How is volumetric strain defined/used for lower-dimensional (1D) fractures in tightly-coupled THM, and how should it couple to aperture?

[Yulongggggg]

Hi PorousFlow developers,

I’m modeling water injection into a lower-dimensional fracture (1D elements) embedded in a reservoir, using a tightly-coupled THM setup (single app, not MultiApps). I want to implement an aperture evolution model driven by pressure, temperature, and mechanics.

1. My confusion is about how “volumetric strain” is defined for a 1D fracture in this context and how it should couple to aperture/porosity. Is it meaningful for 1d

2. For lower-dimensional (1D) fracture elements that share nodes with the matrix, do they share the same displacement DOFs (single-valued displacement field)? If so, is fracture deformation only tangential (axial strain along the line).

For lower-dimensional fracture flow in PorousFlow, is there an existing or recommended mechanism/material to couple mechanics to aperture (normal opening)