(a) (b)
Fig. 12. Longitudinal cross section of specimen subjected to IPD-TMF
showing internal damage development; (a) band contrast, (b) EBSD image.
3.3.2. OPD-TMF cycling
The previous results on the damage evolution in OP-TMF cycling20 revealed substantially different damage mechanism
than in IP-TMF cycling. The cracks started much later than in IP-TMF
cycling, i.e. only when a thick oxide layer was formed on the specimen
surface. The thick oxide layer cracked perpendicularly to the stress
axis. Similar damage mechanism was present in OPD-TMF cycling. The
surface relief of the specimens exposed to OPD-TMF loading with total
strain amplitude 6×10−3 is presented in Fig. 13. The
specimen surface is highly oxidized and moreover the islands of thicker
oxide layers can be distinguished within some grains. The cracks
developed mainly in the islands of the thick oxide layer and are again
oriented perpendicularly to the stress axis. The cracks in the oxide are
well open. The crack in the grain boundaries appeared as well but the
fraction of grain boundary cracking was smaller in comparison to the
transgranular crack formation on the thick oxide layer.