Slip Accommodation Research Articles

Investigation of strain localization in sheared granular layers using 3-D discrete element modeling

In this work, we investigate slip localization in sheared granular faults at seismic velocities using 3-D numerical simulations with the discrete element method (DEM). An aggregate of non-destructive spherical particles is subjected to direct shear by using two moving boundaries in a sandwich configuration to identify the impact of particle-scale parameters on slip localization. We impose a thin layer of fine-grained particles with variable contrast in thickness and grain size adjacent to the boundary as well as in the middle of the granular layer to simulate boundary and Y shears observed in both natural and laboratory fault gouges. The results show that larger amounts of strain is accommodated within the pre-described finer-grained layer even with a small (< 10%) contrast in grain size. Up to 90% of the displacement is localized in a finer-grained layer when the contrast ratio of the grain size is 50%. Based on the concept of the average spreading velocity of particles and squeeze expulsion theory in granular flow, we suggest that the phenomenon of localization is likely from result from the contribution of larger grains collisions with smaller grains. Since the amount of frictional heat generated depends on the degree of localization, the results provide crucial information on the heat generation and associated slip accommodation in sheared gouge zones. We conclude that the occurrence of a weaker, fine-grained layer within a dense fault zone is likely to result in self-enhanced weakening of the fault planes.

Seismo-hydro-mechanical modelling of the seismic cycle: Methodology and implications for subduction zone seismicity

Slip accommodation in subduction zones ranges from aseismic slip phenomena to regular megathrust earthquakes, and strongly depends on pore fluid pressure. We develop a new fully coupled poro-visco-elasto-plastic seismo-hydro-mechanical numerical model, allowing for coupled modelling of tectonic and seismic processes in the presence of fluids. A combination of fully staggered finite differences and marker in cell techniques is used to solve mass and momentum conservation equations for solid matrix and fluid coupled to a poro-visco-elasto-plastic rheological constitutive relationship. Brittle/plastic deformation is resolved through global Picard-iterations and adaptive time stepping is introduced to resolve time scales from milliseconds to thousands of years involved in the hydro-mechanical seismic cycle.We demonstrate how and why the presence of pervasive fluid flow in the deforming poro-visco-elasto-plastic subduction interface causes localisation of deformation and nucleation of seismic events with slip rate up to m/s. The nucleation of fault slip is controlled by rapid fluid pressure increase due to visco-plastic compaction of a spontaneously forming fault balanced by the simultaneous elastic decompaction of deforming pores inside the fault. Subsequent post- and inter-seismic slow fluid pressure release by elastic compaction of the stressed pores allows recovery of subduction interface strength. The events nucleate downdip in the brittle-ductile transition zone and propagate updip in form of highly localized, spontaneous ruptures. The model reproduces the broad spectrum of transient phenomena ranging from slow slip to seismic ruptures on a subduction interface with homogenous elastic and frictional properties that do not depend on slip rate. The degree of locking of the megathrust interface, the coseismic stress drop and the dominant slip regime during subduction are critically dependent on effective large-scale and long-term rock permeability. A decrease of permeability leads to a decrease of degree of locking, leading to smaller stress drop and enhancing the occurrence of stable aseismic slip.

Segmentation pattern and structural complexities in seismogenic extensional settings: The North Matese Fault System (Central Italy)

We investigated the northern slope of the Matese Mts. (Molise, Central Italy) with the aim of characterizing the N- to NE-dipping active normal fault system in the Bojano basin, a sector of primary importance from a seismic hazard perspective. We collected field data to define the geometry and segmentation pattern of two sub-systems (Patalecchia-Colle di Mezzo and Bojano–Campochiaro). New evidence of late Quaternary faulting was obtained by exploiting well log interpretations. Kinematic analysis revealed the interaction of pre-Quaternary inherited (mainly E-W-striking) and newly formed (NW-SE-striking) normal faults. Slip accommodation through linkage was clearly noted in the case of the Patalecchia-Colle di Mezzo sub-system. Detailed topographic profiles across the active fault segments provided post-LGM (15 ± 3 kyr) slip rates up to ∼2 mm/yr which agree with the high deformation rates based on different approaches in the literature. Finally, the instrumental seismicity analysis constrained the bottom of the seismogenic layer to depths of 13–14 km, and the gathered information allowed us to reconstruct the North Matese seismogenic source. Its 3D geometry and dimensions agree with both the dimension-magnitude relationships and macroseismic information available for the 1805 earthquake (Mw 6.6), the main historical earthquake to have struck the Bojano basin.

