Mechanical Testing : Testing Of Materials

Ns
MECHANICAL TESTING
VII SEMESTER
[R 2017 - Open Elective]
OML751 - TESTING OF MATERIALS
S.Thirumalvalavan, Assistant Professor
Department of Mechanical Engineering
E-Mail : thirumalbemech@ gmail.com

Ns
UNIT-IT : MECHANICAL TESTING
> Introduction to Mechanical testing,
» Hardness test (Vickers, Brinell, Rockwell),
> Tensile test,
>Impact test (Izod, Charpy) — Principles, Techniques, Methods,
Advantages and Limitations, Applications.
> Bend test,
» Shear test,
>Creep and Fatigue test - Principles, Techniques, Methods,
Advantages and Limitations, Applications.
For Basics, refer: https://www.youtube.com/watch?v=NYRILITXUFIM

Ns
UNIT -
INTRODUCTION TO MECHANICAL TESTING
* Mechanical properties are obtained by mechanical testing.
* The study of deformation and fracture in materials is called mechanical behavior
of materials.
* Mechanical testing is used for developing design data, maintaining quality
control, assisting in alloy development programs and providing data in failure
analysis.
* Mechanical testing is usually destructive and requires test specimens of the
material to be machined or cut to the specific shape required by the test method.
¢ Measurement of the characteristics and behavior of such substances as metals,
ceramics, or plastics under various conditions. The data thus obtained can be
used in specifying the suitability of materials for various applications.

Malleability
Mechanical Properties of
Material
Brittleness
Strength &
Hardness Toughness

Ns
(i) Hardness test
(ii) Tensile test
(iii) Impact test
(iv) Bend test
(v) Shear test
(vi) Creep test
(vii) Fatigue test

Steel BallES Q
O Hardness is the resistance of material to permanent deformation
of the surface. It is the surface property of a metal, which gives it
the ability to resist being permanent surface deformation when a
load or stress is applied.
O The hardness of a surface of the material is, a direct result of inter
atomic forces acting on the surface of the material.
O Hardness is not_a fundamental property of a material, but a
combined effect of compressive, elastic and plastic properties
relative to the mode of penetration, shape of penetration etc.
O Simply, Hardness may be defined as the ability of a material to
resist scratching, abrasion, cutting or penetration.

Ns
Classification of Hardness
Depending on the manner in which the hardness test is conducted, hardness
may be classified as follows,
¥ Indentation hardness — Ex: Brinell, Meyer, Rockwell, Knoop..
Y Rebound hardness — Ex: Shore scleroscope hardness test
¥ Scratch hardness — Ex: Mohs scale (used in the field of mineralogy)
Y Wear or abrasion hardness — Ex: File hardness
¥ Cutting hardness — Ex: Bauer drill test (To find machinability of the
materials)

Ns
O Hardness measurement can be in Macro, Micro & Nano —
scale according to the forces applied and displacements
obtained.
O Measurement of the Macro-hardness of materials is a quick and
simple method of obtaining mechanical property data.
O The Macro-hardness measurement will be highly variable and
will not identify individual surface features. It is here that
micro-hardness measurements are appropriate.

Ns
O Micro hardness is the hardness of a material as determined by
forcing an indenter into the surface of the material under load,
usually the indentations are so small that they must be
measured with a microscope.
O Micro hardness measurements are capable of determining the
hardness of different micro constituent with in a structure.
O Nano hardness tests measure hardness by using indenter, on
the order of nano scale.
O These tests are based on new technology that allows precise
measurement and control of the indenting forces and precise
measurement of the indentation depth.

Macro-hardness tests
(Rapid routine hardness
measurements)
Micro-hardness tests
(Hardness of coatings, surface
hardness, or hardness’ of
different phases in the multi
phase material is measured)
Nano-hardness test
Rockwell, Brinell,
Vickers test
Micro-Vickers test,
Knoop test
SON to 30000N
10 to 1000gf
1 nano-Newton

Ns
Selection criteria of Hardness tester
Main elements to consider before choosing a hardness tester
1.Test load
2.Hardness range
3.Accuracy level
4.Adaptability of the device

Ns
The following are the hardness test methods
O Rockwell hardness test
O Brinell hardness
Oo Vickers

Ns
O The hardness is measured from an indentation produced in the
component by applying a constant load on a specific indenter in
contact with the surface of the component for a fixed time.
O An indenter is pressed into the surface of the material by a slowly
applied known load, and the extent of the resulting impression is
measured mechanically or optically.
O A large impression for a given load and indenter indicates a soft
material, and a small impression indicates a hard material.

Ns
_ HardnessMeasurement— Rockwell Hardness Test
O Rockwell hardness test is probably the most widely used methods
of hardness testing.
O The principle of Rockwell test differs that of the others in that the
depth of the impression is related to the hardness rather than the
diameter or diagonal of the impression as shown in fig.

Rockwell Hardness Scales
Total Material for
Scale Symbol Indenter indenting which the scale
load is used
A HRA Diamond cone 60 kg Thin hardened
steel strip
B HRB 1/16 inch diameter 100 kg Mild steel and
steel ball non-heat treated
medium carbon
steels
= ——_—____ 7 =< is usea
Cc TRC Diamond cone , 150 ke Hardened and
tempered stec!s
1 sat and alloy steels
D HRD Diamond cone 100 ke Case hardened
}— Steels
E HRE 1/8 inch diameter 100 kg Cast iron.
Steel ball aluminium alloys
| and magnesium
| alloysrT
| F HRF 1/16 inch diameter 60 ke Copper and brass
| steel ball
| G HRG 1/16 inch diameter 150 ke Bronzes. gun
steel ball metal and
}
beryllium copper
| oH HRH 1/8 inch diameter 60 kg Soft aluminium
steel ball and
thermoplastics
K HRK 1/8 inch diameter 150 ke Aluminium and
steel ball magnesium
alloys
E. HRL 1/4 inch diameter 100 kg Thermeptastics
stecl ball
R HRR 1/2 inch diameter 60 ke Ver son
stec! ball thermoplastics 15

Most commonly used scales are,
OB-Scale (1/16 inch diameter steel ball Rockwell Hardness Scales
Diamond cone (Brale)
indenter ; 100kg load), used to measure Load Scale
—s 60kg Ra
the hardness (HRB) of non-ferrous 100kg R,
150kg Re.
metals.
: : Steel ball, 1/16 inch dia.
OC-Scale (120°diamond cone indenter, Load Scale
O 60kg Ry
100kg R
called a BRALE ; 150kg load), used to i50ke R
measure the hardness (HRC) of steels.
Steel ball, 1/8 inch dia.
Load Scale
O 100kg Ri

Ns
_ Testing Procedure - Rockwell Hardness Test
Y The material to be tested is held on the anvil of the machine
v The test piece is raised by turning the hand wheel, till it just touches the
indenter
Y A minor load of 10kg is applied to seat the specimen. Then the dial
indicator is set at zero.
Y Now the major load (100 kg for B-scale or 150 kg for C scale) is
applied to the indenter to produce a deeper indentation.
v After the indicating pointer has come to rest, the major load is removed.
Y With the major load removed, the pointer now indicates the Rockwell
hardness number on the appropriate scale of the dial.

