This paper presents an experimental study of an underwater pulsed plasma discharge in pin-to-pin electrode configuration. Time resolved refractive index-based techniques and electrical measurements have been performed in order to study the pre-breakdown and breakdown phenomena in water. A single high voltage pulse with amplitude of a dozen of kV and duration of [0.1-1] ms is applied between two 100 µm diameter platinum tips separated by 2 mm. This novel experimental work reports that different cases of electrical discharge in water occurs for a unique set of experimental conditions such as (i) bush-like channels from the cathode that do not span the electrode gap, (ii) bush-like channels from the cathode leading to breakdown and (iii) filamentary structures from the anode leading to a stronger breakdown. Two breakdown mechanisms, anode and cathode regimes, have been clearly identified and related to the two principal schools of thoughts to explain discharge propagation in liquid.

A parametric study of an underwater pulsed plasma discharge in pin-to-pin electrode configuration has been performed. The influence of two parameters has been reported, the water conductivity (from 50 to 500 µS/cm) and the applied voltage (from 6 to 16 kV). Two complementary diagnostics, time resolved refractive index-based techniques and electrical measurements have been performed in order to study the discharge propagation and breakdown phenomena in water according to the two parameters. A single high voltage of duration between 100 µS and 1 ms is applied between two 100 µm diameter platinum tips separated by 2 mm and immersed in the aqueous solution. This work, which provides valuable complementary results of paper [1], is of great interest to better understand the mechanisms of initiation and propagation of pin-to-pin discharge in water. For low conductivity (from 50 to 100 µS/cm) results have confirmed two regimes of discharge (cathode and anode) and the increase of the applied voltage first makes the breakdown more achievable and then favors the apparition of the anode regime. For 500 µS/cm results have highlighted cathode regime for low applied voltage but a mixed regime (anode and cathode) for high applied voltage.

This paper reports the analysis of the energy balance of micro-second pin-to-pin discharges in water as well as the bubble dynamics involved in the electrical breakdown. This approach, complementary to previous electrical and optical measurements, is of primary interest to better understand the phenomena leading to different discharge regimes despite having the same applied conditions. This work indicates that different discharge regimes are related to different energy distributions through thermal and mechanical components despite the same initial energy; as a consequence, the breakdown involves different maximum radius of the cavitation bubble as well as bubble dynamics. For the cathode regime the initial energy is mainly converted into thermal energy (70%) that involves the formation of a small vapor filled bubble (∼1 mm) followed by many rebounds. For the anode regime, the initial energy is evenly distributed into thermal and mechanical components (43%/57%) which results in a higher empty bubble (∼2 mm) that collapses. The bubble growth is directly related to the energy dissipated during the breakdown, following same power law but with different constant. This work validates the hypothesis of the thermal regime for cathode breakdown whereas the anode regime is mainly driven by mechanical effects. Despites these discrepancies, simulations using a modified Rayleigh–Plesset model show that the growth of the first bubble is controlled by inertial effects for both regimes.

Disk Pressure Tests on polycrystalline Armco iron flat samples provided fracture pressure values within a wide range of applied hydrogen pressure rise rates. FE simulations of the disk bulging and hydrogen transport before fracture have been performed assuming isotropic elastoplasticity. The diffusive and trapped hydrogen concentrations fields in the zone of interest for failure show the effect of the applied pressure rise rate on the coupling between plastic strain and hydrogen transport, and permit to deduce a phenomenological relationship between the failure stress and the hydrogen concentration, as a first approximation to model the embrittlement process. Submodelling with 3D synthetic polycrystals, obeying crystal plasticity, permits to exhibit statistically the effects of local heterogeneities on the hydrogen distribution, and their consequences on the phenomenological failure stress evolution with the hydrogen content.