Cryogenic Testing

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.

Depending on the operating temperature and application, temperature chambers and immersion cryostats are used for testing in the cryogenic range. Based on the type or version of this cryogenic testing equipment, you can reach test temperatures in the cryogenic range between 77 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 tests on composites

• ISO 527-4, ISO 527-5, ASTM D3039: Tensile tests
• ISO 14126, ASTM D3410, ASTM D6641, ASTM D695: Compression tests
• ISO 14129, ASTM D3518: In-plane shear (IPS)
• ISO 14230, ASTM D2344: Interlaminar shear strength , ILSS
• ISO 14125, ASTM D7264: Flexure tests
• EN 1465, ASTM D3164: Determination of tensile lap-shear strength of bonded assemblies
• ASTM D7905: Interlaminar Fracture Toughness Mode II
• ISO 13003, ASTM D3479: Fatigue behavior under tensile cyclic loading
• ISO 13003 Annex A: Flexure fatigue test

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

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.

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:

• Proper material selection for specimen grips
• 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.