Semi-circular Bending Research Articles

Another look at the semi-circular bend test for the performance ranking of hot mix asphalt

Semi-circular bend (SCB) testing has been widely used to evaluate the cracking resistance of asphalt mixtures. However, it remains unresolved which of the various SCB protocols can provide the most sensitive and precise ranking of different asphalt mixtures. This study aims to improve the current SCB protocol, emphasizing mixture preparation, testing condition and evaluation indices. The effect of aging methods in terms of reversible aging and irreversible aging and loading rates on the intermediate and low temperature cracking resistance of asphalt mixtures was analyzed. Subsequently, the response of different cracking evaluation indices was analyzed. Finally, the repeatability of different cracking evaluation indices was discussed. The results show that higher loading rates are slightly more sensitive to mix composition, possibly due to the toughening effect of polymer modifiers. However, whether higher or lower rates are more accurate remains to be investigated with ongoing field studies. Cracking initiation and resistance indices (CII and CRI) were superior from a repeatability and dynamic range perspective in comparison to flexibility and balanced cracking indices (FI and BCI). Thermoreversible aging over three days of cold conditioning at − 20 °C was found to significantly affect binder properties in selected samples. However, none of the SCB parameters after testing appropriately cold conditioned samples at room temperature reflected this deterioration. Hence, it remains to be investigated whether mixture testing at ambient temperatures properly ranks superior quality Alberta, Canada and Venezuelan sourced materials that are low in wax.

Analyzing the Damage–Healing Performance of Asphalt Mixture from Macroscopic and Mesoscopic Scales

Owing to their self-healing ability, asphalt mixtures have great potential for improving the service life of asphalt pavement, and can play an important role in sustainable infrastructures. In this study, a semicircular bending damage-healing-fracture experiment was conducted to evaluate the healing effect of a 90# asphalt mixture and styrene-butadiene-styrene copolymer modified asphalt mixture. The digital image correlation technique was used to synchronously observe the semicircular specimen to analyze its surface displacement and strain changes during the healing process. The experimental results showed that, on the macroscopic scale, the healing index HIFE based on fracture energy density was recommended to evaluate the healing effect of asphalt mixtures, and the healing effect of the 90# asphalt mixture was better than that of the styrene-butadiene-styrene copolymer modified asphalt mixture. On the mesoscopic scale, the horizontal displacements of the notch of the specimen position gradually recovered under self-healing conditions. The strain concentration at the crack gradually weakened, and the strain degree in the non-cracked area of the specimen also decreased gradually under self-healing conditions. The recovery of the macromechanics of the specimen is related not only to the healing of the crack area but also to the recovery of the non-cracked area.

Cracking resistance evaluation of epoxy asphalt mixtures with 100% reclaimed asphalt pavement (RAP)

Despite the recycling of reclaimed asphalt pavement (RAP) providing great economic and environmental benefits, conventional technologies exhibit poor cracking resistance and cannot achieve high RAP content. The major objective of this paper is to evaluate the cracking resistance of recycled mixtures containing 100% RAP using epoxy asphalt, and to determine an optimal production method. In this paper, the effects of temperatures, epoxy proportions, and gradations on cracking resistance of epoxy-recycled mixtures with 100% RAP were explored through semi-circular bending (SCB) tests, and the mechanism was explained by laser scanning confocal microscopy (LSCM) tests. Subsequently, the validity of SCB parameters was examined using statistical analysis, and a new index (Cracking Resistance Comprehensive Score, CRCS) for evaluating the cracking resistance of epoxy-recycled mixtures was proposed through principal component analysis (PCA). The results indicate that the graded mixture with an epoxy proportion of 30% could be the best solution to recycle 100% RAP, which can restore the cracking resistance to the level of virgin mixtures, outperforming the conventional recycling technique. Statistical analysis shows that temperatures and epoxy proportions have significant effects on SCB parameters, and the fracture energy until failure (Gf1) or crack resistance index (CRI) is valid for characterizing the cracking behavior of epoxy asphalt mixtures. Moreover, CRCS contains over 90% of the original information and effectively identifies the effects of temperatures, epoxy proportions, and gradations, which can be applied to evaluate and rank the cracking resistance of epoxy-recycled mixtures.

The fracture behaviour of cement bitumen emulsion mixture through the digital image correlation (DIC) method

ABSTRACT With the merits of low carbon emission and energy consumption, cement bitumen emulsion mixture (CBEM) is more and more preferred in the construction and rehabilitation of pavement engineering. However, CBEM has a high risk of cracking damage due to the inappropriate addition of cement. To deeply know and then reasonably design the cracking resistance of CBEM, the fracture behaviour of CBEM is studied by a semi-circular bending test, and the global displacement and strain of the specimen during the whole loading process are measured through digital image correlation (DIC). The results indicate that the three fracture stages of CBEM (i.e. crack initiation, crack propagation and failure) can be accurately identified by DIC data. The maximum value of the mean global horizontal strain, representing that the specimen affords the maximum tension energy, is proposed to identify the fracture failure point for the accurate calculation of fracture parameters. The fracture energy, J integral and fracture toughness of CBEM increase and then decrease with the increasing cement dosage. To obtain a good comprehensive fracture behaviour for CBEM, it is recommended that cement dosage can be between 2% and 3%, and the optimum cement dosage is 2.6%.

Use of Random Forest to Predict Intermediate Temperature SCB Jc Parameter of Long-Term Aged Asphalt Mixtures

With the growing use of reclaimed asphalt pavement (RAP), recycled asphalt shingles (RAS), and other innovative additives in asphalt mixtures, there are concerns about the accuracy of Superpave volumetric-based mixture design. Performance tests in mix design procedures are needed to ensure desirable pavement performance. The Louisiana Department of Transportation and Development (LADOTD) has adopted the semi-circular bend (SCB) critical strain energy release rate parameter, Jc, to evaluate the cracking resistance of asphalt mixtures. To address practical shortcomings, this study aims to develop a predictive model to estimate SCB Jc at intermediate temperature of long-term aged (LTA) asphalt specimens from volumetric properties of plant-produced asphalt mixtures, and rheological and chemical properties of unaged asphalt binder by using a machine-learning model, random forest. Asphalt mixture SCB test and asphalt binder rheological and chemical properties tests were conducted. Stepwise correlation analysis was used to determine the most influential variables to SCB Jc. Random forest model was optimized using grid search with hyperparameter combinations. Results show that the developed random forest model was able to predict the SCB Jc fracture parameter of asphalt mixtures and a good agreement was observed between predicted and measured SCB Jc values. Variable importance scores based on mean decrease in impurity (MDI) showed that the Δ Tc had the most influence on SCB Jc prediction accuracy, and other inputs such as coarse aggregate type, LTA days, Fourier transform infrared spectroscopy test carbonyl index, and linear amplitude sweep test ALas also had significant effects on SCB Jc.

Fracture performance and damage behavior of fly ash-based geopolymers toughened by molybdenum tailings based on acoustic emission and digital image correlation

This research proposes the utilization of molybdenum tailings (MT) to toughen fly ash-based geopolymers. To fully understand the effect of MT on the fracture performance and damage behavior of fly ash-based geopolymers, the effect of mode I fracture toughness and fracture energy of fly ash-based geopolymers with different MT contents were researched by using semi-circular bending (SCB) tests. The damage and strain behavior of the semi-circular bending with notches (NSCB) specimens during the fracture process were evaluated by acoustic emission (AE) technology and digital image correlation (DIC) technology, and finally the micromorphology of the fracture surface of NSCB specimens after the loading finished were observed by scanning electron microscopy (SEM). The results show that the tensile strength, fracture toughness and fracture energy of fly ash-based geopolymers were significantly increased with the addition of MT, and the peak value was reached at 30 wt% MT contents, with the increase amplitude reaching 77%, 31% and 45% respectively. This was attributed to the filling effect of MT, which made the structure of fly ash-based geopolymers more dense, reduced the formation and expansion of internal microcracks, shifted the growth direction of microcracks from the pore structure to the gel phase and the interface between the gel phase and glassy microspheres during the fracture process, improved the stability of the structure and the fracture resistance.

