The Effect of High Tensile Mean Stress on Fatigue of Unnotched and Notched SAE <uri xlink:href="http://www.sae.org/technical/papers/1045" xmlns:xlink="http://www.w3.org/1999/xlink">1045</uri> Steel

Abstract

<div class="htmlview paragraph">The objective of this research was to determine fatigue behavior of SAE <a href="http://www.sae.org/technical/papers/1045" target="_blank">1045</a> steel subjected to very high tensile mean stress for unnotched, mildly notched, and sharply notched test specimens, and to determine if common S-N<sub>f</sub> and ε-N<sub>f</sub> mean stress fatigue life models are applicable. High tensile mean stress fatigue tests for R ratios of 0.8 and 0.9 were conducted using unnotched and notched, K<sub>t</sub>=1.65 and K<sub>t</sub>=3.65, axial loaded SAE <a href="http://www.sae.org/technical/papers/1045" target="_blank">1045</a> steel specimens with hardness levels of Rc=10, 37, and 50. The monotonic notch strength ratio, NSR, for 5 of 6 test conditions was greater than 1, which allowed many notched cyclic test values of S<sub>max</sub> or S<sub>m</sub> to exceed the unnotched ultimate tensile strength. Much notched specimen fatigue resistance at these high R ratios was superior to that of unnotched specimens. However, cyclic creep/ratcheting, particularly for Rc=10 and 37, was a predominant cause of failure. Scanning electron microscopy examination revealed many cyclic failures were similar to monotonic failures consistent with cyclic creep/ratcheting measurements. Specimens failed due to cyclic creep/ratcheting, to an interaction between cyclic creep/ratcheting and fatigue, or to surface fatigue crack nucleation and growth only. Fatigue life calculations based on S-N<sub>f</sub> and ε-N<sub>f</sub> models were up to 5 orders of magnitude in error, and thus completely unreasonable at these high R ratios. For a given test condition, this inaccuracy was due to cyclic creep/ratcheting, scatter, notch strengthening, material properties used, inherent inaccuracy of S-N<sub>f</sub> constant life diagrams at high R ratios, and/or extrapolation of the Ramberg-Osgood equation beyond S<sub>u</sub> and σ<sub>f</sub>.</div>