Adhesive cracking

Enamel Cracking Adhesion Cracking Adhesion Cracking Adhesion ... 

The application of surface treatments to mbbers should produce improved wettability, creation of polar moieties able to react with the adhesive, cracks and heterogeneities should be formed to facilitate the mechanical interlocking with the adhesive, and an efficient removal of antiadherend moieties (zinc stearate, paraffin wax, and processing oils) have to be reached. Several types of surface preparation involving solvent wiping, mechanical and chemical treatments, and primers have been proposed to improve the adhesion of vulcanized SBR soles. However, chlorination with solutions of trichloroisocyanuric acid (TCI) in different solvents is by far the most common surface preparation for mbbers. 

Mechanical adhesion. Cracks and pits are produced on the treated rubber surface which favor the mechanical interlocking with the adhesive. Eurthermore, unreacted solid prismatic TCI crystals on the treated rubber surface can be dissolved by the organic solvent into the adhesive, favoring the reaction with the adhesive. 

Fig. 2 Typical force-displacement trace for a CFRP joint bonded with the epoxy-film adhesive. (Crack length, a, values in mm.)...

Fig. 3 Adhesive cracked surfaces observed in shear and experimental load deflection curves (a) high roughness surface, (b) low roughness surface, (c) solid wood.

Structural adhesives General-purpose adhesives Crack injecting systems Anchoring systems Grouts... 

Results of accelerated testing of circuits prepared with this polymer as the dielectric layer are satisfactory. Circuits comprising two metallization and one polymer layer withstand a minimum of 500 cycles from -40 to +130 C with no loss of adhesion, cracking of the polymer, or open circuits. No mechanical or electrical failures occur during 1000 hours at 150°C in air, and the insulation resistance remains above 200 megohm during 500 hours at 85 C/85% RH/60 V (THB). Tantalum nitride resistors under the polymer change < 0.5% under these THB conditions. 

Now assume that in the example of interest here, preliminary analysis of the motor case shows that at the most likely points of debonding. Mode I loading dominates adhesive crack behavior. Two test methods that could appropriately be used to determine Ya Mode I are the blister test and the double cantilever beam (26). If one assumes that the first is chosen, then a layer of the adhesive (the solid propellant) is cast on a plate of the material from which the motor case is made. This plate contains a hole that has been fitted with a pressure connection. This connection is used to pressurize (with air, inert gases, fluids, etc.) the region between the plate and propellant, arid thereby produces a blister. The value of Ya can be determined from measurement of the pressure at which this blister grows as a function of its diameter (26). In our case, let us assume a series of tests yields a "least value" for Ya of J/m2. 

Figure 11, Energy release rate as a function of adhesive crack size for a hypothetical rocket motor.

Spongiosis occurs primarily in cast iron pipes laid in acid earth or in calcareous loamy soil. It can be avoided by applying adhesive, crack-free protective coatings, such as those based on tar. Damage caused by spongiosis is also observed in cast iron feed water preheaters and pumps for drinking water and seawater (Schmitt-Thomas and Fenzel 1965). 

The first truly reversible adhesion cracking experiments were carried out by Obreimoff in 1930 on mica and by Johnson et al in 1971 using smooth elastic rubber spheres. The diameter of the black contact zone was measured in reflected light, and plotted against the applied force to compare with the thermodynamic cracking theory. The results were reasonably reversible and fitted the thermodynamic work of adhesion theory. These experiments are described more fully in Chapters 4 and 9. 

Figure 8.24. (a) Cavitation in a rubber film stretched between rigid plates (b) adhesion cracks giving same effect (c) finger cracks give crazing. 

Figure 1.2 In an overloaded toughened adhesive, crack propagation is stopped by the dispersed rubbery phase.

EP copolymer (reactor blend, with 12 mol% C2) forms the matrix HDPE particles are well distributed void formation because of low adhesion crack/fracture follows the particle/matrix interfaces, falsely implying a larger particle content than 5% ... 

When cementing plastic parts to metal parts, a room temperature cure is recommended to avoid creating strains in the adhesive caused by the difference in coefficient of thermal expansion. This differential can cause adhesive cracking and decrease the bond strength. Also, curing temperatures must not exceed the heat distortion temperature of the plastic. 

If T is a characteristic relaxation time of the substrate, creep will be important when ta T. However, if td T, then adhesion (crack tip) effects at the contact periphery will dominate. Johnson (S) has shown that, in the JKR limit, these two times are very different which allows the two effects to be treated independently. 

Fig. 26. Adhesive crack propagation in DCB specimens along paths of maximum Gc...

Unstable damage development of thick bond-lines (lower plastic strain development) resulted in adhesive cracking in multiple locations with a cohesive-type failure and lower failure strengths. 

On the triple border refractory—air—molten Al (Fig. 3.2), molten A1 interacts with oxygen from the air, giving corundum (here the term corundum is for the mix of alumina with alumina, partly reduced by aluminium). This process is activated by magnesium and other metals. As a result, the outgrowth appears on the walls, which decreases the volume of the furnace. These outgrowths are removed mechanically, but as its strength is big, as well as the adhesion, cracks appear in the lining. These cracks allow the infiltration of Al. 

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