Fiber Bridging Effect Research Articles

Acoustic emission for exploring loading rate impact on type I fracture mechanism of engineering cementitious composites after subjected to sub-high temperature

To investigate the crack extension mechanism of engineered cementitious composite (ECC) after sub-high temperature. This paper studies the mode I fracture properties of ECC after sub-high temperatures (20 °C, 100 °C, 200 °C) at three strain rates (2 × 10-5s-1,10-4s-1,10-3s-1), and monitors the damage process in real-time by acoustic emission (AE) technology. Crack evolution and failure modes of ECC during loading are investigated by AE signal parameters. The results show that compared with 20 °C, the fracture energy of ECC increases after 100 °C, and decreases after 200 °C. The damage characteristics of ECC after 100 °C are similar to those at room temperature. Due to the bridging effect of the PVA fibers, their internal microcracks spread around from the tip of the prefabricated cracks. The percentage of shear type cracks gradually increases. The damage types are dominated by matrix cracking and fiber pull-out. While the PVA fibers deteriorated after 200 °C, the damage within the ECC was more concentrated, and the width of the fracture process zone was reduced. The crack types were dominated by tensile cracks. The failure mode was dominated by fiber pull-off and matrix cracking. Construction of an AE energy-based damage model. The b-value can reflect the whole process of crack evolution from multi-cracks to main cracks within ECC.

Seismic performance and flexural strength prediction of high-strength bar reinforced UHPFRC walls under high axial load ratio

A new-type shear wall built with high-strength reinforcement and ultra-high performance fiber reinforced concrete (UHPFRC) is proposed in this paper, aiming to solve the problem of too large cross-section size of bottom shear wall members of high-rise and super high-rise buildings. Four UHPFRC wall specimens built with high-strength reinforcement were tested under cyclic loading to examine the influence of the parameters of fiber volume fractions (0 vs. 1.5% vs. 2.5%) and the spaces of horizontal distribution reinforcing bars (150 mm vs. 300 mm) on the seismic performance. In addition, a calculation method of flexural strength of UHPFRC shear wall is established. The research results show that: the UHPC wall specimen without fibers failed in the axial compression brittle mode, and the UHPFRC wall specimens with fibers failed in flexure-dominated ductile mode. The bridging effect of steel fibers at the cracks kept the integrity of the UHPFRC. The drift ratio capacities of the UHPFRC wall specimens under high axial compression ratio of 0.5 were 3.56%. Increasing the fiber volume fractions can improve the strength, initial stiffness, and cumulative energy consumption of the wall specimens, reduce the damage degree of the UHPFRC at the toes of the wall specimens, and can also reduce the reinforcement ratio of horizontal reinforcement without influence on the seismic performance. The proposed method of flexural strength of UHPFC walls shows reasonable prediction accuracy.

Research on surface modification of polypropylene fiber and its application evaluation in oil well cement

In order to improve the compatibility and dispersibility of polypropylene fiber (PPF) in oil well cement and enhance the mechanical properties of cementing cement sheath, the surface modification of PPF was carried out in this study. After surface impurity removal of PPF, reaction of catechol with polyamines, and subsequent treatment with γ- aminopropyltriethoxysilane (KH-550) and Ethyl orthosilicate (TEOS), modified polypropylene fiber (MPF) was obtained. The MPF was characterized and analyzed using FTIR, XPS, SEM, and EDS, showing a transformation in the surface properties of the fibers. The most significant change was that the contact angle between MPF and water decreased from 123.87 ° to 46.32 °, marking the successful transition of the fiber from hydrophobicity to hydrophilicity. Image analysis showed that the dispersion of MPF improved significantly. The dispersion coefficient of MPF in water and cement stone was increased by 86.90% and 14.48% compared with PPF, respectively. The effect of MPF on mechanical properties of oil well cement stone was studied. The comparative analysis results showed that MPF can significantly improve the flexural and compressive strength of the cement matrix. The flexural and compressive strength of cement stone mixed with MPF were increased by 10.25% and 6.43%, respectively, compared to cement stone mixed with PPF. SEM, MIP, and fiber single fiber pull-out experiments showed that the porosity of cement stone added with MPF was lower, the rough surface structure of MPF enhanced the bonding between fibers and cement stone, and the bridging effect of fibers in cement stone ultimately improved the mechanical properties of oil well cement stone.

