1. Methods to Maximize Design Life
1. By minimizing initial flaws, especially surface flaws. Great care is taken to produce fatigue-insusceptible surfaces through processes, such as grinding or polishing, that leave exceptionally smooth surfaces. These surfaces are then carefully protected before being placed into service.
2. By maximizing crack initiation time. Surface residual stresses are imparted (or at least tensile residual stresses are relieved) through manufacturing processes, such as shot peening or burnishing, or by a number of surface treatments.
3. By maximizing crack propagation time. Substrate properties, especially those that retard crack growth, are also important. For example, fatigue cracks propagate more quickly along grain boundaries than through grains (because grains have much more efficient atomic packing). Thus, using a material that does not present elongated grains in the direction of fatigue crack growth can extend fatigue life (e.g., by using cold-worked components instead of castings).
4. By maximizing the critical crack length. Fracture toughness is an essential ingredient. (The material properties that allow for larger internal flaws are discussed in Chapter 6.)

2. Stress Cycle and Test Specimen
Figure 7.3 R.R. Moore machine fatigue test specimen. Dimensions in inches.
Figure 7.2 Variation in nonzero cyclic mean stress.

3. Cyclic Properties of Metals
Table 7.1 Cyclic properties of some metals.

4. Fatigue Crack Growth
Figure 7.4 Illustration of fatigue crack growth. (a) Size of a fatigue crack for two different stress ratios as a function of the number of cycles; (b) rate of crack growth, illustrating three regimes.

5. Fatigued Part Cross-Section
Figure 7.5 Cross-section of a fatigued section, showing fatigue striations or beachmarks originating from a fatigue crack at B.

6. Fatigue Fracture Surfaces
Figure 7.6 Typical fatigue fracture surfaces of smooth and notched cross-sections under different loading conditions and stress levels.

7. Fatigue Strength of Ferrous Metals
Figure 7.7 Fatigue strengths as a function of number of loading cycles. (a) Ferrous alloys, showing clear endurance limit.

8. Fatigue Strength of Aluminum Alloys
Figure 7.7 Fatigue strengths as a function of number of loading cycles. (b) Aluminum alloys, with less pronounced knee and no endurance limit.

9. Fatigue Strengths of Polymers
Figure 7.7 Fatigue strengths as a function of number of loading cycles. (c) Selected properties of assorted polymer classes.

10. Endurance Limit vs. Ultimate Strength
Figure 7.8 Endurance limit as function of ultimate strength for wrought steels.

11. Endurance Limit
Table 7.2 Approximate endurance limit for various materials.

12. Notch Sensitivity Usage:
Figure 7.9 Notch sensitivity as function of notch radius for several materials and types of loading.

13. Surface Finish Factor Table 7.3 Surface finish factor.
Figure Surface finish factors for steel. (a) As function of ultimate strength in tension for different manufacturing processes.

14. Roughness Effect on Surface Finish Factor
Figure Surface finish factors for steel. (b) As function of ultimate strength and surface roughness as measured with a stylus profilometer.

15. Reliability Factor
Table 7.4 Reliability factor for six probabilities of survival.

16. Shot Peening
Figure The use of shot peening to improve fatigue properties. (a) Fatigue strength at two million cycles for high strength steel as a function of ultimate strength; (b) typical S-N curves for nonferrous metals.

17. Example 7.4
Figure Tensile loaded bar. (a) Unnotched; (b) notched.

18. Influence of Non-Zero Mean Stress
Gerber Line
Goodman Line
Soderberg Line
Figure Influence of nonzero mean stress on fatigue life for tensile loading as estimated by four empirical relationships.

19. Modified Goodman Diagram
Figure Complete modified Goodman diagram, plotting stress as ordinate and mean stress as abscissa.

20. Modified Goodman Criterion
Table 7.5 Equations and range of applicability for construction of complete modified Goodman diagram.

21. Modified Goodman Criterion
Table 7.6 Failure equations and validity limits of equations for four regions of complete modified Goodman diagram.

22. Example 7.7
Figure Modified Goodman diagram for Example 7.7.

23. Alternating Stress Ratio for Cast Iron
Figure Alternating stress ratio as function of mean stress ratio for axially loaded cast iron.

24. Properties of Mild Steel
Figure Mechanical properties of mild steel at room temperature as a function of strain rate.

25. Example 7.10
Figure Diver impacting diving board, used in Example (a) Side view; (b) front view; (c) side view showing forces and coordinates.

26. Brake Stud Design Analysis
Figure Dimensions of existing brake drum design.
Figure Press brake stud loading. (a) Shear and bending-moment diagrams for applied load; (b) stress cycle.