Characteristics and properties of asphalt mortar containing FO filler

As a preventive maintenance method, microwave self-healing is an effective method for early repairing small cracks in asphalt mixture, reducing maintenance costs and prolonging service life. However, asphalt mixtures containing aggregate-type wave-absorbing agents have a common problem of uneven heating. Asphalt mixture was prepared by partially replacing the limestone filler with ferroferric oxide (Fe3O4, FO), and the healing performance under microwave heating was studied. The rheological properties of asphalt mortars with FO and the fluidity of conventional asphalt mortars were studied by dynamic shear rheometer (DSR). Then, the magnetostatic properties and electromagnetic parameters of FO fillers were analyzed by vibrating sample magnetometer (VSM) and vector network analyzer (VNA). Finally, the semicircular bending (SCB) samples were studied by the breaking-microwave healing experiment. The results show that FO fillers have obvious ferromagnetism, and they mainly rely on magnetic loss for energy conversion. FO is beneficial to the improvement of high temperature rheological properties of asphalt mortar but not to low temperature rheological parameters; the replacement ratio of FO to limestone filler should not exceed 50%. The flow healing behavior of asphalt mortar is greatly affected by temperature, which is the fundamental reason for the self-healing of asphalt mixture. The heating efficiency of the asphalt mixture containing FO filler is significantly improved under microwave irradiation; the FO filler has a slightly negative effect on the initial strength of SCB samples but can significantly improve the healing rate after microwave.

Laboratory Investigation of Open Graded Friction Courses with Different Binder Grades and Aggregate Gradations

This study investigates the effectiveness of the volumetric mix design procedure used by the New Mexico Department of Transportation based only on the tensile strength ratio (TSR) test for designing open graded friction course (OGFC) materials. It also evaluates the effects of different binders (or binder grades) and aggregate gradations on the performance of OGFC materials. Nine OGFC mixes were produced using three different binders (PG 64-28+, PG 70-28+, and PG 76-22+) and three different aggregate gradations (two of 9.5 mm nominal maximum aggregate size [NMAS] and one of 12.5 mm NMAS). Next, a thorough laboratory investigation was conducted to evaluate their various performances. This study found that the traditional volumetric mix design based solely on the TSR test cannot ensure adequate durability performance. The PG 64-28+ OGFCs, for example, met all the volumetric and TSR requirements during mix design but performed poorly in all performance tests. This suggests that additional performance testing for OGFCs is required to ensure adequate longevity. Cantabro abrasion and semi-circular bending tests are found to be useful in characterizing the durability and moisture conditioning, and the fracture performances of an OGFC, respectively. Furthermore, a new test is proposed for evaluating the durability performance of an OGFC in wet conditions using the Micro-Deval device. According to this study, OGFC's durability and fracture performance improves as the grade of binder used increases. The OGFCs prepared with aggregates of smaller NMAS outperformed the OGFCs prepared with larger NMAS aggregate in both durability and fracture tests despite having similar asphalt contents.

Use of the Iron Chloride Type of Lewis Acid Catalyst in High Reclaimed Asphalt Pavement Content Asphalt Mixtures

