Slip line characteristics of insonated copper and aluminum



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A study is made to determine the surface microstructure behavior of high purity polycrystalline copper and aluminum specimens after being subjected to high frequency fatigue. These metals are chosen to represent face-centered cubic structure usually extremely ductile when pure. The high frequency fatigue is performed by means of a piezoelectrically driven exponential horn at a frequency of 20 KHz., and a displacement amplitude of about 2 mils. The static mechanical behavior of these metals is determined from a conventional tensile test. The surface microstructure is investigated using a Cambridge scanning electron microscope as well as an optical microscope. After a short interval of insonation, slip lines with uniform distribution are observed on the surface of both metals. These slip lines are afterwards grown into slip bands where nucleation of microcracks begins. The microcracks are found to lie and propagate on one of the available slip systems, similar to that previously observed in Stage I growth in low frequency fatigue. At larger insonation intervals striations are observed indicating the growth of these microcracks to Stage II. The average distance between these striations is found to increase as the insonation time is increased. This means that the crack growth velocity increases with the increase of insonation doses. The calculations of the rate of crack growth using measured mechanical properties indicate the importance of strain hardening in ultrasonic fatigue as a mechanism affecting crack propagation in Stage II. The comparison between the calculated fatigue life time and that obtained from the experiment shows agreement in the case of aluminum, and an order of magnitude discrepency in the case of copper. This discrepency is attributed to the high elastic stress of copper.