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The remaining fatigue life for aluminum aerospace alloys is greatly reduced by the presence of corrosion damage; prediction of this deleterious influence via standard fracture mechanics methods is non-trivial. Several corrosion morphologies can develop depending on the local chemistry, pH and electrochemical potential along the geometry of the structure. Previous studies demonstrate that such variations in corrosion morphology can affect the fatigue behavior. This study aims to quantify the effect of corrosion morphologies relevant to a fastener-hole geometry of AA7050-T7451 on the initiation life, propagation life and total fatigue life. Four corrosion morphologies are considered: shallow and deep discrete pits widely distributed on the corroded surface, deep fissure-like features, and general corrosion with surface recession. Corrosion morphologies are carefully characterized using optical microscope, white light interferometer and X-ray computed tomography. The samples are subjected to fatigue using loading protocols that create identifiable marks on the fracture surface at known cycle count. These marker bands are used to determine the small scale crack growth rate (da/dN) of the samples, as well as the cycles to initiate a 10 μm crack. Results showed that initiation life plateaus to near constant values for varied corrosion morphologies, and crack growth rates converge to comparable values. The macro-scale corrosion metrics are considered and correlated to the location of the fatigue initiation. Severe macro-scale metrics such as pit or fissure depth, pit mouth surface area, pit volume, pit density, root mean square, peak density, maximum valley depth, do not uniquely correlate with the location of fatigue crack initiation. This suggests a strong role of micro-scale corrosion features and underlying microstructural features on the location of the fatigue crack initiation. [ABSTRACT FROM AUTHOR]

Crack growth rate and S–N fatigue testing of Custom 465-H950 was performed in 0.0006–3 M NaCl with different levels of pre-exposure. Custom 465-H950 exhibited a strong corrosion resistance to various NaCl-based exposures. Concurrent exposure and loading did not result in corrosion damage sufficiently severe to drive crack formation away from microstructure features (TiC particles). The total fatigue life of precorroded specimens (pit depth ≈40 μm, surface diameter ≈175 μm) increased 4–9 fold as [Cl − ] decreased from 3.0 M to 0.0006 M. Detailed analysis demonstrated that the varying [Cl − ] most strongly influenced the initiation life, with secondary contributions from the long- and short-crack propagation regimes. [ABSTRACT FROM AUTHOR]

Fatigue crack growth rates (L–T) for 102 and 178 mm plates of 7085-T7451 are established at R = 0.1 and 0.5 for a wide-range of Δ K and environments. At R of 0.1, the 178 mm plate exhibits slower crack growth rates independent of environment. The crack growth trends correlate with changes in the fracture surface roughness and compliance-based closure indications, suggesting a dominant role of roughness induced closure. At R of 0.5 compliance-based metrics indicate that minimal closure exists, but plate thickness dependent crack growth behavior persists; grain-scale crack path tortuosity and crack branching govern this behavior. Increased crack path roughness, tortuosity, and branching in the 178 mm plate is attributed to strain localization/cracking along precipitate free zones formed in the thicker plate due to a slower quench rate. [ABSTRACT FROM AUTHOR]

The influence of the applied loading rate (dK/dt) on the environment-assisted cracking (EAC) behavior of sensitized AA5456-H116 in 0.6 M NaCl at applied potentials ranging from −800 to −900 mV SCE is assessed via a rising-K testing framework. The applied potential strongly affects the dK/dt-dependence of EAC with results suggesting a minimal influence for potentials more positive than −830 mV SCE and a stronger dK/dt-dependence for potentials more negative than −830 mV SCE. Crack growth rates measured using rising versus static K testing are compared, which demonstrates that rising K methods consistently yield conservative EAC metrics with increased efficiency. • The effect of dK/dt on EAC in sensitized AA5456-H116 depends on applied potential. • EAC is effectively dK/dt-independent for potentials more positive than − 830 mV SCE. • EAC is strongly dK/dt-dependent for potentials more negative than − 830 mV SCE. • Differences in dK/dt dependence are linked to EAC mechanism for Al-Mg alloys. • Rising K-based testing consistently yields efficient and conservative EAC metrics. [ABSTRACT FROM AUTHOR]

