Size Selection of Crack Front Defects: Multiple Fracture-Plane Interactions and Intrinsic Length Scales

Area of Science:

• Materials Science
• Fracture Mechanics
• Soft Matter Physics

Background:

• Material failure is often governed by crack propagation.
• In real 3D materials, cracks frequently involve multiple intersecting fracture planes.
• The interaction between these planes leads to complex, poorly understood 3D crack structures like step defects.

Purpose of the Study:

• To investigate the formation and characteristics of out-of-plane crack structures, specifically step defects.
• To identify the key parameters governing the size of these step defects in brittle materials.
• To establish a fundamental understanding of multiple-crack interactions in three dimensions.

Main Methods:

• Experimental investigation of crack propagation in numerous brittle hydrogels.
• Analysis of fracture surfaces to characterize step defect formation.
• Measurement of crack velocity-dependent fracture energy (Γ(v)) and shear modulus (μ).

Main Results:

• Observed that step defects form when crack fronts segment into overlapping planes separated by a stabilizing distance (h_max).
• Demonstrated a linear relationship between h_max and intrinsic material length scales.
• Found h_max varies linearly with both a nonlinear elastic length (Γ(v)/μ) and a dissipation length (ξ).

Conclusions:

• The size of step defects in brittle hydrogels is determined by intrinsic material length scales.
• These findings provide a pathway to fundamentally understand 3D multiple-crack interactions.
• Identified key parameters (fracture energy and dissipation length) for predicting stable out-of-plane fracture structures.