Influence of Indentation on the Fatigue Strength of Carbonitrided Plain Steel

To study the influence of indentation on the fatigue strength of untreated and carbonitrided specimens of S38C steel, the fatigue limit of specimens with and without indentations was tested. Fracture surfaces were observed using scanning electron microscopy (SEM). The results show that the fatigue strength of the untreated specimen decreases with increasing dimension of indentation, without significant difference compared to the predicted results. Compared to the fatigue limit of the untreated specimen, those of the carbonitrided specimen and the carbonitrided specimen whose compound layer was polished were improved by 12% and 40%, respectively. The fatigue strength of the carbonitrided specimen decreased sharply with increasing indentation size because of the presence of microcracks in the compound layer. When the compound layer was removed, the fatigue limit was observed to be less sensitive to indentation than that of the carbonitrided specimen

A novel machine learning-based imputation strategy for missing data in step-stress accelerated degradation test

The presence of missing data is a significant data quality issue that negatively impacts the accuracy and reliability of data analysis. This issue is especially relevant in the context of accelerated tests, particularly for step-stress accelerated degradation tests. While missing data can occur due to objective factors or human error, high missing rate is an inevitable pattern of missing data that will occur during the conversion process of accelerated test data. This type of missing data manifests as a degradation dataset with unequal measuring intervals. Therefore, developing a more appropriate imputation method for accelerated test data is essential. In this study, we propose a novel hybrid imputation method that combines the LSSVM and RBF models to address missing data problems. A comparison is conducted between the proposed model and various traditional and machine learning imputation methods using simulation data, to justify the advantages of the proposed model over the existing methods. Finally, the proposed model is implemented on real degradation datasets of the super-luminescent diode (SLD) to validate its performance and effectiveness in dealing with missing data in step-stress accelerated degradation test. Additionally, due to the generalizability of the proposed method, it is expected to be applicable in other scenarios with high missing data rates

Influence of laminar plasma quenching on rolling contact fatigue behaviour of high-speed railway wheel steel

This study aims at determining the influence of laminar plasma quenching (LPQ) on the rolling contact fatigue (RCF) behaviour under water condition of high-speed railway wheel steel. The wheel and rail test rollers were treated by LPQ with different scanning speeds (700, 900, 1100, 1300 mm/min), which generated different depth of heat affected zones (HAZs) and residual compressive stresses in the surface layer. To obtain an insight of the microstructures and their corresponding thermal treatment applied, the temperature evolution was simulated during the LPQ by Finite Element Modelling and the micro-structures of cross-sections below the roller surfaces were characterized. The results showed martensite, retained austenite and undissolved cementite in the HAZ as a consequence of the ultrafast heating and cooling rates. The produced residual compressive stresses resulted in an increase of ~30% of the RCF life indicating that LPQ could effectively improve the RCF resistance. Further investigations showed that the high density dislocation martensitic structure of the HAZ of reduced the plastic deformation, which could delay the RCF crack initiation and decrease the depth of RCF crack growth. During the RCF test, some retained austenite of the LPQ treated wheel roller transformed into twin martensite under the effect of strain induced martensitic transformation. The plastic deformation in the untreated wheel roller resulted in a refined microstructure showing lots of sub-grains generated in both pearlite and ferrite regions. Furthermore, we have observed that the lattice structures of the martensite in the LPQ treated sample and sub-grains of the plastically deformed untreated sample were similar to the BCC structure of the ferrite at the substrate in the high-speed railway wheel steel. We conclude from the obtained results that the LPQ processing enhances the RCF life and that this improvement is influenced by the obtained microstructure in the HAZ