Adding hydrogen to fossil fuels is an important step toward decarbonization of combustion processes in large engines and gas turbines. The materials used in these applications are exposed to extreme temperature changes, mechanical stresses, and hydrogen gas. Depending on its proportion in the fuel and the sensitivity of the materials, hydrogen further reduces fatigue strength. Scientists at Fraunhofer IWM have demonstrated how thermomechanical fatigue under the influence of hydrogen can be determined economically using hollow specimens.

 

The addition of hydrogen is considered a transitional technology for making gas turbines for power generation as well as large engines for ships and vehicles "greener" without having to completely replace the equipment. The goal of many research projects and companies is to operate combustion engines such as motors and turbines entirely on hydrogen. The demand for gas turbines for power generation is increasing because, with the expansion of renewable energies, gas-fired power plants are in demand to stabilize the power grid in the event of fluctuations in energy production, as they allow for flexible operation.

 

As the hydrogen concentration within the combustion process increases, CO2 savings grow, as do the challenges for the materials that come into contact with the highly diffusive hydrogen. Consequently, requirements for safety and reliability assessments are increasing. The key is the design regarding so-called thermomechanical fatigue (TMF), which directly affects service life. The current consequences are higher safety margins in component design, the use of expensive materials, and costly component testing. What is needed are sound bases for decision-making that take into account the complexity of the stresses involved.

 

Testing materials and components in the laboratory and qualifying them for use in contact with hydrogen is obvious. However, the practical conditions must be represented on a laboratory scale, and the significance of the material properties determined must be transferable to industrial practice in order to ensure service life and optimize component design.

 

Combine hydrogen diffusion, temperature changes, and tensile-compressive stress independently of each other

The option of testing high-temperature materials in a hydrogen pressure chamber is ruled out because the high and varying operating temperatures cannot be combined with the pressurized hydrogen inside. Scientists at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg therefore use so-called hollow specimens, in which hydrogen flows through a borehole in the interior of the specimen and the specimen material is subjected to the thermomechanical operating conditions from the outside. This allows the hydrogen pressure, temperature changes, and tensile and compressive stress cycles to be varied experimentally independently of each other, enabling conclusions to be drawn about materials suitability for a wide range of operating conditions.

 

Recent tests (and publicly funded projects) have shown that hollow specimens are suitable for determining the effects of hydrogen gas on material properties at constant temperatures (isothermal test conditions) and at varying temperatures (anisothermal test conditions).

 

Gradual implementation of the test results in practice

The test results (stress, strain, service life) from the TMF tests initially provide a basis for deciding on suitable materials. When designing components for use in combustion engines, it is important to strike a balance between conservative design and expected service life, i.e., stress cycles until materials fatigue leads to the formation of cracks. Finally, hydrogen-related reductions in service life depending on pressure can and must be evaluated and taken into account. The various interrelationships to be optimized are represented in a materials model and are thus used in component simulations.

 

This is another milestone on the road to safe and resource-efficient components in contact with hydrogen.

With the experimental combination and variation of material loads in turbines and engines using hydrogen-containing fuels, the effects of hydrogen can now be evaluated quickly and economically. Materials technology decisions for the decarbonization of combustion processes thus become calculable.

 

Questions for Heiner Augenstein and Dr. Michael Schlesinger on the evaluation of thermomechanical fatigue of materials in contact with hydrogen.

 

How are the loads that occur in turbines and engines translated into the test procedure?

The exact interaction of temperature and mechanical stress in the affected components can be determined by simulations. Most often, blocked thermal expansion during start-up leads to compressive stresses; this is called "out-of-phase TMF." We can independently record and control temperature and mechanical load on the hollow specimen. The induction heating used also allows rapid temperature changes. Mechanically, we can control either strain or force. This allows us to simulate almost any load profile in a manner that is close to the actual application.

 

Are the hollow specimens standardized or are the test specimens manufactured specifically for each application?

Hollow specimens for hydrogen testing are an innovative technology and therefore not yet fully standardized. An initial international standard for tensile testing was published in 2024 (ISO 7039). There is still no standard for hollow fatigue testing. Fraunhofer IWM is active in national and international committees that are actively promoting standardization.

 

For fatigue testing, we were able to develop a specimen shape for which simulation and experiment show the comparability of the results (deformation and service life) with classic fatigue specimens. However, since the specimens are best taken directly from the component, application-specific adaptation is sometimes necessary and possible within limits.

 

How is it ensured that the measurement results are correct and meaningful?

A current publicly funded IGF research project addresses precisely this issue and investigates microstructure-dependent materials behavior under the influence of hydrogen at elevated temperatures. In this project, corresponding tests are carried out on hollow specimens for different materials and manufacturing variants, compared with results from reference tests, and their significance confirmed.

 

Source:  Fuel Cells Works