Crazing

This article is about crazing in polymers. For crazing in ceramic glazes, see Glaze defects § Crazing.
Crazes in polystyrene (PS)

Crazing is a yielding mechanism in polymers characterized by the formation of a fine network of microvoids and fibrils.[1][2] These structures (known as crazes) typically appear as linear features and frequently precede brittle fracture. The fundamental difference between crazes and cracks is that crazes contain polymer fibrils (5-30 nm in diameter[3]), constituting about 50% of their volume,[4] whereas cracks do not. Unlike cracks, crazes can transmit load between their two faces through these fibrils.

Crazes typically initiate when applied tensile stress causes microvoids to nucleate at points of high stress concentration within the polymer, such as those created by scratches, flaws, cracks, dust particles, and molecular heterogeneities. Crazes grow normal to the principal (tensile) stress, they may extend up to centimeters in length and fractions of a millimeter in thickness if conditions prevent early failure and crack propagation.[5] The refractive index of crazes is lower than that of the surrounding material, causing them to scatter light. Consequently, a stressed material with a high density of crazes may appear 'stress-whitened,' as the scattering makes a normally clear material become opaque.[6]

Crazing is a phenomenon typical of glassy amorphous polymers,[7] but can also be observed in semicrystalline polymers.[8][9] In thermosetting polymers crazing is less frequently observed because of the inability of the crosslinked molecules to undergo significant molecular stretching and disentanglement,[10][11] if crazing does occur, it is often due to the interaction with second-phase particles incorporated as a toughening mechanism.[12][13]

Crazing and fracture during a tensile test on a transparent polymer (x0.25 speed)
  1. ^ Kambour, R. P.; Holik, A. S. (1969). "Electron microscopy of crazes in glassy polymers: Use of reinforcing impregnants during microtomy". Journal of Polymer Science Part A-2: Polymer Physics. 7 (8): 1393–1403. Bibcode:1969JPoSB...7.1393K. doi:10.1002/pol.1969.160070809. ISSN 0449-2978.
  2. ^ Kambour, R. P.; Russell, R. R. (1971-04-01). "Electron microscopy of crazes in polystyrene and rubber modified polystyrene: use of iodine-sulphur eutectic as a craze reinforcing impregnant". Polymer. 12 (4): 237–246. doi:10.1016/0032-3861(71)90048-6. ISSN 0032-3861.
  3. ^ Kramer, E. J., & Berger, L. L. (1990). Effect of Molecular Variables on Crazing and Fatigue of Polymers. In H. H. Kausch (Ed.), *Crazing in Polymers, Volume 2: Advances in Polymer Science* (pp. 1-67). Springer-Verlag.
  4. ^ Kambour, R. P. (1964-01-01). "Structure and properties of crazes in polycarbonate and other glassy polymers". Polymer. 5: 143–155. doi:10.1016/0032-3861(64)90128-4. ISSN 0032-3861.
  5. ^ Kinloch, A. J.; Young, Robert J. (1983). Fracture behaviour of polymers. London; New York : New York, NY, USA: Applied Science Publishers; Sole distributor in the USA and Canada, Elsevier Science Pub. Co. ISBN 978-0-85334-186-4.
  6. ^ Haward, Robert Nobbs; Haward, Robert N., eds. (1997). The physics of glassy polymers (2 ed.). London: Chapman & Hall. ISBN 978-0-412-62460-5.
  7. ^ Sauer, J. A., & Chen, C. C. (1983). Crazing and Fatigue Behavior in One- and Two-Phase Glassy Polymers. In H. H. Kausch (Ed.), *Crazing in Polymers, Volume 1: Advances in Polymer Science* (Vol. 52/53, pp. 169-223). Springer-Verlag Berlin Heidelberg.
  8. ^ Friedrich, K. (1983). Crazes and Shear Bands in Semi-Crystalline Thermoplastics. In H. H. Kausch (Ed.), *Crazing in Polymers, Volume 1: Advances in Polymer Science* (Vol. 52/53, pp. 226-271). Springer-Verlag Berlin Heidelberg.
  9. ^ Narisawa, I., & Ishikawa, M. (1990). Crazing in semicrystalline thermoplastics. In H. H. Kausch (Ed.), *Crazing in Polymers Volume 2 Advances in Polymer Science* (pp. 355-389). Springer-Verlag.
  10. ^ Arends, Charles B., ed. (1996). Polymer toughening. Plastics engineering. New York: Marcel Dekker. ISBN 978-0-8247-9474-3.
  11. ^ Kinloch, A. J. (1985). Mechanics and Mechanisms of Fracture of Thermosetting Epoxy Polymers. In R. J. Morgan (Ed.), Epoxy Resins and Composites I (pp. 45-67). Springer-Verlag.
  12. ^ Yee, A. F.; Pearson, R. A. (1986-07-01). "Toughening mechanisms in elastomer-modified epoxies". Journal of Materials Science. 21 (7): 2462–2474. Bibcode:1986JMatS..21.2462Y. doi:10.1007/BF01114293. ISSN 1573-4803.
  13. ^ Sue, Hung-Jue (1992-06-01). "Craze-like damage in a core-shell rubber-modified epoxy system". Journal of Materials Science. 27 (11): 3098–3107. Bibcode:1992JMatS..27.3098S. doi:10.1007/BF01154125. ISSN 1573-4803.