The degrading effect of hydrogen on metals has been known since the 19th century and is known as hydrogen embrittlement, due to its detrimental effect on the ductility of materials. Modern material testing machines for mechanical testing in hydrogen gas are equipped with an autoclave, in which a specimen can be tested at different hydrogen gas pressures and in some cases also different temperatures. A well-known problem with material testing inside autoclaves is seal friction, which can make it difficult to determine the test force applied to the specimen. In order to solve the problem of seal friction a patent for a specific tension rod was filed by employees of MPA. The tension rod is connected and screwed in series with the test specimen, and extends into the autoclave, thereby creating the sealing surface for the hydrogen-resistant seal used. The tension rod is precisely hollow drilled and internally instrumented with strain gauges so that the force on the specimen can be measured without the influence from seal friction from inside the autoclave, but without the strain gauges being exposed to hydrogen gas.

As part of this research project, a test autoclave was developed in which the tension rod can be calibrated and further examined under hydraulic water pressure up to 300 bar. In addition, the geometric shape of the tension rod was designed with a pressure-compensating notch. When calibrated correctly, this notch can compensate for the influence of the hydraulic pressure on the test arrangement. The strain gages must be applied precisely in the tension rod as even small deviations in position and orientation of the strain gages in relation to the pressure compensation notch can lead to considerable deviations in the compensation signal. To achieve this, strain gages are applied with the help of a specially designed, 3D-printed and precision milled application device. The desired compensation of seal friction has already been successfully verified and quantified. The so adjusted force signal without the undesired friction force, now shows a test force that is up to 10% lower for the same elongation.