Displacement Of Specimen Research Articles

3D study of the vibrational behaviour of lithic flint blades

Stone Age sites are well known to often contain many lithic flint blades and flakes, which may provide important information about early European Stone Age cultures and their environment. Understanding the mechanical behaviour of lithic flint blades represents an important problem for scientists in general and archaeologists in particular. In this study, the structural behaviour of lithic flint blades is studied. Ten specimens with different geometric shapes (tilted, curved, with bumping surfaces) were studied and tested. Their natural frequencies, damping ratios, and mode shapes (that is how the specimen deforms under any external excitation) are estimated using two models: an analytical model that accounts for the specimen's curvature and a 3D Finite Element (FE) method. Advanced experimental methods, including ultrasound techniques, were used to measure the mechanical properties of the specimens. The experimental set-up was built around a laser vibrometer that measured the specimen's displacement. The model predictions were compared with the experimental data to validate their effectiveness. A good agreement is observed between the models and the real data. It is particularly observed that despite their complicated geometries, the specimens still follow a structured pattern in their dynamic response. The presented study supports the use of acoustic methods as an effective tool to characterize and detect submerged prehistoric materials. This work contributes to the dynamic characterization of submerged Stone Age materials.

Seismic behavior of autoclaved aerated concrete masonry walls reinforced with glass-fiber geogrid

Reinforcing horizontal mortar joints with steel bars can improve the seismic performance of autoclaved aerated concrete (AAC) masonry walls. Nevertheless, the diameter of steel bars can require a large thickness of the mortar joints, which in turn results in the thermal bridging effect. Glass fiber geogrids (GFGs) can be employed to replace steel bars to reduce the thickness of mortar joints, and thus can mitigate the thermal bridging effect. Despite considerable advancements in understanding the mechanical performance of GFGs in masonry walls, their effect on the seismic performance of AAC walls remains unknown. Therefore, this study investigates the effect of different GFG configuration rates on the seismic performance of AAC masonry walls using reciprocating load tests. Furthermore, the effect of different vertical compressive stresses on the seismic performance of GFG-reinforced AAC masonry walls was studied via comprehensive finite element simulations. The test results show that the ultimate load-bearing capacity and ultimate displacements of the wall specimens were enhanced as the GFG configuration rate increased, approaching that of benchmark specimens reinforced with steel bars. On the other hand, the cracking and ultimate loads, along with ultimate displacements of GFG-reinforced AAC masonry walls were significantly improved compared to those of the unreinforced AAC masonry walls. It was also evidenced that the crack formation, failure mode, ductility and energy dissipation capacity of the specimens simulated through the finite element method were highly comparable to that of experimentally tested specimens. Accordingly, the vertical compressive stresses were in the range of 0.1 MPa ∼ 0.6 MPa, while the seismic performance of GFG masonry walls was improved with the increase in vertical pressure. The results of the current study confirm that embedding GFG in horizontal mortar joints can delay the cracking of AAC masonry walls and improve their seismic performance. Ultimately, an empirical equation was developed to calculate the seismic shear strength of GFG-reinforced AAC masonry walls.

Effect of the Slit on the Mechanical Tearing Behavior of High-Density Polyethylene and Polyester Geocell Strips

Geocells are widely applied in numerous infrastructure constructions, like heavy-haul railways and ports. The mechanical tearing behavior of a geocell strip is crucial to the stability of the geocell-reinforced soil structures. At present, extensive studies have been conducted on the tensile characteristics of geocell strips, while limited research has been performed to investigate the post-damage mechanical tearing behavior of geocell strips. Meanwhile, there is also a lack of research on the comparison of performance of strips before and after damage. In this paper, a series of trapezoidal tearing tests were performed on high-density polyethylene (HDPE) and polyester (PET) geocell strips. The tearing test results and failure mode of trapezoidal specimens with a slit were investigated, and the effect of the slit on the strength and deformation characteristics of the specimen were discussed by introducing the “damage ratio of tearing force (RTF)” and “damage ratio of tearing displacement (RTD)”. In addition, the mechanical tearing behavior of HDPE and PET trapezoidal specimens was also compared. The test results indicated that the failure mode of HDPE and PET specimens subjected to tearing force was ductile and brittle failure. The strength and deformation characteristics of post-damage HDPE and PET trapezoidal specimens decreased. The slit had a significant impact on the tearing displacement of HDPE and PET specimens, especially the post-peak tearing displacement. The post-peak tearing displacement of HDPE was 10.99 times that of PET. The peak tearing force of the HDPE specimen without the slit was about 1.61 times that of specimen with the slit. Before local tearing, the peak tearing force of the PET specimen without the slit was about 3.27 times that of the specimen with the slit. The strength damage to the HDPE and PET geocell strips caused by the slit was 38.0%, and 69.46%. The impact of the slit on the tearing force of the PET specimen was greater than that of the HDPE, and was 1.82 times for the HDPE. This study can enhance our understanding of the mechanical tearing behavior of the geocell strip after damage and develop effective mitigation measures.

Comparisons of flexural, split tensile, double punch, and direct tension tests on high-performance concrete reinforced with different fiber types

The study investigates the variation of tensile strength of high-performance concrete (HPC) based on various tensile tests, such as the flexural test, splitting tensile test, double punch test, and direct tension test, conducted on dog bone specimens. The test samples were HPC concrete reinforced with three different fibers in a fraction ratio ranging from 0 % to 2 %. The three kinds of fibers utilized in this investigation are those most frequently employed in Vietnam, including straight steel fibers with copper plating, polypropylene (PP), and hook steel fibers. The study additionally investigates the similarities between the results of tensile strength experiments on HPC concrete by examining the average tensile strengths, tensile failure modes, and material behavior graphs of tensile stress versus recorded specimen displacement. Overall, The average tensile strength results of bending and splitting tensile tests were significantly higher than those of the direct tensile and double punch tests. All tests could capture the behavior of HPFRC in the working periods. Splitting and flexural tensile test results followed a normal distribution. However, direct and double punch test results did not. The results of this study show that double punch tests can be used instead of direct tensile tests to measure the tensile strength of fiber concrete.

Compressive stress–strain behaviour of masonry prisms made of low elastic modulus burnt clay bricks

Masonry, being a quasi-brittle material, displays mechanical properties and manifests non-linear behaviour, which is influenced by the specific attributes of its constituent units and mortar. The compressive stress–strain behaviour of masonry is a vital factor in the realistic analysis of masonry structures when subjected to ground deformations arising from construction activities such as excavation, underground tunnel construction, utility installations, and for evaluating their seismic vulnerability. In India, the commonly used masonry unit is burnt clay bricks, which are highly localized due to changes in the constituent material and manufacturing process. This leads to variations in the strength and stiffness of brick units from region to region. An experimental investigation was conducted using twenty-four brick specimens, seventy-two mortar cube specimens and forty-eight four-unit single-wythe masonry prism specimens, subjected to displacement-controlled uniaxial compressive load. The specimen displacements of brick units and masonry prisms were obtained using Digital Image Correlation analysis. The stiffness of the brick units was significantly lower than those reported in studies conducted in India or elsewhere. Statistical analysis has been performed, and relations between the respective compressive strength and elastic modulus of brick units, mortar and masonry prism specimens have been obtained. An equation has been proposed to estimate masonry prisms’ elastic modulus from its components. An analytical model has been proposed for predicting the compressive stress–strain curve of masonry prisms, incorporating the elastic modulus of masonry prisms, which can be predicted using the expression proposed in this study. This is a fundamental improvement over the expressions available in the literature.