Highlights: [•] Fracture surface measurement techniques enhance microstructure scale characterization of cracking. [•] Low temperature and high alloy purity increase fatigue resistance in 7xxx-series alloys. [•] Fatigue behavior reflects the effect of low temperature on the hydrogen embrittlement process. [ABSTRACT FROM AUTHOR]

Abstract: High fidelity measurements of constituent particle or corrosion topography nucleated fatigue crack growth rates (da/dN) are established for 7075-T651 in humid air. Values of microstructure-scale da/dN are determined by microscopy of programmed load-induced crack surface markers, rather than surface-only measurements. Both pristine and corroded specimen da/dN from various applied stress levels are successfully correlated using continuum-elastic stress intensity (ΔK or ΔK and K max) or dislocation-based (Bilby–Cottrell–Swindon) crack tip opening displacement (cyclic ϕ and ϕ max), with the former accounting for the gradient of elastic stress concentration due to the initiating feature. Values of da/dN vary by an order of magnitude at each fixed driving force due to microstructural influences that result in a locally irregular crack front. Grain-scale models using stress intensity closure or slip-based crystal plasticity do not capture experimental da/dN variability. Due to an inadequate mechanistic basis, mechanics-inspired models of da/dN do not predict multiple growth regimes that are typical of environment enhanced cracking. An elastic ΔK-based description of long crack da/dN data for a given alloy-environment can be transformed to a continuum elastic–plastic ϕ c basis to provide a mean crack growth rate description. Coupling mean rates with a statistical description of microstructure sensitive variability, and dislocation or crystal plasticity-finite element modeling of component ϕ c for non-continuum cracking, will enhance prognosis in the MSC regime. [Copyright &y& Elsevier]

Abstract: The effect of existing-localized corrosion on fatigue cracking of 7075-T6511 was established using crack surface marker-band analysis and a fracture mechanics model. The substantial reduction of fatigue life due to EXCO solution L–S surface pre-corrosion is nearly independent of exposure time after initial-sharp degradation, scaling with the evolution of pit-cluster size and initial stress intensity range with exposure time. Independent of exposure time, formation of a resolvable fatigue crack (∼10μm) accounts for a similar-low (∼5%) fraction of total fatigue life at low stress range (σ max =150MPa, R =0.1). Crack formation occurs at microscopic protrusions into the corroded volume. A corrosion-modified-equivalent initial flaw size (CM-EIFS); predicted with the AFGROW tool using measured initial aspect ratio, initiation cycles, and total fatigue life inputs; accurately represents the corrosion damage effect on fatigue for a range of exposures. The similar deleterious effect of several corroding environments for various-exposed surfaces is described by a lower-bound CM-EIFS with a 300μm depth and 1200μm surface length suggesting fatigue is governed by a microscopic pit-based topography. Either an approximate lower-bound, or specific CM-EIFS calibrated by limited measurements of fatigue life for service-environment exposed specimens, can be used to assess the impact of corrosion in a damage tolerant framework. Complexities (e.g., local H embrittlement, 3D pit geometry, topography dependent initiation, and microstructure sensitive small-crack growth) do not compromise the CM-EIFS estimation, but must be better understood for refined modeling. [Copyright &y& Elsevier]

Abstract: The effect of precorrosion on the fatigue life of aluminum alloy 7075-T6511 was measured, physical characteristics of corrosion topography plus fatigue damage were established by microscopy, and a corrosion modified equivalent initial flaw size (CM-EIFS) was established using fracture mechanics modeling. Fatigue life is reduced by clustered corrosion pits on the L–S surface from laboratory-EXCO exposure. Cracks initiate from pits clustered as a semi-elliptical surface micronotch rather than the deepest pits, consistent with shape-dependent stress intensity. Marker band analysis establishes that the number of cycles to form a crack about a pit cluster can be a significant fraction of total fatigue life. The CM-EIFS, back-calculated from fracture mechanics analysis of measured fatigue life, equals measured initiating-pit cluster size provided that important inputs are provided; such favorable comparison validates this approach to corrosion-fatigue interaction. Calculated CM-EIFS provides a metric to characterize alloy corrosion damage, and can be used to forward-model the effects of stress and loading environment on fatigue life distribution, critical for efficient alloy development. Use in prognosis of the fatigue performance of a service-corroded surface is hindered by uncertain non-destructive characterization of corrosion topography. [Copyright &y& Elsevier]

