1. Ozen Engineering Inc.
1210 E. Arques Ave, Suite 207
Sunnyvale, CA 94085
info@ozeninc.com
Can Ozcan (can@ozeninc.com)
Fracture Mechanics in Ansys R16
Session-01
Introduction to Fracture
Mechanics

2. Design Philosophies
• It is critical to decide on a design philosophy
– Safe-Life
– Fail-Safe
– Damage-Tolerant
22

3. What is Fracture Mechanics?
• It is the study of flaws and cracks in materials
• Can be seen as a special case among different
design philosophies
• Mostly deals with crack growth and life-time
estimation
23

4. Why is Fracture Mechanics important?
• Strength of Material approach does not anticipate the
presence of a crack or does via concentration factors
• Presence of cracks can significantly decrease the
structural strength and reliability
4
𝜎 =
𝐹
𝐴
𝜎 ≫
𝐹
𝐴
4

5. Why is Fracture Mechanics important?
• Flaw Size (a) is an important parameter in
fracture mechanics approach
• Fracture Toughness replaces strength of material
5
𝜎 ≫
𝐹
𝐴
a
– For Linear-Elastic Fracture
Mechanics (LEFM), fracture
toughness of a material is
determined from “Stress
Intensity Factor”
– For Elastic-Plastic Fracture
Mechanics(EPFM), fracture
toughness is determined via
energy required to grow a crack
𝑲 𝑰𝑪
𝑱 𝑰𝑪
5

6. Industries
• Probably encountered in any industry dealing
with structures
– Automotive
– Electronics
– Healthcare
– Aviation
– Civil
– Nuclear
– Defense
– Maritime
6

7. Historical Background
• Comet Airplanes
7
[1] “The DeHavilland Comet Crash | Aerospace Engineering Blog.” [Online]. Available:
http://aerospaceengineeringblog.com/dehavilland-comet-crash/. [Accessed: 17-Mar-2015].

8. Historical Background
• Liberty Ships
• ~1,500 instances of
brittle fracture during
WWII
• Studies revealed ships in
North Atlantic were
exposed to cold
temperatures were
subject to brittle fracture
8

9. Effect of Temperature on Fracture Toughness
• Fracture Toughness of materials are affected
by temperature
• Ships experiencing cold weather conditions
may be prone to fracture type failures
9

10. Historical Background
• Aloha Flight #243 in 1988:
• German Train ICE Accident in 1998:
10

11. • Life span definition:
Crack Initiation & Propagation
11

12. • Life span definition for fatigue analysis:
Crack Initiation & Propagation
The initiation stage
can represent a large
part of the life span
During the stable propagation stage the
crack speed grows exponentially with the
crack size
The crack grows
unstably until the
ductile fracture
Simulation context
12

13. CRACK INITIATION
• Physically, cracks initiate from;
– An imperfection
– An already existing crack
– A damaged (locally weakened) area
• A failure analysis must include;
– Stress analysis
– Failure criterion
13

14. Theoretical Developments
• A. Griffith (1893-1963)
published the results of his
studies on brittle fracture
• He found the strength of glass
depended on the size of
microscopic cracks
CONSTANTaS 
14

15. 15

16. Three Modes of Fracture
• Mode I denotes a symmetric opening (opening or tension mode)
• Mode II denotes an antisymmetric separation (In-plane shear mode)
• Mode III denotes an antisymmetric separation (out-of-plane shear or
tearing mode)
• Crack growth usually takes place in mode I or close to it.
• The crack “adjusts” itself such that the load is perpendicular to the crack
faces.
16

17. Fracture Mechanics Parameters
• J-Integral
• Energy-Release Rate
• Stress-Intensity Factor
• T-Stress
• Material Force
• C*-Integral
– UMM Method (Unstructured Mesh)
17

18. Energy Release Rate and J-Integral
 Energy release rate can be calculated from stress intensity
factors
 The J-integral can be defined as a path-independent line
integral that measures the strength of the singular stresses
and strains near a crack tip.
Ozen Engineering, Inc. confidential information
18

19. Stress Intensity Factor
• The magnitude of stress field in the immediate vicinity of the crack tip is measured
by the “Stress Intensity Factor”
• Stress Intensity Factor is a quantity determined analytically and varies as a function
of the crack configuration and the external loads are applied
• Critical stress intensity factor is independent of the crack geometry and loading
and may be regarded as a material constant.
• Typical critical stress intensity factor values are;
Ultimate Critical Stress
Strength Intensity Factor
• AL 7075-T651 83 ksi 26 ksi in1/2
• AISI 4340 280 ksi 40 ksi in1/2
19

