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The aim of this study is to elucidate post-elastic mechanism and the nature of failure of a machine-made paper under the condition of uniaxial tensile or edgewise compression loading. In order to obtain a direct evidence of the inelastic deformation of the sheet related to its structural changes, observations were made both at the sheet surface and on cross sections of the sheet after reaching prescribed strain levels.In this study, the oriented 80 g/m2, and the 240 g/m2 NBKP machine-made sheets were used. The damaged sites, that is microcracks, on the specimen surface were clearly detected as bright spots, known as electron charging, under SEM observing conditions. From the preliminary test, it was found that these charging spots were not caused by artifacts, but were generated only by the applied external force. The damages occurring within the sheet under loading were counted by using a digitizer along a set of scan lines depicted on the enlarged photo mosaic, which covered the whole sectional view of the test specimen.As a result, the strain at elastic limit of the tested sheets was 0.2%, and the residual strain was increased proportionally as increment of the applied strain. The sheets always behaved as a orthotropic body such as a oriented-fiber reinforced material. This may suggest that mechanical properties for MD and CD highly depend on the amount of load-bearing fibers, thick-walled fibers, laid down in MD and CD, which correspond to the framework components within a sheet. On the other hand, the matrix components, such as thin-walled fibers, fines, loosened fibrils, and exudated hemicelluloses etc., should contribute to carry or to redistribute stress concentrated locally among the load-bearing fibers. As the strain increased into the inelastic regime of the stress-strain curve, partial debondings at the interfiber crossings took place and developed into the minute cracks. The number of the damages increased drastically in the inelastic region. As the strain level approached to the failure point, the fiber breakages began to take place, and they always occurred on the thin-walled fiber aligned parallel to the direction of loading. This damage-developing process was clearly observed on the CD specimen. Furthermore, the internal structure of a sheet also changed by means of the externally applied force. The bonding-ratio (BR) of specimen strained close to the failure point was approximately 7% less than that of the unstrained specimen.These experimental evidences strongly suggest that the post-elastic region of sheet deformation is characterized by the permanent interfiber deformation, accompanying with the partial debonding between the load-bearing fibers and the matrix substances, and the breakage of fiber itself.On the contrary, compression failure is a very localized phenomenon. The appearance of the failure zone shows pronounced out-of-plane deformation such as creases of bucklings. At the failure zone, fiber debondings and fiber bucklings were prominent, but there is no evidence of such damages even at the site 2 mm apart from the crease.

Inelastic behavior of machine-made paper related to its structural changes.