1. Screws, Fasteners, and the Design of Nonpermanent Joints
Chapter 8
Screws, Fasteners, and the Design of Nonpermanent Joints
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

2. Chapter Outline
8-1 Thread Standards and Definitions 8-2 The Mechanics of Power Screws 8-3 Strength Constraints Joints-Fasteners Stiffness
8-5 Joints-Member Stiffness 8-6 Bolt Strength 8-7 Tension Joints-The External Load 8-8 Relating Bolt Torque to Bolt Tension
8-9 Statically Loaded Tension Joint with Preload
8-10 Gasketed Joints
8-11 Fatigue Loading of Tension Joints
8-12 Shear Joints
8-13 Setscrews
8-14 Keys and Pins
8-15 Stochastic Considerations
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

3. LECTURE 38 8-11 Fatigue Loading of Tension Joints 8-12 Shear Joints
8-13 Setscrews
8-14 Keys and Pins
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

4. Example 5 (Example 8-4 Textbook)
The grip is l = 1.50 in. From Table A-31, the nut thickness is 35/64 in.
Adding two threads beyond the nut of 2/11 in gives a bolt length of
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

5. From Table A-17, the next fraction size bolt is 2 1/4 in.
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

6. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

7. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

8. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

9. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

10. 8-7 Tension Joints-The External Load
Fatigue Analysis
a) External Load
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

11. 8-11 Fatigue Loading of Tension Joints
Fatigue Analysis
In general, bolted joints are subject to 0-Pmax, e.g pressure vessels, flanges, pipes, …
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

12. 8-11 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.
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.
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

13. 8-11 Fatigue Loading of Tension Joints
In thread-rolling the amount of cold-work and strain strengthening is unknown to the designer; therefore, fully corrected (including Kf) axial endurance strength is reported in Table 8-17.
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

14. 8-11 Fatigue Loading of Tension Joints
Fatigue-loaded bolted joints subjected to Fatigue action can be analyzed directly by the methods of Chapter 7.
Fatigue loading is the one in which the externally applied load fluctuates between zero and some maximum force P.
Fmax = Fb and Fmin = Fi
Fa = (Fmax – Fmin)/2 = (Fb – Fi)/2
σa = Fa/At
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

15. 8-11 Fatigue Loading of Tension Joints
Fmax = Fb and Fmin = Fi
Fa = (Fmax – Fmin)/2 = (Fb – Fi)/2
Fm = (Fmax + Fmin)/2 = (Fb + Fi)/2
σa = Fa/At
(8-34)
(8-35)
(8-36)
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

16. On the designer’s fatigue diagram, shown in Figure 8-20, the load line is .
High Preload is especially important in fatigue. σi is a constant the load line at Fi/At has a unit slope, r = 1.0
Figure 8-20
Designer’s fatigue diagram showing a Goodman failure locus and how a load line is used to define failure and safety in preloaded bolted joints in fatigue.
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

17. 8-11 Fatigue Loading of Tension Joints
Next, find the strength components Sa and Sm of the fatigue failure locus. These depend on the failure Criteria.
Goodman
(8-37)
Gerber
(8-38)
ASME-Elliptic
(8-39)
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

18. Goodman Gerber ASME-Elliptic
For simultaneous solution between Eq. (8-36), as Sa = Sm + σi and each of Eqs. (8-37) to (8-39) gives
Goodman
(8-40)
(8-41)
Gerber
(8-42)
ASME-Elliptic
(8-43)
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

19. When using relations of this section, be sure to use Kf for both σa and σm. Otherwise, the slope of the load line will not remain 1 to 1.
The factor of safety guarding against fatigue is given by
Applying this to the Goodman criterion, for example, with Eqs. (8-34) and (8-40) and gives
(8-44)
(8-45)
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

20. With no preload, C=1, Fi=0, and Eq. (8-45) becomes
Preload is beneficial for resisting fatigue when is greater than unity. For Goodman, Eqs.(8-45) and (8-46) with puts an upper bound on the preload Fi of
If this cannot be achieved, and nf is unsatisfactory, use the Gerber or the ASME-elliptic criterion to obtain a less conservative assessment.
After solving Eq. (8-44), you should also check the possibility of yielding, using the proof strength
(8-46)
(8-47)
(8-48)
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

21. Example 6 (Example 8-5 Textbook)
Figure 8-21
Pressure-cone frustum member model for a cap screw. For this model the significant sizes are
Where l = effective grip. The solutions are for =30o and dw=1.5d.
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

22. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

23. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

24. Sut=120 kpsi. The coordinates are A: σi=63.72 kpsi
Figure 8-22
Designer’s fatigue diagram for preloaded bolts, dr showing the modified Goodman locus, the Gerber locus and the Langer proof strength locus, with an exploded view of the area of interest, The strengths used are
Sp= 85 kpsi, Se= 18.6 kpsi, and
Sut=120 kpsi. The coordinates are
A: σi=63.72 kpsi
B: σa=3.1 kpsi, σm=66.82 kpsi
C: Sa=7.55 kpsi, Sm=71.29 kpsi
D: Sa=10.64 kpsi, Sm=74.35 kpsi
E: Sa=11.32 kpsi, Sm=75.04 kpsi
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

25. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

26. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

27. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints