Decrease In Shear Strength Research Articles

Compression-shear performance of steel fiber reinforced rubber concrete

Rubber powder can be produced from waste tires through processing and its inclusion in concrete, either with steel fiber or alone, has been shown to improve impact and shear toughness and reduce environmental pollution. However, the mechanical behaviors and failure criteria of steel fiber reinforced rubber concrete (SFRRC) under combined compression-shear loading, which is essential for structural application, is lacking. This study investigates the compression-shear properties of SFRRC, considering steel fiber and rubber powder content, and compressive stress ratio. The effects of these parameters on shear strength, shear peak displacement, and shear toughness were analyzed. Results indicate that as the compressive stress ratio increases, SFRRC's failure pattern changes from tensile failure to compression-shear failure when k ≥ 0.4, exhibiting ductile failure. The equivalent initial energy density increases with rubber powder content, while equivalent shear residual toughness increases after the peak value, but shear strength and peak displacement decrease. With the addition of steel fiber, initial shear strength, peak displacement, and equivalent residual strength increase. A calculation criterion for compression-shear strength was proposed, with theoretical values matching well with experimental values.

Physical and Mechanical Properties of Expanded Polystyrene (EPS) Particle Lightweight Soil under Freeze-Thaw Cycles.

In seasonally frozen regions, the bearing capacity of soil decreases and gradually deteriorates after undergoing freeze-thaw cycles. To resolve this problem, based on the idea of frost-resistant filling materials, a filling scheme of expanded polystyrene (EPS) particles lightweight soil in cold regions was proposed. Unconfined compressive strength, direct shear, and micro-SEM tests were carried out to study the physical and mechanical properties of EPS particles lightweight soil under freeze-thaw cycles. The results indicate that the EPS particle lightweight soil has good frost resistance and can be used as frost-resistant filling material in cold regions. Under freeze-thaw cycles, EPS particle lightweight soil maintains good integrity; EPS particles can effectively reduce the frost heave rate, mass loss rate, and compressive strength loss rate of lightweight soil. The compressive strength depends on the EPS and cement contents: it decreases with an increase in the EPS content and increases with an increase in the cement content. The strength loss rate decreases with an increase in both. When the content of EPS is larger (more than 2%), the soil cement bound with EPS particles is limited, and the performance of lightweight soil decreases. The shear strength and cohesion decrease with an increase in freeze-thaw cycles and EPS content, and the internal friction angle follows no obvious rule with regard to the increase in freeze-thaw cycles but decreases with an increase in the EPS content. Based on the experimental results, an empirical formula for the compressive strength of EPS particle lightweight soil under freeze-thaw cycles was proposed. This study can provide a reference for the engineering design and application and provide new ideas for resolving freeze-thaw problems in construction engineering in cold regions.

Soil mechanical reinforcement by the fibrous roots of selected rangeland plants using a large soil-root shear apparatus

Soil reinforcement by fibrous roots of rangeland plants is essential for soil conservation. This study was done to: i) determine the effect of two genotypes 75B and 75C of Festuca arundinacea, Bromus inermis, and Bromus tomentellus on soil reinforcement at saturated/unsaturated conditions, and ii) explore the relationships between soil shear strength, and root area ratio (RAR) determined by image analysis and root chemical composition and mechanical properties in a sandy clay loam soil. Two genotypes of F. arundinacea were chosen based on their different morphological/physiological responses to environmental stresses. Rooted soils were sheared at three depths, 10, 25, and 40 cm. Peak shear strength and shear displacement-at-failure in the rooted soils were about four times greater than in the non-planted ones. B. inermis (with highest numbers of stretched roots) showed the highest shear strength (i.e., 35.7 kPa), and the roots of B. tomentellus were as effective as F. arundinacea. However, their effects on shear displacement-at-failure were significantly lower. Highest and lowest shear displacements were observed in the rooted soils of F. arundinacea and B. tomentellus, respectively. Greatest soil reinforcement was observed in the shallow soil (i.e., depth 10 cm). Soil reinforcement and the intercept and slope of shear strength vs. RAR were greater in the unsaturated soil (i.e., matric potential –10 kPa), indicating higher mobilization of root strength in soil reinforcement and lower root slippage. Shear strength differences of rooted soil were better noticeable in unsaturated condition. More than fivefold decrease in shear strength was observed from unsaturated to saturated state in control, but it was in the range of 1.4–1.7 for planted soils. This revealed that the reinforcing effects of roots moderated the role of soil water status in mechanical strength. Little changes in the slopes of shear strength vs. RAR relations for F. arundinacea 75C and B. inermis imply that they are effective in soil reinforcement irrespective of water status. However, the slope of relations for F. arundinacea 75B and B. tomentellus considerably increased from saturated to unsaturated state, showing that the roots of these two plants effectively reinforced unsaturated soils. Highest soil reinforcement by the roots of B. inermis was partly related to their greater lignin and hemicellulose contents and tensile strain-at-failure. Highest and lowest values of shear displacement-at-failure were observed in F. arundinacea 75C and B. tomentellus, respectively; it means that the roots of F. arundinacea 75C with higher cellulose/lignin ratio, tensile strength, and strain-at-failure stretched greater under shear.

