In nanoindentation of brittle solids cracking can occur. AE monitoring provides an effective method to study this even when there are no features seen on the indentation curves.

In 500 mN nanoindentation testing of fused silica with a Berkovich indenter there are no features on the loading curve to indicate cracking but as ISO 14577 notes, cracking typically occurs from 75-100 mN. AE monitoring shows discrete bursts of AE during loading and additionally during unloading from higher loads.

AE envelopes for fused silica indentation at 100, 300 and 500 mN (loading time = 10 s, hold at peak load before unloading = 5 s)

Acoustic Emission monitoring provides improved failure onset detection.

For example, when first cracks occur below the surface in a scratch test they are not visible and the critical load is incorrect. 

A 500 mN nano-scratch test on 2.7 µm SiCN coating deposited using magnetron sputtering on silicon with a 5 µm diamond probe. 

Strong AE is detected from the interface cracking at a lower critical load (point "A") than the first angular cracks which appear on the surface at point B. Focussed Ion Beam milling through the scratch track confirmed the cracking at point A.