• Corrosion and microstructure characterized a fatigue initiation sites in AA7050. • No independent correlation between individual features and initiation locations. • Strongly suggests a coupled interaction of features governs the initiation behavior. The fatigue resistance of aerospace structures is degraded by the presence of corrosion damage. This work characterizes the micro-scale (<50 µm) corrosion morphologies and the underlying microstructure of AA7050 that impact fatigue crack formation. Corroded specimens were cycled to failure using a fracture surface marking spectrum. Pre-test corrosion characterization, fractography and EBSD were performed. No corrosion damage or microstructure metric independently correlates to the location of fatigue crack initiation from corroded surfaces. However, there is a modest correlation between the grain that initiates the fatigue crack and grains that have a high misorientation angle with respect to adjacent grains. [ABSTRACT FROM AUTHOR]

Airframe structural components experience coupled load–environment spectra that are currently not captured in traditional damage‐tolerant management approaches. The effect of incorporating fatigue crack growth kinetics relevant to high‐altitude flight environments into LEFM‐based predictions of fatigue progression for aluminum alloy 7075‐T651 was quantitatively assessed for a variety of load–environment spectra using AFGROW. The results of this study demonstrated that in all conditions, the incorporation of high‐altitude‐relevant growth kinetics enhanced the fatigue life predictions. However, the magnitude of the impact on the overall life is dependent upon the subtleties of the coupled load–environment spectrum, despite the fact that environmental effects are most potent in the near‐threshold regime of the da/dN versus ΔK relationship. The potential impact to the structural integrity management community is discussed, and remaining challenges and opportunities are identified. Highlights: Applying low water vapor pressure fatigue growth rates extended fatigue life in LEFM modeling.When applied to in‐service loading spectrum for aerospace applications, this extension was maintained.Subtleties with environment loading and fatigue life were investigated. [ABSTRACT FROM AUTHOR]

Corrosion processes are often discussed as stochastic events. Here, it is shown that some of these seemingly random processes are not driven by nanoscopic fluctuations but rather by the spatial distribution of micrometer‐scale heterogeneities that trigger fast reactions associated with corrosion. Using a novel excitable reaction‐diffusion model, corrosion waves traveling over the metal surface and the associated material loss are described. This resulting nonuniform corrosion penetration, seen as a height loss in modeling, exposes buried intermetallic particles, which depending on the local electrochemical state of the surface trigger or block new waves. Informed by quantitative experimental data for the Mg–Al–Zn alloy AZ31B, wave speeds, wave widths, and average material loss are accurately captured. Morphogenic mitigation based on wave‐breaking microparticles is also simulated. While AZ31B corrosion is identified as a process driven by rare‐wave events, this study predicts several other corrosion regimes that proceed via spots or patchy patterns, opening the door for new protection, design, and prediction strategies. [ABSTRACT FROM AUTHOR]

Understanding of the microstructure and mechanical properties of additively manufactured (AM) 17-4PH stainless steel has matured significantly, but combined wrought-level strength and ductility has yet to be achieved. Identifying the microstructural obstacle(s) impeding this milestone is critical for many future applications and industries interested in this AM alloy. In this work, microstructure, mechanical properties, and fracture behavior is evaluated for AM 17-4PH built via the laser powder bed fusion technique (LPBF) and heat treated via a broad range of techniques to achieve wrought-level 17-4PH strength. The highest strength/ductility was achieved with a two-stage hot isostatic press (HIP) and solution anneal (SA) treatment followed by an aging heat treatment, but wrought-level ductility could not be matched. Persistent nano-porosity (0.06–0.23 μm) was identified and correlated to fracture surface dimpling (R2 = 0.73–0.85) indicating a possible role in this ductility limitation. Contextualization of this study's mechanical properties dataset within the literature suggests that this observation is endemic to the current state-of-the-art processing techniques for AM 17-4PH. • HIP minimizes larger pores, but nanometer-scale pores (60 nm) persist and coarsen. • Reviews 40+ AM 17-4PH mechanical properties studies grouped by heat treatment. • Apply a SA+460 °C/1 h treatment for balanced strength, ductility, and cost. • Apply HIP + SA+460 °C/1 h for strength/ductility and wrought-like microstructure. [ABSTRACT FROM AUTHOR]

