Page 130 - Handbook of Adhesives and Sealants

Joint Design 99

1. Adhesive material properties
2. Adhesive thickness
3. Geometry of the bond area
4. Adherend properties

The effect of non-uniform stress distribution is that the average
stress (i.e., the load divided by bond area) is always lower than the
maximum stress at localized areas within the joint. Only in cases
where there is a near uniform stress does the average stress approach
the maximum stress. Failure in the bond always begins at the maxi-
mum stress regions. Therefore, an understanding of the stress distri-
bution in a joint is of primary importance in the design of adhesive
joints.

3.3.1 Adhesive properties
The simplest example of how adhesive properties affect joint design
efficiencies can be demonstrated by the stress distribution analysis of
a simple lap shear or peel joint, Fig. 3.3. The maximum shear stress
is dependent on the rheological characteristics of the adhesive. Tough,
flexible adhesives have less of a maximum stress, but the average
stress is generally higher. Since typically high elongation adhesives
have lower cohesive strengths, the advantage of high elongation and
peel strength is usually compromised by a corresponding decrease in
the adhesive’s internal shear strength.
Adhesive modulus also influences the stress distribution; however,
it is not a direct effect. For two adhesives of the same strength and
elongation, the higher modulus adhesive would carry more load. How-
ever, the higher elongating adhesives that have good peel and cleavage
strength, tend to have lower moduli and poorer shear strength.
Crack propagation sensitivity is greater with brittle adhesives (i.e.,
those having low elongation and high modulus). Fatigue life is, gen-
erally, lower with brittle adhesives. If the applied fatigue stress is
measured as a percentage of ultimate, then the fatigue life of joints
fabricated from high elongation adhesives is considerably superior to
more brittle adhesives. This is due to both uniform stress distribution
and high internal energy damping with more flexible adhesives.
If high stress nonuniform distributions are expected in service ei-
ther because of external loading, such as peel or cleavage, or from
internal stress, such as from thermal expansion differences or shrink-
age, then tough, flexible adhesives and sealants are usually better
than more brittle ones. This is why flexible adhesives are often rec-