Edge Notched Disc Bend Research Articles

Acoustic emission and fracture morphology characteristics of thermal-damage granite under mixed mode I/III loading

To explore the fracture morphology and acoustic emission characteristics of thermally damaged granite under mixed mode I/III loading, a series of heat treatment and mixed mode I/III loading tests were conducted on edge-notched disc bend (ENDB) granite samples. First, the thermal damage samples’ strength and crack opening displacement were analyzed. Subsequently, based on 3D laser scanning technology, the fracture morphology of the sample was studied, and the fractal dimension was calculated. Then, the samples’ acoustic emission characteristics were discussed, including the ringing count, b value, spectrum signature, and AF-RA. Finally, an acoustic early warning index was proposed based on the critical slowing down theory, and the influence of temperature on the early warning effect was analyzed. The results show that under mixed mode I/III loading, the samples formed an antisymmetric curved surface with out-of-plane torsion. As the thermal damage effect is enhanced, the peak strength decreases linearly, and the fractal dimension increases linearly. The failure mode changes from sudden instability to progressive instability. Shear cracks gradually dominate the sample failure. The peak frequency signal of the sample is mainly distributed between 80 kHz and 400 kHz. The appearance of the sudden drop in the b value and a zero-frequency signal has an early warning effect. The quadratic function can describe the relationship between warning time and temperature. As the temperature increases, the deterioration effect of historical loading on the sample is enhanced, the development time of sample failure is prolonged, and the early warning effect is better.

Evaluation of fracture indices of warm mix asphalt (WMA) modified with nano-additive under pure shear and pure tear deformations

Cracks located on the road edge and curve are two pavement crack types that can reduce road safety. Providing a solution to control this type of crack can solve a large part of the concern in this case. This study innovatively bridges the research gap by evaluating the efficacy of nano-reduced graphene oxide powder (NRGOP) in enhancing the fracture resistance of warm mix asphalt (WMA) under various environmental conditions, a critical aspect previously unexplored in the pavement engineering literature. For this goal, asphalt mixtures containing different ratios of NRGOP were constructed. Then, a level of aging and damage induced by freeze–thaw were applied to the semicircular bend (SCB) and edge-notched disc bend (ENDB) specimens to understand how the changes in fracture strength, fracture crispiness, and fracture stiffness are under modes III and II (at + 15 °C and −15 °C). The finding revealed that the introduction of NRGOP increased the fracture properties of WMA mixtures, such as fracture energy (FEII or FEIII) and fracture toughness (KIIC or KIIIC), as well as fracture stiffness under pure shear and pure tear deformations. Laboratory findings at low temperatures showed that it is necessary to investigate KIIC and FEII for asphalt mixtures under non-conditions and freeze–thaw conditions. However, KIIC and FEIII were suggested for aging conditions. Therefore, in two-thirds of the results, FEII was critical, and in 100 % results, KIIIC was critical. At + 15 °C, KIIC and FEIII were more crucial for non-treated and freeze–thaw conditions, while KIIC and FEII were more critical for aging, respectively. Therefore, in two-thirds of the results, FEIII was critical, and in 100 % results, KIIC was critical. The results of this research provide insights to advance the socio-economics of pavements by controlling surface cracks in tropical and subtropical regions, which in turn increases ride quality, driver safety, and pavement maintenance costs.

Laboratory Investigations on Fracture Toughness of Self-compacting Concrete Containing Recycled Materials

This article investigates the fracture toughness in self-compacting concrete containing different percentages of recycled concrete materials using edge-notched disc bend (ENDB) samples. For this reason, 0, 25, 50, 75 and 100 percent of recycled aggregate were used instead of natural aggregate in the mixing designs. To obtain the fracture toughness, ENDB samples were used under loading of mixed mode I/III and pure modes I and III. Cubic samples were also used to obtain compressive strength. The results show that the compressive strength of the samples has a direct relationship with the fracture toughness and the reverse ratio with the percentage of recycled concrete materials. Also, samples containing 25% recycled materials have the highest compressive strength and fracture toughness. Furthermore, as the recycled percentage of the samples increases, the performance of samples against shear forces and compressive strength is reduced. Keywords: Fracture toughness, Mixed mode I/III, Recycled materials, Self-compacting concrete, ENDB sample

