Page 127 - Handbook of Adhesives and Sealants

96 Chapter Three

Compression loads are the opposite of tensile. As with tensile loads,
it is important to keep the loads aligned so that the adhesive will see
purely compressive stress. An adhesive loaded in compression is un-
likely to fail, although it may crack at weak spots due to uneven stress
distribution. Actually, a joint loaded in ‘‘pure’’ compression hardly
needs bonding of any sort. If the compression force is high enough and
there is no movement of the parts, the parts will stay in position rel-
ative to one another unless the adhesive fails cohesively.
The polyamide-imide journal bearing application in Chapter 2 (Fig.
2.13) is an excellent example of this. At elevated temperatures, the
internal cylinder (polyamide-imide) is compressing against the outside
cylinder (steel) because of the thermal expansion coefficient differ-
ences. At elevated service temperatures an adhesive is not necessary
in the classical sense, and the parts are held together by thermal fit.
However, at lower operating temperatures the opposite effect occurs.
The internal cylinder wants to contract more than the steel cylinder,
and the adhesive is exposed to high tensile forces of uneven distri-
bution.
Tensile or compressive stress is measured as force per bonded area
and is usually given in units of pounds per square inch of bonded area
(psi). For example, if two circular rods of 1 in. diameter were bonded
as a butt joint, the tensile strength would be measured as the ultimate
2
load (in pounds of force) divided by the bonded area [ (0.5 in.) ]
and given in units of psi.
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In SI (Systeme International d’Unites) units, stress is given as
megapascals (MPa) One psi is equivalent to 0.006895 MPa; or IMPa
is equivalent to 145 psi.

3.2.2 Shear stress
Shear stress results when forces acting in the plane of the adhesive
try to separate the adherends. Joints that are dependent on the ad-
hesive’s shear strength are relatively easy to make and are commonly
used in practice. Adhesives are generally strongest when stressed in
shear because all of the bonded area contributes to the strength of the
joint and the substrates are relatively easy to keep aligned.
The lap shear joint, shown in Fig. 3.3 left, represents the most com-
mon joint design in adhesive bonding. Shear stresses are measured
similar to tensile forces, as force per bonded area, psi. By overlapping
the substrates, one places the load bearing area in shear. Note that
most of the stress is localized at the ends of the overlap. The center
of the lap joint contributes little to joint strength. In fact, depending
on the joint geometry and physical properties of the adhesive and ad-
herends, two small bands of adhesive at each end of the overlap may