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Acoustic Emission

High frequency Acoustic Emission (AE) monitoring during nanomechanical tests such as nanoindentation, nano-scratch, nano-impact, micro-pillar compression and micro-cantilever bending provides valuable additional information to improve our understanding of material behaviour at the nano-scale.

Acoustic waves emitted during mechanical tests at the nano and micro scale are a rich source of information about the deformation behaviour (e.g. initiation and propagation of cracks, plastic instabilities, etc.) that may be otherwise inaccessible.

Benefits of Acoustic Emission on the NanoTest:
  • Improved accuracy in scratch test critical load
  • Uncover cracking events in nanoindentation where load-depth profiles are smooth
  • Differentiate between slip and fracture in impact
  • Study yield behaviour
  • Differentiate between different cracking processes
  • Improved understanding of nanotribological tests
How does it work?

Acoustic emission activity is continuously recorded and analyzed throughout the tests with a high-sensitivity ZEDO system (Dakel, Czech Republic) with a high dynamic range and sampling frequency of 10 MHz. This specification allows the study of very weak as well as strong AE events at nanosecond scale in a broad frequency range up to 2 MHz.

Direct synchronization with the NanoTest means the depth-load-time records can be related to AE signals. During the test the samples are mounted on the low noise/high-sensitivity AE sample holders that contain dedicated structured piezo-element and inbuilt pre-amplifiers. The AE hit waveform sampled at MHz frequency is described by several characteristics that provide different insights into the origin of the detected signal.

Cracking in nanoindentation of brittle solids

During the nanoindentation of brittle solids cracking can occur during loading, during the hold at peak load or during the unloading process. In 100-500 mN nanoindentation testing of fused silica with a Berkovich indenter there are no features on the loading curve to indicate cracking but the international standard for instrumented indentation (ISO 14577) notes that cracking occurs at over 75-100 mN. AE monitoring shows discrete bursts of AE activity during loading and additionally during unloading from higher loads:

AE envelopes for fused silica indentation at 100, 300 and 500 mN

 

Cracking of chemically toughened glasses (such as Gorilla Glass) can require significantly more load and the NanoTest high load (30 N) head has been used to perform microindentation tests. For Gorilla Glass AE monitoring revealed cracking beginning around 700 mN with discrete bands of cracking activity as the load increases.

Advanced spectral analysis reveals differences in the frequency signatures for the events at different load levels.

AE envelopes for Gorilla Glass indentation at 2 and 10 N. The spectrogram for the 10 N indentation on the right shows a visual representation of the spectrum of frequencies of the AE signal as a function of time.

 

Check out our acoustic emission applications note for more examples of how this advanced AE monitoring can be used in conjunction with other NanoTest capabilities.

 

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