Towards a better understanding of the origin and early growth of cavities in  high pressure hydrogen-exposed rubbers under decompression

The goal of the PhD is to better understand the very early stages of cavitation observed during pressure release in a rubber previously exposed to high-pressure hydrogen. Since it may affect permeation properties and mechanical resistance, such damage is a major concern for industrial components designed for fuel cell vehicles and hydrogen charge stations. This issue is crucial to improve design and formulation of hydrogen-exposed rubbers but faces strong limitations related to hydrogen manipulation for in-situ experiments and to the relevant scales. Only a few laboratories are able today to conduct physical and mechanical experiments under high-pressure hydrogen in polymers. Kyushu University (KU) and Institut Pprime are leaders in this field. Recent developments from the two laboratories provide unprecedented in-situ experiments (in-situ Small-Angle X-Ray Scattering (SAXS), in-situ X-ray tomography) operating during decompression after high-pressure hydrogen exposure, and able to discuss the mechanisms activated during the transition from a heterogeneous gas-polymer equilibrium state towards a damaged state as detected at the micro-scale.

The project will take benefit from the original chain of experimental and numerical tools now available to discuss early growth mechanisms  in an EPDM. In-situ SAXS experiments will be conducted at Kyushu University to characterize the hydrogen content heterogeneity at equilibrium and during the very early stages of decompression, as a function of saturation pressure and cross-link density. Work at Institut Pprime will be focused on upper scales (10-100µm), with the characterization of the spatial distribution of first cavities and to the growth kinetics of close interacting ones by in-situ X-ray tomography. A coupled diffuso-mechanical model of cavity growth will be used to fill the gap between scales. Decompression tests will be simulated from the heterogeneous gas concentration fields evidenced at KU as inputs. Mechanical and gas content fields  evolution will help to discuss the possible scenarii of early growth.