The use of reclaimed asphalt pavement (RAP) as a partial replacement for virgin aggregates and asphalt binders has received considerable attention recently. This approach is one of many promising methods for improving sustainability. However, many state Departments of Transportation are cautious in adopting high RAP content into their designs because of cracking and durability issues resulting from the aged RAP binder. A Lewis acid catalyst has been used to modify the asphalt binder’s chemical composition and disrupt the associated molecules formed in the aged RAP binder. The objective of this study is to assess the effectiveness of the use of a Lewis acid catalyst in improving the cracking resistance of an asphalt mixture containing high RAP contents. A suite of laboratory mechanical tests was performed, namely, the dynamic modulus test for linear viscoelastic properties; the semi-circular bend, Illinois flexible index, ideal cracking tolerance, and simplified viscoelastic continuum damage tests for fracture and fatigue resistance; the loaded wheel tracking test for rutting resistance and moisture susceptibility; indirect tensile creep compliance and strength tests for low-temperature cracking resistance; and the Cantabro abrasion test for durability. Further, AASHTOWare Pavement ME software was used to predict the long-term performance of asphalt pavements containing mixtures with high RAP contents and a Lewis acid catalyst. Results showed that the use of the Lewis acid catalyst can disrupt associated molecules formed in the aged RAP binder. Thus, asphalt mixtures containing high RAP contents (recycled binder ratio > 0.20) and a Lewis acid catalyst exhibited similar performances to conventional mixtures. Further, Pavement ME predicted fatigue cracking was lower for a structure containing a FeCl3-modified mixture as compared to a similar one with no FeCl3, illustrating the effectiveness of FeCl3.

An investigation on failure behavior of semi-flexible composite mixture at different temperatures

Semi-flexible composite mixture (SFCM) is a kind of pavement material formed by pouring cement-based grout material into a porous asphalt mixture with air voids from 20% to 30%. SFCM is widely used for its outstanding anti-rutting performance. Its mechanical performance is complicated due to its heterogeneity and interlocking structure. According to the present study, asphalt deforms at different temperatures, whereas cement-based grout has no similar characteristics. Rare research focuses on the temperature-based performance of SFCM. Therefore, the study was on the thermal performance of SFCM by seven open-graded asphalt mixture skeletons with different porosities and two types of grouts with early strength (ES) and high strength (HS). The test temperatures ranged from −10 ​°C to 60 ​°C. The mechanical investigation was performed using the semi-circular-bending (SCB) and beam bending tests. The strain sensor was used for analyzing the thermal performance of SFCM. The results show that the temperature significantly affected the SFCM's performance. The porosity was selected for three sections based on the trend of fracture energy (Gf) curves at 25 ​°C. The turning points were the porosity values of 20% and 26%. The initiation slope during elastic deformation increases with the porosity increase. This trend was more evident at intermediate temperature. The shrink strain of SFCM was lower than that of the usual asphalt mixture (AC). The thermal stress of the SFCM filled with HS (HS-SFCM) was higher than that of the SFCM filled with ES (ES-SFCM) at −10 ​°C. Moreover, the thermal failure characteristics of SFCM were influenced by porosity.

Low-Temperature Crack Resistance of Basalt Fiber-Reinforced Phase-Change Asphalt Mixture Based on Digital-Image Correlation Technology

DaoLu Tiaowen Cailiao (DTC) phase-change materials are important materials for improving the temperature adaptation of asphalt pavements. In order to improve the low-temperature crack resistance of asphalt pavements after adding DTC phase-change materials, basalt fiber-reinforced DTC phase-change asphalt mixtures with different basalt fiber dosages were prepared, and their mechanical properties, horizontal strain, and crack characteristics were analyzed using semicircular bending tests combined with digital-image correlation technology. The study showed that the basalt fiber increased the fracture strength of the phase-change asphalt mixture, delayed its damage time, improved its deformation capacity, reduced the crack development rate and damage degree of the interface, and enhanced its low-temperature crack resistance. The enhancement of low-temperature crack resistance of phase-change asphalt mixtures increased and then decreased, and the best effect was achieved when the dosage of basalt fiber was 3‰, which delayed the damage time of phase-change asphalt mixtures by 42.53 s, slowed down the crack expansion rate by 54.23%, and reduced the fractal dimension (D) and damage factor (Df) by 4.32% and 29.83%, respectively. In conclusion, basalt fibers can enhance the low-temperature crack resistance of phase-change asphalt mixtures, and the optimal dosage of basalt fiber is 3‰. Also, both D and Df showed a positive correlation pattern with KIC, but Df was more closely related to KIC, which shows that the new index Df based on the image analysis method and MATLAB software is reliable.

