Some modern adhesives are extremely strong, and are becoming increasingly important in modern construction and industry.

 

 

 

 

The first adhesives were natural gums and other plant resins or saps. Sumerian people were the first to use it. The finding of 6000-year-old ceramics brought astounding evidence to archaeologists about the first practical uses and ingredients of the first adhesives. Most early adhesives were animal glues made by rendering animal products such as the use of horse teeth. The eastern United States (or the Native Americans) used a mixture of spruce gum and fat as adhesives to add waterproof seams in their birchbark canoes. During the times of Babylonia, tar-like glue was used for gluing statues. Also, Egypt was one of the most prominent users of adhesives. The Egyptians used animal glues to adhere furniture, ivory, and papyrus. The Mongols also used adhesives to make their short bows. In Medieval Europe/Eurasia, egg whites were used as glue to decorate parchments with gold leaves. Holland, in the early 1700s, founded the first ever glue factory. Later, in the 1750s, the British introduced fish glue. As life and the modern world evolved, several other patented materials, such as bones, starch, fish, and casein, were inroduced and was come to know as alternative materials from glue manufacture. Modern-time glues have improved beyond recognition. Such improvements are noticeable in its flexibility, toughness, curing rate, temperature, and chemical resistance. the bond between two items depends on the shape of the adhesive.

 

 

 

 

 

The strength of attachment, or adhesion, between an adhesive and its substrate depends on many factors, including the means by which this occurs. Adhesion may occur either by mechanical means, in which the adhesive works its way into small pores of the substrate, or by one of several chemical mechanisms.

 

In some cases an actual chemical bond occurs between adhesive and substrate. In others electrostatic forces, as in static electricity, hold the substances together. A third mechanism involves the van der Waals forces that develop between molecules. A fourth means involves the moisture-aided diffusion of the glue into the substrate, followed by hardening.

 

 

When subjected to loading, debonding may occur at different locations in the adhesive joint. The major fracture types are the following

 

 

 

“Cohesive” fracture" is obtained if a crack propagates in the bulk polymer which constitutes the adhesive. In this case the surfaces of both adherents after debonding will be covered by fractured adhesive. The crack may propagate in the centre of the layer or near an interface. For this last case, the “cohesive” fracture can be said to be “cohesive near the interface”. Most quality control standards consider that a “good” adhesive bonding must be “cohesive”.

 

 

 

The fracture is “adhesive” or “interfacial” when debonding occurs between the adhesive and the adherent. In most cases, the occurrence of “interfacial” fracture for a given adhesive goes along with a smaller fracture toughness. The “interfacial” character of a fracture surface is usually to identify the precise location of the crack path in the interphase.

 

 

 

Beside these two cases, other type of fracture are

 

The “mixed” fracture type which occurs if the crack propagates at some spots in a “cohesive” and in others in an “interfacial” manner. “Mixed” fracture surfaces can be characterised by a certain percentage of “adhesive” and “cohesive” areas.

The “alternating crack path” fracture type which occurs if the cracks jumps from one interface to the other. This type of fracture appears in the presence of tensile pre-stresses in the adhesive layer.

Fracture can also occur in the adherent if the adhesive is tougher than the adherent. In this case the adhesive remains intact and is still bonded to one substrate and the remnants of the other. For example, when one removes a price label, adhesive usually remains on the label and the surface. This is cohesive failure. If, however, a layer of paper remains stuck to the surface, the adhesive has not failed. Another example is when someone tries to pull apart Oreo cookies and all the filling remains on one side. The goal in this case is an adhesive failure, rather than a cohesive failure.

 

 

A general design rule is a relation of the type: "Material Properties > Function (geometry, loads)"

 

The engineering work will consist in having a good model to evaluate the "Function". For most adhesive joints, this can be achieved using fracture mechanics. Concepts such as the stress concentration factor K and the energy release rate G can be used to predict failure. In such models, the behavior of the adhesive layer itself is neglected and only the adherents are considered.

 

Failure will also very much depend on the opening "mode" of the joint.

 

 

Mode I is an opening or tensile mode where the loadings are normal to the crack.

Mode II is a sliding or in-plane shear mode where the crack surfaces slide over one another in direction perpendicular to the leading edge of the crack. This is typically the mode for which the adhesive exhibits the higher resistance to fracture.

Mode III is a tearing or antiplane shear mode.

As the loads are usually fixed, an acceptable design will result from combination of a material selection procedure and geometry modifications, if possible. In adhesively bonded structures, the global geometry and loads are fixed by structural considerations and the design procedure focuses on the “material properties” of the adhesive (i.e. select a "good" adhesive) and on local changes on the geometry.

 

Increasing the joint resistance is usually obtained by designing its geometry so that:

 

The bonded zone is large

It is mainly loaded in mode II

Stable crack propagation will follow the appearance of a local failure.

 

 

A wide range of testing devices have been imagined to evaluate the fracture resistance of bonded structures in pure mode I, pure mode II or in mixed mode. Most of these devices are beam type specimens. We will very shortly review the most popular:

 

Double Cantilever Beam tests (DCB) measure the mode I fracture resistance of adhesives in a fracture mechanics framework. These tests consist in opening an assembly of two beams by applying a force at the ends of the two beams. The test in unstable (i.e. the crack propagates along the entire specimen once a critical load is attained) and a modified version of this test characterised by a non constant inertia was proposed called the Tapered double cantilever beam specimen (TDCB).

 

Peel tests measure the fracture resistance of a thin layer bonded on a thick substrate or of two layers bonded together. They consist in measuring the force needed for tearing an adherent layer from a substrate or for tearing two adherent layers one from another. Whereas the structure is not symmetrical, various mode mixities can be introduced in these tests.

Wedge tests measure the mode I dominated fracture resistance of adhesives used to bond thin plates. These tests consist in inserting a wedge in between two bonded plates. A critical energy release rate can be derived from the crack length during testing. This test is a mode I test but some mode II component can be introduced by bonding plates of different thicknesses.

Mixed-Mode Delaminating Beam (MMDB) tests consist in a bonded bilayer with two starting cracks loaded on four points. The test presents roughly the same amount of mode I and mode II with a slight dependence on the ratio of the two layer thicknesses.

End Notch Flexure tests consist in two bonded beams built-in on one side and loaded by a force on the other. As no normal opening is allowed, this device allows testing in essentially mode II condition.