Page 58 - Handbook of Adhesives and Sealants

26 Chapter One

Many benefits are obtained because the adhesive joint is capable of
spreading stress relatively evenly over the entire overlap region of the
bond. Alternatively, mechanical fasteners and spot welding provides
points of attachment in small discrete areas. Consequently, adhesive
shear strengths of as high as 7000 psi can be obtained. The stress-
distribution characteristics and inherent toughness of adhesives also
provide bonds with superior fatigue resistance, as shown in Fig. 1.7.
Generally in well-designed joints, the adherends fail in fatigue before
the adhesive. Holes, needed for rivets or other fasteners, are not re-
quired for an adhesive bond, thereby avoiding possible areas of high
stress concentration. The elimination of holes also maintains the in-
tegrity of the structural members.

1.4.3 Design advantages
Adhesives offer certain valuable design advantages. Unlike rivets or
bolts, adhesives produce smooth contours that are aerodynamically
and aesthetically beneficial. Adhesives also offer a better strength-
to-weight ratio than other methods of mechanical fastening. Adhesives
can join any combination of solid materials regardless of shape, thick-
ness, or mismatch in physical properties such as coefficient of thermal
expansion or elastic modulus. Certain substrates may be too thin or
too small to weld reproducibly without distortion. Thus, medical prod-

Figure 1.7 Fatigue strengths of aluminum-alloy specimens under pulsating tensile
load. 8