arXiv subjects

H. O. Ghaffari

Publications and source records attributed to H. O. Ghaffari.

16 recordsLinked to original sources

arXiv2018

Solitonic State in Microscopic Dynamic Failures

Onset of permanent deformation in crystalline materials under a sharp indenter tip is accompanied by nucleation and propagation of defects. By measuring the spatio-temporal strain field nearthe indenter tip during indentation tests, we demonstrate that the dynamic strain history at the moment of a displacement burst carries characteristics of formation and interaction of local excitations, or solitons. We show that dynamic propagation of multiple solitons is followed by a short time interval where the propagating fronts can accelerate suddenly. As a result of such abrupt local accelerations, duration of the fast-slip phase of a failure event is shortened. Our results show that formation and annihilation of solitons mediate the microscopic fast weakening phase, during which extreme acceleration and collision of solitons lead to non-Newtonian behavior and Lorentz contraction, i.e., shortening of solitons characteristic length. The results open new horizons for understanding dynamic material response during failure and, more generally, complexity of earthquake sources.

physics.geo-ph↗

arXiv2016

Microscopic Observation of the Light-Cone-Like Thermal Correlations in Cracking Excitations

Many seemingly intractable systems can be reduced to a system of interacting spins. Here, we introduce a system of artificial acoustic spins which are manipulated with ultrasound excitations associated with micro-cracking sources in thin sheets of crystals. Our spin-like system shows a peculiar relaxation mechanism after inducing an impulsive stress-ramp akin to splitting, or rupturing, of the system. Using real-time construction of correlations between spins states, we observe a clear emergence of the light cone effect. It has been proposed that equilibration horizon occurs on a local scale in systems where correlations between distant sites are established at a finite speed. The observed equilibration horizon in our observations defines a region where elements of the material are in elastic communication through excited elementary excitations. These results yield important insights into dynamic communication between failing elements in brittle materials during processes such as brittle fragmentation and dynamic stress triggering of earthquake-generating faults.

cond-mat.mtrl-sci↗

arXiv2016

Dynamic Evolution of Microscopic Wet Cracking Noises

Characterizing the interaction between water and microscopic defects is one of the long-standing challenges in understanding a broad range of cracking processes. Different physical aspects of microscopic events, driven or influenced by water, have been extensively discussed in atomistic calculations but have not been accessible in microscale experiments. Through the analysis of the emitted noises during the evolution of individual, dynamic microcracking events, we show that the onset of a secondary instability known as hybrid events occurs during the fast healing phase of microcracking, which leads to (local) sudden increase of pore water pressure in the process zone, inducing a secondary instability, which is followed by a fast-locking phase on the microscopic faults (pulse-like rupture).

physics.geo-ph↗

arXiv2015

Observation of the Kibble-Zurek Mechanism in Microscopic Acoustic Cracking Noises

The fast evolution of microstructure is key to understanding crackling phenomena. It has been proposed that formation of a nonlinear zone around a moving crack tip controls the crack tip velocity. Progress in understanding the physics of this critical zone has been limited due to the lack of hard data describing the detailed complex physical processes that occur within. For the first time, we show that the signature of the non-linear elastic zone around a microscopic dynamic crack maps directly to generic phases of acoustic noises, supporting the formation of a strongly weak zone near the moving crack tips. We additionally show that the rate of traversing to non-linear zone controls the rate of weakening, i.e. speed of global rupture propagation. We measure the power-law dependence of nonlinear zone size on the traversing rate, and show that our observations are in agreement with the Kibble-Zurek mechanism (KZM) .

cs.CE↗

arXiv2014

Faulting of rocks at three-dimensional stress field by micro-anticracks

Using 9-sets of different laboratory earthquake tests, we examined the nature of cracking under true triaxial stress conditions in the lithosphere . We found that 3D stress state can induce oblique nucleation of many fractures, forming final plane of complex polymodal faults. Our fully 3D experiments indicate unconventional fault nucleation with 2-3 times faster slip phase, implying a new slip-weakening mechanism for earthquakes in upper crust. In addition, We compared our observations of irregular cracks with typical anti-cracks signals in multi-anvil High Pressure and Temperature test. For the first time, we showed oblique faulting can change slip-weakening rate and accelerate the rate of energy release. This implicates a sharper source time function for further modeling of our experiments. Indeed, our results can be assumed as an ex tension to detachment fronts in micro-faults to a general concept of quasi-anti rupture fronts. We showed events from deep-focus earthquakes can share some similarity to shallow earthquakes, promoting recent approach on similarity of deep earthquakes to their shallow counterparts .

