Peak Stress Of Specimens Research Articles

Cyclic compressive behavior of high-performance hybrid fiber reinforced recycled aggregate concrete: Degradation law and acoustic emission features

To tackle the issues of inadequate mechanical properties of recycled aggregate concrete and its limited application in load-bearing structural components, a high-performance hybrid fiber reinforced recycled aggregate concrete (HP-HFRAC) was developed in this study. Sixteen groups of HP-HFRAC specimens fabricated with varying replacement rates of recycled coarse aggregate and different fiber parameters were subjected to uniaxial compression, as well as acoustic emission (AE) tests, to investigate the cyclic behavior and uncover the underlying mechanisms. The findings demonstrated a strong correlation between the AE signals and the macroscopic phenomenon throughout the whole compression process. The AE amplitude distribution was found to be capable of identifying damage mechanisms, including matrix cracking (40–50 dB), fiber sliding (50–80 dB), and fiber pull-out (more than 80 dB). Additionally, the inclusion of hybrid fibers contributed to mitigating the degradation of stiffness and the accumulation of plastic strain, resulting in the ductile failure of the specimens and significantly enhancing the energy dissipation capacity. Specifically, the peak stress of specimens with 1.5% steel fibers increased by 19% compared to that of specimens without steel fibers, compensating for the strength loss due to the presence of recycled coarse aggregate. Finally, constitutive equations governing the skeleton curves and the unloading and reloading curves of HP-HFRAC subjected to cyclic compression were formulated, which were validated using both the test results and the predicted results of the physics-based data-driven model previously published in our companion paper. The findings of this study might contribute to the development of sustainable high performance recycled aggregate concrete for reliable application in load-bearing components.

Experimental investigation on the effects of thermal treatment on the physical and mechanical properties of shale

Shale is affected by high temperature and loads in deep mining, shale gas extraction, underground disposal of nuclear waste, and other shale projects. This paper aims to investigate the effects of thermal treatment on the physical and mechanical properties of shale. The uniaxial compression test and acoustic emission (AE) monitoring were conducted on shale specimens with different bedding plane inclinations (0°, 30°, 45°, 60°, and 90°) after thermal treatment of different temperatures. The results show that the P-wave velocity decreases with the increase in bedding plane inclination at each thermal treatment temperature showing anisotropy. A thermal explosion occurs when the specimen reaches the macroscopic thermal damage threshold temperature, and the threshold temperature and intensity of the thermal explosion are linked to the bedding plane inclination. The peak stress of specimens with the bedding plane inclinations of 0°, 45°, and 90° increases with the rise in thermal treatment temperature. After thermal treatment, the relationship curve between the cumulative AE counts of specimens with different bedding plane inclinations and time presents a step-jump trend. The thermal treatment has no significant effect on the failure modes of specimens with bedding plane inclinations of 30°, 60°, and 90°, while specimens with bedding plane inclinations of 0° and 45° begin to splitting failure with a rise in the thermal treatment temperature. The rockburst failure occurs in the shale specimens, and the main phenomena are spalling, particle ejection, and block collapse. The thermal treatment has a significant influence on the rockburst intensity of specimens with different bedding plane inclinations.

Tests on the Mechanical Properties of Corroded Cement Mortar after High Temperature

Durability of cement mortar and concrete materials under sea water condition is always an important research topic. The objective of this work is to understand the mechanical properties of corroded cement mortar after high temperature, the cement mortar specimens after high temperature were placed in water and sodium sulfate solution, and then the uniaxial compression tests were carried out on the cement mortar specimens after corroded. Test results show that both the differences of compressive strength and strain at the peak stress after high temperature caused by high temperature, are relatively small when the specimens are eroded in water, and the differences are relatively high when the specimens are eroded in sodium sulfate solution. The compressive strength of the cement mortar specimens under normal temperature eroded in sodium sulfate solution is highest, and that eroded in water is lowest. The compressive strength of specimen after high temperature eroded in water is highest and that eroded in sodium sulfate solution is lowest. The strain at the peak stress of specimen, whether after high temperature or not, is highest when eroded in sodium sulfate solution, and that eroded in water is lowest. At present, there are few research results about the mechanical properties of concrete first after high temperature and then after sea water corrosion. The work in this paper can enrich the results in this area.

Dynamic Compressive Property of Closed-Cell Mg Alloy Composite Foams Reinforced with SiC Particles

The high-strain-rate mechanical response of Mg alloy/SiCp composite foams has received increased attention in recent years due to their light weight and potential to absorb large amounts of energy during deformation. Dynamic compressive properties of closed-cell Mg alloy/SiCp composite foams with different relative densities (0.162, 0.227 and 0.351) and different SiCp additions (0, 4 and 8 wt%) have been investigated using Split-Hopkinson pressure bar. It is shown that peak stress and energy absorption capacity significantly increase as the relative density increases at the range of testing strain rates. Peak stress and energy absorption display strain rate dependence. The peak stress of specimens with 0 wt% and 4 wt% SiC particles additions grows with increasing strain rate. Meanwhile, the increment in the peak stress of specimens with 8 wt% addition is not significant with strain rate increasing. The increase in strain rate increases the energy absorption capacity. The suitable amount of SiC particles addition has great advantages over increasing the peak stress and energy absorption capacity at the high strain rate. The strain-rate-sensitive matrix, cell morphology, morphological defects and gas pressure have an impact on the strain-rate sensitivity of Mg alloy/SiCp composite foams.