Shallow-generated damage within non-planar strike-slip fault zones: role of sedimentary rocks in slip accommodation, SW Holy Cross Mountains, Poland

We investigated exhumed damage zones of dextral strike-slip faults dissecting the south-western part of the Mesozoic cover of the Late Palaeozoic Holy Cross Mountains Fold Belt. Structural observations allow to examine the top 1–2 km of the fault zones that deformed asymmetrically with the most intense damage controlled by the non-planar geometry of the faults. The deformational style of fault zones and the roughness of slip surfaces on subsidiary faults within deflections of fault traces in the restraining and releasing bends were facilitated by rock fabric and porosity. High porous sandstones enhanced genesis of cataclastic shear bands within the damage zone and the smooth slip surfaces. Low porous limestones enhanced the formation of dilatant structures infilled with calcite within damage zones and rough slip surfaces. The complex structural pattern of damage zones records multiple episodes of slip and shows evidence of continuous seismic–aseismic modes of fault slip behaviour.

Surface Slip Effect on Thermal Environment of Hypersonic Non-Equilibrium Flows

Boundary slip effect caused by low gas density has an important influence on the thermal environment of the vehicles. Numerical studies on the boundary slip effect and accommodation for moment and energy have been carried out in this paper. The simulations considering slip boundary and surface catalysis are validated with Arc-jet test data. Mechanism and rules of impact on surface heat flux by different boundary slip level (Knudsen number from 0.0028 to 0.05) has been investigated in typical hypersonic flow conditions. The results show that mechanisms of boundary slip effect on mass diffusion heat flux and convective heat flux are different; slip boundary diminishes the convective heat, whereas enhances the mass diffusion heat flux. Smaller moment and energy accommodation coefficient is equivalent to more rarefaction. As Knudsen number goes up, the influences of accommodation on heatflux are enhanced, it mainly affects the convective heat flux.

Design of Heusler Precipitation Strengthened NiTi- and PdTi-Base SMAs for Cyclic Performance

For a wide range of actuation applications, the performance of NiTi-based shape memory alloys is limited by cyclic instability associated with accommodation slip. For medical applications, low-Ni compositions are also desirable. Increasing yield strength via precipitation of a coherent nanoscale Ni2TiAl-type Heusler phase from a supersaturated B2 matrix is an effective approach for eliminating slip in order to improve the stability of the functional response and increase the structural fatigue life. Quaternary additions that partition into the L21 Heusler phase, such as Zr or Pd, are favorable for reducing interphase misfit and maintaining coherency during aging. Phase relations and precipitation kinetics in quaternary Ni(TiZrAl), low-Ni (PdNi)(TiAl), and Ni-free (PdFe)(TiAl) systems are summarized from TEM and atom probe tomography data in the literature. Strengthening behavior during isothermal aging is compared in the NiTiZrAl and PdNiTiAl systems, and recent work characterizing a high-strength, low-Ni “Hybrid” (PdNi)(TiZrAl) alloy is presented. A systems design approach is taken in which an optimal microstructure for peak strengthening is identified while other property objectives such as transformation temperature, misfit, radiopacity, and biocompatibility are satisfied.

