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PolyAmide resistance to damage Cumulation from Fatigue loading and high-pressure HYdrogen Cycling – PACIFHYC

Effects of mechanical fatigue combined with repeated decompressions on the damage of a thermoplastic under hyperbaric hydrogen

Repeated decompressions are already identified as critical (potentially damaging) phases of gas exposure of polymer components. Mechanical fatigue may also alter the semi-crystalline microstructure. The question is how the combination of these two types of loading - frequently undergone simultaneously in hyperbaric hydrogen storage applications - affects the initial mechanical properties and the permeation?

Understand the combined effects of fatigue and cumulated gas decompression

Consideration of hydrogen as an alternate energy carrier for ensuring cleaner means of mobility necessitates the design of reliable infrastructure for the transport, storage and dispensing of gaseous hydrogen. In order to make the sector a credible alternative, an immediate goal is to deploy networks of refueling hydrogen stations along with the increase of vehicles. In this emerging context, the need for cost reduction and optimized design of reliable components is a huge challenge for materials used in hydrogen devices. Feedback about high-pressure hydrogen-exposed polymers is lacking. Better understand of their interaction with hydrogen, among which deformation and damage mechanisms, is crucial to formulate polymers combining good mechanical and permeation properties and to develop modeling tools to design durable and safe infrastructures.<br />The PACIFHYC project focuses on hoses for refueling stations, and more precisely on the damage processes experienced by thermoplastic parts of these structures. The work will be conducted in PolyAmide 11 (PA11), an interesting semi-crystalline thermoplastic for hoses in hydrogen stations. It involves Institut Pprime and Hydrogenius (Japan) labs, and will benefit from the feedback of Arkema (France and Japan) about industrial service conditions and specifications. <br />On one hand, decompression failure is known to affect polymers exposed to high-pressure gases, due to the expansion of gas during pressure release. In semi-crystalline thermoplastics, it results in nano-/micro-voiding and cracks. Due to repeated refuelling of vehicles, hoses undergo repeated decompression cycles, with possible cumulated damage, but the phenomenon has not been really addressed in hydrogen. Cumulative laws and consequences on mechanical and permeation properties has never been reported. On the other hand, mechanical loading is known to nucleate damage in semi-crystalline thermoplastics, especially under triaxial conditions. Due to repeated hoop stresses or bending, fatigue loadings may enhance cumulated damage in hoses. Therefore, both types of loadings are bound to act as microstructure change and damage sources, which may affect themselves the mechanical resistance and the permeability of hoses. <br />The most original scientific challenge here is to address simultaneously fatigue and repeated pressure cycles as co-existing sources of microstructure changes and damage, and to better understand their cross-dependent influence on residual mechanical properties and permeability.

Fatigue and pressure cycling will be studied separately first, in the virgin material. Each series of tests will be interrupted after a selection of numbers of cycles. The exposure pressure is 90 MPa. Then, the two damage sources will be variably combined: successively, as alternated blocks and simultaneously. The two latter cases are expected to exacerbate coupling effects between sources.
After each loading history, cumulated microstructure changes and damage mechanisms (by SAXS, WAXS, DSC, density, SEM, tomography) and their consequences on the residual mechanical properties will be investigated. The aim is to highlight how far damage sources activate similar microstructure changes and damage mechanisms or not, with similar consequences on residual mechanical properties or not, and similar dependence on the number of cycles. Damage and/or microstructure indicators will be processed and globally analysed to formulate unified cumulative laws bound to be inserted into modelling. Integration into basic existing models or criteria will be tested in a last prospective part of the project.
The effect of single or combined damage sources on permeability, which is a key part of specifications for hoses, will be investigated in a selected number of critical cases, motivated by the above conclusions.

