Introducing the Analogic framework for business planning applications
Seminar on fatigue
1. Material Engg.
A
SEMINAR
ON
FATIGUE
Presented by- Guided by-
Mr.Sandip Wanave Prof. Nerkar sir
2. An Introduction
• in materials science, fatigue is the progressive and localized
structural damage that occurs when a material is subjected to
cyclic loading. The nominal maximum stress values are less
than the ultimate tensile stress limit, and may be below the
yield stress limit of the material.
• Fatigue occurs when a material is subjected to repeated
loading and unloading. If the loads are above a certain
threshold, microscopic cracks will begin to form at the
surface. Eventually a crack will reach a critical size, and the
structure will suddenly fracture. The shape of the structure
will significantly affect the fatigue life; square holes or sharp
corners will lead to elevated local stresses where fatigue
cracks can initiate. Round holes and smooth transitions or
fillets are therefore important to increase the fatigue strength
of the structure.
3. Fatigue life
• Fatigue life
• ASTM defines fatigue life, Nf, as the number of stress
cycles of a specified character that a specimen
sustains before failure of a specified nature occurs.[1]
• One method to predict fatigue life of materials is the
Uniform Material Law (UML).[2] UML was developed
for fatigue life prediction of aluminum and titanium
alloys by the end of 20th century and extended to
high-strength steels[3] and cast iron.[4] For some
materials, there is a theoretical value for stress
amplitude below which the material will not fail for
any number of cycles, called a fatigue limit, endurance
limit, or fatigue strength.[5]
4. Characteristics of fatigue
• In metals and alloys, the process starts with
dislocation movements, eventually forming
persistent slip bands that nucleate short cracks.
• Fatigue is a stochastic process, often showing
considerable scatter even in controlled
environments.
• The greater the applied stress range, the shorter
the life.
• Fatigue life scatter tends to increase for longer
fatigue lives.
• Damage is cumulative. Materials do not recover
when rested.
5. Characteristics of fatigue
• Fatigue life is influenced by a variety of
factors, such as temperature, surface finish,
microstructure, presence of oxidizing or inert
chemicals, residual stresses, contact (fretting),
etc.
• Some materials (e.g., some steel and titanium
alloys) exhibit a theoretical fatigue limit below
which continued loading does not lead to
structural failure.
6. The S-N curve
• A very useful way to visualize time to failure for a specific
material is with the S-N curve. The "S-N" means stress verse
cycles to failure, which when plotted uses the stress
amplitude, sa plotted on the vertical axis and the logarithm
of the number of cycles to failure. An important
characteristic to this plot as seen in Fig. 2 is the fatigue
limit.
• The significance of the fatigue limit is that if the material is
loaded below this stress, then it will not fail, regardless of
the number of times it is loaded. Material such as
aluminum, copper and magnesium do not show a fatigue
limit, therefor they will fail at any stress and number of
cycles. Other important terms are fatigue strength and
fatigue life. The stress at which failure occurs for a given
number of cycles is the fatigue strength. The number of
cycles required for a material to fail at a certain stress in
fatigue life.
8. Crack Initiation and Propagation
• Failure of a material due to fatigue may be viewed on a
microscopic level in three steps:
• Crack Initiation: The initial crack occurs in this stage.
The crack may be caused by surface scratches caused
by handling, or tooling of the material; threads ( as in a
screw or bolt); slip bands or dislocations intersecting
the surface as a result of previous cyclic loading or
work hardening.
• Crack Propagation: The crack continues to grow during
this stage as a result of continuously applied stresses
• Failure: Failure occurs when the material that has not
been affected by the crack cannot withstand the
applied stress. This stage happens very quickly.
