Adhesive bounding of aluminum components in the aircraft and automotive industry are rapidly replacing many conventional methods of joining two parts togheter. Adhesives bonding is a process where a monomer compound is applied in between joints and is chemically allowed to transform into complex long chained polymers which interlock the mating parts by inter-atomic bounding and diffusion of the adhesive into the adherents. In most common uses, bonds are permanent, such as the thermosetting adhesives, and require machining in order to separate the mating parts. In other cases, the adhesive bond might be directly related to the operating temperature, as in thermoplastic adhesives, or may contain natural resins and form an elastic bond such as elastomeric adhesives. Adhesives are mostly limited to shear stress applications due to their relatively low peel and creep capabilities and require far more surface area to form a joint then conventional methods, however, their benefits in weight reduction and enhancing ergonomics, as well as their still fairly inexpensive and ability to function in a wide range of environments, are attractive traits which may influence a designer to select such a process.
Adhesives have been long known as the oldest means of joining two parts togheter. In fact, archeological evidences has been found showing the use of natural resins by early man to fasten arrows and spear heads to shafts (3, 170). They also found there first structural use well over three thousand years ago in the construction of the Tower of Babel ( 9, 3). However, it wasn’t until the second World War where great advances where made in structural adhesive bonding which applied to metal to metal structures. Redux, an adhesive conceived in England, was the first ever adhesive used in considerable proportion on the lightest fighter plane of that time, the Mosquito. Following in it’s allied foot steps, the Americans also adopted this process to produce the most effective aircraft of the war, the De Havilland Hornet which used the adhesive to bond the aluminum extrusions to the capped wing spars (4, 41).
Adhesives have since grown considerably and since no one universal adhesive exists up to date, the boundaries are limitless. They can be found in mostly all types of industries, from high tech aerospace to dishwashing machines, and are more and more, replacing conventional ways of fastening joints, namely mechanical fasteners and welding process.
In most cases, monomers are applied to it’s adherent (part to be jointed) and are turned into polymers, which are long chain type molecules, by means of diverse catalyst ranging from heat, pressure or some other chemical compound all depending on the type of adhesive used.
The reason why the bond takes place between the mating surfaces however has taken different theories over the past years. The three most common, principle of diffusion, principle of mechanical adherence and the principal of inter-atomic bonding have been well more developed than other explanations of why adhesive bonds occur.
The diffusion principle states that at polymerization, some of the molecules diffuse in the adherent and consequently, some molecules of the adherent diffuse into the adhesive in order to form a solid joint. The mechanical principal states that, the imperfections on the wetted surface of the adherent serve as to grip the adhesive when it turns into it’s solid polymer state and takes advantage of these imperfections to establish a solid bond. However, this might not help in explaining why perfectly strong bonds are also possible on highly polished surfaces. The third and most generally accepted theory attributes the bonding forces to the fact of the sharing of valence electrons between the newly formed polymer and the adhered material. Some may like to contribute all three factors working togheter as the proper definition of the bonding process( 3, 170) .
Adhesives may be categorized in three groups, thermosets, thermoplasts or elastomers.
Thermosets such as phenolics, epoxies and ureas are adhesives capable of withstanding high shear stresses and are capable of maintaining their properties for a wide range of temperatures. Once cured, machining or extremely high temperatures are required in order to break the formed bond and are irreversible processes. Contrarily, thermoplasts adhesives such as polyvinyls, acetates or acrylics have bond strength variation which is directly temperature dependent and for which at low temperatures, the bond gets stiff and brittle, but at high temperatures, ductile and elastic bonds occur. The third and last classification are elastomers which are generally made of natural or synthetic rubbers and latexes which are highly ductile and deformable adhesives at temperatures ranging close to room temperature ( 1, 402).
Most adhesives are furthermore classified under its ability to cover the entire surface of the mating parts, termed wetting ability ( 8,5). The Washburn equation is generally applied in order to classify the wetting property of the adhesive and is given by:
t = (2h/gcosq) * x2/r where
t : rate of filling irregularities
x : length of irregularity
r : capillary radius
h,g : viscosity and surface tension
q : contact angle between adhesive and substrate
Although the basic application are quite commonly simple, where most adhesives today are still spatula applied, automation is quite common and not such a daunting task to perform. The challenge lies in the preparation of the surfaces to be bonded as well as the study of the optimal curing times and temperatures required to obtain the required bonding strengths.
All parts require extensive cleaning with the use of solvents to eliminate any dust or grease deposit, which may interfere with the bonding process. Metals with oxide films are usually grounded and aluminum alloys pickled in alkali baths at temperatures of 170oF up until 10 minutes in order to clean and roughen the surface for the adhesive. Bare hands should never come into contact with the surfaces to be bonded as they will re-introduce grease to the surfaces to be mated (1, 417). As for curing temperatures and times, each vary depending on the type of adhesive used and the required bond strength sought ( 6, 1054). The most common use of adhesives is in shear bearing applications due to their poor resistance to peel, cleavage and tensile stresses (6, 1056).