The effects of texture and grain morphology on the fracture toughness and fatigue crack growth resistance of nanocrystalline platinum films

The fracture toughness and fatigue crack growth resistance of nanocrystalline materials are significantly affected by the thickness of the specimen. In this work we relate the mechanical properties of nanocrystalline platinum films to their texture and grain morphology. Tensile, creep and fatigue testing of annealed, ∼1μm films resulted in mechanical properties similar to the as-received films (yield strength of ∼1.2GPa, fracture toughness ∼17.8MPa√m, and a fatigue crack growth power law exponent of ∼4.2). However, the breakdown of the initially columnar grain morphology had a marked effect on the transition point from an intergranular to transgranular fatigue cracking mode. Finite element modeling suggests that cyclic (fatigue) grain coarsening and the transition from inter- to transgranular cracking modes are a result of the relative importance of dislocation slip accommodation on in-plane and through-thickness oriented slip directions.

Time and spatial resolution of slip and twinning in a grain embedded within a magnesium polycrystal

Plastic yielding in magnesium alloys frequently involves the initiation of both slip and twinning events. A proper understanding of the phenomenon at the grain level requires knowledge of how these two mechanisms progress and interact over both time and space and what the local resolved stresses are. To date, simultaneous collection of such information has not been achievable. To address this shortfall, we have developed a modified Laue based in situ micro X-ray diffraction technique with an unprecedented combination of time and spatial resolution. A ten-fold reduction in data collection times is realized by the refinement of rapid polychromatic Laue “single-shot” mapping. From single Laue patterns, we extract grain depth information, detect onset of yielding and achieve 2×10−4 lattice strain resolution. The technique is employed to examine yielding and twinning in a magnesium grain embedded ∼200μm below the sample surface. We examine 13 time steps and reveal the following behaviour: initial onset of basal slip, subsequent onset of twinning, development of further accommodation slip and evolution of twin shape and size; along with the corresponding values of local resolved shear stresses.

Interface‐Driven Plasticity: The Presence of an Interface Affected Zone in Metallic Lamellar Composites

Large strain deformation is used to make fine nanolayered two‐phase composites from stacks of conventional polycrystalline sheets. The final materials made by this technique possess a crystallographically highly oriented structure containing nearly atomically perfect interfaces prevailing ubiquitously throughout the material. How this ordered structure evolves with strain from the coarser, more disordered state is not known. Here, using microstructural analysis and computational modeling, we discover a local interface‐affected zone (IAZ) possessing the same crystallographically sharp structure in coarse layered composites as the final nanolayered composites. This means that this strongly oriented interface “zone” survives the mechanical work and overtakes the structure as it refines to the nanoscale. In essence, through the formation of this interface zone, the crossover to a highly oriented nanostructure occurs. Using microstructural analysis and crystal plasticity simulations, we show that the IAZ is a consequence of slip accommodation at the interface. This insight is valuable for developing processing strategies for superior interface‐dominated materials.

Accommodation mechanisms for grain boundary sliding as inferred from texture evolution during superplastic deformation

To gain insight into accommodation mechanisms for local stress concentrations produced by grain boundary sliding (GBS), we systematically examined texture evolution within a superplastic magnesium alloy undergoing deformation at a relatively low deformation temperature (at which basal slip is known to be the preferred slip system in magnesium). Although we did observe an overall weakening of the initial basal texture during superplastic deformation, we also observed within the interior of the specimen a convergent evolution that depends on loading direction. We attribute this texture evolution within the bulk to the competing effects of (a) orientation divergence due to grain rotation accompanied by GBS and (b) convergent evolution due to slip, which acts primarily as an accommodation mechanism for GBS. In contrast, at the near-surface, we found the initial orientation to be preserved, indicating that slip accommodation is less important near the surface than within the bulk.

Cavitation behavior of fine grained AZ31 alloy sheet during hot blow forming

Cavitation behaviors of fine grained AZ31 alloy sheet during high temperature blow forming have been investigated in terms of deformation and microstructure evolution. Especially, effect of hydrostatic stress by applying backward pressure on blow formability has been investigated by examining cavitation behavior. A commercial grade AZ31 magnesium alloy sheet with the grain size of 15μm could have an elongation over 300% over 400°C. However, elongation did not show a strong strain rate and temperature dependencies which could be easily found in most superplastic aluminum alloys. It might result from that AZ31 would be vulnerable to cavitation even at the early stage of deformation. It was also found that hydrostatic stress was not effective to prevent evolution of cavities during blow forming, which was much different with the case in most superplastic alloys. Both low strain rate dependency of elongation and ineffectiveness of hydrostatic stress might be originated from poor accommodation slip for grain boundary sliding of AZ31 even at high temperature where non basal slip systems were activated.

