Search Results (21)

This study examines how coarse aggregate volume fraction (V_a) affects the double-K fracture toughness and fundamental mechanical properties of concrete. Wedge-splitting tests were conducted on specimens with six different V_a values: 19%, 25%, 31%, 37%, 43%, and 50%. The results indicate that compressive strength (f_c) and elastic modulus (E) consistently increase with V_a, reaching 59.8 MPa and 37.9 GPa at V_a = 50%, respectively. Conversely, tensile strength (f_t), double-K fracture toughness (including initiation toughness $K_{\mathrm{IC}}^{\mathrm{ini}}$ and unstable toughness $K_{\mathrm{IC}}^{\mathrm{un}}$), and fracture energy (G_IF) initially increase before decreasing, peaking at an optimal V_a of 37%. Specifically, $K_{\mathrm{IC}}^{\mathrm{ini}}$, $K_{\mathrm{IC}}^{\mathrm{un}}$, and G_IF reached their maximum values of 0.54 MPa·m^1/2, 1.20 MPa·m^1/2, and 225.0 N/m at V_a = 37%. The tortuosity of crack paths follows a similar trend, becoming more pronounced up to V_a = 37% before diminishing. Furthermore, quantitative exponential relationships were established between f_t and $K_{\mathrm{IC}}^{\mathrm{ini}}$, $K_{\mathrm{IC}}^{\mathrm{un}}$, and G_IF. A safety warning parameter (δ), derived from the double-K fracture toughness, was proposed to quantitatively assess the pre-peak ductility, with values ranging from 0.88 to 0.72 in this study. The findings offer valuable guidance for optimizing concrete mix design, suggesting that a V_a range of 25% to 31% provides an optimal balance between high crack initiation resistance and adequate safety warning capacity for critical engineering structures. Full article

Fracture failure in quasi-brittle materials poses a persistent challenge in materials science and engineering. This study presents a thorough investigation of the Boundary Effect Model (BEM), offering a nuanced understanding of the size effect on fracture properties. The conceptual framework, evolutionary process, and applicability scope of BEM are elucidated, highlighting its accuracy and reliability in calculating fracture properties across various quasi-brittle materials. Through the integration of BEM with diverse fracture tests—such as three-point bending, four-point bending, and wedge-splitting—a linear correlation between maximum failure loads and material fracture properties is established. Notably, the study demonstrates that fracture properties, determined by BEM, can be regarded as consistent material constants across specimens of varying sizes, initial notch lengths, geometries, and microstructures. Validation of the BEM’s reliability encompasses the analysis of 140 fracture test results involving concrete, hard rocks, and bamboo scrimber. The synergy of non-linear and linear BEM analyses emerges as a robust approach for accurately predicting the fracture behavior of quasi-brittle materials. This comprehensive exploration sheds light on the potential of the Boundary Effect Model as a valuable tool for predicting and understanding fracture mechanics in diverse materials and scenarios. This research serves as an effective approach to accurately evaluating the fracture properties of quasi-brittle materials, which is of great practical significance for material design, engineering construction, and various industrial applications. Full article

In underground engineering, the deformation of surrounding rock caused by “three heights and one disturbance” leads to the failure of grouted rockbolt systems, which causes huge economic losses to the mining industry. The research shows that the failure process of grouted rockbolt systems is the result of energy accumulation and release, but the relationship between failure mode and energy dissipation is rarely studied. Based on this, the load transfer behavior, energy dissipation, failure mode and failure mechanism of the grouted rockbolt systems are investigated from the perspective of energy in this study using the indoor pullout test. Test results show that the load decreases rapidly, and the absorbed energy decreases due to the whole-body splitting crack. The absorbed energy of the specimen in the splitting crack mode is lower than that in the pullout failure mode. When the pullout load reaches its peak, the pullout load of the specimen with split failure mode decreases sharply. Meanwhile, the load of the specimen with pullout failure mode is relatively slow, and the energy absorption rate decreases gradually due to the occurrence of cracks. However, the reduction in the energy absorption rate under pullout failure is lower than that under split failure. The radial pressure in the grouted rockbolt systems increases due to the wedge action. When the radial pressure exceeds the tensile strength of concrete, the specimen will experience split failure, otherwise pullout failure will occur. Full article

