Joints adhesives

The properties of the composite made when two adherends are united by adhesive are a function of the bonding, the materials involved and their interaction by stress patterns. Potential problems implied by the latter stem from the inherent mismatch between adhesives and the materials commonly employed in construction (Table 4.1). For instance, concrete adherends would benefit from being united with flexible and relatively low modulus products in 

Property (at 20°C) Cold-curing epoxy adhesive Concrete Mild steel 

It has been emphasised already that a successful adhesive bonded joint depends upon several factors  

The significance of some of these factors, particularly surface pretreatment, tends to become more apparent with regard to durability and to long-term performance. 

Kinloch(4) observed that the selection of appropriate failure criteria for the prediction of joint strength by conventional analysis is fraught with difficulty. The problem is in understanding the mechanisms of failure of bonded joints, and in assigning the relevant adhesive mechanical properties. Current practice is to use the maximum shear-strain or maximum shear-strain energy as the appropriate failure criterion. However, the failure of practical joints occurs by modes including, or other than, shear failure of the adhesive. This difficulty has led to the application of fracture mechanics to joint failure. 

It is recognized that one requirement for the establishment of strong adhesive joints is that intimate molecular contact occur at the interface. This means that the adhesive must be able to spread over the surface of the substrate, displacing air and contaminants that may be present. The surface and interfacial free energies are of prime importance in determining whether good interfacial contact can occur and in providing a measure of the thermodynamic work of adhesion. The interactions that can occur at the interface and that are discussed in the different mechanisms of adhesion determine how well the adhesive will adhere to the substrate. 

The ability of the adhesive to completely wet the substrate surface depends both on the chemistry and on the microstructure of the surface. It has long been known that surface treatment can greatly affect the strength and durability of an adhesive joint. Some methods of surface preparation merely attempt to remove gross contamination, while others modify the chemistry and/or microstructure of the surface. This subject will be discussed in more detail in Chapter 8. 

In order to achieve intimate interfacial contact, the adhesive must spread easily and wet the substrate surface. Thus at some stage during bond formation, the adhesive must be a mobile, low-viscosity liquid. However, to be useful, a joint must be able to sustain a variety of stresses. Therefore, once applied to the substrate, the adhesive needs to undergo a phase change into a solid which will have the desired mechanical strength.  

With the structural adhesives discussed in this book, the necessary phase change occurs as the result of a chemical reaction. The conditions under which these reactions take place (the cure conditions) are of considerable practical importance, since they frequently determine the suitability of the adhesive for a particular bonding application. 

The basic principles of adhesive joint design are well known and various standard configurations are available. Irrespective of joint geometry, the basic design criteria are the same. They are  

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The present work was done with the aim to evaluate the efficiency of the acoustic emission method as a diagnostic tool for analysing a carbon plastic composite and its adhesive joints. The samples of the carbon plastic type UKN-5000 were used in the test. Non-defected samples and samples with artificial defects were tested. 

By testing the adhesive joints for the shift, the loading curve appears to be almost linear until the destruction point. The acoustic noise curves for the weak samples are describing an increased activity at tbe initial loading moment, and just before tbe destruction. The strong samples are acoustically little active up to the start of the macro-destruction. 

The analysis of the test results shows that non-defect adhesive joints of the carbon plastic are acoustically less active than the glued main material. This can be explained by absence of plasticization effect of the die (adhesive layer). The value of the breaking point ("C ) at the adhesive joints shift is 9,6 M Pa. 

By working trough the method of the AE diagnostics, and as it was with carbon plastic case, the adhesive joint were tested by the step- and two-multiple loading. 

The test with the step loading shows that acoustic activity of the solid adhesive joints in the tested carbon plastic is quite low. The maximum on the endurance area was fixed at the predestructive moment. The last is evidence to the fact that the prevailing defect of the adhesive joints is starting its development at the loading level, which is close to the destruction point. 

To examine the accumulation effect activity ( A ZT) in the adhesive joints of the carbon plastic, the artificial defects were made. The samples were loaded up to the stress of 0,6"Zf. The test has showed (table 2) that in the weak samples the acoustic emission, at the repeated loading, will start at the point, which is smaller, than initial loading. While, the weaker sample, the bigger value of the "S. ... 

