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Chandwani.pdf
2004 UK ANSYS Conference
Stratford Moat house, Warwickshire, UK, Nov 15-16 2004
3D Fracture Mechanics In ANSYS
Ramesh Chandwani, Miles Wiehahn, Chris Timbrell
Zentech International Limited
http://www.zentech.co.uk
ABSTRACT
This paper will address methods of performing truly three-dimensional fracture
mechanics analyses in ANSYS.
Generally available fracture mechanics techniques and their implementation and use
with ANSYS for 3D analysis will be briefly discussed. Techniques include the crack
opening displacement (COD) method for LEFM, crack tip opening displacement
(CTOD) method for EPFM, and the J-Integral method.
A software implementation using the COD method in conjunction with ANSYS will be
presented. This implementation addresses generation of cracked 3D meshes and
crack growth prediction. Examples will demonstrate large-scale crack growth under
generalised mixed-mode loading and the development of complex 3D crack surfaces.
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INTRODUCTION
Crack propagation behaviour is a major issue in a variety of industries. Aerospace
structures, gas turbine engines, pressure vessels and pipelines are obvious
examples where failure could lead to catastrophic consequences and loss of life. For
example, grounding of a fleet of aircraft because of cracks found in a turbine blade of
the engine, or shutting down of a pipeline have huge implications.
The mere presence of a crack does not condemn a component or structure to be
unsafe and hence unreliable. Whether under cyclic or sustained loading, it is
necessary to know how long an initial crack of a certain size would take to grow to a
critical size at which the component or structure would become unsafe and fail. Also
by knowing how a crack evolves and its rate of propagation, one should be able to
estimate the residual service life of a component under normal service loading
conditions. The financial costs involved if an in-service component is found to contain
a defect are a major factor in the search for numerical methods to predict 3D crack
propagation.
Other situations where crack propagation is required include:
•
•
•
Studying the effects of surface treatment such as shot peening, laser shock
peening etc., to enhance the service life of a component.
Studying the effectiveness of crack repair systems, remedial work,
modifications or design changes.
Establishing inspection and maintenance regimes.
Until recently mode I and mixed mode crack growth behaviour was generally
evaluated using experimental tests. However, using the design philosophy based on
reliability and “safe life” criteria, it becomes necessary to test a large number of
materials and structural components in a very short period. This makes the use of
experimental methods rather impractical and implies the benefit of using numerical
computational methods to evaluate 3D mixed mode crack propagation from simple
mode I tests.
Zentech has developed a 3D crack analysis tool called ZENCRACK (Ref. 1).
ZENCRACK reads in an uncracked finite element model and produces a cracked
finite element model. Stress intensities or the energy release rate are calculated
automatically from the results of the cracked finite element analysis. Furthermore
crack growth can be undertaken by extending the crack position. An updated finite
element model is then created and run to simulate crack growth.
ZENCRACK is a mature product that was first released in 1990. However an ANSYS
(Ref. 2) interface has only been created recently.
This paper discusses, and demonstrates with examples:
•
•
•
methods of obtaining the stress intensity factors or energy release rate from
F.E. codes and in particular, ANSYS
topology and meshing in relation to 3D cracks
crack growth prediction
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3D Fracture Mechanics In ANSYS
FRACTURE MECHANICS PARAMETERS
The primary fracture mechanics parameters that may be of interest for crack
propagation are:
•
•
Stress intensity factors, Ki, Kii, Kiii
Energy release rate, G
The stress intensity factor approach was developed by Irwin in the 1950s following
on from the elastic strain energy approach to brittle fracture developed by Griffith
from the 1920s. Irwin’s work led to the foundations for the concept of linear elastic
fracture mechanics (LEFM) which is still fundamental in most crack propagation
analyses.
For linear elastic analysis the concepts of energy release rate and stress intensity
factors are closely linked. The stress intensity factors describe the magnitude of the
elastic stress field at a crack front. The general form of the stress intensity factor is:
K = f (load , crack length, geometry )
Equation 1
For mode I behaviour it can be shown that:
 EG 

K = 
2 
 1 − (αν ) 
1
2
Equation 2
where E is Young’s modulus, ν is the Poisson ratio and α is a value ranging from 0
for plane stress to 1 for plane strain. In a more general form it is possible to write:
G=
B 2
1 +ν
(
K I + K II2 ) + 
E
 E
 2
 K III

Equation 3
where B=1 for plane stress and 1-ν2 for plane strain.