Effect of nano-reduced graphene oxide (NRGO) on long-term fracture behavior of Warm Mix Asphalt (WMA)

Recent studies have shown that in addition to ambient temperature, time-dependent factors such as temperature cycles and freeze–thaw cycles (FTCs) affect the speed of creation and growth of low-temperature cracks (LTCs) and intermediate-temperature cracks (ITCs) by creating the aging process in the asphalt mixture. Therefore, it is necessary to study the cracking behavior of asphalt mixtures by considering these factors. In the present study, an attempt was made to evaluate the long-term fracture behavior of Warm Mix Asphalt (WMA) modified with nano-reduced graphene oxide (NRGO) containing vertical and angular cracks under mode I loading conditions at low and intermediate temperatures using Semi-circular bend (SCB) geometry. In order to simulate long-term performance, two conditions of 3 FTC and aging process (for 6 days) were applied to the mixtures with and without NRGO, and the results were compared with samples subjected to 0 FTC (short-term performance). The results showed that adding 0.3% and 0.6% NRGO to the WMA mixture increased fracture toughness (KIC), fracture energy (GF), and stiffness under mode I at low and intermediate temperatures. At +15 °C and for the geometry containing the vertical crack, the NRGO-modified WMA mixture had better flexibility under 3 FTC than 0 FTC; while, the flexibility of the NRGO-modified WMA mixture decreased under aging process. For geometry containing angular crack, flexibility indices improved for NRGO-containing mixtures under 3 FTC and aging process (at +15 °C temperature). On the other hand, the results showed that the addition of NRGO to the WMA mixture, while increasing the maximum load at the moment of fracture, caused a decrease in the displacement at the fractured moment at low and intermediate temperatures. This result was obtained for the mixtures that were conditioned under aging process. However, the results showed that applying 3 FTCs decreased the maximum load at the moment of fracture and increased the displacement at the fractured moment. Finally, the results showed that the reduction of fracture indices was higher for WMA and NRGO-modified WMA mixtures under 3 FTCs (at temperatures ±15 °C); therefore, applying 3 FTC instead of aging process can be used to evaluate the long-term performance of asphalt mixtures.

On the evaluation of entropy threshold for debonding during crack prorogation using DIC technique

The accumulated entropy at the crack tip of a semi-circular bending (SCB) specimen is investigated to obtain a threshold for material fracture and interatomic debonding. The presented approach uses the experimental data obtained from the Digital Image Correlation (DIC) technique and strain maps superposed to SCB specimens to measure the plastic flow at the crack tip. The Swift strain hardening model is utilized to calculate the stress domain at the crack tip and, consequently, the plastic strain energy and entropy accumulation. Results show that the crack surface propagates when the generated entropy at the crack tip reaches a critical limit. This critical value can be considered as the material capacity beyond which bond breakage occurs.

Effect of mastic specifications on the fatigue and low-temperature performance of hot mix asphalt

ABSTRACT This study aimed to evaluate the effect of mastic specifications on the intermediate and low temperatures performance of hot mix asphalt. In this research study, four different asphalt mixtures were fabricated with two types of aggregate and bitumen at three different mastic densities (defined as F/B ratios). At intermediate temperature, Four-Point Bending (4PB) beam fatigue tests were carried out at 8 Hz frequency and the strain levels of 460, 750, and 1100 microstrain. Also, the low-temperature performance of mixtures was evaluated by using Semi-Circular Bending (SCB) tests at −6°C, −18°C and −12°C, −24°C for PG 64–16 and PG 58–22 fabricated mixtures, respectively. Fatigue test results showed that the fatigue life of asphalt mixtures was more influenced by the mastic density at the lower strain level. Moreover, a minimum mastic density was necessary for improving fatigue performance. The SCB test results revealed that increasing the mastic density makes higher values in the mixture’s fracture energy, fracture toughness, cracking resistance index (CRI), and tensile strength. Also, the low-temperature performance of asphalt mixtures containing aggregate with a high abrasion value (AAV) was more dependent on the mastic density.