Saturated concrete is significantly different from dry concrete in fracture mechanical properties. Using the wedge-splitting tensile method to research the rule of change in moisture content, double-K fracture toughness and fracture energy of three strength grades (C20, C30, and C40) of concrete immersed in a free water environment for 0 h, 2 h, 5 h, 24 h, and 120 h were studied in order to provide support for the safety evaluation of concrete structures in a water environment. The initial cracking fracture toughness of C20, C30, and C40 concrete in saturated state were, respectively, 29.6%, 23.2%, and 33.4% lower than that in dry state. The unstable fracture toughness of C20, C30, and C40 concrete in saturated state were, respectively, 22.7%, 23.9% and 33.8% lower than that in dry state. The fracture energy of C20, C30, and C40 concrete in saturated state are only 71.99%, 70.29%, and 66.11% of that in dry state, respectively. The initial cracking fracture toughness and unstable fracture toughness of concrete all show a linear, decreasing trend with an increase in moisture content. Before the crack initiation, the measured P–CMOD curve had an obvious linear elastic stage, stable expansion stage, and unstable expansion stage. The critical crack opening displacement gradually decreases with an increase in moisture content; the deformation capacity and toughness of concrete are shown to decrease. The humidity state should be fully considered when evaluating the fracture mechanical properties of concrete. Full article

The low permeability of coal seams has always been the main bottleneck restricting coalbed gas drainage. To improve the permeability of a coal seam, a high-energy gas fracturing technology is proposed. Firstly, based on the high-energy gas fracturing mechanism and gas production principle of fracturing agent, a fracturing agent applicable to coal reservoirs was developed, and its performance and sensitivity were tested. Then, a high-energy gas-fracturing simulated coal sample test was conducted, and the drilling wall pressure and strain of the simulated coal sample were tested. The results show that high-energy gas fracturing technology is a safe and efficient technical means for improving the permeability of coal reservoirs. The pressure–time curve of the borehole wall under the action of high-energy gas can be divided into three stages, namely, the rapid pressure-rising stage, steady pressure stage, and falling stage; the maximum pressure in the borehole is about several hundred MPa, and the pressure distribution in the borehole is not uniform. Compared with explosives blasting, the stress wave intensity in coal caused by the action of high-energy gases is low, the duration is short, and the peak stress attenuates slowly with increasing distance. Under the action of high-energy gas, no crush zone is generated near the borehole; the number of radial cracks produced is small but long. The extent of the fracture zone depends mainly on the quasi-static splitting wedge effect of the high-energy gas. Full article

Background: Existing prediction models for post-transplant mortality in patients bridged to heart transplantation with temporary mechanical circulatory support (tMCS) perform poorly. A more reliable model would allow clinicians to provide better pre-operative risk assessment and develop more targeted therapies for high-risk patients. Methods: We identified adult patients in the United Network for Organ Sharing database undergoing isolated heart transplantation between 01/2009 and 12/2017 who were supported with tMCS at the time of transplant. We constructed a machine learning model using extreme gradient boosting (XGBoost) with a 70:30 train:test split to predict 1-year post-operative mortality. All pre-transplant variables available in the UNOS database were included to train the model. Shapley Additive Explanations was used to identify and interpret the most important features for XGBoost predictions. Results: A total of 1584 patients were included, with a median age of 56 (interquartile range: 46–62) and 74% male. Actual 1-year mortality was 12.1%. Out of 498 available variables, 43 were selected for the final model. The area under the receiver operator characteristics curve (AUC) for the XGBoost model was 0.71 (95% CI: 0.62–0.78). The most important variables predictive of 1-year mortality included recipient functional status, age, pulmonary capillary wedge pressure (PCWP), cardiac output, ECMO usage, and serum creatinine. Conclusions: An interpretable machine learning model trained on a large clinical database demonstrated good performance in predicting 1-year mortality for patients bridged to heart transplantation with tMCS. Machine learning may be used to enhance clinician judgement in the care of markedly high-risk transplant recipients. Full article

Poly (methyl methacrylate) (PMMA) bone cement is an excellent biological material for anchoring joint replacements. Tensile strength $f_{\mathrm{t}}$ and fracture toughness $K_{\mathrm{IC}}$ have a considerable impact on its application and service life. Considering the variability of PMMA bone cement, a three-parameter Weibull distribution method is suggested in the current study to evaluate its tensile strength and fracture toughness distribution. The coefficients of variation for tensile strength and fracture toughness were the minimum when the characteristic crack of PMMA bone cement was ${\alpha }_{\mathrm{ch}}^{\ast }=8d_{\mathrm{av}}$. Using the simple equation ${\alpha }_{\mathrm{ch}}^{\ast }=8d_{\mathrm{av}}$ and fictitious crack length $\Delta {\alpha }_{\mathrm{fic}}=1.0d_{\mathrm{av}}$, the mean value $\mu$ (= 43.23 $\mathrm{MPa}$), minimum value $f_{\mathrm{t}}^{\min }$ (= 26.29 $\mathrm{MPa}$), standard deviation $\sigma$ (= 6.42 $\mathrm{MPa}$) of tensile strength, and these values of fracture toughness ($\mu$ = 1.77 $\mathrm{MPa}\cdot {\mathrm{m}}^{1/2}$, $K_{\mathrm{IC}}^{\min }$ = 1.02 $\mathrm{MPa}\cdot {\mathrm{m}}^{1/2}$, $\sigma$ = 0.2644 $\mathrm{MPa}\cdot {\mathrm{m}}^{1/2}$) were determined simultaneously through experimental data from a wedge splitting test. Based on the statistical analysis, the prediction line between peak load $P_{\max }$ and equivalent area $A_{\mathrm{e}}^1\left(A_{\mathrm{e}}^2\right)$ was obtained with 95% reliability. Nearly all experimental data are located within the scope of a 95% confidence interval. Furthermore, relationships were established between tensile strength, fracture toughness, and peak load $P_{\max }$. Consequently, it was revealed that peak load might be used to easily obtain PMMA bone cement fracture characteristics. Finally, the critical geometric dimension value of the PMMA bone cement sample with a linear elastic fracture was estimated. Full article

