1. I guess you have already read ATENA Troubleshooting, 188.8.131.52 The influence of the volume change during crushing (Beta) and ATENA Theory, 2.4 Drucker-Prager Plasticity Model?
2. I am not sure which variable or formula you denote as "c" (there is no such symbol in the referred equations)?
3. I assume you have already checked the most common problem sources following Troubleshooting, 2.4.6 Floating point problems - Division by zero, Loss of >7 digits on the matrix diagonal, CCFEModelExc: CCPardiso solver error: zero pivot, Warning: Sign of diagonal changed when reducing equation, Floating point exception: Multiple floating points traps, Zero or negative jacobian, InvertA: zero determinant, CCMaterials Extended AExc: CC5ParamYield F: project_stress_on_f_division by zero, and similar?
Please also see Troubleshooting, 2.4.2 I get the message "The execution is killed due to violation of stop iteration criteria", what does it mean? and 2.1.19 Problems reaching convergence and understanding ATENA convergence parameters.
If you still have problems, you can follow Troubleshooting, 2.1.1 to send us your model etc.