Mechanical APDL Structural Analysis Guide

program uses this group of nodes to form the necessary information for the VCCT calculation auto-

matically.

3-D Flat Crack Geometry

For 3-D flat crack surfaces, define a crack-tip node component that includes all of the nodes along

the crack front. At each node location, however, only one node can exist.

All nodes in the crack-tip node component must be connectable, and they must form a line based

on the element connectivity associated with it.This line is the crack front. The program uses it to

determine the elements and the nodes needed for the VCCT calculation automatically.

VCCT is not applicable in the case of a collapsed crack-tip mesh.

The command syntax is:

CINT,CTNC,Par1,Par2,Par3

where CTNC specifies a crack-tip node component,Par1 is the crack-tip node component

name,Par2 is the crack-extension direction calculation-assist node (any node on the

open side of the crack), and Par3 is the crack front’s end-node crack-extension direction

override.

The Par1 and Par2 values help to identify the crack-extension direction. Although the program
automatically calculates the energy-release rat e at the crack tip using the local coordinate system, it is
usually best to use Par2 to define a crack face node to help align the ext ension directions of the crack-
tip nodes.

By default, the program uses the external surface to determine the crack-extension direction and normal
when the crack-tip node hits the free surface. You can use Par3 to override this default.

After the crack-tip node component is defined, define the normal of the crack plane. The program
automatically converts it into the crack-extension vector q, based on the element information. The
crack-extension vector is taken along the perpendicular direction to the plane formed by the crack-
plane normal and the tangent direction of the crack-tip node, and is normalized to a unit vector.

The command syntax is:

CINT,NORM,Par1,Par2

where Par1 is the coordinate system number and Par2 is the coordinate system axis.

Example 1:

! Local coordinate system

LOCAL,11,0,,,,

! select nodes located along the crack front and

! define it as crack front/tip node component

NSEL,S,LOC,X,Xctip

NSEL,R,LOC,Y,Yctip

CM,CRACK_TIP_NODE_CM

! Define a new the energy-release rate calculation

CINT,NEW,1

CINT,TYPE,VCCT

CINT,CTNC,CRACK_TIP_NODE_CM

CINT,NORM,11,2

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Fracture Mechanics