Adhesive bonding is capable of being used in load bearing engineering applications, particularly for joining sheet materials. Modern structural adhesives are strong, but will only work effectively if surface preparation is carried out correctly. It is possible to use simple design rules to predict the strength of some joints, but care must be used, especially where advanced composites are to be joined.
Adhesive bonding is an increasingly attractive alternative method of joining materials for load-bearing applications. Care has to be taken to choose s
uitable surface treatments and adhesives for a given application. It is possible to use simple, quantifiable design rules for predicting joint strength.
One of the major advantages of adhesive bonding is that it enables dissimilar materials to be joined, even when one of these is non-metallic. A major application of bonding is therefore where composite materials are concerned. Composites take many forms. In aerospace applications these materials usually consist of highly aligned layers of carbon or glass fibres, each oriented to accommodate the expected loads. Some aerospace composites are woven or stitched so that the fibres are not perfectly aligned. High-quality chemical plant may be made from satin-weave glass fibre reinforced polyester or epoxy resin, while lower grade composites usually consist of random glass fibres in polyester resin.
Bolts and rivets can sometimes be used with composites, but it is then often necessary to have load-spreading inserts bonded into the structure. Adhesive bonding is attractive since it allows for a more gentle diffusion of the load into the structure, thus reducing the localised stresses encountered in the use of bolts and rivets. In the so-called co-curing technique, the composite is bonded without the use of an adhesive. To do this, the composite is prepared in its uncured (pre-impregnated fibre) form and heat and pressure applied. As the composite cures, the excess matrix material (usually a high performance laminating epoxy) is squeezed out in a liquid form and contacts the adjacent component. When curing is complete, the bond is made with due saving in production costs.
Contact adhesives achieve an instant bond on impacting the two surfaces. This precludes movement of the parts with respect to each other from the moment of contact. Jigs or fixtures are frequently necessary to ensure correct placement of the parts in a single, decisive movement. The strength of the bond does, however, take time to develop and, although component members may be self-supporting after 10-30 minutes, full bond strength may take 24 hours to develop. Anaerobic and cyanoacrylate adhesives are both monomeric materials which polymerise rapidly at room temperature (208C) within the glue-line once the bond is formed. Setting can be very rapid indeed with cyanoacrylates and this implies that rapid and correct location of the adherends is necessary. Anaerobics also set rapidly, but movement of the surfaces is possible after polymerisation has commenced. However, such movement reduces the highest achievable bond strengths and, because of its possibility, jigging is desirable. In the most common use of such adhesives, namely thread-locking, the application is, in effect, its own jig.
Heat-setting adhesives such as epoxides achieve high structural strength by forming a cross-linked, three-dimensional, molecular network by chemical reaction. During this curing reaction, the adhesive becomes, or is already, liquid and able to shear easily. The surfaces being joined must therefore be held until the reaction has proceeded sufficiently for the parts to be self-supporting. Such curing is frequently accomplished between the platens of a heated hydraulic press. Indeed, for some adhesives, hydraulic pressure is an additional requirement since water vapour or other gaseous material is evolved during curing and must be retained in solution. The period of occupancy of the press can sometimes be shortened by removing the components when it is safe to handle them and completing the chemical cross linking in an oven either still contained in a jig or merely supported in the oven atmosphere. This operation is frequently called post-curing.
There is no alternative but to undertake a cost analysis of the alternative designs. In general, designs involving adhesive bonding tend, because of the more uniform stress distribution and the absence of the mass of the mechanical fasteners, to be lighter and more economical of materials. Structural adhesives are expensive but so are nuts and bolts, threads, etc. Surface preparation costs may be greater or less than machining costs in given circumstances.
An adhesively-bonded structure will most probably not be capable of being dismantled and re-assembled, at least without considerable cleaning and repetition of surface preparation. Since this can imply removal of material, parts may no longer be to tolerance or even fit. Sub-assemblies manufactured with adhesives instead of bolting and threading may be economically designed as throw-away parts not ever needing to be dismantled.
Reference:
R.D. Adams and J. Comyn, Joining using adhesives, Assembly Automation, Volume 20. Number 2. 2000. 109-117
Adams, R.D., Comyn, J. and Wake, W.C. (1997), Structural Adhesive Joints in Engineering, 2nd ed., Kluwer, Dordrecht.
