Choosing Explicit vs. Implicit Solvers in Ansys Mechanical for Vehicle Crash Simulations

Decision: Which Solver for High‑Deformation Crash Tests?

In transient structural analyses that involve large deformations, high strain rates, and complex contact, the choice between an explicit or an implicit solver can dramatically affect accuracy, convergence, and computational cost. The decision hinges on the following constraints:

Key Constraints

• Large deformations (e.g., >10% strain) that produce strong geometric nonlinearity.
• High strain‑rate loading (e.g., impact or crash) where wave propagation dominates.
• Highly nonlinear contact interactions (e.g., friction, separation, interpenetration).
• Material models with strain‑rate dependence or damage evolution.
• Available computational resources (CPU, memory, wall‑clock time).

Supported Solver Options

Feature	Explicit Solver	Implicit Solver
Time‑step control	Fixed, derived from element size and wave speed (Δt = 0.5·h/c)	Adaptive, governed by convergence of nonlinear equations
Convergence behavior	No nonlinear iteration; energy‑based stability only	Newton–Raphson iterations; may need relaxation or damping
Memory footprint	High (stores velocity, acceleration, and mass matrix)	Lower (stores only current state and Jacobian)
CPU time per step	Fast, but many steps required	Slow per step, but fewer steps if convergence is achieved
Typical use cases	Crashes, impacts, ballistic, high‑strain‑rate tests	Quasi‑static, moderate deformation, material testing
Failure modes	Time‑step too large → numerical instability; memory exhaustion	Nonlinear convergence failure; step rejections

Trade‑Off Analysis

• Explicit guarantees stability for highly nonlinear events but can become prohibitively slow if the model is large or the time step is extremely small. It requires generous RAM and careful partitioning.
• Implicit offers larger stable time steps and can be efficient for moderate deformations, but convergence can stall when contact or material nonlinearity is severe. Damping, relaxation, or a reduced time step may be necessary.
• Both solvers demand accurate contact definitions. Wrong friction coefficients or contact activation can dominate the result, leading to energy loss or unrealistic penetration.

Implementation in Ansys Mechanical

Step 1: Set Solver Type

// In the Mechanical tree, right‑click “Analysis Settings” and choose “Solver Type”.
// Select “Explicit” or “Implicit” accordingly.

Step 2: Define Material with Strain‑Rate Dependence

// Example: Define a Johnson‑Cook material
Material: Steel_JC
  Elastic Modulus: 210 GPa
  Poisson Ratio: 0.3
  Strength (σ0): 250 MPa
  Strain‑Rate Coefficient (C): 0.02
  Hardening Exponent (n): 0.15
  Thermal Softening (m): 0.02

Step 3: Configure Time Step

• Explicit: Δt = 0.5 * (h_min / c) where h_min is the smallest element length and c is the longitudinal wave speed (c = sqrt(E/ρ)). Use the “Explicit Time Step” option in the solver settings.
• Implicit: Enable automatic time step control. Optionally set a maximum time step (e.g., Max Time Step = 0.1 s) to avoid overly large steps in highly nonlinear regions.

Step 4: Define Contact

// Contact pair: Vehicle Body vs. Road Surface
Contact Pair: Body_Road
  Contact Type: Surface‑to‑Surface
  Friction Coefficient: 0.5
  Contact Method: Penalty (Explicit) or Augmented Lagrangian (Implicit)
  Enable Contact: Yes

Step 5: Run the Simulation

Execute the solve command from the command line or the GUI. Monitor the solver log for stability warnings (explicit) or convergence failures (implicit).

Validation Workflow

1. Create a Benchmark Plate‑Impact Model – a 2‑D rectangular plate struck by a rigid impactor. Use the same geometry, material, and initial conditions for both solvers.
2. Extract Displacement vs. Time – export the node displacement at the plate center to a CSV file from both runs.
3. Overlay Curves – in a spreadsheet or plotting tool, plot the two displacement histories. For a well‑behaved explicit solution, the curves should agree within 5% for the same time step.
4. Check Solver Logs – explicit logs should show a constant Δt; implicit logs should show the number of iterations per step and any step rejections. If the implicit solver fails to converge, reduce the maximum time step or enable damping.
5. Perform Time‑Step Sensitivity – halve the explicit time step and confirm that the peak displacement and velocity converge (difference <1%). This demonstrates that the explicit solution is not dominated by numerical noise.

Limitations

• Explicit solver memory usage scales with the number of nodes; large vehicle models may require >32 GB RAM or partitioning into sub‑domains.
• Implicit solver may still diverge if contact is highly nonlinear; consider using a contact pre‑processor or adjusting the penalty stiffness.
• Both solvers are sensitive to mesh quality; poorly shaped elements can degrade wave speed calculations and convergence.

By following this decision guide, you can confidently select the solver that balances accuracy, convergence, and computational cost for your vehicle crash simulation.