Peak Stress Increase Research Articles

The dependence of Zener-Hollomon parameter on softening behavior and dynamic recrystallization mechanism of a biodegradable Zn-Cu-Mg alloy

The Zn-Cu-Mg alloy exhibits good strength, ductility, anti-aging and antibacterial properties, which lays the foundation for developing high performance Zn-based biodegradable alloys. However, the constitutive equation and dynamic recrystallization (DRX) behavior of this alloy remain unclear, making the optimization of hot processing parameters almost dependent on trial and error. This work aims to address these issues by investigating the hot compression process. The calculated average activation energy Q of this alloy is 141.338 KJ⋅mol-1, exhibiting excellent heat resistance. The deformed microstructure strongly depends on the Zener-Hollomon parameter (Z=ε˙exp(QRT)). Discontinuous DRX (DDRX) dominates at low lnZ, which has a significantly different orientation from the parent grain. Continuous DRX (CDRX) occurs within the grain and at grain boundaries, and is dominant at middle lnZ, mainly through activation 〈a〉 or/and 〈c+a〉 slip systems. Additionally, the activation of prismatic slip further promote CDRX, and most CDRX grains inherit the 30°[0001] orientation from the parent grains. The volume fraction of DRX demonstrates a decreasing trend followed by an increasing trend with increasing lnZ. At high lnZ, the increase of DRX grains is conducive to weakening the texture, and twin-induced DRX (TDRX) is significantly promoted, leading to an increase in both peak stress and strain hardening rate. Furthermore, the grains with c-axis aligned parallel to the compression direction (CD) are more prone to twinning, while the c-axis perpendicular to CD are the hard orientation of basal slip and compression twins. TEM results reveal that a decrease of c/a value promotes the activation of non-basal slip near the twin boundary, and the highly active 〈c〉 and 〈c+a〉 slips contribute to the increase of strain hardening rate. The results of this study are significant for understanding the workability of Zn-Cu-Mg alloys at high lnZ due to its high efficiency and low cost.

Effects of waste oyster shell replacing fine aggregate on the dynamic mechanical characteristics of concrete

Waste oyster shells (WOS) have the potential to serve as a construction material, offering a sustainable alternative to traditional fine aggregates in the production of WOS concrete. This can play a critical role in reducing environmental issues resulting from the overexploitation of river sand and the haphazard disposal of WOS. Although existing research has predominantly focused on understanding the static mechanical characteristics of concrete when WOS is employed, the dynamic mechanical properties have still received less attention. To understand the impact of WOS as a substitute for fine aggregates on the dynamic mechanical properties of concrete, a series of tests employing Split Hopkinson Pressure Bar (SHPB) were carried out. The findings demonstrate that the peak stress and elastic modulus increase as the WOS substitution ratio (Sr) increases from 0 to 20% but exhibit an exponential decline as Sr increases from 20 to 100%. This response can be explained by the joint effects of the pore-filling effect caused by WOS sand and the increasing air content caused by WOS sand. As Sr increases from 0 to 20%, the pore-filling mechanism becomes predominant as the water absorption rate decreases slightly from 4.31 to 3.83%. However, for Sr increasing from 20 to 100%, the negative influence of the air content becomes the primary contributing factor, where the water absorption rate increases from 3.83 to 14.68%. Furthermore, under the same impact pressure, the concrete with Sr = 20% absorbed the most energy, providing the best dynamic mechanical performance. These findings highlight the potential use of WOS in concrete for improving its dynamic characteristics, promoting both sustainable construction and enhancing the material properties in impact-resistant structures.

Investigating the effect of zirconium dioxide micro and nano particles on the mechanical characteristics of Al-7075 alloy

