9.Welding Bonding Permanent Joints

8/10/2019 9.Welding Bonding Permanent Joints

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MECopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 9Welding,

Bonding, andthe Design of

Permanent

Joints


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ME Overview

91 Welding Symbols

92 Butt and Fillet Welds

93 Stresses in Welded Joints in Torsion

94 Stresses in Welded Joints in Bending

95 The Strength of Welded Joints

9

6 Static Loading

97 Fatigue Loading

98 Resistance Welding

99 Adhesive Bonding


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ME Welding Symboles

- A weldment is fabricated by welding together a collection of metal shapes, cut to

particular configurations.

- During welding, the several parts are held securely together, often by clamping

or jigging.

- The welds must

be precisely

specified on

working drawings,and this is done by

using the welding

symbol as

standardized by the

American Welding

Society (AWS).


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ME Welding Symboles

- The arrow of this symbol points to the joint to be welded.

- The body of the symbol contains as many of the following elements as are

deemed necessary:

Reference line

Arrow

Basic weld symbols as in Fig. 9-2

Dimensions and other data

Supplementary symbols

Finish symbols

Tail

Specification or process


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ME Welding Symboles

- Figures 93 to 96 illustrate the types of welds used most frequently by

designers.

-For general machine elements most welds are fillet welds, though butt welds

are used a great deal in designing pressure vessels.

- The parts to be joined must be arranged so that there is sufficient clearance for

the welding operation.

- Since heat is used in the welding operation, there are metallurgical changes inthe parent metal in the vicinity of the weld. Also, residual stressesmay be

introduced because of clamping or holding or, sometimes, because of the order

of welding.

- Usually these residual stresses are not severe enough to cause concern; insome cases a light heat treatment after welding has been found helpful in

relieving them. When the parts to be welded are thick, a preheating will also be

of benefit.


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ME Welding Symboles

F il let welds. (a) The number indicates the leg size; the arrow should point only to one

weld when both sides are the same. (b) The symbol indicates that the welds are

intermittent and staggered 60 mm along on 200-mm centers.

The cir cle on the weld symbol indicates

that the welding is to go all around.


7/87ME Welding Symboles

Butt or groove welds:

(a) square butt-welded on both

sides

(d) Single bevel(c) double V

(b) single V with 60obevel and root

opening of 2 mm


8/87ME Welding Symboles

Special groove welds:

(c) corner weld (may also have a bead

weld on inside for greater strength but

should not be used for heavy loads);

(b) U and J welds for thick plates;(a) T joint for thick plates;

(d) edge weld for sheet metal

and light loads.


9/87ME Overview

91 Welding Symbols





9

6 Static Loading97 Fatigue Loading


99 Adhesive Bonding


10/87ME Butt and Fillet Welds

Figure 97a shows asingle V-groove weld

loaded by the tensile or compression force F,

the average normal stress is

where h is the weld throat and l is the length of the weld.

- Note that the value of h does not include the reinforcement. The reinforcement

can be desirable, but it varies somewhat and does produce stress concentration at

pointA.

- If fatigue loads exist, it is good practice to grind or machine off the reinforcement.

F

hl



The average stress in a butt weld due to shear

loading (Fig. 97b) is

- For a typical transverse fillet weld asshown, from a free body at angle the

forces on each weldment consist of a normal

forceFnand a shear forceFs.

- Summing forces in thex andy directionsgives

F

hl

sin

cos

s

n

F F

F F



- Using the law of sines for the triangle in the

figure yields

2

sin 45 sin 135 cos sin

t h h

- Solving for the throat length t gives

cos sin

ht

- The nominal stresses at the angle in the weldment, and , are

2sin cos sin

sin cos sins s

FF F F

A tl hl hl

2

cos cos sincos sin cos

n n FF F F

A tl hl hl



- The von Mises stress at angle is

1 2

1 2 2 22 2 2 2

3 cos sin cos sin sin cosF

hl

- The largest von Mises stress (differentiating the equation(f))occurs at = 62.5

with a value of

- The corresponding values of and are

= 1.196F/(hl) and

= 0.623F/(hl)

2.16F

hl



- The maximum shear stress can be found by differentiating the equation (d)

with respect to and equating to zero.