Adhesive bonding is an increasingly attractive alternative method of joining materials for load-bearing applications. Care has to be taken to choose s
uitable surface treatments and adhesives for a given application. It is possible to use simple, quantifiable design rules for predicting joint strength.
One of the major advantages of adhesive bonding is that it enables dissimilar materials to be joined, even when one of these is non-metallic. A major application of bonding is therefore where composite materials are concerned. Composites take many forms. In aerospace applications these materials usually consist of highly aligned layers of carbon or glass fibres, each oriented to accommodate the expected loads. Some aerospace composites are woven or stitched so that the fibres are not perfectly aligned. High-quality chemical plant may be made from satin-weave glass fibre reinforced polyester or epoxy resin, while lower grade composites usually consist of random glass fibres in polyester resin.
Bolts and rivets can sometimes be used with composites, but it is then often necessary to have load-spreading inserts bonded into the structure. Adhesive bonding is attractive since it allows for a more gentle diffusion of the load into the structure, thus reducing the localised stresses encountered in the use of bolts and rivets. In the so-called co-curing technique, the composite is bonded without the use of an adhesive. To do this, the composite is prepared in its uncured (pre-impregnated fibre) form and heat and pressure applied. As the composite cures, the excess matrix material (usually a high performance laminating epoxy) is squeezed out in a liquid form and contacts the adjacent component. When curing is complete, the bond is made with due saving in production costs.
Contact adhesives achieve an instant bond on impacting the two surfaces. This precludes movement of the parts with respect to each other from the moment of contact. Jigs or fixtures are frequently necessary to ensure correct placement of the parts in a single, decisive movement. The strength of the bond does, however, take time to develop and, although component members may be self-supporting after 10-30 minutes, full bond strength may take 24 hours to develop. Anaerobic and cyanoacrylate adhesives are both monomeric materials which polymerise rapidly at room temperature (208C) within the glue-line once the bond is formed. Setting can be very rapid indeed with cyanoacrylates and this implies that rapid and correct location of the adherends is necessary. Anaerobics also set rapidly, but movement of the surfaces is possible after polymerisation has commenced. However, such movement reduces the highest achievable bond strengths and, because of its possibility, jigging is desirable. In the most common use of such adhesives, namely thread-locking, the application is, in effect, its own jig.
Heat-setting adhesives such as epoxides achieve high structural strength by forming a cross-linked, three-dimensional, molecular network by chemical reaction. During this curing reaction, the adhesive becomes, or is already, liquid and able to shear easily. The surfaces being joined must therefore be held until the reaction has proceeded sufficiently for the parts to be self-supporting. Such curing is frequently accomplished between the platens of a heated hydraulic press. Indeed, for some adhesives, hydraulic pressure is an additional requirement since water vapour or other gaseous material is evolved during curing and must be retained in solution. The period of occupancy of the press can sometimes be shortened by removing the components when it is safe to handle them and completing the chemical cross linking in an oven either still contained in a jig or merely supported in the oven atmosphere. This operation is frequently called post-curing.
There is no alternative but to undertake a cost analysis of the alternative designs. In general, designs involving adhesive bonding tend, because of the more uniform stress distribution and the absence of the mass of the mechanical fasteners, to be lighter and more economical of materials. Structural adhesives are expensive but so are nuts and bolts, threads, etc. Surface preparation costs may be greater or less than machining costs in given circumstances.
An adhesively-bonded structure will most probably not be capable of being dismantled and re-assembled, at least without considerable cleaning and repetition of surface preparation. Since this can imply removal of material, parts may no longer be to tolerance or even fit. Sub-assemblies manufactured with adhesives instead of bolting and threading may be economically designed as throw-away parts not ever needing to be dismantled.
Reference:
R.D. Adams and J. Comyn, Joining using adhesives, Assembly Automation, Volume 20. Number 2. 2000. 109-117
Adams, R.D., Comyn, J. and Wake, W.C. (1997), Structural Adhesive Joints in Engineering, 2nd ed., Kluwer, Dordrecht.
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