It is a good design in that it performs well under high compressive loads and affords a clean appearance. Like the square butt joint the common T-joint may not provide adequate bending area hence various methods of improvement shown in Fig. The corner joints are subjected to both peel and cleavage stresses and the joint is relatively weak when the loading on a corner joint is at right angle to the adhesive.
Methods of strengthening the comer joints are shown in Fig. Adhesive bonding is also used for tube joints some of which are shown in Fig.
Large bonded areas give strong joints with clean appearance but processing may be Complicated with some while edge preparation may be costly for some others. The strength developed in an adhesive joint depends upon the joint design, type of loading, service temperature, adherend material, etc. In making adhesive joints there are essentially three steps viz. Surfaces to be bonded should be cleaned by the method that ensures that the bond between the adhesive and metal surface is as strong as the adhesive itself.
Failure, if it occurs, should be in the adhesive rather than at the bond- line between the adhesive and adherend. Metal surfaces may be cleaned by chemical etching or by mechanical abrasion.
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Steels are first shot-blasted to remove rust and mill-scale and then degreased. For obtaining maximum strength on aluminium, surfaces are prepared by vapour degreasing and then dipped in chromic-sulphuric acid bath or anodised in chromic acid followed by careful rinse in clean water, and then air- dried. Alternatively, the metal may be roughened with abrasive to increase the effective bonding area.
Grinding, filing, wire-brushing, sanding and abrasive blasting are some of the mechanical methods used for the purpose. Certain types of plastics such as fluoro carbon isomer and polyethylene are difficult to bond and may require chemical treatment. Glass can be easily cleaned with a 30 percent hydrogen peroxide solution. The prepared surfaces are usually tested by their affinity lo be wetted by water. It is called the water-break test.
Smooth spread of water is an indication that the surface is chemically clean while the collection of droplets indicates the possibility of oil film on the surface. To avoid the possibility of contamination of the prepared surface during storage, it is desirable to use it within a few hours. If storage is unavoidable the metal should be kept tightly wrapped or in airtight containers to minimise contamination.
The etched surface must never be touched with bare hands. The operator should wear clean cotton gloves to handle the prepared surfaces as even a thumb print on an otherwise clean surface will impair adhesion. Adhesives may be applied to the prepared surfaces by hand brushing, spraying, roller coating, knife-coating and dipping.
They are also applied as sheet or powder, generally on a precoated surface. The adhesive can either be applied in one thick layer on one of the parts, or in one thin layer on each of the surfaces before assembly. The latter method is preferred as it leads to a stronger bond with a longer tack life. Adhesive bonds with optimal joint strength are achieved when to microns of solvent-free adhesive remains after two smooth, flat, parallel surfaces are bonded together. Lay-down thickness depends upon the porosity and smoothness of the surfaces to be bonded, the fit-up of the joint and the strength required.
If the surface is porous allowance must be made in the lay-down solvent lo be absorbed by the surface, to achieve the desired glue-line thickness. Similarly, allowance must be made while coating rough surfaces so as to fill up all small depressions and attain the desired glue-line thickness; this is normally done in a single coat. Apart from the above described general bonding procedure there are certain well established procedures for achieving optimum joint strength for specific applications.
One such technique is called Redux Bonding in which the metal is first given a coat of phenol formaldehyde in a suitable solvent and then polyvinyl formaldehyde powder is scattered over the precoated surfaces before they are brought together and cured. Though polyvinyl resin is the main adhesive but precoating with phenol formaldehyde is essential to bond it to the metal.
Redux Bonding is widely used, since long, for making adhesive joints for aircraft manufacture. The aim should be to assemble the parts when the applied adhesive is at its optimum consistency. Solvent evaporation rate may be increased by moderate heating using infra-red lamps or a hot air oven. Provision should be made for positioning the components for mating during curing and assembly fixtures are normally used for the purpose.
Care should be taken to align the parts accurately before they are mated because a strong bond is created instantly when the coated surfaces are brought together. The assembly fixtures used for positioning should be light-weight for ease of handling. A heavy fixture is not only difficult to handle it may also act as a heat sink which can retard the heating and cooling rates during curing.
The expansion rate of the fixture material should be as nearly as possible match with that of the expansion rate of assembly to minimise distortion of components and subsequent stressing of the adhesive. Sometimes adhesive bonding is combined with resistance welding or mechanical fastening to improve the load carrying capacity of the joint.
With certain adhesives it is essential to apply and maintain adequate pressure during curing. The pressure should always be uniformly distributed over the entire joint. Generally, it is desirable to use as high a clamping pressure as the adherends can withstand without being crushed. Normally a moderate pressure of to 10 MPa applied in a suitable press serves the purpose well.
After the application of pressure, the surplus adhesive is heated through the cooling cycle preferably in an oven though electric heating pads may be lised for large components. Heat transmitted to the adhesive being dependent upon the thermal conductivity of the adherend, the curing temperature is measured at the glue-line. Curing limes may be reduced at the expense of bond strength if an accelerator is added to the adhesive.
Pressure is provided by compressed air while heating is done by steam heated tubes or electrical elements. For judging the joint quality in adhesive bonding the most commonly used destructive test is the lap shear test in which a 25 mm wide lap joint with an overlap of Such a test is generally satisfactory for control of mixing, priming and bonding.
Peel test is recommended to ascertain the adequacy of cleaning procedures; alternatively the recently developed crack extension or wedge test may be used. The test specimen and the method adopted for wedging action are shown in Fig. The required number of specimens are cut from the adhesive bonded panel. The wedge is forced between the adherend at the glue- line.
This separates the adhesive and produces cleavage loading at the tip opening. The location of the apex of the sheet separation is recorded. The distance that the apex moves during exposure is measured within two hours after exposure. The wedge test is used for surface preparation, process control and procedures by comparing the test results with a maximum acceptable increase in adhesive crack length.
Though the test was originally designed for adhesive bonded aluminium, it may be used for other metals with design modifications to account for differences in stiffness and yield strength. However, the bonding of metal to non-metals especially plastics is gaining utmost importance and is the major application of adhesive bonding. Industries involved with aircraft and automobile construction are the major users of adhesive bonding of metals. Typical applications include fastening of stiffeners to the aircraft skin and in assembling honeycomb structures wherein honeycomb core is bonded between two sheet metal skins.
Many of the joints made in the fabrication of aircraft wing and tail assemblies are by adhesive bonding; increased use is also evident in the fabrication of aircraft internal structures as well as for providing the required smooth surfaces for supersonic planes, making complex designs possible. Adhesive bonded assemblies may comprise over 50 percent of the total area of a modern airplane. They include about major assemblies including sections measuring 75 mm by mm, tapered spar caps over 10 m long and panels measuring upto m by m.
Bonded stiffeners are used on single curvature panels forming the fuselage skin. Cost of fabrication in many of these cases is reduced by 33 to 75 percent.https://rankcutthhelri.tk
Adhesive Joints: Formation, Characteristics and Testintg - Google книги
Double shell panels are bonded with a high strength vinyl plastisol adhesive. Language English. Other Creators Mittal, K. Physical Description viii, p. Subjects Adhesive joints -- Congresses.
Notes "This volume chronicles the proceedings of the Second International Symposium View online Borrow Buy Freely available Show 0 more links Set up My libraries How do I set up "My libraries"? University of Sydney Library. Open to the public ; None of your libraries hold this item. Tags What are tags? Add a tag. Public Private login e.