W9-Welding.pdf

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Shigley’s Mechanical Engineering Design 11th Edition
MECHANICAL ENGINEERING
DESIGN
MACHINE ELEMENTS

Chapter 9
Welding

Welding
A weld is a joint between two surfaces produced by the application
of localized heat.
Here, we briefly discuss only welding between metal surfaces:
thermoplastics can be welded much like metals.
A weldment is fabricated by welding together a variety of metal
forms cut to particular configuration.
Nearly all wielding is by fusion processes. Establishment of
metallurgical bond between two parts by melting together the
base metals with a filler metal is called the fusion process.

Heat is brought about usually by an electric arc, electric
current, or gas flame.
Metals and alloys to arc and gas welding must be properly
selected.
Properties of welding filler material must be matched with
those of base metal when possible.
The joint strength would then be equal to the strength of
the base metal, giving an efficiency of almost 100% for
static loads

Welding Processes and Properties
Metallic arc welding, the so-called shielded metal arc welding
(SMAW), refers to a process where the heat is applied by an
arc passing between an electrode and the work.
The electrode is composed of suitable filler material with
coating ordinarily similar to that of base metal.
It is melted and fed into the joint as the weld is being formed.
The coating is vaporized to provide a shielding gas-
preventing oxidation at the weld as well as acts as a flux and
directs the arc.

Either direct or alternating current can be used with this process.
A weld thickness greater than about 10 mm. is often produced on
successive layers.
In metal– inert gas arc welding or gas–metal arc welding (GMAW),
heat is applied by a gas flame.
In this process, a bare or plated wire is continuously fed into the
weld from a large spool.
The wire serves as electrode and becomes the filler in the union.
Uniform-quality welds are attainable with metal–gas welding.

Resistance welding uses electric-current-generated heat that
passes through the parts to be welded while they are clamped
together firmly.
Filler material is not ordinarily employed.
Usually, thin metal parts may be connected by spot or continuous
resistance welding.
A spot weld is made by a pair of electrodes that apply pressure
to either side of a lap joint and devise a complete circuit.

Laser beam welding, plasma arc welding, and electron beam
welding are utilized for special applications.
The suitability of several metals and alloys to arc and gas
welding is very important.

Strength of Welded Joints
Butt weld: (a) tension loading and (b) shear loading.

Fillet weld: (a) shear loading and (b) transverse tension loading. Notes: h is
the length of weld leg, t is the throat length, and L is the weld length.

The usual equation of the factor of safety n applies for
static loads:
The quantities Sy and Sys represent tensile yield and shear yield
strengths of weld material, respectively.

Stress Concentration and Fatigue in Welds
Abrupt changes in geometry take place in welds, and
hence, stress concentrations are present. The weld
and the plates at the base and reinforcement should
be thoroughly blended together.

Example:
Design of a Butt Welding for Fatigue Loading
The tensile load P on a butt weld fluctuates
continuously between 20 and 100 kN. Plates are 20
mm thick. Determine the required length L of the weld,
applying the Goodman criterion.

Assumptions: Use an E6010 welding rod with a factor of
safety of 2.5.
Solution
By Table 15.8 for E6010, Su = 427 MPa. The endurance limit of
the weld metal is;
Kf: Stress Concentration Factor

9–5 The Strength of Welded Joints
The matching of the electrode properties with those of the
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 the minimum properties for some electrode classes.

It is preferable, in designing welded components, to select a
steel that will result in a fast, economical weld even though this
may require a sacrifice of other qualities such as machinability.
Under the proper conditions, all steels can be welded, but best
results will be obtained if steels having a UNS specification
between G10140 and G10230 are chosen.
All these steels have a tensile strength in the hot-rolled
condition in the range of 410 to 480 MPa.

The designer can choose factors of safety or permissible
working stresses with more confidence if he or she is
aware of the values of those used by others.
One of the best standards to use is the American
Institute of Steel Construction (AISC) code for building
construction.

The permissible stresses are now based on the yield
strength of the material instead of the ultimate strength,
and the code permits the use of a variety of ASTM
structural steels having yield strengths varying from 230 to
340 MPa.
Provided the loading is the same, the code permits the
same stress in the weld metal as in the parent metal.

Table 9–4 lists the formulas specified by the code for
calculating these permissible stresses for various loading
conditions.
The factors of safety implied by this code are easily
calculated.
For tension, n = 1∕0.60 = 1.67.
For shear, n = 0.577∕0.40 = 1.44,
using the distortion-energy theory as the criterion of failure.

It is important to observe that the electrode material is often
the strongest material present.
If a bar of AISI 1010 steel is welded to one of 1018 steel, the
weld metal is actually a mixture of the electrode material and
the 1010 and 1018 steels.
Furthermore, a welded cold-drawn bar has its cold-drawn
properties replaced with the hot-rolled properties in the vicinity
of the weld.
Finally, remembering that the weld metal is usually the
strongest, do check the stresses in the parent metals.

The AISC code, as well as the AWS code, for bridges includes
permissible stresses when fatigue loading is present.
The designer will have no difficulty in using these codes, but
their empirical nature tends to obscure the fact that they have
been established by means of the same knowledge of fatigue
failure already discussed in Chapter 6.
Of course, for structures covered by these codes, the actual
stresses cannot exceed the permissible stresses; otherwise the
designer is legally liable.
But in general, codes tend to conceal the actual margin of
safety involved.

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

9–6 Static Loading
Some examples of statically loaded joints are
useful in comparing and contrasting the
conventional method of analysis and the welding
code methodology.

Example 9-2:

with interpolation

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