Mixed-mode fracture behavior in deep shale reservoirs under different loading rates and temperatures

In the last years, shale gas has gradually substituted oil and coal as the main sources of energy in the world. Compared with shallow shale gas reservoirs, deep shale is characterized by low permeability, low porosity, strong heterogeneity, and strong anisotropy. In the process of multi-cluster fracturing of horizontal wells, the whole deformation process and destruction modes are significantly influenced by loading rates. In this investigation, the servo press was used to carry out semi-circular bend (SCB) mixed-mode fracture experiments in deep shales (130, 160, 190 °C) with prefabricated fractures under different loading rates (0.02, 0.05, 0.1, 0.2 mm/min). The fracture propagation process was monitored using acoustic emission. The deformation characteristics, displacement–load curve, and acoustic emission parameters of shale under different loading rates were studied during the mixed-mode fracture propagation. Our results showed that during the deformation and fracture of the specimen, the acoustic emission energy and charge significantly increased near the stress peak, showing at this point the most intense acoustic emission activity. With the increase in loading rate, the fracture peak load of the deep shale specimen also increased. However, the maximum displacement decreased to different extents. With the increase in temperature, the effective fracture toughness of the deep shale gradually decreased. Also, the maximum displacement decreased. Under different loading rates, the deformation of the prefabricated cracks showed a nonlinear slow growth–linear growth trend. The slope of the linear growth stage increased with the increase in loading rate. In addition, as the loading rate increased, an increase in tension failure and a decrease in shear failure were observed. Moreover, the control chart showing the relationship between tension and the shear failure under different temperatures and loading rates was determined.

Aggregate size effects on fracture behavior of concrete SCB specimens

Fracture in concrete and rock-like materials is the result of applying external loads, initiation, and propagation of the cracks, and finally, creating the failure surface in the material. The most common type of fractures in solid materials are usually formed under combined tensile/shear loading conditions. The stress intensity factors KI and KII, corresponding to opening and sliding modes (I and II), are the main fracture parameters, and their critical values (i.e., fracture toughness) show the material's resistance against the initiation and propagation of cracks. In this paper, the fracture toughness of concrete specimens is determined by considering different aggregate sizes and using the three-point bending test on semi-circular bend (SCB) specimen. The fracture toughness parameters have been determined by different fracture criteria and compared with the values obtained in the laboratory on concrete specimens. The effect of aggregate size on fracture parameters and crack propagation path has been evaluated. To predict thecrack initiation angle and fracture toughness values, themodified maximum tangential stress (MMTS) and extended maximum tangential strain (EMTSN) criteria along with conventional fracture criteriawere used. The results showed, that with the increase in aggregate size, the fracture toughness value under mode I, II, and mixed-mode (I-II) has increased. The laboratory results showed that specimens with aggregate sizes of 0–19 mm have the highest fracture load, and specimens with the aggregate sizes of 0–5 mm have the lowest fracture load

The Effect of Microwave Radiation on the Self-Healing Performance of Asphalt Mixtures with Steel Slag Aggregates and Steel Fibers.

Self-healing in asphalt mixtures is a property that can be enhanced by external heating, which causes a thermal expansion that increases the flow of bitumen with reduced viscosity through the cracks. Therefore, this study aims to evaluate the effects of microwave heating on the self-healing performance of three asphalt mixtures: (1) conventional, (2) with steel wool fibers (SWF), and (3) with steel slag aggregates (SSA) and SWF. After evaluating the microwave heating capacity of the three asphalt mixtures with a thermographic camera, their self-healing performance was determined with fracture or fatigue tests and microwave heating recovery cycles. The results demonstrated that the mixtures with SSA and SWF promoted higher heating temperatures and presented the best self-healing capacity during the semicircular bending test and heating cycles, with significant strength recovery after a total fracture. In contrast, the mixtures without SSA presented inferior fracture results. Both the conventional mixture and that containing SSA and SWF presented high healing indexes after the four-point bending fatigue test and heating cycles, with a fatigue life recovery of around 150% after applying two healing cycles. Therefore, the conclusion is that SSA greatly influences the self-healing performance of asphalt mixtures after microwave radiation heating.