20. T-Stress
• Is the stress acting parallel to the crack faces
• Unlike J-integral, it can have both negative and
positive values
• Positive T-stress values promotes fracture,
where negative T-stress values results in larger
plastic zones)
20

21. Material Force
• “Material Force” method provides parameters for
global material and dissipation forces that yield
near-tip crack-driving force
• Can be applied to all kind of material models
(elastic, visco-elastic, plastic…)
21

22. C*-Integral
• Creep crack growth becomes a concern when
structural components are operated at high
temperatures (ie Nuclear Industry)
• C*-Integral can be used to evaluate the crack
tip fields under creep deformation
• C*-Integral is a path-independent integral,
which is the steady-state creep analogue of J
integral.
22

23. UMM Method
• The unstructured mesh method (UMM) is a
numerical tool used for evaluating fracture
mechanics parameters on unstructured hex or
tet element meshes.
• Unstructured mesh method supports:
– J-integral
– Stress-intensity factor
23

24. Fracture Mechanics Table
• Which parameters is applicable for which assumptions:
24
Method Material Behavior
J-Integral Linear isotropic elasticity
Isotropic plasticity
Energy-Release Rate
(VCCT Method)
Linear isotropic elasticity
Orthotropic elasticity
Anisotropic elasticity
Stress-Intensity Factor Linear isotropic elasticity
T-Stress Linear isotropic elasticity
Isotropic plasticity
Material Force Various (including plasticity,
viscoelasticity)
C*-Integral Creep

25. Crack-Growth Simulation
• VCCT-Based Interface Element Method
• Cohesive Zone Method
• XFEM-Based Method
25

26. Fracture Mechanics Meshing & Element Support
 KSCON command is used to skew
mid-side nodes around the crack tip.
 This way the crack tip 1/(r1/2)
singularity is captured.
PLANE183
SKEWED NODES
SOLID186
PLANE183
26

27. Fracture Mechanics Meshing & Element Support
27

28. 2-D Edge Crack Propagation
 2 D Edge cracked plate
analysis
 can be solved both using
fracture mechanics tools
and cohesive zone
elements in ANSYS.
Ozen Engineering, Inc. confidential information

29. 2-D Edge Crack Propagation
Crack Propagation Animation, β =15° (a) Crack Propagation Path, (b) SIF, FI; β =15°
• Fracture parameters can be calculated directly using KCALC and CINT command in ANSYS.
• Crack initiation angle and energy release rate can be calculated from stress intensity factors.
• Cracks propagate straight (parallel to loading direction) to attain pure mode-I conditions.
Ozen Engineering, Inc. confidential information
29

30. Interface modeling
Problem Geometry Finite Element Model
 Edge crack propagation in a thin film-substrate(SiO2/Si) is studied.
 Effect of interface layer on crack propagation path is examined using cohesive zone elements.
Ozen Engineering, Inc. confidential information
30

31. 3-D Edge Crack Analysis
In Thin Film-Substrate Systems
(a) Propagation Animation; Cut
Length=150
(b) Propagation Path; Cut Length=150
 Crack propagation is performed quasi-statically. Fracture parameters are calculated
at each propagation step and the geometry is updated for the next propagation step.
 Cohesive zone elements are placed between thin film and substrate to model
interface.
Ozen Engineering, Inc. confidential information
31

32. Industrial Example: Pump Housing Fracture
• Pump housing subjected to internal pressure with a through
crack
Crack
0
0.04
0.08
0.12
0.16
0 1 2 3 4
Path 1
Path 2
Path 3
Path 4
Path 5
Inner surface
Outer surface
J Integral
Calculations
32

33. Industrial example: Cracked helicopter flanged plate
• Automatic crack growth simulation under variable loading
33

34. Following Week Schedule
• March 26th: 10AM-10:45AM PT How to Perform
Fracture Mechanics Analysis in Workbench
• April 2nd: 10AM-10:45AM PT XFEM Method –
Meshless Fracture Mechanics
• April 9th: 10AM-10:45AM PT Fracture
Mechanics with Cohesive Zone and Delamination
• April 16th: 10AM-10:45AM PTFatigue Crack
Propagation
34
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35. END
Thanks for your attention !!!
Questions ?
CONTACT:
Attention: Can Ozcan
OZEN ENGINEERING, INC.
1210 E. ARQUES AVE. SUITE: 207
SUNNYVALE, CA 94085
(408) 732-4665
info@ozeninc.com
www.ozeninc.com