elmes i bab 9.pptx

7/28/2019 Elmes I Bab 9.pptx

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Welding, Bonding, and Design of Permanent Joints


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Introduction

Welding is the process of joining two pieces of metaltogether by hammering, pressure or fusion. Filler metalmay or may not be used.

The strongest and most common method of permanentlyjoining steel components together.

Arc welding is the most important since it is adaptable to

various manufacturing environments and is relativelycheap.

A weldment is fabricated by welding together a collectionof metal shapes.


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Introduction

A pool of molten metal in which the components andelectrode material coalesce, forming a homogeneous whole(ideally) when the pool later resolidifies.

The materials of components and electrode must becompatible from the point of view of strength, ductility andmetallurgy.

The form of a welded joint is dictated largely by the layout

of the joined components.

Two most common forms are:1. The buttjoint2. The fillet

joint


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Welding Symbols


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Figure 9-2

Arc and gas-weld symbols

There 2 general types of welds:

1. Fillet welds for general machine elements.

2. Butt or groove welds for pressure vessels, piping systems,...

There are also others such as: ,

Fillet

groove

Bead

Bead Plug or slot

Plug

or slot

Types Of Welding


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Parts to be joined must be arranged so that there is sufficientclearance for welding operation.

Due to heat, there are metallurgical changes in the parent metal in

the vicinity of the weld.

Residual stresses may be introduced because of clamping or holding.

These residual stresses are not severe enough to cause concern.

A light heat treatment after welding is done to relive these stresses.

When the parts to be welded are thick, a preheating will also be ofbenefit.


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Figure 9-3

Fillet welds


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Figure 9-4

The circle on the weld symbol indicates that the welding is to

go all around.


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Figure 9-5


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Figure 9-6


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Butt and Fillet Welds

where h is the weld throat and lis the length of the weld. Notice that the

value ofh does not include the reinforcement.


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The reinforcement can be desirable, but it varies somewhat and doesproduce stress concentration at point A in the figure. If fatigue loadsexist, it is good practice to grind or machine offthe reinforcement.


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Stresses in Fillet Welds

At angle q the forceson each weldmentconsists of a normal

forceFnand a shear

forceFs

sin ,

cosn

sF F

F F

q

q

Fig. 9-8 illustrates a typical

transverse fillet weld.

In Fig. 9-9 a portion of thewelded joint has beenisolated from Fig. 9-8


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Stresses in Fillet Weldssin (cos sin )

cos (cos sin )n

sF F

A hlF F

A hl

q q q

q q q

The nominal stresses at the angle

in the weldment, and , are


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The von Mises stresse at angle is

max occurs at = 62.5owith a value ofmax = 2.16 F/(hl).

The corresponding values of and , are = 1.196 F/(hl) and =

0.623 F/(hl).

max can be found by solving the equation [d()/d]=0.

The stationary point occurs at = 67.5o with a corresponding max =

1.207 F/(hl) and = 0.5 F/(hl).

1 2

2 21 2

2 2

2 2

(cos sin cos )' 3

3(sin sin cos )

F

hl

q q q

q q q


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We have no analytical approach that predicts the existing stresses.

The geometry of the fillet is crude by machinery standards.

The approach has been to use a simple and conservative model,

verified by testing as conservative.


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Consider the external loading to be carried by shear forces on thethroat area of the weld. By ignoring the normal stress on the throat, the

shearing stresses are inflated sufficiently to render the model

conservative.

Use the distortion energy for significant stresses

Circumscribe typical cases by code

For this model, the basis for weld analysis or design employs

which assumes the entire force Fis accounted for by a shear stress in

the minimum throat area.

The approach has been to:

1.414

0.707

F F

hl hl (9.3)


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Notice that this inflates the maximum estimated shear stress by a

factor of1.414/1.207=1.17.

Further, consider the parallel fillet welds shown in Fig. 9-11 where, as

in Fig.9-8, each weld transmits a force F. However, in the case ofFig. 9-

11, the maximum shear stress is at the minimum throat area and

corresponds to Eq. (9-3).


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Under circumstances of combined loading we:

Examine primary shear stresses due to external forces.

Examine secondary shear stresses due to torsional and bendingmoments.

Estimate the strength(s) of the parent metal (s).

Estimate the strength of the deposited weld metal.

Estimate the permissible load(s) for parent metal(s).

Estimate permissible load for deposited weld metal.


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Stresses in Welded Joints in TorsionFigure 9-12 illustrates a cantilever of length l welded to a column by 2 filletwelds.

The reaction at the supportof a cantilever alwaysconsists of shear force V anda moment reaction M.

The shear force produces aprimary shearin the weldsof magnitude

where A is the throat area ofthe welds.

' VA

(9.4)


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The moment at the supportproduces secondary shearor

torsion of the welds, and thisstress is given by

where

r: distance from the centroid ofthe weld group to the point inthe weld of interest.

J: second polar moment of areaof the group about the centroidof the group.

