Using strain field mining to reveal the spatial distributions of tensile, fatigue, and fracture damage accumulation in paper

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Na, Yoon Joo
Muhlstein, Christopher L.
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The most common nonwoven fiber composite material, paper, has a porous, heterogeneous fiber network structure and complicated mechanical properties. The mechanical properties of commercial, machine made papers are orthotropic and are sensitive to loading rate, moisture content, and temperature. Thus, defining the constitutive relationship of paper has remained as a challenge due to the stochastic nature of the structure and countless variables that affect the mechanics of paper. Moreover, the technology to non-destructively characterize the three-dimensional network topography at the fiber length scale is not readily available. This presents a critical barrier to establishing the structure-property relationships of paper. Here, I approached the problem with a fundamentally different strategy and used the structure of the strain fields as a proxy for the network topography. The strain fields of paper from tensile, fatigue, tearing experiments revealed new information about each damage mechanisms. During the tensile deformation, the interplay between the axial and the transverse motions in the fiber network resulted in specimen-orientation-dependent (MD and CD) parameters such as Poisson's ratio, hot spot length scales, and the degree of nonaffinity, D. These metrics were direct manifestation of the anisotropic fiber network in paper. Next, I used strain field mining to track the fatigue crack lengths and quantified crack growth rates during cyclic and constant loading conditions. The fracture profiles and the crack growth rates revealed that there was a unique fatigue damage mechanism in paper which induced the fiber fracture rather than the fiber pull-out. Moreover, I found that the pre-applied creep damage in paper can significantly reduce the fatigue crack growth rate and extend paper's high cycle fatigue life. Lastly, from the strain fields of tearing specimens, I was able to characterize paper's crack tip process zone and the zone of active plasticity (ZAP) whose shape depended on the orientation of the fiber network. Although paper has a completely different structure and failure mechanism from metals, I found that tearing of paper also followed a steady-state process, which was previously observed in thin sheet aluminum foils.
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