Crack pinning model

In summary, the crack pinning model, as described so far, explains why fracture energy increases with filler volume fraction and it accounts for the particle size effect. In the next section a rationalisation of the maxima in the plots of G]c (c) against Vf is presented. 

Extensions to the Crack Pinning Model and the Effect of Particle Matrix Adhesion... 

Some of the models used to rationalize the toughening effect associated with plastic deformation of the matrix and second-phase particle at the crack tip are represented in Fig. 1. This deformation is achieved through crack pinning and broadening, particle bridging and cavitation, crack path deflection, shear band formation and crazing or microcracking for stress relaxation at a crack tip. 

Figure 2 Mechanics model of z-pins bridging delamination, in which Lc is the delamination crack length P is the load corresponding to the applied displacement, 5.

The presence of Z-pins as the through-the-thickness reinforcement has been shown to result in dramatic increases in the apparent resistance to crack propagation under Mode I and Mode II loading conditions, in laboratory tests on standard unidirectional (UD) beam samples [2]. The Z-pin pull-out has been identified as the dominant energy micro-mechanism responsible for the resistance to delamination under Mode I conditions. The behaviour of individual Z-pins in pull-out from a UD-laminate has been characterised and modelled and the single Z-pin pull-out curves used as input into a 2D Finite Element (FE) model of delamination under Mode I [3, 4]. 

Preliminary trials in complex aerospace type structures suggest that selectively placed blocks of Z-pins, of dimensions similar to those used in the laboratory specimens, have a significant capacity to resist or even stop the propagation of an inter-laminar crack in the structure. The success of even relatively simple FE models in predicting the behaviour of any specific block of Z-pins in the laboratory specimens would seem to indicate that the same approach will be successful in more sophisticated local-global models of structures containing such reinforcement. 

The work presented here has confirmed the ability of Z-pin bloeks to arrest a propagating delamination crack, for a range of crack speeds. From the perspeetive of modelling and predictions of behaviour of more eomplex Z-pinned structures, it is eneouraging to see that the... 

The eoneept of eraek pinning was developed after it was observed that the crack (16) fiont changed in length after interaction with a particulate, producing toughening. With this model, toughening is described by... 

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