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The relationship between crack tip opening displacement, local strain and specimen geometry

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Abstract

A method is described whereby values of crack tip opening displacement (COD) can be measured at any section of a pre-cracked or slotted specimen. The relationship between COD and the degree of transverse strain at the notch root of a Charpy specimen has been investigated. A specimen will show some portion in plane strain up to a COD of about one twenty-fifth of the thickness. Transverse contraction results in smaller COD values, for a particular clip gauge displacement, at the outside than at the specimen midsection.

An equation is presented relating COD to longitudinal notch root strain for Charpy specimens with root radius 0.002 in. COD and notch root strain are found to be not directly proportional. This finding is discussed in relation to the concept of a “miniature tensile specimen” at the notch root. The effect of specimen geometry on the value of COD at fracture initiation, (COD)c, has been investigated. Neither a change in specimen thickness nor a change from a fatigue pre-crack to a 0.002 in. root radius notch has been found to significantly affect (COD)c. The material used in this research was, for the most part, a low alloy pressure vessel steel, A533B.

Résumé

On décrit une méthode suivant laquelle il est possible de mesurer le COD de l'extrémité d'une fissure en une section quelconque d'une éprouvette préfissurée ou nantie d'une entaille. On a étudié la relation entre le COD et le degré de déformation transversale à la racine de l'entaille d'éprouvettes Charpy. Dans certains cas, on a constaté un état plan de déformation sur une portion appréciable d'éprouvettes dont le COD atteignait 1/25° de l'épaisseur. Une contraction transversale se traduit par des valeurs du COD plus faibles vers les bords que à mi section de l'éprouvette, dans le cas d'un déplacement donné de la clip jauge. On présente une relation entre le COD et la déformation longitudinale à la racine d'une entaille d'éprouvette Charpy avec rayon d'arrondi de 0,05 mm. Cette relation n'exprime pas une proportionnalité directe. Cette constatation est discutée en se reliant au concept d'une “éprouvette de traction miniature” qui se situerait au niveau de la racine de l'entaille. L'influence de la géométrie de l'éprouvette sur le COD critique correspondant à l'amorçage de la fissure—(COD)c—a été étudiée. On a trouvé que le (COD)c n'était pas affecté de façon significative par un changement de l'épaisseur de l'éprouvette, ou par le passage d'une préfissuration par fatigue à une entaille d'arrondi égal à 0,05 mm. La plupart des essais afférant à cette recherche ont été exécutés sur de l'acier ASTM A533 grade B-Classe 1.

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Author notes

  1. J. N. Robinson

    Present address: Department of Metallurgy and Materials Science, Imperial College, London, England

Authors and Affiliations

  1. Materials Department, School of Engineering and Applied Science, University of California, 90024, Los Angeles, California, USA

    J. N. Robinson & A. S. Tetelman

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  1. J. N. Robinson
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  2. A. S. Tetelman
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Robinson, J.N., Tetelman, A.S. The relationship between crack tip opening displacement, local strain and specimen geometry. Int J Fract 11, 453–468 (1975). https://doi.org/10.1007/BF00033532

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  • DOI: https://doi.org/10.1007/BF00033532

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