Effect of support friction on pure mode I, II, and III fracture toughness of cement concrete tested with edge-notched disc bend specimen

In this paper, the effect of friction on the interactions between the supports and test specimen is investigated in the fracture tests under pure mode I, pure mode II, and pure mode III loadings. 3D finite element analyses are performed on the ENDB (edge-notched disc bend) specimen by modeling the friction at the interactions, and SIFs (stress intensity factors) are computed. The results show that friction affects the values of SIFs significantly. Next, mode I, mode II, and mode III fracture tests are performed on the ENDB specimens made of concrete by using three different supports including the R (roller), RG (roller-in-groove), and F (fixed)-supports. Based on the results, mode I, mode II, and mode III fracture toughness are considerably dependent on the type of supports. Such that the F-supports and R-supports show the highest and lowest values of the fracture toughness, respectively due to having the highest and lowest friction coefficients at the interactions. Finally, the friction coefficients at the interactions are computed for the mode I, mode II, and mode III loadings using the FE and experimental data. Accordingly, mode I fracture tests of the concrete are more sensitive to the friction coefficient at the interactions than the mode II and mode III ones.

Experimental investigation on the effect of grain size of granitic rocks on the fracture roughness and toughness

The fracture surface characterization, such as the fracture roughness, is essential to flow and heat transport through the fracture in the geothermal, oil, and gas reservoirs stimulation by hydraulic fracturing. This research presents an experimental approach to assess the correlation between the fracture roughness and mode I fracture toughness (KIc) of the granitic rock with different grain sizes, which is crucial in geothermal reservoirs. Three types of granite with the same mineralogical compositions but fine, medium, and coarse-grained structures were selected and prepared. Fracture toughness values were measured using a novel specimen called ENDB (Edge Notched Disc Bend), in which a cracked disc shape specimen was loaded in three-point bending. The photogrammetric method was used to estimate the value and distribution of surface roughness after ENDB testing and investigate the relationship between fracture morphology and fracture toughness. Based on the experiments, a clear correlation between the roughness and fracture toughness was observed in the three investigated granite types with different grain sizes. The FPZ around the crack tip, tortuosity, intergranular, and transgranular cracks were examined to study the effect of grain size on fracture toughness and roughness precisely. The results show that by increasing the grain size, KIc and surface roughness increased. The results also showed that the roughness parameter for the three types of rocks in the direction of the crack growth path is greater than the direction perpendicular to the crack growth path.

Assessment of mode I/III fracture toughness of bi-material rock-like ENDB and ENDC specimens

Mode I/III fractures at the bi-material interface are common and present unique challenges in civil and geotechnical engineering. However, there are few studies on the mechanical characteristics of bi-material rock-like specimens under mixed-mode I/III loading. This study used bi-material rock-like Edge-notched disc bend (ENDB) specimens and Edge-notched diametrically compressed (ENDC) specimens to investigate their mechanical and fracture characteristics under pure mode I, III, and mixed mode I/III loading. The results showed that the fracture toughness KI > KIII, KI and KIII decreased overall with increasing loading angle(β, α), the KI and KIII of the ENDB specimens are generally lower than that of ENDC specimens. The fracture energy (Wi) of ENDB specimens is gradually increased with the loading mode changed from pure mode I to pure mode III, while the ENDC specimens have a trend of first decreasing and then increasing. Besides, the ENDB specimens exhibit significant torsion fracture, and the torsional angle(θ) gradually increased with loading angle β, in which low-strength material has larger θ than high-strength material. In contrast, ENDC specimens fractured along the interface in pure mode I and mixed mode I/III loading, while fractures along the loading point to the centre of the specimen under pure mode III loading. Moreover, the acoustic emission results show that tearing cracks are more prominent in ENDC specimens than in ENDB specimens. Generally, ENDB specimens show more pronounced tensile/torsional fracture characteristics than ENDC specimens, it’s recommended to be adopted in mode I/III fracture toughness testing for bi-material rock-like materials.