Study on Cynodon dactylon root system affecting dry–wet cracking behavior and shear strength characteristics of expansive soil

Expansive soil exhibits remarkable characteristics of water absorption expansion and water loss shrinkage, rendering it susceptible to cracking under the alternating dry–wet environments of nature. The generation and development of cracks in expansive soil can result in catastrophic engineering accidents such as landslides. Vegetation protection is an important approach to stabilizing expansive soil slopes and fulfilling ecological protection requirements. In this study, through indoor experiments and theoretical analysis methods, the effects of Cynodon dactylon roots on the crack development and shear strength of expansive soil subjected to dry–wet cycles were analyzed, and the relationship between the crack development and shear strength decay in root-doped expansive soil was explored. Furthermore, the mechanism of vegetative root system action was elucidated. The results show that the Cynodon dactylon root system exerts a significant inhibitory effect on crack development in expansive soil. The crack indexes of root-doped expansive soil exhibit significant phase characteristics during the process of dry–wet cycles. The crack-blocking and reinforcing effect of the root system becomes pronounced as the root-to-soil mass ratio increases and the root diameter decreased. Moreover, the process of crack development in expansive soil is accompanied by a decrease in soil shear strength. The quantitative relationship between crack development and shear strength decay can serve as a basis for predicting the stability of slope soil. Overall, the results highlight the potential of vegetation-based approaches in protecting slopes with expansive soils and have practical implications for ecological protection and engineering design in areas with expansive soils.

Effects of chloride salt erosion and freeze–thaw cycle on interface shear behavior between ordinary concrete and self-compacting concrete

Self-compacting concrete (SCC) has been widely used for concrete jacketing to strengthen RC structures due to its high workability, fluidity, and segregation resistance. In this study, a test was carried out to study the effects of substrate curing including chloride salt erosion and the freeze–thaw cycle on the interface shear strength between ordinary concrete and the SCC. Eighty-four Z-type specimens were designed and tested through direct shear experiments, and the influence of interface treatment (implanting steel bar and cutting groove), concrete type, and substrate curing on the interface shear strength were analyzed. Four calculation models of interface shear strength based on different codes are compared, the applicability of these models is also analyzed. The results show that: For close the substrate concrete and the SCC strength, the effect of chloride salt erosion and freeze–thaw cycle will cause a slight decrease in the interface shear strength compared with the specimen that substrate is not cured, the influence of chloride ion erosion and freeze–thaw cycle on the degeneration of interface shear strength for specimen with implanting steel bar is not more obviously than specimen with cutting groove. Any single code model can not accurately predict the interface shear strength of all specimens with different interface treatments and substrate curing. The fib model can accurately predict interface shear strength of specimens which the average compressive strength of old and new concrete is less than 40 MPa and the substrate is not cured. Whether the substrate is cured or not, EC2 and CSA A23.3–14 models can quite accurately predict the interface shear strength of specimens with an implanting steel bar, but for specimens with cutting groove, the modified AASHTO model can accurately predict the interface shear strength. Except modified AASHTO model, the factor considering the effect of interface treatment should be taken into account when using fib, EC2 and CSA A23.3–14 models to calculate the interface shear strength.

