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The existing damage ratio strength theory using the damage ratio variable is only applicable for concrete, and the deviatoric traces are concave for high hydrostatic pressures. To expand the theory s application scope and make up for the deficiency, this paper proposes a six-parameter expression of the damage ratio variable considering the effect of both the Lode angle and the hydrostatic pressure. The values of the variable obtained by the proposed expression are verified by the experimental stress-strain curves of concrete under uniaxial, biaxial, and triaxial stress states. The modified criterion for concrete using the six-parameter damage ratio variable better represents the corresponding characteristics of the failure surface compared to the previous one. The proposed criterion is compared to the experimental data, against some strength criteria, which confirm the validity of the proposed criterion for concrete and lightweight aggregate concrete. Furthermore, for biaxial stress states, simplified criterion curves are also proposed. Keywords: damage ratio; lightweight aggregate concrete; multi-axial stress; strength criterion.

To deeply understand the rock failure characteristics under actual engineering condition, in which static geo-stress and dynamic disturbance usually act simultaneously, impact tests were conducted on sandstone subjected to axial static pre-stresses varying from 0 to 75 MPa by a modified split Hopkinson pressure bar. The fracturing process of specimens was recorded by a high speed camera. Dynamic parameters of sandstone, such as strain rate, dynamic strength and energy partition were acquired. Fracture mechanisms of pulverized specimens were identified by the method combining the displacement trend line and digital image correlation technique. Moreover, fragments of failed specimens were sieved to obtain the fragment size distribution. Test results revealed that, under the same incident energy, the dynamic compressive strength increases first, then decreases slowly and at last drops rapidly with the increase of pre-stress, and reaches the maximum under 24.4% of uniaxial compressive strength due to the closure of initial defects. Four final patterns were observed, namely intact, axial split, rock burst, and pulverization. The rock burst only occurs when the pre-stress lies in the elastic deformation stage or initial stable crack growth stage and the incident energy is intermediate. For pulverized specimens, the fracture mechanism is transformed into shear/tensile equivalent from tensile-dominated mixed mode as the pre-stress increases. Specimens with 75 MPa pre-stress release strain energy during failure process, contrary to specimens with lower pre-stresses absorbing energy from outside. The crushing degree of pulverized specimens exhibits a positive correlation with the pre-stress as a consequence of higher damage development in rock.

In this study, structural model tests using cubic granite specimens of 100 mm × 100 mm × 100 mm with a centered horizontal circular hole of 50 mm diameter were used to investigate the influence of axial stress (5–160 MPa) on borehole strainburst under true-triaxial stress conditions. A camera and an acoustic emission (AE) monitoring system were used to capture the failure process. The experimental results indicate that the micro- and macroscopic failures are significantly influenced by the axial stress. As the axial stress increases, the vertical stress increment and the duration of particle ejection (from particle ejection to spalling) and spalling failure (from spalling to rockburst), as well as particle ejection to the occurrence of rockburst tend to decrease first and then increase. However, the cracking mechanisms around the V-shaped failure zone and the features of the trans-granular and the extensile micro-cracking in the cross-section perpendicular to the borehole axis are unaffected by the axial stress. For the specimens tested, the rock failure stresses corresponding to crack initiation, particle ejection, crack damage, spalling and rockburst, as well as the cumulative AE absolute energy during the entire test increase as the axial stress increases from 5 to 100 MPa and then they all decrease as the axial stress continues to increase from 100 to 160 MPa. It was also observed in our tests that an increase in the axial stress suppressed the propagation of radial cracks but promoted the development of axial extensile cracks. As a result, rock fractures were developed more in the axial direction in a smoother and more planar fashion and rock fragments with elongated dimension along the borehole axis were produced in the rockburst process. These results suggest that a higher axial stress enhances axial fracturing or spalling during strainburst.HighlightsThe physical tests of the influence of axial stress (5 to 160 MPa) on borehole strainburst under true triaxial stress conditions were successfully carried out. The borehole rockburst process is significantly influenced by the axial stress.Stresses corresponding to crack initiation, particle ejection, crack damage, spalling failure and rockburst, as well as cumulative AE absolute energy are a function of the borehole axial stress.Axial fracturing in rockburst was observed to be significantly enhanced by increasing axial stress.

