Monitoring small-crack growth by the replication method

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. The calculated crack growth rate versus Delta K relationship for small cracks was compared to that for large cracks obtained from middle-crack-tension specimens. The primary advantage of this techinque 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 micron. 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.