Strain-at-break, is defined as the ratio of the increase in length until the onset
of rupture to the original length. Stretch-at-break is the ratio of the length at
rupture to the original length. Both has units of m/m or as a percentage.
Tensile energy absorption, TEA, or tensile strain energy, is the amount of
energy per unit area of the paper absorbed during straining until the onset of
rupture in a tensile test. It can be illustrated as the area under the loadelongation
curve and has the units J/m2
.
Tensile stiffness is determined from the slope of the initial linear elastic part
of the load-elongation curve. Tensile stiffness is the force divided by the
elongation and the width of the strip, and it has the unit kN/m. The elastic
modulus can be obtained by dividing the tensile stiffness by the thickness of
the strip. The unit for elastic modulus is MN/m2
, or MPa.
The nature of the fracture is dependent on the degree of bonding in the sheet
(Page 1969, Davison 1972). A high degree of bonding leads to a large
propagation of fibre breaks, whereas in a poorly bonded sheet the dominating
process in the fracture zone is that fibres are pulled out of the network as the
fibre-fibre bonds break. Refining improves the tensile properties in several
ways. The fibres become more flexible, which facilitates the formation of
fibre-fibre bonds. Also the fines content in the pulp is increased, leading to an
increase in bond strength. Wet pressing increases the density and improves
the tensile properties as a result of an increase in bonded area. The stretch-atbreak
is highly dependent on the strain during the drying of the paper; freely
dried paper has a much higher stretch-at-break than paper dried under
restraint. Seth (1990) showed that the tensile strength increased with
increasing density and decreased with decreasing fibre strength. He also
showed a similar correlation for stretch-at-break. Further, Seth showed that
both tensile strength and stretch-at-break increased with increasing fibre
length. Paavilainen (1993a) saw a decrease in tensile strength with increasing
coarseness and suggested that the most important factors for high tensile
strength are good bonding ability and high intrinsic fibre strength. In a
different study, Paavilainen (1993b) continued the discussion, suggesting that
the tensile strength is determined by the bonded area, thus collapsibility,
external fibrillation, amount of fines and especially wet fibre flexibility.
Mohlin at al. (1996) studied the impact of fibre deformation on sheet
strength. They saw a decrease in tensile and tensile stiffness indices with
increasing total number of fibre deformations, while the stretch-at-break was
increased with decreasing shape factor.