It helps in organizing assignment.

1. Research Paper
Aspects of Fatigue & its Finite element Analysis
By
Fasih Ur Rehman
Submitted To: Engr. Fakhr Ul Islam
MED Fasih Ur Rehman 19JZMEC0309
TRW3 Semester/Section:6@B 24 Jun, 2022

2. Contents
1. Introduction
2. Stages of Fatigue
 Crack Initiation
 Crack Growth
3. Rate of Crack propagation & Growth
4. Finite Element Analysis & Fatigue Simulation
5. Stress-life (S-N) method
6. Microstructural Attributes of Fatigue Material
7. Life improvement

3. Aspects of Fatigue & its Finite element Analysis (Simulation)
Introduction:
Fatigue failure is defined as the tendency of a material to fracture by means of progressive
brittle cracking under repeated cyclic stresses of an intensity considerably below the normal
strength.[1]
Fatigue is also defined as the initiation and propagation of cracks in a material due to cyclic
loading. Once a fatigue crack has initiated, the crack will continue to grow until stress
concentration factor exceeds endurance limit.
In the nineteenth century, the sudden failing of metal railway axles was thought to be caused
by the metal crystallizing because of the brittle appearance of the fracture surface, but this
has since been disproved.[2]
Fatigue cracks usually start at the outer surface of a material for several reasons:
1). Inhomogeneous stress distribution always implies a higher at the surface.
2). Fatigue resistance of a material will be at the material surface. This can be due to
geometrical effects or the material structure being more prone to crack initiation,
3). At the outer surface the environment may contribute to the nucleation of micro cracks.[3]
Stages of fatigue:
Crack initiation
The material will develop cell structures and harden in responseto the applied load. This
causes the amplitude of the applied stress to increase strain. These newly formed cell
structures will eventually break down with the formation of persistent slip bands (PSBs).
Slip in the material is localized at these PSBs, and the exaggerated slip can now serve as a
stress concentrator for a crack to form. Nucleation and growth of a crack to a detectable size
account for most of the cracking process. Itis for this reason that cyclic fatigue failures seem
to occurso suddenly where the bulk of the changes in the material are not visible without
destructive testing. [4]
This slip is not a microstructural change within the material, but rather a propagation of
dislocations within the material due to intensive cyclic loading.

4. Fig From Macro & Microstructural element’s.[5]
Crack growth
The rate of growth is primarily driven by the range of cyclic loading although additional
factors such as mean stress, environment, overloads, and underloads can also affect the rate
of growth. Crack growth may stop if the loads are small enough to fall below a critical
threshold. When the stress intensity exceeds a critical value known as the fracture
toughness, unsustainable fast fracture will occur, usually by a process of micro-void
coalescence.
Rate of Crack Propagation& Growth:
The following effects change the rate of growth:
 Meanstress effect. Higher mean stress increases the rate of crack growth.
 Environment. Increased moisture increases the rate of crack growth. In the case of
aluminum, cracks generally grow from the surface, where water vapor from the
atmosphere is able to reach the tip of the crack and dissociate into atomic hydrogen
which causes hydrogen embrittlement. Cracks growing internally are isolated from the
atmosphere and grow in a vacuum where the rate of growth is typically an order of
magnitude slower than a surface crack.[3]
 Short crack effect. In 1975, Pearson observed that short cracks grow faster than
expected.[6]
 Overloads. Overload usually increases cracks propagation.
 Fatigue life is influenced by a variety of factors, such as temperature, surface finish,
metallurgical microstructure, presence of oxidizing or inert chemicals, residual
stresses, scuffing contact (fretting), etc.

5. Finite Element Analysis & Fatigue Simulation:

6. Stress-life (S-N) method
S-N curve for a brittle aluminumwithanultimatetensilestrengthof 320 MPa[2]
Materials fatigue performance is commonly characterized by an S-N curve. This is often
plotted with the cyclic stress (S) against the cycles to failure (N) on a logarithmic scale.[7] S-
N curves are derived when a regular sinusoidal stress is applied by a testing machine which
also counts the number of cycles to failure. The progression of the S-N curve can be
influenced by many factors such as stress ratio[8] loading frequency, temperature, corrosion,
residual stresses, and the presence of notches. A Constant Fatigue Life (CFL) diagram is
useful for stress ratio effect on S-N curve.[8] Also, in the presence of a steady stress
superimposed on the cyclic loading, the Goodman relation can be used to estimate a failure
condition.
With body-centered cubic materials (bcc), the S-N curve often becomes a horizontal line
with decreasing stress amplitude, i.e., there is a fatiguestrength that can be assigned to these
materials. With face-centered cubic metals, the S_N curve generally drops continuously, so
that only a fatiguelimit can be assigned to these materials. [9]

