Under longitudinal compression and shear, matrix damage due to shear may not occur

The model is implement so that only one damage mode (fiber tension, fiber compression, or matrix damage) is activate at each integration point. Under combined stress cases, the first failure criteria to be satisfied determine which damage mode is activated.

When the following conditions are satisfied, matrix damage will not occur when it would be expected:

• matrix damage and fiber kinking theory are enabled
• a small amount of longitudinal compressive strain is introduced (>1e-6)
• the element size is larger than the width of the kink band so that the fiber kinking decomposition is required

A work around for this issue in structural models is to use the alternating section definition method (Hyder et al, 2018) with the strips of elements designated for matrix cracking having the fiber kinking model disabled.

Fundamentally, a better understanding of the material behavior under combined longitudinal compression and shear is needed.

The issue names no source files, tests, or entry points. Start by reproducing the combined longitudinal compression and shear conditions with matrix damage and fiber kinking enabled, then inspect how the first satisfied failure criterion selects the damage mode. Done means establishing and implementing a justified behavior for matrix damage under these combined conditions, with verification against the reported case.