Chapter 9

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Dr. A. Aziz Bazoune

King Fahd University of Petroleum & Minerals

Mechanical Engineering Department

CH-9 LEC 40 Slide 1

Dr. A. Aziz Bazoune Chapter9: Welding, Bonding, and the Design of Permanent Joints

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CH-9 LEC 40 Slide 2


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CH-9 LEC 40 Slide 3


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I IntroductionIntroduction

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

The strongest and most common method of permanently joining steel components together.

Arc welding is the most important since it is adaptable to various manufacturing environments and is relatively cheap.

A weldment is fabricated by welding together a collection of metal shapes.

CH-9 LEC 40 Slide 4


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I IntroductionIntroduction

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

The materials of components and electrode must be compatible from the point of view of strength, ductility and metallurgy.

CH-9 LEC 40 Slide 5


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CH-9 LEC 40 Slide 6


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

Two most common forms are: 1. the butt joint2. the fillet joint

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CH-9 LEC 40 Slide 7


A weld is fabricated by welding together a collection of metal shapes, cut to particular configurations.

The weld must be precisely specified on working drawing and this is done by welding symbol, Fig. 9-1.

The arrow of this symbol points to the joint to be welded.

The body of the symbol contains as many of the following elements as are deemed necessary:

9-1 Welding Symbols9-1 Welding Symbols

Reference lineArrowBasic weld symbols in Fig. 9-2Dimensions and other data

Supplementary symbolsFinish symbolsTailSpecification or process.

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CH-9 LEC 40 Slide 8


Welding SymbolsWelding Symbols

WELD

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CH-9 LEC 40 Slide 9


Figure 9-2 Arc and gas-weld symbols

There 2 general types of welds:1.1. Fillet weldsFillet welds for general machine elements.2. Butt or groove welds for pressure vessels, piping

systems,...There are also others such as: ,

Types of Types of WeldingWelding

FilleFillett

Fillet Fillet weldswelds groove

welds

grooveBeaBea

dd

BeaBeadd

Plug or Plug or slotslot

Plug Plug or or slotslot

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CH-9 LEC 40 Slide 10


Parts to be joined must be arranged so that there is sufficient clearance 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 of benefit.

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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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9-2 Butt and Fillet 9-2 Butt and Fillet WeldsWelds

where h is the weld throat and l is the length of the weld. Notice that the value of h does not include the reinforcement.

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The reinforcement can be desirable, but it varies somewhat and does produce stress concentration at point A in the figure. If fatigue loads exist, it is good practice to grind or machine off the reinforcement.

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Stresses in Fillet Stresses in Fillet WeldsWelds

At angle the forces on each weldment consists of a normal force Fn and a shear force Fs sin ,

cosn

sF FF F

Fig. 9-8 illustrates a typical transverse fillet weld.

In Fig. 9-9 a portion of the welded joint has been isolated from Fig. 9-8

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Stresses in Fillet WeldsStresses in Fillet Weldssin (cos sin )

cos (cos sin )n

sF FA hlF FA hl

The nominal stresses at the angle θ in the weldment, τ and σ, are

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The von Mises stresse σ’at angle θ is

σ’max occurs at θ = 62.5o with a value of σ’max = 2.16 F/(hl).

The corresponding values of τ and σ, are τ = 1.196 F/(hl) and σ = 0.623 F/(hl).

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

The stationary point occurs at θ = 67.5o with a corresponding τmax = 1.207 F/(hl) and σ = 0.5 F/(hl).

1 22 2

1 22 22 2

(cos sin cos )' 3

3(sin sin cos )Fhl

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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 the throat 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 stressesCircumscribe typical cases by codeFor this model, the basis for weld analysis or design employs

which assumes the entire force F is accounted for by a shear stress in the minimum throat area.

The approach has been to:The approach has been to:

1.4140.707

F Fhl hl

(9.3)

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Notice that this inflates the maximum estimated shear stress by a factor of 1.414/1.207=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 of Fig. 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 bending moments.

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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9-3 Stresses in Welded Joints in 9-3 Stresses in Welded Joints in TorsionTorsion

Figure 9-12 illustrates a cantilever of length l welded to a column by 2 fillet welds.

The reaction at the support of a cantilever always consists of shear force V and a moment reaction M.

The shear force produces a primary shear in the welds of magnitude

where A is the throat area of the welds.

' VA

(9.4)

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The moment at the support produces secondary shear or torsion of the welds, and this stress is given by

where r: distance from the centroid of the weld group to the point in the weld of interest.

J: second polar moment of area of the group about the centroid of the group.

" MrJ

(9.5)

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

Weld 1 has a throat width b1 = 0.707 h1

Weld 2 has a throat width d2 = 0.707 h2

Throat area of both welds together is A = A1 + A2 = b1d1 + b2d2

which is the area to be used in Eq. (9-4)

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The x-axis passes through the centroid G1 of the weld 1.

The second moment of area about this axis is

Similarly, the second moment of area about an axis passing through G1 parallel to the y-axis is

The second polar moment of areas of weld 1 and weld 2 about their centroids are

31 1

12xb dI

31 1

12yd bI 3 3

1 1 1 11

3 32 2 2 2

2

12 12

12 12

G x y

G x y

b d d bJ I I

b d d bJ I I

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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 21 1

1 22 22 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 area of 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 be measured 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 of Ju same regardless of weld size.

Since throat width of a fillet weld is 0.707h, the relation between J and the unit value is

0.707 uJ h J (9.6)

31b 3

2d

31 1 12b d 3

2 2 12d b

Ju : is found from table 9.1 Page 472

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Chapter 9

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Transcript of Chapter 9