Texture and mechanical properties of superplastically deformed magnesium alloy rod

The texture and tensile properties at room temperature of a fine-grained Mg–Al–Zn alloy rod processed by extrusion were examined before and after superplastic deformation. The tensile properties at room temperature were influenced by the high-temperature deformation conditions. The specimen after being exposed to superplastic deformation under tension tend to show higher ductility and lower strength at room temperature as the strain rate during high-temperature deformation decreased or as deformation temperature increased. The pre-existing basal texture of the rod slightly weakened after superplastic deformation. However, the texture weakening in the rod was not significant compared with that observed in the counterpart alloy in sheet-form probably because the initial distribution of basal planes differs between them. It is suggested that the rotation of crystallographic orientation induced by slip accommodation for grain boundary sliding plays an important role in texture randomization in superplastic magnesium alloys.

Formation of a misorientation band in the elastic field of a disclination dipole

The formation of a misorientation band in the elastic field of a disclination dipole is considered and simulated using a kinetic approach. The dipole is artificially located on a grain boundary or induced by plastic deformation on a double ledge in a grain boundary in a bicrystal. The simulation results agree well with the results obtained earlier in a continuum approximation. The dislocation structure of the misorientation band that forms during accommodation slip is shown to be a dynamic steady-state structure, and its appearance has a kinetic nature.

EBSD Study of Delamination Fracture in Al–Li Alloy 2090

Aluminum–lithium (Al–Li) alloys offer attractive combinations of high strength and low density for aerospace structural applications. However, a tendency for delamination fracture has limited their use. Identification of the metallurgical mechanisms controlling delamination may suggest processing modifications to minimize the occurrence of this mode of fracture. In the current study of Al–Li alloy 2090 plate, high quality electron backscattered diffraction (EBSD) information has been used to evaluate grain boundary types exhibiting delamination fracture and characterize microtexture variations between surrounding grains. Delamination was frequently observed to occur between variants of the brass texture component, along near-Σ3, incoherent twin boundaries. EBSD analyses indicated a tendency for intense deformation along one side of the fractured boundary. A through-thickness plot of grain-specific Taylor factors showed that delaminations occurred along boundaries with the greatest difference in Taylor factors. Together, these suggest a lack of slip accommodation across the boundary, which promotes significantly higher deformation in one grain, and stress concentrations that result in delamination fracture.

Measuring coseismic deformation on the northern segment of the Bam-Baravat escarpment associated with the 2003 Bam (Iran) earthquake, by correlation of very-high-resolution satellite imagery

SUMMARY The role that the oblique reverse fault projecting to the surface at the Bam-Baravat escarpment (BBE) played in the slip accommodation of the 2003 (Mw 6.5) Bam earthquake is still unclear, regardless of many seismological and geodetic studies following this event. In this study, we correlate pre- and post-seismic very high spatial resolution panchromatic satellite images to map coseismic surface deformation along the northern segment of the BBE, a few hundred meters east of the urban area of Bam. Using a new approach based on principal component analysis (PCA) on offset measurements, we obtain 1.8 ± 0.6 cm east slip component and 2–6 ± 0.6 cm south slip component along the fault segment. Our results are consistent with ground observations over the study area and support the idea of the reactivation of the shallow part of this fault segment.

