welded joints complete
DESCRIPTION
Book about the welded joints used in machine designTRANSCRIPT
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Mechanical Engineering Dept. CEME NUST 1
Ch-4: Design of Welded Joints
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Mechanical Engineering Dept. CEME NUST 2
Design of Welded Joints
Welded Joint is a permanent joint which is obtained by the fusionof the edges
of the two parts to be joined together, with or without the application of pressureand a filler material
Heat required for the fusion of the material may be obtained by burning of Gas
(in case of Gas Welding) or by an Electric Arc (in case of Electric Arc Welding)
Advantages and Disadvantages of Welded Joints over Riveted Joints
Advantages
Welded Structures are usually Lighterthan riveted structures (Gussets or other
connecting components are not used)provide maximum Efficiency(up to 100%), not possible in case of riveted joints
Alterationsand Additionscan be easily made in the existing structures
smooth in Appearance, therefore looks pleasing
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Mechanical Engineering Dept. CEME NUST 3
Design of Welded Joints
Advantages and Disadvantages of Welded Joints over Riveted Joints
Advantages--contd--
Welded Joint has a great Strength, usually has the strength of the parent metal
itself
members are of such a shape (i .e. Circular Steel Pipes) that they afford difficulty
for riveting. But they can be easily welded
Welding provides very Rigid Joints
It is possible to weld any part of a structure at any point. But riveting requires
enough clearance
Process of welding takes less time than the riveting
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Mechanical Engineering Dept. CEME NUST 4
Design of Welded Joints
Advantages and Disadvantages of Welded Joints over Riveted Joints
Disadvantages
Uneven Heating and Cooling during fabrication, therefore the members may get
distorted or additional stresses may develop
Requires a Highly Skilled Labor and supervision
Inspection of welding work is more difficult than riveting work
No provision is kept for expansion and contraction in the frame there is a
possibility of cracks developing in it
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Mechanical Engineering Dept. CEME NUST 5
Design of Welded Joints
Types of Welded Joints
Lap Joint or the Fillet Joint is obtained by overlapping the plates and then
welding the edges of the plates
Lap Joint
o Cross-sectionof the fillet is approximately Triangular
Single Transverse Fillet Double Transverse Fillet Parallel Fillet Joints
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Mechanical Engineering Dept. CEME NUST 6
Design of Welded Joints
Types of Welded Joints
Butt Joint
Butt Joint is obtained by placing the plates edge to edge
If the Plate Thickness is 5 mm to 12.5 mm, the edges should be beveled to V or
U-groove on both sides
plate edges do not require Bevelingif the thickness of plate is less than 5 mm
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Mechanical Engineering Dept. CEME NUST 7
Design of Welded Joints
Types of Welded Joints
Other Joints
Other type of Welded Joints are Corner Joint, Edge Joint and T-joint
Main considerations involved in the selection of weld type are:
o Shapeof the welded component required
o Thicknessof the plates to be welded
o Directionof the forces applied
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Mechanical Engineering Dept. CEME NUST 8
Design of Welded Joints
Basic Weld Symbols
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Mechanical Engineering Dept. CEME NUST 9
Design of Welded Joints
Elements of a Weld Symbols
1. Reference line
2. Arrow
3. Basic weld symbols
4. Dimensions and other data
5. Supplementary symbols6. Finish symbols
7. Tail
8. Specification, process or other references
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Mechanical Engineering Dept. CEME NUST 10
Design of Welded Joints
Representation of welding symbols
Fillet-weld each side of Tee-convex contour
Single V-butt weld machining finish
Double V- butt weld
Plug weld - 30Groove angle- flush contour
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Mechanical Engineering Dept. CEME NUST 11
Further details on Types of Welding Symbols:
Chap-9 (page: 478), Book: ShigleysMechanical Engineering Design, 9th ed.
