Discrete Slip Events Research Articles

On the extraction of yield stresses from micro-compression experiments

It is widely stated that yield stress extraction from micro-compression testing can be unreliable due to factors such as the pillar taper, pillar-punch misalignment and lateral displacement for example through discrete slip events. This study reports on how these factors can be eradicated when using digital image correlation based strain mapping. This approach is demonstrated through a conventional micropillar compression experiment using the hexagonal MAX phase system Cr2AlC. The use of this approach enables the uncertainty of the extracted yield stress values to be significantly reduced and effectively increases the precision of critical resolved shear stress extraction for basal plane slip from ±52% to ±4% of the mean value. The findings suggest that the statistical variations commonly observed in micro-compression tests may frequently be linked to systematic errors in the stress measurements taken, and that precise evaluation of deformation statistics can only be facilitated if the pillar deformation morphology is closely monitored.

Predicting the size scaling in strength of nanolayered materials by a discrete slip crystal plasticity model

The main attraction of metallic nanolayered composites (MNCs) lies not only with their five-to ten-fold increases in strength over that of their constituents, but also in the tunability of their superior strength with nanolayer thickness. While the size scaling in strength prevails in many MNC material systems, the size scaling cannot be accurately predicted with crystal plasticity framework. Here, we present a crystal plasticity based computational method that considers plasticity to occur in grain boundary-controlled discrete slip events and apply it to predict the deformation response and underlying mechanisms in Cu/Nb MNCs. Predicted tensile stress-strain responses are shown to achieve agreement with measurements for four distinct nanolayer thicknesses, without introducing adjustable parameters. The model predicts the Hall-Petch size scaling of strength on layer thickness and the rising plastic anisotropy as the layer thickness reduces. Analysis of the results indicates that the origin of the layer size effect on strength results from the limits layer thickness places on the lengths of dislocations sources lying in the grain boundaries.

Characterization of machine learning algorithms for slippage estimation in planetary exploration rovers

This paper presents a comprehensive comparison of well-known machine learning algorithms for estimating discrete slip events associated with individual wheels in planetary exploration rovers. This analysis is performed with various tuning configurations for each algorithm (55 setups). This research also shows the key role that environment plays in the performance of the learning algorithms: rover speed (0.05–0.25 [m/s]), type of terrain (gravel vs. sand), and tire type (off-road tires vs. smooth tires). These contributions are validated by using a broad data set collected using a planetary rover equipped with proprioceptive sensing. This work not only identifies the best algorithm to be deployed for discrete slip estimation, but it also helps with the selection and the mounting position of the sensing systems to be employed in future robotic planetary missions.

Coupled crystal orientation-size effects on the strength of nano crystals.

We study the combined effects of grain size and texture on the strength of nanocrystalline copper (Cu) and nickel (Ni) using a crystal-plasticity based mechanics model. Within the model, slip occurs in discrete slip events exclusively by individual dislocations emitted statistically from the grain boundaries. We show that a Hall-Petch relationship emerges in both initially texture and non-textured materials and our values are in agreement with experimental measurements from numerous studies. We find that the Hall-Petch slope increases with texture strength, indicating that preferred orientations intensify the enhancements in strength that accompany grain size reductions. These findings reveal that texture is too influential to be neglected when analyzing and engineering grain size effects for increasing nanomaterial strength.

Emergence of grain-size effects in nanocrystalline metals from statistical activation of discrete dislocation sources

Nanocrystalline (NC) metals are exceptionally strong because they contain an unusually high density of grain boundaries (GBs) that act as sources and sinks for dislocations and significantly modify dislocation motion. In this study, we seek to understand the relationship between discrete dislocation emissions from GBs and grain size effects seen in the strength of NC metals. We propose a statistical GB dislocation source model responsible for discrete dislocation slip events. We find that the grain size limitation on dislocation source sizes gives rise to grain size effects on the statistical distribution for the critical resolved shear stress (CRSS) for discrete slip events. To establish its impact on mechanical behavior, this GB source model is integrated into a 3D crystal plasticity finite element model for NC Ni. We observe that a Hall–Petch scaling in yield strength emerges from the calculations. Further the predictions achieve quantitative agreement with experimental data from several studies across a wide range of average nanograin sizes. It is revealed that statistical dispersion in the CRSS for discrete slip events causes (a) strain hardening in the macroscale flow stress–strain response and (b) the fraction of grains that accommodate the applied strain and the fraction of active grains that undergo multi-slip to strongly depend on grain size. It is also found to lead to an unusual texture effect on slip activity that is significant in the finer nanograins but weak in larger grains (>100nm). In addition, it causes increases in heterogeneity in strain concentrations with decreasing grain size, suggesting that plastic instabilities are more likely to form as nano-scale grain sizes decrease.

