To shed light on these stress waves, we looked at the response of a crack growing dynamically in a glass specimen to an ultrasonic shear pulse of known amplitude, frequency, and polarization introduced at specific locations along the crack path. These shear waves were shown to induce a mode III perturbation in the local loading of the crack front, and to twist the crack front without fragmentation /1/2/. The induced undulation, referred to as Wallner line, was found to have a shape linear in amplitude and frequency of the perturbation, and without any persistence when the perturbation disappears.
In a second step, we examined the interaction of a dynamically growing crack front with a localized heterogeneity introduced through a groove scratched at the surface of the specimen. We then observed marks similar to that observed when the perturbation was driven by an acoustic pulse. This showed that these tracks are also Wallner lines, i.e. undulations resulting from interactions of the crack front with the shear waves radiated from the groove, and not with the crack front waves predicted theoretically /3/4/.
It should be emphasized that all the present observations were carried out at small crack growth velocity, ranging from 300 to 1200m/s, i.e. below the threshold above which branching instability is known to occur /5/. It would then be interesting to investigate acoustic emission as the crack growth velocity is getting close to this threshold, and see if this emission can provide the key mechanism for branching instability. Work in this direction is currently in progress with Fabrice Célarié (post-doctoral fellow) and Claudia Guerra (PhD student), in close collaboration with Davy Dalmas from Unité Mixte CNRS/Saint Gobain, Aubervilliers, France. This project is funded by the French National Research Agency (Agence Nationale de la Recherche) with grant ANR-05-JCJC-0088-01.