Crack Paths 2006

Creation of an automatic crack insertion tool for mixed mode

crack propagation calculations

D. Bremberg1 and G. Dhondt2

1 M T UAero Engines GmbH,Postfach 50 06 40, D-80976 Munich, Germany

K T HHållfasthetslära, Osquars Backe 1, S-10044 Stockholm, Sweden

Daniel.Bremberg@muc.mtu.de, bremberg@kth.se

2 M T UAero Engines GmbH,Postfach 50 06 40, D-80976 Munich, Germany

Guido.Dhondt@muc.mtu.de

Abstract

This paper presents an automatic crack-insertion tool, which makes part of a larger long

term project attempting to create an automatic tool for mixed-mode crack propagation

calculations. First, a flexible cylinder is created along the crack front. Then, this cylinder is

merged into the uncracked structure generating a so-called perforated structure. Finally,

duplication and insertion of the crack into the perforated structure yields a consistent

triangulation of the cracked surface. In a later step, the cracked structure and the cylinder

have to be filled with tetrahedral and hexahedra, respectively, before finite element

calculations can be performed.

The task has shown to be rather complex. During the work many situations have occurred

changing the course of the project. Most of the problems which arose have been solved and

the outcome of the project is a working crack insertion tool. However, there still remain a

number of questions to be answered regarding certain situations.

1. I N T R O D U C T I O N

During the 1990s, a numerical crack propagation software was developed at Motoren- und

Turbinen-Union (MTU) in Munich, Germany [1,2]. The program named C R A C K T R A C E R

works in a fully automatic way without restrictions for in-plane crack propagation.

C R A C K T R A ChaEsRbecome a standard tool at M T Ufor crack propagation calculations and

predictions in a variety of aircraft-engine components such as disks, blades and vanes.

The two main advantages of C R A C K T R A CarEe Rits automatic nature and the fact that

the user does not have to prescribe the shapes of the crack-fronts. It has also proven to give

accurate results [3]. The cracks are, however, restricted to in-plane motion which requires

that the plane of propagation is determined beforehand. Consequently, the analyses are

limited to Mode-I loading conditions. A third restriction concerns the input to the program:

C R A C K T R A CreEquiRres that the uncracked structure is given as a volume mesh consisting

of hexahedral elements.

2. P R O C E D U R E

The starting point of the present procedure is a volume mesh of the structure to be analyzed

consisting of hexahedral, tetrahedral and/or wedge elements, see Figure 1a. Automatic

remeshing of the free faces of the volume mesh yields a surface triangulation consisting of

linear triangle elements, Figure 1b. A plane initial crack, either elliptical or rectangular, is

generated and triangulated based on the geometrical description, Figure 2.