Standards Development

Scope

This document specifies geometries, preparation, and analysis of test results of hollow tensile test piece for internal pressurized test and the slow-strain rate tensile test (SSRT) method with this test piece in high-pressure hydrogen environment.

The influence of high-pressure hydrogen gas environment (one of the pressurized environments) has been conventionally evaluated using a cylinder or vessel for high-pressure hydrogen gas containing a test piece inside and loading from outside, which is also described in ASTM G142 or ISO 11114-4. However, this kind of method requires facilities of high-pressure hydrogen for mechanical properties testing which are costly and have safety concerns. In addition, it is not easy to change the temperature of the test piece with high-pressure hydrogen gas and the high-pressure vessel, and it is difficult to seal the sliding part between the pull-rod for loading the test piece and the high-pressure gas vessel, especially at lower or higher temperatures.

Hollow-type test pieces containing high-pressure hydrogen gas in a small diameter hole along the axis can also evaluate the influence of the hydrogen gas without high-pressure vessels. Consumption of hydrogen gas for one test is about one litter (one small balloon) for 100 MPa test. So, this method is simpler, safer, and less cost than the conventional method and expected to be standardized in order to widespread commercialization of 70 MPa-class hydrogen re-fuelling stations and fuel cell vehicles.

Purpose

The hollow-test piece test is much simpler and more cost-effective for tensile testing than the conventional use of a test chamber. This would allow many more laboratories of industries and universities to test for the influence of pressurized hydrogen gas on metallic materials and would greatly increase the worldwide knowledge base of hydrogen effects on materials, which leads to commercialization of 70 MPa-class hydrogen re-fuelling stations and fuel cell vehicles.

The lowering of cost and shortening testing time would be a great help to industries. There is consistently high demand for simple, cost-effective determination of accurate values of yield strength, tensile strength, and resistance to hydrogen embrittlement of metals.

Concerning the hollow test piece, it had been known and used occasionally since 50 years ago. But those hollow test piece were mostly thin wall or pipe-like, then the deformation behaviour was different from that of solid test piece, and the elongation nor the reduction of area could not be obtained. Other concerns for the hollow test piece were the hoop stress and the roughness of inner surface of hollow. Recent advances in processing technology have made it possible to drill and polish long and narrow holes. The hollow test piece with a narrow hole was found to be able to evaluate the elongation and the reduction of area almost the same as the solid test piece in the relative evaluation of hydrogen influence.

FEM analysis proved the stress distribution of this hollow test piece is mostly the same as the solid test piece and hollow test pieces evaluate more severe (on the safe side) than solid test pieces for materials that break after the maximum load in hydrogen under high pressure, because after the maximum load, the outer surface (contacted to the environment in the case of solid test piece) hydrostatic stress component of the test piece decreases with the increase of strain during necking.This is very important that the materials used for pipes or vessels contain high-pressure gas inside not outside. Hydrogen influences only in tensile stress condition.

This hollow test piece is an innovation of mechanical testing methods which can evaluate the effects of environments uni-axially even after maximum load point and will be widely used in the mechanical properties evaluation of not only the high-pressure hydrogen environment but also other gaseous and/or corrosive environments, such as CO2 or H2S even at higher temperatures.

It is important to standardize the hollow test piece method as a general purpose basic test piece in IS at this stage to prevent from the confusion of results produced due to the difference of geometries and preparation of hollow test pieces by own design of each institute in future.

The major subjects of standardization for this method are influences of the hole along the axis of bartype test pieces, such as the inner surface roughness, the inner pressure, and other test piece geometries. The influence of high-pressure hydrogen gas for high-strength materials is varied according to the surface roughness of test piece. In the conventional tensile tests in the high-pressure hydrogen gas, the parallel part of tensile test piece is polished. However, for the hollow test pieces, it is not easy to polish the inner surface of the hole and the surface of the hole was finished by threerotating wire-cut generally.

