Adhesive bonding joint stresses

The moisture content of both the wood and the adhesive affect the fracture behavior of adhesive bonded joints. Wood joints are especially sensitive to moisture effects as a result of the porosity and permeability of wood, which allows ready access by water to both the interior of the wood member and the adhesive layer. Irle and Bolton [57] showed that the superior durability of wood-based panels bonded with an alkaline PF adhesive compared to panels bonded with a UF adhesive was due to the ability of the phenolic adhesive to absorb and be plasticized by water. In the plasticized state, the phenolic adhesive is able to reduce stress concentrations that otherwise fracture the wood or the adhesive in urea-bonded panels. 

Joint Design The adhesive bond joint must be considered to provide compressive stress rather than tensile stress and reduce stress concentration and peeling in the bonded joints (Lees 1984). 

Figure 1.2 Effect of residual MMA on the strength of adhesive-bonded joints (1) before and (2) after vacuum treatment and (3) on the internal stresses <7,5 in the adhesive layer.

Thus, the adhesive contacts the substrate via a layer of substances that frequently differ from the adhesive in composition. If the cohesion strength of these substances is less than that of the adhesive, this will determine the failure stress of the adhesive-bonded joint. The adhesive, which has the same composition as that of the adhesive in bulk, can form a weak zone in the substrate surface. Adhesives are polymers and the particular nature of a polymer must have effects at all stages of formation and operation of an adhesive-bonded joint. 

In the course of formation of the adhesive-bonded joint, internal stresses appear in the adhesive layer. These stresses can change the process of formation of the polymer boundary layer and cause the formation of faults. With increase of the internal stresses in polystyr-... 

The fracture stress of adhesive-bonded joints (adhesion strength) is a consequence of processes that occur in the course of their formation, but all attempts to formulate a fundamental relationship between formation and failure of adhesive-bonded joints have so far been unsuccessful. This is mainly due to the lack of methods for meastuing adhesion that would permit determination of the failure equilibrium work. Accordingly, the relationship between the experimentally determined value of the adhesion strength and the thermodynamic characteristics can be one of correlation only. 

Figure 3.3 Adhesion strength of the adhesive-bonded joints for different relationships of the normal and tangential stresses 100 days after cementing (1) unloaded specimens (2) shear load 0.3 GPa (3) normal load 0.36 GPa.

For the monomer polymerization at room temperature, the adhesive was augmented with a redox system of 3% BP and 0.75% DMA. To study, explain, and predict the development of the elastic failure of the polymer in the adhesive interlayer, an improved method of investigating adhesive layer crack resistance with modeling of the formation and growth of a crack at the adhesive-honded joint loading was used [119]. Five adhesive-bonded joints with the adhesive mixture compositions shown in Table 3.1 were subjected to static tests for crack resistance at room temperature. The characteristics of the static crack resistance of the adhesive-bonded joint Kic is the coefficient of the stresses intensity Gic is the intensity of the elastic energy release ic is the opening in the crack tip) were determined at the moment of onset of the crack in double-cantilever specimens DCB (Fig. 3.5). The specimen cantilevers were made of PMMA of TOCH type. 

The strength of adhesive-bonded joints under the combined effect of tangential and normal stresses at 20, 40, 60, 90, and 100 days after cementing of St3 steel is displayed in Fig. 3.9 as surface stress plotted in coordinates cr—Taj time. Surface I characterizes the extreme stresses of the joints using VAK adhesive surface II is the same without addition of ATG to the adhesive. 

Because of high adhesion strength and deformability of the VAK and Sprut-4 adhesives, low internal stresses in the adhesive-bonded joints ensured their serviceability when used as binders for the formation of reinforced coatings on metal and other surfaces. The thickness of such coatings can reach some centimeters and their strength is comparable with that of metals. Such coatings are suitable to... 

Internal Stresses in Adhesive-Bonded Joints and Ways of Decreasing Them... 

The internal stresses in adhesive-bonded joints arise for two reasons. In the course of setting of the adhesive, its volmne decreases due to volatilization of solvents, polymerization or physical structurization. As a result of the adhesion interaction of the adhesive and the substrate, the film can contract only in thickness, which is why stresses that appear in it are parallel to the siuface. The film extends while contraction stresses appear in the substrates. Rapid growth of stresses, which tend to reduce the length of the film, begins from the moment the polymer loses yield. 

The second component of the internal stresses is thermal stress caused by differences of the coefficients of linear thermal expansion of the adhesive and the substrate. They appear in the coiu se of heating or cooling of the adhesive-bonded joint. The mechanism of the internal stresses occurring in adhesive-bonded joints does not generally differ from that in coatings, but because there are two sohd surfaces the magnitude of the stresses in the first case appears to be substantially greater. 

In some cases the adhesive film cannot contract in thickness during ciming—for example, in the csise of tube-in tube adhesive-bonded joints. Then the tangential internal stresses are added to the normal ones. The total stresses in this case increase substantially. The occurrence of internal stresses during formation of the polymeric film on the... 

The internal stresses can be equated to a long-period acting load [161]. They can cause spontaneous peel-off of the polymer and fracture of the adhesive-bonded joint [160—163], even at 15—50% of the instantaneous breaking stress. In this csise there is a linear dependence in semi-log coordinates between the lifetime of the joint and the internal stresses [164]. An analytical dependence has been proposed that relates the strength of the adhesive-bonded joint to the internal stresses in the adhesive layer [160],... 

Determination of Internal Stresses In Adhesive-Bonded Joints... 

Determining thermal internal stresses. Experimentally, the stress with a particular temperature change is determined by the deflection of the free end of the cemented three-layer plate fixed as a cantilever. The deflection of the plate end was determined by means of a projection optical system. Figure 4.3 displays the distribution of the thermal internal stresses in a Sprut-5M adhesive-bonded joint for temperature change from 20 to 42°C. Plates made of IKhlSNlOT steel 0.07 mm thick and aluminum foil 0.02 mm thick were used as substrates. It is evident that the internal stresses are characterized by maximum values on the interfaces. 

Shrinkage internal stresses in adhesive-bonded joints... 

There are established methods for evaluating the shrinkage internal stresses in the adhesive-bonded joint using cantilevered three-layer beams [160, 175], but in practice these methods are frequently too complicated. 

Figure 4.3 Internal stresses (MPa) in the adhesive-bonded joint occurring on temperature elevation from 20 C to 43 C.

A paper of Myshko and Garf [175] was the first to attempt to create a method of calculating the distribution of the shrinkage internal stresses along an adhesive-bonded joint cross-section for a cantilevered specimen made of two plates of different flexural rigidity with an adhesive layer between them—i.e., to generalize the equations from [160] for adhesive-bonded joints. 

Let US consider a method of determining internal stresses that lacks the above shortcomings hy applying it to an adhesive-bonded joint represented by a three-layer plate (substrate-adhesive-substrate). The adhesive layer shrinkage causes deformation of the whole plate. The resistance forces appearing in this case can be represented as a system of normal forces and bending moments, which act layer by layer throughout the plate, and stresses can be represented as... 

The rectangular region shoiild be cross-cut along the coordinate lines. Within the limits of the allowable ratios of the element sides, the coordinate lines can be arbitrarily thickened, but narrow elements must be placed right after the broad ones. These two conditions are important when studying the stress-state of thin interlayers for example, polymers in adhesive-bonded joints. 

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