Use of Shock Waves to Measure Adhesion at Interfaces
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USE OF SHOCK WAVES TO MEASURE ADHESION AT INTERFACES GERALD L. NUTT, and WAYNE E. KING Lawrence Livermore National Laboratory, 7000 East Avenue, P.O.Box 808, Livermore, CA. 94550 ABSTRACT The central problem in the study of composite materials is the adhesive strength of the electronic bond between reinforcement and matrix. We have introduced a unique method of measuring the interface bond strength of a wide variety of engineering interfaces (e.g. metal/ceramic, semiconductor/metal, metal/polymer). Specimens are composed of a relatively thin overlayer on a thick substrate. The specimens are shocked using a magnetic hammer which accelerates a thin metal flyer onto the substrate. The shock, upon reflection at the free surface, is incident on the bonded interface as a tensile wave, spalling the overlayer. The method is unique in using free surface velocity measurements to determine the interface stress at the instant of separation. The debonding process is sufficiently rapid (on the order of 1.0 ns) that debonding occurs by the simultaneous breaking of atomic bonds, rather than by propagation of cracks nucleating at stress concentrations near existing flaws. INTRODUCTION There are essentially two methods of determining the bond strength from the spall stress at the interface. The first, introduced by Snowden [1] determines the minimum shock strength required to spall a film from a substrate. The event is then modeled using a computer code to determine the maximum tensile stress at the interface generated by the known shock input. The tensile stress is taken as the ultimate strength of the bond. This method has been reintroduced by Gupta et.al.[5, 3, 4, 2], generating the shock by deposition of laser energy. The second method, introduced by us [6, 7, 8, 9],generates a plane shock wave parallel to the interface by flyer impact. We do not require the measurement of the threshhold shock profile that produces the spall. All that is needed is that the shock wave produce a spall event. The stress at the bond is found from the free surface velocity history of the spalled film. We measure this velocity using a VISAR laser interferometer. We shall discuss the important features of both types of experiments, illustrating with numerical simulations. There are advantages of this type of numerical study as well as limitations. One advantage is, a modeling calculation can show details of the experiment that cannot be observed experimentally. This allows a quick assessment of the effect of parameter changes on the outcome of the experiment, (i.e. sensitivity studies). As we shall see, we will also be able to evaluate assumptions and graphically display their effect on the outcome of the experiment. At the same time we must be cautious when applying a numerical model to an experiment. The model is always a simplification. Additionally, these techniques invariably introduce numerical artifacts which are unphysical, and which must be allowed for when interpreting the results. Several conclusions can be drawn from our numerical studies.
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