Georgia Institute of Technology Materials Science and Engineering

Committee Members:

Prof. Christopher Muhlstein, Advisor, MSE

Prof. Christopher Luettgen, ChBE

Prof. Richard Neu, ME/MSE

Prof. Preet Singh, MSE

Prof. Aaron Stebner, ME/MSE

Deformation and Structure-Property Relations of Paper during Mode I Steady-State Crack Growth

Abstract:

Paper and pulp fiber-based materials have experienced an expansion in use from commodity applications (such as shipping containers) to integration into medical devices. Understanding how cracks form and grow allows for more reliable tear, such as in perforated papers, or for engineered crack growth resistance, such as prevention of web breaks or product failures. As a thin-sheet material, the effectiveness of single-parameter fracture mechanics crack tip parameters (K and J) break down. This dissertation creates and expands empirical knowledge of Mode I growing cracks in wood-fiber, machine-made paper using digital image correlation and tracking and an incremental strain framework approach. The work is divided into three parts.

The first section describes the mechanistic strain evolution to steady-state crack growth in paper. The crack tip was defined throughout the experiment with thresholded, cumulative strains. Strategically-placed incremental strain measurements of the full-field specimen surface illuminated the three-stage process from early gross plasticity to a transition to contained zones of active plasticity (ZAPs), with a final fast fracture event of a small remaining ligament. Crack growth rates and fracture mechanisms were also tied in to the appropriate damage stages. The next section characterized the full deformation of the two orthotropic extremes (cross-direction and machine-direction) of the fiber network. Polar decomposition of the deformation gradient allowed visualization of rigid-rotation around the forming steady-state crack tip and a zone of reversed plasticity was validated in the crack wake with the Hencky strain definition. A new, simple model was used to characterize the singularity of the incremental process zones.

In the final section, relative humidity (and ultimately the moisture content in the fiber network) was used to alter the stress-strain constitutive relationship to gain structure-performance insights of incremental process zones. The excessive water vapor disrupted the secondary hydrogen bonding and reduced the stress to failure but increased the strain. Therefore, the influences of mechanical properties (such as hardening, tensile strain, tensile stress, etc.) and structure were illuminated by the changes (or invariance) of each incremental process zone at different humidity levels and material orientations. Further, the sequence and magnitude of strain and rotation that formed a crack was identified and measured. The insights from this dissertation provide the foundation for more effective models of growing, steady-state cracks.