Ns
Rockwell Hardness Test - Advantages
v Very simple to use
v Hardness can be read directly in a single step
Y Each measurement requires only a few seconds
Y Zit is suitable for routine tests of hardness in mass
production
Y It can be used on metallic materials as well as on plastics.

Ns
Rockwell Hardness Test - Limitations
Y Not accurate as the Vickers test
Y That’s why the Vickers test is usually preferred for research and
development works.

Rockwell hardness tester
HARONESS NUMBER
DAIL
INDENTOR
HOLDER
ANVIL
HAND WHEEL
eft— LOADING LEVER
Load 15, 30, 45 kg
Spherical diamond-tipped
cone of 120° angle
/ 3
|
“1
Depthofimpression
Load 15, 30, 45 kg
Depthofimpression

From Youtube, Some useful links
Working video: https://www.youtube.com/watch?v=ruLHbGtycho
https://www.youtube.com/watch?v=WMRcK9miefA
Advanced m/c - https://www.youtube.com/watch?v=G2JGNIIvNC4
7
5 Major load Major
; not yet load
’ applied withdrawn
. : Minor
F Major Mirror Major
' load load load load —~
applied left rawnnot yet lied
‘ applie
applied /Stee! ball
indenter
Test piece now has
a firm seating due
Test piece to minor load
Elevating —>S
screw ——| ———
[ yf Y
21

Ns
_—-Brinell Hardness Test
Y One of the earlier standardised methods of measuring hardness was
the Brinell test.
Y In Brinell test, a hardened steel ball indenter is forced into the
surface of the metal to be tested.
Y The diameter of the hardened steel (or tungsten carbide) indenter is
10mm.
Y Standard loads range between 500kg and 3000kg in 500kg
increments.
Y During a test, the load is maintained constant for 10 to 15 seconds.

Brinell Hardness Test
Load
Plunger
Ball
, Specimen
Anvil
Wy
®
Hand wheel
_— Screw
TN
23

Ns
Brinell Hardness Test
(a) (b) (c)

Y The diameter of the resulting impression is measured with the help of a calibrated
microscope.
Y Brinell Hardness Number (BHN): The measured diameter is converted into the
equivalent Brinell hardness number using the following relation.
v BHN = Load on the ball / Area of indentation of steel ball
Y BHN =————
M? (p-vD?—?
Where,
P — Load applied on indenter in kg
D — Diameter of steel ball indenter in mm
d — Diameter of ball impression in mm
If the BHN value is higher, then the material is said to be harder. If BHN is less, then
the metal is soft.
25

Load Diameter of | Duration Metals
application ball
3000 kg 10 mm Tron, steel and alloys
Seconds having hardness similar to
750 kg 5 mm steel
500 kg 10 mm 30 seconds Copper, Annealed brass
and magnesium alloys
etc.,
1000 kg 10 mm 15 seconds Gun metal/ Bronze and
cold worked brass etc.,
26

Ns
__HardnessMeasurement— Brinell Hardness Test _
Y rare F
¥ rare F
[ndenter
Diameter » fo BHN ==
Test method consists of indenting the test material with a
10 mm diameter hardened steel or carbide ball subjected to
a load of 3000 kg.
For softer materials the load can be reduced to 1500 kg or
500 kg to avoid excessive indentation.
2/7

Ns
Full load is normally applied for 10 to 15 seconds in the case of
iron and steel and for at least 30 seconds in the case of other
metals.
The diameter of the indentation left in the test material is
measured with a low powered microscope.
The Brinell harness number is calculated by dividing the load
applied by the surface area of the indentation.
F
2 p(v yom
)
BHN =

Ns
___ Brinell Hardness Test - Limitations
Y It cannot be used on very hard or very soft materials
v The test may not be valid for thin specimens
Y The test is not be valid for case-hardened surfaces.

Ns
Working Videos - BHN
From youtube, Some useful links
https://www.youtube.com/watch?v=dMBZDapS-50
https://www.youtube.com/watch?v=RJXJpeH78iU
https://www.youtube.com/watch?v=u-LauuHeHSM

Ns
LJ The Vickers hardness test is similar to Brinell test, with a square-based diamond pyramid
being used as the indenter.
L) As in the Brinell test, the indenter is forced into the surface of the material under the action
of a static load for 10 to 15 seconds.
LJ The standard indenter is a square pyramid shape with an angle of 136° between opposite
faces. This angle was chosen because it approximates the most desirable ratio of indentation
diameter to ball diameter in the Brinell hardness test.
LJ Because of shape of the indenter, this test is frequently called the diamond-pyramid hardness
test.
LJ An advantage of the Vickers test over the Brinell test is that the accuracy is increased in
determining the diagonal of a square as opposed to the diameter of a circle, as show in
figure,

136° between
opposite faces
Ys
2F sin 136° |
nv=- J ?
Test method consists of indenting the test material
with a diamond indenter, in the form of a right
pyramid with a square base and an angle of 136
degrees between opposite faces.
Full load is normally applied for 10 to 15 seconds.
The two diagonals of the indentation left in the
surface of the material after removal of the load are
measured using a microscope and their average
calculated.
The area of the sloping surface of the indentation is
calculated.
Vickers hardness is the quotient obtained by
dividing the load by the square mm area of
indentation.

Brinell
Vickers
Pp indenter indenter
Component
surface _
' '
' '
' '
' '
‘ '
' '
A!' —— Plan
view of f ’ view of
impression i , ; i ip Diagonal = mpression
' ‘
(a) Brinell indenter and impression (5)
136° :
(c)
(b) & (c) Vickers indenter and impression 33

Hardness Measurement
Vickers Method Brinnell Method
Steel sphere
indenter ~——S
Size of depression
created by the tool
/ iS ameasure of
~ hardness a
y Pulishid entries
of widget
Hardness: Load/AB Hardness: Load/C*
Diamond |
indenter ~~~
B
34

Ns
L) The diamond-pyramid hardness number (DPH) or Vickers hardness number (VHN or VPH)
is defined as the applied load divided by the surfaces area of indentation.
2P sin @/2_ 1.8544 P
~ D (O=136°)LU VHN = Applied load / Surface area of impression =
Where,
P — Applied load in kg
@ — Angle between the opposite faces of diamond = 136°
D-— Mean diagonal length in mm

EYE PIECE
LIGHT R :
SOURCE ENCODE
WORKING TABLE
LCD MONITOR
POWER WORKING
SOURCE PANEL
36

Ns
LJ The diagonals of the square indentation can be measured more accurately than the
diameters of the circles.
LJ This method is suitable for hard materials as well as for soft materials
LJ The Vickers indenter is capable of giving geometrically similar impression with
different loads. Thus, the hardness number is independent of the load applied.
Limitations :
LJ The impression is very small and also it requires careful surface preparation of the
specimen
LJ It takes a relatively long time to perform a Vickers hardness test.

