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Cryogenic Testing

Materials testing at cryogenic temperatures

Cryogenic testing (extreme cold temperatures below <120 K) is particularly important in the growing hydrogen technology sector. The objective: identify material characteristics and gain insights on material behavior at the extremely low temperatures in which the material is used. When transporting and storing liquid hydrogen, the operating temperature is 20 K.

In addition to pure static properties under tensile, compression or shear loads at low temperature, the fatigue behavior or fracture mechanical behavior is also of interest, since hydrogen in contact with oxygen is explosive in even small quantities and a failure of the material could lead to fatal consequences.

For test methods including cryogenic tensile testing, fatigue testing or impact testing, ZwickRoell provides the following options:

Cooling with temperature chambers Cooling with immersion cryostats Cooling with continuous flow cryostats Cryogenic impact test Related cryogenic testers

Cryogenic test objective

For liquid hydrogen storage in particular, the following aspects play a major role from a materials testing perspective:

  • The investigation of the static, dynamic and fracture mechanical material behavior in the cryogenic range and the determination of the characteristic values required for the design and verification of corresponding material structures. Since in certain quantities hydrogen is explosive when in contact with oxygen and material failure could have fatal consequences, the fatigue behavior or fracture mechanical behavior in particular is of significant interest.
  • For the H2 infrastructure, the composite material—unlike metals—is often not in direct contact with the hydrogen medium. For this reason, when testing composites the cooling medium helium, which is far less complex to handle, can also be used to reach the test temperature of 20 K.
  • In the case of composite materials, the very different thermal expansion coefficients of the fiber and matrix in fiber-reinforced plastics lead to frozen stresses in the material during the manufacturing process. The far greater temperature variations in hydrogen technology applications result in strong thermo-mechanical stresses. It is important to have a precise understanding of this behavior at real temperatures, since the strong pressure and temperature fluctuations (e.g. during refueling) can cause micro cracks in the composite material, which can negatively affect its mechanical properties and permeability.

To perform tests in the cryogenic range, temperature chambers, continuous flow cryostats, or immersion cryostats are used, depending on the operating temperature and application. Based on the type or version of this cryogenic testing equipment, you can reach test temperatures in the cryogenic range between 20 K and 130 K.

Since the cost of helium is significantly higher than the cost of nitrogen, you must weigh the costs and benefits to determine which temperature range and which cooling medium should be chosen. The actual temperatures are determined by the application.

Standards for cryogenic test methods

Standards for cryogenic tests on composites

Standards for cryogenic tests on metals

  • ISO 6892-3: cryogenic tensile testing
  • ASTM E1450: Standard test method for tension testing of structural alloys in liquid helium

Cryogenic testing in hydrogen storage

There are three options for particularly effective hydrogen storage, which result in the requirements for different types of vessels or tanks, which are decisive for the selection of test parameters.

  • In the liquid state up to 4 bar, in the hydrogen liquefaction range at a temperature of 20 K
  • In a pressure range of 250 ... 700 bar at ambient temperature
  • In a pressure range of 500 ... 1000 bar between 33 and 73 K

Liquid hydrogen, in particular, presents an alternative to transport hydrogen in large quantities. In addition to metals, composites are often used in liquid hydrogen applications. When compared to metals, these offer a significant advantage: light weight. This aspect plays a particularly important role in aerospace or automotive applications, in order to develop very lightweight hydrogen tanks. This makes applications of liquid hydrogen at cryogenic temperatures of particular interest in the aerospace sector, for example, due to the more efficient storage density. In the automotive sector, on the other hand, the industry is also increasingly relying on containers for storage of gaseous hydrogen at high pressures.

Tests for the determination of characteristic values for the design and testing of composite/metal structures on liquefaction facilities or liquid hydrogen tanks under cryogenic conditions are therefore essential in meeting safety requirements to the highest extent possible, and to understand the thermomechanical stress that results from temperature changes in liquid hydrogen applications. This happens, for example, during refueling, due to different thermal expansion coefficients of fibers and matrix in composite materials.

Cooling with a temperature chamber

Temperatures chambers are ideal for tests at high temperatures and low temperatures down to approx. -170 °C. Here, the low temperature is dependent on the cooled volume in the chamber and the volume of the test rods that extend into the temperature chamber. In the version with temperature chamber, the rods are extended into the chamber from above and below.

Cooling with a nitrogen immersion cryostat

With nitrogen immersion cryostats, the material specimen is immersed in a nitrogen bath. The test temperature range of immersion cryostats is reduced to the temperature of liquid nitrogen. The specimens, along with the specimen grips, are guided into the immersion cryostat from above using a self-contained yoke. As soon as the cryogenic test is completed, the nitrogen is normally emptied or it evaporates into the atmosphere.