physics.geo-ph↗

arXiv2014

Functional-Friction Networks: New Insights on the Laboratory Earthquakes

We report some new applications of functional complex networks on acoustic emission waveforms from frictional interfaces. Our results show that laboratory faults undergo a sequence of generic phases as well as strengthening, weakening or fast-slip and slow-slip leading to healing. Also, using functional networks, we extend the dissipated energy due to acoustic emission signals in terms of short-term and long-term features of events. We show that the transition from regular to slow ruptures can have an additional production from the critical rupture class similar to the direct observations of this phenomenon in the transparent samples. Furthermore, we demonstrate detailed sub-micron evolution of the interface due to the short-term evolution of rupture tip, which is represented by phenomenological description of the modularity rates. In addition, we found nucleation phase of each single event for most amplified events follows a nearly constant time scale, corresponding to initial strengthening of interfaces.

physics.geo-ph↗

arXiv2013

Acoustic-Friction Networks and the Evolution of Precursory Rupture Fronts in Laboratory Earthquakes

We show that the mesoscopic and transport characteristics of networks follow the same trends for the same type of the shear ruptures in terms of rupture speed while also comparing the results of three different friction experiments.The classified fronts obtained from a saw cut Westerly granite fault regarding friction network parameters show a clear separation into two groups indicating two different rupture fronts. With respect to the scaling of local ruptures durations with the networks parameters we show that the gap is related to the possibility of a separation between slow and regular fronts.

physics.geo-ph↗

arXiv2012

On Aperture-Friction Networks

A model based on self-organizing nature of the surfaces is represented to capture the evolution of friction networks. Also, the curvatures of displacement fields are determined and critical curvature profiles are connected through links to form ridge-networks. To generate the ridge-networks, we consider the interactions of pairs though a directed network framework, namely, dilatancy (divergence) and deviotric components (vorticity) of local maxima of curvature profiles. The correlation of the characteristics of generated networks with synthetic acoustic signatures of frictional interface is postulated while the interactions of critical pairs before and after frictional sliding highlighted.

physics.geo-ph↗

arXiv2012

Fluid Flow Complexity in Fracture Networks: Analysis with Graph Theory and LBM

Through this research, embedded synthetic fracture networks in rock masses are studied. To analysis the fluid flow complexity in fracture networks with respect to the variation of connectivity patterns, two different approaches are employed, namely, the Lattice Boltzmann method and graph theory. The Lattice Boltzmann method is used to show the sensitivity of the permeability and fluid velocity distribution to synthetic fracture networks' connectivity patterns. Furthermore, the fracture networks are mapped into the graphs, and the characteristics of these graphs are compared to the main spatial fracture networks. Among different characteristics of networks, we distinguish the modularity of networks and sub-graphs distributions. We map the flow regimes into the proper regions of the network's modularity space. Also, for each type of fluid regime, corresponding motifs shapes are scaled. Implemented power law distributions of fracture length in spatial fracture networks yielded the same node's degree distribution in transformed networks. Two general spatial networks are considered: random networks and networks with "hubness" properties mimicking a spatial damage zone (both with power law distribution of fracture length). In the first case, the fractures are embedded in uniformly distributed fracture sets; the second case covers spatial fracture zones. We prove numerically that the abnormal change (transition) in permeability is controlled by the hub growth rate. Also, comparing LBM results with the characteristic mean length of transformed networks' links shows a reverse relationship between the aforementioned parameters. In addition, the abnormalities in advection through nodes are presented.

cs.CE↗

arXiv2012

Network Theory, Cracking and Frictional Sliding

We have developed different network approaches to complex patterns of frictional interfaces (contact areas developments). Here, we analyze the dynamics of static friction. We found, under the correlation measure, the fraction of triangles correlates with the detachment fronts. Also, for all types of the loops (such as triangles), there is a universal power law between nodes' degree and motifs where motifs frequency follow a power law. This shows high energy localization is characterized by fast variation of the loops fraction. Also, this proves that the congestion of loops occurs around hubs. Furthermore, the motif distributions and modularity space of networks -in terms of within-module degree and participation coefficient- show universal trends, indicating an in common aspect of energy flow in shear ruptures. Moreover, we confirmed that slow ruptures generally hold small localization, while regular ruptures carry a high level of energy localization. We proposed that assortativity, as an index to correlation of node's degree, can uncover acoustic features of the interfaces. We showed that increasing assortativity induces a nearly silent period of fault's activities. Also, we proposed that slow ruptures resulted from within-module developments rather than extra-modules of the networks. Our approach presents a completely new perspective of the evolution of shear ruptures.

physics.geo-ph↗

arXiv2012

Network-Configurations of Dynamic Friction Patterns

The complex configurations of dynamic friction patterns-regarding real time contact areas- are transformed into appropriate networks. With this transformation of a system to network space, many properties can be inferred about the structure and dynamics of the system. Here, we analyze the dynamics of static friction, i.e. nucleation processes, with respect to "friction networks". We show that networks can successfully capture the crack-like shear ruptures and possible corresponding acoustic features. We found that the fraction of triangles remarkably scales with the detachment fronts. There is a universal power law between nodes' degree and motifs frequency (for triangles, it reads T(k)\proptokβ (β \approx2\pm0.4)). We confirmed the obtained universality in aperture-based friction networks. Based on the achieved results, we extracted a possible friction law in terms of network parameters and compared it with the rate and state friction laws. In particular, the evolutions of loops are scaled with power law, indicating the aggregation of cycles around hub nodes. Also, the transition to slow rupture is scaled with the fast variation of local heterogeneity. Furthermore, the motif distributions and modularity space of networks -in terms of withinmodule degree and participation coefficient-show non-uniform general trends, indicating a universal aspect of energy flow in shear ruptures.

