10/11/2021
Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Stress- What’s that mean? – Ứng suất là gì?
• No applied force = Unloading
MACHINE ELEMENTS DESIGN
-CONCEPTS AND ENGINEERING TEMS-
• One solid bar under effect of two
opposite forces (loads) following its
central axis = uniaxial loading period.
• External forces occur internal forces due
to the resistance phenomenon. (imagine
in a cross section within the bar)
Strength of Materials & Durable of Structure
- Stress and Strain
Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM– 0948 777 040 – ducpham@dut.udn.vn
Left side
• For equilibrium: External forces = internal
forces
Right side
• STRESS: quantity to describes the
distribution of internal forces within a body
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Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Stress- What’s that mean? – Ứng suất là gì?
Stress- What’s that mean? – Ứng suất là gì?
• STRESS:
• Tensile stress: Sketch objects.
In SI:
: in ANSI
• Compressive stress: compress objects.
• Normal stress: stress occurs when a member
is loaded by an axial force.
• For example: Strength of steel C30 (JIS:
S30C) C30 = 570(Mpa)
Diameter of pipe 40 (mm)
Failure occurs when >C30
Square of pipe 1,256x10-3(m2)
=
Fext
=
Fext
×
F = 71.59 (kN)
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* Stress of I beam more complex
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10/11/2021
Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Stress & Strain correlation – Mối quan hệ ứng suất & sức căng
Strain- What’s that mean? – Sức căng là gì?
• STRAIN: quantity to describes the
deformation occurring within a body
=
L
• Stress & Strain: In section of Elastic
deformation, this correlation is followed by
HOOKE’s law:
Elastic
region
 = E.

• Based on Stress&Strain graph, all
important properties of material can be
known (Strength, Young modulus,
Ductility and Toughness.
L
Rise
Run
• STRESS and STRAIN show a
correlation showing a property of
material
Plastic region
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Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Shear Stress – Ứng suất cắt
Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Shear Stress – Ứng suất cắt
• Normal stress (): External forces
affect to object in perpendicular to
cross section
• Shear strain ( ): change in angle in
deformed process with shear stress.
• Shear stress ( ): External forces
affect to object in parallel to cross
section
= G.
• At any point in body, stress state always
includes Normal and Shear stresses. But
values of those stresses can be different
due to direction of applied force or
analyzed cross-section.
=
*** Average value is due to
that the internal forces will not
be distributed evenly across
the cross-section.
Deformed
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Hooke’s law for shear
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10/11/2021
Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
True stress-strain curve
– đặc tính thật của đường ứng suất và sức căng
• Principle:
 Use Yield strength of material in chosen of
design.
(Elastic region).
 Knowing some important points of stressstrain curve to manufacturing and analysis
elements.
• True stress-strain curve (hooke’s law) is
different with engineering curve
• Reasons:
 Difference between cross-sections
(necking phenomenon).
Ultimate
tensile
strength
Yield
strength
True stress-strain curve
– đặc tính thật của đường ứng suất và sức căng
Necking process
 Deformation Region.
Engineering
Necking
phenomenon
Elastic
region
Plastic
region
Manufacturing process

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=
=
=
(1 +
)
= ln(1 +
)
=

Simulated engineering
analysis
=
−1
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Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Material properties – Đặc tính vật liệu
Material selection:
• Effected on
strength of
machine
Machinablity
Properties
Understand
material to:
Performance
• Ex:
Welding,
cutting
There are three essential properties of material: Strength, Ductility, and Toughness
• Suitable for
machine
purpose
Avabilily
• Supply and
cost
• Choose suitable type of
material
• Have a knowledge of
limit strength of machine
elements ( working
area).
• Repair and Re-design.
• Maintenance.
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10/11/2021
Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Material properties – Đặc tính vật liệu
Converter among the world standards
• Three main elastic constains:
 Young modulus
 Shear modulus
 Bulk modulus
DIN: Deutsches Institut für Normung
BSI Group: British Standards Institution
ANSIANSI: American National Standards Institute
AISI: The American Iron and Steel Institute
ASME: The American Society of Mechanical Engineers
CSA Group: Canadian Standards Association

=
=
=
(1 +
)
= ln(1 +
)
JISC: Japanese Industrial Standards Committee
GB: Guobiao
Simulated engineering
analysis
Manufacturing process
=

=
−1
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ISO: Int. Org. for Standardization
ASTM International: Ame. Stand. for Testing & Materials
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Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Faculty of Mechanical Engineering
Dr. Anh-Duc PHAM
Sample table
Effects from common Alloyng Elements
#1: Very high carbon steel (0.96%-2.1% carbon)
Produced via a specialized process, very high carbon steel is extremely strong and capable of
resisting localized deformation under high tensions.
#2: High carbon steel (0.55%-0.95% carbon)
Used until recently in railroad track applications, high carbon steel is quite strong, but does
offer more ductility than very high carbon steels. Other examples of use include cutting and
chiseling tools.
#3: Medium carbon steel (0.3%-0.54% carbon)
Used primarily in machine parts (i.e. gears, axels, bolts), medium carbon balances strength and
hardness with ductility.
#4: Low carbon steel (0.05%-0.25%)
low carbon steel provides incredible strength relative to a lighter weight.
• Iron alone is not enough….
- ELEMENTS THAT STRENGTHEN: chromium, vanadium, nickel, titanium, nitrogen & aluminum combo
- ELEMENTS THAT INCREASE MACHINABILITY: Phosphorus, Chromium, Sulfur, Niobium (Columbium)
- ELEMENTS THAT RESIST CORROSION: Phosphorus, Copper, Chromium, Nickel, Zirconium
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