Fig. 2: Correlation of fracture mechanics with underlying microstructure in MNI crystals.
From: Fast self-healing in a layered molecular crystal mediated by stress-induced symmetry breaking
a–f Experimental (SEM) analysis of fracture mechanics on the (010) face of a single crystal. a Application of a vertical mechanical stress on a pristine crystal using a metal pin creates a defect point, from where a microcrack is initiated. This is known as Stage I in fracture mechanics. In Stage II, there is a gradual extension of crack propagation leading to divergent cracks (b), which, if left uncontrolled, lead to catastrophic rupture (c). However, if crack propagation adapts a steady state growth (d), an elliptically shaped crack tip emerges (e), which leads to crack arrest via crack-tip plasticity (e) and consequent self-healing in MNI crystals (f). g–j Investigation of the crystal structure of MNI. g The MNI is a polar molecule with a net dipole moment, but it forms a homodimer via C(sp2)–H···O hydrogen bonding with an inversion center (yellow circle). h The MNI molecules form a 1-D chain via strong N–H···N hydrogen bonds (d/Å; θ/°: 2.07 Å, 159°). i The adjacent 1D tapes are connected on both sides by C(sp2)–H···O hydrogen bonds (2.45 Å, 155°) to form a 2D sheet. j These sheets are further stacked over one another in a ladder-like fashion by relatively weaker C(sp3)–H···O hydrogen bonds (2.52 Å, 174°), forming a hierarchical structure closely resembling a van der Waals material. The weakest bonds, which connect the molecular layers, act as sacrificial bonds whose disruption during force application leads to the generation of a crack.
