Tensile Peak Research Articles

Microstructure and joining mechanism of Al/CFRTP resistance element welded joints

Resistance element welding (REW) is an effective method for joining dissimilar materials by applying an auxiliary element in resistance spot welding (RSW). In this paper, 6061-T6 aluminum alloy/carbon fiber reinforced thermoplastic (CFRTP) joints were produced via REW. The macrostructure, microstructure, microhardness distribution and tensile shear performance of the joints obtained via REW were investigated. The effects of welding parameters on the nugget formation and joint performance during REW were investigated. Studied the joining mechanism by microscopic observation and mapping. Conducted fractography by macroscopic 3D observation and scanning electron microscope (SEM) observation. It is found that in a certain range, the interface nugget diameter increases with the increase of heat input, resulting in the increase of tensile peak load. The maximum peak tensile shear loads of the REWed joints were 3788.06 N. The failure mode of the REW joints changed from interface mode to partial button fracture with an increase in welding current under the same welding time and electrode pressure.

RETRACTED: Evaluation of pseudo-strain hardening behavior of hybrid fiber reinforced ultra-high performance concrete containing coarse aggregates by using micromechanical principles

In this study, pseudo-strain hardening (PSH) behavior of hybrid fiber reinforced ultra-high performance concrete containing coarse aggregates (UHPC-CA) and the synergistic effect of hybrid fibers were investigated. First, three types of fiber hybrid samples, namely, 2% short steel fiber (SSF2), 1.5% short steel fiber + 0.5% long steel fiber (SSF1.5 + LSF0.5) and 1.5% short steel fiber + 0.5% PE fiber (SSF1.5 + PE0.5), were designed and tested for their tensile properties. The results showed that the tensile strength and peak tensile strain of SSF1.5 + LSF0.5 were 19.9% and 183.3% higher than the corresponding values of SSF2, respectively. The tensile strength of SSF1.5 + PE0.5 was 3.6% lower than that of SSF2, but its ductility was 4.5 times higher than that of SSF2. Next, three-point bending tests on notched beams and single-crack tensile tests were conducted to evaluate the PSH behavior of hybrid fiber reinforced UHPC-CA by using micromechanical principles. The results showed that all groups met the strength and energy criteria with PSH index values of 5.86, 11.28, and 14.66 for SSF2, SSF1.5 + LSF0.5, and SSF1.5 + PE0.5, respectively. The higher the PSH index of the specimens, the greater the tensile ductility, which is consistent with the tensile test results. These results suggest that it is reasonable to evaluate the PSH behavior of hybrid fiber reinforced UHPC-CA by using micromechanical principles. Third, mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) analyses were performed to clarify the potential micromechanical properties of the hybrid fibers in improving the tensile properties of UHPC-CA. The MIP results showed that the total porosity was closely related to the initial cracking strength of the matrix; The smaller the total porosity, the higher the initial cracking strength of the matrix. The SEM results showed the characteristics of the PE fibers and the matrix. The bond strength between the PE fibers and the matrix was found to be high, leading to various damage patterns during the pull-out process of the PE fibers. Moreover, the combination of PE fibers and steel fibers showed a good synergistic effect, which explains the reason for the significant improvement in the ductility of the specimens when 1.5% steel fibers and 0.5% PE fibers were mixed.

A Three-Parameter Weibull Distribution Method to Determine the Fracture Property of PMMA Bone Cement.

Poly (methyl methacrylate) (PMMA) bone cement is an excellent biological material for anchoring joint replacements. Tensile strength and fracture toughness have a considerable impact on its application and service life. Considering the variability of PMMA bone cement, a three-parameter Weibull distribution method is suggested in the current study to evaluate its tensile strength and fracture toughness distribution. The coefficients of variation for tensile strength and fracture toughness were the minimum when the characteristic crack of PMMA bone cement was . Using the simple equation and fictitious crack length , the mean value (= 43.23 ), minimum value (= 26.29 ), standard deviation (= 6.42 ) of tensile strength, and these values of fracture toughness ( = 1.77 , = 1.02 , = 0.2644 ) were determined simultaneously through experimental data from a wedge splitting test. Based on the statistical analysis, the prediction line between peak load and equivalent area was obtained with 95% reliability. Nearly all experimental data are located within the scope of a 95% confidence interval. Furthermore, relationships were established between tensile strength, fracture toughness, and peak load . Consequently, it was revealed that peak load might be used to easily obtain PMMA bone cement fracture characteristics. Finally, the critical geometric dimension value of the PMMA bone cement sample with a linear elastic fracture was estimated.

