Transcript Slide 1

Fundamentals of Material
Properties
- Part 1Darrell Wallace
Youngstown State University
Department of Mechanical and
Industrial Engineering
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What is the importance of
understanding material
properties?
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Design
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Manufacturing
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Must meet required product characteristics
Selection of material determines applicable
processes
Processing affects material properties
Costs

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Processing
Manufacturing Processes
End-of-Service (Life Cycle)
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Dimensional and Surface
Characteristics
Size
 Shape
 Surface Roughness
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Intrinsic Material Properties
Thermal properties
 Optical characteristics
 Conductivity
 Chemical reactivity

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Functional Material Properties
Strength
 Toughness
 Hardness
 Durability (Fatigue)
 Formability
 Thermal Properties

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Tensile Test
•Simple, low-cost test
•Provides a wide variety
of information about
material characteristics
•Heavily standardized
under ASTM
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Conducting a Tensile Test
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Prior to the test, the crosssection of the test specimen
is carefully measured so
that the initial area is known.
During the test cycle, an
increasing load is applied to
the test specimen.
The change in length of the
test region is measured
throughout the test, usually
using an instrument called
an extensometer.
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Results of a Tensile Test:
Load-Elongation Curve

The raw output of a tensile test is a Load-Elongation curve.
These data are used to calculate stresses and strains
which are more useful for making comparisons between
materials.
Force (lbf)

Elongation (in)
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Engineering Stress and Strain
In the first step of the tensile test analysis
we evaluate the relationship between the
force applied and the deformation of the
material based on its initial state. These
calculations involve significant
simplification of the problem which will be
discussed later.
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Engineering Stress
• Stress : force per unit Area
•Engineering stress is always calculated based on the
initial area of the test specimen.
F
s
A0
F : load applied in pounds
A0 : initial cross sectional area in in²
s: engineering stress in psi
A
F
F
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Engineering Strain

Ratio of change in length to original
length:
e=DL/L
0)/L0
L 0=(L-LDL
0
L
•Calculation is always based on the original length, L0, regardless
of the size of DL
•The engineering strain does not consider that the incremental
change in length is now being spread over a longer distance.
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Engineering Stress-Strain
Curve
The engineering stress-strain curve looks
very similar to the load-elongation curve.
s (psi)

e (in/in)
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s (psi)
Observable Features on the
Engineering Stress Strain Curve
Elastic Region
Onset of Necking
Plastic Deformation
Begins
Test Start
Fracture
e (in/in)
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“True” Stress and Strain
Engineering Stress and Strain are based
on a critical simplifying assumption: they
neglect the changes that occur in the
length and cross-section of the specimen
as it deforms.
 True Stress and True Strain are
instantaneous values that eliminate this
simplification.

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True Strain
Li=Li-1+DL
The incremental strain, therefore
Is found to be:
Strain = DL / Ln-1
L0=gage
L1 =L0+DL
L2=L1+DL=L0+2DL
...
The true strain is the sum of the
Incremental strains as DL  0.
Thus:
e=ln(1+e)
L n = L0+nDL
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True Stress
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The engineering stress calculation is based on the
assumption that the cross-sectional area remains
unchanged. This violates volume constancy.
The change in cross-sectional area is a function
of strain, thus the “true stress” (flow stress) of the
material is calculated as:
s=s(1+e)
where e is the corresponding value of engineering
strain for each stress/strain data pair.
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True Stress-Strain Curve
Notice that the true stress-strain curve does not reach a
peak value and then decrease. As the area decreases,
the true stress continues to increase.
s (psi)
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e (in/in)
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Interpreting the Tensile Test
Results
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We can extract a lot of information from a
tensile test. Let’s now consider some of
the material characteristics that will be
important for design and manufacturing
and gather information about those
characteristics from the stress-strain
curves.
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Stress-Strain Characteristics –
Perfectly Elastic
s
e
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Stress-Strain Characteristics –
Elastic Perfectly Plastic
s
e
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Stress-Strain Characteristics –
Elastic Strain Hardening
s
e
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Elasticity
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Elasticity is the tendency of a material to return to
its original size and shape after deformation. Most
materials, particularly metals, exhibit a region of
elastic deformation
In this region, the material behaves much like a
spring. Any strain that is created in the part will be
restored when the forces are released.
The behavior of the material is virtually identical
for both engineering and true stresses and strains
in the elastic region.
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s (psi)
Elasticity – Hooke’s Law and
Young’s Modulus
Slope=E
Hooke’s Law:
s=Ee
Young’s Modulus:
E=s/e
e (in/in)
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Elasticity Considerations
For Design: In many applications stiffness,
rather than strength, determines the
suitability of a material. (e.g. fishing pole)
 For Manufacturing: The more elastic a
material is, the more deformation you must
apply before you are actually deforming
the material. This leads to significant
“springback” considerations.
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Strength
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“Strength” has several interpretations, depending
on our particular concern. We can ask:
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How much stress can this material sustain before it
deforms? (Yield Strength)
How much stress can this material sustain before it fails?
(Ultimate Strength)
Though we have shown the approximations of
engineering stress and strain, by convention the
values of Yield Strength and Ultimate Tensile
Strength are based on the engineering values.
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Determining Yield Strength and
UTS
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Other Important Features of the
Stress-Strain Curves