Dynamic Loading of Carrara Marble in a Heated State

Useable land is a finite space, and with a growing global population, countries have been exploring the use of underground space as a strategic resource to sustain the growth of their society and economy. However, the effects of impact loading on rocks that have been heated, and hence the integrity of the underground structure, are still not fully understood and has not been included in current design standards. Such scenarios include traffic accidents and explosions during an underground fire. This study aims to provide a better understanding of the dynamic load capacity of Carrara marble at elevated temperatures. Dynamic uniaxial compression tests are performed on Carrara marble held at various temperatures using a split-Hopkinson Pressure Bar (SHPB) setup with varying input force. A customized oven is included in the SHPB setup to allow for testing of the marble specimens in a heated state. After the loading test, a three-wave analysis is performed to obtain the dynamic stress–strain curve of the specimen under loading. The fragments of the failed specimens were also collected and dry-sieved to obtain the particle size distribution. The results reveal that the peak stress of specimens that have been heated is negatively correlated with the heating temperature. However, the energy absorbed by the specimens at peak stress at all temperatures is similar, indicating that a significant amount of energy is dissipated via plastic deformation. Generally, fragment size is also found to show a negative correlation with heating temperature and loading pressure. However, in some cases this relationship does not hold true, probably due to the occurrence of stress shadowing. Linear Elastic Fracture Mechanics has been found to be generally applicable to specimens tested at low temperatures; but at higher temperatures, Elastic–Plastic Fracture Mechanics will give a more accurate prediction. Another contribution of this study is to show that other than the peak stress of the rock failure type, the strain history experienced by the rock during impact and the post-impact fragment size distribution are also significant distinguishing features of damage caused by dynamic loading on heated rocks.

Three-Dimensional Numerical Investigations of the Failure Mechanism of a Rock Disc with a Central or Eccentric Hole

The diametrical compression of a circular disc (Brazilian test) or cylinder with a small eccentric hole is a simple but important test to determine the tensile strength of rocks. This paper studies the failure mechanism of circular disc with an eccentric hole by a 3D numerical model (RFPA3D). A feature of the code RFPA3D is that it can numerically simulate the evolution of cracks in three-dimensional space, as well as the heterogeneity of the rock mass. First, numerically simulated Brazilian tests are compared with experimental results. Special attention is given to the effect of the thickness to radius ratio on the failure modes and the peak stress of specimens. The effects of the compressive strength to tensile strength ratio (C/T), the loading arc angle (2α), and the homogeneity index (m) are also studied in the numerical simulations. Secondly, the failure process of a rock disc with a central hole is studied. The effects of the ratio of the internal hole radius (r) to the radius of the rock disc (R) on the failure mode and the peak stress are investigated. Thirdly, the influence of the vertical and horizontal eccentricity of an internal hole on the initiation and propagation of cracks inside a specimen are simulated. The effect of the radius of the eccentric hole and the homogeneity index (m) are also investigated.

Effect of microstructure and relaxation behavior on the high temperature low cycle fatigue of near-α-Ti-1100

The influence of lamellar or bi-modal microstructures on high temperature LCF behavior in Ti-1100 was investigated at 600°C. Under various creep-fatigue conditions, as the hold-time increases, the fatigue lives of specimens of both lamellar and bi-modal structures reduce compared with those of continuous cycling of the same specimens. It is understood that longer hold-time results in severe creep damage accumulation due to stress relaxation, leading to reduced fatigue life. Specimens with a bi-modal structure show about double the values of relaxed stress normalized with tensile peak stress of specimens with lamellar structure in the same test conditions, resulting in a higher reduction rate of fatigue life. In addition, as hold-time extends, the difference in total hysteresis loop energy between lamellar and bi-modal structures becomes smaller. Therefore, it can be said that creep damage resulting from stress relaxation during hold-time is more detrimental in bi-modal than in lamellar structures.

The behaviour of concrete columns with interlocking spirals

The objective of this study is experimentally to investigate the strength and the deformability of specimens with interlocking spirals subjected to concentric axial loads. For this purpose, experimental study were conducted for 36 specimens with single spirals and 108 specimens with interlocking spirals. Main test variables were concrete strength, spacing of spirals, yield strength of spirals, and interlocking length of spirals. From test results, the strength and the deformation characteristics were reported. Based on test results, model equations for prediction of the strength and the axial strain at the peak stress of specimens with interlocking spirals were proposed.