Stress - oscar

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Welcome
This is a document to explain the concept of Microscopic behaviour of a
sample during tensile testing to the animator.
2
This will take you through a 5 section process to provide the necessary
details to the animator before starting the animation.
3
The legend on the left will indicate the current status of the document.
The big Black coloured number will denote the current section, the
Grey color would denote the completed sections, and the Turquoise
color would denote the remaining sections.
4
The slides having yellow background (like this one) are the 'Instruction
slides'
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Microstructure changes during
Deformation
This idd shows us the basic microscopic changes that take place in a sample
when tensile forces are applied
Related LOs:
> Prior Viewing –Tensile Deformation of Ductile Metal
Prerequisites: Basic phenomenon of tensile test
Course Name: Mechanical Behaviour of Materials
Level(UG):
Author
Sudip Deb
Mentor
Prof P.Pant
Learning Objectives
After interacting with this Learning Object, the learner will be able to:
Identify the microscopic state of the sample when tensile stress is being applied to it.
Identify the microscopic changes that lead to the failure of a material.
•Differentiate between ductile and brittle fracture
•
•
Definitions and Keywords
1
Stress: Stress is defined as force per unit area. It has the same units as
pressure, and in fact pressure is one special variety of stress.
•
2
3
Strain: Strain is defined as the amount of deformation an object
experiences compared to its original size and shape. For example, if a
block 10 cm on a side is stretched so that it becomes 11 cm long, the
strain is (11-10)/10 or 0.1 (sometimes expressed in percent, in this case
10 percent.) Note that strain is dimensionless.
•
Elastic deformation: This type of deformation is reversible. Once the
forces are no longer applied, the object returns to its original shape.
•
Plastic deformation: This type of deformation is not reversible.
However, an object in the plastic deformation range will first have
undergone elastic deformation, which is reversible, so the object will
return part way to its original shape.
•
4
5
1
Definitions and Keywords
Ductility is a solid material's ability to deform under tensile stress; this is
often characterized by the material's ability to be stretched into a wire
2
Necking The reduction in diameter that occurs as a sample material is
subjected to tensile stresses.
Fracture: Fracture is the (local) separation of an object or material into two,
or more, pieces under the action of stress.
3
4
5
Brittleness: A material is brittle if, when subjected to stress, it breaks
without significant deformation (strain)
Ductile Fracture :In ductile fracture, extensive plastic deformation
(necking) takes place before fracture. The terms rupture or ductile
rupture describe the ultimate failure of tough ductile materials loaded in
tension
Slide 3
Introduction
Tab 02
Tab 03
Tab 04
Tab 05
Tab 06
Tab 07
Name of the section/stage
Interactivity
area
•
•
Ductile
Brittle
Ductile Iron Rod
Brittle glass Rod
Instructions/ Working area
Credits
1
2
Master Layout 1:
Universal Testing Machine
3
4
5
http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Tensile.htm
http://www.directindustry.com/prod/instron/tensile-compression-testing-machine-18463-41713.html
1
Step 1:
T1: Choosing the sample
Fig 1.1
Fig 1.2
Clasps
2
START
3
4
Universal Testing
Machine
Interactivit
y type (IO
1/IO 2)
Instruction
to learners
Button click
Select the
material on
which the test
is to be
performed
Ductile Iron Rod
Instruction to
animators
o
o
o
5
Brittle Rod
o
As the experiment
starts the Universal
testing machine and
the two rods appear on
the screen
The learner needs to
select one among the
two rods either the
ductile specimen or the
brittle one
Once the selection is
done the sample is
shown being placed
between the clamps of
the machine
As the selected
specimen is placed in
the machine and start
button appears and
also the graph of stress
and strain
Results and output
If the ductile specimen is selected the n show the
message
•
Else if the brittle specimen is selected show the
message
1
2
Master Layout 2(Ductile):
Universal Testing Machine
3
4
5
http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Tensile.htm
http://www.directindustry.com/prod/instron/tensile-compression-testing-machine-18463-41713.html
1 Step 2.1:
T1: Tensile Test for ductile material - stress – strain curve
Stress
A0
L0
2
Clasps
Original
START
3
4
Interactivit
y type (IO
1/IO 2)
Instruction to
learners
Instruction to animators
1. Notice the
initial length
L0 and the
initial area of
the specimen
A0.
