Internal stresses, adhesive joints

Curing adhesive Shrinkage Internal stress within joint... 

Non-reactive solution adhesives the solvent wets the surfaces to be assembled, then evaporates involving the cohesion of the parts to be assembled by the adhesive joint. The heat behaviour is generally moderate. If the solvent swells the materials to be assembled, there can be migration of materials and subsequent cracking by residual internal stress relaxation. 

No small molecules such as water are liberated during the curing process. Thus, epoxies exhibit low shrinkage, and they can be cured under very low pressure. This provides an adhesive joint with a very low degree of internal stress when cured. 

These internal stresses often can have a degrading effect on the adhesive properties but little or no effect on the cohesive properties of the adhesive film. They mainly affect the interface area of the joint. 

Loss of theoretical adhesive strength can also arise from the action of internal stress concentrations caused by trapped gas and voids. Griffith11 showed that adhesive joints may fail at relatively low stress if cracks, air bubbles, voids, inclusions, or other surface defects occur as a result of the curing process. 

The high elevated-temperature cures are damaging to adhesive systems due to a mismatch in thermal expansion coefficient that can occur between the epoxy and the substrate. The difference in rate of expansion when returning to room temperature from the cure temperature can lead to significant internal stress within the adhesive joint, which results in poor adhesion. 

Since nonreactive diluents do not enter into the crosslinking reaction, they can be lost due to volatilization, especially when exposed to the elevated temperatures of the exotherm or curing cycle. If vaporization does occur, shrinkage of the adhesive film can result in internal stresses being generated within the joint. These internal stresses reduce the degree of adhesion that is realized on final cure. 

Depending on the substrate, the curing temperatures, and the service temperatures that are expected, the adhesive formulator may want to adjust the coefficient of thermal expansion of the adhesive system. This will lessen internal stresses that occur due to differences in thermal expansion between the substrate and the adhesive. These stresses act to degrade the joint strength. 

There are several occasions when the difference in coefficient of thermal expansion between the substrate and adhesive will result in internal stresses in the joint. Common occurrences are (1) when the cured joint is taken to a temperature that is different from the curing temperature and (2) when the joint is exposed to thermal cycling. 

When a liquid adhesive solidifies, the theoretical strength of the joint is reduced because of internal stresses and stress concentrations that usually develop. The most common cause... 

The improvements in adhesive strength of cured epoxy joints that are attributable to fillers are not as much related to the improved cohesive characteristics of the adhesive as to the reduction in internal stress due to modification of the coefficient of thermal expansion, shrinkage, etc. 

Reducing Internal Stress. Internal stresses are common in joints made with high-temperature adhesives. These stresses can be due to... 

Metal fillers for high-temperature adhesives must be carefully selected because of their possible effect on oxidation, as indicated in the previous section. Carrier films, such as glass cloth, are generally used to facilitate the application of the adhesive, but they also provide a degree of reinforcement and lowering of the coefficient of thermal expansion. Thus, they reduce the degree of internal stress experienced at the joint s interface. 

Basically, there are two major considerations when one is formulating or selecting adhesives or sealants for low-temperature applications. The first is the effect of the low temperature on the bulk properties of the polymer, and the second is the effect of thermal cycling and resulting internal stresses on the joint interface. 

A third difficulty in bonding metal surfaces is that they have a higher thermal coefficient of expansion and thermal conductivity than most epoxy adhesive systems. As explained in other chapters of this book, the difference in rates of thermal expansion results in internal stresses in the adhesive joint, especially when the adhesive bond is cured at elevated temperatures or when it is exposed to low temperatures or repeated thermal cycling. 

Another problem in joining elastomers with adhesives is that since they are deformable materials, it is easy to develop internal stresses at the bond interface. These stresses could adversely affect the bond strength and permanence of the joint. Minimal pressure to achieve close substrate contact with the adhesive is all that is necessary when bonding with elastomers. 

Internal stresses may occur within the adhesive joint as a result of the different shrinkage rates of the adhesive and the work pieces, which will reduce the bond strengths. In general the strength of rigid epoxy resin... 

From the dynamic mechanical properties, resin D is evidently more flexible than resin A. As the resins are cured, the more flexible resin (D) develops less internal stress than the stiffer resin A due to greater molecular mobility. As wood absorbs and releases moisture during cyclic wet-dry treatment, the adhesive joint is subjected to cyclic stress. Resin D, due to its flexibility, is able to respond reversibly to the cyclic stress whereas the stiffer resins begin to degrade after five wet-dry cycles. 

In adhesive joints, for example with epoxy resin adhesives, internal stress in the adhesive layer may result from different coefficients of expansion in the glued materials, whereby their moduli of elasticity are important factors. The glass transition temperature, and thus the curing temperature, also play a role. The reaction shrinkage of the resin is another source of internal stress. Suitable formulations with added fillers, oligomers or copolymers are among the measures taken to reduce these influences. 

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.

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-... 

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... 

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