ABSTRACT The study investigates the influence of zirconium dioxide (ZrO2) particles on the mechanical properties of Al7075 alloy and aims to assess the potential for tailored compositions to meet specific engineering requirements, particularly in aerospace applications. Al7075-ZrO2 composite was fabricated with varying percentages of micro (µ) ZrO2 (1%, 2%, and 3%) and nano ZrO2 (0.17%, 0.34%, and 0.51%) particles using the stir casting technique. Mechanical testing was conducted to evaluate the peak stress, load, and yield strength of the resulting Al7075-ZrO2 composites. Experimental findings reveal that the highest peak stress and load were achieved at 0.51% nano ZrO2 content, indicating a reinforcement effect. Moreover, the composite with 0.34% nano ZrO2 exhibited 59% increased peak stress of 153 MPa and 24% decreased strain 3.7% compared 125 MPa and 4.9% for pure Al7075. Al7075-µZrO2 composite has shown 125 MPa of peak stress and 2.9% maximum strain. These properties highlight the potential of Al7075-ZrO2 composite for applications requiring high resistance to tensile failure. Incorporating nano ZrO2 particles positively impact the load-bearing capacity and strength of Al7075 alloy matrix. Specifically, compositions with tailored ZrO2 content percentages demonstrate the potential for meeting the stringent requirements of aerospace applications.

Failure criterion and elasto-plastic constitutive model of seawater concrete incorporating coral aggregate subjected to triaxial loading

Recently, there has been a growing interest in utilizing coral reef debris as a replacement for river sands and natural coarse aggregates (NA). A novel type of seawater concrete, designated as CAC, was developed to effectively utilise raw coral debris and reduce material transportation costs. This study aimed to examine the mechanical properties of CAC under both uniaxial and triaxial loading conditions. Consequently, 18 prism uniaxial specimens and 90 triaxial cylinder specimens were prepared, considering concrete strength (C20, C40), replacement ratio of coral coarse aggregate (CA) (50 %, 75 % and 100 %), and lateral stress ratio (0, 0.1, 0.2, 0.3 and 0.5). The results indicated that the increase in peak stress and strain is associated with the triaxial stress ratio and the cylinder compressive strength of coarse aggregate. Furthermore, the compressive meridian of CAC is lower than that of conventional concrete. Based on the test results in this study, a modified William-Warnke failure criterion for CAC was established. Additionally, an elasto-plastic model considering a modified hardening and softening law was suggested for CAC under tri-axial loading. The proposed models were found to be well verified by the test results.

Axial compressive behavior of bamboo sheet twining tube-confined concrete columns with an inner original bamboo tube

The bamboo is widely distributed around the world and the product process is developed well, the biomass bamboo fiber composite is potential to providing strong confinement for concrete, therefore in this paper, bamboo sheet twining tube-confined concrete columns with an inner original bamboo tube (BSTCB) is proposed. This column is a new structure that consists of an external bamboo fiber composite tube and an internal original bamboo tube filled with concrete between the outer and inner tubes. A total of 18 specimens were tested in axial compression. The effects of BCT thickness and original bamboo tube diameter on the mechanical properties of the specimens were analyzed. The experimental results suggested that BCT has a strong lateral confinement ability. The strength and deformability of concrete increased significantly when BCT thickness increased. When the number of BCT layers was in the range of 6–18 layers, the increase in peak stress ranged from 7.7 % to 12.8 % and the increase in ultimate strain ranged from 6.4 % to 24.0 %. The increase in the diameter of the original bamboo tubes had a positive effect to the deformation capacity, while limited effect on the concrete compressive strength. Within the change in diameter from 90 mm to 110 mm, the reduction in concrete strength is within 6.0 %, and the change in ultimate strain ranges from 8.5 % to 14.1 %. Finally, the peak stress and strain of the concrete were evaluated using the existing FRP-confined concrete model, and the full stress-strain curve of the concrete was obtained.

Experimental study of mechanical properties of artificial dam for coal mine underground reservoir under cyclic loading and unloading

