Cumulative Damage in Push-Pull Fatigue of Fillet-Welded Mild Steel Plate Subjected to Narrow Band Random Loading

Push-pull fatigue tests have been made in constant amplitude loading and in narrow band random loading on fillet-welded cruciforms made from 3/8 in thick mild steel plate to B.S. 1501-151 Grade 28. The test frequency was 250 Hz, mean tensile stresses of 0, 5 and 10 tonf/in²were employed and tests were extended for endurances of up to 10⁸cycles.

Curves have been fitted to the S-N results using the relationship N(S — S_o)^α= C and on the assumption that this may be extrapolated beyond 10⁸cycles, curves showing the effect of mean stress are given for both constant amplitude loading and narrow band random loading for endurances up to 10¹²cycles. Suitable factors of safety should be applied to these stresses before use in design. For endurances beyond 10⁷cycles, an increase in tensile mean stress from 0 to 10 tonf/in²reduced the fatigue strength by about 50 per cent in both constant amplitude loading and random loading.

Reasonable agreement was found at all mean stresses between the experimental random loading S-N curves and those predicted using the constant amplitude results and the Palmgren-Miner cumulative damage hypothesis.