The document discusses mechanical overload failures, which occur when stresses exceed the load-bearing capacity of a material, leading to either ductile or brittle fractures. It highlights the mechanisms of failure, including manufacturing defects and the impact of temperature and loading rates on material behavior. Additionally, it examines the influence of stress concentrations and size on the likelihood of failure in materials.

1. TRIPURA UNIVERSITY
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MATERIAL SCIENCE AND ENGINEERING
ABHIJIT DEBNATH & SANGITA
DAS
1ST SEMESTER
FAILURE DUE TO OVERLOADING

2. INTRODUCTION
 The failure or fracture of a product or component as a
result of a single event is known as mechanical
overload. It is a common failure mode, and may be
contrasted with fatigue, creep, rupture, or stress
relaxation.
 The terms are used in structural engineering when
analysing product failure.
 Failure may occur because either the product is weaker
than expected owing to a stress concentration, or the
applied load is greater than expected and exceeds the
normal strength, shear strength or compressive
strength of the product 2

3.  Overload failures refer to the ductile or brittle
fracture of a material when stresses exceed the
load-bearing capacity of a material.
BRITTLE FAILURE
 In a brittle overload failure, separation of the two
halves isn’t quite instantaneous, but proceeds at a
tremendous rate, nearly at the speed of sound in
the material. The crack begins at the point of
maximum stress, then grows across by cleavage of
the individual material grains. One of the results of
this is that the direction of the fracture path is
frequently indicated by chevron marks that point
toward the origin of the failure. 3

4. FATIGUE FAILURES
 In a fatigue failure, an incident of a problem can
exceed the material’s fatigue strength and initiate a
crack that will not result in a catastrophic failure for
millions of cycles. We have seen fatigue failures in
1200 rpm motor shafts that took less than 12 hours
from installation to final fracture, about 830,000
cycles. On the other hand, we have also monitored
crack growth in slowly rotating process equipment
shafts that has taken many months and more than
10,000,000 cycles to fail.
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5. MECHANISMS OF FAILURE
Manufacturing defect
 Flaw introduced during manufacturing process
Material overload
 Inappropriate consumer use of the material
 Loading material beyond specification
Normal wear through use
 Fatigue, corrosion,creep,etc.
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6. FAILURE OF MATERIAL OCCUR DUE TO
 Strength
Strength is the most obvious determinant of a
metal's behavior when it is overloaded. In general,
soft tough metals will be ductile. Harder, stronger
metals tend to be more brittle. The relationship
between strength and hardness is a good way to
predict behavior. Mild steel (AISI 1020) is soft and
ductile; bearing steel, on the other hand, is strong
but very brittle. The relationship between strength
and hardness of steel.
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7.  Temperature
Temperature has a significant affect on the ductility of
metals. Low temperature decreases ductility, while high
temperature increases it. When a part is overloaded at
low temperatures, a brittle fracture is more likely to
occur. At high temperatures, a more ductile fracture is
likely to occur.
 Lower strength steel (less carbon and alloys) maintains
ductility (toughness) as temperature decreases. When
steel strength increases (more carbon and alloys),
ductility drops more quickly as temperature decreases.
 The dominant factor causing brittle metals to become
more ductile is high temperature.
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8.  Rate of Loading
When an overload happens slowly, there is enough time
for microscopic movements in the metal to occur. The
metal deforms plastically before finally breaking. Sudden
impact frequently causes a ductile material to behave in
a brittle manner. There is not enough time for
microscopic movements to take place. Brittle behavior is
often seen in a catastrophic failure when the overload is
very sudden.
 Stress Concentrations
Changes in geometry, such as keyways, diameter
changes, notches, grooves, holes and corrosion, result
in localized areas where the stress is much higher than
in the adjacent region of a part.
 In regions where there is no stress concentration, it is
easier for microscopic movements to occur. In this case,
the metal behaves in a ductile manner. A stress
concentration does not allow microscopic movements,
so brittle fracture is more likely. 8

9.  Size
Thin parts are more likely to fail in a ductile manner
when overloaded. Large or thicker parts will behave
more like a brittle metal when overloaded because
the geometry does not allow stress to be evenly
distributed. Figure 3 shows the effect of size.
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11. THANK YOU
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