# The EEPA Parameter Estimator Model

Based on the model parameters inputted and the particle properties a Bond Number is calculated. The Bond Number (Bo) positively correlates with the cohesiveness of the material interaction, where:

The cohesiveness is stated alongside the Bond Number value.

The Bond Number (Capece, 2015) is derived from the ratio of the cohesive forces, *Fcohesion* to the weight of the particles, *Wg*:

The cohesive force is defined as:

where *γ* is the mean normal overlap, r is the mean particle radius, *Δ* is the surface energy and *fo* is the constant pull-off force. The Bond Number will be calculated live as model parameters are populated as follows:

In addition to the Bond Number, critical Time Step information will be displayed in the Estimated Properties, where:

**Current Interaction Timestep**

Shows the maximum allowable Time Step for the current interaction parameters.

**Recommended Timestep**

Shows the maximum allowable Time Step for all interactions. This is found by finding the smallest Current Critical Time step for each of the interactions.

**Critical Timestep Interaction**

States which interaction has the smallest Time Step, and is therefore the critical interaction.

The Minimum Critical Timestep should be used in the Solver when running the simulation.

Each interaction’s Time Step is found using the following equation (O'Sullivan, 2004):

where *Δt* is the Time Step, m is the mean particle mass and *k**2* is the unloading/reloading stiffness. *k2* is defined as:

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