9. Crack Initiation and Propagation
Figure 3
A diagram showing location of the three steps in a fatigue fracture
under axial stress
10. Crack Initiation and Propagation
• One can determine that a material failed by
fatigue by examining the fracture sight. A
fatigue fracture will have two distinct regions;
One being smooth or burnished as a result of
the rubbing of the bottom and top of the
crack( steps 1 & 2 ); The second is granular,
due to the rapid failure of the material. These
visual clues may be seen in Fig. 4:
11. Crack Initiation and Propagation
Figure 4
A diagram showing the surface of a fatigue fracture. Notice that the rough surface
indicates brittle failure, while the smooth surface represents crack propagation
13. Demonstration of Crack Propagation
Due to Fatigue
• The figure above illustrates the various ways in
which cracks are initiated and the stages that
occur after they start. This is extremely important
since these cracks will ultimately lead to failure of
the material if not detected and recognized. The
material shown is pulled in tension with a cyclic
stress in the y ,or horizontal, direction. Cracks can
be initiated by several different causes, the three
that will be discussed here are nucleating slip
planes, notches. and internal flaws. This figure is
an image map so all the crack types and stages
are clickable.
15. Infamous fatigue failures
• Following the King's fete celebrations at the Palace of Versailles, a train
returning to Paris crashed in May 1842 at Meudon after the leading
locomotive broke an axle. The carriages behind piled into the wrecked
engines and caught fire. At least 55 passengers were killed trapped in the
carriages, including the explorer Jules Dumont d'Urville. This accident is
known in France as the "Catastrophe ferroviaire de Meudon". The
accident was witnessed by the British locomotive engineer Joseph Locke
and widely reported in Britain. It was discussed extensively by engineers,
who sought an explanation.
• The derailment had been the result of a broken locomotive axle. Rankine's
investigation of broken axles in Britain highlighted the importance of
stress concentration, and the mechanism of crack growth with repeated
loading. His and other papers suggesting a crack growth mechanism
through repeated stressing, however, were ignored, and fatigue failures
occurred at an ever increasing rate on the expanding railway system.
Other spurious theories seemed to be more acceptable, such as the idea
that the metal had somehow "crystallized". The notion was based on the
crystalline appearance of the fast fracture region of the crack surface, but
ignored the fact that the metal was already highly crystalline.
16. Factors that affect fatigue-life
• Cyclic stress state: Depending on the complexity
of the geometry and the loading, one or more
properties of the stress state need to be
considered, such as stress amplitude, mean
stress, biaxiality, in-phase or out-of-phase shear
stress, and load sequence,
• Geometry: Notches and variation in cross section
throughout a part lead to stress concentrations
where fatigue cracks initiate.
• Material Type: Fatigue life, as well as the
behavior during cyclic loading, varies widely for
different materials, e.g. composites and polymers
differ markedly from metals.
17. Factors that affect fatigue-life
• Residual stresses: Welding, cutting, casting, and other manufacturing
processes involving heat or deformation can produce high levels of tensile
residual stress, which decreases the fatigue strength.
• Size and distribution of internal defects: Casting defects such as gas
porosity, non-metallic inclusions and shrinkage voids can significantly
reduce fatigue strength.
• Grain size: For most metals, smaller grains yield longer fatigue lives,
however, the presence of surface defects or scratches will have a greater
influence than in a coarse grained alloy.
• Environment: Environmental conditions can cause erosion, corrosion, or
gas-phase embrittlement, which all affect fatigue life. Corrosion fatigue is
a problem encountered in many aggressive environments.
• Temperature: Extreme high or low temperatures can decrease fatigue
strength.
18. Stopping fatigue
• Fatigue cracks that have begun to propagate can
sometimes be stopped by drilling holes, called
drill stops, in the path of the fatigue crack.[14] This
is not recommended as a general practice
because the hole represents a stress
concentration factor which depends on the size
of the hole and geometry, though the hole is
typically less of a stress concentration than the
removed tip of the crack. The possibility remains
of a new crack starting in the side of the hole. It is
always far better to replace the cracked part
entirely.
19. Stopping fatigue
• Material change
• Changes in the materials used in parts can also improve
fatigue life. For example, parts can be made from better
fatigue rated metals. Complete replacement and redesign
of parts can also reduce if not eliminate fatigue problems.
