2. Course Instructor:
Waqas Asghar
Lecturer,
Mechanical Department, UET Taxila
PhD. (2017-2021): University of Chinese Academy of Sciences,
China.
MSc. (2013-2016): University of Engineering & Technology, Taxila.
BSc. (2008-2012): University of Engineering & Technology, Taxila.
2
Email: waqas.asghar@uettaxila.edu.pk
Office: Engineering Mechanics Lab
3. Course Contents:
3
โข Stresses & Strains
โข Mechanical Properties of Materials
โข Torsion
โข Shearing Force & Bending Moment
โข Centroids & Moment of Inertia
โข Bending of Beams
โข Beam Deflection Methods
5. Grading Policy:
5
โข Quizzes or Assignments: 12%
โข Semester Project or CEP: 13%
โข Mid-semester Exam: 25%
โข Final Exam: 50%
6. Course Books:
6
โข Mechanics of Materials by Gere, 6th Edition
โข Mechanics of Materials by R. C. Hibbler, 8th Edition
Reference Books:
โข Strength of Materials by R. K Rajput
โข Mechanics of Materials by Debabrata Nag and Abhijit Chanda
โข Mechanics of Materials by Ferdinand P. Beer & Russel Johnston
Jr. McGraw-Hill
7. Introduction
Mechanics of Material: Branch of mechanics that deals with behaviour of materials subjected
to various type of loadings also called Mechanics of solids or Mechanics of deformable bodies.
Main objective of MMT: To find stresses, strains and displacement in structures for the complete
evaluation of mechanical behaviour of materials necessary for safe designing of structures.
8. Introduction
Axial force
Force or Load acting on the axis of member which leads to tension or compression.
Example:
Connecting road of engine, spokes of bicycle, columns of buildings, tow bar to pull aeroplane,
landing gear of plane, etc..
In order to have uniform tension or compression, axial force must act through the centroid of
cross sectional area.
9. Normal Force
โข Normal forces act perpendicular to the cross sectional area.
โข Angle between the forces and longitudinal axis is 0ยบ
โข It tries to changes the axial dimensions of object
10. Shear Force
โข Shear forces act parallel to the cross sectional area.
โข Angle between the forces and longitudinal axis is 90ยบ
โข It tries to changes the shape of object
No rotation
14. Identify the Type of Force??
Elongated spring Crushing failure of Can
Columns of building
Rope in tug of war
15. Types of Bars
Prismatic bar:
A bar having constant cross sectional area throughout its length.
Non-Prismatic bar:
A bar which does not have constant cross sectional area throughout its length.
Prismatic bars Non-Prismatic bars
Cross Section: Section taken perpendicular to the longitudinal axis of the bar, is called a cross section.
16. Stress (๐)
In an externally loaded material, stress is the intensity of internal resisting force
which holds the each segment of body in equilibrium against deformation.
OR
Force per unit area is called stress.
Stress may remain constant throughout the body or may change from one point
to another
Direct or Normal Stress:
โข Stress produced due to normal force acting on a bar.
โข In direct stress, load acts perpendicular to the cross sectional area of body.
โข e.g. Axial Stresses, Bending Stresses, Radial Or Hoop Stresses.
Types: Tensile stress (+ive) or compressive ( ฬถ ive)
๐ =
๐๐๐๐ ๐๐ ๐ญ๐๐๐๐
๐๐๐๐
=
๐ท
๐จ
๐๐ง๐ข๐ญ๐ฌ = เต
๐
๐2 ๐๐ ๐๐ ๐๐ ๐๐ ๐.
๐ =
๐ต๐๐๐๐๐ ๐ญ๐๐๐๐
๐๐๐๐
=
๐ท
๐จ
17. Deformation: Force applied on body changes the shape and size of body, which
is called deformation
Strain: Measure of deformation produced in material due to applied load.
Direct or Normal Strain: Measure of materialโs deformation produced due to
normal or direct load.
