Teoría general de la flexión de vigas planas

Structural adhesive bonding is becoming an increasingly important joining technique in modern engineering applications. Adhesive bonds can have a number of distinct advantages over other joining techniques, which include:

i) Adhesive bonding produces a more uniform stress distribution over the area of the bond than other localised connectors such as rivets, bolts or spot- welds. This eliminates large, localised stress concentrations (that would be found around these connectors) and hence increases the strength of the bond. This can also make the joint less susceptible to fatigue and vibration.

ii) The stiffness of the joint is usually increased when adhesive bonding is used. This can increase the overall strength of the complete structure

iii) The joint can provide an integral seal against the unwanted ingress of air or water, offering better environmental resistance and reducing structural degradation.

iv) Adhesive bonding reduces the machining of the components, saving costs.

v) Adhesive bonding allows joints to be made between dis-similar materials that could not be joined by welding, soldering or brazing techniques. It also greatly reduces galvanic corrosion problems between dis-similar metals, and does not introduce any distortion.

vi) By increasing joint strength, adhesive bonds allow the use of lighter components, both reducing initial materials costs, and reducing the weight of the finished product (this is particularly relevant to the aerospace industry).

pleasing structure.

Adhesive bonds can produce very strong bonds in both compression and shear modes, but do not work well in peel modes. The relative ability of a joint to withstand loading in the compression, shear and peel modes is crudely reflected by the ratio 1000:100:1‘. This affects the designs of adhesive joints, which try to reduce, or eliminate any peel stresses, a number of common engineering adhesive joints have been given by Adams and Cawley2, and are shown in Figure 2.1.

2.3 Defect types in adhesive joints

A simple adhesive joint consist of adherends (the components being adhered together) and adhesive layers, as shown in Figure 2.1. Some of the defects that can occur in these bonds are shown schematically in Figure 2.2. These defects can be separated into three regions:

(al Within the adherend

Techniques for testing these are the conventional ultrasonic methods. (b) Within the adhesive layer

A number of different types of defects can occur within this layer, the more common forms being;

i) Cracks, these are often caused by poor curing, by thermal stresses during manufacture, or by impact damage during service. Brittle adhesives are predominantly at risk with this type of defect.

ii) Porosity, caused by either volatiles or entrained gases, (principally air and water vapour).

=i — S in g le la p L" _______ —> D o u b le la p i--- am n v y --- j — > S c a r f 1 ---_J B ev el

___

~

a

____

Figure 2.1 Some common Engineering adhesive joints, after Adams and Cawley2.

A d h e r e n d

C o m p le te d isb ond P oor bond

Figure 2.2 Schematic diagram of typical defects in an adhesively bonded joint

iii) Voids, caused by either coalescence of porosity, by air entrapment during lay-up, by volatile compounds being evolved by the adhesive, or by insufficient or unevenly applied adhesive.

iv) Areas of poor cure, which give a poor cohesive strength. These can occur because of aged/defective adhesives, contaminants, incorrect mixing, or by an incorrect thermal curing cycle. This is sometimes a localised defect, but more commonly occurs throughout the whole of the adhesive layer.

icl At the adhesive-adherend interface

Defects at this interface can cause either a complete disbond (known as a zero- volume disbond) or a partial disbond (known as a poor-bond). Complete disbonds are possible to detect nondestructively using one of a variety of techniques, which are discussed in greater detail later in this chapter. The second group of adhesive- adherend defects, namely those with partial disbonds, are notoriously difficult to detect, and a great deal of research has been undertaken, and is continuing to try to develop techniques to be able to detect this type of defect reliably. To date, no single method has been demonstrated to be able to do this reliably, indeed the ability to do this has been described as the "holy grail" of nondestructive testing3.

Interface defects can be caused by contaminants (such as oil, grease or unremoved surface coatings such as peel ply), a loose coating of oxides on the adherend, faulty adhesives (caused for example, by the adhesive being left unadhered too long, and drying out) and in-service degradation, caused by either the adherend corroding, or aggressive chemicals and water degrading the adhesive.