Fracture Toughness Of Granite Research Articles

Effect of Cyclic Loading on Mode I Fracture Toughness of Granite under Real-Time High-Temperature Conditions

Under hot dry rock development, rock formations undergo the combined challenges of cyclic loading and high temperatures, stemming from various sources such as cyclic hydraulic fracturing and mechanical excavation. Therefore, a fundamental understanding of how rocks fracture under these demanding conditions is fundamental for cyclic hydraulic fracturing technology. To this end, a series of three-point bending tests were conducted on granite samples. These tests entailed exposing the samples to cyclic loading under varying real-time high-temperature environments, ranging from 25 °C to 400 °C. Furthermore, different upper load limits (75%, 80%, 85%, and 90% of the peak load) obtained in monotonic three-point bending tests were used to explore the behavior of granite under these conditions. The analysis encompassed the study of load–displacement curves, elastic stiffness, and mode I fracture toughness under cyclic loading conditions. In addition, the microscopic features of the fracture surface were examined using a scanning electron microscope (SEM). The findings revealed notable patterns in the behavior of granite. Cumulative vertical displacement in granite increased with the growing number of cycles, especially at 25 °C, 200 °C, and 300 °C. This displacement exhibited a unique trend, initially decreasing before subsequently rising as the cycle count increased. Additionally, the critical damage threshold of granite exhibited a gradual decline as the temperature rose. As the temperature ascended from 25 °C to 200 °C, the damage threshold typically ranged between 80% and 85% of the peak load. At 300 °C, this threshold declined to approximately 75–80% of the peak load, and at 400 °C, it fell below 75% of the peak load. Within the temperature ranging from 25 °C to 300 °C, we noted a significant increase in the incidence of cracks, crystal microfracture zones, and the dislodging of mineral particles within the granite as the number of cycles increased.

Influence of specimen size on granite fracture characteristics and acoustic emission phenomena under mode I loading conditions

The rock mode I fracture characteristics and the size effect are of great significance in understanding the crack initiation, nucleation, propagation, and characterizing the fracture toughness of rocks which is essential for predicting the behavior of rock masses and designing underground structures. In this paper, three-point bending fracture experiments were carried out using single-edge notched beam specimens of different sizes. The fracture characteristics of the entire loading stage, including the post peak stage, were analyzed using digital image correlation (DIC) technology and acoustic emission (AE) technology. Specifically, the calculation of the b value uses the Fisher optimal split and global search algorithm (FGS) method. The results show that both the Fracture process zone (FPZ) length and fracture toughness of granite are size-dependent parameters, and the FPZ length growth rate increases with increasing load level. The failure mode of the source during fracture is mainly tensile failure, and the proportion of tensile failure tends to increase with the increase of specimen size. The temporal b value, average AF value and RA value show corresponding stage changes with time, and the minimum b value, minimum AF value and maximum RA value can characterize the maximum of macroscopic growth rate.

Assessment of thermal effect on the fracture toughness of granite

The effect of temperature on rock properties has attacked wide attention with the development of deep earth engineering. The change of temperature results in brittle-ductile transformation and makes the fracture toughness different. In the current study, a modified elastic–plastic damage model is proposed to predict the constitutive relation of granite in tension at different temperatures. The model is applied to predict the J-integral of a double-edge-notched plate. Unloading at the point where slope of the secant line is 0.95 times of the initial linear segment slope, the elastic and plastic part of the J-integral can be distinguished. It is noted that the J-integral decreases with elevated temperature and the downward trend changes obviously at around 600 °C. The plastic part takes 57–73 % of the total J-integral for quasi-brittle materials at different temperatures. The fracture toughness predicted based on different methods is compared, it shows that the fracture toughness predicted by the J-integral method is about 1.43 times of the ASTM load-based method.

Effect of real-time high temperature and loading rate on mode I fracture toughness of granite

An in-depth understanding of the effect of real-time high temperature and loading rate on the fracture toughness of rocks is highly important for understanding the fracture mechanism of Hot Dry Rock (HDR). Three-point bending tests on notched semi-circular bending (NSCB) samples at the real-time temperatures (25, 100, 200, 300, 400 and 500 ℃) and different loading rates (0.1, 0.01 and 0.001 mm/min) were performed to characterize the temperature and rate dependence of the mode I fracture toughness. Besides, the characteristic of the fracture surface morphology was investigated by scanning electron microscope (SEM) and crack deviation distance analysis. Results show that the temperature has a significant effect on the development of intergranular and transgranular cracks. The fracture toughness and peak load are similarly influenced by temperature (i.e., they both decrease with increasing temperature). At the loading rates of 0.1 mm/min and 0.01 mm/min, from 25 to 400 °C, the fracture toughness decreases slightly with decreasing loading rates. However, at a loading rate of 0.001 mm/min, the fracture toughness values above 200 °C are very similar, and the fracture toughness does not strictly follow the law of decreasing with decreasing loading rate. Especially at 500 °C, fracture toughness and loading rate are negatively correlated. Our study also indicates that the effect of loading rate on macroscopic crack propagation path at real-time high temperature is not obvious. This study could provide an important basis for evaluating the safety and stability of geothermal engineering.

