißkorrosion und Ermüdung bei der Schwingungsrißkorrosion eines hochfesten Stahles

Conjoint Action of Stress Corrosion Cracking and Fatigue on Corrosion Fatigue of a High Strength Steel The corrosion fatigue characteristics of a high strength, martensitic steel in 0.5 n NaCl solution is investigated with regard to the fatigue and stress corrosion cracking behaviour of the material. Test parameters are stress ratio and frequency, testing is carried out with fracture mechanics methods, the crack surfaces are examined fractographically. An analysis of the results reveals that corrosion fatigue in high strength steel is caused by fatigue or by stress corrosion cracking, depending on the kinetics of the two processes. Fatigue and stress corrosion cracking do not act cumulative or additive. Instead, the kinetically faster process causes crack advance. The crack growth characteristics are interpreted with respect to the fractographic appearance of the crack surfaces. Corrosion fatigue cracks propagate either intergranular relative to the prior austenite grain boundaries as stress corrosion cracks do or transgranular like fatigue cracks, depending on the crack growth rates of the two processes. Fatigue and stress corrosion cracking do not interact, at least in a measurable degree, because of the different crack path of the two fracture processes. Results can be assessed quantitatively with the “process competition model”.     

On the stress corrosion cracking mechanisms of austenitic stainless steels

In this paper, experimental results on stress corrosion cracking in austenitic stainless steels are described. Crack growth data in sodium chloride solution for AISI 304 steel obtained for different metallurgical conditions, acoustic emission data recorded during crack growth and fractographic observations have been discussed with a view to identifying the operating mechanism. Some of the experimental observations such as crack propagation occurring in discontinuous jumps of the order of a few microns, lowering of the threshold stress intensity andJ-integral values on sensitization and cold working, typical transgranular fractographic features, transition in mode of fracture from transgranular to intergranular in sensitized conditions and activation energies of the order of 50 to 65 kJ/mol can all be accounted by hydrogen embrittlement mechanism. Hydrogen generated at the crack tip by corrosion reaction diffuses ahead of the crack tip under hydrostatic stress and influences the deformation process at the crack tip and also leads to the brittle component of the crack advance in jumps.     

A MICROCRACK GROWTH LAW FOR MULTIAXIAL FATIGUE

Within the past decade, critical plane approaches have gained increasing support based on correlation of experimentally observed fatigue lives and microcrack orientations under predominately low cycle fatigue (LCF) conditions for various stress states. In this paper, we further develop an engineering model for microcrack propagation consistent with critical plane concepts for correlation of both LCF and high cycle fatigue (HCF) behavior, including multiple regimes of small crack growth. The critical plane microcrack propagation approach of McDowell and Berard serves as a starting point to incorporate multiple regimes of crack nucleation, shear growth under the influence of microstructural barriers, and transition to linear crack length-dependent growth related to elastic-plastic fracture mechanics (EPFM) concepts. Microcrack iso-length data from uniaxial and torsional fatigue tests of 1045 steel and IN 718 are examined and correlated by introducing a transition crack length which governs the shift from nonlinear to linear crack length dependence of da/dN. This transition is related to the shift from strong microstructural influence to weak influence on the propagation of microcracks. Simple forms are introduced for both the transition crack length and the crack length-dependence of crack growth rate within the microcrack propagation framework (introduced previously by McDowell and Berard) and are employed to fit the 1045 steel and IN 718 microcrack iso-length data, assuming preexisting sub-grain size cracks. The nonlinear evolution of crack length with normalized cycles is then predicted over a range of stress amplitudes in uniaxial and torsional fatigue. The microcrack growth law is shown to have potential to correlate microcrack propagation behavior as well as damage accumulation for HCF-LCF loading sequences and sequences of applied stress states.     

Stress corrosion cracking of GRP pultruded rods in acid environments

Stress corrosion cracking of GRP pultruded rods has been investigated in 0.0001 to 5.0 N hydrochloric acid environments under bending and tensile loading modes. Crack initiation takes place at exposed glass fibres in the surface of the rod, and crack propagation is planar and at right angles to the rod axis. Leaching of calcium and aluminium from the fibres takes place during the cracking process, and time-to-failure is dependent on the acid concentration, the stress level and the ease of access of the acid to the glass fibre surface. Possible mechanisms of crack propagation through the glass fibres and resin are discussed.     

Molecular Dynamics Simulation of Porous Layer-enhanced Dislocation Emission and Crack Propagation in Iron Crystal

The internal stress induced by a porous layer or passive layer can assist the applied stress to promote dislocation emission and crack propagation, e.g. when the pipeline steel is buried in the soil containing water, resulting in stress corrosion cracking (SCC). Molecular dynamics (MD) simulation is performed to study the process of dislocation emission and crack propagation in a slab of Fe crystal with and without a porous layer on the surface of the crack. The results show that when there is a porous layer on the surface of the crack, the tensile stress induced by the porous layer can superimpose on the external applied stress and then assist the applied stress to initiate crack tip dislocation emission under lowered stress intensity KI, or stress. To respond to the corrosion accelerated dislocation emission and motion, the crack begins to propagate under lowered stress intensity KI, resulting in SCC.     

Engineering property comparisons of 7050-T73651, 7010-T7651 and 7010-T73651 aluminium alloy plate

A comparison of some engineering properties of 7050-T73651, 7010-T7651 and 7010-T73651 plate has been made. The properties investigated were strength, stress corrosion resistance, fracture toughness and fatigue crack propagation resistance under flight simulation loading.

It was found that both 7050 and 7010 are high strength deep hardenable alloys with only minor differences in crack tolerance properties. The fracture toughness of both alloys is equivalent, while 7050 possesses slightly better resistances to stress corrosion cracking and fatigue crack propagation under flight simulation loading.