Fiber-reinforced polymers (FRPs) are widely used within civil structural applications either for structural retrofitting or new constructions. This is due to their appreciable mechanical properties such as high stiffness and strength, resistance to environmental effects, as well low density. Through the years, such peculiarities have encouraged researchers to apply FRP cables within the design of prestressing systems, where steel cables are systematically adopted. However, the brittleness intrinsic to FRP materials necessitates additional efforts to design the anchorage devices. In fact, tendons are here subjected to stress peaks, which need to be controlled in order to prevent the premature failure of the cable. Following this goal, authors recently studied an optimized split-wedge anchorage, for 12 mm-diameter pultruded-carbon-fiber-reinforced polymer (PCFRP) tendons, adopting double-angle (DA) wedges, and compared its performance with a single-angle (SA) wedge configuration. Tensile tests were performed on 3 SA and 2 DA prototypes, respectively, through a universal testing machine: the DA configuration exploited the average cable capacity (257 kN) once, denoting a maximum efficiency. The obtained experimental results are utilized, in the framework of the present work, to calibrate contact parameters of nonlinear finite element models. The presented numerical results helped to assess benefits of the proposed configurations and the behavior of the anchorage components: the DA configuration turned out to satisfactorily avoid stress peak superpositions on the cable, with a reduction in pressure in the loading end of the cable with respect to the SA model. Full article

Although highly desirable, the experimental technology of the dynamic mechanical properties of materials under multiaxial impact loading is rarely explored. In this study, a true-biaxial split Hopkinson pressure bar device is developed to achieve the biaxial synchronous impact loading of a specimen. A symmetrical wedge-shaped, dual-wave bar is designed to decompose a single stress wave into two independent and symmetric stress waves that eventually form an orthogonal system and load the specimen synchronously. Furthermore, a combination of ground gaskets and lubricant is employed to eliminate the shear stress wave and separate the coupling of the shear and axial stress waves propagating in bars. Some confirmatory and applied tests are carried out, and the results show not only the feasibility of this modified device but also the dynamic mechanical characteristics of specimens under biaxial impact loading. This novel technique is readily implementable and also has good application potential in material mechanics testing. Full article

In a naturally saturated state, rocks are likely to be in a stress field simultaneously containing static and dynamic loads. Since rocks are more vulnerable to tensile loads, it is significant to characterize the tensile properties of naturally saturated rocks under coupled static–dynamic loads. In this study, dynamic flattened Brazilian disc (FBD) tensile tests were conducted on naturally saturated sandstone under static pre-tension using a modified split-Hopkinson pressure bar (SHPB) device. Combining high-speed photographs with digital image correlation (DIC) technology, we can observe the variation of strain applied to specimens’ surfaces, including the central crack initiation. The experimental results indicate that the dynamic tensile strength of naturally saturated specimens increases with an increase in loading rate, but with the pre-tension increases, the dynamic strength at a certain loading rate decreases accordingly. Moreover, the dynamic strength of naturally saturated sandstone is found to be lower than that of natural sandstone. The fracture behavior of naturally saturated and natural specimens is similar, and both exhibit obvious tensile cracks. The comprehensive micromechanism of water effects concerning the dynamic tensile behavior of rocks with static preload can be explained by the weakening effects of water on mechanical properties, the water wedging effect, and the Stefan effect. Full article