The global approach is to apply complex loading histories, combining fatigue tests at the Pprime Institute (France) and series of exposure to hyperbaric hydrogen at Hydrogenius (Japan). The same samples will therefore undergo various tests, intermediate storage steps and transport from one place to another. However, the material is very sensitive to the plasticizing effect of humidity, which can significantly modify its properties, independently of the damage enhanced by loading histories. Moreover, the volume of the exposure chamber at these hydrogen pressures, and its time of use, are limited.
=> Therefore, the type and number of tests carried out (and therefore the type and number of needed samples) had to be anticipated from the beginning of the project for the entire test matrix, taking into account the limitation of the number imposed by the geometry of the hyperbaric chamber. This work was carried out jointly by the two partners, in close interaction with the material supplier.
Furthermore, it is crucial to define conditionning and packaging protocols that are (i) reproducible from one sample to another, (ii) stable throughout the campaign and compatible with sending samples between laboratories, and ( iii) the least impacting for the micromechanisms activated during fatigue or pressure loadings in order to be able to remain comparative. This last requirement limits, for example, the admissible temperature rise for drying. Several drying protocols have thus been tested and protocols for initial drying, storage, shipping and re-conditioning before testing have been developed. Mass measurements performed byHydrogenius just after reception and later after storage in their room confirm a low water uptake of the samples. This is a very important result for the global reliability of the project.

To be completed later in the project.

To be completed later in the project.

Consideration of hydrogen as an alternate energy carrier for ensuring cleaner means of mobility necessitates the design of reliable infrastructure for the transport, storage and dispensing of gaseous hydrogen. In order to make the sector a credible alternative, an immediate goal is to deploy networks of refueling hydrogen stations along with the increase of vehicles. In this emerging context, the need for cost reduction and optimized design of reliable components is a huge challenge for materials used in hydrogen devices. Feedback about high-pressure hydrogen-exposed polymers is lacking. Better understand of their interaction with hydrogen, among which deformation and damage mechanisms, is crucial to formulate polymers combining good mechanical and permeation properties and to develop modeling tools to design durable and safe infrastructures.
The PACIFHYC project focuses on hoses for refueling stations, and more precisely on the damage processes experienced by thermoplastic parts of these structures. The work will be conducted in PolyAmide 11 (PA11), an interesting semi-crystalline thermoplastic for hoses in hydrogen stations. It involves Institut Pprime and Hydrogenius (Japan) labs, and will benefit from the feedback of Arkema (France and Japan) about industrial service conditions and specifications.
On one hand, decompression failure is known to affect polymers exposed to high-pressure gases, due to the expansion of gas during pressure release. In semi-crystalline thermoplastics, it results in nano-/micro-voiding and cracks. Due to repeated refuelling of vehicles, hoses undergo repeated decompression cycles, with possible cumulated damage, but the phenomenon has not been really addressed in hydrogen. Cumulative laws and consequences on mechanical and permeation properties has never been reported. On the other hand, mechanical loading is known to nucleate damage in semi-crystalline thermoplastics, especially under triaxial conditions. Due to repeated hoop stresses or bending, fatigue loadings may enhance cumulated damage in hoses. Therefore, both types of loadings are bound to act as microstructure change and damage sources, which may affect themselves the mechanical resistance and the permeability of hoses.
The most original scientific challenge here is to address simultaneously fatigue and repeated pressure cycles as co-existing sources of microstructure changes and damage, and to better understand their cross-dependent influence on residual mechanical properties and permeability. A second originality of the project is the unusual pressure range for now (90 MPa), currently under consideration to extend operating conditions and made accessible by Hydrogenius facilities.
Fatigue and pressure cycling will be studied separately first, in the virgin material. Each series of tests will be interrupted after a selection of numbers of cycles. Then, the two damage sources will be variably combined: successively, as alternated blocks and simultaneously. The two latter cases are expected to exacerbate coupling effects between sources.
After each loading history, cumulated microstructure changes and damage mechanisms (by SAXS, WAXS, DSC, density, SEM, tomography) and their consequences on the residual mechanical properties will be investigated. The aim is to highlight how far damage sources activate similar microstructure changes and damage mechanisms or not, with similar consequences on residual mechanical properties or not, and similar dependence on the number of cycles. Damage and/or microstructure indicators will be processed and globally analysed to formulate unified cumulative laws bound to be inserted into modelling. Integration into existing models or criteria will be tested in a last prospective part of the project.
The effect of single or combined damage sources on permeability, which is a key part of specifications for hoses, will be investigated in a selected number of critical cases, motivated by the above conclusions.

Project coordination

Sylvie CASTAGNET (Institut P' : Recherche et Ingénierie en Matériaux, Mécanique et Energétique)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

Pprime Institut P' : Recherche et Ingénierie en Matériaux, Mécanique et Energétique
Université de Kyushu / Laboratoire HYDROGENIUS

Help of the ANR 239,738 euros
Beginning and duration of the scientific project: March 2021 - 48 Months

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