CH-9 LEC 40 Slide 1

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Chapter 9
Welding, Bonding, and the Design of Permanent
Joints
Dr. A. Aziz Bazoune King Fahd University of
Petroleum Minerals Mechanical Engineering
Department
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Chapter Outline
9-1 Welding Symbols 9-2 Butt and Fillet
Welds 9-3 Stresses in Welded Joints in
Torsion 9-4 Stresses in Welded Joints in
Bending 9-5 The Strength of Welded
Joints 9-6 Static Loading 9-7 Fatigue
Loading 9-8 Resistance Welding 9-9 Bolted and
Riveted Joints Loaded in Shear 9-10 Adhesive
Bonding
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LECTURE-40
9-1 Welding Symbols 9-2 Butt and Fillet
Welds 9-3 Stresses in Welded Joints in
Torsion 9-4 Stresses in Welded Joints in
Bending 9-5 The Strength of Welded Joints
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Introduction

• 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.

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Introduction

• 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.

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• The form of a welded joint is dictated largely by
  the layout of the joined components.
• Two most common forms are
• the butt joint
• the fillet joint

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9-1 Welding Symbols

• 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

• Reference line
• Arrow
• Basic weld symbols in Fig. 9-2
• Dimensions and other data

• Supplementary symbols
• Finish symbols
• Tail
• Specification or process.

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Welding Symbols
WELD
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Types of Welding

• There 2 general types of welds
• Fillet welds for general machine elements.
• Butt or groove welds for pressure vessels,
  piping systems,...
• There are also others such as ,

Fillet welds
groove welds
Bead
Plug or slot
groove
Plug or slot
Fillet
Bead
Figure 9-2 Arc and gas-weld symbols
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• 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 Welds
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 Welds

• 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

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

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Stresses in Fillet Welds

• The nominal stresses at the angle ? in the
  weldment, t and s, are

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• The von Mises stresse sat angle ? is
• smax occurs at ? 62.5o with a value of smax
  2.16 F/(hl).
• The corresponding values of t and s, are t
  1.196 F/(hl) and s 0.623 F/(hl).
• tmax can be found by solving the equation
  d(t)/d?0.
• The stationary point occurs at ? 67.5o with a
  corresponding tmax 1.207 F/(hl) and s 0.5
  F/(hl).

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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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• The approach has been to

• 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
  stresses
• Circumscribe typical cases by code
• For 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.

(9.3)
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• Notice that this inflates the maximum estimated
  shear stress by a factor of 1.414/1.2071.17.
• 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 Torsion

• 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.

(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.

(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

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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

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

• 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

(9.6)
Ju is found from table 9.1 Page 472
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QUESTIONS ?
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