๐ซ๐๐๐๐๐ ๐๐ ๐ต๐๐๐๐๐ ๐บ๐๐๐๐๐ ๐บ =
โ๐
๐
Units: Dimensionless
Types & Sign Convention:
โข Tensile strain (+ive) or Compressive strain ( ฬถ ive)
Mostly, extension of material under load is very small that's why it is expressed as a %age strain, as
๐๐๐๐๐๐ ๐บ =
โ๐
๐
ร ๐๐๐%
19. Uniaxial stress
1D state of stress in which only one normal stress act on body, along any
single direction.
Stress components acting on uniaxial stress system should be equal in
magnitude but opposite in direction.
Biaxial stress
2D state of stress in which two normal stresses act along two different
directions is called biaxial stress
Triaxial stress
3D state of stress in which three normal stresses act along three different
directions is called triaxial stress
20. Shear Stress (๐):
โข It is defined as the stress applied to the parallel or tangential face of material
๐๐๐๐ =
๐ฐ๐๐๐๐๐๐๐ ๐๐๐๐๐ ๐๐๐๐๐
๐จ๐๐๐ ๐๐๐๐๐๐๐๐๐ ๐๐๐๐๐
=
๐ฝ
๐จ
โข Shear stress (ฯ) acts in the same direction as V
โข Units: same as normal stress
โข Shear stress acts in various types of simple connections, like : bolts, pins, rivets, keys,
welds, and glued joints.
โข Normal stress tends to change the size of material and shear stress changes the shape of
material.
22. Shear Strain (ฮณ):
โข Measure of change in angle between two lines segments, which were originally perpendicular to
each other.
โข It is measured in degrees or radians
โข E.g. Bicycle brake block deformed due to application of shear force.
Shear Force
โข Sign Convention: ฮณ is positive ----- angle b/w two faces is reduced
ฮณ is negative ---- angle b/w two faces is increased
โข Normal strain changes volume/ dimension (elongation or compression) of object.
โข Shear strain changes shape of object.
23.
24. Example 1-1 of Gere
Short post constructed from a hollow a Al circular tube supports a compressive load of
26 kips. Inner and outer diameter of tube are di= 4in and do=4.5 in and its length is 16
inch. shortening of post due to load is 0.012 inch. Find compressive stress and strain.
Solution
l=16 in , โ๐= 0.012in,
di=4 in , do=4.5 in
Area of tube= ๐๐2 =
๐๐2
4
=
๐
4
๐๐2 โ ๐๐2 =
๐
4
4.52 โ 42 = 3.38๐๐2
๐๐๐๐๐ =
๐
๐ด
=
26๐๐๐๐
3.38 ๐๐2
=
26000 ๐๐
3.38 ๐๐2
= 7790 ๐๐ ๐
๐๐๐๐๐ =
โ๐
๐
=
0.012
16
= 750 ร 10โ6
25. Curve plotted by using stress and strain data obtained from tensile test of specimen is
called stress strain (ฯ-ฮต) diagram or curve.
๐ โ ๐ diagram gives important information about the mechanical properties and
behaviour of material under various stages of loading.
Tensile test on material is performed by gradually applying load (using m/c) and
resulting deformations are measured until failure.
Gauge Length: Part of test specimen, actually being measured for elongation, during
a tensile test.
Extensometer: Device used to measures the elongation produced tensile test.
Stress-Strain Diagram (ฯ-ฮต)
Test conducting standards: American Society for Testing and Materials (ASTM), American
Standards Association (ASA) and National Institute of Standards and Technology (NIST).
28. Elastic Region:
โข Region in which ฯ & ฮต exhibit linear and
proportional relationship.
โข Material exhibits exhibit linear elastic behaviour in
this region.
โข Slope of Elastic region = Youngs Modulus (E).
โข Youngs Modulus (E) = Measure of materialโs
stiffness, in its elastic range and has the units of
stress.
