Interpreting erosion frequency and magnitude from luminescence profiles in boulders

<p>Exposed bedrock is ubiquitous on terrestrial and planetary landscapes, yet little is known<br>about the rate of bedrock erosion at a granular scale on timescales longer than the<br>instrumental record. As recently suggested, using the bleaching depth of luminescence<br>signals as a measure of bedrock erosion may fit these scales. Yet this approach assumes<br>constant erosion through time, a condition likely violated by the stochastic nature of erosional<br>events. Here we simulate bleaching in response to power-law distributions of removal<br>lengths and hiatus durations. We compare simulation results with previously measured<br>luminescence profiles from boulder surfaces to illustrate that prolonged hiatuses are unlikely<br>and that typical erosion scales are sub-granular with occasional loss at mm scales,<br>consistent with ideas about microflaws governing bedrock detachment. For a wide range of<br>erosion rates, measurements are integrated over many removal events, producing<br>reasonably accurate estimates despite the stochastic nature of the simulated process. We<br>hypothesize that the greater or equal erosion rates atop large boulders compared to rates at<br>ground level suggest that subcritical cracking may be more influential than aeolian abrasion<br>for boulder degradation in the Eastern Pamirs, China.</p>

Breaking it Down: Mechanical Processes in the Weathering Engine

The vast diversity of landscapes found on Earth results from interplay between processes that break rock down, produce mobile regolith, and transport materials away. Mechanical weathering is fundamental to shaping landscapes, yet it is perhaps less understood at a mechanistic level than chemical weathering. Ubiquitous microfractures in rock propagate and grow through a slow process known as subcritical cracking that operates at the low applied stresses common in the near-surface. Subcritical cracking is the most likely explanation for the mechanical processes associated with thermal stress, ice lens growth, mineral alteration, and root growth. The long timescales over which critical zone architectures develop require an understanding of slow processes, such as subcritical cracking.

Fracture Threshold Measurements of Hydrogen Precharged Stainless Steel Weld Fusion Zones and Heat Affected Zones

Austenitic stainless steels are used in hydrogen environments because of their generally accepted resistance to hydrogen embrittlement; however, hydrogen-assisted cracking can occur depending on the microstructures or composition of the stainless steel. One area that has not been well researched is welds and in particular heat affected zones. The goal of this work was to measure the subcritical cracking susceptibility of hydrogen precharged gas tungsten arc (GTA) welds in forged stainless steels (21Cr-6Ni-9Mn and 304L). Welds were fabricated using 308L filler metal to form 21-6-9/308L and 304L/308L weld rings, and subsequently three-point bend specimens were extracted from the fusion zone and heat affected zone and precharged in high-pressure hydrogen gas. Crack growth resistance curves were measured in air for the hydrogen precharged fusion zones and heat affected zones under rising-displacement loading, revealing significant susceptibility to subcritical cracking. Fracture thresholds of 304L/308L welds were lower than 21-6-9/308L welds which was attributed to higher ferrite fractions in 304L/308L since this phase governed the crack path. Fracture thresholds for the heat affected zone were greater than the fusion zone in 21-6-9/308L which is likely due to negligible ferrite in the heat affected zone. Modifications to the weld joint geometry through use of a single-J design were implemented to allow consistent testing of the heat affected zones by propagating the crack parallel to the fusion zone boundary. Despite low hydrogen diffusivity in the austenitic stainless steels, effects of displacement rates were observed and a critical rate was defined to yield lower-bound fracture thresholds.

Measurement of Fracture Properties for Ferritic Steel in High-Pressure Hydrogen Gas

Recently, the measurement of threshold stress intensity factors for various low alloy ferritic steels in high-pressure hydrogen gas of 103 MPa was performed, and it was revealed that the subcritical cracking threshold under rising displacement was lower than the subcritical cracking threshold for crack arrest under constant displacement. These experimental results demonstrate the importance of the testing method for evaluating the fracture properties in high-pressure hydrogen gas. We measured the subcritical cracking threshold under rising displacement for ASME SA-372 Grade J ferritic steels in high-pressure hydrogen gas at pressure up to 115MPa. In contrast to other reported procedures where the applied displacement was increased continuously, in this study crack length was determined using an unloading elastic compliance method. The values of the subcritical cracking threshold measured by the unloading elastic compliance method are consistent with previous measurements in which the applied displacement continuously increased. These results suggest the possibility that subcritical cracking thresholds do not depend on the applied displacement path, i.e., periodic unloading vs. continuously rising displacement.