Efficient and economical hydrogen use may require its transportation and storage at high pressure.  Effects of hydrogen on the degradation of material have been studied; however, the available data on the effects of high-pressure gaseous hydrogen on fracture processes in structural materials are relatively limited.  FRASTA is a tool that can help understand the mechanisms of degradation in materials, assess risk, and understand the potential impact of failed components and structures exposed to high pressure hydrogen. This information is particularly useful for the design of high-pressure hydrogen systems and for the development of codes and standards.

Sandia National Laboratories in Livermore has several test programs for measuring fracture properties of structural materials in high-pressure gaseous hydrogen. Test results indicate a significant effect of hydrogen on crack initiation and growth in steels used in pressure systems.  Fracture surface examination reveals, however, that deformation and fracture mechanisms and their relationship to microstructural features are difficult to delineate.

An SRI International-developed technology, FRASTA (fracture surface topography analysis), reconstructs fracture processes in microscopic details utilizing conjugate fracture surface topographs.  The basic principle of FRASTA is to reconstruct the details of fracture processes based on differences in local plastic deformation. 

Fracture surface topography is characterized by a confocal-optics based scanning laser microscope, and the conjugate topographs are juxtaposed by a computer.  Spacing between the conjugate topographs is initially adjusted so that the conjugate surfaces overlap everywhere within the observation area. The conjugate surfaces are then separated incrementally, and after incremental separation the area is scanned to see if a gap developed.  Gap development signifies microcrack formation.   Locations of gapped areas are plotted in the observation area.  Successive incremental separations produce expansion of gapped area(s), formation of new gaps, coalescence of gapped areas, and eventual formation of a crack front that sweeps through the material.   The plot of a gapped area in the observation area is called a fractured area projection plot (FAPP) and is similar to an x-radiograph of that area.  The FAPPs can be superimposed on the scanning electron microscope photograph of fracture surfaces, revealing microcrack initiation sites and microstructural features affecting crack growth. 

We can also produce plots that cut conjugate surfaces vertically.  These plots are called cross sectional plots (XSP), and indicate the crack-face opening profiles behind the crack tip and the amount of plastic deformation needed to induce fracture ahead of that crack tip.  Using the crack-face opening profiles, we can assess not only the local fracture toughness of the microstructural constituents but also macro fracture toughness.

Finally, the gapped area increase rate can be plotted as a function of spacing between conjugate topographs.  This plot is named a fracture progression plot (FPC).  The slope of the FPC is sensitive to the crack growth rate and influenced by not only the loading condition but also the environment.  With this information, we can determine the location where changes in crack growth conditions occur on the fracture surface images.

This paper presents FRASTA results from materials tested in high pressure gaseous hydrogen and discusses the details of the observed fracture mechanisms.