physics.geo-ph↗

arXiv2012

Topological Complexity of Frictional Interfaces: Friction Networks

Through research conducted in this study, a network approach to the correlation patterns of void spaces in rough fractures (crack type II) was developed. We characterized friction networks with several networks characteristics. The correlation among network properties with the fracture permeability is the result of friction networks. The revealed hubs in the complex aperture networks confirmed the importance of highly correlated groups to conduct the highlighted features of the dynamical aperture field. We found that there is a universal power law between the nodes' degree and motifs frequency (for triangles it reads T(k)\proptokβ (β \approx2\pm0.3)). The investigation of localization effects on eigenvectors shows a remarkable difference in parallel and perpendicular aperture patches. Furthermore, we estimate the rate of stored energy in asperities so that we found that the rate of radiated energy is higher in parallel friction networks than it is in transverse directions. The final part of our research highlights 4 point sub-graph distribution and its correlation with fluid flow. For shear rupture, we observed a similar trend in sub-graph distribution, resulting from parallel and transversal aperture profiles (a superfamily phenomenon).

physics.gen-ph↗

arXiv2011

Motifs of Networks from Frictional Interfaces

We have developed different network approaches to analyze complex patterns of frictional interfaces (contact area developments). Network theory is a fundamental tool for the modern understanding of complex systems in which, by a simple graph representation, the elementary units of a system become nodes, and their mutual interactions become links. With this transformation of a system into a network space, many properties of the system's structure and dynamics can be inferred. The rupture sequence of shear fractures were studied using a transformation form of contact patterns to complex networks; subsequently, sub-graph abundance within the corresponding networks was analyzed. To distinguish the different roles of collective deformation of an interface's elements, pure and non-pure contact patches (i.e., aperture) were mapped onto the nodes. The contact patches were connected with each other by using measurements of similarities as well as constrained geometrical distance and amount of net-contact area per patch, which yielded directed and non-directed networks. A universal trend in sub-graph distribution was observed. We confirmed that super-family phenomena are independent from rupture types in shear processes (as well as in slow or sub-Rayleigh fronts). Furthermore, global features of frictional interfaces as well as shear strength or hydraulic properties were scaled with motifs evolution. In particular, it was found that more common transitive motifs indicate residual shear strength stages, where fluctuations of stored potential energy surrounding rupture tip were minimal. Our approaches were tested over different available data sets, and it was found that discrete as well as real-time contact measurements resulted in the same universal patterns of sub-graphs

physics.gen-ph↗

arXiv2010

Complex Aperture Networks

A complex network approach is proposed to study the shear behavior of a rough rock joint. Similarities between aperture profiles are established and a general network in two directions (in parallel and perpendicular to the shear direction) is constructed. Evaluation of this newly formed network shows that the degree distribution of the network, after a transition stage falls into a quasi stable state which is roughly obeying a Gaussian distribution. In addition, the growth of the clustering coefficient and the number of edges are approximately scaled with the development of shear strength and hydraulic conductivity, which can be utilized to estimate, shear distribution over asperities. Furthermore, we characterize the contact profiles using the same approach. Despite the former case, the later networks are following a growing network mode.

physics.geo-ph↗

arXiv2009

Knowledge Discovery of Hydrocyclone s Circuit Based on SONFIS and SORST

This study describes application of some approximate reasoning methods to analysis of hydrocyclone performance. In this manner, using a combining of Self Organizing Map (SOM), Neuro-Fuzzy Inference System (NFIS)-SONFIS- and Rough Set Theory (RST)-SORST-crisp and fuzzy granules are obtained. Balancing of crisp granules and non-crisp granules can be implemented in close-open iteration. Using different criteria and based on granulation level balance point (interval) or a pseudo-balance point is estimated. Validation of the proposed methods, on the data set of the hydrocyclone is rendered.

cs.AI↗

arXiv2009

Complex Networks on a Rock Joint

A complex network approach on a rough fracture is developed. In this manner, some hidden metric spaces (similarity measurements) between apertures profiles are set up and a general evolutionary network in two directions (in parallel and perpendicular to the shear direction) is constructed. Also, an algorithm (COmplex Networks on Apertures: CONA) is proposed in which evolving of a network is accomplished using preferential detachments and attachments of edges (based on a competition and game manner) while the number of nodes is fixed. Also, evolving of clustering coefficients and number of edges display similar patterns as well as are appeared in shear stress, hydraulic conductivity and dilation changes, which can be engaged to estimate shear strength distribution of asperities.

nlin.AO↗