(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.