Crack Paths 2009

Indeed, we observe in experiments that z varies linearly with d (Fig.8) when 0.1

0.3. W ecould not investigate a wider range of masses because, whenm becomes too

large, a stick-slip-like phenomenonappears, such that crack propagation is not quasi

static anymore. The force intensity is obtained with a linear fit as

0.25 0.05 .

W ecompare this intensity with the minimal force necessary to propagate a crack in this

material, as given by the critical toughness . In the experiment, we observed that a

force is applied in the horizontal plane far from crack tip. The typical distance l between

the effective application point of this force and the crack tip is of the order of the

thickness. The necessary force to propagate the crack is given by

. For the

30 thickness film, we find

0.3 which coincides with the measured

value 0.25 within the experimental uncertainty.

δ

Figure 8. The height of the crack tip z as a function of the added mass m at the bottom

of the film, with

30 ,

2.6 and 4 .

C O N C L U S I O N

This experimental study suggests that an isotropic, homogeneous material locally breaks

in mode I, whatever the imposed mode (I, II or III) at a larger scale, because this

opening mode is the most efficient to separate two surfaces. However the large scale

configuration dictates the crack stability: we show that the crack is always stable under

our (large scale) modeIII loading whereas it can be unstable under modeI.

R E F E R E N C E S

1. Goldstein, R.V. and Sagalnik, R.L. (1974), Int. J. Fract. 10, 507.

2. Mai, Y.W.and Cotterell, B. (1984), Int. J. Fract. 24, 229-236.

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