Page 135 - Analysis and Design of Machine Elements

Solution: Permanent Connections 113

Steps Computation Results Units
1. Determine the Using the maximum distortion energy theory, we have = 202 MPa
s
yield strength in the yield strength in shear as
shear 1 1
= √ = √ × 350 = 202 MPa
s
s
3 3
202
2. Determine the [ ]= s = = 67.3 MPa [ ] = 67.3 MPa
allowable shear S 3
stress
3. Determine the According to Eq. (5.8), we have k = 6 mm
required length of F 20000
k ≥ = = 5.25 mm
the weld leg 0.707 ∑ l[ ] 0.707 × 2 × 40 × 67.3
Select the value as 6 mm.

5.3 Brazing, Soldering and Adhesive Bonding

5.3.1 Applications, Characteristics and Structure
Brazing and soldering are similar to welding yet accurately belong to bonding [2]. Braz-
ing and soldering use heat to melt filler materials that flow into the space between parts

to be joined. They differ from welding in that the temperature is always below the melt-
ing point of base metal. Brazing heats the base metal to a specified temperature above
∘ ∘
450 C; while soldering is similar to brazing except at a temperature below 450 Cand has
a relatively low strength. In both processes, the filler materials act like molten metal glue
that solidifies immediately upon cooling. Common filler materials for brazing are alloys
of copper, silver or nickel, while most solders are tin-lead alloys. Brazed joints require
little or no finishing and can braze dissimilar metals and even nonmetals [2]. Soldering
has wide applications in plumbing and in joining electrical and electronic parts.
Adhesive bonding is one of permanent joints, using polymeric adhesives to join plates
or tubes. Compared with fasteners, the use of adhesives not only significantly reduces
weight, but also eliminates stress concentration associated with drilled holes. Besides,
adhesive bonded joints can effectively seal against leakage as long as the liquid or gas
does not react with the adhesive [2]. And finally, adhesives can be used to join dissimilar
materials or relative thin components. Nevertheless, it is more temperature sensitive
than mechanical fasteners. Safety and environmental factors are important concerns in
plumbing and in adhesive bonding.

5.3.2 Types of Adhesive and Their Selection
Adhesives may be classified in a variety of ways depending on their chemical compo-
sition (e.g. epoxies, polyurethanes, polyimides), physical form (e.g. paste, liquid, film,
tape), adhesion method (e.g. reactive, nonreactive), origin (e.g. natural, synthetic) or load
carrying capability (e.g. structural, semistructural, nonstructural) [1, 8].
Structural adhesives are strong adhesives that are normally used to carry, especially,
shear loads. Most structural adhesives are thermosetting, becoming irreversibly hard-
ened upon being cured. Typical examples include epoxies, urethanes, anaerobics, acrylic
and cyanoacrylates.