This letter reports the application of ultrasonic force microscopy (UFM) to investigate subsurface fracture mechanisms during tensile loading of nanocomposite films consisting of a brittle glass on a ductile polyethylene terephthalate (PET) substrate. Such materials are used in packaging applications where a gas barrier is required to maintain the product quality. Cracking or debonding of the surface glass layer results in destruction of the gas barrier properties of the film. Accurate evaluation of the continuity or discontinuity at the crack edge within the layered material is crucial for the correct characterization of both adhesive failure and crack propagation. Here simultaneous atomic force microscopy and UFM images are compared for a range of tensile strains to identify debonded regions of the glass film from the PET. Debonding occurred for strains greater than 6%. In some films, this debonding increased with applied strain. (C) 2002 American Institute of Physics.