EAS 209 – Fall 2010
Instructor: Human
8/24/2010
1
02FDB/Normal Stress/Shear Stress
Lecture 2  Concept of Stress
•
Study of mechanics of materials provides future
engineers with the means of analyzing and designing
various machines and load bearing structures.
•
Both the analysis and design of a given structure
involve the determination of
stresses
and
deformations
.
This chapter is devoted to the concept
of stress.
Today’s Objectives:
•
Review concepts of equilibrium learned in Statics
•
Free Body Diagram (FBD)
•
Force triangles
•
Understand concept of normal stress,
σ
and shear
stress,
τ
•
Calculate normal stress in bars
•
Calculate shear and bearing stresses in bolted
connections
Today’s Homework
EAS 209 – Fall 2010
Instructor: Human
8/24/2010
2
02FDB/Normal Stress/Shear Stress
Review of Equilibrium
The structure below consists of a boom and rod joined
by pins (zero moment connections)
Construct FreeBody Diagram:
Detach the structure
from supports and apply the loads and reaction forces.
Solve for reactions
: four
unknown forces, three equations
of equilibrium
The structure is not statically
indeterminate because both AB
and BC are twoforce members
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EAS 209 – Fall 2010
Instructor: Human
8/24/2010
3
02FDB/Normal Stress/Shear Stress
Definition
twoforce member:
a member that has pins
or hinge supports at both ends and carries no load in
between
.
For a twoforce member the internal force is
directed along the member.
If we recognize that both the rod and boom are twoforce
members, we can solve for
F
AB
and
F
BC
by considering
equilibrium at joint B
F
AB
=40 kN (compression)
F
BC
=50 kN (tension)
0
30kN
4
5
3
40kN
50kN
B
BC
AB
AB
BC
F
F
F
F
F
=
=
=
=
=
∑
r
Sign convention:
Tension is +ve (force points away from the joint)
Compression is –ve (force points towards the joint)
For the joint to be
in equilibrium, the
force triangle
must close
EAS 209 – Fall 2010
Instructor: Human
8/24/2010
4
02FDB/Normal Stress/Shear Stress
Note:
use force triangle whenever you know the
orientation of all three forces, and the magnitude of one.
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 Spring '08
 Force, Shear Stress, Stress, 02FDB/Normal Stress/Shear

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