STATISTICAL VARIATION OF PAPER STRENGTH

The variability of paper strength was measured using a total of 4000 s pecimens of newsprint and copy paper, in cross-direction (CD) and mach ine direction (MD). The results were analyzed statistically and compar ed to computer simulations, The coefficient of variation of grammage w as the same in the two paper grades, but the coefficient of variation of tensile strength in the newsprint was almost two times larger than in the copy paper. In both papers, the strength variation is smaller i n CD than in MD. The probability distribution function of strength was best described by a double logarithmic distribution. The coordinates of breaking points form an oblong pattern along the average stress-str ain curve of each paper. In computer simulations, similar results were obtained only when local failures were assumed to be strain controlle d rather than strength controlled. In real paper, such variations in b reaking strain could follow from a spatially nonuniform drying shrinka ge.