The effect of grain boundary normal on predicting microcrack nucleation using fracture initiation parameters in duplex TiAl

In a prior study of a duplex Ti–48Al–2Cr–2Nb, Simkin et al. [B.A. Simkin, M.A. Crimp, T.R. Bieler, Scripta Mater. 49 (2003) 149], 11 cracked and 11 intact boundaries between γ grains were analyzed to determine why some boundaries that were similarly stressed cracked, and others did not. This led to a geometrically based fracture initiation parameter that considered the tensile stress axis and the deformation systems in both grains. In order to further examine the physical basis of this fracture initiation parameter, we have measured the grain boundary orientation of the previously studied boundaries. In contrast to the expectation that incorporation of the grain boundary orientation into this parameter would improve the ability to discriminate between cracked and intact boundaries, the orientation of the grain boundary normal was found to be statistically less significant than the orientation of the Burgers vectors of the most highly stressed twinning systems, quantified by | b ˆ tw ⋅ t ˆ | , where t ˆ describes the tensile axis direction. Grain boundary microcracking was found to be most likely when b ˆ tw was in the range of 30–50° from the tensile axis. Combining the Schmid factor with | b ˆ tw ⋅ t ˆ | and the effect of ordinary dislocation slip accommodation provides a simple and effective predictor of the propensity for microcrack nucleation.

Continuum mesoscale modelling of nanocrystalline fcc metals under shock-loading using an spectral formulation fed by molecular dynamics results

In this paper we present a micromechanical approach based on Fast Fourier Transforms to study the role played by dislocation glide and grain boundary (GB) accommodation in the determination of the yield strength of nanostructured materials under shock. For this, we construct unit cells representing self-similar polycrystals with different grain sizes in the nanometer range and use local constitutive equations for slip and grain boundary accommodation. We study the effect of grain size and shock pressure on the local and effective behavior of nanostructured fcc materials with parameters obtained from experiments and atomistic simulations. Predictions of a previous pressure-sensitive model for the effective yield strength behind a shock front are improved by considering strain partition between slip and GB activity.

Horizontal coseismic deformation of the 2003 Bam (Iran) earthquake measured from SPOT‐5 THR satellite imagery

On 26 December 2003, a destructive Mw 6.5 earthquake occurred in Kerman province, Iran, killing more than 30,000 people around the ancient city of Bam. We map the fault zone and determine the slip distribution using sub‐pixel correlation of images taken by the SPOT‐5 THR sensor before and after the main shock. Our results show that the surface displacement occurred on a blind fault oriented N174 and located South of Bam. A 15 km along‐strike profile depicts a distribution with a maximum displacement of 1.2 ± 0.15 m near the centre of the fault. The mean slip that we infer (∼0.77 ± 0.05 m) is much higher than the ground displacement measured in the field (∼0.2 m). We suggest that field‐measured displacements are underestimated due to slip accommodation within an 500 m wide shear zone. Furthermore, we demonstrate for the first time the relevancy of 2.5 m SPOT‐5 THR images for sub‐pixel ground displacement measurements.

Slip transfer across grain boundaries in Fe–Si bicrystals

In situ X-ray diffraction topography using synchrotron radiation (SRXRT) observation of deformation of Fe–4 at.% Si bicrystals Σ3, Σ9 and Σ15 was completed by TEM observation of deformed samples. The results were discussed from the viewpoint of the criteria allowing prediction of slip accommodation at the grain boundary. The common slip system in a Σ3 bicrystal is impeded by dissociation of slip dislocations into grain boundary dislocations. In Σ9 bicrystals, the transfer with highest probability appears simultaneously with independent deformation in both grains. In Σ15 bicrystals, no residual dislocations were observed in the boundary. The slip dislocations contribute to creation of dislocation sources in the boundary.

Deformation Mechanism of Fine-Grained Superplasticity in Metallic Materials Expected from the Phenomenological Constitutive Equation

The superplastic behavior of fine-grained metals is well described by the deformation model in which grain boundary sliding (GBS) is accommodated by slip. This slip accommodation process involves the sequential steps of glide and climb, with climb assumed to be the rate-controlling process. The climb distance during GBS is often considered to be on the order of grain size in the conventional theoretical models. However, these models have not been able to predict quantitatively the strain rates actually observed in fine-grained superplastic materials. Therefore, the deformation model was reviewed by comparing the theoretical and phenomenological equations in order to accurately understand the mechanism of the accommodation process. The analyses revealed that the climb process is governed by the effective diffusivity. The climb distance through the grain boundary is of the order of the grain size, and that through the lattice close to the dislocation core size was quantitatively in agreement with the phenomenological relation.