Thus helicopter rotor blades and propellers in metal are
being replaced by composite equivalents. They are not only
lighter, but also much more resistant to fatigue. They are
more expensive, but the extra cost is amply repaid by their
greater integrity, since loss of a rotor blade usually leads to
total loss of the aircraft. A similar argument has been made
for replacement of metal fuselages, wings and tails of
aircraft.[15
20. Fatigue Test
• A method for determining the behavior of materials under
fluctuating loads. A specified mean load (which may be
zero) and an alternating load are applied to a specimen and
the number of cycles required to produce failure (fatigue
life) is recorded. Generally, the test is repeated with
identical specimens and various fluctuating loads. Loads
may be applied axially, in torsion, or in flexure. Depending
on amplitude of the mean and cyclic load, net stress in the
specimen may be in one direction through the loading
cycle, or may reverse direction. Data from fatigue testing
often are presented in an S-N diagram which is a plot of the
number of cycles required to cause failure in a specimen
against the amplitude of the cyclical stress developed.
21. Fatigue Test
FATIGUE TEST
• OBJECTIVE:
• The main objectives of this experiment are:
•
• 1) Perform the fatigue test on the given specimens using
the Fatigue tester MT 3012 to predict the fatigue life.
•
• 2) Determine the safe stress level for the specimens if a
fatigue life of 1,000,000 reversals had to be withstood.
•
• APPARATUS REQUIRED:
• Fatigue tester MT3012, Vernier caliper Aluminum
specimens.,
22. :
DESCRIPTION OF THE APPARATUS
• fatigue tester MT 3012 shown in Fig.1.is driven by an induction squirrel
cage motor at 3000rpm. Power supply provided is 220V single phase. The
motor is connected on one side to a counter mechanism, which can record
7 figure numbers. Attached to the shaft at the other end is a fixture. The
loading device consists of a spherical ball bearing and a micro switch,
which automatically switches off the motor when the fracture occurs.
• The apparatus is supplied with a recommended standard specimen. The
bending stress for a load P (N) is:
• Where, L…distance from neck to specimen’s contact point with
bearing
• d…Diameter of the neck
• P…Load applied (measured by digital read out)
•
• By turning the loading wheel clockwise the loading on the test piece can
be increased. A cell load which a digital read out measures the loading
value. The fatigue tester, which is designed to be placed on a bench, is
very stable on 8 feet, weighing 24kg. Dimensions are 980x280x460 mm.
25. EXPERIMENTAL PROCEDURE:
• As fatigue fracture experiments may run on for half an hour or so the usual procedure is for each group in a
class to set up and start two aluminum specimens and for all the results to be shared at the end. The load
sets will be provided in the lab session.
• 1. Measure the diameter at the neck of the specimen and inspect the surface roughness.
• 2. Slide one end of the specimen into the adapter at the shaft end and slide the other end into the
adapter at the load end.
• 3. Measure the distance from the neck to the specimen’s contact surface with the bearing.
• 4. Apply the given load. Check with the lab instructor about loading the specimen in order to
have a precise bending loading condition.
•
• Don't put any excessive force on the loading arm!!! It will damage the specimen.
•
• Results from other load cases will be collected an made available to each group after all groups have
completed the experiment.
•
• 4. Set the revolution counter to zero and start the motor.
•
• 5. Normally the test terminates itself through the fracture of the specimen opening the micro switch and
hence stopping the motor. As the onset of fracture approaches the specimen will bend more, and this may
open the micro switch before complete fracture occurs. In this case move the micro switch down slightly and
restart the motor.
•
• 6. Collate the results and plot them as they occur on a graph of stress range, s, against logl0 number of
reversals N. Note that in the case of a rotating cantilever the stress range is twice the applied bending stress.
26. RESULTS:
• After obtaining the results for your load cases
and getting the results of the remaining cases
from other groups, plot σ against logl0 N on a
suitable graph paper and look for best-fit lines
and also determine the safe stress level if a
fatigue life of 1,000,000 reversals had to be
withstood. Also discuss the ruptured cross-
section and identify the cause of the rupture
and analyze the factors, which will affect the
results.