Cooling with nitrogen and helium in a continuous flow cryostat

Nitrogen and helium continuous flow cryostats are operated in a range of ambient temperature to low temperatures of approximately 20 K, depending on the cooling medium. Here, it is critical to keep the volume and the bodies that extend into the cryostat to a necessary minimum. The rule of thumb is: the less (metal) volume that protrudes out of the continuous flow cryostat, the lower the temperatures that can be achieved.

Based on cost factor, continuous flow cryostats are pre-cooled using nitrogen. Once the lowest possible temperature of the nitrogen has been reached, it is cooled with helium from a Dewar vessel until the final temperature of approximately 10 K to 20 K (-253 °C) is reached. The ambient medium around the specimen is always helium. To save on the cost, it is possible to capture and recover the gas and either compress it or re-liquefy it.

A special version of the ZwickRoell continuous flow cryostat can also be operated with hydrogen. In this case, hydrogen is the ambient medium around the specimen. Provided the appropriate safety precautions are taken when handling hydrogen, the ZwickRoell continuous flow cryostat only requires a few technical modifications.

Pure immersion cryostats for operation with liquid helium are not part of the ZwickRoell product portfolio.

Cryogenic pendulum impact tester with helium cooling

If hydrogen comes in contact with oxygen, in certain amounts it can be explosive. Material failure of hydrogen-carrying components would have fatal consequences. Therefore, the strength properties of a material, in addition to mechanical properties and the fatigue and fracture mechanical behavior, are of great interest in material research.

The cryogenic pendulum impact tester is used for the determination of strength properties under cryogenic conditions. With the help of a special cooling device, a Charpy specimen is cooled until it reaches a temperature of 20 K. A traditional Charpy impact test is then performed on an extremely cold metal specimen according to DIN EN ISO 148-1.

An instrumented pendulum impact tester measures the force during impact, provides data on stress and strain, and provides the information on fracture mechanics toughness parameters. Instrumentation, therefore, allows us to determine the failure mode not just the failure energy.

Use in static and dynamic testing machines

ZwickRoell offers the three cryogenic testers for both static testing machines and dynamic testing machines. The following principle applies: The lower the temperature, the more complex the mechanical effort.

In order to keep the cost for the coolant manageable and keep the temperature gradient across metallic feedthroughs as low as possible, we recommend ensuring that masses to be cooled, such as specimen grips and feedthroughs, have the lowest possible materials volume. In addition, the maximum test load should be as low as possible. This is because, contrary to testing at ambient temperature, generously selected dimensions not only result in high cost, they also affect the maximum attainable cryogenic temperature, temperature controllability and ultimately the reliability and reproducibility of test results.

The rule “only as much as necessary” is of particular significance in this case, and must be considered starting with the system's project planning phase. The cryogenic testing systems in the ZwickRoell product portfolio have a maximum load of 100 kN.

When designing a cryogenic testing system, the following points must be given special consideration:

  • Correct specimen grip material selection.
  • Lowest possible volume in the low-temperature area so that the smallest possible amount of coolant is required.
  • Keep temperature losses caused by the rods inserted in the cooling tank as low as possible.
  • Prevent ice buildup with special heating sleeves.
  • Protect the testing machine against condensation buildup.
  • Ensure the alignment and alignment capability of the load string.
  • Ensure the calibration capability of the system.
  • Proper extensometer selection.
  • Compensate for force shunts with the use of seals.
  • Compensate for thermal expansion.

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Interesting customer projects

The ROSEN Group specializes in research, development, manufacturing and use of inspection tools for pipelines and other complex technical equipment. At their location in Lingen (Ems), Germany, the company is establishing their first dedicated hydrogen testing laboratory. It is part of the new, approximately 4000 m² test center building on the company’s test site.
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Cryogenics is the technology used to generate ultra-low temperatures. Temperatures of 120 K (-153 °C) or lower are considered to be in the cryogenic range.

Materials testing under cryogenic conditions provides material characteristics at extremely low temperatures. This technology is used in different industries to investigate material behavior under real operating temperatures. Cryogenics is used in materials testing of composites, metals, aerospace, automotive and energy storage (hydrogen) applications.

Cryogenic temperatures are 120 K (-153 °C) and below. These temperatures are normally expressed in Kelvin.

Cryogenic cooling is used to generate extremely low temperatures. It is most commonly achieved using liquid gases such as nitrogen or helium.