The strength of adhesion joints

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.

Many investigations of the molecular structure of thin films formed by y-APS deposited onto inorganic substrates from aqueous solutions have been carried out. Ondrus and Boerio [2] used reflection-absorption infrared spectroscopy (RAIR) to determine the structure of y-APS films deposited on iron, 1100 aluminum, 2024 aluminum, and copper substrates from aqueous solutions at pH 10.4. They found that the as-formed films absorbed carbon dioxide and water vapor to form amine bicarbonate salts which were characterized by absorption bands near 1330, 1470, 1570, and 1640 cm-1. y-APS films had to be heated to temperatures above about 90°C in order to dissociate the bicarbonates, presumably to free amine, carbon dioxide, and water. Since the amine bicarbonates failed to react with epoxies, the strength of adhesive joints prepared... 

Another way moisture can degrade the strength of adhesive joints is through hydration or corrosion of the metal oxide layer at the interface. Common metal oxides, such as aluminum and iron, can undergo hydration. The resulting metal hydrates become gelatinous, and they act as a weak boundary layer because they exhibit very inadequate bonding to their base metals. Thus, the adhesive or sealant used for these materials must be compatible with the firmly bound layer of water attached to the surface of the metal oxide layer. 

Falconer, D. J., et al., The Effect of High Humidity Environments on the Strength of Adhesive Joints, Chemical Industry, July 4, 1964. 

The presented data is a confirmation that it is expedient to use oil-soluble Cl as components of polymer coatings. If their concentration is optimal, the strength of adhesive joints coating metal ware can improve by 10-20%. The inhibited coatings are more resistant than those made of initial polymers. The crevice corrosion rate of the metal substrate recorded by the polarization resistance method presented in [80] lowers on transfer from the initial polyethylene coating to an inhibited one by three to ten times [81]. 

The strength of adhesive joint was determined by testing of single overlapped adhesive joints in shear using a dynamometer Instron 4301 with aluminum slabs having dimensions 10 X 60 mm, the thickness was 2 mm, the length of overlapping area was 15 mm and the thickness of deposited adhesive was 0.1 mm. 

Let us consider the influence of various combinations of stress conditions, namely, normal fracture, shear, normal compression, and the combined action of direct and tangential stresses of specified ratio, on the strength of adhesive joints, as well as their dependences on materials, adhesion conditions, and polymerization kinetics. Adhesives that were analyzed in Chapter 3 were taken as the objects of investigation. 

Results under normal compression for steel specimens glued with Sprut-5M and VAK compositions in air, under water, and in oil are given in Fig 7.1. As is obvious, the strength of adhesive joints under normal fracture is practically the same up to a certain critical value of precompression stress. Evidently, the applied normal compression stresses do not result in essential fractures in the adhesive joint the fracture occurs in the elastic zone. Using Sprut-5M composition (Fig. 7.1a), these compression stresses remain within 0-43.0 MPa in air, within 0-50.0MPa under water, and within 0-52.0 in oil the corresponding values for VAK are 0-104.0 MPa, 0-90.0 MPa, and 0— 84.0MPa (Fig. 7.1b). 

Figure 7.7 Methods of determining components of the strength of adhesive joint adhesion (T3dh) and friction Tf ) under the combined action of direct compression and shear, (a) Shear deformation of (1) initial specimen (2) precompressed specimen, (h) Shear deformation of the specimen joined hy fractured surfaces and repeatedly compressed to the initial level.

Physical adsorption contributes to all adhesive bonds and so is the most widely applicable theory of adhesion. The basis for adsorption is that van der Waals forces, which occur between all atoms and molecules when they are close together, exist across interfaces. These are the weakest of all intermolecular forces, but their strengths are more than adequate to account for the strengths of adhesive joints, van der Waals forces are of three types, namely the forces of attraction between molecules with permanent dipoles, those between a permanent dipole and a nonpolar molecule, and those between nonpolar molecules. The nature of these forces is outlined below, and more detail is given by Atkins and de Paula [42]. 

G. A. Budenkov, Yu. V. Volegov, V. A. Pepelyaev, and V. I. Redko, The possibility of testing the strength of adhesive joints by means of ultrasonic interference waves. Ultrasonics 1 194 (1977). 

Voyutskii O has also levelled objections at the thermodynamic approach to the strength of adhesive joints his comments and their rebuttal are well summarized by Crocker ... 

In the preceding sections, a number of tests have been described that are intended to provide information on the strength of adhesive joints. There are a number of other standard tests intended to provide other information on adhesives and adhesive joints that might relate to durability. A few of these standard tests will be reviewed briefly here. 

Arnold, W., Nondestructive determination of the strength of adhesive joints. In Trends in NDE Science and Technology. Proc. 14th World Conf. on NDT, 1, 1997, pp. 93-98. 

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