Potential of Multi-temporal InSAR for Detecting Retrogressive Thaw Slumps: A Case of the Beiluhe Region of the Tibetan Plateau

Permafrost degradation due to climate warming is severely reducing slope stability by increasing soil pore water pressure and decreasing shear strength. Retrogressive thaw slumps (RTSs) are among the most dynamic landforms in permafrost areas, which can result in the instability of landscape and ecosystem. However, the spatiotemporal characteristics of surface deformation of RTSs are still unclear, and the potentials of deformation properties in mapping large-scale RTSs need to be further assessed. In this study, we applied a multi-temporal Interferometric Synthetic Aperture Radar (MT-InSAR) method to map the spatiotemporal variations in surface deformation of RTSs in the Beiluhe region of the Tibetan Plateau by using 112 scenes of Sentinel-1 SAR data acquired from 2017 to 2021. The deformation rates of RTSs ranged from − 35 to 20 mm/year, and three typical motion stages were inferred by analyzing the deformation variation trend of the headwall of RTSs: stable, abrupt thaw, and linear subsidence. A total of 375 RTSs were identified in the Mati Hill region by combining InSAR-based deformation results with visual interpretation of optical remote sensing images. Among them, 76 RTSs were newly developed, and 26% more than the inventory derived from the optical images alone. This study demonstrated that the combination of InSAR-derived deformation with optical images has significant potential for detecting RTSs with high accuracy and efficiency at the regional scale.

Effects of build orientation and strain rate on the tensile-shear behaviour of polyamide-12 manufactured via laser powder bed fusion

Laser powder bed fusion (L-PBF) of engineering thermoplastics is a well-established additive manufacturing technique. In this work, an experimental investigation of the tensile-shear behaviour of L-PBF processed polyamide-12 (PA12) is presented, with a particular focus on the concurrent effect of build orientation and rate dependency at strain rates ranging from 10−3 to 103s−1. Results show that the deformation behaviour features significant anisotropy at both low and high strain rates, while quasi-static ultimate strength in either tension or shear shows no dependence on the build direction. As the strain rate increases, the tensile strength is characterised by apparent positive rate dependence but remains insensitive to the build orientation; however the shear strength displays significant build orientation-dependent anisotropy, highlighted by the decreasing shear strength of specimens printed in the loading direction. Post-mortem scanning electron microscopy (SEM) demonstrates torn dimples with stretched filaments in quasi-static fractography, while cleavage facets with flake structures retrieved from high-rate loading. A fibrillar structure is observed from high-rate torsional loading indicating adiabatic decohesion during rapid crack propagation. These findings draw attention to the significant anisotropy of L-PBF PA12 parts in a high-rate loading regime and provide directions to improve the current L-PBF techniques.

Minor Ag inducedshear performance alternation in BGA structure Cu/SnBi/Cu solder joints under electric current stressing

In this work, the effect of minor Ag addition on shear performance of ball grid array (BGA) structure Cu/SnBi/Cu solder joints under electric current stressing was investigated. The results show that the shear strength of solder joints decreased with increasing current density, and the effect of Ag addition on shear strength changed from an elevating state to a deteriorating state. The elevating effect of Ag addition was due to the dominance of Sn/Bi phase refinement and Ag3Sn dispersion, which was weakened gradually with the increase in current density. The deteriorating effect of Ag addition was attributed to the more inhomogeneous distribution of current density and higher thermal gradient induced by the finer phase of the solder matrix as well as the consequent severer strain mismatch at the Sn/Bi phase interface. The finer Sn/Bi phase was also conductive to easier lattice atom migration under current stressing, resulting in a rapid decrease in shear strength. Moreover, the dominant factor on the shear strength reduction tended to change from Joule heating to athermal effect of current stressing. The fracture of solder joints indicated an insignificant influence by the Ag addition, which all occurred in the solder matrix in a ductile mode.