Ns
Vickers Hardness Test — Working Video
From Youtube, some useful links
https://www.youtube.com/watch?v=7Z900Z7C2jI
https://www.youtube.com/watch?v=0oQMNUAkzo7w
Vickers Micro Hardness Tester —
https://www.youtube.com/watch ?v=riz1Duu3Nss
https://www.youtube.com/watch?v=qhSbCLw9npo

Ns
> Knoop Hardness Test: The quantitative determination of hardness on
materials over a very small area under the application of a constant static
load, the diamond indenter knows as the “knop” indenter and hardness
test is called KNOOP hardness test (micro hardness).
Nano Hardness test : Nano hardness or Nano indentation tests, in which
the magnitudes of applied forces are usually in the milli-newton range,
may be as low as 0.1 mN. Majority of nano indentation tests aim to
obtain young’s modulus along with measurement of hardness of the
specimen material from the load displacement data obtained in tests.

Micro Hardness Tester
ce
Test sei handwheel Quautest %
a

Nano Hardness Tester
Reference : http://home.iitk.ac.in/~kbalani/vl-kb/nano.html
Principle
> State-of-the-art technique
» Nano-mechanical properties of different phases
» Continuous measurementof force (in the range of mN) and displacement(in range of nm).
>» Material can be in any form (thin film or small volume)
» Advantage: Probe the surface and map its properties on spatially resolved basis.

Comparison of hardness tests
Test
—T
Indenter
Typical
applications
Brinell (HB) 1— 10mm
diameter steel or
tungsten carbide
ball
Upto 3000 keg for
steel ball,
depending upon
P/D2 ratio of
Forged. rolled,
cast components
in ferrous and
non-ferrous alloys
material
Vickers (HV) Square based 1 — 120 kg All metal alloys
diamond and ceramics,
pyramid needs surface
preparation
Rockwell 1/16 inch 10 kg minor load Low-strength
B-Scale (FIRB) diameter steel
ball
100 kg major load steels and non-
ferrous upto HV
of 240
Rockwell
C-Scale (HRC)
Diamond cone
or Brale
10 kg minor load
150 kg major load
All metals with a
machined surface
finish or
equivalent. High
strength steels
from HV 240—
1000
i
42

Shape of Indentation
Formula for
Test Indenter Side View Top View Load Hardness Number“
Brinell 10-mm sphere 0 HB = en
of steel or ald be P mD[D — VD? - d?]
tungsten carbide >
Vickers Diamond 136° » % P HV = 1.854P/dj
microhardness pyramid <a> ei
bKnoop Diamond ¢ - P HK = 14.2P/P
microhardness pyramid et —
Wb =7.11 f ae
Rockwell and Diamond 120° 60 kg
Superficial cone SN , 100 | Rockwell
Rockwell ts, A, 4, 4 in. Sy se 150 kg
diameter & 1S kg
steel spheres 30 kg }Superficial Rockwell
45 kg

|. 2,000
H— 1,000
1000 a
800 7
600 -— 500
100
Knoop hardness
5
Brinell hardness
Diamond zi
Corundum
or
sapphire—
Nitrided steels
Topaz —
Cutting tools
Quartz —|
File hard
Orthoclase —
Apatite —
Qnvnov
Filuorite—
Calicite—
Brasses
and
aluminum
alloys
Most
plastics
Taici 1
Mohs hardness

Ns
O Tensile test is a measurement of the ability of a material
to withstand forces that tend to pull it apart and to
determine to what extent the material stretches before
breaking.
O Tensile modulus, an indication of the relative stiffness of
a material, can be determined from a stress-strain
diagram.
O The tension test is the most common method for
determining the mechanical properties of materials, such
as strength, ductility, toughness, elastic modulus, and
strain-hardening capability.

Ns
O This is the measure of conventional stress that can be
sustained by the metal for any deforming forces.
O It ranges from zero load (zero deformation) to ultimate
strength in tension which corresponds to the maximum load in
a tension test.
O It is usually measured by the highest point on the conventional
stress-strain curve. In engineering tension tests this strength
provides the basic design information on the materials.
O The tensile strength of a material is the maximum amount of
tensile stress that it can be subjected to before failure.

Ns
O There are three typical definitions of tensile strength.
Yield strength
The stress at which material strain changes from elastic
deformation to plastic deformation, causing it to deform
permanently is known as yield strength.
Ultimate strength
The maximum stress a material can withstand is known as
ultimate strength.
Breaking strength
The strength co-ordinate on the stress-strain curve at the point of
rupture is known as breaking strength.

Ns
Load - The force applied to a material during testing.
Strain gage or Extensometer - A device used for measuring
change in length (strain).
Engineering stress - The applied load, or force, divided by the
original cross-sectional area of the material.
Engineering strain - The amount that a material deforms per
unit length in a tensile test.

Limit of proportionality
Yield point or yield strength
Maximum tensile strength
Breaking strength
Percentage elongation
Percentage reduction in area
Modulus of elasticity
OOOOOOUOO
The tensile test is usually carried out with the help of a
“Universal Testing Machine” (UTM)
49

OVERALL LENM&TH
DISTANCE BETWEEN SHOULDERS
_
GAGE
LENGTH
GRIP SECTION P
Thee, ss
DIA. OR WIBTH y;
_—
WIDTH OF
GRIP SECTION
“REDUCED” SECTION
The accurate measurement of dimensional change achieved by
attaching the sensitive measurement device to test piece. The
devices used to measure longitudinal strain are termed as
extensometer.

Schematic arrangement of a UTM
UNIVERSAL TESTING MACHINE (UTM)
Upper cross Mead
Space for Tensile
Spacemen
Movable Cross Head
Space for Compressive
51

Tensile Strength - Extensometer
Reduced section
i 25 1‘ Aa —F Specimen
-—— -+— - —- p05 OS Diameter —— -4—--——-; a Diameter
- _ ie
-—r7-— frGauge length S Radius
Extensometer y/
‘
Specimen
Extensometer
52

Ns
Tensile Strength
Wd | a | |
& ¢

Ns
_ Equipment's/Tools Required
Universal Testing Machine (UTM)
Extensometer
Scale
Vernier Caliper
Punching tools

Ns
The specimen to be tested is fastened to the two end-jaws of the UTM.
Now the load is applied gradually on the specimen by means of the
movable cross head, till the specimen fractures.
During the test, the magnitude of the load is measured by the load
measuring unit (load cell).
A strain gauge or extensometer is used to measure the elongation of
the specimen between the gauge marks when the load is applied.

