The use of artificial defects in the 5-ball-rod rolling contact fatigue experiments

The fatigue performance of steels under rolling contact loading is normally evaluated using time consuming rigs, though accelerated tests are also used. The present work reports the analysis carried out on tests accelerated by using artificial defects (ADs) over the wear track. Two modifications were introduced to the 3-ball-rod testing protocol. The first modification reduces the time required to produce a spall by substituting 5-ball for the traditional 3-ball mounted on a retainer. The second modification is intended to study surface resistance to rolling contact fatigue (RCF) by furnishing the wear track with artificial defects which promote surface nucleated failures. Three different types of pedigreed artificial defects, i.e. Knoop, Vickers and Rockwell-C indentations were used. The experiments were conducted on specimens of AISI 440C. The failures obtained were optically analyzed and the exact location for spall nucleation was determined to be inside the AD rather than outside. An attempt was then made to correlate the results with the specific oil film parameter (lambda) but life changes could not be predicted by this criteria. The results indicate that the combination of ADs in conjunction with the five balls methodology does not bias the results, but it considerably reduces the time required to test materials.