T

Tension Cleavage Peel

(Butt Joints in Thin Sheet)

Fig. 8.5. Load Paths to Avoid in Bonded Structure1

Table 8.1 Considerations for Designing Adhesively Bonded Joints1

• The adhesive must be compatible with the adherends and be able to retain its required strength when exposed to in-service stresses and environmental factors.

• The joint should be designed to ensure a failure in one of the adherends rather than a failure within the adhesive bondline.

• Thermal expansion of dissimilar materials must be considered. Due to the large thermal expansion difference between carbon composite and aluminum, adhesively bonded joints between these two materials have been known to fail during cool down from elevated temperature cures as a result of the thermal stresses induced by their differential expansion coefficients.

• Proper joint design should be used, avoiding tension, peel, or cleavage loading whenever possible. If peel forces cannot be avoided, a lower modulus (nonbrittle) adhesive having a high peel strength should be used.

• Tapered ends should be used on lap joints to feather out the edge-of-joint stresses. The fillet at the end of the exposed joint should not be removed.

• Selection tests for structural adhesives should include durability testing for heat, humidity (and/or fluids), and stress, simultaneously.

stiffness and strength depends on the matrix properties and not on the higher properties of the fibers. The exaggerated deformations in a composite laminate bonded to a metal sheet under tension loading are shown in Fig. 8.6. The adhesive passes the load from the metal into the composite until, at some distance "L," the strain in each material is equal. In the composite, the matrix resin acts as an adhesive to pass load from one fiber ply to the next. Because the matrix shear stiffness is low, the composite plies deform unequally in tension as shown. Failure tends to initiate in the composite ply next to the adhesive near the beginning of the joint, or in the adhesive in the same neighborhood. The

Composite Plies

Fig. 8.6. Uneven Strain Distribution in Composite Plies1

Composite Plies

Strain Differential Between Top and Bottom Composite Plies

Fig. 8.6. Uneven Strain Distribution in Composite Plies1

highest failure loads are achieved by an adhesive with a low shear modulus and high strain to failure, as previously shown in Fig. 8.4.

It should also be noted that there is a limit to the thickness of the composite that can be loaded by a single bondline. However, multiple steps in the composite thickness, giving multiple bondlines, can be used for thick material, as in a step-lap joint. The effects of adherend thickness and joint configuration on failure mode are shown in Fig. 8.7. For thick adherends, it is necessary to use either a cocured scarf or a step-lap joint to carry the load. The other option for thick joints is to use mechanical fasteners. Note that the double scarf joint shown in Fig. 8.7, while extremely efficient, is rarely used because it is extremely difficult to fabricate. The step-lap joint configuration, while not easy to fabricate, contains discrete steps that can be used for accurate ply location during fabrication.

Basic design practice for adhesive bonded composite joints should include making certain that the surface fibers in a joint are parallel to the load direction to minimize interlaminar shear, or failure, of the bonded adherend or substrate layer. In designs in which joint areas have been machined to a step-lap configuration, for example, it is possible to have a joint interface composed of fibers

Fig. 8.7. Effect of Adherend Thickness on Failure Modes of Adhesively Bonded Joints4

at an orientation other than the optimal 0° orientation to the load direction. This tends to induce substrate failure more readily than would otherwise occur.

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