ABSTRACT
Flaw Size A wide variety of flaws may be found in newsprint: pinholes, shives, slime holes, calender cuts, crepe wrinkles, etc. A general discussion of defects is presented by Snider [79]. A number of older studies identified shives, or thick fiber bundles, as the principal cause of newsprint failure during conversion. Shives are typically bonded quite poorly to the surrounding network and can debond comple tely at low stress, so they do act as cracks. In 1965 Sears et al. [80] studied paper breaks in a specially designed web strainer that imposed gradually increasing strain on the web as it passed through a series of rolls at high speed. They found that the web typically failed at strains as low as 20% of the measured stretch. All but five of the 3200 breaks they observed could reasonably be attributed to some type of defect:
21 were due to slime spots or holes 14 seemed to be due to hairs or hair-induced cuts 2 were caused by preexisting wrinkles 3 appeared to be due to roll damage
3155 were attributed to shives
The effect of shives was most significant for shives near or on the edge of the sheet. Note that LEFM tells us that a crack at the edge of the sheet produces the same stress intensity as a crack approximately twice as long in the center of the sheet (see Appendix). Further analysis by Macmillan et al. [81] identified a potentially harmful shive as one “greater than 3.5 mm long, greater than 0.12 mm wide, and approaching half the thickness of the paper.” Recent theoretical work by Ferahi et al. [30] con firmed the importance of small cracks such as those that would result from a debonded shive and provided quantitative results of the effects of crack orientation and thickness on the stress intensity factor. The data in Fig. 23 demonstrate that a reduction in the thickness of shives would be far more effective than a reduction in the length of the shives.