Highlights • FIB damage during Pt deposition on Al alloys depends on the Pt deposition protocol. • Use of a combined E-beam/I-beam Pt deposition approach reduces observed damage. • A minimum E-beam Pt depth of 100 nm is recommended to effectively prevent damage. Abstract It is well known that damage induced by impinging Ga+ ions during focused ion beam (FIB) milling of transmission electron microscopy (TEM) specimens can obfuscate subsequent TEM characterization, especially in the near-surface region of the TEM foil. Numerous strategies for minimizing this damage have been invoked, with the most common being the deposition of a Pt 'strap' at the area of interest. However, damage can still occur in the near-surface region during this Pt deposition step and the variation in the character and extent of this damage with applied Pt deposition parameter, especially in complex structural alloys, is not well characterized. In this study, the damage induced in an aerospace Al alloy (AA7075-T651) during five different Pt deposition protocols is examined using TEM. Results indicate significant variations in damage character and depth amongst the applied Pt deposition protocols, with damage being effectively eliminated using a combined electron-beam/ion-beam Pt deposition strategy. These experimental results are found to be in good agreement with Monte Carlo-based simulations of ion implantation and the implications of these findings on recent experiments in the fracture mechanics community are explored. [ABSTRACT FROM AUTHOR]

• Fatigue crack growth testing of AA7075-T651 is performed under ultra-high vacuum. • The role of temperature is evaluated between 23 and −65 °C. • A temperature-dependent transition in da/dN is observed for all conditions. • This transition correlates with the onset of slip band cracking. • The mechanism proposed for this transition behavior is related to slip inhibition. It is well-established that the fatigue crack growth behavior of aerospace aluminum alloys in high altitude applications is sensitive to both temperature and water vapor pressure. To isolate the role of temperature, fatigue crack growth experiments were performed on AA7075-T651 under ultra-high vacuum conditions (<6x10−6 Pa) at temperatures ranging from 23 °C to −65 °C. All da/dN–ΔK relationships exhibit the following: (1) a temperature-independent Paris regime at high ΔK, (2) a transition from the Paris regime to a sharply reduced da/dN, which occurs at higher ΔK as the temperature decreases, and (3) a coalescence in da/dN for all temperatures in the threshold region. Interestingly, the point of deviation from the Paris regime is found to correlate with a transition from a flat, transgranular fracture morphology to slip band cracking (SBC) across all conditions. The likely mechanisms governing the observed behaviors are discussed, focusing on the role of slip. [ABSTRACT FROM AUTHOR]

