When we fold a paper and then tear it, why does the tear

Paper is an aggregated web of vegetable cellulose fibres usually prepared from wood pulp. The fibres in this planar web are bound to each other by a kind of bonding known as hydrogen bonding.

The internal structure of paper can be described as follows. There is a distribution of the fibre sizes; the finer ones interpenetrate into the web of larger ones and small clusters of these hold together the larger ones and so on.

The fibre size ranges from sub micron to millimetres. This can be seen at the freshly torn edges of a paper.

In the writing paper, its surface has a top layer of fine fibre particles, which gives the glaze to it. While the good varieties of paper have finer particles to provide better glaze, the relatively inexpensive varieties like the news print paper have very little or no glazing layer.

That is why the larger fibres of the news print paper are fairly clearly visible to the naked eye.

The fine fibre particles, binding the larger ones, provide strength to the paper by distributing the tearing load in many directions. In the process of tearing, we apply a shear load locally and the paper is sheared.

Actually, the individual fibres are stronger than the aggregate; that is why when a paper is torn, the individual fibres are not torn but only the bonding between adjacent fibres is snapped. When we fold the paper, the fiber-to-fiber bonding along the fold gets loosened and so the shear load acts more effectively along this direction rather than getting diverted by the larger and stronger bonded fibres.

The folded paper not only tears along the fold, but also requires a lower force to tear than an unfolded paper.