  We studied the low speed fracture regime (0.1mm/s - 1nm/s) in different
glassy materials (soda-lime glass, glass-ceramics) with variable but controlled
length scale of heterogeneity. The chosen mechanical system enabled us to work
in pure mode I (tensile) and at a fixed load on DCDC (double cleavage drilled
compression) specimen. The internal residual stresses of studied samples were
carefully relaxed by appropriate thermal treatment. By means of optical and
atomic force (AFM) microscopy techniques fracture surfaces have been examined.
We have shown for the first time that the crack front line underwent an
out-of-plane oscillating behavior as a result of a reproducible sequence of
instabilities. The wavelength of such a phenomenon is in the micrometer range
and its amplitude in the nanometer range. These features were observed for
different glassy materials providing that a typical length scale characterizing
internal heterogeneities was lower than a threshold limit estimated to few
nanometers. This effect is the first clear experimental evidence of crack path
instabilities in the low speed regime in a uniaxial loading experiment. This
phenomenon has been interpreted by referring to the stability criterion for a
straight crack propagation as presented by Adda-Bedia et al. (Phys. Rev.
Letters (1996) 76} p1497).