Evaluating fracture resistance of basalt fiber reinforced mortar to mode I/III load using edge notched disc bend (ENDB) specimen: Insights from acoustic emission and morphological analysis

Failures of the topcoat in pavement structures often occur due to mixed mode I/III traction; however, incorporating basalt fibers (BFs) into cement mixtures can effectively enhance mortar toughness and improve service conditions. To gain a deeper insight into the fracture behavior of BF-reinforced cement mixture, the Edge Notch Disc Bend (ENDB) testing with acoustic emission (AE) technique was first conducted on BF reinforced mortar specimens considering three different dosages of BFs (0%, 0.3% and 0.8% by weight). Four fracture parameters, including effective fracture toughness (Ke), effective work of fracture (Wp), effective critical fracture toughness (Keff) and effective critical work of fracture (Wp-eff), were employed to assess the fracture performance of BF-reinforced mortars. The results show that the Ke for mode III is significantly lower than that for mode I, while Wp is critical for mode III. Interestingly, the increase in Ke, Keff, Wp and Wp-eff is closely associated with the micro behavior involved in reducing the effective critical distance. Then, the digital reconstruction of fracture surfaces was performed, revealing that the fractal and roughness characteristics of the fracture surface subjected to mode ІІІ loading are more pronounced than those under mode I loading. These results can be further enhanced by the fiber crack-bridging effect. Moreover, it is validated that the out-of-plane shear failure induced by tensile stress occurs specifically under pure mode ІІІ loading state (KІc>KІІІcand 90% shear-type waveforms), which illustrates the feasibility of applying pure mode III load under ENDB testing. By analyzing variations in dominant frequency and b value, we demonstrate that mortars with higher BFs content exhibit a greater prevalence of shear-type microfractures characterized by smaller rupture scales and weaker heterogeneity under mode ІІІ loading conditions. These findings can offer a novel perspective on elucidating the crack-bridging effect of fibers during the rupture of fiber-reinforced mixtures.

Experimental investigation on mixed mode I-III fracture behaviors and degradation mechanism of sandstone under thermal-chemical stimulation

The presence of mode III fracture component and multi-field coupling environments may occur in certain practical rock engineering scenarios, but the combination of these factors has rarely been reported. In this study, the edge-notched disc bend (ENDB) specimen was used to evaluate the mixed mode I-III fracture characteristics of sandstone under thermal-chemical coupling stimulation. The evolution of mixed mode I-III fracture toughness and failure patterns were analyzed. The degradation mechanism under thermal-chemical stimulation was verified using the scanning electron microscope (SEM) technique and chemical kinetics. The results show a consistent decrease in all mixed mode I-III fracture components as temperature and acidity levels increase. The fracture trajectory of the ENDB specimen converts from a self-similar linear shape to a twist and anti-symmetric plane as it shifts from pure mode I loading to pure mode III. The deterioration degree on the mixed mode I-III fracture toughness components at 75 °C is approximately 1.1–1.4 times larger than at 25 °C. Furthermore, the chemical corrosion exhibits a more significant impact during the thermal-chemical coupling process, where the weakening of sandstone’s micro-structures serves as an internal mechanism leading to the deteriorated physicomechanical properties.

Designing a simple and suitable laboratory test specimen for investigating the general mixed mode I/II/III fracture problem

A novel test specimen was designed for experimental investigations of pure and mixed mode fracture. This novel test configuration is called “modified edge notched disc bend” (M-ENDB) specimen. The M-ENDB specimen has a circular disc shape with an edge crack along its diameter that is loaded using asymmetric three-point bending. Different fracture parameters including: stress intensity factors (SIFs), geometry factors, T-stress, biaxiality ratio (BR), and out-of-plane constraint factor (Tz) were determined for the suggested disc bend specimen using the extensive finite element (FE) analyses. According to the obtained results, the proposed specimen can present all combinations of mode mixities, including pure modes I, II, and III as well as mixed-mode I/II, I/III, and I/II/III deformations. Afterwards, a large number of pure and mixed-mode fracture toughness experiments were carried out on polyurethane (PUR) foam to examine the capabilities of the suggested M-ENDB specimen. Fracture path, fracture surface, fracture toughness, work of fracture, and critical T-stress of broken foams under general mixed mode conditions were investigated. Mixed mode I/II/III fracture toughness value was smaller than the pure mode I fracture toughness (KIc) value showing the weakness of brittle materials against multi-axial fracture compared to pure tensile type fracture.

