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Testing of Composites at Cryogenic Temperatures

with temperature chambers, immersion cryostats or continuous flow cryostats

Cryogenic composites testing (temperatures below <120 K) is required in the growing hydrogen technology sector in particular. The development of new materials and material combinations in the different material groups always requires a test of the material at the respective operating temperature, which is 20 K for liquid hydrogen.

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 cryogenic composites testing, ZwickRoell offers three options:

Cooling with temperature chambers Cooling with immersion cryostats Cooling with continuous flow cryostats

What is cryogenics?

Cryogenics is the technology for generating ultra-low temperatures. Temperatures starting from 120 K (-153 °C) or lower are considered to be in the cryogenic range. At these temperatures, gases become liquid.

Materials testing in the cryogenic environment delivers material characteristic values at ultra-low temperatures. This technology is used in different industries to investigate material behavior under real operating temperatures. Cryogenics is especially used in the areas of composites, metals, aerospace, automotive and energy storage (hydrogen).

Purpose of the test

When using composites for liquid hydrogen storage in particular, two 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 composite structures. Since, unlike metallic structures for the H2 infrastructure, the composite material is often not in direct contact with the hydrogen medium, the cooling medium helium, which is far less complex to handle, can also be used to reach the test temperature of 20 K.
  • The very different thermal expansion coefficients of the fiber and matrix in fiber-reinforced plastics also lead to frozen stresses in the material during the manufacturing process of today’s composite materials and applications. 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 test composites in the cryogenic range, temperature chambers, continuous flow cryostats, and immersion cryostats are used depending on the operating temperature and application. Based on the type or version of this equipment, you can reach test temperatures in the cryogenic range between 4 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 test temperatures themselves are determined by the application of the composite material.

Standards

The following standards for static analyses for composites testing are also used when testing with cryogenics:

The following standards for dynamic analyses for composites testing are also used when testing with cryogenics:

  • ISO 13003, ASTM D3479:
  • ISO 13003 Annex A: Flexure fatigue test

Hydrogen storage options

In the area of hydrogen storage there are three ranges, dependent on pressure and temperature, in which storage is particularly effective:

  • In the liquid state at pressures 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.

These ranges result in the respective requirements for different types of tanks: from a pure hydrogen pressure tank at ambient temperature up to a 20 K liquid hydrogen tank at low pressure. These requirements also determine the
ultimate test parameters to be used for the material test.

Composites have a significant weight advantage when compared to metals. To use this advantage, work is underway to develop hydrogen tanks with the highest possible composite content, especially for aerospace and automotive applications. For applications in the aerospace industry, the improved storage density brings greater interest to the concept of storing liquid hydrogen at cryogenic temperatures. In the automotive sectors, containers are increasingly being used to store gaseous hydrogen at high pressures.

Cooling with a temperature chamber

Conventional design temperature chambers are used for tests at high temperatures as well as low temperatures down to approx. -170 °C. Here, the low temperature is dependent on the cooled volume in the chamber, or 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 composite specimen is immersed in a nitrogen bath. The test temperature range of immersion cryostats is reduced to the temperature of liquid nitrogen. The specimens, including the specimen grips, are guided into the immersion cryostat from above using a self-contained yoke. After the test, the nitrogen is normally emptied or it evaporates into the atmosphere.

Cooling with nitrogen, helium and hydrogen continuous flow cryostats

Nitrogen/helium and hydrogen continuous flow cryostats can be operated from ambient temperature to low temperatures of approximately 20 K, depending on the cooling medium. Here, it is very important to keep the volume and the bodies that extend into the cryostat to a necessary minimum. The less (metal) volume that protrudes out of the continuous flow cryostat, the lower the temperatures that can be achieved.

Continuous flow cryostats are pre-cooled with nitrogen for cost reasons. 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 is reached. The ambient medium around the specimen is always helium.

A special version of ZwickRoell continuous flow cryostats can also be operated with hydrogen. Here, the ambient medium around the specimen is hydrogen. Provided the appropriate safety precautions are taken when handling hydrogen, the ZwickRoell continuous flow cryostat only requires a few technical modifications to be operated with hydrogen. Due to the high cost for helium, it is possible to capture and recover the helium as it escapes the cryostat and compress it again or even re-liquefy it. Both solutions, however, are complex and costly and are the responsibility of the system operator.

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

Use in static and dynamic testing machines

ZwickRoell installs the above mentioned temperature conditioning devices in both static testing machines and dynamic testing machines, whereby the lower the temperature, the more complex the mechanical efforts. To keep the costs for cooling mediums low and obtain the lowest possible temperature gradient across metallic feedthroughs, the masses to be cooled, such as specimen grips, and the feedthroughs should be designed with the smallest possible volume of material. It is therefore recommended that the test load be kept as low as possible. In contrast to testing at ambient temperature, generously selected dimensions are strongly reflected in the costs, the maximum achievable temperature, the temperature controllability and ultimately, in reliable and reproducible test results. The precept “only as much as necessary” is of particular significance in this case, and must be considered as part of the system planning phase. The cryogenic testing systems in the ZwickRoell product portfolio are limited to 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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