| Previously a laser spallation experiment was developed to measure the tensile strength of thin film interfaces. In this experiment, a laser-produced compressive stress pulse in the substrate, reflecting from the coating's free-surface pulls the interface in tension and leads to its failure if the tensile amplitude is high enough. The interface stress is determined directly by recording the coating free-surface velocities using a Doppler interferometer. Since the interface separation occurs at a strain rate of almost 10{dollar}sp7 secsp{lcub}-1{rcub}{dollar}, the plastic deformation that usually accompanies the interface decohesion process is largely suppressed, and the measured tensile strength can be related to the intrinsic interface fracture energy by using a phenomenological atomic stress-separation relationship.; The aim of this thesis work was to extend the laser spallation technique to measure directly the interface fracture energy by implanting well-defined interfacial flaws. To this end, penny-shaped interfacial cracks were generated by encouraging buckling of the overlayer from selected spots on the substrate, by utilizing the intrinsic compressive stresses within the coating. The laser-produced shock wave was then used to load these flaws, and the critical stress amplitude responsible for their propagation was recorded. The key element of the work was to relate the measured free surface velocity to the local stress intensity factor for a well defined flaw, by using the concepts of dynamic fracture mechanics. This was implemented via a finite element simulation process. Besides acting as a reference to the strength measurements accomplished previously, this work now allows the measurement of the fracture energies of several metal/ceramic interfaces of interest to the microelectronic and composites industries.; In addition to the above, the potential of low-amplitude laser-generated stress pulses for non-destructive evaluation of flaws in composites and other engineering solids is also discussed. |
