LS-DYNA Metal Forming Simulation: Stamping, Forging, and Springback Analysis

LS-DYNA Metal Forming Simulation: Stamping, Forging, and Springback Analysis

Metal forming simulation saved my neck on a project once — we had a bracket that kept tearing during stamping, and the die shop was getting frustrated with trial and error. I ran the simulation in LS-DYNA, found the problem was insufficient binder force in one area, and the next die tryout worked. That's when I became a believer. Let me walk you through how I set up stamping simulations and predict springback.

Forming Process Overview

Sheet Metal Stamping

1. Tools: Die (lower), Punch (upper), Binder/Blankholder (holds sheet edge)
2. Process:
   • Blank placed on die
   • Binder closes and applies holding force
   • Punch descends, forming sheet into die cavity
3. Challenges:
   • Wrinkling: Excess material compresses (insufficient binder force)
   • Tearing: Material stretches beyond forming limit (excessive binder force or deep draw)
   • Springback: Part elastically recovers after tool release
   • Thinning: Material thins at high-strain regions

Model Setup

Blank (Sheet Metal)

1. *PART: Blank
2. Elements: *ELEMENT_SHELL (Belytschko-Tsay, ELFORM 2)
3. Material: *MAT_PIECEWISE_LINEAR_PLASTICITY (MAT_024)
   • RO: Density (7.85×10⁻⁶ kg/mm³ for steel)
   • E: Young's modulus (210 GPa)
   • PR: Poisson's ratio (0.3)
   • SIGY: Yield stress (e.g., 180 MPa for DC04 steel)
   • ET: Tangent modulus (e.g., 800 MPa)
   • LCSS: Load curve for true stress-strain
4. Thickness: 0.8-2.5mm (typical auto body)
5. Integration points: 5 (through thickness, for accurate bending)

Stress-Strain Curve

1. *DEFINE_CURVE:
   • True stress vs. true plastic strain
   • Example (DC04 steel):

| Plastic Strain | True Stress (MPa) | |----------------|-------------------| | 0.000 | 180 | | 0.020 | 320 | | 0.050 | 380 | | 0.100 | 430 | | 0.200 | 480 | | 0.400 | 540 | | 0.600 | 580 |

2. Must use true stress and true strain (not engineering)
3. Convert: σtrue = σeng(1+εeng), εtrue = ln(1+εeng)

Tools (Die, Punch, Binder)

1. *PART: Die, Punch, Binder
2. Elements: *ELEMENT_SHELL (rigid)
3. Material: *MAT_RIGID (MAT_020)
   • Tools are rigid (no deformation)
   • Faster computation
4. Tool geometry:
   • Die: Lower tool with cavity
   • Punch: Upper tool that forms the sheet
   • Binder: Ring that holds blank edge
5. Tool mesh:
   • Fine enough to capture tool geometry
   • Coarser than blank mesh (rigid, no accuracy concern)

Contact

1. *CONTACT_FORMING_SURFACE_TO_SURFACE:
   • Blank to die: Contact between sheet and die
   • Blank to punch: Contact between sheet and punch
   • Blank to binder: Contact between sheet and binder
2. Friction:
   • FS (static): 0.12 (typical for steel-steel with lubrication)
   • FD (dynamic): 0.10
3. Contact formulation:
   • Constraint method: For forming (more accurate than penalty)
   • Or Soft penalty: For complex geometry

Adaptive Meshing

Why Adaptive Meshing?

1. During forming, blank deforms significantly
2. Initial mesh may be too coarse in high-strain regions
3. Adaptive meshing refines where needed:
   • Split elements: Divide one element into 4 (2D) or 8 (3D)
   • Based on angle: Refine when element angle exceeds threshold

Setup

1. *CONTROL_ADAPTIVE:
   • ADPTIME: Time interval for adaptation (e.g., 0.001s)
   • ADPOPT: Adaptivity option (1 = angle-based)
   • ADPASS: Maximum number of adaptation passes (3-5 typical)
   • ADPEN: Angle threshold for refinement (7-10 degrees)
2. *PART:
   • ADPOPT: 1 (enable adaptivity for blank part)
3. Results:
   • Mesh refines in high-strain regions (corners, draw beads)
   • Mesh stays coarse in low-strain regions (flat areas)
   • Total element count increases (check memory)

Forming Simulation

Step 1: Gravity Loading

1. Apply gravity to blank (settles onto die)
2. *LOAD_BODY_Y (gravity in -Y direction)
3. Duration: 0.01s (short, just to settle)
4. Damping: *DAMPING_GLOBAL (to prevent oscillation)

Step 2: Binder Close

1. Move binder down to close on blank
2. *BOUNDARY_PRESCRIBED_MOTION (binder, Y-direction, velocity)
3. Binder velocity: 1-5 m/s (typical)
4. Binder force: 100-500 kN (typical for auto body panels)
5. Duration: 0.05-0.1s

Step 3: Punch Stroke

1. Move punch down to form blank
2. *BOUNDARY_PRESCRIBED_MOTION (punch, Y-direction, velocity)
3. Punch velocity: 1-10 m/s (scaled for simulation speed)
4. Stroke depth: Per part geometry (e.g., 100mm for deep draw)
5. Duration: 0.05-0.2s
6. Check:
   • Wrinkling: Look for out-of-plane waves
   • Tearing: Check for element failure (if erosion enabled)
   • Thinning: Check thickness reduction

Step 4: Springback

1. Remove tools (release blank)
2. Switch to implicit solver for springback:
   • *CONTROL_IMPLICIT_GENERAL
   • IMFLAG: 1 (enable implicit)
   • DT0: Initial time step (0.01s)
   • TTF: Termination time (0.1s)
3. Or use dynamic relaxation:
   • *CONTROL_DYNAMIC_RELAXATION
   • Damps out vibrations to find static equilibrium
4. Springback results:
   • Displacement: Difference between formed shape and final shape
   • Typical: 1-5mm (auto body panels)
   • Compensation: Adjust tool geometry to compensate for springback

Forming Limit Diagram (FLD)

What is FLD?