Another important relationship for stress intensity factors in linear elastic analysis is
based on the Westergaard equations that link the stress intensity factors to the
displacement field around the crack tip (COD) giving:
KI =
EVi
4B
EVii
2π
, K II =
4B
r
2π
π
E
, K III =
Viii
2(1 + ν )
2r
r
Equation 4
where B is defined as above and Vi, Vii and Viii are the relative opening
displacements at a radius r from the crack front for an orthogonal system aligned with
the mode I, II and III directions. This approach is widely used by practitioners of both
the finite element and boundary element methods and has the benefit that it requires
relatively little additional effort on top of the basic stress solution. The drawback to
the method is that it requires a state of stress assumption.
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Calculation of the magnitudes of energy release rate and / or stress intensity factors
do not directly provide directional information regarding crack growth. A number of
criteria have been developed to specify the direction. They include maximum energy
release rate, maximum tangential stress and the normal to the maximum principal
stress. In the context of numerical calculations of energy release rate and stress
intensity factor, the two most useful criteria are maximum energy release rate and a
direction based on stress intensity factors e.g.:
 K II
 KI
θ = tan −1 



Equation 5
The J-integral concept was first described by Rice in the late 1960s. It is an energy
based concept in which the J-integral, J, can be considered a non-linear elastic
equivalent of the energy release rate, G. By definition G and J are the same for
elastic behaviour. The J-integral can be calculated as a post processing exercise
after completion of a finite element analysis. However, this capability is currently not
available for 3D fracture mechanics in ANSYS.
For similar reasons that significant additional post-processing coding would be
required, methods such as the virtual crack closure method do not present an
attractive proposition.
For elastic plastic fracture mechanics the crack tip opening displacement (CTOD) is a
measurement of the crack opening displacement at the crack tip. The relationship
between J and CTOD ( δ ) is given by the equation:
J = mσ ys δ
Equation 6
where m varies between 1.15 and 2.95 (Ref. 3). For such elastic plastic cases the
difficulty of using this method is choosing a valid value of m. Hence the method
cannot readily be applied as a means of evaluating J from displacements for EPFM.
In the interface between ZENCRACK and ANSYS, the COD method is used for
evaluating the stress intensity factors (see Equation 4) for linear elastic materials.
Extensive testing has shown that COD agrees well with the J-Integral when using
ZENCRACK with other f.e. codes (see verification examples later in the paper). COD
can also be used for problems with residual stresses where current J-Integral
implementations have shortcomings (See Ref. 7).
Note that ZENCRACK is also capable of generating meshes suitable for EPFM
although in these cases there is no calculation of fracture mechanics parameters.
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TOPOLOGICAL ISSUES
A critical issue that must be addressed in 3D F.E. fracture mechanics analysis is that
of mesh generation. In the simplest of geometric cases where symmetry can be
used, it may be possible to utilise standard mesh generation tools to produce a crack
of the required size. In the general case, however, the use of standard tools poses
several time consuming problems including:
•
•
•
•
•
Component geometries are often complex and time consuming to model in
their uncracked forms.
Defects often occur at geometrically difficult locations e.g. corners, welds,
chamfers.
Initial cracks of the correct size and shape must be inserted into the
component at the correct location.
Cracks may develop in a non-planar fashion depending upon the loading.
These problems are compounded if more than one crack size must be
analysed or if there are multiple cracks in a component.
The approach that has been successfully adopted at Zentech is the use of ‘crackblocks’ which model the details of the crack region. Figure 1 and Figure 2
demonstrate the use of the crack-block methodology in generating a cracked mesh
from a user-supplied intact component. The method works by replacing one or more
elements in the uncracked mesh by crack-blocks that contain sections of crack front.
The interface to ANSYS operates via the ANSYS batch file for the uncracked
component. This file is read and processed by ZENCRACK. A new batch file is
created for the cracked mesh. Each batch processing keyword in the ANSYS input
library has been given an associated status value. When a keyword in the uncracked
mesh is identified, the action taken by ZENCRACK depends upon this status value
(e.g. the SFE option is checked to see if load updates are required on the crackblocks).
