Lwy

(b) Tapered Single Lap Joint Very Good

(f) Tapered Double Strap Joint Excellent

(c ) Single Strap Joint Fair

(g) Scarf Joint Excellent

(d) Double Lap Joint Very Good

(h) Step Lap Joint (Cocured Only) Excellent

Fig. 8.2. Typical Adhesively Bonded Joint Configurations3

Fig. 8.3. Typical Bondline Shear Stress Distribution1

portion of the load. Therefore, adhesives designed to carry high loads need to be strong and tough, especially if there is any bending in the joint that would induce peel loads. In order to improve fracture toughness and fatigue life, adhesives are frequently modified with rubber or other elastomers that reduces the adhesive

Fig. 8.4. Typical Stress-Strain Behavior for Brittle and Ductile Adhesives4

modulus. A comparison of a "brittle" high strength, high modulus adhesive with a "ductile" lower strength, lower modulus adhesive is shown in Fig. 8.4. While the brittle high strength adhesive has the highest strength, the tough ductile adhesive, which has a much larger area under the shear stress-strain curve, would be a much more forgiving adhesive, particularly in structural joints that often experience peel and bending loads. The joint design must insure that the adhesive is loaded in shear as much as possible. Tension, cleavage, and peel loading (Fig. 8.5) should be avoided when using adhesives. Actually, tension loading is acceptable as long as there is appreciable surface area, but certainly not in the butt joint shown. Some further considerations for joint design are summarized in Table 8.1.

Bonding to composites rather than metals introduces significant differences in criteria for adhesive selection for two reasons: (1) composites have a lower interlaminar shear stiffness compared to metals, and (2) composites have much lower shear strength than metals. This occurs because the interlaminar shear

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