Ch16 Welded Joints

8/10/2019 Ch16 Welded Joints

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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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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?


4/25Adapted from Hamrock Text

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.

..



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..?



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.



Types of Rivets

pop rivets!

Setting rivets



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



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)



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, ..



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.



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 -?

Documents

Ch16 Welded Joints