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Lecture

Effects of surface and material imperfections on the durability and structural integrity of bearing components

Thursday (28.05.2020)
16:40 - 17:00

Bearings are critical machine components for mechanical power transmission. The challenge of ever increasing power density from modern equipment manufacturing imposes a higher demand for the load-carrying capacity on one hand and reliability of bearings on the other hand. Catastrophic failure such as fracture of bearing components in applications must be avoided.

High-strength materials such as hardened bearing steels, can suffer in general from lack of damage tolerance in the form of sensitivity to pre-existing imperfection features. The detrimental effects of the imperfection features must be properly accounted for in the design and structural integrity evaluation of engineering components. The present study focuses on a specific type of imperfection feature, i.e. surface roughness resulting from machining, which is relevant for bearing flanges and flanged parts of bearing units.

Rotating bending fatigue (RBF) tests were conducted on a high-carbon steel in martensitic and bainitic conditions, and a tough-tempered medium-carbon steel. The specimens were surface-finished to different conditions: polished surface and ground surface with a range of roughness levels. The experimental results indicate that the hardened high-carbon steel specimens with rough surface failed predominantly by surface crack initiation, whereas the fatigue fracture of the specimens with smoother surface tend to fail by subsurface crack initiation from non-metallic inclusions. Moreover, the fatigue strength of the martensitic specimen is lower than that of the bainitic specimen in the low stress-cycle range in which failure is dominated by surface initiated fatigue fracture, whereas this difference diminishes in the high-cycle fatigue regime where subsurface initiated fatigue prevails. The non-hardened medium-carbon steel samples, however, fail only by surface crack initiation, and the fatigue strength is much less sensitive to surface roughness.

A unified model is developed to predict the fatigue strength of structural components with different microstructures and surface finish. The model is used to predict the fatigue strengths of flanged bearing rings subjected to thrust loading. A fairly good agreement is achieved between the model prediction and experiments

 

Speaker:
Additional Authors:
  • Dr. Hanzheng Huang
    SKF