So far, fatigue life of components in a hydrogen atmosphere can only be estimated with great uncertainties. "For valid lifespan predictions, many influencing factors must be considered: hydrogen pressure, gas purity, temperature, material and surface conditions, mechanical stress," states the Fraunhofer Institute for Mechanics of Materials (IWM). "However, their interaction cannot be experimentally represented in a single test, which contributes to the high development effort as many experiments are necessary."
In addition, different degradation and damage processes occur simultaneously in the material, not least hydrogen-induced, leading to material fatigue and reducing lifespan. "The better and, above all, the more completely these processes can be described and simulated, the better the performance limits of the materials can be exploited and the component safety can be set."
Conventional models underestimate lifespan
According to Fraunhofer IWM, classical fracture mechanics calculations often turn out to be too conservative and provide too short lifespans. This complicates the development of hydrogen-powered turbines, whose demand is growing both in the power plant sector and in aircraft construction.
Therefore, the IWM has introduced a new simulation model for predicting the fatigue life of components in a hydrogen atmosphere. According to the researchers, it is the first crack-based lifespan model that physically represents the influence of hydrogen on so-called Low Cycle Fatigue (LCF) – short-term strength. The model is intended to be used especially for the design of hydrogen turbines and engines.
Tiny cracks determine over 90 percent of lifespan
The formation and growth of short cracks from about 20 micrometers to the technical crack of around one millimeter account for up to 90 percent of the actual component lifespan, according to the institute. Hydrogen can increase crack growth by a factor of 100 or more. This influence could not previously be represented on a physical basis, which is why components had to be designed with high safety factors.
The new model describes hydrogen-influenced short crack growth based on elastic-plastic fracture mechanics. The HELP effect (Hydrogen Enhanced Localized Plasticity) is considered as the central damage mechanism: hydrogen reaching the crack tip occupies dislocations in the metal lattice, increasing local plasticity and accelerating crack growth. At temperatures above 400 degrees Celsius, oxygen diffusion along grain boundaries contributes as an additional damage factor.
From test bench to simulation: Fewer tests for turbine manufacturers
The model can consider a wide range of influencing factors, including temperature, hydrogen pressure, load frequency, and mechanical load amplitude. "Components used in hydrogen atmospheres are exposed to a variety of different operating conditions, resulting from combinations of hydrogen pressure, temperature, and changing load cycles. It is generally not feasible to cover every possible scenario experimentally. Our simulation model is capable of mathematically capturing this complexity and reliably estimating lifespan even under extreme operating conditions," explains Fabien Ebling, project manager at Fraunhofer IWM.
According to the institute, the model requires temperature-dependent cyclic material properties such as modulus of elasticity, yield strength, and hardening exponent as input data, as well as hydrogen-specific parameters like diffusion rate, hydrogen solubility, and hydrogen binding energy. These can be experimentally determined with a few tests or derived from the literature.
Target markets: gas turbines, H2 engines, aircraft engines, compressors, and H2 infrastructure
Other hydrogen damage mechanisms such as HEDE (hydrogen-enhanced decohesion), pressure theory, or hydride phase formation are not yet represented by the model, according to Ebling, but can be added if needed. Since it is based on physical relationships, it can also be transferred to other materials with few experimental data. Initial results show, according to Fraunhofer IWM, that the transfer from cubic face-centered to cubic body-centered materials works.
Calibration and validation are carried out at Fraunhofer IWM using test capacities in the temperature range from minus 196 to 1,000 degrees Celsius with hollow samples and up to 1,000 bar hydrogen pressure in the autoclave test stand. The institute names applications with cyclically highly stressed components in a hydrogen environment as target markets, such as gas turbines, hydrogen engines, aircraft engines, compressors, and hydrogen infrastructure.
The research results were developed within the projects AdHyBau and AdHyBau2, funded by the Federal Ministry for Economic Affairs and Energy (BMWE), which deal with hybrid construction methods for high-performance electric motors and the development of an approved hydrogen-electric propulsion system for aircraft.
