Reduction of the fatigue strength due to the influence of hydrogen

Hydrogen embrittlement, corrosion fatigue strength, service life assessment

Fraunhofer LBF supports companies in the fields of transportation, energy as well as mechanical and plant engineering in the development of reliable, lightweight and efficient products that are exposed to hydrogen. To this end, customer-specific or individual analysis and test concepts are used to meet the increased requirements for the verification of structural durability and to represent real operating conditions in an optimal manner.

The European Union is pursuing the goal of going carbon neutral by 2050 and massively reducing CO2emissions. However, it should be noted that Germany was only able to meet its national targets for 2020 due to the severe limitations placed on public life and the economy as a result of the coronavirus pandemic. By 2030, greenhouse gas emissions are to reduced by at least 40%, relative to 1990 (as a matter of fact, a reduction of up to 55% under discussion) and by as much as 80-95% by 2050. Analysis has shown that 75% of the EU’s greenhouse gas emissions stem from energy consumption. Reaching the goals that have been set to reduce greenhouse gas emissions by reducing energy needs will require a complete overhaul of the energy system. As well as developing a “circular energy system” – typically using waste heat from factories – the “electrification of end-use sectors” is to be driven forward. This includes the using of renewable power for uses such as heat pump systems and vehicle electrification (electric cars). As part of the third pillar of this strategy program, there is to be an increase use of clean, renewable fuels, such as hydrogen and biofuels. In this way, hydrogen can support the decarbonization of industry, vehicles, power generation and buildings all over Europe as part of an integrated energy system. Here, strengthening transport development and use of hydrogen, which will mainly be generated with the help of wind and solar power, is considered to be the primary goal. This is to be used in sectors that are not suitable for direct electrification, e.g. commercial vehicles and trains.

The upheaval of the energy economy and development of a hydrogen-based economy offers the chance to have a sustained positive influence on the environment and the fight against global climate change. However, a sudden change is bound up with hurdles, obstacles and bottlenecks, as it is not easy to transition from an established energy economy based on fossil fuels. In this context, potential bottlenecks include the issue of hydrogen availability – from production and transport to storage at the users’ premises. Safety also represents a fundamental consideration in this regard. This is the case not only for the infrastructure that is to be developed, but also for the subsequent use of fuel cell road vehicles, trains and airplanes. Due to the known embrittling effect of hydrogen when it comes into contact with metallic materials, known as “hydrogen embrittlement”, it must be ensured, when it comes to the safe operation of components exposed to hydrogen, that there is no danger of premature component failure. For this reason, Fraunhofer LBF has been operating individual test facilities for several years to investigate the reduction of the ability of materials to withstand stress, namely external loads, due to the effect of corrosive environmental influences (e.g. hydrogen or biofuels).

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Testing under pressurized hydrogen

Individual and variable test concepts enable Fraunhofer LBF staff to perform corrosion fatigue tests in order to identify relevant damage mechanisms and to establish characteristic values for modeling or deriving suitable design concepts.

For several years, Fraunhofer LBF has been using a special test facility for performing force and strain-controlled tests under pressurized hydrogen with gas pressures of between 10 and 50 bars, in order to investigate cyclic material behavior under the medium of hydrogen. As well as performing reference tests in an inert nitrogen atmosphere under a pressure of 10 bar, it is also possible to control the temperature of the autoclaves, with testing temperatures that can be adjusted between -40 °C and +130 °C.

Testing under electrochemically-applied hydrogen

As part of the H2-D project – “A hydrogen economy for Germany”, 25 Fraunhofer Institutes are conducting research to find answers to the main issues regarding the building of a hydrogen economy in Germany. This includes the production of hydrogen using electrolysis (focus 2) as well as a secure infrastructure and safe technologies (focus 3) for its transport, storage, distribution and use. There are many areas of application in which hydrogen can be used. The project therefore aims to investigate select, highly relevant example applications from industry and business, with their design and operation optimized on the basis of digital twins. In the H2DIGITAL focus point (focus 4), key components of the value chain are being given a model-based depiction. In addition, a coherent data and model space for a future hydrogen economy is being created that spans the Fraunhofer-Gesellschaft. As part of the overall consideration of the system (focus 1), all the individual focal points are taken into account, with emphasis on a future hydrogen economy and the integration of this technology into the energy system as a whole.

In this context, Fraunhofer LBF has developed a test facility based on an electrochemical cell as part of focus point 3 (secure infrastructure), allowing it to perform fatigue strength tests with simultaneous electrochemical hydrogen charging. Using this test facility allows for conclusions to be drawn in relation to the potential susceptibility of metallic materials to hydrogen in a way that is faster and more cost-effective than exposing them to pressurized hydrogen.

Material qualification and resistance analysis under compressed hydrogen

Strain-controlled tests under exposure to 50 bar of pressurized hydrogen were carried out to investigate the influence of pressurized hydrogen on the cyclic material behavior of 1.4521 (X2CrMoTi18-2) stainless steel. Comparing the results of tests performed under air with those carried out under pressurized hydrogen shows how the fatigue-strength reducing influence of hydrogen comes into effect, especially in relation to short-term strength characteristics and at high strain amplitudes a,t. Comparing service life for a total strain amplitude of a,t = 0.8 % shows how fatigue life until the initiation of a crack is reduced by a factor of 20.

The evaluation of the cyclic deformation curves identified in the performance of strain-controlled tests makes it clear how, compared to tests under air, failure under the medium of pressurized hydrogen happens rather suddenly and without a clear cracking stage. A sudden failure of the sample occurs, with a significantly shorter service life and without a recognizable drop in strain. This change in the material properties, especially the increase in embrittlement, is caused by hydrogen penetrating and being deposited in the metal lattice and is known as “hydrogen embrittlement”.

To evaluate the potential susceptibility of different materials to hydrogen, it is absolutely essential to carry out the corresponding tests. It is only in this way that it is possible to prevent the premature failure of components and system components, which could have potentially fatal consequences for the user. Using hydrogen for energy storage therefore places very high demands on safety technology, as having even small amounts of hydrogen in the ambient air (>4 vol.-%) form an inflammable mixture, with amounts greater than 18 vol.-% forming an explosive mixture. Due to the fact it is odorless, escaping gas goes unnoticed, meaning that it is necessary for the individual components and joints to be manufactured to a high standard.

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Area Of Expertise Reliability Design