Energy dissipation around a propagating crack is the primary mechanism for the enhanced fracture
toughness in viscoelastic solids. Such dissipation is spatially non-uniform and is highly coupled to
the crack propagation process due to the history-dependent nature of viscoelasticity. We present
an experimental approach to map the dissipation field during crack propagation in soft viscoelastic
solid. Specifically, we track randomly distributed tracer particles to measure the evolving
deformation field. The measured deformation field is then put into a nonlinear constitutive model
to determine the dissipation field. Our methodology was used to investigate the deformation and
dissipation fields around a propagating crack in a Polyampholyte (PA) hydrogel. The deformation
field measurements allowed us to assess whether the commonly assumed translational invariance
in viscoelastic fracture theories holds true in practical experiments. Furthermore, by combining
the obtained deformation fields with a nonlinear viscoelastic model, we captured the complete history of the dissipation field during crack propagation. We found that dissipation occurred even
at material points that are a few millimeters away from the crack tip. The mapped dissipation field
also enabled the separate determination of the intrinsic and dissipative components of fracture
toughness for the viscoelastic hydrogel.
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Time dependent fracture of soft materials: linear versus nonlinear viscoelasticity
Toughness of soft materials such as elastomers and gels depends on their ability to dissipate energy and to reduce stress concentration at the crack tip. The primary energy dissipation mechanism is viscoelasticity. Most analyses and models of fracture are based on linear viscoelastic theory (LVT) where strains are assumed to be small and the relaxation mechanisms are independent of stress or strain history. A well-known paradox is that the size of the dissipative zone predicted by LVT is unrealistically small. Here we use a physically based nonlinear viscoelastic model to illustrate why the linear theory breaks down. Using this nonlinear model and analogs of crack problems, we give a plausible resolution to this paradox. In our model, viscoelasticity arises from the breaking and healing of physical cross-links in the polymer network. When the deformation is small, the kinetics of bond breaking and healing are independent of the strain/stress history and the model reduces to the standard linear theory. For large deformations, localized bond breaking damages the material near the crack tip, reducing stress concentration and dissipating energy at the same time. The damage zone size is a new length scale which depends on the strain required to accelerate bond breaking kinetics. These effects are illustrated by considering two cases with stress concentrations: the evolution of spherical damage in a viscoelastic body subjected to internal pressure, and a zero degree peel test.
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- Award ID(s):
- 1903308
- PAR ID:
- 10167757
- Date Published:
- Journal Name:
- Soft Matter
- ISSN:
- 1744-683X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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A finite strain nonlinear viscoelastic constitutive model is used to study the uniaxial tension behaviour of chemical polyampholyte (PA) gel. This PA gel is cross-linked by chemical and physical bonds. Our constitutive model attempts to capture the time and strain dependent breaking and healing kinetics of physical bonds. We compare model prediction by uniaxial tension, cyclic and relaxation tests. Material parameters in our model are obtained by least squares optimization. These parameters gave fits that are in good agreement with the experiments.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0SM00097C
