scholarly journals Effect of Crack Geometry on Dislocation Nucleation and Cleavage Thresholds

1998 ◽  
Vol 539 ◽  
Author(s):  
L.L. Fischer ◽  
G.E. Beltz
Keyword(s):  
Continuum Model ◽  
Crack Tip ◽  
Critical Energy ◽  
Dislocation Nucleation ◽  
Mode I ◽  
Critical Mode ◽  
Critical Energy Release Rate ◽  
Material Behaviors ◽  
Mode I Loading ◽  
Crack Tip Blunting

AbstractA continuum model based upon the Peierls-Nabarro description of a dislocation ahead of a crack is used to evaluate the critical mode I loading for dislocation nucleation at the tip of a finite, pre-blunted crack. A similar approach is used to evaluate the critical mode I loading for atomic decohesion. Results are presented for various crack tip root radii (a measure of bluntness), for several crack lengths. It is shown that increasing the crack length increases the critical energy release rate for both material behaviors. Increasing the bluntness of a crack tip always increases the required loading for atomic decohesion but nucleation thresholds are initially decreased by very small increases in crack tip bluntness. Nucleation thresholds are later increased after reaching significant crack tip blunting. Implications for ductile versus brittle competition are discussed by comparing the ongoing competition between these two different material behaviors.

Dislocation Nucleation Versus Cleavage in Ni3AI and Ni

1990 ◽  
Vol 213 ◽  
Author(s):  
Yuemin Sun ◽  
James R. Rice ◽  
Lev Truskinovsky
Keyword(s):  
Energy Release Rate ◽  
Energy Release ◽  
Release Rate ◽  
Crack Tip ◽  
Dislocation Core ◽  
Critical Energy ◽  
Dislocation Nucleation ◽  
Critical Energy Release Rate ◽  
Cutoff Radius ◽  
Extended Dislocation

ABSTRACTRecent advances in the modelling of dislocation nucleation from a crack tip are used here to compare the critical energy release rate associated with emission in Ni3Al and Ni. The method for analyzing nucleation makes use of a Peierls-type stress versus displacement relation ahead of the crack tip. It has been shown recently by Rice [1] that the energy release rate for emission scales with γus, the “unstable stacking” energy associated with the sliding of atomic planes past one another. The advantage of this approach is that it allows for an extended dislocation core during nucleation, thus eliminating the need for a core cutoff radius. Preliminary calculations which take into account only the shear stress on a slip plane show that it is more difficult to emit a dislocation in Ni3Al than in Ni. Working within the framework of the competition between atomic decohesion and blunting by dislocation emission, the implications for explaining the brittleness of Ni3Al are also discussed.

Effect of Characteristic Parameters of Exponential Cohesive Zone Model on Mode I Fracture of Laminated Composites

2012 ◽  
Vol 525-526 ◽  
pp. 409-412 ◽  
Author(s):  
Guo Wei Zhu ◽  
Yu Xi Jia ◽  
Peng Qu ◽  
Jia Qi Nie ◽  
Yun Li Guo
Keyword(s):  
Energy Release Rate ◽  
Energy Release ◽  
Release Rate ◽  
Laminated Composites ◽  
Peak Load ◽  
Interfacial Strength ◽  
Critical Energy ◽  
Mode I ◽  
Zone Model ◽  
Critical Energy Release Rate

Delamination is a particularly dangerous damage mode of high performance laminated composites. In order to describe the composites ductile cracking and its progressive evolution accurately, the adjusted exponential cohesive zone model (CZM) is adopted, which correlates the tensile traction with the corresponding interfacial separation along the fracturing interfacial zone. At first the adjusted exponential CZM is used to simulate the mode I delamination of the standard double cantilever beam (DCB). The simulated results are in good agreement with the corrected beam theory and the corresponding experimental results. Then in order to research how the interfacial properties influence the mode I fracture, the interfacial strength and the critical energy release rate are studied. The main results are obtained as follows. The interfacial strength plays a crucial role in the laminated composites delamination onset, and it affects the peak load significantly if there is not a pre-crack. Once the delamination propagation begins to occur in the laminated composites, the responses of the load-displacement plots are relatively insensitive to the interfacial strength, and only the critical energy release rate is of critical importance. Furthermore, the peak load increases with the increase of the critical energy release rate and interfacial strength.

