Joints internal stress

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-... 

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. 

Curing adhesive Shrinkage Internal stress within joint... 

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... 

Figure Q13.12 shows the mechanical properties of two urea-formaldehyde (UF) resins cured with NH4CI (bottom) variation of the shear strength of bonded wood joints with cyclic wet-dry treatments of joints (middle) development of internal stress with duration of resin cure at room temperature (top) dynamic mechanical properties of resins. Discuss the interrelationships between the observed mechanical properties. 

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. 

Internal stress may detract significantly from the apparent strength of a joint even if it is insufficient to fracture the joint. For example, if the internal tensile stress in a joint is equal to one-half the ultimate stress or strength of the weakest material, the available tensile strength of the joint is lowered by 50%. 

R. H. Gillespie, Effect of internal stresses on bond strength of wood joints, NTIS PB-258 832j 5ST, prepared for Department of Housing and Urban Development, U.S. Department of Agriculture Forest Service, Forest Products Laboratory, Madison, WI, 1976. 

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. 

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