Y-shaped cutting as a characterization method for the failure of soft elastic solids

Zhang, Bingyang

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https://hdl.handle.net/2142/113945 Copy

Description

Title

Y-shaped cutting as a characterization method for the failure of soft elastic solids

Author(s)

Zhang, Bingyang

Issue Date

2021-08-24

Director of Research (if dissertation) or Advisor (if thesis)

Hutchens, Shelby B.

Doctoral Committee Chair(s)

Hutchens, Shelby B.

Committee Member(s)

• Wagoner Johnson, Amy
• Elbanna, Ahmed
• Lambros, John

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Fracture mechanics
• Soft elastic solids
• Cutting
• Tearing
• Failure criteria

Abstract

Although both cutting and tearing create new surfaces at a crack tip through necessary energy investments, many questions regarding the fracture mechanics of highly deformable, soft materials under these failure scenarios remain. For example, in general, cutting energies cannot be quantitatively related to fracture energies measured from far-field tearing samples. This gap in the knowledge stems from the complex interplay between local failure process, mesoscale energy dissipation, and material constitutive nonlinearity during soft fracture. Additional complications arise from differences in loading geometry. These challenges motivated us to re-engineer a promising but under-utilized cutting characterization technique originally designed by Lake and Yeoh [Int. J. of Fracture, 1978] to test natural rubbers. We extend its applicable range and advantageous features to softer, silicone-based hyperelastic materials. The cutting method adopts a pre-stretched Y-shaped geometry which achieves a nominally frictionless cutting environment, while providing control over the crack tip geometry and cutting rate. At low cutting rates and wide leg angles, we observe a stick-slip cutting response. However, in general, the cutting energy is rate independent in the highly elastic materials we focus on. Most interestingly, we report, for the first time, a nonlinear, radius-dependent cutting energy response determined using a range of blade radii from nanoscale to microscale. At large blade radii, the sensitivity of cutting energy to radius increments depends on the material's constitutive response; In contrast, at a threshold length scale, cutting energy transitions to a plateau regime suggesting a threshold for failure. We find order-of-magnitude agreement between the threshold cutting energy within the plateau and the threshold fracture energy postulated by a modified Lake-Thomas theory. The core of the new theory predicts that microscopic damage at the crack tip necessary to create new surfaces extends beyond a classic single-molecular plane into a super-molecular damage zone whose minimum length scale is defined by the tip radius at the onset of cutting energy threshold transition. The combination of the threshold fracture energy and the minimum damage volume defines failure criteria for soft fracture which enables finite element implementations using a predetermined strain energy density cutoff. Quantitative agreement between the simulated results and experimental data for the sensitivity of cutting energy to increasing blade radius outside of the plateau suggests the role of material constitutive response in achieving local stress concentration necessary for fracture. Different materials exhibit different capabilities to accommodate large deformation. A more stretchable, neo-Hookean-like material more easily deforms around the crack tip and thus requires a larger failure force for the same blade radius increment as observed experimentally. These unique findings of cutting energy in silicone elastomers necessitate the incorporation of both threshold failure response and nonlinear material properties into a new physical picture of soft fracture that qualitatively and quantitatively connects far-field tearing with contact cutting. This picture of material deformability-mediated failure facilitates the discovery of a new dimensionless group that quantitatively maps the threshold cutting energy to the tearing energy in a linear proportionality by using ultimate properties obtained in uniaxial tension.

Graduation Semester

2021-12

Type of Resource

Thesis

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http://hdl.handle.net/2142/113945

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Copyright 2021 by Bingyang Zhang. All rights reserved.

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