Ns
Ex: Result of a Tensile test (12.5mm dia, Cross sectional area, Ao=122.7mm_2)
Material: Aluminium alloy test bar
Load, P(KN) Gauge Length Engineering stress, Engineering strain,
(mm) o = P/Ao (MN.m-2) ¢ = Al/l (min.mm-1)
0 50 0 0
5 50.03 40.7 0.0006
10 50.06 81.5 0.0012
15
20, 25, 30
35.6 (Max.) 53.00 290.1 0.06
33.8 (Fracture) 55.30 275.4 0.104

Ns
Ex: The stress-strain curve for an Al alloy plotted (Data from previous table)
Pp Ultimate tensile
/ strength
(
AS
a = Modulus of elasticity
0 Vo 4 I {xn | | 4 Se |
0.002 0.004 0.02 0.06 0.10
Engineering Strain (mm.mm-1)

551 MPa ———_—
Aluminium alloy 7075-T6
BS EN 10087 11SMn30
Strong Grey Cast Iron
BS EN1561 EN-GJL-350
Magnesium alloy
Weak Grey Cast Iron
BS EN1561 ENGJL 150
Pure Aluminium-Annealed
| 1 | i Ll
0,008
Strain
0,004 0,012 0.016 0,020 0,024 0,028 0,032
59

Mechanical Testing : Testing Of Materials

es Stress Strain curve of Mild-steel
=
. Neckin
Ultimate tensile strength 9
' Fracture
be nn ne enn nnn nn :.
baie minetcencs ' '
i
!
‘ '
Young's Modulus ‘ ‘
' = slope : :
' = stress/strain : {
' ' '
i 1 '
' ! '
' i '
' ' '
'Non-Uniform!on-Uniform
Elastic Uniform Plastic Plastic
eformatioa Deformation ' ?
Plastic Strain}

Tensile Strength
= SS 5
es : ienene —P
¢ Localized deformation of a ductile material during a tensile test
produces a necked region.
¢ The image shows necked 62

TABLE 2.2
Metals (Wrought) E (GPa) Y (MPa) UTS (MPa) Elongation in Poisson's
50 mm (%) ratio (
Aluminum and its alloys 69 -79 35 —550 90 -600 45 4 0.31 —0.34
Copper and its alloys 105 —150 76 -1100 140 -1310 65 -3 0.33 -0.35
Lead and its alloys 14 14 20 -55 50-9 0.43
Magnesium and its alloys 41 -—45 130 -305 240 —380 21-5 0.29 —0.35
Molybdenum and its alloys 330 -360 80 —2070 90 —2340 40 -30 0.32
Nickel and its alloys 180 —214 105 —1200 345 —1450 60 -5 0.31
Steels 190-200 205 —1725 415 -1750 65 —2 0.28 —0.33
Titanium and its alloys 80 —130 344 —-1380 415 -1450 25 7 0.31 -0.34
Tungsten and its alloys 350 -—400 550 -690 620 —760 0 0.27
Zinc and its alloys 50 25 -180 240 —550 65 —5 0.27
Nonmetallic Materials
Ceramics 70 —1000 = 140 —2600 0 0.2
Diamond 820 —1050 = _ a —
Glass and porcelain 70 -80 - 140 0 0.24
Silicon carbide (SiC) 200 —500 — 310-400 — 0.19
Silicon nitride 280 —310 — 160-580 - 0.26
Rubbers 0.01 -0.1 a — - 0.5
Thermoplastics 1.4 -3.4 = 7-80 1000 —5 0.32 —0.40
Thermoplastics, reinforced 2 —50 = 20-120 10-1 a
Thermosets 3.5 -17 = 35-170 0 0.34
Boron fibers 380 — 3500 0 _
Carbon fibers 275 -415 = 2000 —3000 0 a
Glass fibers 73 -85 oe 3500 —4600 0 =
Kevlar fibers 62 -117 = 2800 0 —
Spectra Fibers 73 -100 a 2400 —2800 3 os
Note: In the upper table, the lowest values for
(GPa) by 145,000 to obtain pounds per square in. (psi) and megapascals (MPa) by 145 to obtain psi.
E, ¥Y, and UTS and the highest values for elongation are for pure metals. Multiply gigapascals

Ns
Factors affecting — Tensile testing
¥ Specimen preparation and specimen size
v Rate of straining
v¥ Temperature
Y Hydrostatic pressure effects
Y Radiation effects

Ns
Tensile testing — Working video
https://www.youtube.com/watch?v=D8U4G5kcpcM
https://www.youtube.com/watch?v=CVJWdtp4EiA
https://www.youtube.com/watch ?v=8e0a7RSQGA4
Stress-Strain Relations: Tensile Testing, Yield & Ultimate Strengths, Elastic Modulus,
Safety Factor — Refer: https://www.youtube.com/watch?v=0502uqywKKg

Ns
O The impact test is performed to study the behavior of materials under dynamic
load i.e., suddenly applied load.
O Impact strength defined : The capacity of a metal to withstand blows without
fracture, is known as impact strength or impact resistance.
O The impact test indicates the toughness of the material 1.e., the amount of energy
absorbed by the material during plastic deformation.
O The impact test also indicates the notch sensitivity of a material. The notch
sensitivity refers to the tendency of some normal ductile materials to behave like
a brittle materials in the presence of notches.
O Jn an impact test, a notch is cut in a standard test piece which is stuck by a single
blow in a impact testing machine. Then the energy absorbed in breaking the
specimen can be measured from the scale provided on the impact testing
machine.

Dn Aca GeO)
| In mechanics, an impact is a high force or shock applied over a short
time period when two or more bodies collide. Such a force or
acceleration usually has a greater effect than a lower force applied
over a proportionally longer period.
_! At normal speeds, during a perfectly inelastic collision, an object
struck by a projectile will deform, and this deformation will absorb
most or all of the force of the collision
_| The effect depends critically on the relative velocity of the bodies to
one another.
_| Example : car crush, wind force, earthquake etc.
67

a: *
ra aor
‘ s ™%
. - s
2 ‘
. o of ‘
Scale with ou ‘
pointer y
Pendulun
Striking
edge
Specimen
Specimen suppo
r l nig
| x :
ee
68

Ns
O Impact strength is the resistance of a material to
fracture under sudden load.
O It is a complex characteristic which takes into
account both the toughness and strength of a
material.
O It is defined as the specific work required to fracture
a test specimen with a stress concentrator in the mid
when broken by a single blow of striker in pendulum
type impact testing machine.