Symmetrization of contact stresses in coating under frictional sliding indentation

Frictional contact problem of a layer-substrate structure is an important topic in coating optimization design. The present work carries out a comprehensive analysis on the coupling effect of coefficient of friction, Poisson’s ratio, and layer thickness on contact stresses distribution. A physical property namely full decoupling under interaction between material and geometric couplings is explicitly observed. Eccentricity in contact could be virtually diminished, and symmetrization on contact pressure of non-Hertzian type is presented at a specific Poisson’s ratio. Additionally, position of tensile peak within in-plane surface stress could not be affected by friction under the full decoupling effect. HIGHLIGHTS Coupling effects of Poisson’s ratio, friction coefficient, and layer thickness on eccentric contact involving soft bonded coating are studied systematically. A physical property namely full normal-tangential decoupling at a specific Poisson’s ratio in thin coating is explicitly observed and analyzed comprehensively. The tensile peak of in-plane stress as a crack initiator at the coating surface is studied under the full decoupling effect mentioned above.

A micro–macro method for evaluating progressive and direct tensile fractures in brittle rocks

A micro–macro method for evaluating the behaviors of direct tensile fractures during progressive loadings in brittle rocks is proposed in this study. The method consists of the suggested equation of the stress intensity factor of the mode-I crack that considers crack initiation, growth, and coalescence subjected to triaxial tensile loadings and the expression of axial strain relating to the extended length of the wing crack. The direct tensile correlation of stress and strain for depicting the complete initial elasticity, strain hardening, strain softening, and fracture stages is also studied. The reasonability of the presented method is proved by contrasting published results of experiment. Furthermore, the sensitivities of the density, inclination angle and size of the initial crack on the axial stress–strain curve, axial stress–crack length curve, tensile strength, crack initiation stress, and elastic modulus are determined. The tensile peak stress, tensile stress at crack initiation, and tensile elastic modulus descend with the increment of the inclination angle or size of the initial crack. The tensile peak stress initially descends and then remains constant, finally reaching a critical value with the increment of the density of the initial crack. The tensile elastic modulus descends with the increment of the density of the initial crack. The calculated results have a great significance for the safety evaluation of surrounding rocks in deep-buried underground engineering.

Lithological controls on fault damage zone development by coseismic tensile loading

During subshear earthquake rupture opposing sides of the fault experience transient impulses of tensile and compressive peak mean normal stresses exceeding the strength of the rock resulting in off-fault damage. This damage is expected to be influenced by lithology and occur preferably on the nominally tensile side of ruptures because rocks are weaker in tension than compression. Whereas the static tensile strength of many rocks is known, our capacity to relate parameters used to quantify brittle deformation in fault damage zones commonly measured in the field, including fracture density and fragment size, to earthquake rupture conditions is poor. This is due to a lack of experimental data relating tensile strength, fracture density, and fragment size to lithology under transient loading conditions. To relate brittle damage to coseismic tensile loading conditions, we use a modified sample configuration for a Split Hopkinson Pressure Bar (SHPB) designed to induce tensile rock fragmentation. Experiments cover a range of rock types including granite, diabase, welded tuff, and sandstone. All crystalline rocks fail predominantly via mode-I fracture, while sandstone fails through localized dilation bands, distributed pore space expansion, and less common mode-I fractures. Measurements of fracture density, fragment size, and porosity change are compared to tensile strength and strain rate at failure. We derive empirical relationships for (1) the transition from fracture to dilation banding as a function of initial porosity and (2) the strain rate dependence of tensile strength. These empirical relationships are used to estimate the rock strength during earthquake rupture on the side of the fault loaded under tension for comparison to predicted stress decay with distance from a fault to estimate the lithology dependent extent and nature of coseismic fault damage. Our experiments offer new insights into the lithological controls on the formation of asymmetric fault damage zones by dynamic rupture.

