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Adapted from Hamrock Text
Ch 16: Joining MethodsRivets and Welded
Joints
Mir M. Atiqullah, Ph.D. Mechanical Engineering Technology Southern Polytechnic State University
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Adapted from Hamrock Text
Why Join ?
Cant make them big enough
Cant make the shape Cant transport
Cheaper Different materials
Different manufacturer at different locations ummm makes manufacturing sense..???
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Joining can be done by..
Major Joining Methods
Screws
Rivets
Adhesive
Welding
Some are permanent, some are not, some are in-between..
PressFit
How else can you assemble/join 2 parts together?
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Riveted Fasteners
Rivets are used to join thin or sheet materials, speciallyductile metals and alloys.
It is a semi permanentjoint, meaning normally it is
permanently joined with rare occasions of disassembly.
..
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Why use rivets..?
Will not come off due to shaking/vibration etc Inexpensive, when done in assembly setup Lightweight Does not necessarily need through hole Can be done w/o access to other side
Not as strong as threaded fastener Can not be disassembled, normally. Causes stress concentration/crack..
Why not use rivets..?
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Failure Modes for Fasteners in Shear
Figure 16.18 Failure modes due to shear loading of riveted fasteners. (a)
Bending of member; (b) shear of rivet; (c) tensile failure of member; (d)bearing of member on rivet.
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Types of Rivets
pop rivets!
Setting rivets
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Figure 16.19 Group of riveted
fasteners used in Example 16.9.
(a) Assembly of rivet group; (b)
radii from centroid to center of
rivets; (c) resulting triangles; (d)direct and torsional shear acting
on each rivet; (e) side view of
member. (All dimensions in
inches.)
Example 16.9
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Welded Joints
Defn: Joining materials (metals, plastic) by causing diffusionat the joint/interface by applying heat and/or pressure.
Often a filler material is used, to strengthen the joint. This is in contrast with soldering and brazing, which involvemelting a lower-melting-point material between the workpieces to form a bond between them, without melting the
work pieces. But HOW???
Surface diffusion (atomic)
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Welding Methods
Electric Arc Welding ( TIG, MIG, ..)
Gas Welding ( Oxy Acetylene,..) Resistance Welding ( Spot welding, roll welding)
Energy Beam - laser, electron beam, etc
Solid State Welding- does not melt the parentmaterials. E.g.
Ultrasonic ( most common for polymers), explosion,
friction, hot pressure, induction, ..
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Weldability
The quality of a weld is also dependent on the combination of
1. materials used for the base material and2. the filler material.
3. Not all metals are suitable for welding, and not all filler
metals work well with acceptable base materials.
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Common Welding Joint Types
(already covered in Mfg. course)
1. Square butt joint,
2. Single-V preparation
joint,
3. Lap joint,
4. T-joint.
5. Corner
5
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(a) Cross-section of weld
showing throat te and legs he
Fillet Weld Lap or corner
Throat- the thinnestsection of the weld-45
from sides.
Under Tensile load asshown, the shear stress
occurs along the throatparallel to the load.
See Welded Joints handout forDesign calculations.(b) shear planes.
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Parallel and Transverse
Loading Welded joints fail by shear at both parallel
and transverse locations.
( )weldsy
we
wewewe
S
Lt
P
Lh
P
Lh
P
Lt
P
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Bending-Normal stress
In bending, welds experience transverse shear as well asnormal stress.
Effective moment of inertia wuewue LIhLItI 707.0==
Where,Iu =unit moment of inertia (?) andLw=Length of weld. See table 16.12 again.
Force per unit length of weld is w=P.a/Iu
where a= distancefrom weld to applied load. Finally,
I
Mc=
Now calc principal stresses and
then use MSST or DET for design.
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Table 16.12 Geometry of welds and parameters usedwhen considering various types of loading.
Geometryof Welds
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Torsion-Shear Stress
Loads on the weld include both direct d and torsional tstresses.
J
Tr
A
Vtd +=+=
r= distance from centroid of weld to the farthest point of weld
J= polar moment of inertia=teJu=0.707 heJu
Ju= Unit polar moment of inertia, as given in Table 16.12 p-747
weldsytd S
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G
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Table 16.12 (cont.) Geometry of welds and parameters used when
considering various types of loading.
Even more Geometry of Welds
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Table 16.12 (cont.) Geometry of welds and parameters used when
considering various types of loading.
Final Geometry of Welds
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Table 16.14 Fatigue strength reduction factors for welds.
Welds in Fatigue
Heat Affected Zone (HAZ)- area around weld whereheat treatment has been altered due to high temp.
Use the following fatigue strength reduction factors tostandard strengths of welding rod and parent materials fordesign.
Q: Where would you use this value ofKf?
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Weld Quality Assurance
Defects Voids
Cracks Improper thickness
Inclusion (of foreign materials, slugs.)
Testing Methods Visual
X-ray Ultrasound Destructive tensile, bending, peeling(sheet metal) etc.
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Summary
Rivets
Welded Joints Types, throat, leg, toe ! Total sh. Stress = direct stress + T..
Unit moment of inertia Unit polar moment of inertia
Welding Inspection HAZ -?