ThinKing February 2021: Precise measured values - Pushing to the limits, safely

01.02.2021

 

Anyone who wants to push the limits of performance has to know their materials in detail. This is true of extreme athletes, just as it is for lightweight design engineers. Because of this, ThinKing went to the Material Testing Institute (MPA - Materialprüfungsanstalt) at the University of Stuttgart in February of 2021 ...

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A research team there developed a load cell that delivers significantly more precise material measurements for material testing in a hydrogen atmosphere. These measurements are used for safer, more efficient lightweight component design – which is essential, for instance, for future hydrogen technology in mobility.

The Baden-Württemberg State Agency for Lightweight Design will present the ThinKing for this innovation in February of 2021. Each month, Leichtbau BW GmbH uses this label to award innovative lightweight design products or services from Baden-Württemberg.

At a glance:

  • Independent precision: interior sensors conduct precise measurements, regardless of the ambient medium
  • High repeat precision: The sensors are protected and do not show drift.
  • Indirect weight savings: Precise material measurements result in better design and more efficient products in the use of materials.
  • Safe lightweight design: The material parameters are more precise, helping the user achieve both safety and material efficiency.

Lightweight design pushes material loads to their limits for the most weight-saving construction possible. To do so, however, designers have to have a very thorough understanding of material parameters.

Lighter components thanks to improved material parameters
This is the only way for lightweight design engineers to push materials to their load limits and ensure material-saving construction, without negatively impacting component safety or service life. The material parameters serve as the basis for design in simulation tools.
Therefore, reproducible and standardised conditions are essential to exactly determine parameters, so that sensors’ measured values are comparable. Vibration resistance, elongation at break, and crack growth, in particular, are influenced greatly by the ambient medium. This must be taken into account in the test arrangement. One current example are materials used in hydrogen technology in the mobility of the future.

Hydrogen adds complexity
Hydrogen, in particular, is a unique ambient medium from material mechanics, technological, and ecological standpoints. The gas can diffuse into even high-strength steels and damage them. To design hydrogen tanks, gas lines, injection nozzles or fuel cells in a way that uses materials as efficiently as possible, material samples need to be tested in specialised testing machines in a hydrogen atmosphere.
Previous testing systems have used autoclaves with load cells outside of the test chamber. The strain gauges used as sensor elements can be damaged or destroyed in some cases by the aggressive media in the autoclave, and will at a minimum experience a drop in precision. Therefore, the forces measured by previous load cells are affected by sealing forces, which regularly causes drifting values and a lack of precision.
“These kinds of challenges usually make me extremely curious” says Martin Werz, smiling when he talks about starting development work. He heads up the department of Joining Technology and Additive Manufacturing at the MPA. Several years, and many intensive technical discussions later, his employee Alexandra Oßwald implemented one of the potential solutions in a demonstrator.

From outside to inside
The new, patented load cell in the prototype test arrangement continuously delivered highly precise measurements – in particular in hydrogen testing systems.
How does it work? The patented load frame, which transmits the test force onto the sample in the autoclave, is designed to house the sensor elements inside. All disruptive force components are therefore compensated for either intrinsically or electronically. Sealing forces no longer play a role.
Although the load cell extends into the autoclave, the sensor elements inside are exposed to a standard pressure. This results in continuously high precision.
Hydrogen pressures of up to 1,000 bar, which simulate usage conditions for the material, are no longer a significant challenge for Stefan Zickler, Department Head of Operational Behaviour under Media Influence and his team, as well as Martin Werz. Parameters of the materials – mainly metals – can be measured precisely and with high repeatability. This makes the MPA one of three institutes around the world that can complete material and component testing under these conditions.
“In addition to testing services, we offer large companies with high demands and their own laboratories the option of licensing this specialised measurement and testing technology” says Martin Werz.

Significant indirect lightweight design potential
“Our sensor indirectly has a lightweight design potential estimated at around seven percent that can be used in a variety of industries and applications” he says, identifying its importance for lightweight design development.
Alexandra Oßwald explains this estimate as follows: “The percentage of friction force, and therefore falsification of measurement results, is around +/- 3 to 4 percent, compared to our previous testing systems. Today's components have been designed to be this much heavier than necessary.” However, it could be even much more, if the previous parameters were measured on other, less precise testing systems.
This potential for lightweight design can be used in many industries and applications. Exact material parameters allow all affected components to be designed more precisely, with the same light, safe design.
In addition, the test arrangement can also be used to test load and structural strength parameters of processing and joining variants – this is a good sign for future material combinations used in lightweight design.

About the University of Stuttgart Material Testing Institute
The University of Stuttgart Material Testing Institute offers testing services to determine material and component behaviour under mechanical and/or corrosive load. The spectrum of testing ranges from structural tests to high pressure tests, burst testing, fatigue and vibration testing with test forces of up to 5 MN, and gas pressures of up to 1,000 bar. Hydrogen testing technology is one specific area of focus. Testing autoclaves at the institute allow for material and component tests in hydrogen atmospheres, highly pure water, and other media within a wide temperature and pressure range.

https://www.mpa.uni-stuttgart.de/