The influence of recycled asphalt pavement materials on low and intermediate fracture behavior of asphalt concrete (AC) after exposing to different thawing and freezing periods

The effects of compaction process, mixture type and environmental conditions on mode I cracking resistance of asphalt concrete (AC) materials was investigated experimentally. To do that, a large number of edge notched disc bend specimens containing 0, 25 and 50% recycled asphalt mixture (RAP) material and exposed to different numbers of thawing and freezing cycles (0 to 10 cycles) were fractured. The critical values of stress intensity factor and fracture energy indexes were obtained for the AC mixtures at three test temperatures of −25, 0 and + 25 °C. Based on the results, the fracture resistance capacity of the AC mixture was decreased by increasing the RAP content, number of thawing/freezing cycles and decreasing the test temperature. The effect of thawing/freezing cycles on decreasing the cracking resistance was more pronounced for the first cycles and the variations of fracture toughness and fracture energy values with the numbers of thawing/freezing cycles showed a plateau form for higher numbers of cycles. The fracture curves of the tested samples manufactured using the Gyratory compaction method was upper than the related curves of samples made by Marshall compaction method. The sensitivity of fracture results (i.e., gradient of reduction in the fracture resistance versus freeze/thaw cycles) became low by increasing the test temperature.

Low temperature cracking behavior of modified asphalt mixture under modes I and III

The study of the short- and long-term cracking behavior of asphalt pavement under tensile and tear deformations, and the production of sustainable mixtures using recycled additives, were the two main goals of this study. For this purpose, two types of additives, namely pyrolysis carbon black (PCB) and pyrolysis hydrocarbon (PHC), which were made from hospital waste recycling as a result of the COVID-19 pandemic, were used to study the fracture behaviour of asphalt mixture under modes I and III (at −15 °C). For simulating the actual pavement conditions with the Edge Notched Disc Bend (ENDB) samples, 0 and 3 freeze–thaw damage (FTD) cycles were applied to mixtures. Laboratory findings indicated that the addition of 6, 12, and 18 % PCB, 1.25, 2, and 2.75 % PHC, and 18 % PCB + 2.75 % PHC into the asphalt mixtures resulted in an increase in the total fracture energy of mixtures at −15 °C by increasing the GIfpre-peak and GIIIfpre-peak as well as increasing the fracture toughness, for both FTD cycles. Also, the results of brittleness indices revealed that using 18 % PCB + 2.75 % PHC in the asphalt mixture had more flexible behaviour than the mixture containing PCB (due to a higher average brittleness index) and more brittle behaviour than the mixture containing PHC (due to a lower average brittleness index) under mode I. While the mixtures containing PCB, PHC, and 18 %PCB + 2.75 %PHC were more flexible under mode III. Finally, the mixture containing 18 % PCB + 2.75 % PHC had the best fracture performance under mode I. It increased the asphalt mixture's fracture energy and toughness respectively under 3 FTD cycles by 62 and 54 %, in comparison with the unmodified mixture and without applying FTD cycles. Also, it was indicated that the modified mixture with 18 %PCB + 2.75 %PHC can compensate for 97 % of the fracture energy decrease due to the influence of FTD cycle (under mode III), while in terms of the fracture toughness index, a 12 % increase was obtained.

An experimental study and theoretical evaluation on the effect of specimen geometry and loading configuration on recorded fracture toughness of brittle construction materials

