Pinning cracks by microstructure design in brittle materials

In this work, we explore the possibility of controlling toughening mechanisms in brittle materials through microstructure design to enhance their resistance to crack growth. First, a computational framework is established using the distinct element method (DEM) with J integral implemented to simulate complex crack propagation and characterize the effective fracture energy of brittle materials. The fracture behavior of a soda-lime glass plate containing pre-existing voids under tensile loading is simulated, and toughening mechanisms associated with the existence of voids, including blunting-induced pinning and deflection-induced pinning, are identified in brittle materials. Then we seek to modulate the fracture toughness of brittle materials by introducing either randomly distributed voids or sinusoidally distributed voids and controlling the pinning events. Our findings reveal that randomly distributed voids with different volume fractions induce only modest toughening in brittle materials. Sinusoidally distributed voids with specific amplitude and wavelength trigger both crack blunting and deflection in the vicinity of voids, increasing the probability of crack pinning and leading to a significant increase in crack growth resistance. Moreover, we identify the critical conditions for the "embrittlement-toughening transition" and "maximized toughening". Finally, we discuss the difference between void toughening for brittle materials and ductile materials due to distinct toughening mechanisms activated, and also extend the toughening strategy to nacre-like materials. The stairwise herringbone structure is demonstrated to be a promising candidate for structure optimization due to unique stiffness, strength, and toughness.