MECHANISTIC THEORY OF THE LOAD-ELONGATION PROPERTIES OF PAPER - 1. A UNIFORM STRAIN THEORY FOR PAPER.

The theory of the load-elongation of paper presented at the 1965 Cambridge symposium of the (then) British Paper and Board makers' Association has been improved. It has developed into a mechanistic theory whose final expressions do not contain any empirical constants. The theory inter-relates the elastic modulus, tensile strength, failure strain, and zero-span tensile strength of papers with both a random and a biased fiber-orientation distribution. Agreement is obtained between the theory and data for handsheets of five widely-differing pulps, after assuming that increases in sheet modulus with refining reflect a proportionate rise in the number of active fibers developing a load during initial sheet straining. The theory proposes that the proportion of active fibers falls off during further straining to failure, and that, in well-bonded handsheets, as few as half as many fibers are active at failure as during elastic straining. For publication purposes, the paper has been broken into four parts. The first describes the approach employed in developing the theory, the basic assumptions, and the characteristics of the fibers taken into account.