module 4 lecture 5 final

7/30/2019 Module 4 Lecture 5 Final

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Module

4

Design for Assembly


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Lecture

5

Design for Adhesive bonding


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Instructional objectives

This lecture is intended to outline (a) the principle of adhesive bonding. (b) applications of

adhesive bonding, and (c) recommended joint designs for adhesive bonding to achieve good

quality joints economically.

What is Adhesive Bonding?

Adhesive bonding is a process of joining materials in which an adhesive (liquid or a semi

solid state material) is placed between the faying surfaces of the workpiece / parts (adherents)

to be joined. Either heat or pressure or both are applied to get bonding. The detailed

explanation on the steps followed to get an adhesive joint is given below. Three essential

steps are required to follow to make an adhesive joint including (a) preparation of the

surfaces, (b) application of the adhesive on to the mating surfaces, and (c) assembly of

workpieces / parts and curing the joint.

Preparing the surface

The workpiece surfaces are cleaned by chemical etching or mechanical abrasion. Grinding,

filing, wire brushing, sanding and abrasive blasting are some of the mechanical cleaning

methods. Next the prepared surfaces are tested by their affinity to be wetted by water. It is

called 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 in the surface.

Application of the adhesive to the surface

Adhesives are applied on to the workpiece surface by hand brushing, spraying, roller casting,

knife coating and dipping. They are also applied as sheet or tape type coating to the surface.

The adhesive are either applied as one thick layer on one of the workpiece surface, or as a

thin layer on surfaces of both the workpiece.

Assembly of workpieces

After the application of the adhesive, the workpieces are assembled and held together by

means of clamps, tools, tack welds, or other fixtures. During the assembly process, sufficient

care should be taken so that the open time of the adhesive is not exceeded, the parts are put

together in the proper sequence, the bonding is performed under specified environmental

conditions, and the parts are held together until cured.


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Curing the joint [1, 2]

Curing is the process during which an adhesive changes from a liquid state to solid. The

curing of the adhesive is performed with pressure and either heat or addition of suitable

catalyst. The pressure should always be uniformly distributed over the entire joint. The curingtemperature, time and the pressure combinations for different adherents and adhesive

combinations are given elsewhere.

Classification of Adhesives

Adhesives are broadly classified into two groups structural adhesives and non-structural

adhesives. Structural adhesives are having high load carrying capacity while the non-

structural adhesives are having low load carrying capacity. As the structural adhesives are

most famous among the two, the same is discussed in the present section.

Structural adhesives

Structural adhesives are generally classified into two groups thermoplastic and

thermosetting. The thermoplastic adhesives get soften at high temperatures. The most

commonly used thermoplastic adhesives include polyamides, vinyls and non-vulcanizing

neoprene rubber. The vinyls are especially very popular for structural applications. For

example, polyvinyl acetate is used to form strong bonds with metals, glass and porous

materials. The thermosetting adhesives usually do not get soften at high temperature. Once

they harden, these adhesives cannot be remelted and a broken joint cannot be rebounded by

heating. Thermosetting adhesives are mainly available in two variants phenolic resin and

the epoxy resin. Phenolic resins are the best bonding materials for waterproof plywood.

Epoxy resins produce the joint with high strength, toughness, chemical inertness, and low

shrinkage. These adhesives can be cured at room temperature. Other thermosetting adhesives

include melamin-formaldehyde, polyurethanes, polysters, phenolic rubber, and neoprene

rubbers.

Adherents and adhesives suitable for adhesive bonding

Almost all solid materials can be joined with adhesive bonding. Teflon, polyethylene and

polypropylene are difficult to join due to their sticking inability. Detailed list of the adherents

and their suitable adhesives are easily available in open references elsewhere [1-3]. Table

4.5.1 explains the advantages and the disadvantages of typical adhesive bonded joints.


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Table 4.5.1 Advantages and disadvantages of the adhesive bonded joints

Advantages Disadvantages

Fast and cheap joining technique

The adherents are not affected by heat Uniform stress distribution Possibility to join large structures Ability to join different materials Possibility to join very thin adherents Gas proof and liquid tight joints No crevice corrosion No contact corrosion Good damping properties High dynamic strength

Limited stability to heat

High strength adhesives are often brittle Long term use may alter the properties Cleaning and surface preparation of the

adherents is necessary in many cases

Specific clamping devices are oftenrequired to fix the joint

Difficult disassembly of joined parts

Applications of Adhesive Bonded Joints

1. Bonding of metal to non-metals especially plastics is the major application of adhesivebonding.

2. Used as an alternative to riveting for aircraft structures.3. Widely applicable in fastening of stiffeners to the aircraft skin and in assembling

honeycomb structures in aircraft [Figure 4.5.1(a)].