- The stationary point occurs at = 67.5with a corresponding

max= 1.207F/(hl) and

= 0.5F/(hl)


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Stress distri bution in f il let

welds:

(a) stress distri bution on

the legs as reported by

Norris;

(b) distri bution of

principal stresses and

maximum shear stress as

reported by Salakian.


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ME Butt and Fillet Welds

- The most important concept here is that we have no analytical approach that

predicts the existing stresses.

- The approach has been to use a simple and conservative model, verified by

testing as conservative. The approach has been to

Consider the external loading to be carried by shear forces on the

throat areaof the weld. By ignoring the normal stress on the throat, the shearing

stresses are inflated sufficiently to render the model conservative.

Use distortion energy for significant stresses.

Circumscribe typical cases by code.

- The geometry of the fillet is crude by machinery standards, and even if it wereideal, the macrogeometry is too abrupt and complex for the methods.


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- For the model, the basis for weld analysis or design employs

1.414

0.707

F F

hl hl

- Note that this inflates the maximum estimated shear stress by a factor of

1.414/1.207 = 1.17.

- Further, consider the parallel fillet welds shown in this Figure, where, as in

Fig. 98, each weld transmits a forceF. However, in the case of this Figure,the maximum shear stress is at the minimum throat areaand corresponds to

Eq. (93).

which assumes the entire forceF is

accounted for by a shear stress in the

minimum throat area.


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Under circumstances of combined loadingwe

Examine primary shear stresses due to external forces.

Examine secondary shear stresses due to torsional and bending moments.

Estimate the strength(s) of the parent metal(s).

Estimate the strength of deposited weld metal.

Estimate permissible load(s) for parent metal(s).

Estimate permissible load for deposited weld metal.


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ME Overview

91 Welding Symbols





9



99 Adhesive Bonding


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ME Stresses in Welded Joints in Torsion

- The figure illustrates a cantilever of

length l welded to a column by two

fillet welds.

- The reaction at the support of a

cantilever always consists of a shear

force V and a momentM.

- The shear force produces aprimary

shear in the welds of magnitude V

A

whereA is the throat area of all the welds.

- The moment at the support producessecondary shear or

torsion of the welds, and this stress is given by the equation

Mr

J

where ris the distance from the centroid of the weld group to the point in the weld

of interest and Jis the second polar moment of area of the weld group about the

centroid of the group.

r


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- When the sizes of the welds are

known, these equations can be solved

and the results combined to obtain

the maximum shear stress.

- Note that r is usually the farthest

distance from the centroid of the

weld group.

- The figure shows two welds in a

group. The rectangles represent the

throat areas of the welds.

- Weld 1 has a throat thickness t1= 0.707h1, and weld 2 has a throat thickness t2=

0.707h2. Note that h1and h2are the respective weld sizes. The throat area of both

welds together is

- This is the area that is to be used in equation of the primary shear.

btdtAAA2121


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- Thex axis in the figure passes through

the centroid G1of weld 1. The second

moment of area about this axis is

- In a similar manner, the second polar moment of area of weld 2 about its centroid is

- Similarly, the second moment of area

about an axis through G1parallel to they

axis is

- Thus the second polar moment of area of

weld 1 about its own centroid is

12

3

1dt

Ix

12

3

1dtIy

1212

3

1

3

1

1

dtdtIIJ yxG

1212

3

2

3

2

2

btbtJ

G


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- The centroid G of the weld group is

located at

1 1 2 2A x A x

xA

- From the figure, we see that the

distances r1and r2from G1and G2to G,

respectively, are

1 2

2 2

1 1r x x y

1 1 2 2A y A y

yA

1 2

2 2

2 2 2r y y x x

- Using the parallel-axis theorem, we find the second polar moment of area of the

weld group to be

2 2

1 1 1 2 2 2G GJ J A r J A r

- This is the quantity to be used to calculate secondary shear. The distance r must

be measured from G and the momentM computed about G.


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- The reverse procedure is that in which

the allowable shear stress is given and

we wish to find the weld size. The usual

procedure is to estimate a probable weld

size.

- The quantities t13and t2

3which are the

cubes of the weld thicknesses, are small

and can be neglected.

