Isao Ohkawa

Short Crack Growth under Cyclic Torsion with Static Tension

The superposition of static loading on cyclic torsion has been recognized to affect the crack growth behavior and fatigue life of materials, due to the mean stress acting on the crack planes. However, the effect of static loading on torsional fatigue is different depending on the material type. This appears to be related to the difference in microstructure and cyclic hardening/softening characteristics of the materials.

Fatigue Behavior of Circumferentially Notched Round Bars of Austenitic Stainless Steel Under Torsional and Axial Loads

Fatigue tests of smooth and notched round bars of austenitic stainless steels SUS316NG and SUS316L were conducted under cyclic tension and cyclic torsion with and without static tension. Fatigue strength under fully reversed (R=−1) cyclic tension once increased with increasing stress concentration factor up to Kt =1.5, but it decreased from Kt =1.5 to 2.5. Fatigue life increased with increasing stress concentration under pure cyclic torsion, while it decreased with increasing stress concentration under cyclic torsion with static tension. From the measurement of fatigue crack initiation and propagation lives using electric potential drop method, it was found that the crack initiation life decreased with increasing stress concentration and the crack propagation life increased with increasing stress concentration under pure cyclic torsion. Under cyclic torsion with static tension, the crack initiation life also decreased with increasing stress concentration but the crack propagation life decreased or not changed with increasing stress concentration then the total fatigue life of sharper notched specimen decreased. It was also found that the fatigue life of smooth specimen under cyclic torsion with static tension was longer than that under pure cyclic torsion. This behavior could be explained based on the cyclic strain hardening under non-proportional loading and the difference in crack path with and without static tension.Copyright

Effect of ageing on fatigue life of low carbon steel under two-level stress loading.

In the case of two-level cyclic stress loading, it is generally recognized in steels that the cumulative cycle ratio (ΣN/Nf) is greater than unity for a low to high stress sequence (σL→σH) but less than unity for a high to low stress sequence (σH→σL). This kind of deviation from the Palmgren-Miners rule (linear damage rule) is expected to be related to the fatigue damage accumulation process and age-hardening ability.Two kinds of low carbon steels were prepared; specimen A, in which the age-hardening ability was attained through low temperature quenching, and specimen B, in which the ageing process in specimen A was completed by leaving them at room temperature. In specimen B, the deviation from Palmgren-Miners rule was a little for both σL→σH and σH→σL sequences. On the other hand, in specimen A which had age-hardening ability, the deviation from Palmgren-Miners rule for σL→σH sequence was greater than that in specimen B.The change of internal friction energy and propagation of fatigue crack were observed during cyclic loading. Based on the experimental results of accumulation process of internal friction energy, the following has been assumed.(1) A part of the internal friction energy is dissipated for the accumulation of fatigue damage.(2) Crack growth corresponds to fatigue damage.(3) The accmulation process of fatigue damage is expressed by the Marco-Starkey and New-mark models in which an exponent depends on stress amplitude and increases with progress of ageing.Based on the above assumptions, the cumulative cycle ratio was calculated, and the result showed a similar tendency as the experimental one.