1. Dr. Alagiriswamy A A, (M.Sc, PhD, PDF) Asst. Professor (Sr. Grade),
Dept. of Physics, SRM-University,
Kattankulathur campus,
Chennai
MECHANICS OF MATERIALS
UNIT V
Lecture 2
November 22, 2018November 22, 2018

2. Outline of the presentation
Features of ductile/brittle materials
Destructive testing & explanations
Fundamental mechanical properties
Stress-strain relation for
different engineering materials
Examples
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3. Stress measures the force required to deform or break a material
Ductility; the property of a metal by virtue of which it can be drawn into an elongated state before RUPTURE takes place. Percentage of elongation =
Stress measures the force required to deform or break a material
s = F/A
Strain measures the elongation for a given load
e = (L-Lo)/Lo
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4. Issues of ductile material
Materials
Percentage of Elongation
Low-Carbon -37%
Medium-Carbon 30%
High-Carbon- 25%
A ductile material is one with a large Percentage of elongation before failure
Ductility increases with increasing temperature.
Easily drawn into wire
Moldable,
Easily stretchable without any breakage
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5. Quiz time
Ductility is the ability of a metal to ________ before it breaks.
A: Bend
B: Stretch or elongate
C: Be forged
D: Be indented

6. Features of Brittle material
A specified amount of stress applied to produce desired strain
Grey cast iron (example)
A brittle material is one with a low % of elongation before failure
Brittleness increases with pressure
≤ 5 % elongation
Dislocations/defects/imperfections could be the probable reasons
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7. Fundamental Mechanical Properties
(i)Tensile strength
(ii) Hardness
(iii) Impact strength
iv) fatigue
(v) Creep

8. Destructive testing (i)Tensile strength (Alloy steel ; 60-80 kg/mm2)
provides ultimate strength of a material
maximum withstandable stress before breakage
just an indication of instability regime
provides the basic design information to the test of engineers
Yield strength (elastic to plastic deformation)
Ultimate strength (maximum stress that can withstand)
Breaking strength (strength upto the rupture)
November 22, 2018November 22, 2018

9. Destructive testing Yes, you could use AFM tip as a nanoindenter
(ii) Hardness factor
Ability of a material to resist before being permanently damaged
Direct consequences of atomic forces exist on the surface
This property is not a fundamental property (like domain boundary)
Measure of macro/micro & nano- hardness factors provide the detailed analyses
Hardness
Measurement Methods
Rockwell hardness test
Brinell hardness
Vickers
Knoop hardness
Shore
Yes, you could use AFM tip as a nanoindenter
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10. Destructive testing Brinell, Rockwell and Vickers hardness tests;
to determine hardness of metallic materials to check quality level of products, for uniformity of sample of metals, for uniformity of results of heat treatment.
Knoop Test;
relative micro hardness of a material
Rock well hardness;
a measure of depth of penetration
Shore scleroscope ;
in terms of the elasticity of
the material.
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11. Vickers hardness tests
Microhardness test involves using a diamond indenter to make a microindentation into the surface of the test material, the indentation is measured optically and converted to a hardness value
Metalography; viewing of samples through high powerful microscopes
HV = 1.854(F/D2);
F is the force applied,
d2 is the area of the indentation
November 22, 2018November 22, 2018

12. The _______ type hardness test leaves the least amount of damage on the metals surface.
A: Rockwell
B: Brinell
C: Scleroscope
D: Microhardness

13. The ability of a material to withstand shock loading
Destructive testing
Try to pull it --
tensile strength
Try to compress it --
compressional strength
Try to bend (or flex) it --
flexural strength
Try to twist it --
torsional strength
Try to hit it sharply and suddenly -- (as with a hammer)    
impact strength
Affected by the rate of loading, temperature variation in heat treatment,
alloy content
Impact Strength
The ability of a material to withstand shock loading
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14. Destructive testing
(i)Fatigue
Fatigue is the name given to failure in response to alternating loads (as opposed to monotonic straining
expressed in terms of numbers of cycles to failure (S-N)
Occurs in metals and polymers but rarely in ceramics.
Also an issue for “static” parts, e.g. bridges.
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15. November 22, 2018November 22, 2018

16. Destructive testing (i)Fatigue
Repeated/cyclic stress applied to a material
An important mode of a failure/disaster
Loss of strength/ductility
Increased uncertainty in service
SEM Fractograph (Aluminum alloy)
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17. Will you be embarrassed by reviving “Who you are??????????”
You are the message (based on several consequences)
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18. Factors affecting Fatigue
What causes fatigue?
Fatigue is different for every person. Here are some causes of fatigue:
Chemotherapy/Pain
Sleep problems/Radiation
Certain medicines/Lack of exercise
Surgery/Not drinking enough fluids
Not being able to get out of bed/Nausea
Eating problems
Surface roughness/finishing
thermal treatment
Residual stresses
Strain concentrations
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19. Undergo a time-dependent
Creep
Adopts this kind of relationship
property of a material by virtue of which it deforms continuously under a steady load
slow plastic deformation (slip) of material
occurs at high temperatures.
Iron, nickel, copper and their alloys exhibited this property at elevated temperature.
But zin, tin, lead and their alloys shows creep at room temperature.
Undergo a time-dependent
increase in length

20. Different stages of creep
1) Primary creep is a period of transient creep. The creep resistance of the material increases due to material deformation. Predominate at low temperature test such as in the creep of lead at RT.
2) Secondary creep provides a nearly constant creep rate. The average value of the creep rate during this period is called the minimum creep rate.
3) Tertiary creep shows a rapid increase in the creep rate due to effectively reduced cross-sectional area of the specimen
Logarithmic Creep (low temp)
Recovery Creep (high temp)
Diffusion Creep (very high temperatures)
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21. Factors affecting Creep
Dislocations
Slips
Grain boundaries
Atomic diffusion
Heat Treatment
Alloying
Grain size
Types of stress applied
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22. Types of Fracture Brittle Fracture Ductile Fracture Fatigue Fracture
Fracture; a disaster occurs after the application of load,
Local separation of regions
Origin of the fracture (in two stages):
initial formation of crack and
spreading of crack
Types of Fracture
Brittle Fracture
Ductile Fracture
Fatigue Fracture
Creep Fracture
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23. Fracture
Depending on the ability of material to undergo plastic deformation before the fracture two fracture modes can be defined - ductile or brittle
• Ductile fracture - most metals (not too cold):
Extensive plastic deformation ahead of crack
Crack is “stable”: resists further extension unless applied stress is increased
• Brittle fracture - ceramics, ice, cold metals:
Relatively little plastic deformation
Crack is “unstable”: propagates rapidly without increase in applied stress
Ductile fracture is preferred in most applications
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24. Different stages of Fracture
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25. Equation governing fracture mechanisms
 =
Where,
e is half of the crack length,
 is the true surface energy
E is the Young's modulus.
the stress is inversely proportional to the square root of the crack length.
Hence the tensile strength of a completely brittle material is determined by the length of the largest crack existing before loading.
For ductile materials (additional energy term p involved, because of plastic deformations
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26. The Ductile – Brittle Transition
Surface energy increases as temperature decreases.
The yield stress curve shows the strong temperature dependence

27. On recalling/revisiting
Make sure you understand language and concepts:
Roughness/ductility/Brittleness/hardness
Isotropy/anisotropy/orthotropy/elasticity
Resilience/endurance
Brittle fracture
Corrosion fatigue
Creep
Dislocation/slip
Ductile fracture
Ductile-to-brittle transition
Fatigue /Fatigue life
Fatigue limit/Fatigue strength
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