- Develop a test method capable of assessing the fitness-for-purpose of precipitation-hardened nickel alloys (PHNAs) with respect to hydrogen embrittlement (HE).
- Develop a technique that reliably captures crack initiation during slow strain rate tensile and incremental step load testing and which can later be used to define strain-based failure criteria.
Recent observation of slip bands on the brittle fracture surfaces of slow strain rate tensile (SSRT) test specimens has suggested that plastic strain is required in order to initiate HE cracking. Interrupted SSRT tests have suggested that more than 1.8% plastic strain is required to initiate HE cracking at the root of notched specimens. Further work using fracture toughness based testing methodologies on fatigue pre-cracked specimens has also shown the resistance of PHNAs to crack propagation is very low under environmental conditions known to embrittle the material in SSRT tests. This has led to the hypothesis that crack initiation may be the dominant factor controlling resistance to HE and a critical strain may be required for crack initiation to occur. If correct and measurable, the resistance of PHNAs could be quantitatively defined by this critical strain. Critical strain could also be used to develop a design criterion to increase the resistance of components to HE.
In order to measure the critical strain at which cracking occurs a method of detecting crack initiation is required. Previous work has investigated incremental step load (ISL) testing combined with direct current potential drop (DCPD) measurements as a means of detecting crack initiation. ISL testing has proved more sensitive to the effects of strain localisation on specimens with different notch acuities but it has not been possible to identify crack initiation using either mechanical test or DCPD data.
This project will investigate techniques for detecting crack initiation in notched specimens, including: DCPD, acoustic emission, ultrasonic techniques and strain gauges across the notched area. These approaches will be validated using interrupted testing at loads before and after the detection of cracking. Previous work will be extended using the ISL test method to investigate the influence of strain localisation on resistance to HE in high strength nickel alloys.
Previous work has focused on Alloy 718, the ‘workhorse’ alloy widely considered to be relatively resistant to HE; however, newer higher strength alloys are thought to be more susceptible to HE and the effects of strain localisation. This project will investigate whether the notch sensitivity of higher strength alloys is greater than Alloy 718 when tested using the ISL test method developed.
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Benefits to Industry
The acceptance/design criteria could be developed to prevent future failures of PHNAs due to hydrogen embrittlement. The expertise developed in this project will enable industry to understand, approve and adopt the testing techniques as part of their current materials approval processes. Based on the outcomes, further work will allow industry to develop failure criteria for the materials currently given in API Standard 6ACRA.
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