Mechanical APDL Structural Analysis Guide

For 2-D crack problems, the crack ext ension is the summation of length of interface elements that are
currently open (a). For 3-D problems, the crack ext ension is measured at each crack front node and is
the summation of the length of the interface element edges that follow the crack ext ension direction
(b).

Crack ext ension ∆a is available as CEXT as part of the crack solution variable associated with the crack-
calculation ID, and can be postprocessed similar to energy release rat es via POST1 and POST26 postpro-
cessing commands (such PRCINT,PLCINT, and CISOL).

12.1.3. Fracture Criteria

To model the crack growth, it is necessary to define a fracture criterion for crack onset and the subsequent
crack growth. For linear elastic fracture mechanics (LEFM) applications, the fracture criterion is generally
assumed to be a function of Mode I (GI), Mode II (GII), and Mode III ((GIII) critical energy-release rates,
expressed as:

I

C

II

C

III

C

= ( I II III )

Other parameters may be necessary for some models.

Fracture occurs when the fracture criterion index is met, expressed as:

≥c

where fc is the fracture criterion ratio. The recommended ratio is 0.95 to 1.05.The default is 1.0.

The following fracture criteria are available:

12.1.3.1. Critical Energy-Release Rat e Criterion

12.1.3.2. Linear Fracture Criterion

12.1.3.3. Bilinear Fracture Criterion

12.1.3.4. B-K Fracture Criterion

12.1.3.5. Modified B-K Fracture Criterion

12.1.3.6. Power Law Fracture Criterion

12.1.3.7. User-Defined Fracture Criterion

The user-defined option requires a subroutine that you provide to define your own fracture criterion.

To initiate a fracture criterion table without the critical energy-release rat e criterion, issue the TB,CGCR
command.

12.1.3.1. Critical Energy-Release Rate Criterion

The critical energy-release rat e criterion uses total energy-release rat e (GT ) as fracture criterion. The
total energy-release rat e is summation of the Mode I (GI), Mode II (GII), and Mode III ((GIII) energy-release
rates, expressed as:

T

T

=  T= 
+ 

+ 

where 



is the critical energy-release rate.

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Interface Delamination and Failure Simulation