Assembly technique
Mechanical engineering - Semester 6 - ULFGII
Dr. Stéphanie Chahine
" Design of non-permanent joints,
I
fasteners joints: thread standards,
the mechanics of power screws,
threaded
fasteners,
fastener
and
member stiffness, bolt strength.
" Design
of
permanent
joints,
welding
and
bonding:
welding
symbols,
welding
in
welded
stresses
bending,
in
type,
stresses
in
torsion,
joints
welded
strength
joints
of
in
welded
Jjoints, static and fatigue loading,.
I
* Understand
the
Principle
andI
Applications of Permanent Joints
» Explore
the Principle and Versatile
Uses of Temporary Joints
* Recognize
the
Importance
of
Thermal
Effects
in
Welding
and
Implement
Remedial
Measures
for
Residual
Stress
and
Distortion
Proficiency
in
Assessing
Reduction
e Gain
Weldability, Designing Weld Joints,
and Automating
Welding Processes
for Various Materials
I
CHAPTER I
INTRODUCTION
Mechanical
Engineering
Design
* Mechanical design is a challenging task, requiring many skills.
* Design is an iterative process with many interactive phases.
* Codes
and
standards,
economic
factors,
safety
concerns,
all
play roles in the design process.
* The
survival
of a mechanical
component
is often
related
through stress and strength.
* Matters of uncertainty are presented in engineering design by
the design factor and factor of safety.
Mechanical
* Mechanical
Engineering
engineers
work
on
Design
generating
energy,
manufacturing goods, developing transportation methods, and
implementing automation technologies.
* Real-world problems are not easily divided into separate parts.
* Designing a simple structure involves a range of factors like
fluid flow, heat transfer, and material selection.
* Design may
seem distinct, but they're all part of mechanical
engineering and rely on similar knowledge and skills.
Mechanical
Engineering
Design
* What is the design process?
* How does it begin?
Identification of need
* Does the engineer simply write
Definition of problem
down some ideas?
Synthesis
* What happens next?
Analysis and optipnization
e What
factors
influence
or
Evaluation
control the decisions that have
to be made?
* How
end?
does the design process
Presentation
Mechanical Engineering Design
Design Considerations
* Some
of these
characteristics
have
to do
directly with the
dimensions, the material, the processing, and the joining of the
elements of the system.
* Several characteristics may be interrelated, which affects the
configuration of the total system.
1
Fu
14
Noise
Mechanical Engineering Design
Tools and Ressources
* Engineer has a great variety of tools and resources available to
assist in design.
* A
robust
computer
software
provide
tools
capability
for
design,
analysis,
and
mechanical
components.
the
of
immense
simulation
of
This will allow the development of
three-dimensional that gives rapid and accurate calculations.
* The
engineer
always
needs
technical
textbooks, brochures or catalogs.
information
such
as
Mechanical
Engineering
Design
Engineer Responsabilities
* Design
engineers
must meet the needs
of interested parties
competently, ethically, and responsibly.
Effective
= communication
1is
vital,
with
journaling
recommended for clarity and record-keeping.
* Adopting a systematic problem-solving approach is essential.
* Ongoing
professional
growth
is encouraged:
participation in
societies, and staying updated.
* Ethical
conduct,
prioritizing human
fundamental for success.
welfare
and honesty,
is
Mechanical
Engineering
Design
Engineer Responsabilities
The consideration of cost plays an important role in the design
decision process that we could easily spend as much time in
studying the cost factor as in the study of the entire subject of
design.
Mechanical Engineering
Uncertainty
* Engineers must accommodate
Design
uncertainty, with mathematical
methods deterministic or stochastic.
* Examples of uncertainties:
» Composition of material
» Effect of temperature
* Intensity and distribution of loading.
» Validity of mathematical models used to represent reality.
* Intensity of stress concentrations
* Influence of time on strength and geometry
« Effect of corrosion. ..
Mechanical Engineering Design
Stress and Strength
* The durability of products relies on designers ensuring that the
maximum stress within a component is lower than its strength
at critical points, with a safety margin to prevent failure even
under uncertain conditions.
* Strengths
is a property
of a material
or of a mechanical
element. The strength of an element depends on the choice, the
treatment, and the processing of the material.
* Stress 1s a state property
which
is
a function
at a specific point within a body,
of load,
manufacturing processing.
geometry,
temperature,
and
Material
Strength
and
Stiffness
:
P
:
.
* The load is converted to stress: o = . where A, is the original
0
area
and P is the load.
* The normal strain ;& =
Strain €
(a)
B)
Typical stress-strain diagrams
(a) for ductile and (b)brittle materials
Material
Strength
and
In the linear range, stress-strain
relation
is given
by
Hooke’s
law:
a="E=r
where
E
is the
modulus
of
elasticity (Young’s modulus) is
the slope.
Stiffness
Material
Strength
and
* Pl: proportional limit:
s
point at which the curve first ;
Stiffness
Y[
:
|
|
E
!
/i
4
begins
to
straight line.
deviate
from
a
g-:
|
Pl
1
1
.
!
!
el‘
"
%r
Material
Strength
and
Stiffness
* ¢l: elastic limit:
beyond
this
point,
the
deformation is plastic and the
material
will
take
on
a
permanent set when the load is
removed.