Geometry and loading condition of cracked specimens may affect the fracture toughness results and hence the evaluation of differences in measured fracture toughness values is necessary for many construction materials. In the absence of standard methods for measuring the general mode fracture resistance of asphaltic concrete materials, in this study the fracture toughness values for such materials were measured and compared by means of three different test specimens and methods. All three fundamental fracture modes (i.e. pure I, pure II, and pure III) and related mixed-mode loading cases (i.e. I/II and I/III) were evaluated. The test specimens were edge-notched disc bend (ENDB), edge-notched diametrically compressed disc (ENDC), and semi-circular bend (SCB). During the experimental program the specimens made of asphaltic concrete were tested at two temperatures of −5 and −25 °C and analyzed using the linear elastic fracture mechanics (LEFM) principles. Also, several theoretical frameworks were compared and examined to evaluate their applicability and accuracies for predicting the trends of experimentally obtained test data. The results showed that the geometry and loading configuration can significantly affect the measured fracture toughness values under pure mode II and pure mode III. The value of mode I fracture toughness measured by three specimens was almost the same (with small differences of 4%) and equal to 0.68 MPa√m. At pure mode II, the fracture toughness evaluated by the ENDB specimen was about 20% higher than that of the SCB specimen (with averages of 0.83 and 0.68 MPa√m for the ENDB and SCB samples, respectively). For pure mode III case, the measured fracture toughness by the ENDC specimen was approximately 60% higher than that of the ENDB specimen (with averages KIIIc = 0.62 MPa√m for the ENDC and 0.42 MPa√m for the ENDB specimen). Among the investigated mixed mode fracture criteria, the GMTS criterion provided accurate predictions for KIIc/KIc ratios. Also, 3D-MTSN, and 3D-MTSED criteria were more precise than others for mode I/III predictions. The evaluations indicated that the discrepancy between the measurements is mainly due to sign and magnitude of T-stress parameter.

Experimental investigation on Hybrid Fibre-Reinforced Concrete (HFRC) fracture toughness and impact resistance with partial replacement of nano-silica

This paper focuses on the compressive strength, fracture toughness and impact resistance of hybrid fibre-reinforced concrete. A total of 78 numbers of compressive strength test cubes, 24 numbers fracture Edge Notched Disc Bend (ENDB) specimens, and 20 numbers of impact disc specimens were constructed and tested. The replacement of Nano-silica was kept constant at 2% based on literature. Glass and steel fibres of 50 mm length were used with 1–4% by weight of cement content. Based on the obtained results, Steel Fibre (SF), Glass Fibre (GS) and Nano-Silica (NS) combinations gave better strength and performance in concrete. The observation made herein is that NS reduces the porosity of concrete as verified with the ultrasonic pulse velocity (UPV) test and microstructural studies including Scanning Electron Microscope (SEM) analysis. The results indicate that fibre combinations achieved better fracture toughness and high-impact resistance while maximum compressive strength was attained for 2% NS + 0.5% SF and 0.5% GF combinations.

Comparison of mixed mode I/III fracture toughness data obtained from the ENDB specimens manufactured by gyratory cylinders with two diameters of 100 and 150 mm

ABSTRACT Different specimens have been developed to characterise the fracture parameters. Recently, a disc shape specimen entitled ‘edge notched disc bend’ (ENDB) sample was introduced; it is used for conducting pure mode I, III, and related fracture mode mixities. In this paper, to extend the application of ENDB specimen, a large number of mode I, III, and I/III fracture toughness experiments were conducted on HMA at three different low-temperatures (−5, −15, and −25°C) and by applying two rates of loading (1 and 5 mm/min). The effect of ENDB radius (i.e. 50 and 75 mm) for a constant thickness of 30 mm was investigated experimentally. Based on the data, both ENDB diameters result in nearly the same mixed I/III fracture toughness values,. The difference between the fracture toughness of both ENDB sizes was less than 8%. Hence smaller size of ENDB sample with a diameter of 100 mm can be employed successfully instead of 150 mm (that is commonly used in many asphalt fracture toughness studies). The results also indicated that the fracture toughness values in the full ranges of mode I/III case increase as the test temperature decreases (about −7% and −15% by increase from −25 to −15°C and −5°C).

Experimental and theoretical investigation of mixed-mode I/II and I/III fracture behavior of PUR foams using a novel strain-based criterion

Brittle fracture in Polyurethane (PUR) foam is experimentally and theoretically investigated under mixed-mode I/II and I/III loading conditions. Original test data are generated to examine different criteria in prediction of the mixed-mode fracture behavior of PUR foam. To this end, edge notched disc bend (ENDB) specimen which has been proven to be a suitable test configuration for such loading conditions is used, and subsequently, fracture angles and fracture toughness values are obtained experimentally. Three different fracture criteria, including minimum strain energy density (SED), maximum principal stress (MPS), and the recently introduced strain-based criterion called ECD-SN – which is established based on considering an effective critical distance – are considered to theoretically estimate the fracture behavior of PUR foam. A comparison between the experimental and theoretical results revealed that the ECD-SN criterion gives a better prediction both for the fracture angles and the mixed-mode fracture toughness of PUR foam. The results indicated that considering effective critical distance instead of the traditional approach, which assumes the critical distance is constant as a material property and independent of the loading mode, can considerably increase the accuracy of the theoretical estimation.

