Crystallographic fatigue crack growth in incompatible aluminum bicrystals: Its dependence on secondary slip

The secondary slip behavior ahead of crystallographic fatigue cracks and its effect on the crack growth near the grain boundaries (GBs) in tilt nonsymmetrical aluminum bicrystals under constant cyclic stress amplitude have been systematically examined. The displacement field ahead of short crack tips near the interfaces in two specimens has been measured by using a microfiducial grid technique. It has been observed that the critical persistent slip band (PSB) ahead of a short crack tip near the GB in a middle misoriented bicrystal was able to develop as long as the primary one and resulted in a temporary stage II growth. As a longer crystallographic crack grew into the grain boundary affected zone (GBAZ), activation of the critical slip ahead of the crack front and crack branching along the critical PSB occurred in all groups of the aluminum bicrystals, which reveals a crucial role of the critical slip in increasing the crack opening and triggering the slip in the adjacent grain. On the other hand, cross slip became the dominant slip mode ahead of the crystallographic crack front near the GB in a bicrystal of larger misfit angles and drove the crack along the cross PSB, a steep path with a remarkably high growth rate, until it propagated into the GBAZ. The resultant stress on the secondary slip system ahead of a crack front near the interface contributed by the internal stress due to both intergranular and intragranular incompatible strain, as well as the enhanced crack tip stress, has been evaluated and rationalizes the activation of the secondary slip systems. On leave from Taiyuan University of Technology, Taiyuan, Shanxi 030024, People’s Republic of China.     

Crystallographic fatigue crack growth in incompatible Aluminum Bicrystals: Its Dependence on Secondary Slip

The secondary slip behavior ahead of crystallographic fatigue cracks and its effect on the crack growth near the grain boundaries (GBs) in\([12\bar 1]\) tilt nonsymmetrical aluminum bicrystals under constant cyclic stress amplitude have been systematically examined. The displacement field ahead of short crack tips near the interfaces in two specimens has been measured by using a microfiducial grid technique. It has been observed that the critical persistent slip band (PSB) ahead of a short crack tip near the GB in a middle misoriented bicrystal was able to develop as long as the primary one and resulted in a temporary stage II growth. As a longer crystal- lographic crack grew into the grain boundary affected zone (GBAZ), activation of the critical slip ahead of the crack front and crack branching along the critical PSB occurred in all groups of the aluminum bicrystals, which reveals a crucial role of the critical slip in increasing the crack opening and triggering the slip in the adjacent grain. On the other hand, cross slip became the dominant slip mode ahead of the crystallographic crack front near the GB in a bicrystal of larger misfit angles and drove the crack along the cross PSB, a steep path with a remarkably high growth rate, until it propagated into the GBAZ. The resultant stress on the secondary slip system ahead of a crack front near the interface contributed by the internal stress due to both intergranular and intragranular incompatible strain, as well as the enhanced crack tip stress, has been evaluated and rationalizes the activation of the secondary slip systems.     

A THREE-DIMENSIONAL FINITE ELEMENT ANALYSIS FOR A CRYSTALLOGRAPHIC CRACK NEAR THE INTERFACE OF AN INCOMPATIBLE BICRYSTAL

On the basis of modelling bicrystal deformation, using three-dimensional, anisotropic finite elements, the problem of a crystallographic crack approaching the interface in a [111] tilt bicrystals of 90? misfit angle under shear loads is solved. The influence of thickness direction material heterogeneity across the interface on the distribution of the stresses, strains and crack sliding displacements along the crack front near the interface has been revealed. As the crack approaches the interface, those mechanical parameters are considerably changed by the heterogeneity across the interface. Remarkable variations in the stresses and strains along the crack front have also been identified and are referred to the different constraint across the thickness. The maximum stress may shift from the crack tip to the interface ahead of it, where, as suggested by numerical results and previous experimental observation, a new fracture process core may be activated. The interface-induced crack shielding or antishielding under mode II and III loading is analyzed and discussed.     