Modified fractional-Zener model—Numerical application in modeling the behavior of asphalt mixtures

To seek a constitutive model that can both describe the viscoelastic response of asphalt mixtures well and be applied to finite-element simulation, a modified fractional-order Zener model (MFZM) was adopted from the mathematical expressions describing viscoelastic materials based on fractional calculus theory in this study. The MFZM was verified by the generalized fractional-Zener model, and the corresponding model configuration was provided. In this model, the dynamic response and relaxation-modulus function were derived. Based on the relaxation-modulus function, a numerical algorithm for the MFZM that can be implemented in a finite-element environment was proposed. The ability of the fractional-Zener model (FZM) and MFZM to characterize the dynamic response of asphalt mixtures was compared using dynamic modulus test. The dynamic modulus and semicircular bending (SCB) tests of the asphalt mixture were simulated using the proposed numerical algorithm in the software ABAQUS. The results demonstrate that, unlike FZM, MFZM can characterize the asymmetric characteristics of viscoelastic response of the asphalt mixture in the frequency domain. The simulation result of the dynamic modulus test agreed with the analytical solution of the MFZM in the frequency domain, which illustrated the availability of the proposed numerical method for the response of the asphalt mixture in the frequency domain. Moreover, the consistency between the actual experimental and virtual results of SCB proved the accuracy of the model and applicability of the proposed numerical method, allowing the model to be used in a finite-element environment.

Experimental investigation on effect of loading rate on fracture behavior of coal-seam roof rock

The fracture behavior of coal-seam roof rock in coal mining is a key and controlling factor for the mode optimization of the artificial roof caving. However, the fracture mechanism of roof rock under loading is not clear. In the work, the split Hopkinson pressure bar (SHPB) experiment was carried out using semicircular bending samples from the sandstone of coal-seam roof rock in the Junger mining area of Inner Mongolia at the loading rate of 0.35–3.78 GPa·m0.5·s−1, and the dynamic fracture behavior and energy dissipation mechanism of samples under different loading rates were investigated. The result shows that the dynamic stress–strain process of the hard roof rock includes four stages: linear instantaneous compaction, linear elastic compression, failure, and fracture extension, in which the failure forms changes from brittle fracture to ductile fracture with the increase of loading rate. The mode I fracture toughness increases linearly under confining pressure. In addition, the propagation orientation of induced fractures is parallel to the loading direction, and the gravel in the samples can inhibit fracture extension, resulting in changing the fracture extension path. Further, the energy absorption efficiency of the samples during the fracture process decreases with the increase in the loading rate.

Numerical study and experimental validation of the size effect of smooth and mode I cracked semi-circular bend specimens

The edge-cracked semi-circular bend (SCB) specimen subjected to three-point bending loading is used in many applications to measure the fracture behavior of quasi-brittle materials. The main objective of the present work was to study the effect of the crack length to SCB specimen radius ratio (a/R), span to specimen diameter ratio (S/D), and specimen size on its flexural and mode I crack growth behavior. The contour integral method was implemented using the 3-D finite element method to determine the mode I stress intensity factor. In addition, high-strength concrete specimens were experimentally studied to validate the numerical results. The results show that the maximum compression stress is not sensitive to the S/D value, while the tensile stress is very sensitive. The value of S/D is the main parameter controlling the crack driving force (i.e., the crack mouth opening displacement (CMOD) and the normalized stress intensity factor, YI). For the same S/D, the SCB specimen diameter value change has a marginal effect on CMOD and YI. The specimen with S/D = 0.8 showed that it is the most compatible specimen with three-point bending test conditions, regardless of the SCB specimen size. A good agreement between the numerical and experimental results was achieved.