Experimental Research on Mechanical Properties of Carbon Fiber-Reinforced Reactive Powder Concrete after Exposure to Cryogenic Temperatures.

This study aims to evaluate the mechanical properties of carbon fiber-reinforced reactive powder concrete (CFRPC) after exposure to cryogenic temperature. The mechanical properties of plain RPC and CFRPC with carbon fiber volume contents of 0, 0.5%, 1.0%, and 1.5% were examined after exposure to 20 °C, −5 °C, −15 °C, and −25 °C for 72 h. The effect of fiber contents and exposure temperatures on the cubic and axial compressive strength, splitting tensile strength, elastic modulus, and peak strain were systematically reported and analyzed. The results showed adding carbon fiber to RPC could significantly enhance the strength and slightly improve ductility performance. Additionally, CFRPC with 1.0% fiber content showed the best mechanical properties. The maximum increases in cubic and axial compressive strength and tensile strength were 26.0%, 25.7%, and 21.8%, the elastic modulus was 13.2%, and the peak strain was 13.0% over the plain RPC. Additionally, all mechanical properties continued to degrade with decreasing temperature. After exposure to −25 °C, the cubic, axial compressive strength, and tensile strength of CFRPC degraded to 82.2–84.9%, 80.7–87.5%, and 72.7–73.7% of the normal temperature strength, respectively. In addition, the linear relationship equation between the discount factor of each mechanical property and the temperature was established. Finally, the equation for the stress–strain ascending curve of CFRPC described by a quadratic polynomial was proposed, which fitted well with the experimental results.

Effect of hold time on resistance spot weldability of aluminium to steel

The effects of welding hold time on weld quality of aluminium alloy to dual-phase steel resistance spot welds were studied. Results demonstrated that there existed an optimum range of hold time (350∼500 ms), within which the largest weld nugget diameter (7.6 mm), thinnest intermetallic compound (IMC) layer (1∼5 µm) and maximum tensile shear peak load (4.7 kN) were obtained. Too short hold time led to shrinkage, small weld nugget and thick IMC layer due to reduced heat dissipation after removing electrodes, thus resulting in low tensile peak load and brittle fracture along the IMC layer. While too long hold time, i.e. 750 ms, produced a smaller weld nugget diameter due to more rapid heat dissipation.

Effects of Steel Fiber and Specimen Geometric Dimensions on the Mechanical Properties of Ultra-High-Performance Concrete.

Ultra-high-performance concrete (UHPC) is an advanced concrete with superior mechanical strength, ductility and durability properties. However, the influence of steel fiber on its constitutive laws and the specimen geometric dimension effect on its strength had not been paid enough attention. To investigate the effect of steel fibers on the properties of UHPC, specimens with different fiber volume contents and fiber types were tested. Meanwhile, the mechanical properties of UHPC at different ages from 3 days to 28 days were conducted. Moreover, specimens with various geometric dimensions were also prepared to study the effect of specimen geometric dimensions (dog-bone-shaped, prism and cylinder specimens) on the properties of UHPC. The results indicated that elastic modulus, tensile peak stress and the corresponding strain increased as the fiber volume content and curing age increased. Specimens with hooked-end fibers exhibited better tensile performance than those with straight fibers. Furthermore, different geometric dimensions of specimens significantly influenced the tensile properties of UHPC. Based on the experimental results, conversion factors were suggested for the transformation of strength obtained from specimens with different geometric dimensions to reference specimens. In addition, both compressive and tensile constitutive laws were proposed to generate the stress–strain relationship of UHPC.