Carbon fiber-reinforced polymers (CFRP) have been actively employed as lightweight materials; yet, evaluating the material’s reliability under multi-axis stress states is still challenging owing to their anisotropic nature. This paper investigates the fatigue failures of short carbon-fiber reinforced polyamide-6 (PA6-CF) and polypropylene (PP-CF) by analyzing the anisotropic behavior induced by the fiber orientation. The static and fatigue experiment and numerical analysis results of a one-way coupled injection molding structure have been obtained to develop the fatigue life prediction methodology. The maximum deviation between the experimental and calculated tensile results is 3.16%, indicating the accuracy of the numerical analysis model. The obtained data were utilized to develop the semi-empirical model based on the energy function, consisting of stress, strain, and triaxiality terms. Fiber breakage and matrix cracking occurred simultaneously during the fatigue fracture of PA6-CF. The PP-CF fiber was pulled out after matrix cracking due to weak interfacial bonding between the matrix and fiber. The reliability of the proposed model has been confirmed with high correlation coefficients of 98.1% and 97.9% for PA6-CF and PP-CF, respectively. In addition, the prediction percentage errors of the verification set for each material were 38.6% and 14.5%, respectively. Although the results of the verification specimen collected directly from the cross-member were included, the percentage error of PA6-CF was still relatively low at 38.6%. In conclusion, the developed model can predict the fatigue life of CFRPs, considering anisotropy and multi-axial stress states.

This study proposes a novel method for predicting the axial stress of bolts by analyzing their nonlinear ultrasonic characteristics. We first derive the theoretical relationship between the relative nonlinear coefficient and stress. Then, we build a detection system based on nonlinear ultrasonic technology to measure the axial stress of bolts. We find that the relative nonlinear coefficient of bolts increases with the stress, and the increase rate becomes faster when the stress exceeds a certain value. This indicates that the relative nonlinear coefficient is very sensitive to the stress change and can be used to accurately characterize the bolt axial stress. We normalize the relative nonlinear coefficient and fit the experimental data to create a bolt axial stress prediction model. We also use both nonlinear ultrasonic and linear ultrasonic methods to predict the bolt axial stress. The prediction results shows that when the axial stress exceeds 200 MPa, the prediction error of nonlinear ultrasonic is less than 5%, indicating that the prediction accuracy is significantly better than that of the linear ultrasonic method. In summary, this study demonstrates the effectiveness of nonlinear ultrasonic technology in detecting axial stress in bolts and offers a novel approach for the evaluation and prediction of bolt axial stress under service conditions.

In this research study, the performance of geopolymer concrete (GPC) in-filled fibre-reinforced polymer (FRP) composite (GPC-FRP) columns exposed to compressive loading is examined using the finite element (FE) analysis. The load–deflection behaviour is investigated by considering the impact of the strength of concrete, different fibre orientations and thicknesses of FRP tubes in terms of the diameter/thickness (D/t) ratio, surface friction in between the concrete and enclosing FRP tube, the lateral confinement and the axial stress distribution characteristics. The load-carrying capacity (LCC) of the GPC-FRP composite columns and cement concrete (CC) in-filled FRP composite (CC-FRP) columns is compared and the results imply that the LCC of the GPC-FRP composite columns is (0.9 to 2.04%) greater than the CC-FRP composite columns. The improvement in the LCC and lateral confining pressure of the GPC-FRP composite columns is observed as the thickness of the FRP tube increases. The LCC of the GPC-FRP composite columns with a D/t ratio of 30 was almost (12.70 to 14.23%) greater than the GPC-FRP composite columns with a D/t ratio of 50. The GPC-FRP composite columns with a fibre orientation in the axial and hoop directions (0°) exhibit (8.4 to 11.39%) better performance than the columns with any other orientations (30° and 53°). The LCC of the GPC-FRP composite columns with a coefficient of friction of 0.25 and 0.5 are quite comparable. The axial stress distribution in the GPC-FRP composite columns with different tube thicknesses is explored in this research. This FE model is validated with the experimental results obtained by Kim et al., (2015) and the load and deflection are predicted with the validation error of 6.5 and 6.1%, respectively.

