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The suitability of the acetate replication method for monitoring the growth of small cracks is discussed. Applications of this technique are shown for cracks growing at the notch root in semicircular-edge-notch specimens of a variety of aluminum alloys and one steel. Cracks were allowed to initiate naturally along the surface or at the corner of the notch root, a stress condition representative of a fastener hole or fillet in aircraft components. The calculated crack growth rate versus ΔK relationship for small cracks was compared to that for large cracks obtained from middle-crack-tension specimens. The advantages and limitations of the acetate replication method in comparison to other commonly used methods for small crack research are delineated. The primary advantage of this technique is that it provides an opportunity, at the completion of the test, to go backward in time towards the crack initiation event and “zoom in” on areas of interest on the specimen surface with a resolution of about 0.1 (μm (0.0001 mm). The primary disadvantage is the inability to automate the process. Also, for some materials, the replication process may alter the crack-tip chemistry or plastic zone, thereby affecting crack growth rates.
fatigue (materials), crack propagation, short crack, replicas
Research engineer, Lockheed Engineering & Sciences Company, Hampton, VA