•Mark and show L0 andA0 in the sample
•Show the sample being placed in the machine
• Show the zoomed up Lo part of the rod in
between the machine clasps.
2. Click on red
button to
start the
experiment
• The learner clicks on the red button
• As the red button is clicked the button
vanishes the graph starts but no significant
change in the sample is seen and it stops as
the graph reaches the orange dot
• Show instruction to learner 2.
•
5
•
On reaching the orange dot a pop up is
shown from the magnified image of the
specimen which shows the microstructure
of steel
Display test 3 to the learner and start
button reappears
Text to be displayed
1.
Stress is a function of
area (A0)
2.
Strain is defined as the
change in length/
original length on
L1- L0
L0
3. This is the
microstructure of steel
within the elastic limit
1 Step 2.2:
T1: Tensile Test for ductile material - stress – strain curve
Stress
A rod
L1
L
1
2
Clasps
START
3
4
Interactivit
y type (IO
1/IO 2)
Instruction to
learners
Click the start
button for
1.
Instruction to animators
o
o
o
5
o
Once the start button is clicked the
microstructure is removed and also
phenomenon starts again and also
the graph starts moving
This time there is some change in the
dimensions of the sample its length
increases by 1 cm
This continues till the graph reaches
the blue dot where this stops the text
1 is displayed to the learner
Then a pop up is shown from the
elongated sample showing the
microstructure of the elongated
sample and also text 2 is displayed
and start button reappears
Text to be displayed
The maximum elastic limit
of the sample has been
reached and plastic
deformation in the sample
has started as length
increased from Lo to L1 .
1.
2.This is the microstructure
of the sample in the plastic
region .the grains are
elongated.
1 Step 2.3.1:
T1: Tensile Test for ductile material - stress – strain curve
Stress
Necking
2
Clasps
START
3
Interactivit
y type (IO
1/IO 2)
Instruction to
learners
Click the start
button for
1.
Instruction to animators
o
o
4
o
o
5
Once the start button is clicked the
microstructure is removed and also
the start button is removed and the
material starts being stretched
the graph also starts moving also
the sample starts to elongate further
a blue dot appears on the screen
(text 1 appears)
midway after sometime
necking is seen to appear in the
sample and text 1 is displayed to the
learner
Text to be displayed
The Ultimate Tensile
Strength of the material has
been reached and is
indicated by the blue dot
2. 2.Necking is seen to
appear on the surface of
the material
1.
1
Step 2.3.2:
Deformation due to dislocation
2
2
3
Interactiv
ity type
(IO 1/IO
2)
Click
Instruction to animators
Results and output
•As necking start appearing in the earlier slide
simultaneously show the first fig with the dots in
it
• then as necking progress the show necking
occurring in this sample also with the size of the
voids increasing
4
•Next show the voids increasing in size
•After that show the voids coalescing with each
other and growing in size also show the area
near the voids narrowing up.
•Finally Show the sample ruptured and label it as
deformation due to dislocation
5
1. Deformation of a material by
the method of void formation.
1 Step 2.4:
T1: Tensile Test for ductile material - stress – strain curve
Stress
Crack
2
L
4
Clasps
START
3
4
5
Interactivit
y type (IO
1/IO 2)
Instruction to
learners
Click the start
button for
1.
Instruction to animators
o
o
o
o
o
Once the start button is clicked it is
removed
the graph starts moving also the
sample starts to tear up from the
necked region
Text one is displayed
As the material starts tearing apart
Text 2 and text 3 are displayed
Text to be displayed
Cracks being formed on
the sample
1.
Facture or Failure
The material starts to
crack once it has crossed
its ultimate tensile strength
until the point of complete
failure or fracture of the
material.
2.Microstructure of the
fractured sample
1.
1 Step 3.1:
T1: Tensile Test for brittle material - stress – strain curve
Stress
A0
L0
2
Clasps
Original
Fractured Region
START
3
4
Interactivit
y type (IO
1/IO 2)
Instruction to
learners
Notice the initial
length L0 and
the initial area of
the specimen
A0.