Microstructure and high-temperature mechanical properties of TP310HCbN welded joint

The microstructure of TP310HCbN welded joints and its effect on the tensile and low-cycle fatigue properties were investigated in the temperature range 20–700 °C, particularly in the operating range of 500–700 °C. The results showed that the welded joint was fully austenitic; however, a significantly different microstructure was formed in each welding zone, inducing material inhomogeneity. The material inhomogeneity was most pronounced in the weld metal, wherein an austenitic columnar grain structure composed of dendrite cores and interdendritic regions was developed. A combined analysis of the nanoindentation, failure mechanism, and kernel average misorientation using electron backscatter diffraction revealed that the dendrite core was the softest region in the welded joint and the deformation was localized here, thereby serving as a crack nucleation site and propagation path. This promoted premature failure of the welded joint, resulting in a reduction in the tensile strength, ductility, and the fatigue resistance, in comparison to the base metal. As both the welded joint and base metal showed significant cyclic hardening behavior (more than two-fold increase in the tensile peak stress) during fatigue deformation, the plastic strain energy density was found to be a suitable fatigue parameter, which was invariant throughout the fatigue life. An energy-based unified fatigue life prediction model using material toughness was developed, and its validity was demonstrated by successfully predicting the temperature dependence of the fatigue life as well as the fatigue life reduction of the welded joint.

Effects of freeze-thaw cycles on the quality of frozen raw noodles

The effects of freeze-thaw cycles (0–8) on the quality of frozen raw noodles (FRN) were investigated. Three types of wheat flour with different protein contents were used to prepare FRN. Freeze-thaw treatment decreased the break force and increased the tensile distance of FRN with higher protein content. More mobile water and less compact microstructure were detected in FRN. Freeze-thaw cycles exerted negative effects on the cooking and texture performance of FRN indicated by increases in the cooking loss (CL) and broken noodle (BN) percentage and decreases in the hardness and chewiness. After four freeze-thaw cycles, FRN with lower protein content exhibited higher CL and BN ratios and less chewiness. The tensile distance and peak force of the gluten isolated from FRN decreased during freeze-thaw cycles, which caused the quality deterioration of FRN. FRN with higher protein content and greater gluten strength demonstrated better resistance to the freeze-thaw treatment.

Ultra-High-Performance Concrete Crack Propagation Based on Fiber Random Distribution Model

The tensile strength, crack behavior, and strain-hardening properties of ultra-high-performance concrete (UHPC) are mainly affected by the steel fibers distributed in the matrix. In this study, a mesoscopic model of UHPC was established using a numerical simulation method to study the effects of steel fibers on the crack propagation and tensile properties of UHPC. First, the exponential cohesive model was used to simulate the pull-out of a fiber embedded in a UHPC matrix. Then, the distribution and variation of the interfacial shear stress during the fiber pull-out process were obtained, and the UHPC fiber-pull-out load–displacement curve was obtained. Using the Monte Carlo method, a meso-scale finite element model of UHPC was established by modeling randomly distributed steel fibers in the UHPC matrix. After verification, the model was used to study the effects of fiber characteristics, interface strength, and matrix strength on the crack propagation path and tensile properties of UHPC. The results showed that the exponential cohesive constitutive model with a softening coefficient of –1 can effectively characterize the mechanical behavior of the interface between the steel fibers and matrix in UHPC. Affected by the random distribution of fibers, the main propagation mode of cracks in UHPC was that the cracks bypassed the fiber-dense area and extended to the fiber-sparse area, and the crack propagation path was mainly affected by the fiber distribution. The distribution of fibers significantly affected the tensile strength and peak strain of UHPC. When the fiber inclination angle was in the range of 15°–30°, the comprehensive tensile properties of UHPC were the best. With increasing fiber volume fraction and fiber length, UHPC gradually began to exhibit multi-cracking and strain hardening (when the fiber volume fraction was more than 2.5% or fiber length was more than 15.5 mm). The interface strength and matrix strength had little effect on the crack propagation path, but had significant effects on the tensile strength and toughness of the UHPC. The higher the strength of the UHPC matrix, the more fully the anti-cracking effect of the steel fiber that can be achieved.

