Page 136 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
114
Epoxies are the most versatile and widely used structural adhesives, with a large vari-
ety of formulations and properties available. One-part epoxies require heat curing; while
two-part types cure at room temperature but require premixing. Urethanes are similar
to epoxies in great versatility, relatively high strength, good toughness, flexibility and
impact resistance. Anaerobics cure in the absence of oxygen. Acrylic adhesives are pre-
ferred for many metals and plastics under ordinary industrial conditions as they are
tolerant of dirty surfaces. Cyanoacrylates are particularly appropriate for fast curing
requirement [2, 3]. Most structural adhesives can carry significant stresses.
Nonstructural adhesives provide cost effective means required for assembly of fin-
ished products where failure would be less critical. They have some strength at normal
application. However, at elevated temperature, their properties decrease rapidly. Typical
examples include contact adhesives and pressure sensitive adhesives. They are generally
intended for light duty applications [1].
Although almost any solid materials can be bonded with a suitable adhesive, the
selection of proper adhesive depends on operating environment and applications. Since
exposure to water, solvents, ultraviolet light and high temperature can significantly
degrade adhesive performance, environmental limitations and effects must be recog-
nized while selecting adhesives. Other special properties, like rust-proofing, insulating,
electrical conducting, transparent and super high or super low temperatures, should
also be considered for specific applications.

5.3.3 Analysis and Design of Adhesive Joints
Adhesive joints function as one component when carrying loads. Normally, adhesive
joints have larger load carrying capacities for tension and shear than for peer and cleav-
age. When adhesive joints are loaded with tension or shear (Figure 5.5a), stresses are
uniformly distributed over the bonded area, with minor stress concentration at the edge
of contact. However, when adhesive joints take peel or cleavage (Figure 5.5b), peel stress
and cleavage stress can be quite large and often account for debonding failure.
The design of adhesive joints need to consider their load carrying capability. Since
adhesive bonds perform better when loaded in shear, design the joint to carry shear
load instead of peel load. Wherever possible, bond to multiple surfaces in preference to a
single surface, as bonding to multiple surfaces permits loads to be carried predominantly
in shear [1].
The adhesive, bonded materials and their interaction influence the load carrying
capacity of bonded joints. The mechanical properties of adhesives are affected by adhe-
sive composition, curing conditions (including curing temperature, pressure and time),
surface treatments, and operation environment. Detailed data can be found in design

F Figure 5.5 Forces on adhesive bonded joints.

F
F s
s
F
(a) (b)