BEE1043 Engineering Materials

Unit 4: Metal Fatigue

Dr. Maisarah Binti Lutfi
Mechanical Engineering, FOEBEIT
maisarah@mahsa.edu.my
 Learning Outcomes

i. What is Fatigue?
ii. Cyclic Stresses
iii. Mechanism of Fatigue failure
iv. Factors that affect Fatigue life
v. The S–N Curve – Fatigue test
vi. Surface treatment to control Fatigue
 What is Fatigue?
1. Fatigue is a form of failure that occurs in
structures subjected to dynamic and fluctuating
stresses (e.g., bridges, aircraft, and machine
components).

2. Under these circumstances it is possible for

failure to occur at a stress level considerably lower
than the tensile or yield strength for a static load.

3. The term fatigue is used because this type of

failure normally occurs after a lengthy period of
repeated stress or strain cycling.
4. Fatigue is catastrophic and insidious, occurring very
suddenly and without warning.

5. Fatigue failure is brittle like in nature even in

normally ductile metals, in that there is very little, if
any, gross plastic deformation associated with failure.

6. The process occurs by the initiation and

propagation of cracks, and ordinarily the fracture
surface is perpendicular to the direction of an applied
tensile stress.
Fatigue failure of bridge
Fatigue failure of bridge
Fatigue failure of I-beam bridges
 Fatigue failure of Aircraft Component
There are several key components on an aircraft that are
susceptible to metal fatigue. aircraft components experience
vibration, high-frequency rotation, and other forms of in-flight
stress.

✓ Engine Components
✓ Fan Blades
✓ Wing Attachments
✓ Brakes/ Landing Gears
✓ Welded Components
✓ Aging Components
✓ Fastener Holes

Cracks due to continuous stress on these parts can cause

catastrophic structural failure (Completely failed).
Fatigue failure in gear
Classification of various types of fatigue failure
occurring in engineering materials.
 CYCLIC STRESSES
1. The applied stress may be axial (tension–
compression), flexural (bending), or torsional (twisting)
in nature.

2. Three different fluctuating stress–time modes are:

(a) Reversed stress cycle
(b) Repeated stress cycle
(c) Random stress cycle
Parameters used to characterize the fluctuating stress
cycle:
(a) mean stress σm, :the average of the maximum and
minimum stresses in the cycle

(b) range of stress σr is just the difference between σmax

and σmin

(c) stress amplitude σa : one-half of this range of stress

(d) stress ratio R: the ratio of minimum and maximum

stress amplitudes
1. Reversed stress cycle

the amplitude is symmetrical about a mean zero stress level,

for example, alternating from a maximum tensile stress
(σmax) to a minimum compressive stress (σmin) of equal
magnitude.
2. Repeated stress cycle

the maximum and minimum are

asymmetrical relative to the zero stress level.
 3. Random stress cycle

the stress level may vary randomly in amplitude and frequency.

 MECHANISM OF FATIGUE FAILURE
1. Process of fatigue failure is characterized by three
distinct steps:
(a) crack initiation
(b) crack propagation
(c) final failure

2. Crack initiation: a small crack forms at some point of

high stress concentration.

3. Crack propagation : crack advances incrementally with

each stress cycle.

4. Final failure: occurs very rapidly once the advancing

crack has reached a critical size.
MECHANISM OF FATIGUE FAILURE
5. Cracks associated with fatigue failure almost always
initiate (or nucleate) on the surface of a component at
some point of stress concentration.

6. Crack nucleation sites include surface scratches,

sharp fillets, keyways, threads, dents, and the like.

7. Crack propagation step may be characterized by

two types of markings:
(a) Beachmarks
(b) Striations
8. Both of these features indicate the position of the
crack tip at some point in time and appear as
concentric ridges that expand away from the crack
initiation site(s), frequently in a circular or semicircular
pattern.

9. Beachmarks: macroscopic dimensions

Striations: microscopic in size

10. The presence of beachmarks and/or striations on a

fracture surface confirms that the cause of failure was
fatigue. Nevertheless, the absence of either or both
does not exclude fatigue as the cause of failure.
a) Striations indicating fatigue failure, b) Beach marks indicating fatigue failure.
 FACTORS THAT AFFECT FATIGUE LIFE
1. Factors that affect fatigue life include:
(a) mean stress level
(b) geometrical design
(c) surface effects
(d) environment

2. Mean stress level: increasing the mean stress

level leads to a decrease in fatigue life.

3. Geometrical design :any notch or geometrical

discontinuity can act as a stress raiser and fatigue crack
initiation site. The sharper the discontinuity (i.e., the
smaller the radius of curvature),the more severe the stress
concentration.
4. Surface Effects: most cracks leading to fatigue
failure originate at surface positions, specifically at
stress amplification sites. Fatigue life is especially
sensitive to the condition and configuration of the
component surface.

5. Environment: Thermal fatigue & corrosion fatigue.

-Thermal fatigue is normally induced at elevated temperatures by fluctuating
thermal stresses; mechanical stresses from an external source need not be
present. The origin of these thermal stresses is the restraint to the dimensional
expansion and/or contraction that would normally occur in a structural member
with variations in temperature.

