High Cycle Fatigue Testing of Structural Materials in High Pressure Hydrogen Atmosphere

Abstract

For the development of space engines based on the combustion of liquid hydrogen and liquid oxygen (such as for a future launcher) materials are required that withstand the chemical environment as well as the thermal and mechanical loads. In that context, the sensitivity of the materials to Hydrogen Environmental Embrittlement (HEE) is a critical factor when selecting the material for a particular component. This is particularly important since most of the high strength Fe and Ni based alloys frequently used in space applications are prone to HEE. One aspect of the characterization of materials in hydrogen environment is their behaviour with regards to high cycle fatigue. Fatigue tests in the high cycle regime (up to 107 cycles and more) are often carried out by submitting the specimen to the stress field of a standing ultrasonic sound wave. Because of its origin in a standing wave, the load applied by the stress field is symmetric, i.e. the mean load is 0. To overcome this limitation, we have developed a device that permits superposing a constant tensile stress and the symmetric stress generated by an ultrasonic wave. The constant tensile stress is generated by means of a gas pressure difference inside an autoclave which applies a constant force on the specimen. This concept leads to a relatively small device that is completely closed to the outside. The latter is especially important when testing in hazardous environments like hydrogen at high pressure. This setup will be used to characterize a number of superalloys with regard to hydrogen embrittlement at a gas pressure up to 30 MPa. We present the setup as well as first test results on Inconel 718.