The inner surface roughness Rz of the hole after the wire-cut was about 10 m. The inner surface roughness Rz is improved by polishing and Rz after the abrasive flow polishing was less than 0.5 m. Details are described in the proceedings of ASME PVP2019-93492. The hollow-test piece test is much simpler and more cost-effective for tensile testing than the conventional use of a test chamber. This would allow many more laboratories of industries and universities to test for the influence of pressurized hydrogen gas on metallic materials and would greatly increase the worldwide knowledge base of hydrogen effects on materials, which leads to commercialization of 70 MPa-class hydrogen re-fuelling stations and fuel cell vehicles. The lowering of cost and shortening testing time would be a great help to industries.

There is consistently high demand for simple, cost-effective determination of accurate values of yield strength, tensile strength, and resistance to hydrogen embrittlement of metals. Concerning the hollow test piece, it had been known and used occasionally since 50 years ago. But those hollow test piece were mostly thin wall or pipe-like, then the deformation behaviour was different from that of solid test piece, and the elongation nor the reduction of area could not be obtained. Other concerns for the hollow test piece were the hoop stress and the roughness of inner surface of hollow. Recent advances in processing technology have made it possible to drill and polish long and narrow holes. The hollow test piece with a narrow hole was found to be able to evaluate the elongation and the reduction of area almost the same as the solid test piece in the relative evaluation of hydrogen influence.

FEM analysis proved the stress distribution of this hollow test piece is mostly the same as the solid test piece and hollow test pieces evaluate more severe (on the safe side) than solid test pieces for materials that break after the maximum load in hydrogen under high pressure, because after the maximum load, the outer surface (contacted to the environment in the case of solid test piece) hydrostatic stress component of the test piece decreases with the increase of strain during necking. This is very important that the materials used for pipes or vessels contain high-pressure gas inside not outside. Hydrogen influences only in tensile stress condition.

This hollow test piece is an innovation of mechanical testing methods which can evaluate the effects of environments uni-axially even after maximum load point and will be widely used in the mechanical properties evaluation of not only the high-pressure hydrogen environment but also other gaseous and/or corrosive environments, such as CO2 or H2S even at higher temperatures. It is important to standardize the hollow test piece method as a general purpose basic test piece in IS at this stage to prevent from the confusion of results produced due to the difference of geometries and preparation of hollow test pieces by own design of each institute in future.

The major subjects of standardization for this method are influences of the hole along the axis of bartype test pieces, such as the inner surface roughness, the inner pressure, and other test piece geometries. The influence of high-pressure hydrogen gas for high-strength materials is varied according to the surface roughness of test piece. In the conventional tensile tests in the high-pressure hydrogen gas, the parallel part of tensile test piece is polished. However, for the hollow test pieces, it is not easy to polish the inner surface of the hole and the surface of the hole was finished by threerotating wire-cut generally.

The inner surface roughness Rz of the hole after the wire-cut was about 10 m. The inner surface roughness Rz is improved by polishing and Rz after the abrasive flow polishing was less than 0.5 m. Details are described in the proceedings of ASME PVP2019-93492. After the examinations by the different surface roughness test pieces and the FEM analysis of stressstrain curves and stress distribution for the hollow test pieces and the solid test pieces during tensile  tests, the geometries and preparation of the test pieces were determined as follows:

1 Geometries of test pieces:

1) round bar type generally,

2) diameter of parallel section: 4~8 mm,

3) inner diameter of hole: 1~2 mm, 2 Preparation of test pieces: 1) the inner surface of the hole of the test piece shall be polished to remove the work-hardened or heataffected layer, as a rule, for the higher strength and/or hydrogen- sensitive materials by a diamondelectroplated wire (file) or an abrasive flow polishing.

2) joints for piping is required according to the environments. 3 Testing conditions

1) temperature: 20 K ~ 1000 K, as customer required.

2) Inner pressure: more than 100 MPa as customer required.

3) strain rate is less than 5.0 ×10-5 s-1.