Effect of epoxy resin addition on properties and corrosion behavior of sintered joints in power modules serviced offshore

Power modules applied in offshore applications are facing risks of corrosion failures on die-attach materials due to high humidity and H2S exposure. To investigate such corrosion behavior for sintered die-attach materials, we conducted a study with four groups of samples fabricated using copper and silver metal particles under different solvent systems. Such samples were firstly subjected to high-humidity-H2S conditions for 168 h to simulate the harsh offshore environment. After undergoing corrosion, the primary compounds formed were CuO/Cu2O and Ag2S through SEM, XRD, and XPS analysis. Notably, the incorporation of epoxy resin into sintered copper joints resulted in a remarkable reduction in corrosion and a substantial improvement in electrical conductivity after the reaction. In contrast, while the addition of epoxy did not evidently reduce corrosion in silver joints, it did lead to a significant increase in shear strength. Furthermore, to gain further insights into the effect of epoxy resin on corrosion behavior, electrochemical analysis, and molecular dynamics simulations were conducted. Finally, the mechanical reliability of the corroded copper and silver joints was evaluated through thermal shock tests. In summary, sintered copper joints exhibited better anti-corrosion behaviors than sintered silver under high humidity and H2S exposure, especially with the addition of epoxy resin. However, the corrosion products of sintered copper suffered from a sharp decrease in shear strength after thermal shock tests than sintered silver, which is probably due to the coefficient of thermal expansion mismatch.

Effects of annealing on the interfacial microstructure and mechanical properties of explosively welded AZ80 magnesium alloy and A6005C aluminium alloy

Effects of annealing on the interfacial microstructure, mechanical properties, and residual stress of the explosively welded AZ80 magnesium alloy and A6005C aluminium alloy cladding plate were investigated. By explosive welding, a thin interlayer composed by an intermetallic compound, i.e. γ-Mg17Al12 phase, was formed at the interface of the cladding plate. After annealing at both 373 K and 473 K, the thickness of the interlayer increased. After annealing at 473 K, the interlayer changed from a single layer of γ-Mg17Al12 phase to a double layer of γ-Mg17Al12 phase and β-Al3Mg2 phase, resulting in a decrease in shear strength. As a result of nanoindentation measurement at the interface, the hardness was remarkably high in the β-Al3Mg2 phase. It was suggested that this phase became the crack initiation site for brittle fracture and the shear strength is decreased. Measurements of the residual stress using synchrotron radiation X-rays at the interface of cladding plate revealed the tendency of the generation of tensile residual stress on AZ80 magnesium alloy side and compressive residual stress on A6005C aluminium alloy side. After annealing at 473 K, residual stress in AZ80 magnesium alloy side and A6005C aluminium alloy side changed to compressive and tensile stresses, respectively, and these stress values became smaller in both cases. On the other hand, after annealing at 373 K, compressive residual stress was observed in both AZ80 magnesium alloy side and A6005C aluminium alloy side.

TOTAL AND MATRIC SUCTION MEASUREMENT OF UNSATURATED SOILS IN BAGHDAD REGION BY FILTER PAPER METHOD

Soil suction is one of the most important parameters describing the moisture condition of unsaturated soils. The measurement of soil suction is crucial for applying the theories of the engineering behavior of unsaturated soils.The filter paper method is one of the soil suction measurement techniques In this paper, five soil samples were collected from five sites within Baghdad city – al-Rasafa region. These soils have different properties and they were prepared at different degrees of saturation. For each sample, the total and matric suction were measured by the filter paper method at different degrees of saturation. Then correlations were made between the soil properties and the total and matric suction. It was concluded that the suction increases with decrease of the degree of saturation. The relationships between the total and matric suction and the filter paper water content are approximately linear and indicate decrease of suction with increase of the filter paper water content. The total and matric suction increase with the decrease of the soil shear strength.

