Micromechanics of the through-thickness deformation of paperboard

Abstract

An experimental investigation of the behavior of paperboard has been performed, focusing on the identification of the mechanisms of through-thickness deformation. Experiments have been conducted at the microscopic and macroscopic levels, the difference between the two being the length scale. Experiments at the microscopic level were performed in a scanning electron microscope, allowing concurrent viewing of the deformation as it took place with acquisition of load vs. displacement data. The experiments at the macroscopic level confirm the mechanisms observed at the microscopic level and provide more accurate, continuum-level stress-strain data. The motivation for the investigation is the modeling of the creasing process, in which a sheet of paperboard is punched and folded along a narrow line to create a corner for packaging. Creasing experiments indicate that out-of-plane shear damage during punching and normal delamination during folding govern the quality of the resulting crease. Experiments in out-of-plane tension, compression, and simple shear have been performed to investigate the behavior of paperboard under these simple loading conditions. The results show that normal and tangential delamination at the interfaces between layers is of extreme importance in the behavior of paperboard in tension and shear. Damage in the form of micro cracks occurs almost from the onset of strain and culminates in large-scale delamination coincident with a large decrease in the strength of the material. In compression, the behavior is mostly elastic and is governed by densification of the material. The brief initial stages of deformation involve plastic elimination of voids. This is followed by non-linear elastic stiffening of the material via densification.