Thus, carrying out tests of the samples shows that the acoustic emission method is quite effective at the quality estimation of carbon plastic and its adhesive joints. Depending on the chosen diagnostic diagram of the construction material loading, the criteria parameters are K, S or AS (a C). 

Acoustic emission test of adhesive joints of the carbon plastic UKN-5000. 

The performance of the classifier has been verified using a number of practical applications, such as civil engineering [3], inspection of aerospace composite structures, ball bearings and aircraft multi-layer structures. Here we present shortly some results, focusing on detection of disbonds in adhesively joint multi-layer aerospace structures using Fokker Bond Tester resonance instrument, details can be found in [1]. 

Fig. XII-15. Diagram of peel test. A and B, adhesive joint C, double Scotch adhesive tape and D, rigid support. (From Ref. 107. By permission of IBC Business Press, Ltd.)...

J. J. Bikerman, The Science of Adhesive Joints, Academic, New York, 1961. 

Pie2oelectric materials are used in many different types of sensing—actuating devices. A few appHcations include printing, monitoring of performance behavior of adhesive joints, and intelligent processing. 

The principal type of shear test specimen used in the industry, the lap shear specimen, is 2.54 cm wide and has a 3.23-cm overlap bonded by the adhesive. Adherends are chosen according to the industry aluminum for aerospace, steel for automotive, and wood for constmction appHcations. Adhesive joints made in this fashion are tested to failure in a tensile testing machine. The temperature of test, as weU as the rate of extension, are specified. Results are presented in units of pressure, where the area of the adhesive bond is considered to be the area over which the force is appHed. Although the 3.23-cm ... 

Table 1 provides the approximate load bearing capabiUties of various adhesive types. Because the load-bearing capabiUties of an adhesive are dependent upon the adherend material, the loading rate, temperature, and design of the adhesive joint, wide ranges of performance are Hsted. 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

R. D. Adams and W. C. Wake, Structural adhesive Joints in Engineerings Elsevier, New York, 1984. 

A sealant s adhesion is commonly studied by 180 degree peel tests such as ASTM C794 or by tensHe/adhesion joints tests such as ASTM C719. The adhesion test protocol should simulate actual field conditions as closely as possible. Sealants often have good adhesion to dry substrates, but this adhesion may be quickly destroyed by water. Because most sealants are exposed to water over their lifetime, adhesion testing should include exposure to water for some length of time. ASTM C719 is one of the better tests to determine a sealant s adhesion durabHity as it exposes sealants to seven days of water immersion. 

The path of failure of an adhesive joint can give information about the mechanism of failure if analysis of the elemental and chemical composition can be conducted along the path. Several authors have performed such analyses by loading the adhesive joint until it fractures and then using XPS to analyze each side of the fracture. 

Fig. 1, Schematic of commonly u.sed methods for testing the strength of adhesive joints, (a) Peel test. Note that the peel angle can be changed depending on the test requirements, (b) Double overlap shear test. In this test, the failure is predominantly mode II. (c) Single overlap shear test. In this test the failure mode is mixture of mode I and mode II. (d) Blister test.

Usually tj/ is very much larger than Fq. This is why practical fracture energies for adhesive joints are almost always orders of magnitude greater than works of adhesion or cohesion. However, a modest increase in Fq may result in a large increase in adhesion as and Fo are usually coupled. For some mechanically simple systems where is largely associated with viscoelastic loss, a multiplicative relation has been found ... 

To be effective, there must be a certain minimum concentration of inhibitor at the interface to be protected. Therefore, there must be sufficient inhibitor in the primer, and these inhibitors need to be soluble enough in water to enable transport of inhibitor to the oxide surface as water permeates the adhesive joint. However, too high of a solubility will rapidly deplete the primer layer of inhibitor resulting in a loss of protection. One of the fortuitous properties of zinc and strontium chromates is the limited solubility of these compounds in water (about 1.2 g/1 at 15°C [33]). 

Fig. I. Comparison of unprimed and eleetroprimed single lap-shear adhesive joint strengths for steel coupons bonded with imidazole-cured epoxy [43].

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