Experimental study on the compression-shear performance of a new-type circumferential joint of shield tunnel

To investigate the compression-shear behavior of a new circumferential joint based on the sleeve-straight bolt combination type connection of large-diameter shield tunnels, a series of full-scale joint experiments was carried out. In the process of the experiment, more attention was paid to the specimen displacement, bolt stress and joint damage mode. On the basis of these experiment phenomena, this study discussed the compression-shear bearing process of the new connector, analyzed the damage mode of the joint structure, and finally evaluated the performance of the new connector. It is found that the bearing process of the joint can be divided into four stages: the transitional stage for overcoming the friction of the concrete, the sleeve bearing stage for the sleeve bearing shear loads alone, the combined bearing stage for bearing shear loads by the connector system, and the structural damage stage for structural instability and damage. Generally speaking, affected by connector position and hand hole, the positive compression-shear stiffness of the joint is less than the negative compression-shear stiffness, and the positive shear strength of the joint is greater than the negative shear strength. The increase of longitudinal axial force will improve the compression-shear performance of the joint. The relationship between longitudinal axial force and joint stiffness is a logarithmic function. The use of new type of connector can effectively improve the compression-shear stiffness of joints under low shear loads, but the application of straight bolts will lose part of the strength performance.

Stress and strain calculation method for orthotropic polymer composites under axial and bending ultrasonic fatigue loads

Accelerated fatigue testing is one potential solution to evaluate the very high cycle fatigue behavior of composite materials within a reasonable amount of time. The ultrasonic fatigue testing methodology can be adopted to realize fatigue experiments up to 109 cycles at 20 kHz, compared to conventional fatigue experiments usually carried out between 5–50 Hz. The determination of cyclic stresses during ultrasonic loading remains to be one of the major challenges. The cyclic stresses during ultrasonic fatigue loading were investigated for a carbon fiber 5H satin fabric reinforced in Polyetherketoneketone (CF-PEKK) composite material. Two experimental setups were developed to perform ultrasonic testing under uni-axial and three-point bending loading conditions. A 3D-Scanning Laser Doppler Vibrometer (3D-SLDV) and a single-point Laser Doppler Vibrometer (LDV) were integrated into the test systems to measure the oscillation displacement of the CF-PEKK specimens during ultrasonic cyclic loading. These displacement measurements were used to calculate the resulting strains and stresses under elastic loading conditions. The experimental results were found to be in good agreement with those obtained from finite element models, providing evidence for applying the proposed method.

Seismic behavior along the asymmetrical axis of T-shaped multi-partition steel–concrete shear walls

Five specimens have been tested under cyclic lateral loads to explore the seismic behavior of T-shaped multi-partition steel–concrete shear walls along the asymmetrical axis. The loading has been applied by considering the torsion effect from non-coincidence between the lateral loads and the shear center in this study. The main parameters of the test are the axial compression ratio and section height-to-breadth ratio aiming to study the vertical load and web constraint effect. The test results indicate that the failure mode of all specimens along the asymmetric axis is bending failure with minor shear deformation measured during loading. The yield drift (1/138) and the ultimate drift (1/41) of the T-shaped multi-partition steel–concrete shear walls could meet the requirement of the corresponding code, namely 1/350 and 1/70, respectively. The increase in axial compression ratio increases the out-of-plane displacement but decreases the ductility of the specimens, whilst the increase in section height-to-breadth ratio decreases the out-of-plane displacement of the specimens. The relatively small torsion angle of less than 0.6° indicates that the T-shaped multi-partition steel–concrete shear walls have good torsion resistance. Under the cyclic lateral loads along the asymmetrical axis, the out-of-plane displacement of the T-shaped specimen will occur on the side without the web. The web elastic stress distribution model can predict the lateral resistance more accurately, whilst the full-section plastic stress distribution model overestimates the lateral resistance of T-shaped multi-partition steel–concrete shear walls.

Hybrid Simulation for Evaluation of Seismic Performance of Highway Bridge with Pier Retrofitted Using Fe-SMA Strips