"Standard" processing is performed if the batch file contains explicit element and
node specifications. Alternatively, the batch file may be constructed using the ANSYS
solid-modelling and mesh generation capabilities. ZENCRACK then automatically
performs an initial ANSYS analysis to generate an ANSYS database file for the
uncracked model. This is then queried to generate explicit node, element and
boundary condition data. ZENCRACK then creates a modified batch file and
interprets this data in the "standard" way.
When the cracked mesh is constructed, output requests are incorporated to allow
displacements at key nodes to be extracted by ZENCRACK for use in calculating
stress intensity factors.
ZENCRACK has two types of crack-blocks:
•
Standard crack-blocks.
o These crack-blocks have a “clean” face on three faces.
o The crack-blocks are designed to replace elements in the mesh by
updating element connectivities and node numbers (see Figure 1).
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•
3D Fracture Mechanics In ANSYS
o The crack-blocks consist of “through” and “quarter circular” crackblocks.
Large crack-blocks.
o These crack-blocks do not have “clean” faces.
o These crack-blocks use tied contact to tie the crack-blocks to the model
and can therefore straddle several standard elements (see Figure 2).
o The crack-blocks consist of “through” and “quarter circular” crackblocks.
The crack-blocks have a varying number of “rings” of elements around the crack
front. The innermost ring contains “collapsed” elements to represent the singularity in
the stress and strain field at the crack front. ZENCRACK offers full control of the
nodes along the crack front and the radial nodes closest to the crack front in order to
generate a singularity best suited to LEFM or EPFM (Ref. 5).
Although the crack-blocks are referenced as “quarter circular” or “through” crackblocks, the user has control of the initial crack front shape which may be defined by
fitting a spline through a series of points for the greatest flexibility in definition.
The use of crack-blocks allows loading (e.g. pressure load) and boundary conditions
to be updated as the crack is incorporated (and advanced through the mesh).
CRACK GROWTH PREDICTION
By extending the mesh generation scheme described above and adding a crack
growth algorithm, it is possible to carry out automatic crack growth prediction, as
summarised in Figure 3.
The remeshing of a fixed region in space was discussed in by Cook et al in Ref. 8.
The original method only allowed growth to occur within the volume occupied by the
original element of the uncracked mesh. The current implementation in ZENCRACK
allows greater flexibility by (see Figure 4):
•
•
•
•
Shifting of the boundaries of the crack-blocks.
Relaxing elements surrounding the crack-blocks.
Transferring crack-blocks from one location to another.
Allowing use of “large” crack-blocks to increase the volume in which growth
may occur.
In order to complete the crack growth prediction, it is necessary to integrate using the
results from a f.e. analysis along with the crack growth data. This procedure cannot
be fully described here (see ZENCRACK documentation for details) but the salient
features of the implementation are:
•
Standard forms of crack growth data are allowed such as Paris and Walker
equations in addition to tabular data (as a function of stress ratio and
temperature) and a completely general user subroutine option for proprietary
data or complex equation forms.
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•
•
•
•
•
•
3D Fracture Mechanics In ANSYS
Threshold and fracture material properties can be defined.
The integration is carried out in such a way that a consistent dn is evaluated
for all crack front nodes although in general the da values vary from node-tonode (thus allowing the possibility of complex crack shape development).
The step size between f.e. analyses is adapted based on the accuracy of
previous integration steps.
Constant amplitude or variable amplitude fatigue loading can be analysed.
A sustained load crack growth option is also available (i.e. da/dt vs K growth
data rather than da/dn vs ∆K).
Static load may be incorporated with the cyclic load.
STRESS INTENSITY FACTOR VERIFICATION EXAMPLES
Two different verification examples are shown to demonstrate the basic calculation of
stress intensity factors:
•
•
A SEN specimen
A fully embedded crack
The SEN model is shown in Figure 5. A comparison of the ANSYS COD values with
J-Integral values and a theoretical solution (Ref. 4) is shown in Figure 6 (a is the
crack length, W is the specimen width, K 0 = σ πa where σ is the stress). The COD
line agrees well with the theoretical and J-Integral results.
