VCCT (Virtual Crack Closure Technique)
VCCT (Virtual Crack Closure Technique): Theoretical Foundations
What is VCCT?
Professor, what is VCCT?
VCCT (Virtual Crack Closure Technique) is a method that directly calculates the energy release rate $G$ from nodal forces and opening displacements at the crack tip. It is a crack propagation technique alongside CZM.
$F_y$ is the nodal force at the crack tip, $\delta_y$ is the opening displacement behind the crack tip, and $\Delta a$ is the element size.
VCCT vs. CZM
Characteristic VCCT CZM Crack Nucleation × ○ Propagation of Existing Crack ○ ○ Parameters Only $G_c$ Strength + $G_c$ + Stiffness Mesh Dependency Present Low (regularized by $G_c$) Computational Cost Low High
VCCT is only for propagation of existing cracks. CZM can also handle nucleation.
VCCT is optimal for delamination propagation (problems where cracks propagate from known crack tips). CZM is more versatile.
Summary
| Characteristic | VCCT | CZM |
|---|---|---|
| Crack Nucleation | × | ○ |
| Propagation of Existing Crack | ○ | ○ |
| Parameters | Only $G_c$ | Strength + $G_c$ + Stiffness |
| Mesh Dependency | Present | Low (regularized by $G_c$) |
| Computational Cost | Low | High |
VCCT is only for propagation of existing cracks. CZM can also handle nucleation.
VCCT is optimal for delamination propagation (problems where cracks propagate from known crack tips). CZM is more versatile.
Rybicki and Energy Release Rate
VCCT (Virtual Crack Closure Technique) was proposed in 1977 by Rybicki and Kanninen (US invention). It is a method to calculate the energy release rate when a crack propagates by a small amount Δa, using the forces at the crack tip nodes and the displacement differences when the crack is virtually closed. Its efficiency in calculating GI, GII, and GIII simultaneously from a single FEM calculation has made it the standard method for interface delamination analysis in composite materials.
Computational Methods for VCCT (Virtual Crack Closure Technique)
FEM for VCCT
```
*DEBOND, SLAVE=crack_surface, MASTER=intact_surface
*FRACTURE CRITERION, TYPE=VCCT, MIXED MODE BEHAVIOR=BK
G_Ic, G_IIc, G_IIIc, eta
```
Define the upper and lower surfaces of the crack as slave/master. Nodes are automatically released when $G \geq G_c$.
Summary
VCCT Mesh Requirements and Error Evaluation
VCCT depends on the element size Δa at the crack tip. Smaller Δa improves accuracy but increases computational cost. Practically, Δa = t/10 to t/20 relative to plate thickness t is recommended; coarser meshes can lead to errors exceeding 10%. Also, because it's a simple calculation of the product of nodal forces and displacements rather than an integral path, its calculation speed is 2-3 times faster than the J-integral.
VCCT (Virtual Crack Closure Technique) in Practice
VCCT in Practice
Used for delamination in composite materials (simulation of DCB, ENF tests), and adhesive joint debonding.
Practical Checklist
Example of VCCT Application for CFRP Wing Skin Delamination
VCCT is used to evaluate delamination growth at the joint between CFRP wing skin and ribs on the Airbus A320. It combines the mixed-mode fracture criterion (GI/GIc+GII/GIIc=1) with VCCT calculated values to predict delamination front propagation. Boeing's internal standard BGS-33 "Composite Delamination Evaluation Procedure" has adopted VCCT as a standard method since the 1990s and is also used for CFRP component certification on the 737MAX.
VCCT (Virtual Crack Closure Technique): Software & Solver Comparison
VCCT Tools
ANSYS Mechanical VCCT Implementation
ANSYS Mechanic
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