**6. Fatigue failure**

Turning now to a practical example. **Figure 3** shows an image of the microstructure of a main outer bearing ring of carbon-chrome steel which had failed by rolling contact fatigue [5]. It seems to have been generally assumed that the large array of cracks had been formed by fatigue. This is not true.

The large array of cracks is a typical tangle of bifilms, introduced into the steel as a result of the turbulence during pouring of the originating ingot. This was probably top poured for the economy, but even if uphill teemed (bottom gated) the turbulence and air entrainment issues are immensely damaging and certainly capable of creating such extensive defects. The bifilm population in the solidifying ingot will tend to be segregated into the ingot centre because of the 'pushing' action of the advancing solidification front (advancing dendrites cannot grow through the 'air layer' in the bifilm). Probably, the solidified ingot is now forged, opening it into the shape of a ring. The inner working surface of the bearing will naturally contain the highest density of bifilm defects from the centre of the original ingot, typical of those seen in **Figure 3**.

The enlarge detail provided in **Figure 3b** shows a fractured inclusion together with light etching cracks and 'wings' on either side, as in a classical fatigue structure. The 'fractured' inclusion appears fractured because of its growth either side of a bifilm (it is worth emphasising that the 'fracture' of inclusions is not normally the result of stress, but of growth on bifilms). However, this diminutive region constitutes the real fatigue failure. One can imagine that among the massive bifilm array, of the order of millimetres in size, large blocks of metal will be stressed by the passing of the rollers, and the stress will be concentrated in those small remaining regions which connect the block to the main mass of the bearing. The gradual failure of these ligaments by fatigue will eventually release the block into the rollers, causing catastrophic failure. The size of the 'butterfly wings' is of the order of 10 μm—only 1% of the size of the pre-existing bifilm cracks, but, of course, necessary for releasing the final failure.

#### **Figure 3.**

*(a) Array of bifilm cracks under the bearing surface; (b) inclusion with butterfly wings and adjacent white-etching cracks (courtesy ref. [5]).*

In summary, extensive pre-cracks (bifilms) provide major weakening of wind turbine bearings, but final failure is by the fatigue of microscopic ligaments in which the rolling stresses are concentrated. The ligaments may or may not contain inclusions. It seems that extensive bifilm pre-cracks and microscopic fatigue cracks may be expected to be common conditions for failure. Work on the newer bainitic steels [6] is expected to *reduce* the fatigue failures of wind turbine bearings which is, of course, welcome. However, the *complete* elimination of failures is only to be expected if casting techniques can be improved [3].
