Bondline

Corrosion-inhibiting primers based on this technology have been in continuous service since they were first utilized with nitrile epoxies in the late 1960s. These inhibitors function by passivating the aluminum. In this process, water permeating the adhesive bondline carries a certain amount of inhibitor to the oxide surface. 

At elevated temperatures where titanium alloys could be the adherend of choice, a different failure mechanism becomes important. The solubility of oxygen is very high in titanium at high temperatures (up to 25 at.%), so the oxygen in a CAA or other surface oxide can and does dissolve into the metal (Fig. 12). This diffusion leaves voids or microcracks at the metal-oxide interface and embrittles the surface region of the metal (Fig. 13). Consequently, bondline stresses are concentrated at small areas at the interface and the joint fails at low stress levels [51,52]. Such phenomena have been observed for adherends exposed to 600°C for as little as 1 h or 300°C for 710 h prior to bonding [52] and for bonds using... 

Davis, G.D. and Kolonko, L.L., Process control methodology — process sensitivity evaluation of the case to insulation bondline. In Proc. JANNAF Structure and Mechanical Behavior Subcommittee Meeting, Pub . 591. Chemical Propulsion Information Agency, Columbia, MD, 1992, p. 67. 

StiTJCtural adhesives are also available in paste form. Bondline thickness control and void elimination are more difficult with paste materials but they can be very useful for unusual designs and innovative manufacturing methods. 

Primers are required to be resistant to all of the same fluids and environments as the adhesive, and are in addition expected to be compatible with secondary finishes such as corrosion and fluid resistant primers applied to cured bond assemblies. The most commonly used primers for 250°F cured epoxy adhesives also have active corrosion inhibitors themselves to combat corrosion at bondlines. This last requirement is somewhat dated, evolving from the severe corrosion and delamination problems experienced before U.S. airframe manufacturers adopted durable surface treatments. 

Vacuum bag bonding is restricted by definition to less than 1 atm (<15 psi) which can be insufficient to achieve good part contact and prevent bondline voids. Hence vacuum bag bonding is rarely used outside of repair situations. 

During the temperature ramp period, pressure is applied. How much pressure is applied depends on the adhesive and the type of assembly. Honeycomb assemblies are limited by the compression strength of the honeycomb core, so cure pressure is typically limited to 50 psi for aluminum core of standard density. Metal to metal assemblies can withstand higher pressures and usually have fewer bondline voids when cured at higher pressures. Metal-to-metal assemblies bonded with standard modified epoxies are cured at 90 psi. 

For a typical assembly, first part qualification begins with a rigorous dimensional check and painstaking prefit of all details on the bond tool. The assembly details are placed on the tool without adhesive, close contact between bond surfaces is verified and any detail or tool interference is corrected prior to proceeding. This is followed by fabrication of a verification film , or a simulated bond cure cycle of the assembly to allow measurement of the adhesive bondline thickness. 

After a satisfactory verification film is produced, an assembly may be fabricated specifically for destructive inspection to validate that the verification film was accurate. This correlation allows the use of verification film rather than more expensive destructive inspection for future changes such as duplicate tool fabrication and tool or detail modification. Simple assemblies are usually not destructively inspected because of high confidence that the verification film is entirely representative of the expected bondlines. Complex or large parts may not be destructively inspected because of the cost of the details and assembly time. In these cases other means of validating the verification film are used. Meticulous pre-bond detail and post-bond assembly thickness measurements may be sufficient to prove bondline thickness control. Ultrasonic inspection and X-ray photography (discussed previously) may be sufficient to prove that details are in the correct places and bonds are good. 

If an assembly is destructively inspected, the verification film is used to identify potential problem areas for particular attention. These areas and others randomly selected are cross-sectioned to determine bondline thickness (Fig. 22), bond details are peeled apart to inspect for voids and honeycomb core bonds are... 

Application of adhesive primer presents another challenge. Most adhesive primers require a high-temperature cure, which presents the potential for damaging existing bondlines. Infrared curing can be used to reduce the likelihood of problems. 

J. Gough and A.H. Muhr, Initiation of failure of rubber close to bondlines, in Proceedings of International Rubber Conference, Maastricht, The Netherlands, June 2005, lOM Communications Ltd., London, 2005, pp. 165-174. 

Bondline readout, 7 122 Bond number (Bo), 15 687t Bond orientational order, of liquid crystalline materials, 15 85 Bonds, fullerene, 12 233-234 Bond strength, in thermal bonding, 17 510 Bone, 7 273t. See also Bones citric acid in, 6 632t Bone fractures, 3 725 treatment, 3 725... 

Fig. 1. The crack propagated rapidly and erratically through the adhesive as observed visually. Although the initial crack length in the control samples was 2.5 cm, the crack propagated an additional 6.5 cm through the bondline under the test environment. The results for the Ti(IV)B samples under these conditions were comparable to the results reported in Fig. 1.

The results above suggest that it may be possible to apply fracture mechanics data to determine failure loads of more complex structures, provided that (i) the adhesives used are not too ductile, (ii) bondline thickness is known and controlled, (iii) non-linear behaviour due to adherend and interface damage is limited, and (iv) the specimens employed to determine... 

The two components cure without evolution of any by-products, little or no volume change occurs and the system is ideally suited for use in thick and variable thickness bondlines. Solventless and 100% solid systems are possible which cure at room temperature within a few minutes. Viscosity, curing rate and cure temperature requirements can be tailored to meet the user s needs. Application of the system is generally made by two-part metering and mixing equipment. 

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