UMAT for Abaqus written by Nicolò Grilli and Ed Tarleton based on UEL by Fionn Dunne.

Grilli, N., Tarleton, E. & Cocks, A.C.F. Coupling a discrete twin model with cohesive elements to understand twin-induced fracture. Int J Fract 227, 173–192 (2021). https://doi.org/10.1007/s10704-020-00504-9

define a user material in Abaqus with 125 state dependent variables (SDV) 11 material constants

The first constant indicates the crystal type:

0 = HCP 1 = BCC 2 = BCC 3 = Carbide 4 = Olivine 5 = Orthorombic

The constants 2-10 contains the components of the rotation matrix that transforms a vector from the crystal reference frame to the sample (Abaqus) reference frame The order of the components in the input file must be R11, R12, R13, R21, R22, R23, R31, R32, R33

The 11th constant is the grain index different for different grains Therefore a polycrystal should be made with different materials in abaqus

The parameters that must be set are in the variable declaration part of the following files: umat.for kmat.f kMaterialParam.f

The total number of bulk elements and cohesive interface elements must be set in: mycommon.f

If the twins are activates, it is necessary to set the number of neighbouring points (NUpDown) in mycommon.f and then set ArrayNUpDown as the product: NUpDown * nElements * nintpts A preliminary simulation must be run, if the number of neighbouring points is not high enough for the specific mesh, warning will be given in the .log file The last warning line will contain the number that must be assigned to NUpDown

IMPORTANT: when using discrete twin model be sure no more such warnings are present in the .log for reliable simulation results

kMaterialParam.f includes the elastic and plastic parameters for different materials. If a new material is needed, new parameters and material name must be introduced in this file

kRhoTwinInit.f is preset, but can be modified if it is necessary to introduce a specific initial value (for instance space dependent) of the dislocation density or twin phase field