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Fracture phenomena in polystyrene

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Abstract

Fracture mechanics concepts, in terms of stress intensity factors, have been used in a study of the growth of cracks and crazes in Crystal Polystyrene in air at 293°K. Single edge notch tension specimens and tapered cleavage specimens have been tested over a wide range of strain rates and also at constant load.

It was initially found that the critical stress intensity factor K Ic (evaluated at crack instability) which should be a constant, independent of test method and specimen geometry, could apparently vary over a wide range. This phenomenon was shown to be caused by the presence of craze bunches at the crack tips; the size of these bunches being dependent upon the method of notching employed. A notching technique producing pure cracks was devised and as a result the lower bound value of K Ic at instability for the material was shown to be 1.05 MN/m3/2. A curve showing the relationship between crack speed and crack toughness K c was also obtained and a lower value of K c for crack initiation was extrapolated as being 0.78 MN/m3/2.

The role of inherent flaws in the fracture of unnotched tensile specimens and the consequences of crazing are discussed in terms of fracture stresses predicted from the notched tests using a Dugdale model.

Résumé

On a utilisé les concepts de mécanique de rupture, sous la forme de facteurs d'intensité des contraintes, pour l'étude de l'extension de fissures et d'irrégularités dans le polystyrène transparent, sollicité dans l'air à 293°K. Des éprouvettes de traction à simple entaille latérale et des éprouvettes profilées entraînant une rupture par clivage ont été essayées sous une gamme étendue de vitesses de déformations ainsi qu'à charge constante.

On a trouvé dès l'origine des essais que le facteur critique d'intensité des contraintes, KIc, mesuré au stade d'instabilité de la fissure, pouvait apparement subir d'importantes variations, alors qu'il devrait se maintenir constant, indépendant de la méthode d'essai et de la géométrie de l'éprouvette. Cette particularité s'est avérée être causée par la présence d'accumulations de discontinuités aux extrémités de la fissure. Leur dimension dépend du mode d'entaillage de l'éprouvette.

On a mis au point une technique d'entaillage qui produisit des fissures sans irrégularités et on a obtenu, dans ces conditions, la valeur limite inférieure de KIc à l'instabilité, égale à 1,05 MN/m3/2.

On a également obtenu une courbe montrant la relation entre la vitesse de propagation de la fissure et la ténacité Kc; par extrapolation, on en a tiré la valeur plancher de Kc pour l'amorçage de la fissure, égale à 0,78 MN/m3/2.

Le rôle que jouent les défauts internes dans la rupture d'éprouvettes de traction non entaillées et les conséquences qui peuvent découler de la formation d'irrégularités, sont discutés sur la base des contraintes de rupture que laisse prédire l'exploitation d'essais sur éprouvettes entailées à l'aide du modèle de Dugdale.

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Authors and Affiliations

  1. Mechanical Engineering Department, Imperial College of Science and Technology, S.W. 7, London

    G. P. Marshall, L. E. Culver & J. G. Williams

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  1. G. P. Marshall
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  2. L. E. Culver
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  3. J. G. Williams
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Marshall, G.P., Culver, L.E. & Williams, J.G. Fracture phenomena in polystyrene. Int J Fract 9, 295–309 (1973). https://doi.org/10.1007/BF00049197

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  • DOI: https://doi.org/10.1007/BF00049197

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