- Setting the weld thicknesses t1and t2to

unity leads to the idea of treating each

fillet weld as a line.

- The resulting second moment of area is then a unit second polar moment of area Ju.

- The advantage of treating the weld size as a line is that the value ofJuis the same

regardless of the weld size.


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- Since the throat width of a fillet weld is 0.707h, the relationship betweenJ and

the unit value is

in whichJuis found by conventional methods for an area having unit width.

- Table 91 lists the throat areas and the unit second polar moments of area for the

most common fillet welds encountered. The example that follows is typical of the

calculations normally made.

0.707u

J hJ


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= 10.4 mm

= 95 mm

b = 56 mm

d = 190 mm


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AA r

CC r

DD r

BB r


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= (2+ 2)1/2


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ME Overview

91 Welding Symbols





9



99 Adhesive Bonding


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ME Stresses in Welded Joints in Bending

- A rectangular cross-section

cantilever welded to a support at

the top and bottom edges.

- The shear force produces aprimary shear in the welds of

magnitude

V

A

whereA is the total throat area.

- The momentM induces a throat shear stress component of 0.707in the welds.

- Treating the two welds of Fig. 917b as lines we find the unit second moment ofarea to be2

2u

bdI

Th d f I


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- The second moment of areaI,

based on weld throat area, is

- The nominal throat shear stress is now found to be

2

0.707 0.7072

u

bdI hI h

2

2 1.414

0.707 2

Mc Md M

I hbd bdh

- The model gives the coefficient of 1.414, in contrast to the predictions of Sec.

92 of 1.197 from distortion energy, or 1.207 from maximum shear.

- The second moment of area is based on the distance dbetween the two welds. If

this moment is found by treating the two welds as having rectangular footprints,the distance between the weld throat centroids is approximately (d + h). This

would produce a slightly larger second moment of area, and result in a smaller

level of stress.

Thi th d f t ti ld li d t i t f ith th ti


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- This method of treating welds as a line does not interfere with the conservatism

of the model. It also makes Table 92 possible with all the conveniences that

ensue.

Bending

Properties

of Fi ll et

Welds


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Bending Properties of F i l let Welds (conti nued)


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Bending Properties of F i l let Welds (conti nued)


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Th t hi f th l t d ti ith th f th t t l i ll


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ME The Strength of Welded Joints

- The matching of the electrode properties with those of the parent metal is usually

not so important asspeed, operator appeal, and the appearance of the completed

joint. The properties of electrodes vary considerably, but this Table lists the

minimum properties for some electrode classes.

- It is preferable, in designing welded components, to select a steel that will result in

a fast, economical weld even though this may require a sacrifice of other qualities

such as machinability.

Min imum Weld-Metal Properties


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Thi T bl li h f l ifi d b h d f l l i h


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- This Table lists the formulas specified by the code for calculating these

permissible stresses for various loading conditions.

- The factors of safety implied by this code are easily calculated.

- For tension, n = 1/0.60 = 1.67. For shear, n = 0.577/0.40 = 1.44, using the

distortion-energy theory as the criterion of failure.

*The factor of safety nhas been computed by using the distortion-energy theory.Shear stress on base metalshould not exceed 0.40Syof base metal.

Stresses permi tted by the AI SC code for weld metal


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- It is important to observe that the electrode material is often the strongest material

present.

- If a bar of AISI 1010 steel is welded to one of 1018 steel, the weld metal is

actually a mixture of the electrode material and the 1010 and 1018 steels.

- The weld metal is usually the strongest, do check the stresses in the parent metals.

- The fatigue stress-concentration factors listed in Table 95 are suggested for use.

These factors should be used for the parent metal as well as for the weld metal.

Fatigue Stress-Concentration Factors, Kfs

Al lowable steady-load information and minimum f il let sizes


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Al lowable steady load information and minimum f i l let sizes.


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ME Overview

91 Welding Symbols





96 Static Loading

97 Fatigue Loading


99 Adhesive Bonding


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ME Static Loading


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ME Static Loading

Eq. 9-1

Table 9-4


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ME Static Loading


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ME Static Loading

Table 9-4

Table A-20


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ME Static Loading

75*4


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ME Static Loading


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ME Static Loading

Eq. 8-56

o

Table 9-3

Weld metal: E70, Sut = 70 kpsi

Weld metal


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ME Static Loading

Eq. 9-3: assumed that F is

accoun ted by a shear stress

in the min imum throat area

Weld metal

Base metal


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ME Static Loading


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ME Static Loading

Table 9-4


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ME Static Loading


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ME Static Loading

Stresses in welded joints

in bending

r = d/2


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ME Static Loading

Eq: 5-21


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ME Static Loading

AI SI 1018 HR steel


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ME Static Loading


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ME Overview

91 Welding Symbols





96 Static Loading

97 Fatigue Loading


99 Adhesive Bonding


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ME Fatigue Loading

- In fatigue, the Gerber criterionis best; however, the Goodman criterionis in

common use.

- Recall, that the fatigue stress concentration factors are given in Table 95. For

welding codes, see the fatigue stress allowable in the AISC manual.


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ME Fatigue Loading


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ME Fatigue Loading

ka= aSutb

288


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ME Fatigue Loading

completely reversed


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ME Fatigue Loading


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ME Fatigue Loading

repeatedly appli ed

Type 1 Table 9-1


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ME Fatigue Loading


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ME Overview

91 Welding Symbols





96 Static Loading

97 Fatigue Loading


99 Adhesive Bonding


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ME Resistance Welding

- The heating and consequent welding that

occur when an electric current is passed

through several parts that are pressed

together is called resistance welding.

- Spot weldingand seam weldingare forms of resistance welding most often used.

- The advantages of resistance welding over other forms are the speed, the accurate

regulation of time and heat, the uniformity of the weld, and the mechanical

properties that result. In addition the process is easy to automate, and filler metal

and fluxes are not needed.

- Seam welding is actually a series of overlapping spot welds, since the current is

applied in pulses as the work moves between the rotating electrodes.

(a) Spot welding; (b) seam welding.


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75/87

ME Overview

91 Welding Symbols


9




96 Static Loading

97 Fatigue Loading


99 Adhesive Bonding

- The use of polymeric adhesives to join components for structural, semi-structural,

d t t l li ti h d d tl i t lt f


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ME Adhesive Bonding

Diagram of an

automobi le body

showing at least 15

locations at which

adhesives andsealants could be used

or are being used.

and nonstructural applications has expanded greatly in recent years as a result of

the unique advantages adhesives may offer for certain assembly processes and the

development of new adhesives with improved robustness and environmental

acceptability.

- The increasing complexity of modern assembled structures and the diverse types

of materials used have led to many joining applications that would not be possible

with more conventional joining techniques.

- Adhesives are also being used either in conjunction with or to replace


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ME Adhesive Bonding

d es ves a e a so be g used e e co ju c o w o o ep ace

mechanical fasteners and welds. Figure 924 illustrates the numerous places

where adhesives are used on a modern automobile. Indeed, the fabrication of

many modern vehicles, devices, and structures is dependent on adhesives.

- In well-designed joints and with proper processing procedures, use of adhesives

can result in significant reductions in weight.

- Eliminating mechanical fasteners eliminates the weight of the fasteners, and

also may permit the use of thinner-gauge materials because stress concentrationsassociated with the holes are eliminated.

- The capability of polymeric adhesives to dissipate energy can significantly


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ME Adhesive Bonding

- The capability of polymeric adhesives to dissipate energy can significantly

reduce noise, vibration, and harshness (NVH), crucial in modern automobile

performance.

- Adhesives can be used to assemble heat-sensitive materials or components that

might be damaged by drilling holes for mechanical fasteners.

- They can be used to join dissimilar materials or thin-gauge stock that cannot be

joined through other means

Types of Adhesive


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ME Adhesive Bonding

- There are numerous adhesive types for various applications. They may be

classified in a variety of ways depending on their chemistry (e.g., epoxies,

polyurethanes, polyimides), their form (e.g., paste, liquid, film, pellets, tape), theirtype (e.g., hot melt, reactive hot melt, thermosetting, pressure sensitive, contact), or

their load-carrying capability (structural, semi-structural, or nonstructural).