The influence of microwave treatment on the mode I fracture toughness of granite

Microwave assisted mechanical rock breaking is a hybrid rock breaking method that combines microwave heating and mechanical rock breaking technologies and is proven to have potential applications in mineral processing, mining, and geotechnical engineering. It is, thus, important to understand the effect of microwave-induced damage on rock fracture characteristics. In this study, granite specimens of the Cracked Straight Through Brazilian Disc (CSTBD) were used to carry out the mode I fracture test of the samples irradiated by microwave for different time and thermal cycles. The results showed that the heating rate of the surface temperature of granite is about 1 °C/s, under microwave power of 0.9 kW. When the heating time exceeds the 60 s and after 3 cycles, the fracture toughness of the granite hardly changes with the heating time and cycles. Before 100 °C, the fracture toughness and fracture energy of granite decrease by about 40% with the increase of temperature. Under microwave irradiation, the medium absorber (biotite) in granite is heated and expanded, which results in thermal damage to the rock. Moreover, the polar materials in fluid inclusions move more violently than under conventional heat conduction heating, resulting in a lower decrepitation temperature of the inclusions than under conventional heating and decrepitation degree is more severe, which further leads to an increase in the density of microcracks in mineral crystal and a significant decrease in fracture toughness. When the surface temperature is 300 °C, the bending degree of the crack increases and deviates from the straight line obviously.

Experimental and numerical studies on fracture characteristics of notched granite beams under cyclic loading and unloading

In underground engineering, such as mining engineering and deep tunnel engineering, the rock is often loaded and unloaded repeatedly. The strength of rock under cyclic load is lower than that under static load. To obtain the fracture response of the rock, the three-point bending tests of notched granite beams under cyclic loading and unloading were carried out with Electro-hydraulic Servo Material Test System. The acoustic emission technology was adopted to monitor the acoustic emission events of sample in the process of fracture. It is revealed that the fracture toughness of granite under cyclic loading and unloading is lower than that under static loading. Based on the acoustic emission energy obtained from monitoring, the damage evolution during cyclic loading and unloading was analyzed. The fracture mode of granite samples is analyzed by the RA value-average frequency correlation method. And the Felicity ratio during the loading and unloading cycle was calculated to evaluate the severity of initial damage of the material. It is revealed that Kaiser effect appears only in the elastic deformation stage of cyclic loading unloading bending. The Holmquist–Johnson–Cook damage constitutive model and Weibull distribution were used to establish the heterogeneous granite model. And the three-point bending of the model under cyclic loading and unloading was simulated to disclose the crack growth mechanism of rock. The study may provide some references for rock instability control in geotechnical engineering construction.

Calculation for tensile strength and fracture toughness of granite with three kinds of grain sizes using three-point-bending test.

Tensile strength and fracture toughness, important parameters of the rock for engineering applications are difficult to measure. Thus this paper selected three kinds of granite samples (grain sizes = 1.01mm, 2.12mm and 3mm), used the combined experiments of physical and numerical simulation (RFPA-DIP version) to conduct three-point-bending (3-p-b) tests with different notches and introduced the acoustic emission monitor system to analyze the fracture mechanism around the notch tips. To study the effects of grain size on the tensile strength and toughness of rock samples, a modified fracture model was established linking fictitious crack to the grain size so that the microstructure of the specimens and fictitious crack growth can be considered together. The fractal method was introduced to represent microstructure of three kinds of granites and used to determine the length of fictitious crack. It is a simple and novel method to calculate the tensile strength and fracture toughness directly. Finally, the theoretical model was verified by the comparison to the numerical experiments by calculating the nominal strength σn and maximum loads Pmax.

Determination of Tensile Strength and Fracture Toughness of Granite Using Notched Three-Point-Bend Samples

Direct tensile strength and fracture toughness of rock and concrete, important properties for many applications, are cumbersome to measure directly. In this study, granite is chosen as an example to show how the tensile strength and fracture toughness can be measured from small three-point-bend samples of a single size but with different notches. An existing fracture mechanics model has been extended to include the stable fictitious crack growth before peak loads, which is then linked to the granite grain size. Both tensile strength and fracture toughness of granite can be estimated by the maximum load measurements from those notched three-point-bend samples. In total, 72 three-point-bend granite samples with different notches have been tested, and the estimated tensile strength and fracture toughness are compared with those available in the literature. The modified fracture mechanics model is then used to predict the fracture behaviour of smaller samples of the same granite. The theoretical prediction is confirmed by the experimental results of those smaller samples. Finally, the fracture model and its relation with the American Society for Testing and Materials (ASTM) standard on fracture toughness are discussed.