  Due to the oscillatory singular stress field around a crack tip, interface
fracture has some peculiar features. This paper is focused on two of them. One
can be reflected by a proposed paradox that geometrically similar structures
with interface cracks under similar loadings may have different failure
behaviors. The other one is that the existing fracture parameters of the
oscillatory singular stress field, such as a complex stress intensity factor,
exhibit some non-objectivity because their phase angle depend on an arbitrarily
chosen length. Two objective and independent fracture parameters are proposed
which can fully characterize the stress field near the crack tip. One parameter
represents the stress intensity with classical unit of stress intensity
factors. It is interesting to find that the loading mode can be characterized
by a length as the other parameter, which can properly reflect the phase of the
stress oscillation with respect to the distance to the crack tip. This is quite
different from other crack tip fields in which the loading mode is usually
expressed by a phase angle. The corresponding failure criterion for interface
cracks does not include any arbitrarily chosen quantity, and therefore is
convenient for comparing and accumulating experimental results, even existing
ones.