Sustainable terpene triblock copolymers with tuneable properties for pressure sensitive adhesive applications

A series of triblock copolymers in a hard-soft-hard block configuration with varying hard block α-pinene methacrylate content and molecular weight and butyl acrylate soft segment have been synthesised and investigated for viability in pressure sensitive adhesive (PSA) applications. The morphologies vary from pockets of hard phase distributed within a continuous soft matrix, through to lamellar with co-continuous phases, and finally continuous hard phase with pockets of soft phase dispersed. Uniaxial tensile properties, probe adhesion performance and cyclic adhesive behaviour are presented for seven compositions including four short chain and three long chain copolymers, alongside a commercial benchmark PSA. Structure-property relationships for the novel elastomers are evaluated, establishing that short chain materials with 20–25 wt% poly(α-pinene methacrylate) offer similar tensile and adhesion performance to the commercial elastomer. Raising the hard phase concentration has been observed to provide a considerable increase in ultimate tensile strength, stiffness and peak tack force, but at the expense of significant reductions in ultimate tensile strain, adhesive bond displacement and vibrational dissipation. The results suggest that the performance of these sustainable materials can be tuned to produce viable PSAs with a range of useful properties.

Proteomic, mechanical, and biochemical characterization of cartilage development

The objective of this work is to examine the development of porcine cartilage by analyzing its mechanical properties, biochemical content, and proteomics at different developmental stages. Cartilage from the knees of fetal, neonatal, juvenile, and mature pigs was analyzed using histology, mechanical testing, biochemical assays, fluorophore-assisted carbohydrate electrophoresis, and bottom-up proteomics. Mature cartilage has 2.2-times the collagen per dry weight of fetal cartilage, and fetal cartilage has 2.1-times and 17.9-times the glycosaminoglycan and DNA per dry weight of mature cartilage, respectively. Tensile and compressive properties peak in the juvenile stage, with a tensile modulus 4.7-times that of neonatal. Proteomics analysis reveals increases in collagen types II and III, while collagen types IX, XI, and XIV, and aggrecan decrease with age. For example, collagen types IX and XI decrease 9.4-times and 5.1-times, respectively from fetal to mature. Mechanical and biochemical measurements have their greatest developmental changes between the neonatal and juvenile stages, where mechanotransduction plays a major role. Bottom-up proteomics serves as a powerful tool for tissue characterization, showing results beyond those of routine biochemical analysis. For example, proteomic analysis shows significant drops in collagen types IX, XI, and XIV throughout development, which shows insight into the permanence of cartilage's matrix. Changes in overall glycosaminoglycan content compared to aggrecan and link protein indicate non-enzymatic degradation of aggrecan structures or hyaluronan in mature cartilage. In addition to tissue characterization, bottom-up proteomics techniques are critical in tissue engineering efforts toward repair or regeneration of cartilage in animal models. Statement of significanceIn this study, the development of porcine articular cartilage is interrogated through biomechanical, biochemical, and proteomic techniques, to determine how mechanics and extracellular matrix composition change from fetal to mature cartilage. For the first time, a bottom-up proteomics approach is used to reveal a wide variety of protein changes through aging; for example, the collagen subtype composition of the cartilage increases in collagen types II and III, and decreases in collagen types IX, XI, and XIV. This analysis shows that bottom-up proteomics is a critical tool in tissue characterization, especially toward developing a deeper understanding of matrix composition and development in tissue engineering studies.