Mode I Interlaminar Fracture Characterization of CFRP Composite Laminates

2012 ◽  
Vol 488-489 ◽  
pp. 552-556 ◽  
Author(s):  
S.S.R. Koloor ◽  
H. Hussin ◽  
M.N. Tamin
Keyword(s):  
Composite Laminates ◽  
Critical Energy ◽  
Fracture Plane ◽  
Mode I ◽  
Critical Energy Release Rate ◽  
Fracture Characterization ◽  
Delamination Crack ◽  
Deformation Response ◽  
Dcb Specimen ◽  
Cfrp Composite

This study examines the deformation response of CFRP composites with preexisting delamination crack under Mode I loading. A DCB specimen is used in a series of Mode I interlaminar tests, each having a different initial delamination crack length. The 48-ply composite laminate has a symmetric ply sequence with 0/0 fiber orientation at the mid-plane. Apparent toughness is indicated by the composite specimen with a starter film insert, likely due to the presence of a neat pocket of resin at the front of the starter crack. The compliance of pre-delamination cracked specimens increases faster beyond the normalized delamination length, a/L of 0.68 due to effects of severe deflection of the longer DCB specimen arm. The critical energy release rate, GIC = 0.5 N/mm is determined based on pre-cracked DCB specimens. Fractographic analysis revealed a smooth fracture plane that indicates interface delamination as the primary failure mode.

Mode I Interlaminar Fracture Toughness of Through-Thickness Reinforced Laminated Structures

2011 ◽  
Vol 423 ◽  
pp. 154-165 ◽  
Author(s):  
M. Tarfaoui ◽  
L. Hamitouche
Keyword(s):  
Fracture Toughness ◽  
Laminated Composite ◽  
Element Model ◽  
Critical Energy ◽  
Mode I ◽  
Interlaminar Fracture Toughness ◽  
Critical Energy Release Rate ◽  
Interlaminar Fracture ◽  
Laminated Composite Materials ◽  
Laminated Structures

The main objective of the study is to understand the mechanisms of the preform reinforcement (2D, stitched and 2.5D) in laminated composite materials. The study is focusing on the mode I interlaminar fracture toughness for glass/vinylester based composites. Starting from DCB tests we quantify the critical energy release rate for the various cases of reinforcement, conclusive that 2.5D reinforcement can increase resistance x7 in comparison with the standard composite. Moreover, the existence of z-fibres made the fracture more complex and caused several characteristic phenomena, so that the required fracture energy for crack propagation was strongly increased. It is shown that a finite element model is successful in reproducing qualitatively the cracking initiation and propagation through the un-reinforced and 3D reinforced sample provided that the action of the through-thickness reinforcement is modelled by discrete nodal forces so as to replicate the physical phenomena.

Effect of Crack Blunting on the Ductile-Brittle Response of Crystalline Materials

1998 ◽  
Vol 539 ◽  
Author(s):  
D.M. Lipkin ◽  
G.E. Beltz ◽  
L.L. Fischer
Keyword(s):  
Crack Tip ◽  
Dislocation Nucleation ◽  
Crack Advance ◽  
Dislocation Emission ◽  
Crystalline Materials ◽  
Self Consistent ◽  
Crack Blunting ◽  
Cohesive Stresses ◽  
Crack Tip Geometry ◽  
Crack Tip Blunting

AbstractWe propose a self-consistent criterion for crack propagation versus dislocation emission, taking into account the effects of crack-tip blunting. Continuum concepts are used to evaluate the evolving competition between crack advance and dislocation nucleation as a function of crack- tip curvature. This framework is used to classify crystals as intrinsically ductile or brittle in terms of the unstable stacking energy, the surface energy, and the peak cohesive stresses achieved during opening and shear of the atomic planes. We find that ductile-brittle criteria based on the assumption that the crack is ideally sharp capture only two of the four possible fracture regimes. One implication of the present analysis is that a crack may initially emit dislocations, only to reinitiate cleavage upon reaching a sufficiently blunted crack-tip geometry.

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