scholarly journals Expansion-Induced Crack Propagation in Rocks Monitored by Using Piezoelectric Transducers

Sensors ◽  
2020 ◽  
Vol 20 (21) ◽  
pp. 6054 ◽  
Author(s):  
Chi-Hyung Ahn ◽  
Dong-Ju Kim ◽  
Yong-Hoon Byun
Keyword(s):  
Crack Propagation ◽  
Wave Velocity ◽  
Rock Specimen ◽  
Maximum Amplitude ◽  
Heating System ◽  
Piezoelectric Transducers ◽  
Rapid Expansion ◽  
Crack Control ◽  
Expansion Pressure ◽  
Rock Specimens

The objective of this study is to develop a new vibration-free excavation method based on vermiculite expansion for rock cracking and to evaluate the performance of the heating system via elastic wave monitoring. Natural vermiculites expand rapidly in volume when heated above 800 °C. MgO powder is used to evenly transmit the surface temperature of a heater rod, which can attain high temperatures rapidly, to the vermiculites. The insertion direction of the heater rod greatly affects the expansion pressure. Three cuboid rock specimens are prepared and equipped with the heating system at different hole-to-face distances. Crack propagation is monitored by a pair of disk-shaped piezoelectric transducers. For short hole-to-face distances, the wave velocity and maximum amplitude rapidly decrease after certain time. For the greatest hole-to-face distance, the shear wave velocity remains constant during the test, while the maximum amplitude decreases after a certain time. The time taken for the velocity and amplitude of the shear waves to decrease reasonably corresponded to that taken for detectable crack propagation to occur on the surface of the rock specimen. The proposed method and materials may be useful from the viewpoints of rapid expansion, economy, and crack control.

scholarly journals Investigations on Crack Propagation Under Cyclical Isothermal and Thermo-Mechanical Loadings for a Type 304-L Stainless Steel Used for Pressurized Water Reactor

2015 ◽  
Author(s):  
C. Gourdin ◽  
F. Rossillon ◽  
P. Le Delliou ◽  
G. Perez ◽  
A. Fissolo
Keyword(s):  
Crack Propagation ◽  
Crack Opening ◽  
Current Method ◽  
Heating System ◽  
Thermal Fluctuations ◽  
Pressurized Water Reactor ◽  
Pressurized Water ◽  
Running Water ◽  
Water Reactor ◽  
New Device

The integrity of structures exhibiting flaws in Pressurized Water Reactor (PWR) has to be assessed to meet safety criteria. This paper deals with crack-propagation under cyclic thermo-mechanical loadings, as encountered in class I austenitic pipes of PWR’s. To have a conservative and reliable assessment of the crack propagation due to the in-service loading, various codes and standards use simplified method. For example, RSE-M introduces a plastic correction depending on the proportion of the mechanical loading. An improvement of the current method requires additional investigations. Moreover, components loaded with transient or thermal fluctuations are not really in strength-controlled conditions. To this end, a new device called PROFATH was designed. The specimen is a pre-cracked thick-walled tube undergoing a set of thermal cycles and loaded with a static mechanical force. During the first part of the cycle, a high frequency induction coil heats the external wall. Then, the heating system stops and the specimen is cooled down by running water inside the tube. Finite element calculations show that only a region half-way along the tube should be heated to ensure adequate structural effect. In the heated zone, the machining of a sharp circumferential groove ensures the propagation of a unique crack. An electro-mechanical jack controls the level of the mechanical static load. To obtain a very precise thermal mapping, a specific specimen with dedicated instrumentation is used: 20 thermocouples are appropriately located on the outer surface and along the wall thickness. During the test, the crack-propagation is estimated through crack-opening measurements (compliance method). Now, three first tests have been carried out. These tests allow having an evaluation of the pertinence of the correction proposed by the RSE-M for a significant plasticity. Following tests are planned in order to have a confirmation or to propose an extension of the validity domain.

scholarly journals Dynamic Indentation Characteristics for Various Spacings and Indentation Depths: A Study Based on Laboratory and Numerical Tests