1. FLD shows whether a forming process is safe or will fail
2. Plot: Major strain (ε1) vs. minor strain (ε2)
3. Regions:
   • Safe zone: Below FLC (Forming Limit Curve)
   • Marginal zone: Between FLC and 10% below
   • Failure zone: Above FLC (tearing expected)
   • Wrinkle zone: Negative major strain with positive minor strain

FLD in LS-DYNA

1. *MAT_FORMING_LIMIT_DIAGRAM (add to material):
   • LCID: Load curve for FLC (forming limit curve)
2. FLC (Keeler-Brazier approximation):
   • FLC = (23.3 + 14.13 × t) × n / 0.21 (for steel)
   • t: Sheet thickness (mm)
   • n: Strain hardening exponent
3. Post-processing:
   • LS-PrePost > FLD > Plot
   • Each element plotted as point (ε1, ε2)
   • Color: Green (safe), yellow (marginal), red (failure)

FLD Evaluation

1. Safe (green): All elements below FLC → part can be formed
2. Marginal (yellow): Some elements near FLC → risk of tearing
3. Failure (red): Elements above FLC → part will tear → redesign needed
4. Fix for failure:
   • Increase binder force: Reduces material flow (less thinning)
   • Decrease binder force: Allows more material flow (less stretching)
   • Add draw bead: Controls material flow locally
   • Modify tool geometry: Increase radii, reduce depth
   • Change material: Higher formability grade

Thinning Analysis

1. Check thickness after forming:
   • LS-PrePost > Fcomp > Thickness
   • Color contour of thickness distribution
2. Thinning limits:
   • Safe: Thinning < 20% (e.g., 1.0mm → 0.8mm)
   • Marginal: 20-25% thinning
   • Failure: > 25% thinning (risk of splitting)
3. High-thinning regions:
   • Corners and radii (stretch bending)
   • Deep draw areas (punch nose)
   • Wall corners (side wall curl)

Springback Compensation

Springback Prediction

1. After forming and tool release:
   • Part elastically recovers (springback)
   • Typical: 1-5mm for auto body panels
   • More for high-strength steel (HSS) and advanced HSS
2. Measure springback:
   • Compare formed shape to final shape
   • Plot: Displacement vector from formed to final
   • Check: At critical dimensions (trim, mating surfaces)

Compensation Method

1. Iterative compensation:
   • Step 1: Simulate forming → get springback
   • Step 2: Modify tool geometry (add springback in opposite direction)
   • Step 3: Re-simulate → check if springback is reduced
   • Step 4: Iterate until part is within tolerance
2. LS-DYNA tool: *CONTROL_SPRINGBACK_COMPENSATION
   • Automatically adjusts tool geometry
   • Iterative process (3-5 iterations typical)

Process Parameters

Binder Force

1. Too low: Wrinkling (material flows freely)
2. Too high: Tearing (material can't flow, stretches)
3. Optimal: Just enough to prevent wrinkling without tearing
4. Typical: 100-500 kN for auto body panels
5. Optimization: Run multiple simulations with different binder forces

Friction

1. Lubricated: μ = 0.10-0.15 (with drawing oil)
2. Dry: μ = 0.20-0.30 (no lubrication)
3. Higher friction → more stretching → more thinning
4. Lower friction → more sliding → more material flow
5. *CONTACT: FS (static friction), FD (dynamic friction)

Punch Speed

1. Physical speed: 0.1-0.5 m/s (actual press speed)
2. Simulation speed: 1-10 m/s (scaled for efficiency)
3. Check: Kinetic energy < 5% of internal energy (quasi-static)
4. If KE is significant: Reduce punch speed or add mass scaling

Verification Checklist

• [ ] Material stress-strain curve uses true stress and true strain
• [ ] Sheet thickness and integration points are correct (5 recommended)
• [ ] Tools are rigid with correct geometry
• [ ] Contact includes all tool-blank interfaces
• [ ] Friction coefficients match lubrication condition
• [ ] Adaptive meshing is enabled (for blank)
• [ ] Gravity loading is applied before forming
• [ ] Binder force is appropriate (no wrinkling or tearing)
• [ ] FLD shows all elements in safe or marginal zone
• [ ] Thinning is within 20% limit
• [ ] Springback is predicted and compensated
• [ ] Energy balance: KE < 5% of IE (quasi-static)

Wrapping Up

The two things I always check after a forming simulation: the FLD (forming limit diagram) and the thinning distribution. If any elements are in the red zone on the FLD, the part will tear — you need to adjust binder force, add draw beads, or change the material grade. For springback, I always run the implicit solver after the explicit forming step — it's much more stable than dynamic relaxation. And if you're working with high-strength steel, expect more springback than you think. I've seen parts spring back 5mm more than predicted because the material model didn't capture the Bauschinger effect properly.