Nacre-like alumina (NLA) is a ceramic composite composed of monocrystalline alumina platelets (95 vol.%) and a glass phase (5 vol.%). Even though being composed almost exclusively of alumina, NLA shows a increased toughness of 50% as compared to alumina, while keeping a similar flexion strength [1]. This improvement is due to the so-called ‘brick-and-mortar’ microstructure of NLA. Platelets are aligned in one direction and the glassy is present at the interface between the platelets. Due to the high contrast in material properties between alumina and glass (e.g. stiffness, tensile strength) crack initiation and propagation occur in the glassy phase leading to a strong crack deviation by the platelets. Depending on the platelet alignment with respect to the loading direction, the microstructure-crack interaction differs[2][3], thus triggering different extrinsic toughening mechanisms.
In the present work, we studied the influence of microstructure orientation in NLA with respect to the loading direction. Especially, 3 particular orientations are considered, defined by Currey for natural nacre [3]. Among these three orientations, the across configuration presents the highest resistance to crack initiation and a quasi-stable crack propagation is observed. This behavior has been the focus of many studies within the last ten years [5][6]. On the contrary, very few studies were conducted on the two other configurations. They are the focus of the present study. The effects of loading type (i.e. loading in mode I or mixed-mode loading), presence or absence of notch in the testing sample on crack initiation and propagation are assessed. Experimental findings (e.g. fracture test results, microscopic observations, full-field data from Raman spectroscopy and digital image correlation) are used in finite element models to investigate the multi-scale crack-microstructure interactions in NLA. These models rely on the coupled criterion [4]. This criterion extends Griffith’s linear elastic fracture mechanics to crack initiation. It is based on a simultaneous fulfillment of both a stress and an energy condition. Finally, based on the fracture characterization results and finite element models, NLA microstructure optimization guidelines with respect to its resistance to fracture are defined.
 
[1] Bouville, F. et al., DOI : 10.1557/jmr.2019.418
[2] Duminy, T. et al., DOI : 10.1016/j.tafmec.2022.103710
[3] Currey, J.D., DOI : 10.1098/rspb.1977.0050
[4] Leguillon, D., DOI : 10.1016/S0266-3538(01)00067-7
[5] Barthelat, F, DOI : 10.1016/j.jmps.2014.08.008
[6] Pelissari, P.I.B.G.B., DOI : 10.1016/j.jeurceramsoc.2017.10.042