Phase Stability, Microstructure, and Mechanical Properties of Spark Plasma Sintered Nanocrystalline Boron-Doped AlCoFeMnNi High-Entropy Alloy

The microstructure and mechanical properties of mechanically alloyed and spark plasma sintered AlCoFeMnNi-xB (x = 0, 0.5, 1, and 5 at. %) high-entropy alloys (HEAs) have been investigated. Boron-doped HEAs were synthesized using mechanical alloying up to 50 h of milling. Synthesized powders were then consolidated at 850, 900, and 950 °C for 10 min under a uniaxial pressure of 40 MPa using spark plasma sintering (SPS). A scanning electron microscope, which was equipped with energy dispersive spectroscopy (EDS), together with an optical microscope (OM) were used to analyze the microstructural evolution. X-ray diffraction analysis was used to differentiate the phases formed in the solution. The mechanical properties of the sintered specimens were analyzed using the shear-punch test (SPT). The fracture surface of the SPT samples was studied using SEM. Thermodynamic calculations revealed that by employing this process, it is possible to produce solid solution HEAs with a duplex FCC + BCC structure. It was shown that boron-doped AlCoFeMnNi high-entropy alloys contain some unique attributes. SPS at 900 °C for a sample with boron up to 0.5 at. % leads to the formation of an alloy with the highest shear strength. A further increase in the boron content in the boron-doped HEAs exhibited a decrease in the maximum shear strength. Finally, the correlations between the microstructural and mechanical characteristics of the sintered boron-containing high-entropy alloys are discussed.

Study on macro and micro shear strength of continuously graded and gap-graded sand

The particle gradation of sand has a significant influence on its shear strength, yet the similarities and differences between the effects of continuous and gap grading have yet to be fully explored. In this study, the discrete element method (DEM) was used to simulate biaxial tests on granular samples that were both continuously graded and gap-graded. The macroscopic analysis revealed that the shear strength of continuously graded sands increases initially and then decreases as the uniformity of particle size distribution decreases. On the other hand, the lack of medium particles in gap-graded sands amplifies the difference in particle size between coarse and fine particles, leading to a decrease in shear strength. Microscopically, both continuous and gap gradings affect the internal packing structure of the particle assembly, which consequently affects particle stress distributions, contact forces, coordination numbers, stress-induced anisotropies, and contact force networks, thus having an impact on the macroscopic shear strength. The global uniformity of particle size distribution was unidirectionally affected by continuous grading, while gap grading had a locally bidirectional influence. These findings provide a better understanding of the effects of particle grading on the macroscopic shear strength of sands.

Perkuatan Balok Beton Bertulang Yang Mengalami Kegagalan Geser Menggunakan Metode Deep Embedment

Bridges are infrastructure buildings that are commonly used and very functional in everyday. One of the structural components of the bridge is a reinforced concrete beam as a load bearer that will be forwarded to the foundation. The shear capacity of reinforced concrete beam structures sometimes cannot meet the existing requirements. This can be caused by increased loads, inadequate shear strength in the initial design and material damage due to natural factors. There are several methods that have been carried out to overcome the decrease in shear strength in beam structures, namely reinforcement methods by externally bonded (EB) and near-surface mounted (NSM). In reality, shear reinforcement with EB and NSM methods in implementation only relies on epoxy adhesion and concrete blankets, which still causes structural failure. The deep embedment strengthening method (DE) can be proposed as a shear reinforcement for reinforced concrete beams to overcome the previous problems. Reinforcement with DE method is a shear reinforcement that is reinforced in the core of reinforced concrete beams. This research was conducted by embedding 8 reinforcements vertically with a distance of 200 mm along the shear span. In this study, finite element modeling was carried out using ABAQUS. The results of finite element modeling with the DE method showed that the maximum load was 30.646 kN and the maximum deflection was 13.00 mm. The collapse model that occurs from finite element modeling on test specimens with DE reinforcement experiencing flexural failure.