2018 ◽  
Vol 2018 ◽  
pp. 1-12 ◽  
Author(s):  
Jie Liu ◽  
Wen Wan ◽  
Yu Chen ◽  
Jun Wang
Keyword(s):  
Crack Propagation ◽  
Laboratory Tests ◽  
Indentation Depth ◽  
Numerical Study ◽  
Stress Concentrations ◽  
Dynamic Indentation ◽  
Indentation Force ◽  
Numerical Tests ◽  
Crack Tips ◽  
Rock Specimens

Laboratory and numerical study tests were conducted to investigate the dynamic indentation characteristics for various spacings and indentation depths. First, laboratory tests indicate that the increase in the indentation depth first resulted in enlarged groove volumes, caused by fiercer rock breakages between indentations for a fixed spacing; then, the groove volume slightly increased for further increase in indentation depth, whereas the increase in spacing restrained rock breakages and resulted in shrunken grooves. In addition, the numerical study agreed well with laboratory tests that small chips formed at the shallow part of the rock specimen at the early indentation stage, and then, larger chips formed by the crack propagation at deeper parts of the rock specimens when the indentation depth increased. With further increase in indentation depth, crushed powders instead of chips formed. Moreover, the numerical analysis indicates that crack propagation usually leads to the decrease of the indentation force and the dissipation of the stress concentrations at crack tips, whereas the cessation of crack propagation frequently resulted in the increase of the indentation force and the stress concentrations at crack tip with the increase in indentation depth.

Effect of input signal type and time delay in sensors on wave velocity in rock specimens

2019 ◽  
Vol 260 ◽  
pp. 105225 ◽  
Author(s):  
Jin-Yeon Kim ◽  
Jaewon Jang ◽  
Tae Sup Yun
Keyword(s):  
Time Delay ◽  
Wave Velocity ◽  
Input Signal ◽  
Signal Type ◽  
Rock Specimens

scholarly journals Investigation of Energy and Damage Evolutions in Rock Specimens with Large-Scale Inclined Prefabricated Cracks by Uniaxial Compression Test and AE Monitoring

2020 ◽  
Vol 2020 ◽  
pp. 1-12
Author(s):  
Xiaolou Chi ◽  
Ke Yang ◽  
Zhen Wei
Keyword(s):  
Fracture Surface ◽  
Uniaxial Compression ◽  
Damage Evolution ◽  
Strain Energy ◽  
Large Scale ◽  
Rock Specimen ◽  
Intact Rock ◽  
Stress Strain ◽  
Dip Angle ◽  
Rock Specimens

To explore the energy dissipation mechanism and damage evolution characteristics of rock specimens under compressive loading, we performed the acoustic emission (AE) testing under uniaxial compression in intact rock specimens and those with large-scale prefabricated cracks. The basic mechanical properties of both types of specimens were analyzed comprehensively, and the evolution patterns of strain energy indicators (total strain, elastic, and dissipative energies) in rock specimens before the peak on the stress-strain curve were identified. We further revealed the effect of the prefabricated crack dip angle, which controlled the surplus energy conversion of the following peak deformation and failure in the rock specimens. Using the modified equation of rock specimen damage evolution characterized by the AE energy and examining the fracture surface morphology via the scanning electron microscopy (SEM), the AE distribution law for rock specimen damage was revealed. An increase in the prefabricated crack dip angle was shown to reduce the peak stress and strain of rock specimens, which experienced a transition from the tensile and splitting failure mode to shear and slip one. Cracked rock specimens exhibited strain energy accumulation at the elastic deformation stage of the stress-strain diagram and rapid energy consumption at the plastic stage. By contrast, the intact rock specimens had a smoother energy evolution pattern. As the prefabricated crack dip angle increased, the dissipated and surplus strain energies’ shares increased. Moreover, the first peak of the AE energy occurred earlier, and the stress needed for its occurrence decreased as the dip angle increased. According to the damage evolution equation for rock specimens, their damage process can be subdivided into the initial damage, stable damage increase, and the accelerating damage increase stages. An increase in the prefabricated crack dip angle accelerated the damage accumulation in rock specimens. The locking effect of the sawtooth-like structures on the fracture surface was less conspicuous, and the fracture surface roughness increased. Thus, microcracks gradually developed, and rock specimens became more susceptible to sudden unstable failure.

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