This study investigates the stability of an artificial dam used in an underground reservoir in a coal mine under periodic weighting imposed by overlying rock strata. For this purpose, cyclic loading and unloading tests with different stress amplitudes were designed. Differences in the mechanical performance of the artificial dam with and without overlying strata were analyzed using a uniaxial compression test. The mechanical properties of the structure under constant-amplitude cyclic loading and unloading were characterized. Further, the law of influence of stress amplitude on stability was discussed. A formula for predicting the mechanical performance of the artificial dam with its overlying rocks (hereafter referred to as the complex) was finally derived and was suitable for clarifying the law of damage in the complex under cyclic loading and unloading. The results showed that the complex had changed the internal structure of rocks. The strength and deformation of the complex were intermediate to that of either single structure. All three underwent brittle failure. During the constant-amplitude loading and unloading tests, the hysteresis loop could be divided into three phases, namely, sparse, dense, and sparse again, with a shift in the turning point in rock deformation memory effect. As the stress amplitude increased during the test, the damping ratio of the specimens decreased, and the area of the hysteresis loop increased non-linearly. The dynamic elastic modulus decreased first and then increased. The confidence interval for the formula fitted based on the test results was above 97%. Damage to the complex caused by constant-amplitude loading and unloading could be divided into three stages. An increase in peak stress served as a catalyst for the evolution of small cracks within the specimens into median and large cracks, thereby accelerating the damage process.

Utilisation of Athel Leaves to Improve the Unconfined Compressive Strength of Soil

Athel Leaves (AL) are plentiful agricultural waste that might result in a detrimental effect on the environment owing to inadequate disposal. The purpose of this paper is to experimentally assess the impact of Athel leaves as a novel sustainable waste application on the unconfined compressive strength (UCS) of soil. For the purpose of fulfilling this main objective, five AL percentages (1%, 2%, 3%, 4% and 5% by dry weight of soil) and four curing periods (3, 7, 14 and 28 days) were selected. The failure pattern was also studied to better understand the ultimate behaviour of the improved soil. Assessment of the derived conclusions revealed that the inclusion of AL into soil enhanced the UCS. Careful inspection of the stress-strain relationships showed that inclusion of AL resulted in increased peak stress at large strains. Additionally, it was found that bulging and shear were the two patterns observed in this research. This study confirms the possibility of incorporating AL in geotechnical applications with significant environmental benefits.

Study on the calcium dissolution of self-compacting recycled concrete

Considering the effects of the replacement rate of recycled coarse aggregate (RCA), content of fly ash (FA), and solution temperature, the accelerated calcium dissolution test of self-compacting recycled concrete (SCRC) was simulated by using ammonium chloride solution. The results show that the physical properties (the mass loss ratio and the dissolution depth) and the strength degradation of SCRC increase with the increase of the replacement rate of RCA and solution temperature. With the increasing content of FA, the physical properties decrease approximately linearly, while the strength degradation first decreases and then increases. For the SCRC with 50% RCA and a dissolution age of 56 days, the dissolution depth of SCRC with the FA contents of 10%, 20%, and 30% decreased by 3.85%, 10.00%, and 15.56% respectively compared with that of SCRC without FA, and the degradation rate of compressive strength also decreased by 11.60%, 25.91%, and 15.01%, respectively. The peak stress of the stress-strain curve of SCRC under axial compression is negatively correlated with the dissolution age, replacement rate of RCA, and solution temperature, while the peak strain is positively correlated with these variables. When the dissolution age is more than 28 days, the calcium silicate hydrate (C-S-H) gels produced by low-volume (10%, 20%) FA after complete hydration lead to an increase in peak stress and a decrease in peak strain of SCRC, while high-volume (30%) FA has the opposite effect due to incomplete hydration. Finally, the performance degradation model of SCRC under calcium dissolution was established.

Damage constitutive model of polypropylene-fibre-reinforced recycled aggregate concrete

Adding polypropylene fibre (PPF) to recycled aggregate concrete (RAC) not only improves performance but also has economic benefits. The effects of single-blend and double-blend micro- and macro-PPFs in RAC specimens were studied. Micro- and macro-PPFs were used to design and produce 30 groups of PPF-reinforced RAC specimens with 0%, 25% and 50% coarse aggregate (CA) substitution rates. Controlling the fibre mixing proportions, the stress–strain curve, elastic modulus, peak strength, peak strain and acoustic emission amplitude–frequency extremum (AFE) of each group of specimens were obtained. The elastic modulus and peak stress of specimens without PPFs decreases gradually with an increase of the CA substitution rate. However, there was a certain increase in elastic modulus and peak stress when PPFs were added. A damage constitutive model for PPF-reinforced RAC was established. By fitting this model, it was found that although the elastic modulus and peak stress of the RAC specimens increased by a certain extent, the fitting parameters of RAC were greater than those of ordinary concrete and the RAC post-peak strength was lower. The evolution law of acoustic emission AFE of PPF-reinforced RAC was studied. It was found that the cumulative AFE of RAC with PPF was larger, indicating that the PPFs limited crack propagation and increased the fracture energy.