-Small pits may form as a result of chemical reactions between the environment
and material, which serve as points of stress concentration and therefore as crack
nucleation sites.

αi=Thermal expansion, E = Young modulus

 THE S–N CURVE
1. A test apparatus for the fatigue test should be
designed to duplicate as nearly as possible the service
stress conditions (stress level, time frequency, stress
pattern, etc.).

2. Figure 8.18(a) shows a schematic diagram of a

rotating-bending test apparatus, commonly used for
fatigue testing.

The compression
and tensile stresses are
imposed on the specimen as
it is simultaneously bent and
rotated.
3. Tests are also frequently conducted using an
alternating uniaxial tension–compression stress cycle.

4. A series of tests are commenced by subjecting a

specimen to the stress cycling at a relatively large
maximum stress amplitude (σmax), usually on the order
of two-thirds of the static tensile strength; the number
of cycles to failure is counted.

5. The procedure is repeated on other specimens at

progressively decreasing maximum stress amplitudes.
6. Data are plotted as stress S versus the logarithm of
the number N of cycles to failure for each of the
specimens.

7. The values of S are normally taken as stress

amplitudes.

8. Two distinct types of S–N behavior are observed in

Figure 8.19

9. Three important parameters:

(a) Fatigue limit (endurance limit)
(b) Fatigue strength
(c) Fatigue life
10. Fatigue limit represents the largest value of
fluctuating stress that will not cause failure for
essentially an infinite number of cycles. Below fatigue
limit, fatigue failure will not occur.

11. For many steels, fatigue limits range between 35%

and 60% of the tensile strength.

12. Most nonferrous alloys (e.g., aluminum, copper,

magnesium) do not have a fatigue limit, in that the S–
N curve continues its downward trend at increasingly
greater N values
13. Fatigue strength : the stress level at which failure
will occur for some specified number of cycles (e.g.,
107 cycles).

14. Fatigue life Nf : the number of cycles to cause

failure at a specified stress level, as taken from the S–
N plot.

15. Several statistical techniques have been developed

to specify fatigue life and fatigue limit in terms of
probabilities. Example is shown in Figure 8.20.
16. Low-cycle fatigue : occurs at less than about 104 to
105 cycles.

17. High-cycle fatigue : relatively large numbers of

cycles are required to produce fatigue fail. High-cycle
fatigue is associated with fatigue lives greater than
about 104 to 105 cycles.
 FATIGUE CONTROL
1. Surface Treatments by:
(a) Imposing residual compressive stresses
(b) Case hardening
2. Residual compressive stresses are commonly
introduced into ductile metals mechanically by
localized plastic deformation within the outer surface
region.

3. Commercially, this is often accomplished by a

process termed shot peening.

4. Small, hard particles (shot) having diameters within

the range of 0.1 to 1.0 mm are projected
at high velocities onto the surface to be treated.
5. Case hardening is a technique by which both
surface hardness and fatigue life are enhanced for
steel alloys.

6. This is accomplished by a carburizing or nitriding

process whereby a component is exposed to a
carbonaceous or nitrogenous atmosphere at an
elevated temperature.

7. A carbon- or nitrogen-rich outer surface layer

(or“case”) is introduced by atomic diffusion from the
gaseous phase. The case is normally on the order of 1
mm deep and is harder than the inner core of
material.
 PREVENTION OF FATIGUE FAILURE
i. Variation in part or mechanical component sections are special regions where stress will
concentrate, which will affect their mechanical fatigue strength.
ii. A part’s geometry will also affect the speed at which that crack will propagate. A design that
favours the emergence of stress concentration areas, cross-section changes, presence of
keyways, holes or inset corners, will enable a crack to develop sooner.
iii. The design also has a significant influence in fatigue failure. Any geometric discontinuity acts
as a stress concentrator, and may become the point of origin of a crack due to fatigue. The
sharper the discontinuity, the more severe the stress build-up.
iv. Avoiding structural irregularities or revising the design, eliminating sharp changes along the
edges that lead to square edges will lead to a higher fatigue strength.
v. The dimensions of the part also play a part; increasing their size results in a lower fatigue
limit.
vi. Improve the surface finish by polishing, to prevent small scratches or grooves that appear
on a part’s surface due to cutting.
vii. Surface hardening via carburization and nitriding processes where a component is exposed
to a carbon or nitrogen-rich atmosphere at high temperatures. This layer is usually 1mm
deep and harder than the core’s material.
viii.Fluctuating or cyclic stress are another aspect to keep in mind. Their loads act for a great
number of cycles before failure occurs, being one of the main parameters present in material
fatigue strength.
ix. An intermediate tensile stress worsens fatigue-related performance in metals since it widens
the crack. Conversely, compression stress improves it.
x. Increase performance by means of residual compression stress on a thin surface layer. The
overall effect is that the probability of crack nucleation and fatigue failure is reduced.