Uniaxial tensile experiments on hydrogen-charged and non-charged polycrystalline nickel deformed to a true plastic strain of ~0.1 at room temperature and 77 K reveal that hydrogen has no measurable effect on texture evolution. Texture measurements were completed on samples of each condition, as well as an as-received reference specimen, using electron backscatter diffraction data collected from >20,000 individual grains. Crystal plasticity simulations support the conclusion that the similarity is attributable to the insensitivity of small-strain deformation texture evolution to potential hydrogen-induced modifications in deformation mechanisms. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The effect of hydrogen (H) on the deformation behavior of Monel K-500 in various isothermal heat treatment conditions (non-aged, under-aged, peak-aged, and over-aged) was assessed via uniaxial mechanical testing. H-charged and non-charged specimens were strained to failure to facilitate a comparison of ductility, fracture surface morphology, strength, and work hardening behavior. For all examined heat treatment conditions, H charging leads to a significant reduction in ductility, which is accompanied by a consistent change in fracture surface morphology from ductile microvoid coalescence to brittle intergranular fracture. While H charging led to a systematic enhancement in the yield strength of all heat treatments, the three age-hardened conditions exhibited a more than 2-fold increase relative to the non-aged heat treatment. This suggests that H modifies the dislocation–precipitate interactions, which also manifest themselves through changes in work hardening metrics related to the dislocation storage and recovery rates. In particular, the H-charged peak-aged specimen exhibited a significant increase in initial hardening (dislocation storage) rate relative to the H-charged under-aged specimen. Transmission electron microscopy of these samples revealed the onset of widespread dislocation looping in the H-charged peak-aged sample, in addition to the planar slip bands characteristic of the non-charged condition. This result suggests that hydrogen induces the particle shearing-to-looping transition at smaller particle sizes. Possible mechanistic explanations for this observed behavior are presented. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• The validity of the K Tada solution for the pinned SEN(T) is evaluated. • Measured da/dN reductions demonstrate that the true K can deviate below K Tada. • FE calculations quantify the influence of non-linear geometry effects on K Tada. • Most pinned SEN(T) data are valid due to low a/W, low K Tada , and high alloy modulus. • A new K solution is needed to cover broader ranges of a/W, load, and alloy modulus. The validity of the Tada stress intensity factor (K Tada) for pinned-ends single edge notch tension [SEN(T)] specimens is assessed via a combined experimental-modeling approach. Analysis of fatigue crack growth rate reductions during constant-ΔK Tada loading demonstrates that specific combinations of alloy stiffness, geometry, and loading result in the true K deviating below K Tada. Geometrically non-linear, 3-dimensional finite element calculations confirm mild-to-strong influences of these parameters, which are not captured by K Tada. Most existing pinned SEN(T) data are found to use parameters where K Tada is not significantly reduced, but the present results underscore the need for a more broadly applicable K solution. [ABSTRACT FROM AUTHOR]

• Experimental characterizations can be used to generate equivalent computer models. • Fatigue indicator parameters (FIPs) analyzed produce similar spatial fields. • FIPs can adequately predict the location of crack initiation in corroded materials. • The microstructure is vital in affecting the pit-to-crack transition. • Constituent particles play a secondary role in corrosion mediated crack initiation. The onset of fatigue crack initiation is driven by the microstructure and the corrosion morphology; however, these mechanisms are rarely studied synergistically. In this work, characterizations of the microstructural features and the corrosion morphology resulting from two different environmental exposures are instantiated into crystal plasticity models. Fatigue indicator parameters (FIPs) are calculated from the micromechanical fields to quantify failure. The FIPs compare favorably to predict the experimentally observed location of fatigue crack initiation. These results show the potential behind analyzing environmentally-assisted fatigue crack initiation from a multivariable perspective. [ABSTRACT FROM AUTHOR]

Uniaxial mechanical testing conducted at room temperature (RT) and 77 K on hydrogen (H)-exposed nickel was coupled with targeted microscopy to evaluate the influence of deformation temperature, and therefore mobile H-deformation interactions, on intergranular cracking in nickel. Results from interrupted tensile tests conducted at cryogenic temperatures (77 K), where mobile H-deformation interactions are effectively precluded, and RT, where mobile H-deformation interactions are active, indicate that mobile H-deformation interactions are not an intrinsic requirement for H-induced intergranular fracture. Moreover, an evaluation of the true strain for intergranular microcrack initiation for testing conducted at RT and 77 K suggests that H which is segregated to grain boundaries prior to the onset of straining dominates the H-induced fracture process for the prescribed H concentration of 4000 appm. Finally, recent experiments suggesting that H-induced fracture is predominately driven by mobile H-deformation interactions, as well as the increased susceptibility of coherent twin boundaries to H-induced crack initiation, are re-examined in light of these new results. [ABSTRACT FROM AUTHOR]

• Product form effects on fatigue crack growth are better described via grain structure, residual stress, and/or crack path differences. • Quenching-induced residual stress states may promote 3D fatigue crack paths. • Growth rate prediction via residual stress superposition is hampered by 3D crack path-induced closure. • Non-planar crack paths induced by residual stress are resilient to mitigation without heat treatment. Damage tolerant structures require accurate fatigue crack growth rate models for life prediction. Central to these models are fracture mechanics similitude and empirically gathered growth rates. In this work, standard test methods failed to achieve similitude for 17-4PH stainless steel round-rod but succeeded for plate. Material product form differences are interrogated through constant ΔK tests, crack path analysis, residual stress (RS) characterization, and growth rate simulation. Analyses revealed that quench-induced RS in the round-rod promoted a non-planar, 3D crack path with closure effects. Strategies to mitigate the RS/path effects are evaluated by controlling constraint, closure, environment, and heat treatment. [ABSTRACT FROM AUTHOR]

