Influence of mean stress and notches on the very high cycle fatigue behaviour and crack initiation of a low‐pressure steam turbine steel

Abstract

The aim of the present work is to investigate the influence of different stress concentration factors (1.09, 1.31, and 2.42) and several R‐values on the fatigue behaviour of martensitic steel X10CrNiMoV12‐2‐2 up to 2 · 109 load cycles at ambient temperature. The tests were performed using an ultrasonic fatigue testing system developed at the authors' institute. The S‐N curves of specimens with the lowest investigated stress concentration factor of 1.09 show for both R = −1 and R = 0.5 a flat slope with transition from surface to volume crack initiation at about 1–2 · 107 load cycles. The maximum number of load cycles where fracture occurs increases with increasing R‐value. The √area approach by Murakami describes lifetime behaviour within about 4 decades of lifetime and a wide range of load ratios. In case of internal crack initiation at R = −1, the fracture surface contains a fine granular area (FGA) around the inclusion within the typical “fish‐eye” fracture. With increasing notch factor, the fatigue strength up to 2 · 109 load cycles decreases and the slope of the S‐N curve becomes steeper in the low cycle fatigue and high cycle fatigue regime, especially for αk = 2.42. In this case, damage at internal defects is suppressed and fatigue cracks are generally initiated at the notch root.

FGA formation mechanism for X10CrNiMoV12‐2‐2 and 34CrNiMo6 for constant and variable amplitude tests under the influence of applied mean loads

“…Hence, crack closure effects and, consequently, the applied load ratio R as well as residual stresses have a major influence on FGA formation. In recent publications, fracture without FGA formation for increased R values is reported for several materials in contrast to fully reversed loading condition where FGA formation is observed, supporting the NCP model . In the present work, the nomenclature FGA will be used independent on the proposed model.…”

“…The mechanical properties are given in Table and combine both relatively high strength and significant ductility. Samples taken from the root section of steam turbine blades are manufactured with cylindrical gauge length and a slight stress concentration factor of 1.09 . All samples were mechanically polished before testing.…”

“…To limit self‐heating of the specimens, all tests were carried out in pulse‐pause mode with additional air cooling. For detailed information concerning fatigue testing systems, the authors refer to earlier publications . Fracture surfaces of specimens failed from internal defects have been investigated using high‐resolution SEM technique with special attention paid to the crack origin and the surrounding FGA in the fisheye's centre.…”

“…Characteristic for these materials with internal imperfections like non‐metallic inclusions is a change from surface crack initiation in the low cycle fatigue (LCF) and high cycle fatigue (HCF) regime to internal crack initiation in the VHCF regime. For some materials, this change results in a multistep S‐N curve with a significant decrease of the tolerable stress amplitudes while for the investigated steel X10CrNiMoV12‐2‐2, the fatigue strength does not decrease significantly . For the second investigated steel 34CrNiMo6, the S‐N curves show a constant slope independent of the number of cycles until failure or the crack initiation mechanism .…”

A review about the effects of structural and operational factors on the gigacycle fatigue of steels

“…Data summarized in Table show that several investigations about the normal mean stress sensitivity in the VHCF regime already exist for axial and rotational loading types . Generally, the S‐N curves of steels are shifted towards lower stress amplitudes with increasing the load ratio independently of the loading type . An example is depicted in Figure .…”