arXiv subjects

Purusattam Ray

Publications and source records attributed to Purusattam Ray.

29 records · Page 2Linked to original sources

arXiv2006

Gaussian Statistics of Fracture Surfaces

We analyse the statistical distribution function for the height fluctuations of brittle fracture surfaces using extensive experimental data sampled on widely different materials and geometries. We compare a direct measurement of the distribution to a new analysis based on the structure functions. For length scales $δ$ larger than a characteristic scale $δ^*$, we find that the distribution of the height increments $Δh = h(x+ δ) -h(x)$ is Gaussian. Self-affinity enters through the scaling of the standard deviation $σ$, which is proportional to $δ^ζ$ with a unique roughness exponent. Below the scale $δ^*$ we observe an effective multi-affine behavior of the height fluctuations and a deviation from a Gaussian distribution which is related to the discreteness of the measurement or of the material.

cond-mat.mtrl-sci↗

arXiv2005

Breakdown of Heterogeneous Materials

We discuss the threshold activated extremal dynamics that is prevalent in the breakdown processes in heterogeneous materials. We model such systems by an elastic spring network with random breaking thresholds assigned to the springs. Results are obtained from molecular dynamics simulation of the system under constant stress and constant strain conditions. We find that the distribution $P(m)$ of the avalanches of size $m$, caused by the rupturing of the springs till the failure of the network, decays as a power-law: $P(m) \sim m^{-α}$, where $α$ can be closely approximated to 5/2. The average avalanche size $ $ diverges as $ \sim (F_c - F)^{-1/2}$ close to the stress $F_c$ at which the total failure of the network occurs. We study the time evolution of the breakdown process: we find that the bonds rupture randomly over the network at initial times but the rupturing becomes highly correlated at late times to give rise to a well-defined macroscopic crack.

cond-mat.stat-mech↗

arXiv2004

Persistence in extended dynamical systems

Persistence in spatially extended dynamical systems (like coarsening systems and other nonequilibrium systems) is reviewed. We discuss, in particular, the spatial correlations in the persistent regions and their evolution in time in these systems. We discuss the dependence of the persistence behavior on the dynamics of the system and consider the specific example of different updating rules in the temporal evolution of the system. Lastly, we discuss the universal behavior shown by persistence in various stochastic models belonging to the directed percolation universality class.

cond-mat.stat-mech↗

arXiv2004

Response of random field Ising model driven by an external field

We study the dynamics of spin flipping at first order transitions in zero temperature two-dimensional random-field Ising model driven by an external field. We find a critical value of the disorder strength at which a discontinuous sharp jump in magnetization first occurs. We discuss growth morphology of the flipped-spin domains at and away from criticality.

cond-mat.stat-mech↗

arXiv2002

Persistence at the onset of spatiotemporal intermittency in coupled map lattices

We study persistence in coupled circle map lattices at the onset of spatiotemporal intermittency, an onset which marks a continuous transition, in the universality class of directed percolation, to a unique absorbing state. We obtain a local persistence exponent of theta_l = 1.49 +- 0.02 at this transition, a value which closely matches values for theta_l obtained in stochastic models of directed percolation. This result constitutes suggestive evidence for the universality of persistence exponents at the directed percolation transition. Given that many experimental systems are modelled accurately by coupled map lattices, experimental measurements of this persistence exponent may be feasible.

cond-mat.stat-mech↗

arXiv2000

Topological Defects in Size-Dispersed Solids

We study the behavior of the topological defects in the inherent structures of a two-dimensional binary Lennard-Jones system as the size dispersity varies. We find that topological defects arising from the particle size dispersity are responsible for destabilizing the solid as follows: (i) for particle density $ρ\leq 0.9$, the solid melts through intermediate states of decreasing hexatic order arising from the proliferation of unbounded dislocations, (ii) for $ρ> 0.9$, the dislocations form grain boundaries, dividing the system into micro-crystallites and destroying the translational and orientational order.

cond-mat.soft↗

arXiv1999

Avalanches in Breakdown and Fracture Processes

We investigate the breakdown of disordered networks under the action of an increasing external---mechanical or electrical---force. We perform a mean-field analysis and estimate scaling exponents for the approach to the instability. By simulating two-dimensional models of electric breakdown and fracture we observe that the breakdown is preceded by avalanche events. The avalanches can be described by scaling laws, and the estimated values of the exponents are consistent with those found in mean-field theory. The breakdown point is characterized by a discontinuity in the macroscopic properties of the material, such as conductivity or elasticity, indicative of a first order transition. The scaling laws suggest an analogy with the behavior expected in spinodal nucleation.

cond-mat.stat-mech↗

arXiv1997

Dispersity-Driven Melting Transition in Two Dimensional Solids

We perform extensive simulations of $10^4$ Lennard-Jones particles to study the effect of particle size dispersity on the thermodynamic stability of two-dimensional solids. We find a novel phase diagram in the dispersity-density parameter space. We observe that for large values of the density there is a threshold value of the size dispersity above which the solid melts to a liquid along a line of first order phase transitions. For smaller values of density, our results are consistent with the presence of an intermediate hexatic phase. Further, these findings support the possibility of a multicritical point in the dispersity-density parameter space.

cond-mat.mtrl-sci↗

arXiv1997

Effect of Size Dispersity On the Melting Transition

We present a molecular dynamics simulation study of the liquid-solid transition in a two dimensional system consisting of particles of two different sizes interacting via a truncated Lennard-Jones potential. We work with equal number of particles of each kind and the dispersity $Δ$ in the sizes of the particles is varied by changing the ratio of the particle sizes only. For the monodisperse case ($Δ= 0$) and for small values of $Δ$, we find a first order liquid-solid transition on increasing the volume fraction $ρ$ of the particles . As we increase $Δ$, the first-order transition coexistence region weakens gradually and completely disappears at high dispersities around $Δ= 0.10$ . At these values of dispersity the high density phase lacks long range translational order but possesses orientational order with a large but finite correlation length. The consequences of this effect of dispersity on the glass transition and on the melting transition in general are discussed.

cond-mat.mtrl-sci↗

arXiv1996

Scaling for the Coalescence of Microfractures before Breakdown

We study the behavior of fracture in disordered systems close to the breakdown point. We simulate numerically both scalar (resistor network) and vectorial (spring network) models with threshold disorder, driven at constant current and stress rate respectively. We analyze the scaling of the susceptibility and the cluster size close to the breakdown. We observe avalanche behavior and clustering of the cracks. We find that the scaling exponents are consistent with those found close to a mean-field spinodal and present analogies between the coalescence of microfractures and the coalescence of droplets in a metastable magnetic system. Finally, we discuss different experimental conditions and some possible theoretical interpretations of the results.

cond-mat.mtrl-sci↗

arXiv1996

First-Order Transition in the Breakdown of Disordered Media

We study the approach to global breakdown in disordered media driven by increasing external forces. We first analyze the problem by mean-field theory, showing that the failure process can be described as a first-order phase transition, similarly to the case of thermally activated fracture in homogeneous media. Then we quantitatively confirm the predictions of the mean-field theory using numerical simulations of discrete models. Widely distributed avalanches and the corresponding mean-field scaling are explained by the long-range nature of elastic interactions. We discuss the analogy of our results to driven disordered first-order transitions and spinodal nucleation in magnetic systems.

cond-mat.stat-mech↗