Energy evolution and mechanical properties of modified magnesium slag-based backfill materials at different curing temperatures

To study the mechanical properties and failure characteristics of all-solid waste cementitious materials prepared in the intense geothermal environment of deep mines. Uniaxial compression, acoustic emission (AE), SEM and other tests were adopted to characterize the energy and deformation failure of backfill materials from different temperatures (20 °C, 30 °C, 40 °C). The results show that the temperature has a promoting effect on the development of modified magnesium slag-fly ash cemented paste backfill (MFPB) strength. The elastic strain energy and the dissipated energy corresponding to the peak stress increase as the curing temperature increases. The high frequency phase of the energy scatter plot is accompanied by a rapid increase in the cumulative ringing count and cumulative energy. Tensile cracks increase with the increase of curing temperature. The temperature effectively promotes the hydration process of MFPB. The decrease of strength in the later stage of curing temperature 40 °C is due to the harmful thermal stress within the backfill. The results provide theoretical guidance for the practical application of MFPB.

Dislocation-density-related constitutive model and its applicability to cyclic deformation and damage behavior of 316LN SS at 823 K

In the present investigation, an in-depth understanding of the relationship between dislocation kinetics and cyclic plasticity, along with the description of yield asymmetry behavior during loading and unloading in nuclear grade 316LN austenitic stainless steel at 823 K, was examined by employing a constitutive framework using the evolution of physically-based internal-state variables. The formulation encompassed a set of first-order simultaneous differential equations that governed the evolution of dislocation density, mean free path of dislocations, and back stress. These variables were interdependent with the power law which described the kinetics of plastic strain rate. The evolution equations were numerically integrated, and the simulated data were fitted to the experimental data up to stabilized cycle at a temperature of 823 K and total strain amplitudes of ±0.004, ±0.006, ±0.008 and ± 0.010. The predicted cyclic stress response as a function of the cyclic number displayed the continuous cyclic hardening from the first cycle to the initiation of stabilized cycle. Further, good agreement between the predicted and experimental hysteresis loops was found at different cycle numbers. The predicted dislocation density and tensile peak values of back/effective stress increased with cyclic hardening and approached the value of saturation at the stabilized cycle. On the other hand, a rapid decrease in mean free path with cycle number was noticed. An increase in total strain amplitude resulted in a systematic increase in dislocation density, peak stress, back stress and effective stress. At the total strain amplitudes above ±0.006, the predicted higher rates of dislocation accumulation and annihilation, and the faster evolution rate of back stress towards saturation led to the development of stable dislocation substructure configurations with a clear distinction between dislocation high/low dense regions. Furthermore, the applicability of the model was extended by including an empirical relationship between the damage variable and cycle number within the power law, thereby enabling predictions of the cyclic stress-strain response beyond the stabilized cycle.

Effects of initial unloading level on the mechanical, micro failure and energy evolution characteristics of stratified rock mass under triaxial unloading confining pressure

In order to study the strength, micro crack distribution and energy evolution characteristics of stratified rock mass under unloading confining pressure conditions, triaxial compression tests and triaxial unloading confining pressure tests at different initial unloading levels were carried out. The experimental results indicated that the triaxial unloading confining pressure plus axial stress is a stress path between the stress state of uniaxial compression and triaxial compression. With the increasing initial unloading level, the specimen is closer to the stress state of triaxial compression, the peak stress, peak axial strain, peak circumferential strain and peak volume strain increase linearly, Poisson’s ratio decreases linearly, failure type of specimens shows 4 typical characteristics. When the bedding plane inclination is 45°, 3D reconstruction and quantitative characterization of micro cracks of stratified rock mass specimens are conducted based on CT scanning results. Variations of area fraction along the specimen height show 3 typical characteristics. The stratified rock mass mainly stores and dissipates energy before the peak stress, and mainly releases and dissipates energy after the peak stress under different conditions. With the increasing initial unloading level, the input energy, elastic energy, dissipative energy and energy dissipation rate at the peak stress increase approximately linearly.

