Plasticity Failure/Degradation in Explicit Dynamics when Reference Strain Rate is set to 0.001

buaajun

I am simulating the impact behavior of high-strength material using Ansys Explicit Dynamics with the Johnson-Cook model.

I have observed a strange numerical phenomenon:

When Reference Strain Rate = 0.001s^-1: The simulation results show purely linear elastic behavior. Even when the stress exceeds 10,000 MPa, the Equivalent Plastic Strain remains 0. This is physically incorrect, as the calculated dynamic yield stress should be well below 2,000 MPa.

When Reference Strain Rate = 0.01s^-1: The simulation works perfectly. The material exhibits clear plastic deformation, and the results align with physical expectations.

To isolate the issue, I conducted a parameter sweep for the Reference Strain Rate ranging from 0.0001 to 1000. A distinct "watershed" phenomenon was observed between 0.001 to 0.01

WHY?

ram27x

This behavior is numerical, not physical. In EM, the Reference Strain Rate acts as a viscoplastic regularization parameter. When it is set too low (≈0.001 s⁻¹ or below), the plastic strain increment per time step becomes smaller than internal solver tolerances, so the plastic return mapping is never activated. This results in purely elastic behavior even at very high stresses.

At ≥0.01 s⁻¹, the plastic increment is large enough to exceed numerical thresholds, and plasticity activates normally. The sharp transition you observed is due to time‑step and rate‑scaling effects.

As a reaccommodation's The Reference Strain Rate should be chosen for numerical robustness (not as a physical rate).

We also recommend posting this observation, along with time‑step details, on the Ansys Community Forum for broader visibility and confirmation from experts and development.

Cheers,

RAMX27