4. Using extensively in the fabrication of aircraft internal structures and providing thesmooth surface for supersonic planes.

5. Useful in automobile industry for attaching brake lining to shoes, automatictransmission bands, and stiffeners [Figure 4.5.1(b)].

6. Find applications in the fabrication of railway coaches, boats, refrigerators, storagetanks, and microwave reflectors for radar and space communications.


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Figure 4.5.1 Some typical adhesive bonded structures (a) honey comb Panel structure,

(b) automotive brake lining [2,3].

Design recommendations for adhesive joints

1. As adhesive bonds resist shear, tensile, and compressive forces better than cleavage orpeel, design assemblies which will be subjected to similar load bearing applications

[Figure 4.5.2(a)].

2. The width of the joint overlap is more important than the joint length. Bond strengthis proportional to the joint area only in the case of compressive and tensile forces. In

assemblies loaded under shear forces, the stresses are concentrated at the joints end

[Figure 4.5.2(b)].

3. Difference in the thermal expansion coefficient of the materials leads to shear stresseswhen exposed to thermal loading. Hence to reduce the same the adhesive should have

an expansion coefficient midway between that of the adherents. Fillers are often

added to an adhesive to control its coefficient of expansion.

4. The surface preparation is a major step in adhesive bonding. Both the surfaces of thebonded parts should be cleaned properly to get a sound joint. Vapour degreasing and

solvent wiping techniques may be applied to clean the surfaces. Smooth surfaces are

preferred as these are more easily wet by spreading liquid adhesive.

5. Simple butt joints should be used only when fairly large bonded surfaces are involvedand when cleavage stresses cannot be anticipated [Figure 4.5.3].

(a) (b)


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6. Design modifications to minimize cleavage stresses in lap joints subjected to tensionare shown in Figure 4.5.4.

7. Figure 4.5.5 depicts the recommended corner joints for the workpieces of differentthicknesses. It is always recommended to choose a design which involves least

preparation cost.

8. The techniques for joining rods and tubes adhesively are shown in Figure 4.5.6.

Figure 4.5.2 Design recommendations for adhesive joints [3]

Fig. no. Feasible Better and preferred

a

b

c


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Figure 4.5.3 Few modified butt joint designs to reduce the cleavage stresses [3]

Modified butt joint design Performance

Poor

Better in tension, poorin bending

Poor

Better

Better yet

Good in tension and

bending, costs more

Excellent but

expensive, slower


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Figure 4.5.4 Few modified lap joint designs to reduce the cleavage stresses [3]

Modified lap joint design Performance

LapGood

Beveled lap

Better than simple lap

Joggle lap

Good if members are stiff

ScarfGood if members are thick

Half lifeGood if matching is possible

Strap lap

Good in tension

Better

Better

Double lap

Good of load is balanced


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Figure 4.5.5 Recommended corner joint designs [3]

Corner joint design Performance

Poor if the corner is stressed

Better

Better yet

Good in tension and bending

Good but slower to produce

Good for flat members

Excellent but takes time and

material


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Figure 4.5.6 Recommended designs for joining rods and tubes [3]

Rod and tube joint design Performance

Good for rods in tension, compression,

and tension if area is large; poor inbending

Good for rods

Good for rods

Good for rods or tubes

Poor for tubes

Good for rods but slower to produce

Good for tubes if wall thickness permits

machining

Corners of rods or tubes best handled with

elbow


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Exercise

1. Name few adherents and adhesives used in the fuse lag construction of aircraft.2. What is the recommended design for the honey comb panel fabrication in aircraft?

References

1. G. L. Schneberger, Adhesives in manufacturing, Marcel deckker inc, New york.2. R. Houwink and G. Salomon, Adhesion and adhesives, Elsevier publishing

company, New York.

3. J. B. Bralla, Design for manufacturability handbook, McGraw Hill handbooks, 2ndedition, New York.

module 4 lecture 5 final

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