Ns
Classification of Impact test
1. Single Blow Pendulum Impact test
(i) Charpy notched — bar impact test
(ii) Izod notched — bar impact test
Drop weight test (DWT)
Robertson crack-arrest test
Dynamic tear (DT) test
Instrumented puncture testing
Seobf
Tensile impact test

Ns
Types of Impact Tests
LJ Based on the types of specimen used on impact testing machine the
impact tests can be classified into:
1. Izod test, 2. Charpy test
LJ It can be noted that the impact testing machines are designated so
that both types of test can be performed on the same machine with
only minor adjustments.

Izod Test
LJ Izod test used a cantilever specimen of size 75mm x 10mm x
10mm as shown in fig. (a)
LJ The V-notch angle is 45° and the depth of the notch is 2mm.
The Izod specimen is placed in the vise such that it is a cantilever
as shown in fig. (b)
f28 mm
é a Izod impact test <—
7S mm Striking
7 direction
y

y
y
(b)
10mm
I .. <— 10mm
(a) 72

Impact Strength — Izod Test Machine
Point of

Charpy ts
LI The charpy test uses a test specimen of size 55x10x10mm as shown
in fig (a)
LI The V-notch angle is 45° and the depth of the notch is 2mm.
LJ The Charpy specimen is placed in the vise as a simply supported
beam as shown in fig. (b)
(a)
. ssmm —10 |<
- mm
40 mm _
. «
OY Vv OY Striking
(b) WN direction
SS
Anvil —
/
Striking edge
74

LJ The specimen is placed in the vice of the anvil
LI The pendulum hammer is raised to known standard height depending on the
type of specimen to be tested
LJ When the pendulum is released, its potential energy is converted into kinetic
energy just before it strikes the specimen
LJ Now the pendulum strikes the specimen. It may be noted that the Izod
specimen is hit above the V-notch and the Charpy specimen will be hit behind
the V-notch.
LI) The pendulum, after rupturing the specimen, rises on the other side of the
machine.
LI) The energy absorbed by the specimen during breaking is the weight of the
pendulum times the difference in two heights of pendulum on either side of the
machine.
LJ Now the energy i.e., the notched impact strength, in foot-pounds or meter-kg,
is measured from the scale of the impact testing machine.
LJ The pointer is set up to its maximum value (300 J)

Ns
O Impact strength is a measure of ability of a material to absorb
energy during plastic deformation.
O Brittle materials have low toughness as a result of the small
amount of plastic deformation that they can endure.
O Impact strength is affected by the rate of loading, temperature
and presence of stress raisers in the materials. The thickness
of the material also affects its impact strength.
O It is also affected by variation in heat treatment, alloy content,
sulphur and phosphorus content of the material.

Ns
Y Angle of notch
v Shape of notch
Y Impact velocity
v Temperature of the specimen and
Y Dimensions of notch specimen

| eremetyr
Specimen position
Izod impact test
Specimen held at
vertical
Charphy impact test
Specimen held at
horizontal
| Point of strike
| -
At Upper tip of
specimen
At Point of notch but in
opposite direction
| Types of notch V-notch
Farming hammer
V-notch and U-notch
Ball pin hammer| Type of hammer
|
| Specimen dimensionL
75x10 10mm 55x 10x 10mm
Notch face Facing the striker,
fastened in pendulum
Face is positioned away
from the striker
Materials used Plastics and metals Metals
Holding It imitates cantilever
beam
It imitates simply
supported beam
Temperature It is largely affected by
temperature changes
It shows minimum error
to temperature changes
Calculation The Izod impact value
(J/m, kJ/m?) is
calculated by dividing
the fracture energy by
the width of the
specimen.
The Charpy impact value
(kJ/m2) is calculated by
dividing the fracture
energy by the cross-
section area of the
specimen.
Energy The fracture energy is
determined from the
swing-up angle of the
hammer and its swing-
down angle
The fracture energy is
determined from the
swing-up angle of the
hammer and its swing-
down angle. 78

= Pendulum
Vice
a Specimen . Specimen
eThe Specimen is held vertically ina vice ¢The Specimen is held horizontally —
between two supports
eThe notch on the specimen faces the
pendulum °The notch on the specimen faces away
from the pendulum
eThe striking energy of the pendulum is
167 Joules ¢The striking energy of the pendulum is
300 Joules

Ductile to brittle 1 ransition | emperature
Ductile
Transition
Temperature
EnergyAbsorbedonImpact—»
Temperature ——>
80

Ns
Impact test — Working Video
IZOD TEST:
https://www.youtube.com/watch?v=HqD7KtvEgB4
CHARPY TEST:
https://www.youtube.com/watch?v=tpGhqQvfitAo
https://www.youtube.com/watch ?v=JfvLJIIVo450
@ S.Thirumalvalavan, Assistant Professor
E-Mail : thirumalbemech@gmail.com

Ns
O Fatigue is the phenomena of material failure for repeated
pulsating or reversing loads (or) The capacity of material to
withstand repeatedly applied stresses is known as fatigue.
O Fatigue or endurance limit is defined as the maximum stress
which a specimen can endure without failure when the stress is
repeated for a specified number of cycles/times.
O The nominal stress values that cause fatigue failure are less
than the ultimate tensile stress limit, and may be above the
yield stress limit of the material.
O Fatigue strength of a metal is usually tested using a rotating
beam machine.

LJ The stress-life method : A mechanical part is often exposed to a
complex, often random of sequence of loads values of large and small
range.
Types of stress life method
(i) Rainflow analysis
(11) Fatigue damage spectrum
(111) S-N curve
(iv) Miner’s rule.
J The strain-life method : When strains are no longer elastic, such as in
the presence of stress concentrations, the total strain can be used instead
of stress as a similitude parameters. This is known as the strain-life
method.
(i) The crack growth method
(11) Probabilistic methods

Ns
Fatigue Testing — Rotating beam machine
This part of the specimen
Revolution
is In COMpression (~ Vve force) Mator
: . oounter
¥ .
’ ee
= ee
| {) © orl 2 (> ‘ — | = SN
= 7 h— wt
’ DP —1_
This part of the specimen AT et
is.in tension (+¥e force) ~ |
“=
Weight (WW) on specimen
Fatigue specimen is gripped on to a motor at one end to provide
the rotational motion whereas the other end is attached to a
bearing and also subjected to a load or stress.

Bearing Revolution
Shaft Test piece H counter
Fig. 5.27. Rotating beam fatigue testing machine
Tension
85

Ns
When the specimen is rotated about the longitudinal axis, the
upper and the lower parts of the specimen are subjected to tensile
and compressive stresses respectively.
Therefore, stress varies repeatedly at any point on the specimen
surface. The test proceeds until specimen failure takes place.
The revolution counter is used to obtain the number of cycles to
failures corresponding to the stress applied

Stress — Cycle (SN) Curve
350
300
NNVv
o
Stress(MPa
10E+00 1.0E+01 1.0E+02 10E+03 F
Life (cycles}
0E+04
8/7

LI The test specimen is placed on the machine
LI Now the specimen is rotated using an electric motor
LI When the specimen is rotating, it can be noted that the upper surface
of the specimen is subjected to tension and its lower surface
experiences compression, as shown in fig.