2.Click on red
button to start
the experiment
1.
Instruction to animators
Mark and show L0 andA0 in the sample
Show the sample being placed in the machine
• Show the zoomed up Lo part of the rod in
between the machine clasps.
•
•
•
Show instruction to learner 2.
The learner clicks on the red button
As the red button is clicked it vanishes the
graph starts but no significant change in the
sample is seen and it stops as the graph
reaches the orange dot
Text to be displayed
Stress is a function of
(A0)
1.
Strain is defined as the
change in length/ original
length on application of
1.
•
•
5
On reaching the orange dot the sample
suddenly breaks showing the phenomenon of
brittle fracture
•Display test 3 to the learner and mark the
fractured region
•
3. Brittle fracture has
occurred in the element
1
Step 3.2:
T1: Tensile Test for brittle material - Microstructure
2
3
microscopic Image
Zoom
in
type
http://pwatlas.mt.umist.ac.uk/internetmicroscope/micrographs/failure/brittle-steel_z9.html
Interactivi
ty type
4
5
SEM image
Instruction to
learners
Instruction to animators
o
Zoom
in
type
o
o
Take the fractured sample.
Zoom into the fractured region of the
sample and show the schematic image
to the learner
Further zoom into the image shown and
SEM image to the learner.
Text to be displayed
1.
Notice the various
regions of the
fractured sample and
also the region along
which fracture has
taken place
2. SEM image shows more
detailed into the fractured
regions
1
Step 4:
Types of Brittle fracture
Schismati
c Image
2
http://
3
4
Transgranular
Interactivi
ty type (IO
1/IO 2)
Instruction to animators
Show two completely intact images of
grains simultaneously (images on the right
without the black cracks)
Text to be displayed
•
Point
out
In one of the grains show it cracking along
the grain label it as transgranular
•
In the other one show the crack
propagating along the grain boundaries
(blue lines) label it as intergranular
•
5
Intergranular
Observe the Differences
Transgranular Fracture-Fracture
along the grain
Intergranular Fracture-Fracture along
the grain boundary
1.One of the two fractures has occurred
in the above case
1.
APPENDIX 3
1
2
3
4
5
Questionnaire
1. Which of the following components will take the maximum time for fracture?
Answers: a) Steel b)Wood c) Plastic d) Glass
2. Phenomenon of necking occurs in?
Answers: a)Brittle Cast Iron b)Stainless Steel c) Dolomite d) Graphite
3. Ductile Fracture occurs basically due to?
Answers: a)Rusting b)Grain boundary fracture c) Corrosion d) Void Formation
4.Which of this is absent in Brittle metals ?(multiple choice)
Answers: a) Yield Strength b) Plastic deformation c) Fracture d)Necking
1
Instruction to Animators
2
Master layout 1 shows both the ductile and the brittle sample out of which one has to be selected
. If the ductile specimen is selected then the slides 9-14 are shown to the learner.
•The Slide 14 is to be shown if ductile specimen is selected this shows the specific process of void
formation which leads to ductile fracture.
•Slide 15-16 are shown if the brittle specimen is selected.
•In both the cases the microstructures are to be shown as zoomed out from the specimen itself.
•Slides 17-18 should be shown to the user to find the difference in both the processes.
•
•
3
4
5
APPENDIX 2
Reference websites:
http://www.substech.com/dokuwiki/doku.php?id=fracture_toughness
http://www.engineersedge.com/strength_of_materials.htm
http://www.feppd.org/ICB-Dent/campus/biomechanics_in_dentistry/ldv_data/basic.htm
http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Tensile.htm
http://www.feppd.org/ICB-Dent/campus/biomechanics_in_dentistry/ldv_data/basic.htm
Books:
Mechanical Metallurgy – George E. Dieter
Mechanical Behavior of Materials.- Thomas H. Courtney
APPENDIX 3
Summary
•
Elastic deformation:
•
•
The stress and strain initially increase with a linear relationship.
In this region of the curve, when the stress is reduced, the material will return to its original shape.
Yield point: From this point on in the tensile test, some permanent deformation occurs in the
specimen.