Tools › Structures › Fatigue › Spectrum Fatigue Life › Example
MIL-HDBK-5J Figure 3.2.3.1.8(a): best-fit S/N equation of unnotched 2024-T3 sheet
| Tool | Spectrum Fatigue Life |
| Source | U.S. Department of Defense (2003). Metallic Materials and Elements for Aerospace Vehicle Structures. MIL-HDBK-5J, 31 January 2003. Figure 3.2.3.1.8(a), equivalent stress equation for unnotched 2024-T3 sheet, longitudinal. |
| Level | Professional |
Problem
Unnotched 2024-T3 aluminum sheet is cycled from zero to 40 ksi (R = 0). Using the handbook's best-fit equation log N = A − B log(S_eq − C) with S_eq = S_max (1 − R)^p, find the equivalent stress and the cycles to failure; repeat for a fully reversed cycle of ±30 ksi.
Inputs to Enter
- S/N select: 2024-T3 bare sheet, unnotched, Figure 3.2.3.1.8(a) (the default selection), filling A = 11.1, B = 3.97, C = 15.8 ksi, p = 0.56; K_t,sn 1.0, K_f 1.0
- History 0 40 0 (ksi), or 30 −30 30 for the reversed cycle; repeats 1
Answers
- R = 0, S_max = 40 ksi: S_eq = 40 × 1^0.56 = 24.2 ksi above the threshold C = 15.8 ksi
- log N = 11.1 − 3.97 log(24.2) = 5.606, N = 403,900 cycles (tool evaluation of the handbook equation)
- R = −1, S_max = 30 ksi: S_eq = 30 × 2^0.56 = 44.2 ksi
- A cycle with S_eq below 15.8 ksi, or a wholly compressive cycle, gives no damage
Notes
The constants A, B, C and p are the handbook's printed values on the figure; the lives are the tool's evaluation of that equation (the handbook prints the curve, not this point). The default history is the tool's own block, so enter the two-point history to see one cycle.