/ -
{or { { 3 om Cne ci e/ >.< Specimen
r] + | ( € }) 7 -~—Tension ae state LAY] Se / Bearing
NA ma Se Z /
= =

Ns
Fatigue test — Testing procedure
LJAs the specimen rotates, there is sinusoidal variation of stress
between a state of maximum tensile stress and a state of maximum
compressive stress.
LThe cycles of stress are applied until the specimen fractures.
A reduction counter records this number of stress cycles.
LINow a member of specimen of the same material (at lease 6
specimens) are tested in the same manner under different stress
levels and the results are plotted on S-N graph.

Fatigue test — Testing procedure
LI The S-N graph is drawn on a semi-logarithmic scale with the number of
cycles (N) required to cause failure of the specimen on the X-axis and the
stress (S) on the Y-axis. The resulting curve in a S-N graph is called as S-N
curve.
LI The below fig. shows the S-N curve obtained on testing different materials.
Pe ee ene pS
et h j
ites
ee Eadiiran eget het teedeelsteinenet
' MN cnocurdanc s
} ES ~/A - 410 MN/in"
e NN | py ta J
. lool ste¢ ; = it Seee Le eed
“ {eet
< ~ | Laat |
200 } + ;
: I On lm Alurninium alloy 1 1 1a tii4 bs LOO ott | |
Ls. | | | | | | t
1}
| || LI
Q% . rR 5 8
10° 10° 10° 10 10
Number of cycles
Fig. 2.25. S-N curve for steel and aluminum alloy

Stages in Fatigue Failure
Initiation
1. Stage-1 Crack initiation
2. Stage-2 Crack propagation aa :
3. Stage-3 Sudden fracture Catastrophic
rupture
TYPES OF FATIGUE CYCLE
a) High-cycle fatigue —
N > 1044 cycles (20-50 Hz, Commonly
used)
a) Low-cycle fatigue —
N < 1043 cycles, with constant strain
amplitudes typically at 0.01 —5 Hz.
91

LJ The fatigue test can tell us show long a part many survive or the maximum
allowable loads that can be applied to prevent failure.
LU The fatigue test is useful in setting the design criterion with the use of the
endurance limit.
LJ Endurance limit is stress below which there is a 50% probability that
failure by fatigue will never occurs, which is the common design criterion.
LJ Fatigue life tells us how long a component survives at a particular stress.
LJ From the S-N curve, one can find the fatigue life (n) for the applied stress.
LW Endurance ratio = Endurance limit/ Tensile strength = 0.5.
LJ) The endurance or fatigue ratio allows us to estimate fatigue properties from
the tensile test.

Ns
Fatigue Test — Working video
https://www.youtube.com/watch?v=LhUcIxBUV E
https://www.youtube.com/watch?v=ZslwEp574ho
https://www.youtube.com/watch?v=DykiHVrVkKg
NPTEL: https://www.youtube.com/watch?v=OlexdbPETPw

Ns
O Creep is the tendency of a material to deform permanently
under the influence of constant long term stresses, that are
essentially below the yield strength of the material.
O The rate of this deformation is a function of the material
properties, exposure time, exposure temperature and the
applied structural load.
O Creep is a deformation mechanism that may or may not
constitute a failure mode. Moderate creep in concrete is
sometimes welcomed because it relieves tensile stresses that
might otherwise lead to cracking.

Ns
— CREEPTEST
LU 7he continuous deformation of a metal under a steady load is known as
creep.
LIThe purpose of creep tests is to determine the creep limit. The creep
limit or the limiting creep stress is defined as the stress that will not
break the specimen when applied for an infinite period at a specific
constant temperature.
LI The creep tests require the measurement of four variables stress, strain,
temperature and time.
LI The creep tests are simply tension tests run at constant load and constant
temperature. Then the value of strain of the test piece is noted as a
function of time.
LI The specimen for creep testing are usually the same as for conventional
tensile tests.

Ns
Creep Testing Installation
(Ge, e)
’
, '
Wei ght —f—inert = WeightFor Bend For Tensi foalor Ben Gas ee | Monitor TV
i
a ae ---| p= scavsing! -(G7A) -[RecorderRad ant / « = -lAnatyzerar/,
aw aes yew +
Thermocouple : ; (Computer
- Operation ’
Panel
Rotary Turbo |
P P c. 2ump ump Schematic Diagram Equipment
Objective of Creep test is to find the maximum stress that may
be applied for a long period of time at a given temperature.
96

Ns
Constant force
applied
Extension measured :
over gauge length
Heating
element
|
Thermocouple
Constant force
applied
Creep test 1s conducted using a tensile specimen
to which a constant stress 1s applied, often by the
simple method of suspending weights from it.
Surrounding the specimen is a thermostatically
controlled furnace, the temperature being
controlled by a thermocouple attached to the
gauge length of the specimen.
The extension of the specimen is measured by a
very sensitive extensometer since the actual
amount of deformation before failure may be
only two or three per cent

Loading lever
wit |
oe [>
Upper pull rod
Sp
keFS,
hae
Furnace (or)
Heat chamber
Specimen
.CM;
+4
Lower pull rod
Weights —~
Ui
eeeeeeeSeeee
eeSSeeeeee
98

v It must possess means for applying and maintaining a constant tensile
load.
Y There must be a furnace capable of keeping the temperature of the test-
piece at the desired value to within very close limits.
Y There should be means for the accurate measurement of testpiece
extension.
m
—)#
Pa
a
go
4
coowi
S77er
¢
Enee

> The specimen to be tested is placed in an electric furnace. At the
electric furnace, the specimen is heated to the given temperature under
the constant load.
> The strain in the specimen is measured by a strain gauge or an optical
extensometer as a function of time.
> The above test is repeated for 4 to 5 specimens at each temperature
under different loads.
> Now the creep curve i.e., the elongation versus time curves areare plotted
for each specimen, as shown in fig. a
g? 27
637027654
c
2
a
c
o2
x
Ww
Time in hours ————s»

Strain———»>
Primary Tertiary creep
creep Secondary creep
Gy
Initial elastic strain
Failure
Time ———>
Creep Test Curve
General Creep Curve
75 MPa
STRAIN[%l]
mno
fp —
Qo 20 40 60 80 1OO 120
TIME [hours]

} a. elastic deformation
STRAIN b. plastic deformation
— E —.
stage 3
specimen
fracture
primary secondary tertiary TIMEt
creep creep creeps:
102

v Primary Creep: The initial creep stage where the slop is rising rapidly at first in a
short amount of time. After a certain amount of times has elapsed, the slope will
begin to slowly decrease from its initial rise.
Y Secondary Creep: After the primary creep, the creep rate reaches essentially a
steady state, in which the creep rate changes little with time. This region of
approximately constant creep rate.
Y During this stage, the steady state is achieved because of an approximate balance
between two opposing factors: the strain hardening that tends to reduce the creep
rate and the softening or recovery process that tends to increase it.
vY Tertiary Creep: The last stage of creep when the object that is being subjected to
pressure is going to reach its breaking point.
Y In this stage, the objects creep continuously increases until the object breaks. The
slope of this stage is very steep for most materials. During this stage, high stresses
or/and at high temperatures.
vY The creep rate is greater and increases continuously till the material undergoes
fracture.
Y Tertiary creep occurs when the effective cross-sectional area of the specimen is
reduced remarkably either due to localized necking or internal void formation.

