Joint-assembly methods

A number of methods have been developed for assembling bonded joints. The following basic principles apply to all the methods  

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ISO 1872-1 1993 Plastics - Polyethylene (PE) moulding and extrusion materials - Part 1 Designation system and basis for specifications ISO 1872-2 1997 Plastics - Polyethylene (PE) moulding and extrusion materials - Part 2 Preparation of test specimens and determination of properties ISO 1969 2004 Fibre ropes - Polyethylene - 3- and 4-strand ropes ISO 3458 1976 Assembled joints between fittings and polyethylene (PE) pressure pipes -Test of leakproofness under internal pressure ISO 3459 1976 Polyethylene (PE) pressure pipes - Joints assembled with mechanical fittings - Internal under-pressure test method and requirement ISO 3501 1976 Assembled joints between fittings and polyethylene (PE) pressure pipes -Test of resistance to pull out... 

Appropriate test methods for the control of fabrication procedures and non-destructive testing (NDT) are basic requirements for the formation of structural adhesive joints. Such methods should be based upon relatively easily measurable parameters that have a close identity with the properties of the bonded assembly that need to be controlled. However the quality of bonded joints depends upon many factors, requiring a range of very different procedures. 

While specific methods of joint assembly may differ in the details, they can be classified as one of the four basic types of wet assembly, pressure-sensitive bonding, solvent activation, and heat activation. 

These methods are usable only when the adhesive retains some tack when dry. The only difference between the two techniques is that in pressure-sensitive bonding only one adherend is coated versus both surfaces in contact bonding. In both techniques, joint assembly consists of permitting the adhesive coating to dry completely, then aligning the parts and pressing them together to form the bond. 

Press Fit. In press or interference fits, a shaft of one material is joined with the hub of another material by a dimensional interference between the shaft s outside diameter and the hub s inside diameter. Press-fit joints can be made by simple application of force or by heating or coohng one part relative to the other. This simple, fast assembly method provides joints with high strength and low cost. 

ISO 3459 1976 Polyethylene (PE) pressure pipes - Joints assembled with mechanical fittings - Internal under-pressure test method and requirement. 

The distribution of stresses can be clearly revealed in photo-elastic models of joint assemblies made from a transparent material that becomes doubly refractive when subjected to stress. If a beam of polarised white light is shone through a stressed component, coloured lines appear when the object is viewed through a second polarisation filter as a result of interference effects. These lines indicate areas of equal stress. If stress levels are increased, the sequence of coloured lines is repeated. The photographs below show specimen components made from clear acrylic or polycarbonate joined by different methods. In the side view of a bolted coimecfion under load, the stress peaks around the bolt appear very clearly (Fig. 13). 

Automated methods for preparation, adhesive application, and bond assembly give the best reproducibility and best control of durability however, such automation is frequently not possible. Manufacturers recommendations in data sheets and on containers should always be followed and particular attention paid to storage conditions, mixing procedures, adhesive or sealant application, and joint assembly. 

The foil layer and compliant support is attached to the structure by a spot welded attachment joint. High cycle fatigue (HCF) has been shown to cause failure of this joint. More advanced assembly methods may be required to reduce the likelihood of this failure. 

Design. Detailed adhesively bonded joint design and analysis methods are discussed elsewhere in this volume. General issues associated with bonded assembly design will be considered here. 

Pipe runs are normally made up from lengths of pipe, incorporating standard fittings for joints, bends and tees. Joints are usually welded but small sizes may be screwed. Flanged joints are used where this is a more convenient method of assembly, or if the joint will have to be frequently broken for maintenance. Flanged joints are normally used for the final connection to the process equipment, valves and ancillary equipment. 

Lap-joint flanges, Figure 13.33c are used for piped work. They are economical when used with expensive alloy pipe, such as stainless steel, as the flange can be made from inexpensive carbon steel. Usually a short lapped nozzle is welded to the pipe, but with some schedules of pipe the lap can be formed on the pipe itself, and this will give a cheap method of pipe assembly. 

Reactive adhesive after wetting the surfaces to be assembled, there is polymerization of the adhesive joint. The heat behaviour can be better than with the prece ng methods. 

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