LECTURE # 39LECTURE # 39

FATIGUE LOADING OF TENSION FATIGUE LOADING OF TENSION JOINTSJOINTS

Table 8-16 lists average fatigue Table 8-16 lists average fatigue stress-concentration factors for the stress-concentration factors for the fillet under the bolt head and also at fillet under the bolt head and also at the beginning of the threads on the the beginning of the threads on the bolt shankbolt shank

These are already corrected for notch These are already corrected for notch sensitivity and for surface finishsensitivity and for surface finish

Use of rolled threads is the Use of rolled threads is the predominant method of thread-forming predominant method of thread-forming in screw fasteners, where Table 8-16 in screw fasteners, where Table 8-16 appliesapplies

In thread-rolling the amount of cold-In thread-rolling the amount of cold-work and strain strengthening is work and strain strengthening is unknown to the designer; therefore, unknown to the designer; therefore, fully corrected (including Kfully corrected (including Kff) axial ) axial endurance strength is reported in Table endurance strength is reported in Table 8-178-17

Fatigue loading is the one in which Fatigue loading is the one in which the externally applied load fluctuates the externally applied load fluctuates between zero and some maximum between zero and some maximum force Pforce P

FFmaxmax = F = Fb b and Fand Fminmin = F = Fii

FFaa = (F = (Fmax max – F– Fminmin)/2 = (F)/2 = (Fbb – F – Fii)/2)/2

aa = F = Faa/A/Att

Fig. 8.20Fig. 8.20

tt

ii

t

iba A

CP

A

FFCP

A

FF

222

t

i

tm

A

F

A

CP

2

iam

Strength Components SStrength Components Saa and and SSmm of the Fatigue Failure Locus of the Fatigue Failure Locus

Goodman :Goodman :

GerberGerber

1ut

m

e

a

S

S

S

S

12

ut

m

e

a

S

S

S

S

ASME-ellipticASME-elliptic

1

22

p

m

e

a

S

S

S

S

Goodman :Goodman :

eut

iutea

SS

SSS

iam SS

GerberGerber

eiutieeutut

e

a SSSSSSS

S 242

1 22

iam SS

ASME-ellipticASME-elliptic

eiiepp

ep

ea SSSS

SS

SS 222

22

iam SS

Be sure to use KBe sure to use Kff for both for both aa and and mm, , otherwise slope of the load line will otherwise slope of the load line will not remain 1 to 1not remain 1 to 1

The factor of safety guarding against The factor of safety guarding against fatigue is fatigue is

a

af

Sn

Applying this to Goodman criterion, Applying this to Goodman criterion, with with

ii = F = Fii/A/Att

When preload FWhen preload Fii is present is present

eut

itutef SSCP

FASSn

2

With no preload, C = 1, FWith no preload, C = 1, Fii = 0 = 0

Preload is beneficial for resisting Preload is beneficial for resisting fatigue when nfatigue when nf f / n/ nf0 f0 > 1> 1

eut

tutef SSP

ASSn

2

0

For Goodman, last two equations For Goodman, last two equations with nwith nf f / n/ nf0 f0 > 1 puts an upper bound > 1 puts an upper bound on the preload Fon the preload Fii

FFii << (1-C) S (1-C) Sut ut AAtt

If this can not be achieved, and nf isIf this can not be achieved, and nf is

unsatisfactory, use Gerber or ASME-ellipticunsatisfactory, use Gerber or ASME-elliptic

Criterion or additional bolts or different Criterion or additional bolts or different size boltsize bolt

may be usedmay be used

The higher the preload, the betterThe higher the preload, the better

Preloads of 60 percent of proof load Preloads of 60 percent of proof load rarely loosenrarely loosen

Check the possibility of yielding, Check the possibility of yielding, using the proof strengthusing the proof strength

am

pp

Sn

Fig. 8.21Fig. 8.21

Fig. 8.22Fig. 8.22