Ns
Creep Test — Working Video
Some useful youtube links
https://www.vyoutube.com/watch?v=OJ5ERAKOc98
https://www.youtube.com/watch ?v=k8Py4-SdjyvU&t=14s
NPTEL video: https://www.youtube.com/watch?v=zHOSsDLKMoU

Ns
LIn shear test, the shear force is the load that causes two contiguous
parts of the body to slide relative to each other in a direction parallel
to their plane of contact.
UPrinciple: Shear strength measures a materials ability to resist
forces that cause the materials to slide against it. The specimen is
loaded in shear fixtures, load in applied perpendicular to specimen
through plunger.
UUThe phenomenon of shear applies through the shear fixtures
(coupling device) is known as shear test.

Ns
Shear Test
LI Types of shear test:
(1) Single shear test,
(41) Double shear test
LJ Components:
(i) Universal Testing Machine
(ii) Vernier Caliper
(111) Shear fixtures

Ns
Shear Fixtures
LITwo coupling braces which is used for both single or double shear
connection. Both are at similar position with certain distance apart.
For single shear, specimen is routed to single brace and for double
shear, specimen is routed fully.
7 Applied Load <
> ) Z- ad fr i
ge” Ne SO ] Load ~~ Jf J |
6 7 | Bushing | if 4
Sy sey ee LL, || Lf a Groov-Pin ™ / C~
Ll SK | Sr Y (Sk
I iy 4 IN A Pin fs
| Ty |e /
Ph oo | (4-1 — Support /
i Lp .
LN ? Bushing /
Layout of Shear fixtures

Ns
Single & Double Shear Test
LiIn the double-shear method, the specimen is sheared off at two
cross sections. In the single-shear process, the specimen only
shears away at one cross section.
LICalculating the shear strength in the two processes differs in the
cross sectional area to be applied.
LIThe shear strength determined in the shear test is important in the
design of bolts, rivets and pins as well as for calculating the force
required for shears and presses.

Shear Test - Working
LJ The diameter is measured using the Vernier caliper
LJ Mount the shear fixtures on UTM and load the specimen in shear fixture
accordance to need of shear test. Operate (push) buttons for driving the
motor to drive the pump.
rose Load Plunger
a
Specimen
e| Specimen
Shear Testing
Attachment
Single & Double Shear Loading
109

Single Shear Testing
+ne
Specimen
Double Shear Testing
110

Ns
Shear Test — Working
LI Gradually move the head control level direction till the specimen shears.
Take the load at which the specimen shears.
F
Shear stress, T= —
2A
Where,
F — Force at breaking shear
A — Shearing surface

Ns
Shear Test — Working video
From youtube, some useful links
https://www.youtube.com/watch?v=T9LGm4EjF6w
https://www.youtube.com/watch?v=nmZBCOzjT-I
https://www.youtube.com/watch?v=SI75riDXiAk&t=44s
https://www.youtube.com/watch?v=NunYq16C2ic

Ns
LI Bending tests is standard test method for material of smooth bars like
flat metal spring, concrete, natural stone, wood, plastic, glass and
ceramics.
LIIt also called as flexural test (particularly to evaluate tensile strength of
brittle material which is difficult to under estimate in uniaxial tension
test.

Ns
LIPRINCIPLE — Bend tests are conducted by placing a length of
material across a span and pushing down along the span to bend the
material causing a concave surfaces or a bend to form without the
occurrence of fracture and are typically performed to determine the
ductility or resistance to furnace of that material, the elastic modulus of
bending, flexural stress, and flexural strain of a material.

Bend test Equipment
ent
YY
a7 yae Y)
a yG YY] YY) ew
Moving Crosshead :
oad Gell ap Ballscrew
ss . W4 Test Specimen
Bending
YY
Fixtures x A 5G4 Opposing
WY Tapered
i) 4 Roller
Bearings
Machine Base
115

Method of Bend test Based on LOAD position
LJ Single point loading at the free end of a cantilever beam
LI Centre point loading (or) Three point bending test
LI Four point bending test
Three point
Cantilever bend test Four point Bend test
load load load load
Clamp { {
L
b-L/2—-}-- L/2—- ae L/3—+ Ta
l- ——o specimen N specimen “ T
span length = L : support a support
span length = L ' spanlength=L
116

Ns
L) Materials testing systems accurately and reliably measure the flexural
properties of metals, concrete, plastics, medical devices and other
products and components.
LIThe machines can calculate flexural modulus, flexural strength, and
yield point at maximum capacities.

Ns
LI Bend fixtures are used to determine the flexural properties of rigid and
semi-rigid materials.
LIThey are available in a variety of capacities, spans, and support
diameters and widths. It consists of default adjustable load pointers
based on loading position.

Ns
UUThe bending fixture is supported on the platform of hydraulic
cylinder of the UTM.
NIA loading beam that rests on two rollers on the top of beam to be
tested is used to apply the loads.
LIA load applied to the loading beam accurately at the mid-point
between its two supporting rollers for three point loading (or) four
point loading.
The support are generally knife-edge or convex. The load applicator
is a rounded knife-edge with an including angle of 60°, applied
wither at mid span (for 3 point testing) or symmetrically placed from
the supports (for 4 point testing).

Ns
LI These rollers in turn must be spaced accurately at equal distances from
the supporting rollers for the beam to be tested.
LIIf the distance between the supporting rollers of the test-beam is L; the
supporting rollers of the loading beam are often located at L/3 or L/4
distances from the test-beam supports, although any equal location
distances can be used.
LI Load and either deflection or strain are usually recorded in the test.
LI Using this method, a beam mounted on supports is studied under a
applied force to the beam.