The present research investigates the use of Fe-based shape memory alloy (SMA) strips for seismic retrofitting of rehabilitated RC bridge pier models. Four severely damaged bridge pier models were first rehabilitated by RC jacketing to retrieve at least their lost strength. Three configurations of Fe-SMA strips namely hoop (H), end-anchored (EA), and a combination of previous two approaches (EAH) were adopted. Prestrained Fe-SMA hoops, placed in the plastic hinge region of the bridge pier model, aims at utilizing the recovery stress developed upon thermal activation for application of active confinement pressure on the test specimen. The EA Fe-SMA strips utilize precompression developed in the concrete in the plastic hinge region of the test specimen. The EAH reinforcement scheme was studied for both precompression and active confinement. To compare the seismic performance of the various Fe-SMA retrofitted test specimens with respect to the control specimen, hybrid simulation (pseudodynamic) of an existing highway bridge located in Tripura, India was carried out with the scaled bridge pier test models as experimental elements. Earthquake excitations of different intensity levels were applied in a sequential manner to analyze the behavior of the structure over a damage spectrum ranging from minor cracking to major damage. To determine the ultimate capacity of the test specimens, cyclic tests were carried out after completion of a hybrid simulation. Bridge pier specimens with Fe-SMA in the form of EA reinforcement exhibited enhanced load-carrying capacity but marginal improvement in ultimate displacement while Fe-SMA hoops improved the ultimate displacement of the test specimen with marginal improvement in the peak lateral load. A combination of EA and hoop Fe-SMA reinforcement was found to be more efficient in improving all the seismic performance parameters such as lesser damage and higher load-carrying capacity, stiffness, and energy-dissipation capacity.

An Inverse Optimization Method for the Parameter Determination of the High-Temperature Damage Model and High-Temperature Damage Graph of Ti6Al4V Alloy.

Ti6AL4V alloy is widely used in the biomedical and energy vehicle industries, among others. Ti6Al4V alloy cannot be fabricated at ambient temperatures; hence, it requires hot forming. However, this method is susceptible to crack defects. The crack defect problem of Ti6AL4V alloy in the hot-forming process cannot be ignored, so we must develop a precise hot-forming damage prediction model. In this study, three high-temperature damage models of Ti6Al4V alloy were developed, considering the temperature and strain rate. These models were derived from the normalized Cockcroft and Latham (NCL), Oyane, and Rice and Tracey (RT) damage models. The damage parameters of the models were identified using a genetic algorithm combined with finite element simulation. The force accumulation error of the Ti6AL4V alloy specimen, which was obtained from a simulated thermal tensile test and an actual test, was used as an optimization target function. Then, the damage parameters were optimized using the genetic algorithm until the target function reached the minimum value. Finally, the optimal damage model parameter was obtained. Through program development, the three high-temperature damage models established in this paper were embedded into Forge® NxT 2.1 finite element software. The simulated thermal tensile test of Ti6AL4V alloy was performed at a temperature of 800-1000 °C and a strain rate of 0.01-5 s-1. The simulated and actual fracture displacements of the tensile specimens were compared. The correlation coefficients (R) were calculated, which were 0.997, 0.951, and 0.912. Of the high-temperature damage models, the normalized Cockcroft and Latham high-temperature damage model had higher accuracy in predicting crack defects of Ti6Al4V alloy during the hot-forming process. Finally, a fracture strain graph and a high-temperature damage graph of Ti6Al4V alloy were constructed. The Ti6Al4V alloy damage evolution and thermal formability were analyzed in relation to the temperature and strain rate.

An analytical approach to model transverse inter-fibre fracture evolution in cross-ply carbon fibre reinforced polymers laminates under a three-point bending test

The prerequisite to equip structures with in situ electrical health monitoring systems in a meaningful manner is to estimate potential damage locations. In this context, an analytical approach to model the spatial and temporal evolution of transverse inter-fibre fractures (IFF) is exemplified for [Formula: see text] cross-ply laminates under three-point bending. The model is based on the tensile load in the bottom 90°-layer. The effective stress is computed directly from the specimen displacement, whereas the strength is considered to be periodically distributed due to material inhomogeneities along the specimen. The continuous comparison between the sinusoidal strength function and the effective stress allows modelling progressive damage in form of IFF. A modified Hann window is used to consider their effect to the bottom 90°-layer. Experiments with the same configuration are performed and the observed IFF evolution is used to tune the model parameters in an optimisation procedure, e.g., the peak separation of the sinusoid and the length of the Hann window. The comparison between model and experiment shows a high level of agreement. The model is thus capable to reproduce experiments with minimal computational effort. This makes it highly suitable as input for electrical monitoring models that rely on the continuous damage evolution of the investigated structure. In its current form, the model is limited to the specified configuration. However, the simple analytical approach allows it to be easily adapted. It is furthermore shown that progressive composite damage in terms of spatial and temporal evolution can be accurately described using an analytical approach.

