Fracture Energy Release Rate in Nylon Fibers S. MICHIELSEN Georgia Institute of Technology, School of Textile and Fiber Engineering, Atlanta, Georgia 30332-0295 Received 15 April 1997; accepted 16 August 1997 ABSTRACT: The effect of relative humidity on the fracture energy release rate, G Ic , for single nylon 6,6 fibers has been determined previously. In this article, it is shown that G Ic is independent of relative humidity for moisture contents of ú 2.3% once the plastic zone correction is made. G Ic is compared with various proposed mechanisms to account for fracture energy. It is shown that the energy required to disrupt or ‘‘melt’’ the crystals in the plastic zone accounts for the majority of the energy required to break the specimen, and should be considered explicitly in future analyses of fracture in semicrystalline polymers.  1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 1541 – 1544, 1998 Key words: nylon; fracture; plastic zone; fracture energy INTRODUCTION the surface free energy ) . Although this described the case he studied well, additional dissipation through a ‘‘plastic deformation zone’’ around the As industry tries to replace metals with plastics, it has become increasingly important to improve crack tip has been identified to account for the fact that G Ic measured experimentally is typically our understanding of the fracture process in plas- tics. Already, the use of fracture mechanics, bor- much larger than that obtained through only sur- face energy considerations. rowed from the metals industry and adapted to In two previous articles, Michielsen 2,3 described plastics, has found its way into the design process. the failure of high-strength nylon 6,6 monofila- A critical property in this regard is the fracture ments using fracture mechanics. These articles energy release rate, G , the energy released per showed that the fracture energy release rate, G Ic , unit area of crack (or flaw) growth. The fracture depended on both moisture and orientation. G Ic energy release rate under plane strain loading was some eight times larger for a transverse conditions is given as G I . Above its critical value, break in the highly oriented fibers than for unori- G Ic , the energy released as the crack grows is ented nylon plaques, whereas the initial Young’s greater than that required to create additional modulus only increased Ç 2.2 times. In addition, crack area, and the crack grows rapidly to com- it was shown that G Ic was some 10 times larger plete fracture. Due to the importance of G Ic or for a transverse break than for axial splitting of equivalently, K Ic , the critical stress intensity fac- the fibers, confirming the long-held belief that the tor, to the design of parts, there have been many increase in strength along an oriented fiber is ac- attempts to determine what physical features of companied by a loss in strength transverse to the the material control G Ic , starting with the pio- fiber axis. neering work of Griffith. 1 He identified G Ic with It was also shown that, as the relative humidity the energy required to create new surface ( i.e., (RH) increased from 0.2 to 50%, G Ic decreased by a factor of 2. At still higher RH, the G Ic decreased Journal of Applied Polymer Science, Vol. 67, 1541 – 1544 ( 1998 )  1998 John Wiley & Sons, Inc. CCC 0021-8995/98 / 091541-04 only slightly. However, Table I, reproduced in 1541 8E13 4789 / 8e13$$4789 12-17-97 16:34:23 polaas W: Poly Applied