"Mr

J

(9.5)


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Figure 9-13 shows 2 welds in a group. The rectangles represent the throatareas of the welds.

Weld 1 has a throatwidth b1 = 0.707 h1

Weld 2 has a throatwidth d2 = 0.707 h2

Throat area of bothwelds together is

A = A1 + A2 = b1d1 + b2d2

which is the area to be

used in Eq. (9-4)

The x-axis passes through


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The x axis passes throughthe centroid G1of the weld1.

The second moment of

area about this axis is

Similarly, the second

moment of area about anaxis passing through G1parallel to the y-axis is

The second polar momentof areas ofweld 1 and weld2 about their centroids are

3

1 1

12x

b dI

3

1 1

12

y

d bI 3 3

1 1 1 11

3 3

2 2 2 22

12 12

12 12

G x y

G x y

b d d b

J I I

b d d bJ I I

The centroid G of the weld group is located at


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The centroid G of the weld group is located at

The distances r1

and r2

from G1

and G2

are respectively given by

1 22 2

1 1

1 22 2

2 2 2

r x x y

r y y x x

1 1 2 2 1 1 2 2A x A x A y A yx yA A

Using the parallel axis theorem,the second polar moment of areaof the weld group is

2 21 1 1 2 2 2G GJ J A r J A r

This is the quantity to be used in Eq. (9-5). The distance r must bemeasured from G and the moment M computed about G.


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The quantities and , which represent the weld width are small and

hence can be neglected.

The terms and Makes JG1

and JG2

linear in the weld

width.

Setting weld widths b1 and d2 to unity leads to the idea of treating each

fillet weld as line.

The resulting second moment of area is then a unit second polar

moment of area.

The value ofJu same regardless of weld size.

Since throat width of a fillet weld is 0.707h, the relation betweenJ and

the unit value is 0.707 uJ hJ (9.6)

3

1b3

2d

3

1 1 12b d

3

2 2 12d b

Ju

: is found from table 9.1


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Fig. 9-17a shows a cantilever welded to a support by fillet welds at topand bottom.

9-4 Stresses in Welded Joints in Bending

A FBD diagram of

the beam would

show a shear force V

and a moment

diagram M.

The shear force

produces a primary

shear in the welds of

magnitude

' V

A

The moment M introduces a throat shear stress

component of0.707 in the welds.

Treating the two welds of Fig. 9-17b as lines we find

the second moment of area to be2

2u

bdI

(a)

(b)


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The second moment of area I, based on weld throat area, is

The nominal throat shear stress is now found to be

The model gives the coefficient of 1.414, in contrast to the predictionsof Sec.9-2 of1.197 from distortion energy, or 1.207 from maximum shear.

The second moment of area in Eq. (d) is based on the distance dbetween the two welds.

Stresses in Welded Joints in Bending

20.707 7

20. 07 uI h

bdIh

22 1.414

0.707 2

M dMc M

I b d hh bd

(c)

(d)


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The Strength of Welded Joints The matching of the electrode properties with those of parent metal is

usually not so important as speed, operator appeal, and the appearance of

the completed joint.

The properties of electrodes vary considerably, but Table 9-3 lists theminimum properties for some electrode classes.


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It is preferable, in designing welded components, to select a steel that illresult in a fast, economical weld.

Best results are obtained for steels having a UNS specifications betweenG10140 and G10230.

All these steels have a tensile strength in the hot-rolled condition in therange of60 to 70 kpsi.

Permissible stresses are now based on the yield strength of the materialinstead of the ultimate strength, and the code permits the use of a variety ofASTM structural steels having yield strengths varying from 33 to 50 kpsi.

For these ASTM steels, Sy

= 0.5 Su

.

Table 9-4 lists the formulas specified by the code for calculating thesepermissible stresses for various load conditions.


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The fatigue stress concentration factors listed in Table 9-5 are

suggested for use. These factors should be used for the parent

metal as well as for the weld metal.


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Table 9-6 gives steady-load information and minimum fillet sizes.

S i L di


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Static LoadingExample 9-2 (Textbook)

Table A-20, Sy = 27.5 kpsi

h =3/8=0.375 in

t = 1/2 in

l = 2 in


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CH-9 LEC 41 Slide 41Dr. A. Aziz Bazoune Chapter 9: Welding,Bonding, and the Design of Permanent Joints

Example 9-2 (Cont.d)

h =3/8=0.375 in

t = 1/2 in

l = 2 in

E l 9 4 (T b k)Table A-20,


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Example 9-4 (Textbook)

Table 9-3,

Sy = 50 kpsi,

Sut= 62 kpsi

Table 9-2, pattern 2,b=3/8 = 0.375 in

and d= 2in

Sy = 32 kpsi, Sut= 58 kpsi


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Eq. 6-21

Ssy= 0.577 Sy

E6010 electrode

Table 9-3,

Sy = 50 kpsi,


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Given n=3

elmes i bab 9.pptx

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