The uncracked mesh for the fully embedded crack is shown in Figure 7. Figure 8
shows the cracked mesh for a circular crack (a=0.3mm,c=0.3mm) and Figure 9
shows the cracked meshes for an elliptical crack (a=0.3mm,c=0.6mm). The results
can be seen in Figure 10. Again the COD results agree well with the J-Integral and
theoretical solutions (Ref. 6).
EXAMPLES OF CRACK GROWTH
Some of the crack growth capabilities of ZENCRACK are shown in three examples:
•
•
•
A 43° oblique crack.
A lug-pin interaction.
A turbine compressor disk.
It must be noted that each analysis, once started, is completed fully automatically.
The first example is of a 43 degree slanting crack in a plate under cyclic axial tension
(see Figure 11). This is based on a test case for which experimental data is
published (Ref. 9). The crack face triangular facets that are defined by ZENCRACK
for the initial crack, plus those calculated during the analysis, are shown in Figure 12.
The growth is compared against experimental results in Figure 13 and Figure 14. It is
clear that there is excellent agreement in both the calculated growth direction and
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cycle history. Note that although the problem is initially mixed mode, the crack very
quickly re-aligns itself into mode I behaviour.
The second example is a pin-loaded lug (see Figure 15). This problem requires
contact conditions between the pin and lug in the f.e. analysis. The maximum
principal stress in the uncracked mesh is used to determine the initial defect location.
The initial defect is postulated and is grown all the way to the edge of the lug (note
that no Kic is defined in this analysis). The calculated crack growth profiles are
shown in the bottom right corner of Figure 15. The crack that develops is determined
by the loading and materials data. The user does not have to make any assumptions
about the crack direction, or forcing the crack to grow between the particular element
boundaries.
The third example is of a crack growing from a bolt hole in a compressor disk (see
Figure 16). This analysis represents a spin test in which there is bi-axial loading at
the bolt holes. The model was obtained from Ref. 10. Further discussion of the
problem can be obtained from Ref. 11.
CONCLUSIONS
ZENCRACK has been interfaced to ANSYS allowing state of the art 3D fracture
mechanics analysis to be undertaken. COD is used to obtain the stress intensity
factors. A general crack growth scheme allows crack advancement and non-planar
crack growth.
REFERENCES
Ref. 1
ZENCRACK
Zentech International Limited, U.K.
http://www.zentech.co.uk/zencrack.htm
Ref. 2
ANSYS
Ansys Inc, U.S.A.
http://www.ansys.com
Ref. 3
“Fracture mechanics”, H.L.Ewalds and R.J.H Wanhill, Edward Arnold
(Publishers) Limited, 3rd imprint 1986 (ISBN 0-7131-3515-8), pg 156.
Ref. 4
AFGROW,
Air Vehicles Directorate, Air Force Research Laboratory, U.S.A.
http://afgrow.wpafb.af.mil/
Ref. 5
“Use of modified standard 20-node isoparametric brick elements for
representing stress/strain fields at a crack tip for elastic and perfectly
plastic material”, Koers,R.W.J., Int. J. Frac. 40 (1979) 79-110.
Ref. 6
“Fracture mechanics”, H.L.Ewalds and R.J.H Wanhill, Edward Arnold
(Publishers) Limited, 3rd imprint 1986 (ISBN 0-7131-3515-8), pg 49.
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Ref. 7
“Residual Stress in a 3D Finite Element Fracture Mechanics Analysis”, C.
Timbrell, R. Chandwani, FENET Technology Workshops - Durability and
Life Extension, Palma, Majorca, Mar 25-26 2004.
Available at http://www.zentech.co.uk/zencrack_papers.htm
Ref. 8
“Automatic and adaptive finite element mesh generation for full 3D fatigue
crack growth”, G.Cook, C.Timbrell, P.W.Claydon, STRUCENG & FEMCAD
Conference, Grenoble, France, 1990.
Available at http://www.zentech.co.uk/zencrack_papers.htm
Ref. 9
“Fatigue crack propagation under general in-plane loading - I:
Experiments”, M.A. Pustejovsky, Engineering Fracture Mechanics 11
(1979) 9-15.