In a group of deep coal seams, mining of upper multilayers generates cyclic loading–unloading stresses in bottom layers, improving their permeability and enhancing coalbed methane gas drainage. In this study, the respective repeated mining stress path was introduced into the seepage test. A high-resolution 3D X-ray microscopy imaging system was used for computed tomography scanning of coal samples. The crack characteristics and stress–permeability relationships were analyzed based on the experimental results. It was found that the permeability increased approximately by 5.8 times after the loading–unloading cyclic tests. The fracture volume and rate grew obviously, and the complexity degree of the fracture network increased. The permeability evolution during the test was closely related to stress–strain state of the coal sample with variable response characteristics of axial deviatoric stress, confining pressure, and axial pressure. The realization of different numbers of loading–unloading cycles in the tests revealed similar fracturing effects of the initial and later cycles, whereas the largest fracturing effect was obtained within the effective stress range from 4 to 11 MPa. The mechanical deformation was found to play an important role in the stress–permeability relationship. The crack closure and elastic deformation at low stress-state caused the permeability loss, while crack growth and plastic deformation at high-stress state increased the permeability. The plastic strain significantly grew with the number of loading–unloading cycles, but the fracturing effect of different numbers of cycles in the critical fissure point was nearly the same. The results obtained are considered instrumental in studying the pressure relief and extraction of coalbed methane during upper protective layer mining in coal seam groups.

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.

This paper presents an investigation on the cyclic behavior and design aspects of a hybrid self-centering wall (SW), in terms of the wall’s axial stress and post-tensioning tendons’ prestressing ratios. These key parameters are examined extensively within the context of both force and displacement-based design approaches. Finite element models of SWs are developed and verified against experimental results available from the literature. The numerical model is used to perform a parametric study where the wall’s behavior, including cyclic response, damage severity and pattern, energy dissipation, stiffness degradation, period shift, and residual drift are widely discussed in the context of the aforementioned design approaches. Numerical results indicate that an increase in the axial stress ratio intensifies the extent and severity of the wall’s damage, lateral capacity, residual drift, energy dissipation, and structural seismic demand. Additionally, it is shown that by increasing the tendons’ prestressing ratio, the structural seismic demand is increased, while the wall’s damage, residual drift, and energy dissipation capacity are decreased. Based on this study, the wall’s axial stress ratio and the tendons’ prestressing ratio should be limited to a range of 0.075–0.115 and less than 0.75, respectively, to meet the basic objective performance. Higher objective performance criteria could be maintained with using external energy dissipaters, and adopting a suitable design approach.

Frequent rockburst disasters in deep-buried engineering projects severely impact construction. To explore the influence of axial stress on rockburst in deep-buried tunnels, large-scale true triaxial rockburst experiments were conducted under four different axial stress ratio conditions ( η t , axial loading stress/vertical loading stress) using a self-developed true triaxial loading device under the condition of “pre-loading before excavation”. The influence of axial stress on the rockburst process and failure characteristics in deep tunnels was studied using a combination of real-time video monitoring, rockburst debris sieving, and acoustic emission monitoring. The results indicate: (1) all four specimens subjected to different axial stress ratio loading conditions exhibited three stages of macroscopic failure: small particle ejection, flake spalling, and large fragment ejection. Ultimately, “V”-shaped notches appeared on both sides of the tunnel. (2) The failure stress, fragment volume, and fragment size distribution of the rockburst specimens exhibited a clear two-stage failure characteristic with increasing axial stress ratio. In the lower axial stress ratio stage ( η t ≤ 0.7), the increase in the axial stress ratio enhances lateral confinement, thereby increasing the crack initiation strength of the surrounding rock, inhibiting crack formation and propagation, and thus suppressing damage to the surrounding rock of the tunnel. In the higher axial stress ratio stage ( η t > 0.7), the increase in axial stress ratio makes the Poisson effect of the surrounding rock more pronounced, promoting the generation and propagation of cracks along the tunnel axis direction, thereby promoting damage to the surrounding rock. (3) Based on the analysis of acoustic emission parameters (fracture properties), it can be concluded that in the lower axial stress ratio stage ( η t ≤ 0.7), an increase in the axial stress ratio leads to a higher proportion of shear fracture in rockburst damage. Conversely, in the higher axial stress ratio stage ( η t > 0.7), the increase in axial stress ratio gradually reduces the proportion of shear fracture in rockburst damage.