Experimental and numerical tests on shear failure and size effect of CFRP-wrapped RC beams without stirrups: Influence of the shear-span ratio

The target of the current works is to analyze the impact of shear-span ratio on the shear failure of Carbon Fiber Reinforced Polymers (CFRP) wrapped reinforced concrete beams having different depths. Twenty-four simply supported beams with different structure sizes (the maximum beam depth reaching 1200 mm) and distinct Shear span to effective depth ratios of 1.0, 1.5, and 2.0, under the CFRP ratioρfof 0% and 0.0835% were designed and tested. In the experiment, the shear failure process and failure pattern of beams are presented in detail. The strain distribution of the CFRP sheets is described clearly as well. In addition, the shear contribution of CFRP with different beam sizes, especially large beams, and different shear span to effective depth ratio ratios were evaluated, besides, the calculation formulas of shear capacity in design specifications were analyzed and compared. The tested results indicated that all beams with distinct shear-span ratios exhibit a visible size effect. As the shear span to effective depth ratio increases, the generalized shear strength decreases. In addition, the smaller the shear span to effective depth ratio is, the smaller the shear contribution ratio of CFRP is, manifesting the impact of CFRP in a larger shear-span ratio is not as good as that of a smaller shear-span ratio. Finally, a 3-D Meso-scale finite element model is founded to enhance the understanding of the shear failure mechanical response of CFRP shear-strengthened concrete beams without hooping under the impact of a wide range of shear-span ratio.

Analisis Elemen Hingga Terhadap Perilaku Balok Beton Bertulang Yang Diperkuat Dengan Menggunakan Metode Deep Embedment

Bridges are infrastructure buildings that are affected by technological developments. Beam structures on bridges often experience a decrease in strength, one of which is shear strength. Sometimes the shear capacity of reinforced concrete beam structures cannot meet the requirements at this time. This can be caused by increased loads, inadequate shear provisions in the initial design and material damage due to natural factors. Reinforcement methods by means of externally bonded (EB) and near-surfaced mounted (NSM) are methods that have been carried out to overcome the decrease in shear strength in beam structures. Shear reinforcement using the EB and NSM methods in practice only relies on epoxy bonding and concrete covers, this makes it susceptible to structural failure. Reinforcement method deep embedment (DE) can be used as a suggestion as shear reinforcement in reinforced concrete beams to overcome the previous problems. Reinforcement using the DE method is shear reinforcement which is reinforced at the core of reinforced concrete beams. The aim of this research is to analyzq the behaviour of reinforced concrete beams resulting from finite element modelling using ABAQUS software. This research was carried out by embedding 20 bars vertically with a distance of 100 mm along the shear span. The results of finite element modelling with ABAQUS software show that the maximum load obtained is 28,12 kN and the maximum deflection that occurs is 14,10 mm. The final result of the beam failure model is flexural failure. This is indicated by the initial crack pattern that occurs vertically in the middle of the beam span. The crack continues to propagate towards the compression area of the beam until the beam collapses.

Study on the effect of hydro‐chemical dissolution on shear properties of rock fractures using the improved discontinuous deformation analysis

AbstractHydro‐chemical erosion has critical effects on the short‐ and long‐term stability of fractured rock masses in subsurface engineering. Attempts have been made to study this water‐rock interaction process using numerical approaches. However, the majority of the existing approaches quantify the degradation based on the homogeneity hypothesis, which leads to unrealistic results in the case of rock mass with discontinuities. In this study, the hydro‐chemical degradation is represented through geometrical variation of a single crack and reflected in the modified bilinear constitutive model by introducing the hydro‐chemical damage factor. Then, discontinuous deformation analysis (DDA) method embedded in such improvements is chosen to implement the direct shearing process of the rock fracture after hydro‐chemical erosion. The results reveal that the peak shear strength decrease with the increase in the acidity solutions, and a logarithmic relation is found between the decreasing percentage of shear properties and soaking time, which are consistent with the laboratory results. Besides, based on the proposed method, the change in peak shear strength presents synchronism with the variation in the mineral ion and concentration, shown to be in good agreement with the basic understanding of the dissolution process. In addition, the failure patterns of the sheared samples under different normal stress are studied. This research provides an effective tool to quantify the hydro‐chemical damage from the microscale and will be a significant complement to the coupled numerical analysis of the water‐rock interaction process.