Numerical study on compressive mechanical characteristics of filled jointed rock under confining pressure based on PFC

Confining pressure is an important factor affecting the strength and deformation characteristics of rock mass, it is of great significance to study the mechanical and deformation characteristics of jointed rock mass under confining pressure for the construction of deep underground engineering and the prevention of geological disasters. In order to study the mechanical and deformation characteristics of filled jointed rock under confining pressure, based on the laboratory experiment results of static uniaxial compression of filled jointed rock samples, the Particle Flow Code is used to conduct the numerical simulation. The strength characteristics, failure characteristics and micro-cracks development characteristics of filled jointed rock under different confining pressure levels, different joint inclination angles and different sample sizes are analyzed. The results show that the peak stress and peak strain increase with the increase of confining pressure level, and there is a strong linear relationship between peak stress and confining pressure level. The peak stress and initiation stress decrease first and then increase with the increase of joint inclination angle. With the increase of confining pressure level, the change law of initiation stress of filled jointed rock under different joint inclination angles is different. The confining pressure will prolong the development process of micro-cracks in filled jointed rock, which will make the distribution of micro-cracks more dispersed and the total number of micro-cracks increase. The failure mode changes from splitting failure to shear failure with the increase of confining pressure level. The change of joint inclination angle will seriously affect the failure mode and micro-cracks development characteristics of filled jointed rock.

Mechanical properties of a novel hierarchical cellular structure architectured with minimal surfaces and Voronoi-tessellation

Hierarchical structures are commonly found in nature due to their combination of low density, exceptional specific properties, and multifunctionality. Inspired by this, we proposed a novel hierarchical cellular structure based on triply periodic minimal surface (TPMS) by connecting two topologically identical and parallel sheets with a series of connecting ribs. A conformal design guideline was provided by describing the mathematical principles behind the generation of such hierarchical cellular structures. Then the designed structures with different configurations were fabricated via laser powder bed fusion (LPBF). Finally, the mechanical performance of the hierarchical cellular structures was analyzed by both experiment and numerical simulation. The dynamic compressive behavior was investigated using a Split Hopkinson pressure bar (SPHB) at a strain rate of 650 s−1, and the temperature distribution during tests was monitored using a high-speed infrared camera. As for quasi-static compression tests, it is found that the hierarchical cellular structure had good energy absorption characteristics and relatively low first peak stresses. These results showed that the structure could reduce strain rate hardening effects under dynamic loading, thereby mitigating the increase in first peak stress. The proposed hierarchical cellular structures showed good potential for efficient energy absorption under quasi-static and dynamic compression loadings.

Mechanical Characteristics and Particle Breakage of Calcareous Sand under Quasi-One-Dimensional Impact Load

Calcareous sand, a type of marine sediment formed from the skeletal remains of marine life, exhibits unique characteristics such as high porosity and fragility due to its biological origin. Particle breakage is a key attribute of calcareous sand. Given that foundations on calcareous sand islands encounter various types of loads, including pile driving, aircraft loading, earthquakes, and tsunamis, it is imperative to investigate its mechanical properties and particle breakage under high strain rates. This study focuses on assessing the dynamic mechanical properties of calcareous sand under quasi-one-dimensional impact loads using split Hopkinson pressure bar (SHPB) tests. Three particle sizes of calcareous sand with different water contents, strain rates, and relative densities were examined. The particle fragmentation degree of each sand sample was also analyzed quantitatively. The results indicated that stress–strain curves progress through an elastic phase with rapid elevation, followed by a plastic stage with a slower increase under various factors. Within the plastic phase, there are multiple instances of stress drops and recoveries. The stress–strain curves generally decrease as particle size increases, concurrent with an increase in particle breakage. Moisture content has minimal impact on the stress–strain curve; a higher moisture content does correspond to reduced particle breakage. Both the maximum strain and peak stress increase as the strain rate increases, resulting in a higher relative crushing rate. The difference between stress–strain curves under different relative densities diminishes as particle size increases, and greater relative density leads to reduced particle breakage. Functional relationships among peak stress and strain rate, relative fragmentation rate and water content, strain rate and relative density, as well as relative density and peak stress are also established.