Design of Welded Joints
Representation of welding symbols
Staggered Intermittent Fillet Welds
Circle on the weld symbol welding is to go allaround
o welds are intermittent and staggered 40 mm
along on 100-mm centers
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Mechanical Engineering Dept. CEME NUST 12
Design of Welded Joints
Strength of Transverse Fillet Welded Joints
Transverse Fillet welds are designed for Tensile Strength
Single Transverse Fillet Double Transverse Fillet
Assumption: section of fillet is a Right Angled
Triangle ABC with hypotenuse AC making equal
angles with other two sides ABand BC
Leg Or Size Of The Weld: Length of each side (ABor
BC)
Throat Thickness: Perpendicular distance of the
hypotenuse from the intersection of legs (i.e. BD)
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Mechanical Engineering Dept. CEME NUST 13
Design of Welded Joints
Strength of Transverse Fillet Welded Joints
t= Throat thickness (BD)
s= Leg or size of weld= Thickness of plate
l= Length of weld
Throat Thickness = t = s sin 45= 0.707 s
Minimum area of the weld or throat area= A= Throat thickness Length of weld
Minimum Area of the weld is taken because the
stress is maximum at the minimum area
= t l = 0.707 s l
P = Throat area Allowable tensile stress = 0.707 s l t
Tensile Strength of the joint for Single Fillet Weld:
Tensile Strength of the joint for Double Fillet Weld:
P = 20.707 s l t = 0.707 s l t
weld is weaker than the plate due to slag and blow holes, therefore the weld is given aReinforcementwhich may be taken as 10%of the Plate Thickness
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Mechanical Engineering Dept. CEME NUST 14
Design of Welded Joints
Strength of Parallel Fillet Welded Joints
Parallel Fillet Welded Joints are designed for Shear Strength
Double Parallel Fillet Weld Combination of transverse
and parallel fillet weld
= Allowable Shear Stress for the weld metal
P = Throat area Allowable Shear Stress = 0.707 s l
Shear Strength of the joint for Single Parallel Fillet Weld:
P = Throat area Allowable Shear Stress = 20.707 s l = 1.414 s l
Shear Strength of the joint for Double Parallel Fillet Weld:
For Combination of Single Transverse and Double Parallel Fillet Welds:
P =0.707s l1t+ 1.414 s l2
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Mechanical Engineering Dept. CEME NUST 15
Polar Moment Of Inertia and
section modulus of welds
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Mechanical Engineering Dept. CEME NUST 16
Design of Welded Joints
Special Cases of Fillet Welded Joints
1. Circular fillet weld subjected to torsion
d= Diameter of rod,
T= Torque acting on the rod,
s= Size (or leg) of weld,
t= Throat thickness,
J = Polar moment of inertia of the weld section =
Shear Stress of the material is:
maximum shear occurs on the throat of weld which is inclined at 45to the horizontal
plane
Length of throat: t = s sin 45= 0.707 s
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Mechanical Engineering Dept. CEME NUST 17
Design of Welded Joints
Special Cases of Fillet Welded Joints
2. Circular fillet weld subjected to Bending Moment
d= Diameter of rod,
M= Bending moment acting on the rod,
s= Size (or leg) of weld,
t= Throat thickness,
Z= Section modulus of the weld section
Bending Stress:
maximum shear occurs on the throat of weld which is inclined at 45to the horizontal
plane
Length of throat: t = s sin 45= 0.707 s
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Mechanical Engineering Dept. CEME NUST 18
Design of Welded Joints
Strength of Butt Joints
Butt Joints are designed for tension or compression
In case of butt joint, length of leg or size of weld is equal to throat thickness
which is equal to thickness of plates
Tensile Strength of the butt joint (single-V or square butt joint)
P = t l t
Tensile Strength for Double-v Butt Joint
P = (t1+ t2) l t
t1= Throat thickness at the top
t2= Throat thickness at the bottom
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Mechanical Engineering Dept. CEME NUST 19
Figure shows a horizontal steel bar of thickness hloaded in steady tension and
welded to a vertical support. Find the load Fthat will cause an allowable shearstress, allow, in the throats of the welds.