• The effects of low water vapor, low temperature environments, on the FCGR of 2199 AA. • FCGR was found to decrease with decreasing water vapor pressure systematically. • Temperature was varied from 23C to −65 °C and only −30 °C and −50 °C effected FCGR. • The proposed reasoning for low temp changes in FCGR are H-dislocation interactions. The objective of the study is to evaluate the fatigue crack growth of a third-generation Al-Li-Cu alloy (2199-T86) in environments relevant to high-altitude flight. Reduction of P H2O at 23 °C resulted in a general reduction of crack growth rates. A local minimum in growth rates were observed at intermediate ΔK and P H2O and was attributed to roughness induced impedance of water transport through the crack wake. Decreasing temperature had no effect on growth kinetics down to −15 °C, but decreased crack rates were observed at −30 °C and −50 °C. [ABSTRACT FROM AUTHOR]

• HEAC behavior of Monel K-500 in 0.6 M NaCl at −950 mV SCE strongly depends on dK/dt. • K TH at −950 mV SCE was independent of dK/dt ranging from 0.2 to 20 MPa√m/hr. • Two regimes of behavior were noted for da/dt at −950 mV SCE as a function of dK/dt. • Diffusion-mediated crack growth is only observed under specific testing conditions. • Rising K-based testing can provide conservative HEAC metrics in an efficient manner. The effect of loading rate (dK/dt) on the hydrogen environment-assisted cracking (HEAC) behavior of Monel K-500 immersed in 0.6 M NaCl and polarized to −950 and −1200 mV SCE was assessed via a slow-rising stress intensity (K) testing framework at dK/dt ranging from 0.2 to 20 MPa√m/hr. Comparative testing on Monel K-500 in an inert environment (dry N 2 with RH < 5%) at dK/dt ranging from 0.1 to 4 MPa√m/hr revealed a clear dK/dt-dependence of local plasticity effects on the direct current potential difference (dcPD) signal. A method for decoupling this "false" signal caused by local plasticity from the 'true' crack growth kinetics is presented. Analysis of the corrected data showed that the threshold stress intensity for the onset of HEAC (K TH) was nominally invariant with dK/dt. The HEAC growth rates (da/dt) were only mildly dependent on dK/dt under severe environmental conditions (−1200 mV SCE). However, for moderate conditions (−950 mV SCE), a strong influence of dK/dt on HEAC growth rates (da/dt) was observed. For this latter testing environment, two characteristic regimes of behavior were noted depending on the applied dK/dt: (1) a 'plateau' regime where da/dt is independent of dK/dt was observed for dK/dt > 8 MPa√m/hr and (2) a 'linear' regime where log(da/dt) linearly scales with log(dK/dt) was observed for dK/dt < 8 MPa√m/hr. These trends are interpreted using resistance curves calculated from the measured da/dt vs. K J data. The results of this analysis demonstrate that diffusion-mediated crack growth is only observed for dK/dt > 10 MPa√m/hr at −950 mV SCE and all evaluated dK/dt at −1200 mV SCE , while mechanics-based factors govern the da/dt response for dK/dt < 10 MPa√m/hr at −950 mV SCE. The influence of dK/dt is considered in the context of current testing standards for evaluating HEAC susceptibility; specifically, rising K-based frameworks consistently yield conservative HEAC metrics. [ABSTRACT FROM AUTHOR]