The thickness of subduction plate boundary faults from the seafloor into the seismogenic zone

The thickness of an active plate boundary fault is an important parameter for understanding the strength and spatial heterogeneity of fault behavior. We have compiled direct measurements of the thickness of subduction thrust faults from active and ancient examples observed by ocean drilling and fi eld studies in accretionary wedges. We describe a general geometric model for subduction thrust decollements, which includes multiple simultaneously active, anastomosing fault strands tens of meters thick. The total thickness encompassing all simultaneously active strands increases to ~100–350 m at ~1–2 km below seafl oor, and this thickness is maintained down to a depth of ~15 km. Thin sharp faults representing earthquake slip surfaces or other discrete slip events are found within and along the edges of the tens-ofmeters- thick fault strands. Although fl attening, primary inherited chaotic fabrics, and fault migration through subducting sediments or the frontal prism may build melange sections that are much thicker (to several kilometers), this thickness does not describe the active fault at any depth. These observations suggest that models should treat the subduction thrust plate boundary fault as <1–20 cm thick during earthquakes, with a concentration of postseismic and interseismic creep in single to several strands 5–35 m thick, with lesser distributed interseismic deformation in stratally disrupted rocks surrounding the fault strands.

Stratigraphic evidence for millennial-scale temporal clustering of earthquakes on a continental-interior fault: Holocene Mississippi River floodplain deposits, New Madrid seismic zone, USA

The earthquake cycles that characterize continental-interior areas that are far from active plate boundaries have proven highly cryptic and difficult to resolve. We used a novel paleoseismic proxy to address this issue. Namely, we reconstructed Holocene Mississippi River channels from maps of floodplain strata in order to identify channel perturbations reflective of major displacement events on the high-hazard and mid-plate Reelfoot thrust fault, New Madrid seismic zone, U.S.A. Only three discrete slip events are currently documented for the Reelfoot fault (∼ AD 900, ∼ AD 1450, and AD 1812). This study extends this record and, thus, illustrates the utility of stratigraphic proxies as paleoseismic tools. We concurrently offer here some of the first quantified response times for tectonically induced channel pattern changes in large alluvial rivers. We identified at least two cycles of pervasive meandering that were interrupted by channel-straightening responses occurring upstream of the Reelfoot fault scarp. These straightening responses initiated at 2244 BC +/− 269 to 1620 BC +/− 220 and ∼ AD 900, respectively, and each records initiation of a period of Reelfoot fault slip after millennia of relative tectonic quiescence. The second (or New Madrid) straightening response was triggered by the previously known ∼ AD 900 fault slip event, and this initial low sinuosity has been protracted until the modern day by the latter ∼ AD 1450 and AD 1812 events. The first (or Bondurant) straightening response began a period of several hundred to ∼ 1400 years of low river sinuosity which evidences a similar period of multiple recurrent displacement events on the Reelfoot fault. These Bondurant events predate the existing paleoseismic record for the Reelfoot fault. These data offer initial evidence that slip events on the Reelfoot fault were temporally clustered on millennial scales and, thus, offers the first direct evidence for millennial-scale clustering of earthquakes on a continental-interior fault. This carries additional ramifications. Namely, faults that have been quiescent and non-hazardous for millennia could re-enter an enduring period of recurrent hazardous earthquakes with little warning. Likewise, the Reelfoot fault also reveals evidence of temporal clustering of earthquakes on short-term cycles (months), as well as evidence for longer-term reactivation cycles (10 4–10 6 years). This introduces the possibility that temporal clustering could be hierarchical on some continental-interior faults.

Scale-Free Intermittent Flow in Crystal Plasticity

Under stress, crystals irreversibly deform through complex dislocation processes that intermittently change the microscopic material shape through isolated slip events. These underlying processes can be revealed in the statistics of the discrete changes. Through ultraprecise nanoscale measurements on nickel microcrystals, we directly determined the size of discrete slip events. The sizes ranged over nearly three orders of magnitude and exhibited a shock-and-aftershock, earthquake-like behavior over time. Analysis of the events reveals power-law scaling between the number of events and their magnitude, or scale-free flow. We show that dislocated crystals are a model system for studying scale-free behavior as observed in many macroscopic systems. In analogy to plate tectonics, smooth macroscopic-scale crystalline glide arises from the spatial and time averages of disruptive earthquake-like events at the nanometer scale.

The break-back thrust splay of the Main Dun Thrust (Himalayas of western Nepal): evidence of an intermediate displacement scale between earthquake slip and finite geometry of thrust systems

Numerous studies have shown that development of a forward breaking sequence in a thrust belt is accompanied by reactivation of inner faults and development of break-back thrusts. The partitioning of shortening in the Outer and Lesser Himalayan thrust belt follows these rules. Tectonic and sedimentary relationships in this thrust belt exemplify out-of-sequence thrusts in the Lesser Himalayas and break-back imbricate fans close to the MBT (Main Boundary Thrust) and MDT (Main Dun Thrust) which are transported during their development above the Himalayan basal detachment. The displacement along a single splay of the imbricate fan of the MDT was >40 m, before abandonment and nucleation of a new inner splay. The development of the break-back imbricate fan is discontinuous in time. Furthermore, superimposition of ultracataclasite, pressure-solution cleavage, extensional crenulation cleavage and tension gashes in the fault zones suggests that the 40 m displacement occurred by discrete slip events, or most probably by a succession of several seismic/aseismic deformations. These data require several scales of sequential displacement in the Outer and Lesser Himalayas consisting of forward propagation of the thrust system, out-of-sequence reactivation, break-back nucleation of thrust splays, and seismic/aseismic fault slip. Our findings require an intermediate period of tectonism between the <1000 y earthquake cycle and the >500 Ka deformation of the accretionary sediments of the Himalayas.