Structural Properties and Damage Detection Capability of Carbon Nanotube Modified Mortars after Freeze-Thaw

Here we explore the structural properties and damage sensing of cementitious mortars after a freeze-thaw process (F-T) as a function of nano-modification. For this purpose, carbon nanotubes were added at 0.2–0.8 wt.% cement using two different dispersive agents. F-T resulted in reduced fracture energy in nano-modified specimens prepared using superplasticizer as a dispersant while the opposite held true for the surfactant-containing ones. All nano-modified mortars possessed significantly higher fracture energy compared to the plain specimens after F-T (up to 73% improvement). The acoustic emission activity was lower after F-T, while acoustic emission indicators revealed a more tensile mode of fracture in both plain and nano-modified mortars.

Double-layered wrap-around sensor network for three-dimensional positioning of acoustic emission sources and its effect on monitoring crack propagation in rock samples

The present sensor arrangement in a cubic way for monitoring crack propagation in rock samples exhibits shortfalls of blind monitoring zone and large deviation. This study proposes a double-layered wrap-around sensor network, which enhances the monitoring range and improves the location accuracy of acoustic emission source. Furthermore, based on the polar formation algorithm, acoustic emission source was positioned to explore the propagation of microscopic cracks in cylindrical rock samples and this was further validated by the acoustic emission activity index. The results show that: (1) The double-layered wrap-around sensor network exhibits considerably broader monitoring range and enhanced precision. The simulated fracture formed from cracks of high-energy release had a favorable consistency with the macro-failure surface of rock specimens; (2) During the loading process, acoustic emission activity had a significant positive correlation with signal amplitude and the number of events. In addition, acoustic emission activity of medium-grained sandstone showed a tendency of decreasing—remaining at a low value—increasing—remaining at a high value, which exactly corresponds to the four rock loading stages of compaction, elastic deformation, crack development, and crack connection; (3) rock samples experienced micro-cracking of low energy, micro-cracking of high energy, and crack connection in sequence in the failure process, which shows a high consistency between crack development and acoustic emission activity. Thus, acoustic emission activity could be used as an index for assessing the rock failure state.

Comparative experimental studies of acoustic emission characteristics of sandstone and mudstone under the impacts of cyclic loading and unloading

In this article, the acoustic emission tests of uniaxial cyclic load imposed on or released from the sandstone and mudstone were carried out. The deformation and failure characteristics and the law governing the acoustic emission activity were studied. The results of the study show that (1) the variation of acoustic emission events of sandstone and mudstone is law governed and is in agreement with the tendency of stress and strain curve development. (2) The acoustic emission activity of mudstone is most active before peak stress, while sandstone is at peak stress. For the sandstone, when the number of acoustic emission events is the most active, the corresponding acoustic emission energy is not the largest. However, the peak value of acoustic emission events and the peak energy of the mudstone coincide, and the acoustic emission events get to the most intense due to the peak energy. (3) The acoustic emission activity is more severe when a load is imposed on or released from the rock. Compared to loading, the rock damage caused by unloading is even greater. (4) The acoustic emission event at the splitting point is more concentrated. The line of acoustic emission point is basically consistent with the shape of the split.