Investigation of the Effect of Reinforcement Type on the Flexural Performance of Slab Tracks

This paper presents the results of a comprehensive experimental study on the flexural performance of ballastless discrete precast slab tracks with two alternative reinforcement types. These are the precast slab tracks produced using ordinary reinforcement [nonpost-tensioned (NPT)] and the precast slab tracks produced using post-tensioned (PT) reinforcement. The study aims to compare the flexural performances of NPT- and PT-type slab tracks using three-point bending tests, even though they continuously sit on the ground with different rigidities in real-life applications. The tests performed on four full-scale slab tracks in the longitudinal and transversal directions resulted in the first cracking and the ultimate loads were greater in PT specimens. Also, the maximum crack widths and residual displacements of PT specimens were smaller. Although the aforementioned performance parameters of NPT-type slab tracks are relatively behind those of PT-type slab tracks, their overall response is still acceptable, with the advantages of easier production and lower cost.

Mechanical properties of multi-scale germanium specimens from space solar cells under electron irradiation

During long-term service in space, Gallium Arsenide (GaAs) solar cells are directly exposed to electron irradiation which usually causes a dramatic decrease in their performance. In the multilayer structure of solar cells, the germanium (Ge) layer occupies the majority of the thickness as the substrate. Due to the intrinsic brittleness of semiconductor material, there exist various defects during the preparation and assembly of solar cells, the influences of which tend to be intensified by the irradiation effect. In this work, first, Ge specimens for mechanical tests were prepared at scales from microscopic to macroscopic. Then, after different doses of electron irradiation, the mechanical properties of the Ge specimens were investigated. The experimental results demonstrate that electron irradiation has an obvious effect on the mechanical property variation of Ge in diverse scales. The four-point bending test indicates that the elastic modulus, fracture strength, and maximum displacement of the Ge specimens all increase, and reach the maximum value at the irradiation dose of 1 × 1015 e/cm2. The micrometer scale cantilever and nanoindentation tests present similar trends for Ge specimens after irradiation. Atomic Force Microscope (AFM) also observed the change in surface roughness. Finally, a fitting model was established to characterize the relation between modulus change and electron irradiation dose.

BUILDING COMPONENT DEFORMATION TRACKING USING STRUCTURED LIGHT TECHNIQUE

Abstract. It is crucial to fully master the mechanical properties of building components for ensuring the safety of infrastructures during service. The laboratory deformation test is an important way to study the mechanical properties of building components. At present, there is still a lack of a combination of high-efficiency, high-accuracy, and low-cost measurement methods for laboratory deformation tests. To address the issue, a method is proposed to measure the deformation of building components by using the structured light technique in this paper. It is applied to the axial tensile test of the dog-bone specimen, and the 3D full-field displacement of the dog-bone specimen is obtained by calculating the established deformation tracking model. The experimental results demonstrate the deformation measurement accuracy is at the millimeter level and can meet the accuracy requirement in the laboratory. Furthermore, the results are consistent with the way of force and the deformation mechanism of the dog-bone specimen. This paper presents a novel method for the deformation measurement, which provides a reliable analysis basis for studying the mechanical properties of building components.

