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Double cantilever beam (DCB) samples are typically used to estimate the crack growth rates during hydrogen environmentally assisted cracking (HEAC) of 7xxx aluminium alloys. In this investigation, a cohesive zone model was developed to simulate the HEAC behaviour of 7xxx alloys during DCB tests. A coupled mass-diffusion and stress analysis was performed in Abaqus to elucidate key parameters affecting crack growth rates via sensitivity analysis. The threshold stress intensity for cracking was mainly dependent on hydrogen solubility, while the stage II crack growth rate was influenced by the mass-transfer coefficient and diffusivity. Model parameters were fitted to match experimental results of AA7449-T7651 at different temperatures, analysing hydrogen distribution within the samples. Results indicated that hydrogen concentration at external and fractured surfaces remained at saturation levels throughout the tests. The postulated crack propagation mechanism involves enhanced hydrogen diffusion near the crack tip due to a high hydrogen concentration gradient, crack growth driven by hydrogen accumulation, and enhanced hydrogen ingress at newly formed surfaces. [ABSTRACT FROM AUTHOR]

During the initial stages of hydrogen environmentally assisted cracking (HEAC), including crack incubation, initiation and microstructurally short cracking, the geometrical configuration of the microstructure greatly influences the crack growth behaviour. Therefore, there is a big incentive to generate a model which can replicate intergranular HEAC at a microstructural scale. This report provides a general framework to implement a microstructural intergranular HEAC model by using a cohesive zone approach in Abaqus. The parameters of the phenomenological model were fitted by using in-situ synchrotron tomography observations of crack initiation and propagation during HEAC of AA7449-T7651. After fitting the parameters, the real HEAC behaviour of the aluminium alloy 7449-T7651 has been replicated accurately. Several characteristic HEAC features were achieved, including crack segmentation, preferential cracking along grain boundaries with a high resolved normal stress and cracks slowing down at grain boundary triple junctions. Comparisons with experimental observations show the suitability of this approach for the prognosis of crack initiation and propagation at a microstructural scale under HEAC conditions. • Some engineering alloys are sensitive to intergranular hydrogen environmentally assisted cracking (HEAC). • The crack growth behaviour is affected by the microstructure configuration during crack initiation. • Cohesive zone models can be used to recreate intergranular HEAC at a microstructural scale. • Randomly generated synthetic microstructures can be used to analyse the variability in the fracture behaviour. [ABSTRACT FROM AUTHOR]

The performance of UV diamond photocathodes in gaseous environments has been explored with regard to the potential applications, which exist for such devices. Stable performance is exhibited even at gas pressures approximately 0.5 bar for hydrogenated diamond, in gaseous environments such as He, H2, O2, CH4 and CF4. However, for chlorocarbon gases, UV stimulated photodissociation of the ambient gas causes the adsorption of chlorine on the diamond surface, and rapidly degrades device performance. Thermal dissociation of ambient gases brings about a similar effect. Thus, although diamond photocathodes do have potential use in gaseous environments, it is clear that care is required to ensure that the hydrogenated surface layer is not destroyed when in use. [Copyright &y& Elsevier]

• Novel 7xxx alloys with a high Zn/Mg ratio are sensitive to cracking in moist air. • Cracking occurs due to the absorption of hydrogen under sustained loading. • Fracture mechanics samples can be used to estimate the crack growth rates. • The crack growth rates of AA7449-T7651 increase exponentially with temperature. • At higher temperatures, cracking can occur at lower stress levels. The 7xxx series of aluminium alloys are sensitive to intergranular hydrogen environmentally assisted cracking (HEAC) in moist air environments. A new generation of 7xxx alloys used in aircraft components (including AA7449) have been found to be more sensitive to HEAC. This investigation aims to quantify the HEAC crack growth rates of AA7449-T7651 in moist air at different temperatures. Double cantilever beam specimens were loaded at a constant displacement and placed in moist air (80–85% relative humidity) at temperatures between 25 and 80 ° C. Regular measurements of crack length permitted the determination of the crack growth rate as a function of the stress intensity factor (K I) during stage I (high K I dependence of growth rate) and stage II (low K I dependence of growth rate). Increasing the temperature was found to increase the crack growth rate during stage II cracking, following Arrhenius kinetics. The activation energy for the stage II crack growth rate at a stress intensity factor of 14.5 MPa m was estimated as 84.7 kJ/mol. Additionally, the threshold stress intensity factor for HEAC was found to decrease with increasing temperature. This was attributed to a higher hydrogen solubility at higher temperatures. The crack growth rate of AA7449-T7651 at room temperature was similar to that of overaged AA7050 and AA7075-T7651, ranging between 1.3 and 2.1 × 10 − 7 mm/s. It was inferred that the alloy temper has more influence than the composition during stage II cracking at room temperature. [ABSTRACT FROM AUTHOR]