- Structural adhesivesare relatively strong adhesives that are normally used well

below their glass transition temperature; common examples include epoxies andcertain acrylics. Such adhesives can carry significant stresses, and they lend

themselves to structural applications.

- Contact adhesives, where a solution or emulsion containing an elastomeric

adhesive is coated onto both adherends, the solvent is allowed to evaporate, and

then the two adherends are brought into contact. Examples include rubber cement

and adhesives used to bond laminates to countertops.

Types of Adhesive


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ME Adhesive Bonding

-Pressure-sensitive adhesivesare very low modulus elastomers that deform easily

under small pressures, permitting them to wet surfaces. When the substrate and

adhesive are brought into intimate contact, van der Waals forces are sufficient tomaintain the contact and provide relatively durable bonds. Pressure-sensitive

adhesives are normally purchased as tapes or labels for nonstructural applications,

although there are also double-sided foam tapes that can be used in semi-structural

applications.

- As the name implies, hot meltsbecome liquid when heated, wetting the surfaces

and then cooling into a solid polymer (the glue guns in popular use).

- Anaerobic adhesivescure within narrow spaces deprived of oxygen; such

materials have been widely used in mechanical engineering applications to lock

bolts or bearings in place.

yp

This Table presents important strength properties of commonly used adhesives.


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ME Adhesive Bonding

Stress Distr ibutions


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ME Adhesive Bonding

- Good design practice normally requires that adhesive joints be constructed in

such a manner that the adhesive carries the load in shear rather than tension.

Bonds are typically much stronger when loaded in shear rather than in tensionacross the bond plate.

- Lap-shear joints represent an important family of joints, both for test specimens

to evaluate adhesive properties and for actual incorporation into practical designs.

- Generic types of lap joints that commonly arise are illustrated in Fig. 925.

- The simplest analysis of lap joints suggests the applied load is uniformly

distributed over the bond area.


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ME Adhesive Bonding

Common types of lap joints used in

mechanical design:

(a) single lap

(b) double lap

(c) Scarf

(d) Bevel

(e) Step

(f ) butt strap

(g) double butt strap

(h) tubular lap.

Joint Design


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ME Adhesive Bonding

Design toplace bondline in shear, not peel. Beware of peel stresses focused at

bond terminations. When necessary, reduce peel stresses through tapering the

adherend ends increasing bond area where peel stresses occur, or utilizing rivets atbond terminations where peel stresses can initiate failures.

Where possible, use adhesives with adequate ductility. The ability of an adhesive

to yield reduces the stress concentrations associated with the ends of joints and

increases the toughness to resist debond propagation.

Recognize environmental limitationsof adhesives and surface preparation

methods. Exposure to water, solvents, and other diluents can significantly degrade

adhesive performance in some situation, through displacing the adhesive from the

surface or degrading the polymer. Certain adhesives may be susceptible to

environmental stress cracking in the presence of certain solvents. Exposure to

ultraviolet light can also degrade adhesives.

g

Joint Design


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ME Adhesive Bonding

Design in a way thatpermits or facilitates inspections of bonds where possible. A

missing rivet or bolt is often easy to detect, but debonds or unsatisfactory adhesive

bonds are not readily apparent.

Allow for sufficient bond area so that the joint can tolerate some debonding

before going critical. This increases the likelihood that debonds can be detected.

Having some regions of the overall bond at relatively low stress levels can

significantly improve durability and reliability.

Where possible, bond to multiple surfaces to offer support to loads in any

direction. Bonding an attachment to a single surface can place peel stresses on the

bond, whereas bonding to several adjacent planes tends to permit arbitrary loads to

be carried predominantly in shear.

Adhesives can be used in conjunction with spot welding. The process is known as

weld bonding. The spot welds serve to fixture the bond until it is cured.

This Figure presents examples of improvements in adhesive bonding.


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ME Adhesive Bonding

Design

practices thatimprove

adhesive

bonding.

(a) Gray load

vectors are to

be avoided as

resulting

strength is

poor.


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Design practices that

improve adhesive bonding.

(b) M eans to reduce peel

stresses in lap-type joints.

9.Welding Bonding Permanent Joints

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