Mechanical properties of paper are closely related to the geometric factors of fiber assemblages such as the number of fibers, fiber length distribution, fiber weight distribution,
etc., which describe the network structure of paper as well as the single fiber properties. In the present paper, the dependence of in-plane elastic moduli of handsheets made from softwood and hardwood bleached kraft pulps on the basis weight which is associated with the number of fibers per unit area,
Nf, has been studied.
Two independent in-plane elastic moduli, Young's modulus
Exy and Poisson's ratio ν
xy were determined by use of a biaxial tensile tester of Bistron type that was modified for testing paper.
The results obtained are summarized as follows;
(1) The dependence of Young's modulus,
Exy on the basis weight is caused by the associative effect of thickness
T and
Exy (defined by
Exy T), both of which depend on the basis weight.
(2) Since the Young's modulus of paper varies with thickness which can not be defined explicitly, the Young's modulus seems to be unsuitable as a mechanical constant.
(3) The newly defined
Exy is a linear function of the basis weight or
Nf within the limit of this experiment but nonlinearity may appear for weights less than 20g/m
2. The gradient in the linear region of
Exy-Nf relation for the softwood pulp is much larger than that for the hardwood pulp and this quantity may be used as a new characteristic value of paper.
(4) The Poisson's ratio for the hardwood pulp is slightly larger than that for the softwood pulp but both of them do not markedly change with the basis weight or
Nf.
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