Multiparameter Optimization to Improve the Erosion Resistance of Coating by Finite Element Simulation

Abstract Finite element method (FEM) was used to study the stress peak of stress S11 (radial stress component in X-axis) on the steam turbine blade surface of four typical erosion-resistant coatings (Fe2B, CrN, Cr3C2–NiCr and Al2O3–13%TiO2). The effect of four parameters, such as impact velocity, coating thickness, Young's modulus, and Poisson's ratio on the stress peak of stress S11 was analyzed. Results show that the position of tensile stress peak and compressive stress peak of stress S11 is far away from the impact center point with the increase of impact velocity. When coating thickness is equal to or greater than 10 μm, the magnitude of tensile stress peak of stress S11 on the four coating surfaces does not change with the coating thickness at different impact velocities. When coating thickness is equal to or greater than 2 μm, the magnitude of tensile stress peak of stress S11 of four coatings shows a trend of increasing first and then decreasing with the increase of Young's modulus. Meanwhile, the larger the Poisson's ratio, the smaller the tensile stress peak of stress S11. After optimization, when coating thickness is 2 μm, Poisson's ratio is 0.35 and Young's modulus is 800 GPa, the Fe2B coating has the strongest erosion resistance under the same impact conditions, followed by Cr3C2–NiCr, CrN, and the Al2O3–13%TiO2 coating, Al2O3–13%TiO2 coating has the worst erosion resistance.

Straw Paper for Art Applications – A Paper Development Study

Each paper grade, especially art papers have their own unique requirements by the artist using the paper in regard to mechanical integrity, surface topography, optical appearance and printability based on the unique artistic process applied during printing.
 Wheat straw fiber, a left over product from agricultural operations, can be used in many papermaking grades if properly processed prior to papermaking.
 A wheat straw containing art paper for artistic water color, oil, and encaustic (wax) painting applications was development and produced with a 10% and 15% northern bleached softwood content using a 12-inch (304 mm) wide laboratory scale paper machine, operated at a speed of 3.76 ft/min (1.14 m/min) with and without operating a calendar. The basis weight of the paper product was 350 g/m² with a caliper of 1050 μm to 1640 μm,
 The paper products burst load in psi ranged between 2.49 and 4.71. Tensile peak load measured in kilogram-force (kgf) per 15mm wide testing strip ranged from 4.08 to 9.32 for the machine direction and 1.96 to 4.74 for the cross machine direction.
 The roughness range of the wheat straw art paper was 2909 ml to 2963 ml for the top side and 2880 ml to 2954 ml for the bottom side of the paper.
 The ISO brightness and opacity of the yellowish wheat straw art paper was at 22.74% and 100.00%, The ISO color value was for the L*, a*, b* Hunter color scale 69.46%, 02.02, and 15.47 respectively.
 The finished art paper product met the expectations by the artists. The paper was used for several encaustic art studio projects.

Uniaxial tensile behavior and mechanism characterization of multi-scale fiber-reinforced cementitious materials

The uniaxial tensile properties of multi-scale fiber-reinforced cementitious material (MSFRCM) with steel and polyvinyl alcohol (PVA) fibers and calcium carbonate whisker (CW) were studied. The results showed that CW improved the uniaxial tensile stiffness, strength, peak strain, and toughness of the steel-PVA hybrid fiber-reinforced cementitious material. The CW not only played a role in the small deformation stage but also improved the load holding capacity and toughness of the hybrid fiber-reinforced cementitious material during the large deformation stage. Computational models to assess the uniaxial tensile strength and toughness of the MSFRCM were established. Microstructure observations showed that the steel and PVA fibers formed a weak interfacial transition zone (ITZ) due to the “wall effect.” The CW effectively optimized the structure of the ITZ of the steel and PVA fibers through physical and chemical effects, such as filling, bridging, improving Ca(OH)2 orientation, and chemical effects. The steel fibers, PVA fibers, and CW in the MSFRCM bridged cracks at the macro, mesoscopic, and microscopic levels, respectively. As a result, we observed a fiber chain effect that improved the positive hybrid effect between the multi-scale fibers.