We can observe some other important
aspects of the stress-strain curves:
True stress-strain curve for most strainhardening metals can be modeled as an
exponential curve
 Onset of necking can be observed in the
engineering stress-strain curve
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Strength Considerations
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For Design:
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UTS will determine point of catastrophic failure
Yield will determine loading under which permanent
deformation occurs
For Manufacturing:
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Combination of material and manufacturing processes
must achieve required strength characteristics (work
hardening, annealing)
Forces required for forming processes will depend on yield
strength
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Exponential Approximation for
Strain-Hardening Materials
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Formability
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This is an ambiguous term that has a
variety of meanings depending on the
operation(s) to be performed. Some
factors:
Strength
 % cold work
 Strain hardening (n)
 Anisotropy
 Alloying
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Ductility
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Directly for Tensile Test:
Uniform Elongation
 Elongation at Failure
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Secondary Measurements:
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% Area Reduction at failure
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Ductility Considerations
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For Design:
 Ductile materials tend to be able to absorb energy
 These materials will tend not to crack or fail
catastrophically under many impact conditions
 If the design implementation subjects the part to loads
that exceed the yield strength, permanent deformation will
occur.
For Manufacturing:
 Ductile materials tend to be easy to form (particularly in
forging)
 Formability in sheet will depend on strain hardening
exponent
 Very ductile materials tend to be “gummy” and may cause
difficulties in machining or extrusion operations
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Toughness
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“Ability to absorb Energy”
Area under the stress-strain curve
 Can be measured by impact tests
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May be sensitive to a wide variety of
factors
Material purity (internal defects)
 Surface characteristics (notch sensitivity)
 Rate of deformation (strain rate sensitivity)
 Temperature sensitivity (ductile to brittle
temp)
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Charpy Impact Test
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Charpy Impact Test – Ductile
Material
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Charpy Impact Test – Brittle
Material
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Compression Testing
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Some materials exhibit different flowstress characteristics in compression than
in tension. Compression tests are
particularly relevant (from a process
standpoint) for predicting forming behavior
in forging.
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Hardness Tests
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Hardness is defined as a material’s ability
to resist indentation. A variety of tests
exist depending on the hardness of the
material and the circumstances under
which it can be measured.
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Hardness – Indentor Tests
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Brinell (HB, BHN) – round indentor, widely
used, correlates very well to strength:
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Approximation: TS(psi)=500 * HB
Vickers (HV, VHN) – pyramidal indentor
 Knoop (HK) – for checking localized
hardness
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Hardness – Other Tests
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Scleroscope – measures hardness based
on coefficient of restitution (bouncing)
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Scratch Test – relative hardness
measure, most commonly used for very
hard materials such as minerals and
ceramics
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Durometer – indentation test specifically
for polymers and elastomers
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Fatigue Testing
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Under cyclic loading, most materials
exhibit some degradation of strength
characteristics.
Some materials, such as steel, approach
some fatigue limit
 Other materials, such as Aluminum, have no
fatigue limit and will continue to fatigue until
failure
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Creep
Some materials will continue to undergo
strain over time at a given load
 This behavior is often temperature
sensitive
 Very common in polymers and elastomers
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Yield-Point Elongation
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