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Crack Propagation behavior is a significant problem in Industries. Aerospace Structures,
Pressure Vessels, gas turbine engines and pipe lines in oil and gas industry are examples, Where Failure could lead
to damages and loss of life as well. For example Crack found in turbine blade of Aircraft engine and Shutting
down of pipe line of process industry leads to not only economical loss it affects other dependent industries.
The presence of Crack does not mean structure or component is unsafe, it is necessary to
know how long initial crack size take grow at critical size at which component or structure would become fail or
unsafe. And also it is necessary to know how crack evolves and its rate of propagation, hence we should be able to
estimate service life of a component .Cost involved if in service Component found crack, initiated for numerical
methods to predict crack propagation. Not only in above said, crack propagation analysis is required studying effect
of surface treatment etc.
Until recently Crack Propagation behavior is predicted experimentally, which is time consuming,
ever since development of high speed computers and commercially available fem coding, industries using soft
ware’s to predict crack propagation.
In this project obtained stress intensity factor from ANSYS. And Predicting crack growth
prediction. Analysis is carried on component of AERO STRCTURE

 Fracture means the cracking or breaking of a hard object or material.
What is fracture?
Breaking
of
Materials
High sulphur in material
Cold waters
Welding instead of riveting
Continuity of the structure
Applied stress
Microcracks

Factors affecting fracture
Thermal expansion Quality of surfacesMagnitude of stress

 Brittle deformation: it occurs (in rocks) when a rock breaks or fractures due to
stress. This happens in high pressure but low temperature environments
 Elastic deformation: An object can bend or flex temporarily and will “ spring
back" to its original shape, but at some point it will also fail.
 Ductile deformation: Any strain is irreversible and the rock deforms readily
(Think taffy puller). Often in a single fault all three forms of deformation take
place in that order as depth increases.
Brittle & Ductile Fracture

 Fracture mechanics is the field of mechanics concerned
with the study of the propagation of cracks in materials. It
uses methods of analytical solid mechanics to calculate the
driving force on a crack and those of experimental solid
mechanics to characterize the material's resistance
to fracture.
 Fracture mechanics is based on the implicit assumption
that there exists a crack in a work component.
 It is the study of flaws and cracks in materials.
 Mostly deals with crack growth.

• 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
𝜎 =
𝐹
𝐴
𝜎 ≫
𝐹
𝐴

• Flaw Size (a) is an important parameter in fracture
mechanics approach
• Fracture Toughness replaces strength of material
𝑲 𝑰𝑪 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
𝑱 𝑰𝑪
𝜎 ≫
𝐹
𝐴
a

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

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

• 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

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.

 The stress intensity factor K is used in fracture mechanics to
predict the stress state ("stress intensity") near the tip of a
crack caused by a remote load or applied stresses.
 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.

There are two main variables
 σ - Far Field Stress
 a - Crack length
Irwin defined the new variable, K
KI = σ(πa)1/2 Mode-I
KII = τ(πa)1/2 Mode-II
KIII = τ(πa)1/2 Mode-III
G=K2/E (For plane stress)

To observer the crack propagation on a rectangular plate of
length 2m and breadth 1.6m and considering a crack length of
0.6m
 Material: - Aluminum
 Young’s modulus: - 70GPa
 Poisson’s ration: - 0.3
 Density: - 2700 Kg/m^3
 Software: - ANSYS 16.0

 PRE PROCESSING:
 From main menu select Preferences select structural  ok

 From the main menu select Pre-processor
 Element type  Add/Edit/Delete  Add  select solid 
select 4 node 182  ok.

 Options  change K3 into the plane strain  ok  close.

 Material properties Material models Structural Linear
Elastic  Isotropic
 Define EX=70e9
 PRXY=0.33
 Density= 2700  ok  close

 From the main menu select Pre-processor  Modeling.
 Create the key point in the workspace
 Create  Key points  In active CS
X Y Z
0 0 0
1 0 0
1 0.8 0
0 0.8 0
0.3 0 0

 Click APPLY to all the points and for the last point click OK.
 Create LINES using the Key points
 Create  Lines  Straight Line  Select 1-5, 5-2, 2-3, 3-4 &
4-1

 From the main menu select MESHING
 Meshing  Size control  Manual sizing  Lines  Picked
lines select the lines 1, 2 & 5 Apply
 Assign the number of division equal to “4”
 Again select the lines 3 & 4
 Assign the number of division equal “6” and give space ratio as
“0.2”.

 Meshing  Sizing control  Manual size  Concentrate key
points
 Give radius R1=0.025
 No of element around the circumference = 16
 Change node position from no skewed 1/2pt → skewed 1/4pt

 Again go to modeling
 Modeling  Create  areas  Arbitrary  By lines 
select all five lines  ok

 Again go to meshing
 Meshing  mesh  areas  free  select the area  ok

 Defining loads
 Loads  define loads  apply  structural  displacement 
symmetric B.C.  with areas  select the line first and then area
 apply
 Define loads  apply  structural  force/moment  on nodes 
select the nodes on the top side  ok
 Define the direction of the force = “ fy”
 `And the value of forces = 750 N  ok

 SOULTION:
 Solve → current LS → ok

 POST PROCESSING:
 From utility menu → plot →nodes

 Defining path
 General postproc → path operation → define path → by
nodes → select the nodes → ok

 From utility menu → work plane → local coordinate system →
create local cs → by 3 nodes

 Selecting the three nodes as follows

 General postproc → option for output → change global CS to
local CS → local system reference no = 11 → ok

 RESULT:
 General post processing → nodal calculation → stress intensity
factor

 Click OK
 Hence the stress intensity factor (KI) = 922.00

 Hence the stress intensity factor of the given rectangular plate is (K) 922.00.
 Hence by using these software the crack propagation is measured.

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