Abstract
Fully plastic structures are desirable to help detect impending failure. For stability, the displacement per unit drop in load due to any crack growth must be less than the compliance of the surrounding structure. The corresponding crack growth ductility is less with asymmetric configurations, than with the symmetric ones usually studied. Examples are given of face-grooved plates with unequal flank angles, staggered cracks, and single cracks near welds in joints under tension or bending. Further examples are the tendency of cracks on a microscopic scale to become rough, to zig-zag, or even to spiral (in round bars).
The maximum triaxiality can occur on a single slip line between staggered cracks from opposing faces of a plate, according to a slip line analysis. Except at the lowest triaxiality, 1018CF steel with a 3 mm ligament became unstable even when the compliances of the surroundings totalled only 6 × 10−6 mm/N. For higher triaxialities the limit load was not reached and the ensuing unstable fractures usually turned to cleavage.
In plates, the slow,stahle, plastic extension of slant-mode through-cracks sometimes abruptly changed to cleavage fracture, even at 10°Cabove both the 0.51 mm lateral expansion and Charpy 34.1 J (25 ft-Ib) transition temperatures, either sufficient to meet the ASME Code. In structures, this means that fragmentationcan intervene even during slow, “low-triaxiality”, stable crack growth in material meeting Code requirements.
Résumé
Des constructions à caractéristiques totalement plastiques sont souhaitables pour permettre la détection de risques de rupture. Pour conserver la stabilité, il faut que le déplacement par unité de chute de la charge associée à toute croissance d'une fissure soit inférieur à la compliance avoisinante de la construction. La ductilité vis-à-vis de la croissance de la fissure est plus faible en configurations asymétriques que dans les configurations symétriques généralement utilisées dans les études.
Des exemples de plaques comportant des entailles de surface à angles de bordes inégaux, de fissures en quinconce et de fissures simples au voisinage de joints soudés soumis à traction ou à flexion sont fournis. D'autres exemples montrent à l'échelle microscopique la tendance pour les fissures à devenir grossières, à zigzaguer ou même à se spiraler (dans des barreaux ronds).
Conformént à une analyse des lignes de glissements, a la triaxialité maximum peut survenir sur une ligne simple de glissement entre des fissures en quinconce partant de la face opposée d'une plaque. A l'exception du cas de triaxialité la plus faible, l'acier 1018 CF avec un ligament de 3 mm devient instable même lorsque les compliances de la structure avoisinante ne totalisent ue 6.10−6 mm/N. Pour des triaxialités plus importantes, la charge limite n'a pas été atteinte, et les ruptures instables qui résultent se muent généralement en clivages.
Dans les tôles, on a constaté que l'extension lente, stable et plastique de fissures traversantes obliques peut soudain se muer en rupture fragile, même à 10°C au dessus de la température correspondant à une expansion latérale de 0.51 mm, ou au-dessus de la température de transition Charpy V correspondant à 25 ft-lb, ces deux critères étant pourtant conformes au code ASME.
Dans les constructions, ceci signifie qu'une fragmentation peut survenir même au cours d'une croissance lente, stable et “en faible triaxialité” d'une fissure dans un matériau qui satisfait aux prescriptions du code.
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McClintock, F.A. Reduced crack growth ductility due to asymmetric configurations. Int J Fract 42, 357–370 (1990). https://doi.org/10.1007/BF01185957
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DOI: https://doi.org/10.1007/BF01185957