Flexural Behavior of Two-layers Reinforced Concrete Beams

High-strength concrete is used to reduce the size of the beams in addition to enhancing the strength, this leads to overestimated cost in comparison with normal-strength concrete while using normal-strength concrete leads to the overestimated amount of concrete (layer size) of the beam section. For balancing the condition between the cost and size of beams, the benefit of both materials is used, by using beams in two layers, high-strength concrete in the compression zone (top layer), which is more beneficial for beam strength, and normal strength concrete in tension zone (bottom layer), which is no need using high strength concrete the in-tension zone. This study investigates the flexural and shear behavior of reinforced concrete beams consisting of two layers with different concrete strengths (grades), for beams with and without shear reinforcement (stirrups), considering the effect of shear-span ratio, layer thickness, layer compressive strength, and the overlap time casting of the two layers. The experimental program consists of a total of nineteen reinforced concrete beams of dimension (125 mm x 250 mm) with a total length of 1200 mm, the beams are reinforced with longitudinal reinforcement (4Ø12mm) and using (Ø8mm) bar as transverse reinforcement (stirrups). The experimental results show that the crack pattern of the two-layer reinforced concrete is closer to the crack of the control beam with one layer. Increasing the compressive strength of the concrete of the top layer, the ultimate failure load increased by (8.35%, 15.6%, and 18.85%), with respect to the (control beam) with the full depth of normal concrete. By increasing the high-strength layer thickness, the value of shear strength (Vc) and ultimate shear strength (Vu) increased linearly. The casting overlap time of up to (60 min) can be used for casting two-layered reinforced concrete beams, which is recommended, beyond this time the strength of the shear strength (Vc) and, ultimate shear strength (Vu) decreases. With increasing the shear span ratio (a/d) from (1 to 1.5 and 2) the ultimate load failure decreased by (33% and 50%). The shear strength capacity decreases with increasing stirrup spacing.

Study on Interface Mechanical Properties of Graphene/Copper Matrix Composites

Graphene/copper matrix composites have a wide range of application prospects, but the mechanical properties of the interface have been one of the key problems restricting their wide application. In this paper, the mechanical behaviors at the interface of graphene/copper matrix composites, such as pulling up, pulling out, and cohesion, and the effects of temperature and graphene content on them were studied by the molecular dynamics method. The results show that the pull-up force and cohesiveness show two stages in the whole process. The pulling force increases rapidly and then decreases to 0 slowly. The pull-out force shows three stages: it rises rapidly at first, then fluctuates continuously, and finally drops to 0. The mechanical properties of the interface deteriorated with the increase in temperature. When the temperature increased from 0 K to 1100 K, the interface normal strength, shear strength, and cohesion strength of the interface decreased by 26.3%, 32.9%, and 24.8%, respectively. In addition, with the increase in graphene content, the normal strength of the interface increases, the shear strength decreases, and the cohesion strength almost stays the same. When the graphene content increases from 6.71 at% to 11.75 at%, the normal strength increases by 6.8%, while the shear strength decreases by 37.4%. The influence mechanism of temperature and content is explained from the aspects of the influence of atomic thermal motion and the hindering effect of graphene on the dislocation motion of the copper matrix. The relevant results have certain reference values for the engineering application and theoretical research of graphene/copper composites.

Effectiveness of Surface Treatment on Bonding Performance of Starch-Based Aqueous Polymer Isocyanate Wood Adhesive.

The surface of a bonding material plays a key role in the bonding performance of an adhesive. Herein, we evaluated the effect of substrate surface treatment methods (sandpaper polished, chemical oxidation, and coupling agent) on the adhesive properties of starch-based aqueous polymer isocyanate (API) wood adhesive during hygrothermal aging. The birch substrate was processed with three different surface treatments, and the change of surface was analyzed by X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared spectroscopy (FT-IR), and Energy Dispersive Spectroscopy (EDS) methods. The results showed that the surface treatment had a great influence on the change of the shear strength of glued wood under hygrothermal conditions, and the silane coupling agent treatment could effectively reduce the decrease in the compressive shear strength of the adhesive. An XPS analysis indicated that the chemical oxidation modified wood surface polarity, and the coupling agent treatment in the wood surface formed a transition layer. After hygrothermal aging treatment, due to the different surface treatment of adhesive joint surface binding energy, the internal water absorption rate of starch-based API adhesives exhibited different failure modes of the adhesive joint. These findings indicate that the surface treatment effectively improved the durability of the adhesive joints.