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name: "curing-contact" description: "Define contact pairs and friction for composite curing simulation: mold-composite contact (S1↔S4+S6), temperature-dependent friction (0.45→0.2→0.169), HARD contact property. Invoke when setting up contact, friction, or mold interaction in curing models."

Curing Contact Setup

Define the frictional contact between the composite (P8-1) and the mold (TOOL-1). This is the physical mechanism that constrains the composite during curing and creates asymmetric springback upon demolding.

When to Invoke

• User asks to set up contact between composite and mold
• User mentions "接触", "摩擦", "模具接触", "contact pair"
• User needs to change friction coefficients
• User encounters contact-related convergence issues

Contact Pair Definition

*Surface, type=ELEMENT, name=com-surface
P8-1.S1                      # Composite outer surface (contact with mold)

*Surface, type=ELEMENT, name=tool-surface
TOOL-1.S4, S6                # Mold contact surfaces

*Contact Pair, interaction=INTER-1, type=SURFACE TO SURFACE
com-surface, tool-surface    # Master: tool, Slave: composite

Surface assignments:

Surface	Part	Element Face	Role
com-surface	P8-1 (composite)	S1 (outer)	Slave surface
tool-surface	TOOL-1 (mold)	S4 + S6	Master surface

Why S1 on composite?

• S1 is the element face on the outer surface of the composite
• This is the face that physically touches the mold
• S2 is the inner surface (where pressure is applied)

Why S4+S6 on TOOL?

• The mold is an L-shaped bracket with two contact faces
• S4 and S6 correspond to the two inner faces of the mold L-shape
• Both faces must be included to capture full contact

Contact Interaction Property

*Surface Interaction, name=INTER-1
*Friction
0.45                         # Step vis: viscous state friction
*Surface Behavior, pressure-overclosure=HARD

Temperature-Dependent Friction

Friction changes with material state (temperature-dependent). Each step has a different friction coefficient:

Step	Temperature Range	Material State	Friction Coefficient
vis	25°C → 150°C	Viscous (rubbery)	0.45
rub	150°C → 180°C	Rubbery	0.20
glassy	180°C → 25°C	Glassy (cured)	0.169
sp	25°C	—	N/A (contact removed)

Physics behind friction change:

• Viscous (0.45): High friction because resin is viscous and sticky; composite strongly adheres to mold
• Rubbery (0.20): Lower friction as resin becomes more rubbery; some slip occurs
• Glassy (0.169): Lowest friction as material vitrifies; cured composite can slide more freely on mold

How to implement per-step friction:

*Step, name=vis
*Change Friction, interaction=INTER-1
0.45, 

*Step, name=rub
*Change Friction, interaction=INTER-1
0.20, 

*Step, name=glassy
*Change Friction, interaction=INTER-1
0.169, 

HARD Contact Behavior

*Surface Behavior, pressure-overclosure=HARD

• HARD: Standard hard contact — no penetration allowed, no tension
• Pressure transfers only when surfaces are in contact
• Gap opens when pressure goes to zero (no adhesion)

Contact in Springback Step (sp)

In the sp step, contact is completely removed via Model Change:

*Step, name=sp
*Model Change, remove
TOOL-1.EL                    # Remove mold elements
*Model Change, remove
TOOL-1, com-surface          # Remove composite contact surface assignment
*Model Change, remove
TOOL-1, tool-surface         # Remove tool contact surface assignment

After Model Change:

• No contact pair exists
• No friction
• Composite is free to deform (constrained only by Set-2)
• Residual stresses drive springback deformation

Why Contact Creates Asymmetric Springback

1. Friction varies spatially: Different regions of the composite experience different normal pressures, leading to varying frictional resistance
2. L-bracket geometry: The corner region has higher contact pressure than the arm tips, creating non-uniform constraint
3. Thermal expansion: During heating, the composite expands and slides against the mold; friction resists this sliding asymmetrically
4. Demolding release: When the mold is removed, the stored frictional energy releases unevenly, causing asymmetric deformation
5. Result: Maximum displacement at free edges (arm tips), minimum at the corner — this is the correct physical pattern

P8_only vs P8_mold Contact Comparison

Aspect	P8_mold (Correct)	P8_only (Incorrect)
Contact pair	com-surface ↔ tool-surface	None
Friction	0.45 → 0.2 → 0.169	None
Contact behavior	HARD	None
Demolding	Model Change removes contact	No demolding
Composite constraint	Friction (allows slip)	ENCASTRE (no slip)
Springback pattern	Asymmetric (correct)	Center-symmetric (wrong)

Common Errors

Error	Cause	Fix
Penetration in ODB	Contact not properly defined	Verify surface assignments (S1 on composite, S4+S6 on tool)
No springback	Contact not removed in sp	Add *Model Change, remove for TOOL elements and surfaces
Convergence issues	Friction too high	Check friction values: 0.45/0.2/0.169 for vis/rub/glassy
Symmetric deformation	No contact (P8_only model)	Use P8_mold_V2.inp with proper contact pair
CPRESS = 0 everywhere	Wrong surface pair	Verify master/slave assignment; tool is master