Ref. 10
“Prediction of crack growth from bolt holes in a disc”, W. Z. Zhuang,
International Journal of Fatigue, 22 (2000) 241-250.
Ref. 11
“The Application of 3D Finite Element Analysis to Engine Life Prediction”,
G. Cook, C. Timbrell, B. Browning, AeroMat 2001 - 12th Advanced
Aerospace Materials & Processes Conference & Exhibition, Long Beach,
CA, U.S.A., June 11-14 2001.
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Figure 1 - Summary of crack insertion and mesh update process using
standard crack-blocks
The target crack-block is shown in light green.
The target crack-block has been replace a large
crack-block. Contact is applied between the
surfaces of the crack-block and the surrounding
mesh.
Figure 2 - Example of a large crack-block
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USER INPUT
An existing f.e. mesh
of an uncracked
component
ZENCRACK
Creates f.e. mesh
of the cracked
component
USER INPUT
Additional data e.g.
crack location, size
& crack growth data
F.E. CODE
Analysis
ZENCRACK
Evaluates crack growth
ZENCRACK
Updates f.e. model
STOP
no
ZENCRACK
Next f.e. analysis?
yes
Figure 3 - Simplified flow chart for crack growth prediction analysis
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Uncracked mesh
Crack-block inserted into original element definitions
Crack-block inserted with boundary shift
Crack-block inserted with boundary shift and relaxation
of surrounding elements
Crack-block transferred to new location after growth
Advanced position in new location
Figure 4 - Demonstration of boundary shifting, relaxation and crack-block
transfer
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Uncracked mesh
Cracked mesh (a=2.5mm)
Figure 5 - SEN model
Ki/Ko for different crack lengths
11.0
AFGROW Equation
ABAQUS J-Int
ANSYS COD
10.0
9.0
8.0
Ki/Ko
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.00
0.10
0.20
0.30
0.40
a/W
0.50
0.60
0.70
Figure 6 - Comparison of COD with theory for a SEN specimen
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0.80
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The relaxed region
is in orange
Figure 7 - Uncracked mesh for the embedded crack
Relaxed region of the mesh (see Figure 7)
Close-up of crack region
Figure 8 - Cracked mesh with a single standard quarter circular crack-block
(a=0.75mm, c=0.75mm)
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Close-up of crack region (a=0.3mm,c=0.6mm)
Close-up of crack region (a=0.3mm,c=0.6mm)
Figure 9 - Embedded elliptical crack (a=0.3mm,c=0.6mm)
K values for embedded cracks
500
Theory
475
K (MPa-sqrt(mm))
ABAQUS J-Int
450
ANSYS COD
a=0.3mm, c=0.6mm
425
400
375
a=0.3mm, c=0.3mm
350
325
0
10
20
30
40
50
60
70
80
90
Phi (degrees)
Figure 10 - Comparison of COD with theory for a fully embedded crack
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Close-up of the target crack-blocks
Initial cracked mesh
The entire uncracked model
Figure 11 - Mesh for an oblique crack at 43°
Triangular facets used for definition of the initial and extended crack faces
Figure 12 - Crack growth profiles for the oblique crack at 43°
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Crack growth curve - inclined crack
13
Experiment - Left side
Experiment - Right side
ABAQUS J-Int
ANSYS COD
Crack size (mm)
12
11
10
9
8
7
6
0
2000
4000
6000
8000
10000
12000
14000
16000
No of cycles (N)
Figure 13 - Crack growth rate for the oblique crack at 43°
Crack profile - inclined crack
6
Experiment
ABAQUS J-Int
ANSYS COD
y coordinate (mm)
4
2
0
-2
-4
-6
-12
-8
-4
0
4
8
12
x coordinate (mm)
Figure 14 - Comparison of the ZENCRACK and experimental crack growth path
for the oblique crack at 43°
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Maximum principal stress in the uncracked mesh used to
determine an initial defect location
Initial defect position
Crack-blocks transferred into the third row of elements
Calculated crack profiles
Crack-blocks transferred into the sixth row of elements
An initial through crack grows through the lug. Note the crack path is independent of element boundaries in the uncracked mesh
and that the mesh surrounding the defect is modified as the crack advances.
Figure 15 - Pin lug example
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Figure 16 - Compressor disk example
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