Radial cracking at Vickers indentations in pristine flat/square silica fibers

It has been found that the pristine surface of as‐drawn, flat silica fibers can be indented in air (~50%RH) using a Vickers diamond at a load of 11N without severe cracking during loading. Cracking does occur during unloading, but rather than ring and cone cracks as has been found by many others, radial cracks are observed: essentially as found in “normal” silicate glasses. This normal type cracking in silica glass has been also observed in detail at a high magnification by using a self‐made indentation microscope. It is suggested that the ring/cone cracks usually seen in a silica glass plate are the result of the propagation of the flaws normally present on surface, that is, “pre‐existing cracks.” The data are discussed using Yoffe's model which represents the indentation stress field.