On the interaction among stage I short crack,slip band and grain boundary: a FEM analysis

A FEM model of a stage I short crack in polycrystals was constructed based on the micromechanics and the physical process of its growth. The effective stress and effective plastic strain at the growing stage I short crack tip is slightly affected by the coplanar slip except in the vicinity of the grain boundary. Stage I growth is a mode transition process from pure shear to pure tensile, which is governed by the interaction among the crack, the slip at the crack tip and the approached grain boundary. The coplanar slip at the stage I crack tip enhances plastic CTSD and promotes the shear mode growth, nevertheless the grain boundary hindering the coplanar slip promotes the tensile mode growth. The apparent fast growth of stage I short cracks is accounted for by the enhanced CTSD resulting from the micro-plasticity in the coplanar PSB at the crack tip; and the minimum growth tate in the vicinity of a grain boundary is correlated with the expended CTSD and internal stress due to incompatible deformation at the boundary. The closure of stage I short crack in the first grain on the path is negligible, but dramatically increases as the crack deflects at the boundary entering into the second grain on its path.

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Résumé On établit un modéle d'une fissure courte en stade I dans des polycristaux en se basant sur la micromécanique et le processus physique de sa croissance. La contrainte effective et la déformation plastique effective à l'extrémite d'une fissure courte au stade I de croissance sont légèrement influencées par les glissements coplanaires, sauf au voisinage de la frontière des grains. La croissance en stade I est un processus de passage d'un cisaillement pur à une traction pure, régi par l'interaction entre la fissure, le glissement à son extrémité et la présence de la frontière de grain proche. Le glissement coplanaire à l'extrémité de la fissure accroit la déformation plastique à cet endroit, et enconrage le mode de croissance par cisaillement, tandis que la frontière de grain bloque le glissement coplanaire et favorise un mode de croissance par traction. On attribue la croissance apparemment rapide de fissures courtes en stade I à la plastification accrue à fond d'entaille résultant de la micro-plasticité dans les plans de bandes de glissement à l'extrémité de la fissure. Par ailleurs, la vitesse minimale de croissance de la fissure au voisinage d'une frontière de grain est en corrélation avec l'accroissement de déformation plastique et avec la tension interne due à une incompatibilité de déformation à cette frontière. L'accomplissement du stade I est néglicable tant qu'une fissure courte se trouve dans le premier grain de son parcours. II s'accélère de manière spectaculaire lorsqu'elle dévie à la fontière du grain suivant et qu'elle pénètre dans celui-ci.

     

Short crack growth behavior in a particulate-reinforced aluminum alloy composite

Short and long crack propagation behaviors in a coarse A1₂O₃ particulate-reinforced 6061 aluminum alloy composite (Al₂O₃/6061 Al) are investigated and compared under different ranges of tensile-compressive cyclic stress. It is found that short cracks up to 400 μm in length propagate in a shear-dominant mode at maximum cyclic stress level below the fatigue limit until they are permanently trapped by the surrounding particles. The microstructure sensitivity of short crack growth in the composite decreases as the short crack length and/or applied stress range increase. The characteristics of short cracks and the mechanisms of short crack trapping by particles in the material are discussed. leave from Taiynan University of Technology This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

VECTOR CTD CRITERION APPLIED TO MIXED MODE FATIGUE CRACK GROWTH

Abstract— This work is aimed at developing a general parameter based on the deformation intensity at a mixed mode crack tip to predict crack growth behaviour, especially in the near threshold region. Being a mechanisms-related parameter, the vector crack tip displacement (CTD) is defined as a vector summation of CTOD and CTSD_c which act, respectively in the directions of mode I and mode II fatigue crack growth. The basic assumption is that both direction and rate of mixed mode fatigue crack growth are governed by the vector ΔCTD, which represents the resultant of the "driving force"at the crack tip. The analytical predictions obtained by using the vector ΔCTD are in good agreement with the reported experimental results of mixed mode I and II fatigue cracks.