Seismic performance of steel semi-rigid friction-damped joints with replaceable angles in modern Chinese traditional-style buildings

As an important part of world-class architectural art, Chinese ancient timber architecture has a very high level of artistry and technology. However, these buildings have been seriously damaged in the long history. To inherit and reflect the national characteristics and regional culture, the modern Chinese traditional-style building (MCTB) is newly developed to imitate the appearance of Chinese ancient timber architecture. The base metal of the typical joint area in MCTBs is always fractured under earthquakes because of its rigid characteristics, which is difficult to repair once damaged. To prevent joint failure and improve reparability after earthquakes, steel semi-rigid friction-damped joints with replaceable angles in MCTBs are proposed. Four specimens were fabricated and tested under lateral cyclic loading. The test results showed that the failure of all specimens is mainly concentrated in the weak region of the replaceable angles. Compared with simple semi-rigid joints, the yield displacements of specimens with friction dampers (including shear damper and rotational damper) are obviously larger, and the strength, initial stiffness, cumulative energy dissipation and replaceable capacity are significantly improved. The improvement is more pronounced with the increase of the slip force of the dampers. Based on experimental results, the corresponding numerical models were established to optimize the structural design of steel semi-rigid friction-damped joints with replaceable angles in modern Chinese traditional-style buildings.

Mixed mode I/II fracture behavior and surface morphology of hard rock under dynamic loading

To investigate the dynamic mixed mode I/II fracture properties of hard rock, split-Hopkinson pressure bar (SHPB) was utilized to conduct dynamic fracture tests on cracked straight through Brazilian disc (CSTBD) diorite specimens with different mode mixities. The dynamic fracture load and fracture toughness for different crack angles or mode mixities were determined. The dynamic fracture process and crack mouth opening displacement (CMOD) of CSTBD specimens were monitored by high-speed camera and digital image correlation (DIC) technique, and the fractal and morphology characteristics of fracture surfaces were further analyzed using laser 3D scanner and scanning electron microscope (SEM). The test results indicate that the dynamic CMOD behavior is interrelated closely to the force acting on the crack surface, and the allowable CMOD corresponding to the dynamic peak load decreases with increasing crack angle. In addition, the stress-based and strain-based fracture criteria were introduced to analyze the mixed mode fracture mechanism of diorite. It is found that the GMTS criterion with small fracture process zone (FPZ) size and the EMTSN criterion have the best prediction effect for fracture toughness and crack initiation angle respectively. Observation results show that the macro fracture trajectories of primary cracks and secondary cracks are controlled by the prefabricated crack angle, while the variation in mode mixity has little impact on the meso-roughness and micro-morphology of the fracture surface. The fracture surface of CSTBD diorite specimen under dynamic load exhibits the SEM morphology features of mixed tensile-shear cracks.

The effects of tensile strain rate on the dynamic tensile behavior of ultra high toughness cementitious composite

In this study, dogbone-shaped block specimens with cross section aspect ratio of 1.25 were prepared and tested at different strain rates ranging from 10−5 to 10−1 s−1 to investigate the dynamic tensile behavior of ultra high toughness cementitious composite (UHTCC) under different strain rates. The dynamic tensile responses were characterized in terms of the tensile strength, strain capacity, elastic modulus, specific energy absorption (SEA) and crack pattern. The results indicated that, with the increase of strain rate, the ultimate tensile strength increased by 160% but the strain capacity decreased by 80%. As the strain rate increased from 10−5 to 10−2 s−1, the elastic modulus and SEA increased by 40% and 50%, respectively. Reduction in the width of cracks in UHTCC was observed with the increase of strain rate while the number of cracks kept almost constant. Compared with thin planar specimens, the rate of increase in the ultimate tensile strength of UHTCC block specimens caused by strain rate increase was higher whereas that in the strain capacity was lower. Mesoscale numerical model with randomly distributed polyvinyl alcohol (PVA) fibers was built to analyze relationship between the cross section stress and displacement of specimens with different thicknesses. The results showed that the stress in the cross section and their corresponding relative displacement of block specimen were smaller than those of thin planar specimen under the same loading status. In addition, the cross section stress-displacement relationship of thin planar specimen was more sensitive to the strain rate than that of block specimen, which may be the reason for the weaker strain hardening behavior of thin planar specimens under dynamic tensile loading.

