Tools › Structures › Joints › Lug Strength › Example
AFFDL-TR-69-42 Section 9.18: fatigue life of a 7075-T6 lug under an axial stress cycle
| Tool | Lug Strength |
| Source | Maddux, G. E., Vorst, L. A., Giessler, F. J., and Moritz, T. (1969). Stress Analysis Manual. AFFDL-TR-69-42, Air Force Flight Dynamics Laboratory, Wright-Patterson Air Force Base, OH. Section 9.18, Figures 9-22 to 9-27. |
| Level | Professional |
Problem
A 7075-T6 aluminum lug with D = 0.437 in, w = 1.1258 in, e = 0.5625 in and t = 0.25 in on a 0.437 in pin is loaded axially so that the net-section stress cycles between 18.47 and 27.7 ksi. Using the manual's k_1 k_2 k_3 parameters, its R = 0 life curves and the Goodman construction, estimate the fatigue life.
Inputs to Enter
- Check the Fatigue box; alloy 7075-T6 (Figure 9-24); lug 2 with D 0.437, w 1.1258, e 0.5625, t 0.25, w_T 1.1258 in; D_p 0.437 in
- f_max = 27.7 ksi, f_min = 18.47 ksi (form defaults); Advanced mode
Answers
- k_1 = 1.0 at a/c = 1.0, k_2 = 1.33 at D/c = 1.27, k_3 = 0.99 at D = 0.437 in (manual 1.0 / 1.33 / 0.99; product 1.32)
- f_a = 4,615 psi, f_m = 23,085 psi
- Life curve for k_1 k_2 k_3 = 1.32 with F_nuL = 70 ksi: f_a = 24.5 ksi at 10³, 13.5 ksi at 10⁴, 5.70 ksi at 10⁵ cycles (manual table, reproduced within 2–5 %)
- Goodman R = 0 equivalent f_N = f_a F_nuL / (F_nuL − f_m + f_a); life N = 8 × 10⁴ cycles (manual 8 × 10⁴; the manual quotes tests at 8.6 × 10⁴)
Notes
The lug dimensions must be typed (the default lug is the Section 9.6 male lug); the stresses and alloy are the form's defaults. The library check uses the manual's own F_nuL = 70 ksi; the tool computes F_nuL from the lug's net-section efficiency, so its life differs slightly from the manual's.