Failure behaviour and damage evolution of multi-recycled aggregate concrete under triaxial compression

In this paper, the stress–strain behaviour, mechanical properties, characteristic stress, and damage evolution of multi-recycled aggregate concrete (Multi-RAC up to 3 recycling cycles) under triaxial compression were investigated, considering the effects of confining pressure and number of recycling cycles. Furthermore, a parametric sensitivity evaluation of key parameters using grey correlation theory to analyse the peak stress, peak strain, and elastic modulus of Multi-RAC. The results indicate that both peak stress and peak strain increase with rising lateral confining pressure. While the peak stress of Multi-RAC is lower than that of natural aggregate concrete (NAC), the deformation capacity of Multi-RAC is greater. The deformation and failure of Multi-RAC under triaxial loading can be categorized into five stages. Notably, crack initiation stress (σci) and crack expansion stress (σcd) demonstrate nonlinear growth as lateral confining pressure increases. In particular, the second generation of Multi-RAC (RACII) exhibits superior performance in resisting crack initiation and expansion, while the third generation of Multi-RAC (RACIII) performs relatively less favourably. The energy evolution characteristics and division of failure stages of Multi-RAC are effectively characterized by the evolution of elastic strain energy (Ue) and dissipation energy (Ud). The degree of progressive damage aligns with the increase in dissipation energy. Among the five key parameters, the number of recycling cycles shows the most significant grey correlation effect on the peak stress, peak strain, and elastic modulus of Multi-RAC. These findings contribute to a deeper understanding of the mechanical properties and failure behaviours of Multi-RAC under the complex triaxial loading conditions, thus promoting the application of Multi-RAC as a structural concrete.

Experimental investigation of a fissured rock with thermal storage potential subjected to descending amplitude low cycle impacts

As the advancement of deep earth energy engineering in the 21st century, the challenges imposed by dynamic loads and high temperatures will be increasingly regular. This paper studied the dynamic mechanical behaviors and fracture characteristics of defective red sandstone (single fissure inclination of 0°, 30°, 60°, and 90°) subjected to thermal treatments (25 °C, 200 °C, 400 °C, 600 °C, and 800 °C) under descending amplitude low cycle (DALC) impact loading. The experiments were conducted using a modified split Hopkinson pressure bar (SHPB) system, and the fracture surfaces of the failed specimens were characterized through the use of high-resolution 3D laser scanning to obtain point cloud data. Subsequently, the fractal, roughness, texture, and elevation features of fracture surfaces were analyzed using the boxing-counting method, grayscale co-occurrence matrix (GLCM), and elevation-based statistics. The scanning electron microscopy (SEM) was utilized to study the fracture micromorphology as well. Additionally, the mechanisms of damage and fracture evolution were discussed based on the results of X-ray diffractometry (XRD), synchronous thermal analysis (STA), discrete element method (DEM), and strain energy density theory. The results demonstrate that the peak stress and dynamic elastic modulus deteriorate with rising temperatures and increase with larger fissure angles. While the peak strain, strain rate, and peak stress increase factor (PIF) exhibit the opposite trend. The threshold values for the intensification of these events are 400 °C and 60°. The damage is dominated by tensile cracks, which transition to shear at 30° and 60° due to temperature dependence. Meanwhile, the fractal dimension (DC), joint roughness coefficient (JRC), second-order statistics of GLCM, and elevation eigenvalues have been modeled using multiple regression analysis to investigate their interaction with temperature and fissure angle. The best-fit models illustrate significant linear or non-linear correlations between the aforementioned parameters and the mechanical parameters. Furthermore, the magnitude and distribution pattern of thermal cracks, as well as the growth of shear fractures, are governed by the dehydration, thermal expansion, phase transition, plasticization, thermal fracture, and thermal decomposition. Finally, strain energy density theory manifests promising prospects for understanding and predicting DALC impact fracture patterns of specimens.