Pure mode I fracture resistance of hot mix asphalt (HMA) containing nano-SiO2 under freeze–thaw damage (FTD)

• Pure mode I fracture resistance of Nano-SiO2-modified asphalt mixture under FTD was evaluated. • The semi-circular bending and edge notched disc bend tests were performed on samples at ± 15 °C. • Nano-SiO 2 improved the behavior of LTC and ITC asphalt mixtures under mode I loading and FTD conditions. • Nano-SiO 2 improved the fracture energy and fracture toughness of asphalt mixture under FTD. • SCB geometry with an angle of 35°was critical due to lower average fracture toughness. In cold climates, increased traffic load has increased the damage caused by low-temperature cracks (LTCs) in asphalt mixes. Due to the viscoelastic behavior of asphalt mixtures, cracks caused by intermediate temperatures (ITCs) have led researchers to think more about improving the technical properties of asphalt mixtures with additives. On the other hand, asphalt pavements are affected by freeze–thaw cycles during their service life, a process that intensifies distress and reduces crack resistance. However, despite the effect of freeze–thaw cycles on the fracture behavior of LTCs and ITCs, this process has received less attention from researchers. Hence, in this study, the effect of Nano-SiO 2 on the occurrence of low and intermediate temperature cracks under freeze–thaw damage (FTD) conditions in asphalt mixtures has been investigated experimentally. For this purpose, Semi-Circular Bending (SCB) and Edge Notched Disc Bend (ENDB) tests were used under mode I loading to investigate the effect of Nano-SiO 2 on the crack resistance of the asphalt mixture (i.e., vertical and angular cracks) at temperatures of ± 15 °C. Before the fracture test, the samples were subjected to 0 and 3 FTD cycles to investigate the effect of these conditions on the long-term fracture resistance of the samples. The results showed that adding Nano-SiO 2 increased the brittleness of the mixtures, fracture energy, and fracture toughness of asphalt mixtures containing vertical and angular cracks in both 0 and 3 FTD cycles. Also, it was concluded that Nano-SiO 2 could recover a large part of the resistance reduced by the 3 FTD cycles; therefore, using Nano-SiO 2 additive improved the long-term behavior of LTC and ITC asphalt mixtures under mode I loading and FTD conditions. Finally, by comparing all the geometries used in this research, it was concluded that from the point of view of fracture energy, the ENDB geometry, and from the point of fracture toughness, the SCB geometry with an angle of 35°was critical due to lower average fracture energy and lower average fracture toughness, respectively (temperatures of ± 15 °C under 0 and 3 FTD cycles).

Effect of temperature on mixed mode I/III fracture behavior of diorite: An experimental investigation

Rock fracture after high temperature is always a hot topic in the field of rock engineering. However, the mixed mode I/III fracture characteristics of thermal-treated rocks have rarely been reported previously. In this paper, edge-notched disc bend (ENDB) specimens were used to study the mixed mode I/III fracture properties of diorite under different temperatures (25 ℃, 100 ℃, 200 ℃, 400 ℃, and 600 ℃). The P-wave velocities were measured before and after heating, and acoustic emission (AE) signals during fracture were monitored. Furtherly, the fractal dimension was calculated by the box-counting method to quantitatively describe the roughness of the fracture surface. The results showed that the fracture toughness of diorite is significantly affected by the temperature at each mode mixity. When the temperature rises from 25 ℃ to 200 ℃, the fracture toughness at pure mode I is more sensitive to temperature. For the specimens heated under 600 ℃, the reduction in fracture toughness at pure mode III is greater than that at mode I. The AE active period of diorite increases with the increase of temperature, indicating that the brittleness of diorite is weakened by high temperature. Moreover, for pure mode I fracture, the influence of temperature on the fractal dimension is limited. When mode III load is introduced, the fractal dimension is more obviously affected by the temperature and the fractal dimension increases with the increase of temperature. The findings provide new insights into the mechanism of mixed mode I/III fracture in heat-treated rocks.

