Understanding Acoustic Emission Testing- Reading 1 Part B-A

16.6.2 Effect of Loading Devices
As is well known, the end restraint of a compression specimen of concrete
due to the friction between the ends of the specimen and the loading platens
can have a considerable effect on the apparent strength of the concrete.
These differences are also reflected in the acoustic emissions measured
when different types of loading devices are used. For instance, in
compression testing with stiff steel platens, most of the acoustic emission
appears at stresses beyond about half of the ultimate stress; with more
flexible platens, such as brush platens, significant acoustic emission appears
at about 20% of the ultimate stress. This undoubtedly reflects the different
crack patterns that develop with different types of platens, but it nonetheless
makes inter-laboratory comparisons, and indeed even studies on different
specimen geometries within the same laboratory, very difficult.
Charlie Chong/ Fion Zhang