LECTURE # 39

FATIGUE LOADING OF TENSION JOINTS Table 8-16 lists average fatigue stress- concentration factors for the fillet under the bolt head and also at the beginning of the threads on the bolt shank Table 8-16 lists average fatigue stress- concentration factors for the fillet under the bolt head and also at the beginning of the threads on the bolt shank These are already corrected for notch sensitivity and for surface finish These are already corrected for notch sensitivity and for surface finish

Use of rolled threads is the predominant method of thread-forming in screw fasteners, where Table 8-16 applies Use of rolled threads is the predominant method of thread-forming in screw fasteners, where Table 8-16 applies In thread-rolling the amount of cold-work and strain strengthening is unknown to the designer; therefore, fully corrected (including K f ) axial endurance strength is reported in Table 8-17 In thread-rolling the amount of cold-work and strain strengthening is unknown to the designer; therefore, fully corrected (including K f ) axial endurance strength is reported in Table 8-17

Fatigue loading is the one in which the externally applied load fluctuates between zero and some maximum force P Fatigue loading is the one in which the externally applied load fluctuates between zero and some maximum force P F max = F b and F min = F i F max = F b and F min = F i F a = (F max – F min )/2 = (F b – F i )/2 F a = (F max – F min )/2 = (F b – F i )/2  a = F a /A t

Fig. 8.20

Strength Components S a and S m of the Fatigue Failure Locus Goodman : Goodman : Gerber Gerber

ASME-elliptic ASME-elliptic

Goodman : Goodman :

Gerber Gerber

ASME-elliptic ASME-elliptic

Be sure to use K f for both  a and  m, otherwise slope of the load line will not remain 1 to 1 Be sure to use K f for both  a and  m, otherwise slope of the load line will not remain 1 to 1 The factor of safety guarding against fatigue is The factor of safety guarding against fatigue is

Applying this to Goodman criterion, with Applying this to Goodman criterion, with  i = F i /A t When preload F i is present

With no preload, C = 1, F i = 0 With no preload, C = 1, F i = 0 Preload is beneficial for resisting fatigue when n f / n f0 > 1 Preload is beneficial for resisting fatigue when n f / n f0 > 1

For Goodman, last two equations with n f / n f0 > 1 puts an upper bound on the preload F i For Goodman, last two equations with n f / n f0 > 1 puts an upper bound on the preload F i F i < (1-C) S ut A t If this can not be achieved, and nf is unsatisfactory, use Gerber or ASME-elliptic Criterion or additional bolts or different size bolt may be used

The higher the preload, the better The higher the preload, the better Preloads of 60 percent of proof load rarely loosen Preloads of 60 percent of proof load rarely loosen Check the possibility of yielding, using the proof strength Check the possibility of yielding, using the proof strength

Fig. 8.21

Fig. 8.22