Threshold levels of surface sliding and contact pressure have been identified at which the different types of damage were found to initiate at both: points where impact loading was used to pre-damage specimens; points away from impact damage zones. Figure 7-20 displays these thresholds by showing the lowest contact pressures and slip levels at which the damage was found. Figure 7-21 plots the thresholds in terms of contact pressure and traction coefficient rather than slip. Comparing results in this way allows the minimum required levels of different damage initiating factors, taking into account their combined effects, to be estimated. Mean traction coefficients at different slip levels and contact pressures are displayed in Table 4-12. It is proposed that any point to the right or above the plotted lines is above the threshold required to initiate damage. It is clear that increasing contact pressure, slip or traction coefficient within the ranges investigated, caused increasing levels of damage, so it is assumed that if a specimen has experienced damage at a lower contact pressure and slip level/traction coefficient, then it will do so at higher levels, even if that damage was not directly observed from the results of this study. Clearly, this figure should be used with caution as it cannot accurately predict the thresholds between data points and contact pressures below 1.79 GPa, which were not investigated in this study. Figure 7-20a and Figure 7-21a display the results for specimens that had been pre-seeded with impact damage, Figure 7-20b and Figure 7-21b displays results for those with no pre-seeded damage. Both figures show clearly that all forms of damage at MnS inclusions are sensitive to increased levels of contact pressure, slip or traction coefficient as well as to pre-seeded hammering impact damage.
Figure 7-20: Contact pressure and slip (%) threshold identification for different forms of MnS inclusion initiated damage for: a) specimens pre-damaged with hammering impact loading b) specimens with no pre-seeded damage (overlapping thresholds indicated by dashed two-colour lines).
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Figure 7-21: Contact pressure and traction coefficient threshold identification for different forms of MnS inclusion initiated damage for: a) specimens pre-damaged with hammering impact loading b) specimens with
no pre-seeded damage (overlapping thresholds indicated by dashed two-colour lines).
All damage types were more extensive under higher contact pressures and higher traction coefficients (and slip percentages). Separation at inclusions occurred in all specimens, other than the non-impact damaged specimens in disc 1 (defined in Table 4-11), which experienced the lowest slip level and lowest contact pressure of all tested specimens. However, increased levels of traction above certain contact pressure levels were required for the separation to spread away from inclusion tips into the steel matrix as shown in Figure 7-7. This occurred at the lowest contact pressure (1.79 GPa), when the traction coefficient was above 0.079, as shown in Figure 7-2.
Inclusions that were internally cracked were not found at lower levels of contact pressure/slip so it is clear that the pre-seeded hammering impact damage was not extensive enough to directly lead to this form of damage. However, since internally cracked inclusions appeared at lower contact pressures of 2.48 GPa in the pre-damaged areas of discs rather than 3.03 GPa in the areas of no pre-seeded damage, it appears that hammering impact damage did perhaps weaken the inclusions within the impact zone. It is clear that this damage mode is affected by surface traction, since in both specimens that were pre-damaged and in those that were not, a traction coefficient threshold of around 0.07 appeared to exist, below which, no internally cracked inclusions were found.
Crack propagation from inclusion tips occurred at lower contact pressures and slip levels than inclusion internal cracking, which offers further evidence that the latter is not required to initiate the former. It was clear that the pre-damaged specimens had lower contact pressure and traction coefficient threshold for this damage mechanism, further backing up the hypothesis that hammering impact damage may accelerate damage of WTGBs.
As with the other forms of damage, WEA formation at inclusions only occurred above certain contact pressures and traction coefficients. WEA formation appeared to be less affected by the
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hammering impact pre-damage, although at the highest contact pressure, it did appear at a slightly lower traction coefficient (0.070 compared to 0.079). Since only 16 WEAs were found at inclusions however (as shown in Figure 7-10), this result cannot be considered conclusive. It is possible that if further RCF load cycles had been applied to the specimens, the separated/crack initiating inclusions discussed above may have gone on to initiating WECs at the created free surfaces. In which case, it is likely that WEAs may have occurred at lower slip/contact pressure thresholds if the number of load cycles was higher. Evidence to support this hypothesis is presented in Chapter 6, since WEAs were only found in the long term hammering impact tests. WEAs were found at a variety of depths from the contact surface and the variation in the location that they formed is discussed in the following section.
Subsurface cracking appeared above contact pressures of 2.4 GPa and at lower contact pressures when the slip was increased to 10 %. Subsurface cracks were found to be slightly longer under the impact zone, although this result was not conclusive and may have been due to chance as well as over-analysis of IZ sites compared to non-IZ sites. Hammering impact damage seeding did not seem to affect the probability of a subsurface crack initiating in these samples. Surface cracking, perhaps unsurprisingly, was strongly affected by both the level of surface sliding and the contact pressure. All specimens that were exposed to 5 % slip of higher experienced surface cracking, which occurred at lower slip levels when the contact pressure was higher than 2.48 GPa.