”
{
i
‘
=
N.B: Between pl and el the diagram is not a perfectly
straight line, even though the specimen is elastic.
Material
Strength
and
Stiffness
stress. Yield strength S,
B’
point at which the strain begins
to increase very rapidly without
a corresponding increase in
Sress o =: ‘W‘q'n
* y: yield point:
”
!
/
N.B: Not all materials have an obvious yield point,
especially for brittle materials
Material
Strength
and
Stiffness
* u: ultimate point:
Maximum stress reached on the
stress-strain diagram. Ultimate
strength 5,,, Sy¢
N.B: Not all materials have an obvious yield point,
especially for brittle materials
Material
Strength
and
Stiffness
« f: failure point:
Some
materials
show
a
decrease
in
strength
after
reaching maximum stress and
break(failure occur). Failure
strength Sy
N.B: For brittle materials, point u and f are identical.
Temperature
Effects
» Strength and ductility, or brittleness, are properties affected
by the temperature.
* Tensile
strength
remains
1o
relatively stable until a specific
=~ @
temperature, after which it drops
<
rapidly.
0.6
* Yield strength decreases steadily
iy
:
with increasing temperature.
R
gy
200
400
Temperature,°C
600
Impact of temperature on steels
* Ductility
increases
temperatures.
at
higher
Temperature
*
Effects
Creep is the gradual
when
it
1s
deformation of a material over time
subjected
temperatures.
This
to
a
constant
loads,
at
high
deformation
occurs
even
when
the
shape
and
applied stress is below the material's yield strength.
*
Creep
can
lead
to
permanent
changes
in
dimensions of the material and is an important consideration
in the
design
and
engineering
under high temperatures and loads.
of components
operating
Temperature
Effects
» First stage: Includes both elastic and plastic deformation, with
*Second
stage:
Exhibits
a
constant
minimum
creep
rate
attributed
to
the
annealing
effect.
* Third stage: Shows a significant
reduction in area, increased true
stress, and eventual fracture due
to higher creep.
Creep deKirma tion
a decreasing creep rate due to strain hardening.
Hot
and
Cold Working
Proecesses
* Hot working refers to processes
like
rolling,
extrusion,
where
beyond
forging,
and
the
hot
metal
its
hot
100
pressing,
.
is
heated
_
recrystallization
":2:
ql:ml\
E a0
/\
Hat-rolled
= ¥
40
temperature.
* Cold working refers to shaping
o
metal at low temperatures (room
temperature). Unlike parts made
through hot working.
0z
04
Eloagatiom,in
&
Equilibrium
State
* The system refers to any isolated part of a machine or structure
that we want to analyze.
« If a system is motionless or moving at a constant velocity, it
has zero acceleration and is considered to be in equilibrium.
* Equilibrium means that the forces and moments acting on the
system are balanced, keeping it at rest or in a steady state:
* >F=0
*YM=0
Free-Body
*We
can
Diagrams
simplify
the
analysis
of
complex
structures
or
machines by isolating each element and studying it with freebody diagrams.
* After analyzing all elements individually, we can combine the
information to understand the behavior of the entire system.
* Free-body diagramming helps break down complex problems
into manageable
parts for analysis
and then integrates
findings to gain insights into the overall system behavior.
the
Shear
Force
and
Bending
Moments
¥
* The shear force V is obtained by summing the forces on the
isolated section.
* The bending moment
M
is the sum
of the moments
of the
forces to the left of the section taken about an axis through the
isolated section.
_am
(c:;:.‘)
(;::,)
Force
and
Bending
Moments
-
Shear
» If the bending is caused by a distributed load g:
LAV
d?M
1=~
d
N.B: Pay attention to the direction of the force
Cartesian
* A
complete
Stress
state
Components
of
stress
is
defined
by
nine
stress
COMPONENts; Gy, Gy, O, Toeys Tazs Ty Typzs Tznes Ty
* For equilibrium, cross-shears are equal, reduce the problem to
six stress components: Tyy = Tyy, Tyz = Tzy,Txz = Tzy.
*In case of a plane stress : stresses on one surface are zero,
¥
800, = Ty; =Ty, =0
s
Ty
Try
=
Ty
t iy
Elastic
Strain
* In tension, there exists an axial strain and two lateral strain
(negative strain)
» Assuming a linear, homogeneous, isotropic material, and x the
5
.
.
0.
0.
axial direction :g, = F" and g, = ¢, = —v—=
* For stress element in the three directions:
-~E 4, — v(o, W + 0,)]
it
g, = Vi [oy,
Hooke’s law: 0 = E¢
—Vv(oy + 0,)]
il
€, = B [0,
V(O-x + o-J/)]
* Shear strain y is the change of stress element when subjected
to pure shear stress:T = Gy
Uniformly
Distributed
*In
common
design,
distributed,
it's
leading
to
to terms
Stresses
assume
like
"pure
stress
is
evenly
tension,"
"pure
compression," or "pure shear" depending on how the load is
applied.
* "simple" is used instead of "pure" to indicate there are no
other complicating factors.
* The assumption of uniform stress allows us to write: