Weld Joint Design

• Welding

• The process of joining by heating the edges of the pieces to be joined until they melt and join or fuse together is referred to as Welding.

Definitions

WELD JOINT CONCEPTS

• Designed load transfer must take place through the weld.

• Weld at least as strong as parent metal. Eg. Welder test

• Early concepts involved welding as per standard joint norms on proportion with thickness of members.

• But excess welding involves costs and risk of distortion so calculated amount of weld

TYPES OF WELD

• Design for welding access by types of grooves

• Sizing of weld joints as per specifications which vary with application but generally related to parent member thickness

• One guiding principle is that considering greater reliability of parent material and high cost of filler, keep filling low.

• Design information conveyance

INTRODUCTION TO STRUCTURAL WELDING DESIGN

• In butt welds weld treated as strong as parent metal

• In fillet weld shear strength of throat decides weld strength. Formulae for different structural members given.

• Ref. Trg. Hand out 1 from AWS

WELDING SYMBOLS

. 375 (.50)

.06

90º

90º

.06

.375 .50

. 375 (.50)

.06

90º

90º

.06.06

.375 .50

. 375 (.50)

45º

.06

.06

45º

.375

.50

. 375 (.50)

45º

.06

.06

45º

.375

.50

BEVEL GROOVE

J GROOVE

V GROOVE WITH A BACKING Strip

TPES OF WELDING PROCESSES

• Definition

• Arc welding types

• Manual/SMAW

• SAW

• GTAW

• GMAW

• Plasma Arc Welding

SMAW

• Shielded metal arc welding is the most widely used welding process. Versatile.

• Simple eqpt. Portable, Less expensive than other arc welding processes.

• Versatile: Indoors/ Out doors, Any position Even blind areas as back side of pipes in corners.

• No water pipes or gas tubes so with long cables practically any where.

• Make and break and remake easily possible

SMAW

• Examples in field: Storage tanks, Bridges, ship structures, Oil fields, Factories under construction.

• All indoor applications like fabrication of equipment etc.

• In thick plates one uses better alternatives

• Thin sheets have better alternatives

• Reactive metals difficult Eg. Al, Ti, Mg

SMAW LIMITATIONS

• Being manual makes it versatile but economies of automation not possible.

• For example electrodes have to be changed frequently

• Welder skill becomes important

• Deslagging needed after every pass

• To overcome each limitation a modification has been designed. EXAMPLES

WELD DEFECTS

• Slag inclusions:

• Incomplete de slagging of previous pass

• Wide weaving – Restrict width of weave

• Erratic travel speed – use uniform speed

• Excessive amount of slag ahead of arc in deep grooves – shorten arc increase electrode angle or travel speed

WELD DEFECTS

• Porosity scattered along the entire length

• Impurities as S & P. - Consider base metal change

• Surface contamination – cleaning – H2O

• Moisture in electrodes – Dry Electrodes

• Arc length – use correct arc length

• Excessive travel speed – reduce travel speed to permit escape of gases

WELD DEFECTS

• Undercuts - Excessive currents and excessive weaving speeds, excessive electrode size etc.

• Hot cracking – occur at high temp and are intergranular showing an oxidized surface

• If not repaired hot cracks continue through successive deposited layers.

• Most likely in root pass of deep penetrating welds –Preheat, larger root bead,

WELD DEFECTS

• Cold cracking: may occur several days later as stresses adjust themselves

• Related to excessive restraints on joints or martensite transformation. – stress relieve, preheat,

• Low hydrogen electrodes recommended for both cold and hot cracks.

FRACTURE AND FATIGUE

• Risk of catastrophic failure in pressure vessels and ships related to DBTT due to residual stresses.

• Weld defects have a propensity to propagate, Esp. Un fused fissures, Micro cracks etc.

• Welding specs specify stress reliving & Radiography/ DP/ MP examinations. Eg. LPG tanks

FRACTURE AND FATIGUE

• Welding defects, Micro cracks etc involve risk under fatigue loads.

• High factor of safety; Tendency to weld all over, Control of welding process, Qualified process etc.

• Bottom line is that engineers are always suspicious when welded components are subjected to fatigue. Often converted to forgings

• Remember that 85-90% failures are fatigue failures (Total – Not necessarily welded)

• One solution is peening to produce comp. stress

Day - 2

Welded Joint Strength calculations

Difficult to calculate stresses in Welded joints due to- Variable and unpredictable parameters like homogeneity and weld metal

thermal stresses, physical properties (rate of cooling)

Assumptions

Load is distributed uniformly along the entire length of weld Stress is uniform along the cross section of the weldment.

Welded Joint Strength calculations

1. Strength of Transverse Fillet weld2. Strength of parallel Fillet weld3. Circular Fillet welds subjected to Torsion loads4. Fillet welds subjected to Bending loads5. Strength of Butt Welds6. Welded joints with eccentric loading7. Welded Joint design for combined loading

1. Calculating Weld Size and Strength of Fillet Joint

Weld size need not exceed the thickness of the thinner part joined.

t = asin45o(a)

(b)

hl

F

hl

F 414.1

707.0

FF

F

F

F

F

Fillet weld. The circle on the weld symbol indicates that the welding is to go all around

r

yI

M I = Πr3t

rJ

T J= Πr3t + Πr3t

M or T

X

Z

Torsion, T

rJ

T

61212

333 tltltlJ

l

Fig. 6 A typical butt weld

2.0 Butt and Fillet Welds

A single V groove weld loaded by the tensile force F. For either tensile or compression loading , the average normal stress is

hl

F

Where h is the weld throat and l is the weld length as shown in the figure. The value of h does not include reinforcement . The reinforcement though desirable, it may cause stress concentration at point A in the figure. If fatigue load exists, it is good practice to grind or machine off the reinforcement

The average shear stress in a butt weld due to shear loading is

hl

F

2.1 Butt Weld

3.1 Stresses In Welded Joints Eccentric loading

Fig. illustrates a cantilever of length l welded to a column by two fillet welds. Thereaction at the support of a cantilever always consists of a shear force V and amoment M

The shear force produces a primary shear in the weld of magnitude

A

V

Where A is the throat area of all the welds.

The moment at the support produces secondary shear or torsion of the welds, andthis stress is given by the equation

J

Mr

Where r is the distance from the centroid of the weld group to the point in the weldof interest and J is the second polar moment of area of the weld group about the centroid of the group.

Fig.9

(a)

(b)

Fig. 10

Fig. 10 shows two welds in a group. The rectangles represent the throat areasof the welds.

If h1 and h2 are the weld sizes of welds 1 and 2 respectively. Then the throat widths are b1

=0.707h1 and b2 =0.707h2

The throat area of both the welds together is

A = A1 + A2 =b1d1 + b2d2

The second polar moment of area of weld 1 about its own centroid is

1212

3

11

3

111

bddbIIJ yxG

Similarly, the second polar moment of areaof weld 2 about its own centroid is

1212

3

22

3

222

bddbJG

©

(d)

(e)

The centroid G of the weld group is located at

A

xAxAx 2211

A

yAyAy 2211

The distances r1 and r2 from G1 and G2 to G, respectively, are

2122

1 1yxxr

21

2

2

22 xxyyr

Using parallel axis theorem, the second polar moment of area of weld group to be

2

222

2

111 rAJrAJJ GG

Weld throat areas vary as per requirement. For convenience the followingprocedure is followed

• Each fillet throat is assumed to be of unit thickness. • The second term of Eq. (d) and the first term of Eq. (e) are small compared

to the other term and can be ignored

(f)

(g)

(h)

Then, JG1 and JG2 are the second moment of areas for unit throat. But the throat width is 0.707h. The second polar moment of area J can be written as

J = 0.707hJu

Where Ju is the unit second polar moment of area.

Table (2) lists the throat area and the unit second polar moments of area for themost common fillet welds encountered.

As given in Eq. (h) the transfer formula for Ju must be employed when the welds occur in groups.

2122

cos)")('()"('

Resultant shear stress,

Table 2 Torsional properties of fillet welds

3.2 Stresses In Welded Joints In Bending

Fig. Shows a cantilever welded to a support by fillet welds at top and bottom. Thereaction at the support of a cantilever always consists of a shear force V and amoment M.

