Question

A forged 0.85 inch diameter steel bar is sent to test center in a manufacturing factory for safety test. Before the experiment is performed, you wish to have a model of the strength over use and fatigue strength with an infinite life with 99% reliability. The key material properties of this steel bar are S,-160 ksi and Sy = 95 ksi. Plot on the semi-log graph below the estimated S-N curves for: (a) Axial load (b) Bending load (c) Torsional load. For each of the above mentioned cases, determine the fatigue strength corresponding to I. 10 or more cycles II. 2 x 105 cycles Please show your assumptions and analysis/calculations for the values you plot on the graph, you need to use Table 8.1 and Figure 8.13 for your calculations.

Answer 1

For steel, forged component with 99 % reliability s_ut = 160 ksi s_yt = 95 ksi For steel endurance limit, sigma_e = s_e = 0.5 times s_ut = 0.5 times 160 s_e = 80 ksi correction factors: - (1) Surface finish factor, k_1 = 0.35 for forged component (ii) Reliability factor for R = 99% k_2 = 0.814 (iii) Load facor, k_l = 0.8 for axial loading = 1 for bending loading = 0.577 for Torstonal loading Therefore corrected endurance limit: - (i) For axial load s_ea = k_1 k_2 k_L s_e = 0.35 times 0.814 times 0.8 times 80 s_ea = 18.23 ksi (ii) For bending load, s_eb = k_1 k_2 k_L s_e = 0.35 times 0.814 times 1 times 80 s_eb = 22.8 ksi (iii) For Torsional loading, s_er = k_1 k_2 k_L s_e = 0.35 times 0.814 times 0.577 times 80 s_et = 13.15 ksi (i) 10^6 cycles