Example 2.1
Design of Welded Joints
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Mechanical Engineering Dept. CEME NUST 20
Design of Welded Joints
Eccentrically Loaded Welded Joints
Eccentric Load may be imposed on welded joints in many ways
induced stresses are combined depending upon the nature of stresses
When the shear and bending stresses are simultaneously present in a joint, then
maximum stresses are as follows
Maximum Normal Stress
Maximum Shear Stress
b= Bending stress
= Shear stress
a T-joint fixed at one end and subjected to an eccentric
load Pat a distance e
o joint will be subjected to the following two types of stresses
1. Direct shear stress due to the shear force P acting at the
welds
2. Bending stress due to the bending moment P e.
T-joint fixed at one end and subjected to eccentric load
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Mechanical Engineering Dept. CEME NUST 21
Design of Welded Joints
Eccentrically Loaded Welded Joints
T-joint fixed at one end and subjected to eccentric load contd--
A = Throat thickness Length of weld
= t l 2 = 2 t l ... (For Double Fillet Weld)
= 2 0.707 s l = 1.414 s l ... (t = s cos 45= 0.707 s)
Shear Stress in the weld (assuming uniformly distributed)
Section Modulus of the weld metal through the throat
...(For both sides weld)
Bending moment, M = P e
Bending Stress
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Mechanical Engineering Dept. CEME NUST 22
Design of Welded Joints
Stresses in Welded Joints in Torsion
A Cantileverof length l welded to a column by two Fillet Welds
Eccentric Load F can be replaced by a
Shearing Force V and a Moment M
o Shear Force produces a Primary Shear /in
the welds of magnitude:
Ais the Throat Area of all the welds
o Moment at the support produces Secondary
Shearor Torsionof the welds:
r= Distance from the Centroidof Weld Group to the point in the weld of interest
J= Second Polar Moment of Area of Weld Group about the Centroid
//is proportional to its distance from the center of twist (r), (//)max
will occurat
the corners of the weld
Primary Shear /is always directed parallel to P
Secondary Shear Stress //can be added vectorially to the Primary Shear Stress / to
determine the Maximum Shear Stress max
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Mechanical Engineering Dept. CEME NUST 23
Design of Welded Joints
Stresses in Welded Joints in Torsion
Two Welds in a Group
Rectangles represent the Throat
Areas of the welds
Throat Thickness of Weld-1 = t1= 0.707h1
Throat Thickness of Weld-2 = t2= 0.707h2
h1and h2are the respective Weld Sizes
Throat Area of both welds together
A = A 1+ A 2=t1d + t2b
Second Moment of Area of Weld-1through
G1about x-axisis:
Second Moment of Area of Weld-1through G1about y-axisis:
Second Polar Moment of Area of Weld-1 about its own centroid:
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Mechanical Engineering Dept. CEME NUST 24
Design of Welded Joints
Stresses in Welded Joints in Torsion
Two Welds in a Group--contd--Second Polar Moment of Area of Weld-2
about its own centroid:
CentroidG
of the Weld Group is located at
distances r1and r2from G1and G2to G
using the Parallel-axis Theorem, Second Polar Moment of Area of the Weld Group
This is to be used in Torsion Eq.
In a Reverse Procedure, Weld Size can be found for which the Allowable Shear
Stressis given
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Mechanical Engineering Dept. CEME NUST 25
Design of Welded Joints
Stresses in Welded Joints in Torsion
Two Welds in a Group--contd--
Setting the weld thicknesses t1and
t2 to Unity leads to the idea of
treating each fillet weld as a Line
Resulting Second Moment of Area is then a Unit Secon d Polar Moment of Area
Since Throat Width of a fillet weld is 0.707h, the relationship between Jand Juis:
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Mechanical Engineering Dept. CEME NUST 26
Design of Welded Joints
Torsional Properties of Fillet Welds
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Mechanical Engineering Dept. CEME NUST 27
A steel bar of thickness h, to be used as a beam, is welded to a vertical support as
shown in the figure. Find the safe bending force Fif the allowable shear stress in
the welds is 140 MPa
Example 2.2
Design of Welded Joints
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Mechanical Engineering Dept. CEME NUST 28
A 50-kN load is transferred from a welded fitting into a 200-mm steelchannel as illustrated in Fig. Estimate the maximum stress in the weld.
Example 2.3
Design of Welded Joints