The shear force produces a primaryshear in the weld of magnitude

A

V

Where A is the throat area of all the welds.

The second moment of area I based on weld throat area is

2707.0)

2707.0(2

22 bd

hdhbI

The moment at the support produces secondary shear or torsion of the welds, and this stress is given by

bdh

M

dhb

dM

I

Mc 414.1

2707.0

22

The shear magnitudes ’ and ” are acting in perpendicular directions . The shearmagnitude is pythagorean combination

21

22

Table (3) lists the throat area and the unit second moments of area for the most common fillet welds encountered.

Table 3

Table 3 contd.

3. Design for Combined loading

•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 the deposited weld metal•Estimate permissible load(s) for parent metal(s)•Estimate permissible load for the deposited weld metal

Table 4 Minimum Weld-Metal properties

AWS electrodeNumber

Tensile strengthMPa

Yield strengthMPa

Percent elongation

E60xxE70xxE80xxE90xxE100xxE120xx

427482551620689827

345393462531600737

17-25221914-1713-1614

3.3 Strength of Welded Joints

Table 4 gives minimum weld-metal properties as per AWS(American WeldingSociety) code. The matching of the electrode properties with those of parent is usually not so important as speed, operator appeal, and the appearance of completed joint. The properties of electrodes vary considerably.

It is preferable, in designing welded components, to select a steel that will result in a fast, economical weld even though this may require sacrifice of other qualitiessuch as machinability.

Type ofLoading

Type of Weld

Permissible Stress

Factor ofSafety, n+

TensionBearingBendingSimple compressionShear

ButtButtButtButtButt or fillet

0.60Sy

0.90Sy

0.60-0.66Sy

0.60Sy

0.60Sut++

1.671.111.52-1.671.67

+ The factor of safety n has been computed using the distortion-energy theory++ Shear stress on base metal will not exceed 0.40Sy of base metal

Table 5 Stress permitted by the AISC code for weld metal

The stresses permitted by the AISC(American Institute of Steel Construction) forWeld metals for various loading conditions is given in Table 5.

The designer can choose factors of safety or permissible working stresses withmore confidence if he or she is aware of the values of those used by others.

3.4 Design for Fatigue

The fatigue stress concentration factors listed in tables 1 and 6 are suggested for use. These factors should be used for parent metal as well as for the weld metal.The endurance stress limit is modified to account for various aspects like surfacecondition, size, load, environment, and other miscellaneous factors .

Factor of safety method

Se =ka kb kc kd ke eS

Where ka = surface condition factorkb = size modification factorkc = load modification factorkd = temperature modification factorke = miscellaneous-effects modification factor

eS = rotary-beam endurance limit

The modified endurance stress is

The maximum stress obtained from analysis after correcting for stress concentration (if applicable) should be less than Se with appropriate factorof safety.

Type of weld kfs

Reinforced butt weldToe of transverse fillet weldEnd of parallel fillet weldT-butt joint with sharp corners

1.21.52.72.0

Table 6Fatigue stress concentration factors

In fatigue problems, the variable-stress descriptive components are stress range r and K factor, min / max , rather than stress amplitude and steady (midrange) components. Useful relations between shear-stress amplitude a, steady stress m, maximum shear stress max, and minimum shear stress min are

max = a + m min =-( a - m )

Or

max = a + m min =-( a - m )

Broadening the definition of K factor to

ma

ma

ma

ma

V

V

F

F

M

MK

max

min

max

min

max

min

max

min

max

min

AISC code for Fatigue allowable

K

srall

1)( max

K

srall

1)( max

This allowable stress is compared to the extant maximum stress at the criticallocation.

all)( maxmax

all)( maxmax

Or

When this equality is met the design is satisfactory as to weld design fatigue strength.

Stress range sr and sr depends on the required fatigue life range and welded joint configuration. (See AISC code)

Design cases

Maximum Principal Stress in MPa

2.5G(X) & 3G(Y)

Weld 46

BaselineDesign cases

Maximum Principal Stress in MPa

2.5G(X) & 3G(Y)

Weld 14

Design Fix Model

Weld 6