This is where the challenges arise for the engineer who must design or modify an existing joint in order to maximize the load in a shear plane direction. Although they are limited to these applications, a well-designed joint may sustain an impressive amount of stress. However, most load bearing structural adhesives may have quoted performances rated as high as 50% of their ultimate tensile strength, but as a general rule of thumb, these materials should never be loaded more than 10% of the rated capabilities, which make designers skeptical in using adhesives for long term use applications ( 7, 5 ).
Another problem arises at selection of the adhesive since no universal adhesive exist, each adhesive has its own characteristics for which it may perform at better operating conditions then others and therefor, a quite profound knowledge of the different types of adhesives available is required. Furthermore, design must require high surface finish and quality control performed very closely. And in most instances, the conversion from a mechanical fastener to an adhesive usually requires a considerable larger surface area of jointing ( 9 ,10-13).
So why use adhesives one might ask? Some adhesives today are now capable to cover temperature ranges from -55oC to 350oC with service life of 60 000 hours or 20 years (4, 42). Furthermore, there are several beneficial reasons why adhesives may be preferred over conventional joining processes. For instance, when comparing to welding, adhesive joints do not distort the part close to the bonding area and does not leave any surface discoloration’s, which in turn may require further grinding and then painting. The most notable advantage is the possibility to join dissimilar metals, which is impossible in welding due to a high variation in melting temperatures.
As for mechanical fasteners, the replacement with adhesives induces a reduction in overall weight and serves as a corrosion barrier between a joint made of two different materials. Since adhesives are also less stiff then most fasteners, they also serve as vibration dampers and can easily accommodate thermal expansion of the adhering joints. By replacing rivets with adhesives in structural components, it is also possible to eliminate the stress concentrations formed near rivet holes and therefor prevent unforeseeable stress cracks, which may lead to disastrous failures. In an ergonomic point of view, adhesives do not protrude from their assemblies and go virtually unnoticeable and offer great characteristics in aerodynamic applications ( 9, 9).
Although the alternative of opting for an adhesive joint might be appealing in some circumstances, the economics involved can be quite complex. In general, adhesives are less expensive than a welding or fastening procedure. However, the implementation of adhesives in a mass production might prove to be quite expensive when taking for account the surface preparation requirements, the curing times and temperatures and the expertise needed to evaluate and control these processes. However, many companies have established major savings by using adhesives instead of other methods. The Northrop Aircraft Company stated that their conventional riveting operation of fabricating trailing wing sections cost them closely to1312$ where as obtaining similar strengths by using adhesives cost them 149$ per wing (3, 171).
In addition, the use of adhesives permitted the use of 0.020in thick aluminum sheets instead of the 0.051in sheets minimum requirements for rivets. Hence savings in money and weight ( 9, 5).
Adhesives most important contribution to the automotive industry came in the early 50’s when it replaced the rivets used to hold the brake linings to the shoe. This allowed for a far greater life of the brakes as they could know be run down all the way down to the show, instead of at the rivet heads. Also, created no distortion in the lining due to riveting, created better heat dissipation and reduced the risk of brake failure.
Although many new applications have arisen, they do not contribute an important saving to the automobile producer. Welds are still very present and so are mechanical fasteners due to the requirements of mass production, which can not permit itself of stopping the production line in order to let the structural adhesives cure. However, most of the final assemblies such as the trimmings, the side panel guards, the window rims and other sealed surfaces are joined by adhesion just prior to painting such that when the automobile must be put in an oven for the paint to cure, the adhesives cure also. Some manufacturers do use adhesives to fix the top gears of the transmission due to their low torque capabilities, namely Renault, and the arrival of vehicles made of composite material panels have created the need for an increase in adhesive joints (5, 63-65)
The reason why adhesive technology has developed so much is surely due to this industry in which adhesives plays a crucial role. A typical Boeing 747 is composed of over 1300m2 of adhesive material. The entire fuselage is covered by an adhesive film, which helps in the aerodynamics and the acoustic dampening of the vehicle. Furthermore, apart from the hundreds of sealed joints where adhesives can be found, critical structures such as wing sections, thrust reversers, walking panels, elevators, doors and flaps to name some, are all composed of high resistance, light weight, good fatigue and high acoustics dampening honeycomb structures which can only be assembled with high strength adhesives (8, 258). Up until date, not one adhesive failure has ever been directly linked to a fallen airplane (9, 5) .
Helicopter blades consist of plate-to-plate type adhesive bonds which make for a better, more aerodynamic and certainly safer joint then with rivets, which where know to fracture off (4, 44)
As for space travel, no mission in space would have ever been possible without the use of high strength and temperature ranging adhesives which are required to mount the heat shields on the shuttles under side which protect it from flaming up on re-entry into the atmosphere ( 9, 7).
As the cost of life increasingly becomes more expensive, especially when considering the high fuel prices of today, it is no wonder why manufacturers of transportation vehicles try to optimize their product in order to somehow turn out a profit. Some factors, such as weight and drag reduction, may help in turning in some savings, without compromising the overall safety of the consumer. Adhesives have helped alleviate the problem and this can be seen on such a wide scale that the adhesive industry has grown considerably over the years, an estimated 20% increase in production annually (3, 1). And as more composite materials make their appearance and become economically more feasible, so will the appearance of vehicles which will entirely be made of these plastics and will most certainly contain numerous bonded joints which can only be achieved threw adhesives. These days are certainly not far out of reach.
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