Monitoring Hydrogen Embrittlement with Acoustic Emission Sensors
TWI Industrial Member Report 1184-2024 [pdf / 4,126KB]
By Dr. Stephen Grigg PhD
Industrial Need
Although hydrogen is a clean energy source, its interaction with high strength steel alloy containers can provoke rapid deterioration in the material’s microstructure.
Hydrogen Embrittlement (HE) is a type of damage to metal structures caused by the presence of excess hydrogen in the environment and is of increasing importance in the drive towards the ‘Hydrogen Economy’, where hydrogen replaces hydrocarbons as a fuel. TWI has conducted various studies of the HE process on stainless steels using slow strain rate tensile tests. In these tests, the detection of crack initiation in notched specimens is critical and a Non-Destructive Testing (NDT) method for achieving this would be of great benefit.
Acoustic Emission (AE) testing is an established Non-Destructive Testing (NDT) method for detecting crack initiation in metals under tensile and fatigue loads. Microscopic dislocations occur during plasticisation of a notch under load before a crack appears and this is known to create measurable AE burst of stress waves, so called AE ‘hits’. The difficulty is detecting these bursts in the presence of background AE noise. TWI has experience in developing novel AE signal analysis methods that distinguish signals from noise and artefacts (such as internal reflections in small test pieces).
Key Findings
- Modelling of the plastic region of round tensile duplex stainless steel specimens was conducted successfully using material data from a previous project.
- AE testing of the specimens after charging with hydrogen was severely compromised by poor ultrasound coupling of the sensors on the tight curvature of the surfaces.
- The poor coupling led to a sparsity of data to characterise AE features, such as amplitude and energy and correlate these with HE prior to HISC formation
- Localisation of the AE from HISC forming during the tests was improved by using an AIC algorithm to define the leading edge of the AE signals
Impact
The objectives could not be met, because of the ultrasound coupling problems. To minimise them, the smallest available AE sensor was used. Even smaller sensors are available from the manufacturer, that may have reduced the coupling problem, but these could not be acquired within the project budget.
Acoustic Emission sensors attached to Hydrogen Embrittlement specimen