Study on the Acoustic Emission Characteristics and Failure Precursors of Water-Rich Frozen Sandstone under Different Lateral Unloading Rates

The artificial freezing method is used to cross the water-rich soft rock strata in order to exploit deep coal resources. At present, studies that consider both freezing effect and unloading rate are insufficient. To study the influences of the excavation rate using the artificial freezing method on the unloading deformation and failure of the water-rich surrounding rock, we carry out mechanical and synchronous acoustic emission (AE) tests on frozen (−10 °C) sandstone samples under different lateral unloading rates. Combined with the AE signals, the stress, strain and failure process are analysed to determine the mechanical behaviours of frozen rock samples under different lateral unloading rates. The damage difference between normal temperature rock and frozen rock during lateral unloading is studied. According to acoustic emission signals, the damage relationships among acoustic emission amplitude, energy, cumulative acoustic emission energy (CAEE), stress and strain were compared and analyzed. In this paper, acoustic emission 3D positioning system is used to monitor the fracture propagation trajectory in the process of unloading confining pressure of frozen sandstone. The results show that the peak stress of frozen sandstone during lateral unloading is about 2.5 times of that at 20 °C. More than 2 AE amplitudes per second are regarded as the precursor of failure (FP), and point FP is taken as the first level warning. The CAEE of rock samples at 20 °C and frozen rock samples shows the same change law over time, increasing slowly before the FP point and exponentially after the FP point. Peak stress increases and axial strain decreases with the increase of unloading rate of frozen rock sample. The CAEE at point FP and the peak acoustic emission energy (AEE) and the CAEE at the time of failure increase when the unloading rate of frozen rock sample increases. Principal component analysis method was used to extract key characteristic energy to obtain a clearer AEE concentration area, which was defined as second-level early warning. The research results can provide guidance for freezing shaft construction to reduce the occurrence of disasters.

Compressive behavior of seawater coral concrete with varying confinement subjected to axial loading

Coral reef debris have emerged as a promising alternative to conventional natural coarse aggregates, driven by the increasing demand for oceanic structures. To comprehend the compressive behavior of coral aggregate concrete (CAC) with varying confinements which induced by the lateral dilation of concrete, further studies were required. To this end, a total of 81 specimens were preaperd encompassing three concrete grades (C20, C30 and C40). Among these, 9 speicmens without outer confinements, while 36 specimens confined by carbon fiber reinforced polymer (CFRP) and 36 specimens composited with 6061-T6 aluminium alloy tubes (AA). Furthermore, various parameters were explored to investigate their influence on the compressive behavior, including four layers of CFRP wraps (1, 2, 3 and 4) and four thickness of AA tubes (2 mm, 3 mm, 5 mm and 7 mm). Subsequently, an analysis was conducted to examine the effects of different parameters. By utilizing the Weibull damage distribution method, a damage constitutive model was developed to account for variations in confinement. Based on the simplified failure criterion of CAC, bearing models were developed. The findings indicate that both peak stress and peak strain increase with greater external confinement, and an increase in axial and lateral strain was also observed. Moreover, specimens confined with CFRP exhibited elastic behavior in their lateral stress, whereas specimens confined with AA displayed elasto-plasticity behavior. Finally, the varying lateral stress models presented accurate confinement predictions. The proposed damage-constitutive and bearing models showed good agreement with the measured results.

High Strain Rates Impact Performance of Glass Fiber-Reinforced Polymer Impregnated with Shear-Thickening Fluid

This paper examines the behavior at high strain rates of a shear-thickening fluid (STF) impregnated glass fiber-reinforced polymer (GFRP) fabric using a split Hopkinson pressure bar (SHPB). This study involved impact testing of 4 GFRP specimens and 20 GFRP-STF composite specimens at four different strain rates. The STF employed in this study was synthesized by incorporating 20.0 wt.% of 12 nm silica in polyethylene glycol. Rheological tests indicated that the STF exhibited a noticeable shear-thickening effect, with viscosity surging from 3.0 Pa·s to 79.9 Pa·s. The GFRP-STF specimen demonstrated greater energy absorption capacity, deformation ability, and toughness, bearing higher and faster impact loads than neat GFRP. Specifically, the GFRP-STF specimen showed a 21.8% increase in peak stress and a 92.9% rise in energy absorption capacity under high-strain-rate loading. Notably, the stress–strain curve of the GFRP-STF specimen exhibited a distinct yield stage, while the energy absorption curve displayed no significant descending stage features.