Three-Dimensional Finite Element Analysis of Different Connector Designs for All-Ceramic Implant-Supported Fixed Dental Prostheses

All-ceramic fixed dental prostheses (FDPs) tend to fracture at the connector regions due to high stress concentration at these areas influenced by their design. This study was performed as an adjunct to an existing clinical study to evaluate the influence of the different radii of curvature of gingival embrasure on the stress distribution of a three-unit all-ceramic implanted supported FDP. Three three-dimensional (3D) models were created by scanning two titanium dental implants, their suitable zirconia abutments, and a patient-retrieved dental prosthesis using a micro-CT scanner. The radius of curvature of the gingival embrasure for the distal connector of the FDP was altered to measure 0.25 mm, 0.50 mm, and 0.75 mm. A finite element analysis (FEA) software (ABAQUS) was used to evaluate the impact of different connector designs on the distribution of stresses. Maximum Principal Stress data was collected from the individual components (veneer, framework, and abutments). The radius of curvature of gingival embrasure had a significant influence on the stress distribution at the assessed components. The tensile peak stresses at all structures were highest in the 0.25 mm model, while the 0.50 mm and 0.75 mm models presented similar values and more uniform stress distribution.

Experimental Investigation on Dynamic Tensile Behaviors of Engineered Cementitious Composites Reinforced with Steel Grid and Fibers

Engineered cementitious composites (ECC) used as runway pavement material may suffer different strain rate loads such as aircraft taxiing, earthquakes, crash impacts, or blasts. In this paper, the dynamic tensile behaviors of the steel grid-polyvinyl alcohol (PVA) fiber and KEVLAR fiber-reinforced ECC were investigated by dynamic tensile tests at medium strain rates. The mixture was designed with different volume fractions of fibers and layer numbers of steel grids to explore the reinforcement effectiveness on the dynamic performance of the ECC. The volume fractions of these two types of fibers were 0%, 0.5%, 1%, 1.5%, and 2% of the ECC matrix, respectively. The layer numbers of the steel grid were 0, 1, and 2. The dynamic tensile behaviors of the PVA fiber and the KEVLAR fiber-reinforced ECC were also compared. The experimental results indicate that under dynamic tensile loads, the PVA-ECC reveals a ductile and multi-cracking failure behavior, and the KEVLAR-ECC displays a brittle failure behavior. The addition of the PVA fiber and the KEVLAR fiber can improve the tensile peak stress of the ECC matrix. For the specimens A0.5, A1, A1.5, and A2.0, the peak stress increases by 84.3%, 149.4%, 209.6%, and 237.3%, respectively, compared to the matrix specimen. For the specimens K0.5, K1, K1.5, and K2, the peak stress increases by about 72.3%, 147.0%, 195.2%, and 263.9%, respectively, compared to the matrix specimen. The optimum fiber volume content is 1.5% for the PVA-ECC and the KEVLAR-ECC. The KEVLAR-ECC can supply a higher tensile strength than the PVA-ECC, but the PVA-ECC reveals more prominent deformation capacity and energy dissipation performance than the KEVLAR-ECC. Embedding steel grid can improve the tensile peak stress and the energy dissipation of the ECC matrix. For the strain rate of 10−3 s−1, the peak stress of the A0.5S1 and A0.5S2 specimens increases by about 49.1% and 105.7% compared to the A0.5 specimen, and the peak stress of the K0.5S1 and K0.5S2 specimens increases by about 61.5% and 95.8%, respectively, compared to the K0.5 specimen.

Effect of metakaolin on the mechanical properties and pore characteristics of fiber-reinforced tailing recycled aggregate concrete

In this study, based on the fiber-reinforced tailing recycled aggregate concrete, the effects of the metakaolin (MK) content on the mechanical performance were investigated. The pore structures and strain development of concrete were obtained by nuclear magnetic resonance (NMR) and digital image correlation (DIC). In addition, the enhanced effects of MK on concrete matrix were further studied at the micro level. The results uncovered that MK addition significantly improved the cubic compressive strength, axial compressive strength, splitting tensile strength and peak strain. The best performance was obtained by the mix that contained 10% MK. The NMR results showed that MK substitution led to decreased numbers of pores and a coarsening of micropore structure of concrete. The harmless pores (pore radius range 0–0.2 μm) were the most important parameter affecting the compressive strength. The DIC results indicated that the presence of MK particles favored the dispersion of polypropylene fibers in the concrete matrix. Microscopically, the addition of MK promoted the hydration process of matrix, leading to enhanced bonding between fiber and matrix.