NLRM
Numerical Laboratory
of Rock Mechanics
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Acoustic emission uncovers thermal damage evolution of rock

1.Backgroud

The thermal damage evolution of rock closely relates to many important scientific and engineering problems,such as the mechanisms of earthquake and the effective extraction of deep geothermal energy. However, there is lack of rational thermal damage evolution model, and the mechanisms of thermal damage are still unclear especially during cooling. By use of a specially designed acoustic emission (AE) test system, we explored the thermal damage evaluation in a rock and established a thermal damage evolution model which takes into ac-count both heating and cooling processes. The mechanisms of rock damage caused by heating and cooling were analyzed by using a lattice spring model.

The thermal damage of rock is always an interesting topic in geoscience, and has attracted a wide focus. Temperature has substantial effects on rocks. Thermally induced activation of faults may be a probable mechanism of fault movement even earthquake. Moreover, in geothermal exploitation, cold fluid injected into hot rock may cause probable microseismic events. In some particular underground engineering such as nuclear waste disposal14 or fire in tunnel and subway, thermal damage closely relates to project safety. These issues therefore warrant the study on the thermal damage of rock. However, the real geological settings are too complex (e.g. high pressure and high temperature) to objectively find the exact influence of temperature on rock damage.

2.Obiective

To validate the experimental results and the theoretical model, we investigated the thermal damage process of rock by using a lattice spring model. Our numerical results not only well reproduced the thermal damage process, but also revealed the mechanisms of thermal damage.

3. Experiment Process

3.1 Damage procedure and AE monitoring

3.2 Validation of the AE test system

3.3 evaluate thermal damage evolution

3.4 Numerical simulation.

Fig. 13 (a)Flow chart for simulating thermal damage using DLSM.


Fig. 14 (a) Normalized CEDs of the MSM and the MDM, which presents the validation of the MSM for simulating heating damage, the MDM for simulating cooling damage. (b) Damage variables of the incorporated models, experimental and theoretical results, which shows a coincident evolution trend.

Fig. 15. Simulation results for thermal damage evolution. (a) Microstructure model. Thermal damage occurs and develop with the increase in temperature. Most damage occurs in the weak material at the contact regions especially at contact corner. (b) Microdefect model. Damage continuing in cooling process within a defective crystal under compressive stress. The red color presents maximum damage degree. Two tensile cracks occurred at the tips of the defect after heating. With the progress of cooling process, cracks develop and branch finally propagating to crystal boundary. Animations are available online showing damage evolution with temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

4. Conclusions

This paper investigates the thermal damage of rock by using a newly developed AE test system, and derived the damage evolution model. Finally, we discussed the damage mechanisms by numerical modelling. The main conclusions are as follows:  

(1) AE technology is feasible for researching thermal damage of rock. The main advantage of this technology is to determine the evolution of thermal damage variable during both heating and cooling processes.  

(2) During thermal damage process (heating and cooling), there exist two types of AE wave: the broadband waves (include type 1 and 2) and the high-frequent wave. The AE wave shows a broad frequency distribution from 0.1 kHz to 80 kHz.  

(3) Comparing to cumulative count, cumulative energy density is better for characterizing thermal damage, by which we established a thermal damage evolution model. This model describes the damage extent of sandstone at any real-time temperature.

(4) By use of the DLSM, we established a heterogeneous micro-structure model and a microdefect structure model to reproduce heating damage and cooling damage, respectively. The combined numerical result well reproduced the damage evolution as experiment revealed.




Article classification: 英文版热损伤
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