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Crack propagation and renucleation in soft brittle hydrogels

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Crack tip opening displacement (CTOD) and fracture energy are determined from crack geometry and material properties for very slowly propagating cracks, less than 50 \(\upmu \mathrm {m/s}\), in thin brittle hydrogels on the sub-millimeter scale. 2D fluorescent speckle images are captured using confocal microscopy during propagation, and 3D volumetric images are captured both before propagation begins and after the crack arrests. Fracture energy builds up until a critical value is reached and then remains constant as the crack propagates and eventually arrests when the energy is no longer sufficient for propagation. Once a crack arrests, more energy is needed for renucleation, suggesting that local toughening effects are at play. Based on observations of renucleation events and analysis of 3D crack shapes, this local toughening points to a mechanism for fracture surface roughening observed in the literature for slowly propagating cracks. Additionally, through-thickness variation in fracture energy, while expected from linear elastic fracture mechanics (LEFM) theory, suggests local toughening in the process zone which contributes to this roughening of crack surfaces.

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Computational details:

The Digital Image Correlation (DIC) analyses in this chapter were obtained using the following Matlab packages: FIDIC (Fast Iterative DIC): Ncorr v1.2:


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This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1144469 and Award No. DMS-1535083 under the Designing Materials to Revolutionize and Engineer our Future (DMREF) program. Imaging was performed in the Biological Imaging Facility, with the support of the Caltech Beckman Institute and the Arnold and Mabel Beckman Foundation.

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Correspondence to Kimberley Ann Mac Donald.

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Mac Donald, K.A., Ravichandran, G. Crack propagation and renucleation in soft brittle hydrogels. Int J Fract 222, 37–52 (2020).

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  • Brittle
  • Confocal microscopy
  • Fracture mechanics
  • Slow cracks
  • Soft gels