Effect change of extra-wide concrete self-anchored suspension bridge under blast loads and its prediction considering time-dependent effect

In order to study the dynamic response of self-anchored suspension bridge with extra-wide concrete girder under explosive load, a real bridge was taken as the background, which is the widest on in China at present. Firstly, the refined finite element model (FEM) of the whole bridge was established. The reliability of numerical simulation method was validated by the measured data of the bridge at completion state and explosive test results of scaled girder specimen. The vertical displacement, reinforcement strain, acceleration and failure characteristics of the girder specimen under the explosive load of 3 kg TNT were analyzed. Through parametric simulation, the effects of different scale distance, concrete strength and reinforcement ratio on the explosion response of girder specimen were studied. Secondly, the dynamic responses of girder were studied. Finally, the change law of dynamic response at different ages was analyzed. The results show that the elliptical crack is formed on girder top plate. The maximum girder vertical displacement reaches 135.9 mm under 1000 kg, and the maximum increasing ratio of hanger force is 27%. The influence brought by the extra width of girder is significant. The girder vertical displacement changes along transverse direction and the concrete breach sizes are different. In addition, with the increase of service age, the girder displacement increases from 60.50 mm to 106.16 mm under the same explosive load. The research results can provide an important basis for the anti-explosion protection and reinforcement of similar extra-wide concrete self-anchored suspension bridge.

In-plane/out-of-plane seismic performance of masonry infilled RC frame structure made of nano-SiO2 reinforced mortar: Experimental study

Due to the seismic vulnerability of conventional masonry infill wall Reinforced Concrete (RC) frame buildings, which caused the infill wall structure to break and collapse, resulting in serious casualties and property damage. It was necessary to fully understand the action mechanism and seismic strengthening of the infill wall structure. Considering that few researches had paid attention to the effect of mortar performance on the seismic behaviors of infill wall structure, this study was the first to use a nano-SiO2 (NS) reinforced mortar to construct masonry infill wall RC frame structure. The pure out-of-plane (OOP) and in-plane and out-of-plane (IPOOP) loading tests were conducted to validate the structure's seismic performance. The experimental results of comparison with ordinary mortar specimen were shown. Due to the mitigation of the cracking of wall in pure OOP test, the initial stiffness, load-bearing capacity and ultimate displacement of new mortar specimen were increased by 65.88%, 23.57% and 11.12%, respectively. However, both infill wall specimens exhibited the similar cracking characteristics and load-displacement response, those indicated that the improvement in mortar performance was not sufficient to change the failure mode. In the in-plane (IP) tests, the infill walls constructed with new mortar provided a better diagonal bracing mechanism, resulting in a 13.32% and 12.67% increase in the IP load-bearing and energy dissipation capacity. The less side shear cracking of infill wall caused stiffness degrades more slowly. In the IPOOP tests, at approximately the same previous IP damage level, the slower propagation of crack resulted in a slower reduction in the load-bearing capacity and an increase in the deformability during subsequent OOP loading. Therefore, the application of the new mortar improved the shear and bond strength of the members, which mitigated IP damage and improved the stability of the wall, while slowed down the OOP cracking of the wall and reduced the risk of collapse.

Experimental investigation of the creep damage evolution of coal rock around gas extraction boreholes at different water contents.

The creep process of the coal rock around the extraction boreholes under stress-water coupling is an important factor affecting the stability of the boreholes. To study the influence of the water content of perimeter of the coal rock around the boreholes on its creep damage, a creep intrinsic model considering water damage was established by introducing the plastic element model from the Nishihara model. To study the steady-state strain and damage evolution of coal rocks containing pores, and verify the practicality of the model, a graded loading water-bearing creep test was designed to explore the role of different water-bearing conditions in the creep process. The following conclusions were obtained: 1) water has a physical erosion and softening water wedge effect on the perimeter of the coal rock around the boreholes, which affects the loading axial strain and displacement of the perforated specimens; 2) an increase in water content reduces the time taken for perforated specimens to enter the creep phase, making the accelerated creep phase come earlier; 3) the parameters of the water damage model are considered to be exponentially related with the water content. The experimental data are similar to the results of the model parameters, and the model shows some practicality; 4) the damage variables in the accelerated creep phase increase rapidly throughout the creep process, leading to local instability in the borehole. The findings of the study provide important theoretical implications for the study of instability in gas extraction boreholes.