Influence of forging defects on fatigue behaviour of a rotor steelThursday (28.05.2020) 14:30 - 14:50
Structural components produced by casting and forging may contain internal defects detected by means of ultrasonic testing (UT). The decision whether such a component can be taken into operation or should be withdrawn depends on the defects size, applied load and material properties. Currently, the fracture mechanics methodology is widely used to estimate the severity of UT indications, while assuming a sharp initial crack which contour envelops the respective indication. However, such an approach may considerably underestimate the remaining fatigue life of the component, since neither the defect morphology nor the phase of crack nucleation from an individual defect or a defect field are taken into account.
In this study, a series of fatigue tests were performed to quantify the effect of manufacturing defects on the fatigue life of a rotor steel. First, material S-N curves were derived in stress controlled HCF and strain controlled LCF tests, using small cylindrical specimens with no defects. Further tests on the defect-free material were carried out using larger cylindrical specimens with a diameter Ø25 mm. Finally, large cylindrical tension specimens with embedded defects located close to the centre as well as four-point bend rectangular specimens containing sub-surface defects were tested. In all cases, the defect position in the specimen was estimated based on UT measurements on material segments extracted from a rotor disc. After testing, fracture surfaces were examined by SEM to determine the type, size and location of the defect at which crack initiation occurred, as well as to estimate the number of cycles until crack initiation by means of beach-marks produced on the fracture surfaces during the test.
The experimental results were analysed to correlate the defect type and size with the fatigue life, on the one hand. On the other hand, different fatigue assessment approaches were applied to predict fatigue lives of individual specimens based on the respective UT indications and quantitative information derived from fractographic investigations. When doing this, finite-element models were established representing different defect configurations, while varying the size and number of defects in the model. Results of finite-element calculations of the stress and strain fields for different defect configurations and their correlation to the experimentally obtained fatigue lives are discussed in the paper.