The hydrogen environment-assisted cracking (HEAC) behavior of a compositionally complex Co–Ni–Cr–Mo–Fe–Ti–Al alloy (MP98T) proposed for marine fastener applications was evaluated via slow-rising stress intensity testing while immersed in 0.6 M NaCl at applied potentials ranging from −1.0 to −1.2 V SCE (vs. saturated calomel electrode). Results indicate that MP98T is resistant to HEAC at applied potentials representative of cathodic protection. In particular, minimal crack growth was observed across the tested applied potential range, which represented conditions known to promote aggressive HEAC in legacy Ni-based fastener alloys, such as Monel K-500. Measurement of the diffusible hydrogen concentration (C H,Diff) of MP98T as a function of applied potential in 0.6 M NaCl suggests this improved susceptibility may be linked to a reduced efficacy for hydrogen uptake in MP98T at hydrogen overpotentials where Monel K-500 is susceptible. Specifically, the C H,Diff of MP98T at a given applied potential was consistently observed to be less than 50% of that measured for Monel K-500. [ABSTRACT FROM AUTHOR]

Fatigue crack growth experiments were conducted on highly sensitized AA5456-H116 in laboratory air and 0.6 M NaCl at two applied potentials (−900 mV SCE and −1300 mV SCE ; vs. saturated calomel electrode) using frequencies of 5 and 0.1 Hz. At 5 Hz, the fatigue crack growth rate (da/dN) was similar between −900 mV SCE and −1300 mV SCE (~1.5 × 10−4 mm/cycle), which were both approximately 10-fold faster than the da/dN measured in laboratory air. However, significant differences in da/dN were observed between −900 and −1300 mV SCE for testing at 0.1 Hz. Specifically, while a nominally similar da/dN to the 5 Hz data was observed for −900 mV SCE , a 1000-fold decrease in da/dN was noted for −1300 mV SCE at 0.1 Hz. Microscopy of the crack tip region on interrupted fatigue experiments conducted at −1300 mV SCE revealed significant blunting of the crack tip at 0.1 Hz, while the crack tip cycled at 5 Hz remained sharp. A stepped potential experiment demonstrated that the measured da/dN at −1100 mV SCE decreased by multiple orders of magnitude when this potential was approached from the cathodic versus the anodic direction. Collectively, these data suggest a salient role of corrosion-induced blunting on the measured da/dN for highly sensitized AA5xxx-series alloys immersed in marine environments at highly cathodic potentials and low loading frequency. [ABSTRACT FROM AUTHOR]

• New sensitization quantification method without mass loss assumption (ie. G67). • Analysis on samples taken from fracture specimens tested for IG-SCC susceptibility. • More accurate correlation between linear grain boundary β phase coverage and IG-SCC. • Correlation between β coverage and IG-SCC affected by yield strength. • Results provide insight into improving ASTM G67 test method. The ASTM G67 Nitric Acid Mass Loss Test (NAMLT) is not a universally accurate proxy for intergranular stress corrosion cracking (IG-SCC) susceptibility in Al-Mg alloys. This study aims to understand this discrepancy via an electron backscatter diffraction (EBSD) and ammonium persulfate etching/overlay analysis to characterize the linear grain boundary β coverage (LGBBC). Testing at three NAMLT levels of AA5083-H131, AA5083-H116, AA5083-SHTQ, and AA5456-H116 derived from fracture specimens demonstrates that LGBBC is a more accurate metric to describe the IG-SCC susceptibility, however the correlation degrades for materials with divergent yield strengths. These results provide insights necessary to improve upon ASTM G67. [ABSTRACT FROM AUTHOR]

• The onset of sub-critical cracking during slow strain rate tensile (SSRT) testing is assessed. • Hydrogen diffusion alone cannot explain measured intergranular fracture depths. • A new phase field formulation is developed for the coupled hydrogen-deformation problem. • Simulations indicate the onset of sub-critical cracking well before specimen failure. • Sub-critical cracking is shown to obfuscate the interpretation of SSRT metrics. The onset of sub-critical crack growth during slow strain rate tensile testing (SSRT) is assessed through a combined experimental and modeling approach. A systematic comparison of the extent of intergranular fracture and expected hydrogen ingress suggests that hydrogen diffusion alone is insufficient to explain the intergranular fracture depths observed after SSRT experiments in a Ni–Cu superalloy. Simulations of these experiments using a new phase field formulation indicate that crack initiation occurs as low as 40% of the time to failure. The implications of such sub-critical crack growth on the validity and interpretation of SSRT metrics are then explored. [ABSTRACT FROM AUTHOR]