Aging effect on the mixed-mode (I/III) fracture toughness of cement emulsified asphalt composite: Experimental and statistical investigation

Aging can affect the fracture behavior of bitumen-containing composites due to an increase in the total stiffness of the material. In this study, the effect of aging on the behavior of Cement Emulsified Asphalt Mortar (CEAM) under the mixed-mode I/III fractures was investigated at an intermediate temperature, and its fracture behavior was modeled with the aid of Response Surface Methodology (RSM). To collect the experiential data, several mix designs with different cement contents (C: [460, 500] kg/m3), bitumen-to-cement ratios (A/C: [0.3, 0.5]), and water-to-cement ratios (W/C: [0.4, 0.6]) were generated by Central Composite Circumscribed (CCC) method. Then, a series of fracture tests were performed on aged and unaged Edge Notched Disc Bend (ENDB) specimens. The results showed that for both aged and unaged (control) specimens, the highest fracture toughness is obtained under pure tensile mode and by increasing the contribution of mode III the corresponding value of fracture toughness decreases. The developed statistical models revealed that for unaged specimens, the increase in the A/C ratio has a significant effect on the stress intensity factors of mode I, mode III and mixed-mode I/III. However, for aged specimens, the effect of A/C is significantly observed in its interaction with W/C. The effect of aging on the fracture toughness of mortar was also measured with an index called the ratio of Aged to Unaged Fracture Toughness (FTRiAged/Unaged). The results showed that aging decreases the critical stress intensity factor of the mortar in the mode mixities, with the highest decrease of 15% under pure mode I.

Evaluation of low and intermediate temperatures fracture indices for modified Warm mix asphalt (WMA) using edge notched disc bend (ENDB) specimen

Fracture toughness and fracture energy have been used as the main fracture indices to evaluate asphalt mixtures' fracture resistance. Recently, some cracking-related index parameters such as Flexibility Index (FI), Toughness Index (TI), and Cracking Resistance Index (CRI) have proposed the brittle behavior of asphalt mixtures by normalizing the fracture energy obtained from the load–displacement curve. Although previous studies have evaluated fracture indices in a limited range of temperature conditions and on a limited number of geometries, this study tried to evaluate different fracture indices on Warm Mix Asphalt (WMA) using Edge notched disc bend (ENDB) geometry (pure mode I and III) at low and intermediate temperatures. Besides, environmentally friendly additives, including Calcium Lignosulfonate (CL) and polyester fibers (PE), were used in different percentages jointly and separately to improve WMA mixtures' fracture performance. The results showed that although the risk of cracking of pure mode III was higher than pure mode I based on the toughness results, nevertheless, more fracture energy was required to fracture the sample in pure mode III than pure mode I. For the modified WMA mixture, the fracture toughness and fracture energy in both pure modes I and III had a similar procedure and were affected by the presence of PE at −18 °C. At +25 °C, the fracture energy in pure mode I and III were higher for samples containing 0.25% PE-6% CL than for all samples. Finally, considering all the results, 0.25% PE-6% CL mixture due to higher fracture toughness, higher fracture energy, and a more flexible fracture (based on all three indices FI, TI, and CRI) in pure modes I and III were recommended to use WMA at temperatures of −18 °C and +25 °C.

Evaluating the fracture behaviour of modified asphalt concrete composites (ACC) at low and intermediate temperatures using edge notched disc bend (ENDB) specimen

In this study, fracture energy and fracture toughness are used to investigate the fracture behaviour of Asphalt Concrete Composites (ACC) in pure mode I and III using Edge Notch Disc Bend (ENDB) geometry at temperatures of −18°C and +25°C. Also, polyester fibre (PE) and calcium lignosulfonate (CL) additives were used separately and simultaneously to improve the performance of ACC's fracture. In order to evaluate the brittleness of the ACC, Flexibility Index (FI), Toughness Index (TI), and Cracking Resistance Index (CRI) were evaluated. The results showed that the top-down cracking potential due to the pure mode III loading condition for the base sample was higher than the opening cracking at −18°C due to lower fracture toughness and fracture energy of pure mode III than pure mode I. At +25°C, the ACC cracking potential was based on the results of fracture toughness and fracture energy for top-down cracking and opening cracking, respectively.