Versatility and high performance in terms of specific stiffness and strength, as well as non-corrosive sensitivity, make FRP (Fiber-Reinforced Polymer) cables a viable alternative to steel ones in the development of prestressing systems. On the other hand, the orthotropic and brittle nature of FRPs could trigger a premature failure of the cable in the anchorage system, for which several solutions have been proposed so far in civil structural applications. In this context, after a preliminary state of the art, the work introduces a split wedge anchorage for FRP (ϕ = 12 mm) cables proposing two different solutions for steel wedges having the external surface: either (1) a constant (3 degrees) slope or (2) a double slope obtained by shaping it with an angle of 3.0 degrees before and then of 3.1 degrees along the most tapered part. The goal was to exploit the nominal cable capacity (257 kN), avoiding stress peaks that cause its premature failure. The proposed solutions have been experimentally tested and, as far as the double angle solution is concerned, the failure loads were equal to 222 and 257 kN, denoting that the proposed solution can reach the cable capacity. Clearly, further investigations are needed to check the variability of the results and eventually improve the system. Full article

This research evaluates the fracture behavior of concrete with reactive magnesium oxide (MgO). Replacing cement with MgO is an attractive option for the concrete industry, mainly due to sustainability benefits and reduction of shrinkage. Four different MgO’s from Australia, Canada, and Spain were used in the concrete mixes, as a partial substitute of cement, at 5%, 10%, and 20% (by weight). The fracture toughness (K_I) intensity factor and the stress–strain softening parameters of the wedge split test were evaluated after 28 days. The experimental results showed that the replacement of cement with MgO reduced the fracture energy between 13% and 53%. Moreover, the fracture energy was found to be correlated with both compressive strength and modulus of elasticity. A well-defined relationship between these properties is important for an adequate prediction of the non-linear behavior of reinforced concrete structures made with partial replacement of cement with MgO. Full article

The presented research’s main objective was to develop the solution to the global problem of using steel waste obtained during rubber recovery during the tire recycling. A detailed comparative analysis of mechanical and physical features of the concrete composite with the addition of recycled steel fibres (RSF) in relation to the steel fibre concrete commonly used for industrial floors was conducted. A study was carried out using micro-computed tomography and the scanning electron microscope to determine the fibres’ characteristics, incl. the EDS spectrum. In order to designate the full performance of the physical and mechanical features of the novel composite, a wide range of tests was performed with particular emphasis on the determination of the tensile strength of the composite. This parameter appointed by tensile strength testing for splitting, residual tensile strength test (3-point test), and a wedge splitting test (WST), demonstrated the increase of tensile strength (vs unmodified concrete) by 43%, 30%, and 70% relevantly to the method. The indication of the reinforced composite’s fracture characteristics using the digital image correlation (DIC) method allowed to illustrate the map of deformation of the samples during WST. The novel composite was tested in reference to the circular economy concept and showed 31.3% lower energy consumption and 30.8% lower CO₂ emissions than a commonly used fibre concrete. Full article

This article investigates the development of the following material properties of concrete with time: compressive strength, tensile strength, modulus of elasticity, and fracture energy. These properties were determined at seven different hydration ages (18 h, 30 h, 48 h, 72 h, 7 days, 14 days, 28 days) for four pure cement concrete mixes totaling 336 specimens tested throughout the study. Experimental data obtained were used to assess the relationship of the above properties with the concrete compressive strength and how these relationships are affected with age. Further, this study investigates prediction models available in literature and recommendations are made for models that are found suitable for application to early age concrete. Results obtained indicate that the relationship between the splitting tensile strength and concrete compressive strength can be approximated with a power function between 0.7 and 0.8, and this correlation is not affected by age. Fracture energy of the concrete and modulus of elasticity values obtained in this study correlate well with the square root of the compressive strength and it was found that this relationship holds true for all hydration ages investigated in this paper. Inverse analysis on the wedge-splitting test was conducted to determine the direct tensile strength. Values of tensile strength obtained from the inverse analysis have been validated numerically by carrying out finite element analysis on the wedge split, and anchor pull-out tests. The ratio of the tensile strength obtained from the inverse analysis to the splitting tensile strength was found to be in the range of 0.5–0.9 and 0.7 on average. Full article

This paper investigated the fracture properties of concrete in dry environments with different curing temperatures (5, 20, 40, and 60 °C). For each curing condition, the key fracture parameters of concrete were tested using wedge splitting specimens at five different ages (3, 7, 14, 28, and 60 d). The results show that in dry environments, the effective fracture toughness and fracture energy of concrete exposed to elevated temperatures increased at a relatively high growth rate at an early age. Nevertheless, the growth speed of effective fracture toughness and fracture energy decreased more quickly at elevated temperatures in the later stages. As a result, the concrete cured at higher temperature exhibited lower ultimate values of fracture parameters, and vice-versa. Namely, a temperature crossover effect was found in the effective fracture toughness and fracture energy of concrete under dry environments. Considering the early growth rate and ultimate values of fracture parameters, the optimum temperature suitable for concrete fracture properties development under dry condition was around 40 °C. Full article