Fracture Diodes: Directional Asymmetry of Fracture Toughness

Toughness describes the ability of a material to resist fracture or crack propagation. It is demonstrated here that fracture toughness of a material can be asymmetric, i.e., the resistance of a medium to a crack propagating from right to left can be significantly different from that to a crack propagating from left to right. Such asymmetry is unknown in natural materials, but we show that it can be built into artificial materials through the proper control of microstructure. This paves the way for control of crack paths and direction, where fracture—when unavoidable—can be guided through predesigned paths to minimize loss of critical components.

Figure 1

Snapshots of the crack evolution in a metamaterial consisting of a two-dimensional array of voids exhibiting directional asymmetry of its effective toughness.

Figure 2

Asymmetry in toughness in a metamaterial consisting of a two-dimensional array of voids computed using the surfing boundary conditions. Effective toughness is normalized by the value of the base material.

Figure 3

Asymmetry in toughness in PMMA specimens tested on a rail. The insets show failed specimens.

Figure 4

Asymmetry in toughness in 3D printed specimens tested in uniaxial tension tests. (a)–(d) Specimen geometries. (e) Time lapse sequence of images of a test on a uniaxial specimen (The clock begins at crack initiation). (f) Table of observed crack propagation direction for all tests performed ($\mathrm{For}.=\mathrm{Forward}$, $\mathrm{Indet}\mathrm{.}=\text{Indeterminate}$). (g) Specimen geometries used for computations. Images of all specimens (a)–(e),(g) are cropped to show the operative section.

Figure 5

Asymmetry in toughness in the designed fracture diode metamaterial tested in uniaxial tension tests. The first image shows the undeformed geometry. The inclusions have a period of 4.05 mm. The subsequent images show snapshots at 0.5 second intervals (The clock begins at crack initiation at the first inclusion).