๐๐ฉ๐๐๐ข๐๐ข๐ ๐ฌ๐ญ๐ข๐๐๐ง๐๐ฌ๐ฌ =
๐๐จ๐ฎ๐ง๐ โฒ๐ฌ ๐๐จ๐๐ฎ๐ฅ๐ฎ๐ฌ
๐๐๐ญ๐๐ซ๐ข๐๐ฅโฒ๐ฌ ๐๐๐ง๐ฌ๐ข๐ญ๐ฒ
29. OR
Transition point between linear elastic deformation
region and non-linear elastic deformation.
OR
It determines the greatest stress that is directly
proportional to strain.
โข Below proportional limit, material deforms linearly,
elastically & is able regain its original shape on
removal of applied load.
Proportional limit:
Upper stress limit of linear elastic region is called proportional limit (๐๐๐).
30. Yield Point:
โข Point on ๐ โ ๐ curve, where elastic behaviour of
material changes into plastic behaviour.
โข OR Transition point between elastic & plastic region.
โข Below yield point, material deforms non-linearly,
elastically & is able regain its original shape on
removal of applied load.
โข In yielding region, material becomes perfectly plastic and continues to elongate without any
noticeable increase of load. This phenomenon is called yielding.
โข Low carbon steels exhibit two values of yield point i.e. initial upper yield point followed by certain
decrease in load carrying capacity to a lower yield point.
โข Yield strength or Yield stress: Stress corresponding to yield point, after which material begins to
deform permanently or plastically.
31. Strain hardening
โข Strengthening the material by plastic deformation.
โข It occurs when metal is plastically deformed
beyond yield point.
โข Material undergoes change in its crystalline
structure which ultimately increases the strength
of material.
โข During strain hardening, elongation of the test
specimen requires an additional amount of tensile
load.
โข Finally, load (๐ โ ๐ curve) reaches its maximum value, at which corresponding value of stress
(point D) is called the ultimate stress (๐๐ข).
โข Strain hardening is also known as work hardening or cold working.
32. Ultimate stress (UTS):
โข Maximum stress or load that a stretched material can
withstand, before failure.
โข UTS often corresponds to highest point (point D) on
๐ โ ๐ curve
โข Beyond UTS, specimen deforms even load is removed.
โข Materialโs ultimate stress is also called ultimate
strength or ultimate tensile strength of material.
Strength: Capability of structure to resist load.
๐๐ฉ๐๐๐ข๐๐ข๐ ๐ฌ๐ญ๐ซ๐๐ง๐ ๐ญ๐ก =
๐๐ญ๐ซ๐๐ง๐ ๐ญ๐ก ๐๐๐
๐๐๐ญ๐๐ซ๐ข๐๐ฅโฒ๐ฌ ๐๐๐ง๐ฌ๐ข๐ญ๐ฒ
33. Necking:
โข Type of plastic deformation observed in ductile materials.
โข Localized reduction of materialโs cross sectional area,
under tensile load.
โข Before UTS, entire material undergoes uniform plastic
deformation.
โข But just after UTS, cross sectional area of small localized
region decreases more rapidly (by greater proportion), &
eventually adopts neck or V shape.
โข Neck exhibits high concentration of local strain.
โข Necking behavior is considered in calculating true stress
but disregarded in calculating engineering stress.
Neck
34. Fracture Stress:
โข Stress or load at which material failure occurs.
โข At fracture stress, necking region ends & strain
reaches its max. value
โข Ductile materials: Fracture strength < UTS
โข Brittle materials: Fracture strength = UTS
38. Yield Strength of Materials having No Well-defined
Yield Point
โข Aluminum does not show any obvious yield point
โข Arbitrary yield stress can be determined by using Offset
method.
โข In offset method, 0.2% strain (0.002 in/in ) is chosen as a
reference point on strain axis.
โข From this reference point, a line parallel to the initial straight-
line portion of the stressโstrain diagram is drawn.
โข The point where this line intersects the curve defines the yield
strength.
Yield
Strength