CRM/SPH cone drop with LHS-1: unexpected behavior when adding a cohesion parameter

[JüliMari]

Hello Chrono community,

I am working with the CRM/SPH cone drop demo and adapting it for LHS-1 lunar regolith simulant. My research team would like to include cohesion in the LHS-1 material model, but I am seeing behavior that makes me unsure whether I am applying the cohesion coefficient correctly, or whether there is something in the cone drop setup that affects cohesive cases.

I ran two loose LHS-1 cone drop cases with the same setup (other than render_fps, to capture as many snapshots as possible). The only change between cases was manually editing the LHS-1 material construct from cohesion_coeff = 0 Pa to cohesion_coeff = 299 Pa. With 0 Pa, the loose LHS-1 bed behaves smoothly during the cone drop and as expected following other CRM research papers. With 299 Pa, the upper region of the bed becomes highly disturbed, while the lower material remains comparatively uniform. I attached snapshots showing the difference between the two cases.

The LHS-1 material construct I implemented:

struct lhs1_material {
double density_max = 1740; // kg/m^3, dense LHS-1
double density_min = 1340; // kg/m^3, loose LHS-1
double Young_modulus = 35.9e6; // Pa
double Poisson_ratio = 0.3;
double mu_I0 = 0.08;
double mu_fric_s = 0.61;
double mu_fric_2 = 0.61;
double average_diam = 51e-6; // m
double cohesion_coeff = 0; // changed manually to 299 for cohesive case };

And for MU_OF_I rheology,

mat_props.rheology_model = RheologyCRM::MU_OF_I;
mat_props.mu_I0 = mu_i0;
mat_props.mu_fric_s = mu_s;
mat_props.mu_fric_2 = mu_2;
mat_props.average_diam = lhs1_mat.average_diam;
mat_props.cohesion_coeff = lhs1_mat.cohesion_coeff;

My questions are: is this disturbed free-surface behavior expected when adding cohesion to a loose granular bed, or does it suggest that my setup is inconsistent? Could this be caused by the interaction between cohesion, loose packing density, free-surface handling, and the initial pressure/preconsolidation setup? And is there any hard-coded assumption or scaling in the cone drop (or cone penetration) examples that could make cohesion behave unexpectedly in this case?

I can provide the full code file if you need to see it, though I wanted to check first whether this behavior is expected for CRM/SPH cohesion implementation.

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