Abstract:
This paper deals with fracture
mechanics clearly distinguishing between crack and fracture, types of fracture
etc. Here we take into study, an incidence of fracture that occurred in the rear wheel of Karizma bike and
analysing the effect of dynamic fracture in that incidence and influence of
fatigue on it. Discussion of preventive measures to be taken to avoid the
fracture is also discussed.
Contents:
- Introduction
to fracture mechanics.
- Definition
of crack and fracture.
- Types
of fracture.
- Problem
definition.
- Analysis
of Dynamic fracture and fatigue.
- Prevention
of fracture.
- Conclusion.
- References.
Introduction:
Through the ages the application of
materials in engineering design has posed difficult problems to mankind. In the
Stone Age, the problem was mainly in shaping of the material. In the early days
of the Bronze Age and the Iron Age the difficulties were both in production and
shaping. For many centuries metal working was laborious and extremely costly.
Estimates goes that the equipment price of a knight and horse in the 13th
century was of the equivalent price of a centurion tank in Second World War.
With the improving skill of metal working,
applications of metal in structures increased progressively. Then it was
experienced that structure built of these materials did not always behave
satisfactorily and unexpected failures occurred. The vastly increased use of
metals in the 19th century caused number of accidents and casualties
to reach unknown levels. Some of these accidents were due to poor design, but
it was gradually discovered that material deficiencies in the form of pre-existing
flows could initiate Cracks and Fractures.
The failure often occurred under
conditions of low stress(several ships failed suddenly while in the harbor)
which made them seemingly inexplicable. As a result extensive investigation
were initiated in many countries and especially in USA
What is crack?
Partial breaking of a
material is called crack. Initiation of a crack begins due to repeated cyclic
loading which causes localized changes in the structure of the material further
leading to fracture.
What is fracture?
A fracture is the local separation of the
body into two or more pieces under the action of stress. The word fracture is often applied to bones of
living creatures, or to crystals or crystalline materials, such as gemstones or
metal.
Types of Fracture:
Fractures can be mainly
classified into two types they are as below:
1.
Ductile
fracture
2.
Cleavage
fracture
1.
Cleavage Fracture:
Cleavage fracture is the most brittle
form of fracture that can occur in crystalline materials. Brittle cleavage
fractures in ships, bridges and tanks have made it a notorious type of failure.
Cleavage Fracture is encountered in cases like low temperatures and high strain
rates. Cleavage Fracture of metals occurs by direct separation along
crystallographic planes due to simple breaking of atomic bonds. Its main
characteristics are that it occurs with a particular crystallographic plane.
2.
Ductile Fracture:
Fracture occurring under
the single application of continuously increasing load can be either brittle
cleavage fracture or fracture associated with plastic deformation, which is
essentially ductile the most familiar type of fractures by overload in tension,
which produces the classic cup and cone fracture.
Dynamic Fracture:
Fracture
observed in the rear wheel of Karizma bike
How does it happen?
We have seen cases in which fracture
has occurred in static condition after prolonged loading conditions and stress.
But here is another case where in the fracture has occurred in the dynamic
state or in simple words the fracture has occurred during motion.
A bike which looks as if its normal but just
view the crack in the rear wheel, its due to Dynamic Fracture which has occurred when the bike was in motion.
Now let’s analyse how it has actually happened?
Here instability occurs due to crack
extension, the elastic energy release rate remains larger than the crack
resistance “R”. The surplus of released energy (G-R) can be converted into
kinetic energy .The kinetic energy is associated with rapid movement of the
material at each side of the crack path, during the passage of a high velocity crack
the difference between “G” and “R” determines how much energy can become
available as kinetic energy and consequently it governs the speed at which the
crack will propagate through the material.
Both G and R represent
the energies associated with crack extension. The crack resistance is the
function of the plastic behavior of the material at the crack tip and of its
fracture characterstics.The yield stress increases and the fracture strain
decreases at higher strain rates at the tip of crack moving at high velocity
the strain rates are very high, and it must be expected that the material
behaves in a more brittle manner the higher crack speed. Fractures are analyzed using the latest scanning electron
microscopy (SEM) and other metal testing techniques.
Analysis:
- It
is fatigue loading problem.
- Fatigue
loading is given by the equation.
F=mrw2
Where,
m=lumped mass of wheel body
at crack.
r=radius of crack location.
w=angular acceleration of
the wheel.
I
Prevention:
Ø Fractures can be prevented
by proper quality control techniques and suitable design.
Ø Methods like Non-
Destructive testing and X –ray analysis can prevent fracture.
Ø Excessive loading should be
avoided and if possible repeated cyclic loadings should be minimized.
Ø Manufacturing defects like
blow holes should be avoided and suitable manufacturing process should be
adopted.
Ø Research and Developnnent
will further provide scope in prevention of fracture.
Conclusion:
Fracture is an
unpredictable phenomena,the only solution to this is by proper design and using
of modern manufacturing techniques. Quality of material plays an important role
in robustness of the product.
References:
- “Elementary
engineering fracture mechanics” by David Broek.
- www.wikipedia.com
- www.howstuffworks.com
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