Ns
LI The bending test demonstrates the relationship between the load of a
bending beam and its elastic deformation.
LI The loading is held in the middle cross head.
LJAt a particular load the deflection at the center of the beam is
determined by using a dial gauge.

v Flexural strength, also known as modulus of rupture or bend strength or
transverse rupture strength is a material property, defined as the stress in a
material just before it yields in a flexure test.
1. For three point bending test (rectangular cross section)
3FL
2bd?on=
2. For four point bending test where the loading span is 1/2 of the support
span (rectangular cross section)
_ FL
°F ~ ba?
3. . For four point bending test where the loading span is 1/3 of the support
span (rectangular cross section)
_ 3FL
“f ~ 4ba?
o, = Stress in outer fibers at midpoint, (MPa)
= load at a given point on the load deflection curve, (N)
L = Support span, (mm)
b = Widthof test beam, (mm)
d = Depth or thickness of tested beam, (mm) -

Y Deflection, is the degree to which a element is displaced under a
flexural load (due to its deformation). Deflection for three point
bending test,
Modulus of Elasticity (or) Young’s modulus
{TI
II
I = Area moment of inertia of cross section

Ns
Bend Test — Working video
From youtube, some useful links
https://www.youtube.com/watch ?v=VoJXAOAA-VQ
Single point loading at the free end of a cantilever beam :
https://www.youtube.com/watch ?v=Qt-HZv6wTns
Centre point loading (or) Three point bending test :
https://www.youtube.com/watch ?v=iTXghfxjdkQ
Four point bending test :
https://www.youtube.com/watch?v=nTR5od5RfeA

Ns
(i) Effect of Grain size
(ii) Method of heat treatment
(iii) Atmospheric exposure
(iv) Low & High Temperature

Fundamental Check —
Topics to Remember
E-Mail : thirumalbemech@gmail.com

e WHY STUDY Failure?
e Breaking two or pieces- external load
e Two steps in the process of fracture:
-- Crack initiation
-- Crack Propagation
Ductile
Fracture
Brittle

Fracture
Fracture: separation of a body into pieces due to stress, at
temperatures below the melting point.
Steps in fracture:
= erack formation
= erack propagation
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):
em
= Extensive plastic deformation ahead of crack
>» Crack is “stable™’: resists further extension unless
applied stress 1s 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

v
i
Different types of fracture
LJ
fn
Figure 8.1 (a) Highly ductile fracture in
which the specimen necks down to a point.
(b) Moderately ductile fracture after some
necking. (c) Brittle fracture without any
plastic deformation.
129

Necking
microcracks formation
Crack formation
Crack propagation
fracture

Spritefracture
Exhibits little or no plastic deformation and low energy
absorption before failure.
Crack propagation spontaneous and rapid
Occurs perpendicular to the direction of the applied stress,
forming an almost flat fracture surface.
Crack propagation corresponding to Successive and
repeated breaking of atomic bonds along specific
crystallographic planes is called cleavage
This type of fracture is called cleavage fracture
This type of fracture are generally found in BCC and HCP,
but not FCC

(a) (b)
Figure 8.3 (a) Cup-and-cone fracture in aluminum. (b) Brittle fracture in a mild steel.
132

¢ Crack propagation across grain boundaries is known as
Intragranular/transgranular
° While propagation along’ grain boundaries is termed
Intergranular
Grains Path of crack propagation
f 4 |
w /
Grain boundaries Path of crack propagation i
ae
—

Brittle Vs Ductile
Brittte
lL. Occurs with minimum or ro
defortnation
2, Occurs
eneray
warnurte-
suddenty with miniqnuim
absorption and without any
4. Separation occurs normal to the
tensile axia, resulting in a fat surface.
LI. Initiates at the poin
crack is largest
t where mucroe
5S. Movement of
little plastic deformation adjacent to
crack involves very
the crack,
6, Fractured surface shows sharp
Pinar surface.
-?. Commonly observed in BCC and
HCP metals
8 in general, fracture occurs along
‘Cleavage planes.
| alongslip planes.
Ductile
cms with large plastic
deformation
Occurs woth slow tearing of the mrtal
with absorption of enecrey
Crack propagates in a direction at 45°
to tensile aan, sesulting in « e1p arc
| cOtte fracture surface
Inideates in some localised region
} where deformation ts very large
Crack propagstes as a result of highty
localised plustic defoctmation of metal
Fracture surface appear dirty with
rough contour.
Commenty observed in FOC metals.
| After necking and crack propagation
at 45° to tensile asm, fracture occurs
134

Ductiie— brittle: fransition
¢ Ductile materials fracture abruptly and with little plastic
Deformation
¢ Crack propagation takes precedence over plastic deformation
¢ Ductile — Brittle transition occurs when,
1. Temperature is lowered
2. Rate of straining increased
3. Notch or stress raiser is introduced

Bal 5 ‘ g oH
The temperature at which the stress to propagate a crack 6;is equal to
the stress to move dislocations 6,.
When 6, < 6,;material is ductile
When 6, > 6; material is brittle
This transition is commonly observed in materials having BCC and
HCP structures.
For ceramic materials, the transition takes place at elevated
temperatures.
For polymers the transition occurs over a narrow range, below room
temp.

G riftith: theory. ¢ 4 5
¢ Measured fracture strength of most brittle materials are significantly
lower than theoretical strength- what is the reason?
Stress concentration
¢ Brittle materials contains a population of fine cracks which produce a
stress concentration
¢ Stress amplification is assumed to be at the crack tip
¢ Magnitude of this amplification depends on the crack orientation and
geometry

¢ It is assumed that the crack is elliptical in shape and is
oriented with major axis perpendicular to the applied stress
So
;
o
m
|
2 | |= | |
” | |
| |
| |
| |
| |
| |
Sp | |
| |
| |
| |
| |
| |
| |
x 4
| Position along X—X’
(a) : (b)
0
Figure 8.8 (a) The geometry of surface and internal cracks. (b) Schematic stress profile
along the line X—X’ in (a), demonstrating stress amplification at crack tip positions.

¢ Maximum stress at the crack tip
6
om =20, |
p

Ns
¢ Increase in surface energy is required to generate extra surface
area
¢ Source of this increased surface energy is the elastic energy
which is released as the crack spreads
¢ Griffith criterion -A crack will propagate when the decrease
in elastic strain energy is at least equal to the energy required to
create the new crack surface
¢ The change in surface energy due to the change in crack length
must be just equal to the change in elastic strain energy.
. dUE _ du;
dc dc

Introducing compressive stresses
Polishing surfaces
Avoiding sharp corners
Improving purity of the materials
Grain refinement
Avoid precipitation of second phase

ris the-hsciot Hwith the
behavior of materials containing cracks or
small flaws.
¢ Fracture toughness measures the ability of the
material containing a flaw to withstand an
applied load.
¢ Stress intensity factor
K=fo V 1c
Unit is Mpa,/m

ae) 055115 tr: 1s Ver
¢ Used to design and select materials
considering the inevitable presence of flaws

Mode of fracture
Figure 8.10 The
three modes of
crack surface
displacement.
(a) Mode I, opening
or tensile mode;
(b) mode Il, sliding
mode; and (c) mode
III, tearing mode.
(a) (b) (ce)
144

THANK YOU
E-Mail : thirumalbemech@ gmail.com

Mechanical Testing : Testing Of Materials

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