7. Microstructural Attributes of Fatigue Material:
Fracture surface in a glassrod showingbeachmarkssurroundingthe initiationsite.
Micrographsshowinghowsurface fatigue cracksgrow as material isfurthercycled.FromEwing&Humfrey,1903
[5] Macro & Micro Aspect of Fatigue elements

10. Life improvement
1. Change material. Changes in the materials used in parts can also improve fatigue life.
For example, parts can be made from better fatigue rated metals. Thus helicopter rotor
blades and propellers in metal are being replaced by compositeequivalents. They are
not only lighter, but also much more resistant to fatigue. [10]
2. Induce residual stressesPeening a surface can reduce such tensile stresses and create
compressive residual stress, which prevents crack initiation. Forms of peening include:
shot peening, using high-speed projectiles, high-frequency impact treatment (also
called high-frequency mechanical impact) using a mechanical hammer,[11] and laser
peening which uses high-energy laser pulses. Low plasticity burnishing can also be
used to induce compresses stress in fillets and cold work mandrels can be used for
holes. Increases in fatigue life and strength are proportionally related to the depth of
the compressive residual stresses imparted. Shot peening imparts compressive residual
stresses approximately 0.005 inches (0.1 mm) deep, while laser peening can go 0.040
to 0.100 inches (1 to 2.5 mm) deep, or deeper.[12]
3. Deepcryogenic treatment. The use of Deep Cryogenic treatment has been shown to
increase resistance to fatigue failure. Springs used in industry, auto racing and
firearms have been shown to last up to six times longer when treated. Heat checking,
which is a form of thermal cyclic fatigue has been greatly delayed.[13]
4. Re-profiling. Changing the shape of a stress concentration such as a hole or cutout
may be used to extend the life of a component.[14]

11. [1] “Fatigue (material),” Wikipedia. Jun. 11, 2022. Accessed:Jun. 25, 2022. [Online].
Available:
https://en.wikipedia.org/w/index.php?title=Fatigue_(material)&oldid=1092609804
[2] S. Suresh, Fatigueof materials, 2nd ed. Cambridge ; New York: Cambridge
University Press, 1998.
[3] J. Schijve, “Internal fatigue cracks are growing in vacuum,” Eng. Fract. Mech., vol.
10, no. 2, pp. 359–370, Jan. 1978, doi: 10.1016/0013-7944(78)90017-6.
[4] P. Forsyth, “Exudation of Material from Slip Bands at the Surface of Fatigued Crystals
of an Aluminium–Copper Alloy,” Nature, 1953, doi: 10.1038/171172A0.
[5] “MACRO/MICRO ASPECTS OF FATIGUE OF METALS,” p. 50.
[6] S. Pearson, “Initiation of fatigue cracks in commercial aluminium alloys and the
subsequent propagation of very short cracks,” Eng. Fract. Mech., vol. 7, no. 2, pp. 235–247,
Jun. 1975, doi: 10.1016/0013-7944(75)90004-1.
[7] I. Burhan and H. S. Kim, “S-N Curve Models for Composite Materials
Characterisation: An Evaluative Review,” J. Compos. Sci., vol. 2, no. 3, Art. no. 3, Sep.
2018, doi: 10.3390/jcs2030038.
[8] “Kim, H. S. (2016). Mechanics of Solids and Fracture (2nd ed.). Ventus Publishin”.
[9] tec-science, “Fatigue test,” tec-science, Jul. 13, 2018. https://www.tec-
science.com/material-science/material-testing/fatigue-test/ (accessed Jun. 25, 2022).
[10] H. Magazines, PopularMechanics. Hearst Magazines, 1989.
[11] M. Zamanzadeh, E. Larkin, and R. Mirshams, “Fatigue Failure Analysis Case
Studies,” J. Fail. Anal. Prev., vol. 15, Nov. 2015, doi: 10.1007/s11668-015-0044-3.
[12] “Lser Shock Peening - Purdue.”
https://engineering.purdue.edu/LAMPL/research_peening.html (accessed Jun. 26, 2022).
[13] “Search for ‘fatigue’ - Cryogenic Treatment Database.”
https://cryogenictreatmentdatabase.org/?cat_ID=220&s=fatigue (accessed Jun. 26, 2022).
[14] M. Heller et al., “Airframe Life Extension By Optimised Shape Reworking —
Overview Of Dsto Developments,” in ICAF 2009, Bridging theGap between Theory and
OperationalPractice, M. J. Bos, Ed. Dordrecht: Springer Netherlands, 2009, pp. 279–299.
doi: 10.1007/978-90-481-2746-7_17.
Finite Element Analysis are performed in SolidWorks and I have generated a SolidWorks
Driven report soonafter the finite element analysis are simulated.