Data from: Micromechanics of compressive and tensile forces in partially-bonded granular materials
Data files
Mar 02, 2026 version files 14.32 GB
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BiAxial_test.zip
14.32 GB
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README.md
4.40 KB
Abstract
Bulk material properties such as strength and stiffness are highly sensitive to changes in the degree of interparticle cohesion or the introduction of elongated particles, effects that are both associated with an increase in the average number of constraints per particle. By performing an ensemble of isotropic compression experiments, all starting from the same initial particle configuration but with varying fraction of bonded particles, we use photoelastic force measurements to identify the causes of this phenomenon at the particle-scale and meso-scale. By introducing a variable percentage of bonded dimers, we observe the expected decrease in the critical packing fraction at which jamming occurs. Above jamming, the local pressure increases predominantly for the bonded particles, measured relative to the unbonded case. The use of bonded dimers allows us to directly measure the relative contributions of tensile and compressive forces at the bond, which we observe occur with approximately equal probability. Histograms of the magnitude of the interparticle forces become broader for systems with more bonded particles. We measure both pressure and coordination number as a function of distance from a bonded dimer, both of which are locally enhanced for nearest neighbors, indicating that dimers appear to act as areas of concentrated force and connectivity that improve rigidity. These datasets contain information about the particle location, neighbors, and the particle-level vector contact forces.
Dataset DOI: [10.5061/dryad.s7h44j1p5]
Description of the data and file structure
This repository contains analyzed data corresponding to the cyclic biaxial tests on cohesive granular ensemble comprising 832 photoelastic particles. The particles are bonded in pairs (dimers) at concentrations of 0, 10, 15, 20, 25 percent.
Files and variables
File: BiAxial_test.zip
Description: This archive contains two types of files: *_preprocessing.mat and *_contacts.mat. These files exist for each dimer concentration: 0, 10, 15, 20, and 25 percent.
_preprocessing.mat: The primary fields in this file contain information about the particle position, radius, force-bearing contacts and bonded particles.
The fields are: [id, x, y, r, rm, color, fsigma, z, f, g2, forces, betas, alphas, neighbors, contactG2s, forceImage, synthImg, contactIs, bond, parBonded/bondedpar, bond_old]
- id: individual id of one particle that should persist from frame to frame
- x: x position of particle, in pixels
- y: y position of particle, in pixels
- r: radius of particle, in pixels
- rm: radius in meters (image resolution: 0.1344 mm per pixel)
- color: color based on the size of the particle. Large particles are labelled 'r', whereas small particles are labelled 'b'
- fsigma: Fringe value of the photo-elastic material; quantifies the relation between the optical response of the material and the applied stress
- z: number of force-bearing contacts a particle has
- f: average pressure on the particle, guessed based on the intensity gradient values of the particle's green channel
- g2: estimate of the gradient of the intensity field across the particle's green channel
- forces: left empty in this file (see _contacts.mat)
- betas: angular location (in radians) of each force vector, defined in a polar coordinate system centred at the particle centroid
- alphas: inclination of each force vector (left empty; see _contacts.mat)
- neighbors: ids of particles that form force-bearing contacts with the current particle id
- contactG2s: estimate of the intensity gradient (green channel) at the region near the force-bearing contact
- forceImage: green channel of the particle
- synthImg: reconstructed green channel of the particle (left empty)
- contactIs: sum of the intensity (green channel) at the region near the contact
- bond: Boolean (true) indicating whether the particle is bonded
- bondedpar or parBonded: id of the particle that is bonded to the current particle id
- bond_old: Boolean indicating whether the particle was bonded in the preceding dimer concentration dataset
Note: Field 'bondedpar' is named 'parBonded' in 10 percent dimer files
_contacts.mat: The primary fields in this file contain the vector contact force information for each force-bearing contact
The fields are [id1, id2, x, y, fAbs, fNorm, fTan, alpha, beta, contactX, contactY, error]
- id1: id of particle in contact with particle id2
- id2: id of particle in contact with particle id1
- x: x position of particle, in pixels
- y: y position of particle, in pixels
- fAbs: absolute value of the force vector, in Newtons
- fNorm: Normal component of the force vector, in Newtons
- fTan: Tangential component of the force vector, in Newtons
- alpha: inclination of each force vector with respect to the radial vector directed toward the contact point
- beta: Angular location (in radians) of each force vector, defined in a polar coordinate system centred at the particle centroid
- contactX: x coordinate of contact on the particle, defined in a Cartesian coordinate system centred at the particle centroid (in pixels)
- contactY: y coordinate of contact on the particle, defined in a Cartesian coordinate system centred at the particle centroid (in pixels)
- error: error value in the optimisation process while matching the experimental image with the reconstructed/pseudo image
Particle positions and vector contact forces were resolved using the Photoelastic Grain Solver analysis code (PeGS). This code is available at https://github.com/photoelasticity
Some other parameters such as betas and alphas that are present in the files are described in Daniels et al. (10.1063/1.4983049) and the wiki: https://photoelasticity.net/