Mechanical Characteristics and Damage Constitutive Model of Fiber-Reinforced Cement-Stabilized Soft Clay

Marine soft clays are prevalent in coastal regions of China, giving rise to engineering challenges such as salt swelling, corrosion, and load bearing in foundations with soft soil. This study is dedicated to enhancing the mechanical properties of fiber-reinforced cement-stabilized soft clay (FCSSC) and revealing its strengthening mechanism. Uniaxial compression tests are performed to explore the impact of fiber length, fiber amount, and curing ages on mechanical behavior. The stabilization mechanisms of cement and glass fibers are explored through damage analyses and microscopy. Based on the experimental results, a damage constitutive model is formulated for FCSSC, and its validity is established by comparing fitting curves with testing curves. The results demonstrate a significant improvement in the mechanical properties of the stabilized soil, attributed to the synergistic effects of the cement and glass fibers. The growth rate of the unconfined compressive strength decreased with increasing curing ages. Notably, the fiber length significantly impacted the strength index, with short-chopped fibers playing a crucial role in strength enhancement. The compressive strength exhibited an initial increase followed by a decrease with rising fiber content, reaching a maximum between 0.3% and 0.4%. The bridging effect of the glass fibers proved effective in inhibiting compression crack expansion and mitigating structural damage of the soil sample. However, excessive fiber content or length led to improved local porosity, resulting in the deterioration of strength and deformation properties. The stress–strain curves fitted using the proposed damage constitutive model accurately reflected the stress–strain relationship and deformation characteristics of the FCSSC.

Study on bending failure and crack characteristics in ductile fiber‐reinforced concrete beams

AbstractIn this study, we designed six polypropylene fiber‐reinforced concrete (PP‐FRC) beams and three reinforced concrete (RC) beams for experimental studies. Then, we analyzed the effects of incorporating PP fibers on the cracking performance of PP‐FRC beams from the perspectives of load‐displacement response, crack distribution, crack depth, crack width, crack spacing, and deformation coordination. The research results showed that the cracking load and number of cracks were positively correlated with the volume ratio of PP fibers, while the maximum crack width, average crack spacing, and average crack depth were negatively correlated with the volume ratio of PP fibers. Due to the bridging effect of fibers, the PP‐FRC beam body and reinforcement could deform in a coordinated manner within a specific loading range after cracking, which increased the deformation coordination ability between the body and reinforcement. In this study, the cracking mechanism of PP‐FRC beams was investigated. Firstly, the formula for calculating the cracking moment of PP‐FRC beams was derived from the perspective of equivalent bending tensile strength. Then, based on the fiber bridging law and the bond strength between PP‐FRC and reinforcement, theoretical formulas were proposed to predict the average crack spacing and maximum crack width of PP‐FRC beams. Finally, through comparison, it was found that the proposed formulas could be used for characteristic crack prediction.

Shape memory alloy-reinforced UHPC tube confined bridge piers for enhancing the seismic resistance of highway bridges

Multi-span continuous highway bridges supported with reinforced concrete (RC) bridge piers may experience large permanent deformation after strong earthquakes. To overcome the limitations, an innovative UHPC tube-confined SMA reinforced bridge pier, i.e., shape memory alloy (SMA) replacing the steel rebars at the plastic hinge region and a prefabricated UHPC tube substituting the cover concrete, is proposed. To fully utilize the tensile strength of UHPC, the UHPC tube is embedded into the foundation. A performance-based seismic design procedure considering target residual drift as engineering demand parameter (EDP) is proposed to design this novel UHPC-SMA pier. The seismic responses of novel bridge systems supported by UHPC piers (Bridge II) and UHPC-SMA piers (Bridge III) are investigated and compared with reference bridge (Bridge I) having RC piers. The results indicate that both UHPC piers and UHPC-SMA piers are efficient in improving the seismic resistance of highway bridges. Compared with Bridge I, the prefabricated UHPC tube significantly decreases the residual deformation of Bridge II piers by 22.9 % as a result of the bridging effect of steel fibers. Due to the excellent self-centering property of SMA, the residual deformation of Bridge III piers decreases further by 57.8 % in comparison to reference bridge. The UHPC tube increases the low stiffness of bridge piers caused by SMA rebars at the plastic hinge region. The combination of UHPC tube and SMA can effectively prevent core concrete from severe damage with less dissipated energy of bridge piers.

Effect of freeze–thaw cycles on the macrostructure and failure mechanisms of fiber-reinforced clay using industrial computed tomography

Freeze–thaw cycling is a critical issue in cold-climate engineering because these cycles impact the mechanical properties of soils due to the translocation of water and ice at temperatures near 0 °C. Reinforcement methods have been developed to decrease these adverse effects, including the use of polypropylene (PP) fibers. However, few macrostructural investigations have been able to demonstrate the underlying physical basis for their effectiveness. This study used computed tomography (CT) images of clay samples reinforced with 2% PP fibers and subjected to unconfined compression and Brazilian tests before and after up to 10 closed-system freeze–thaw cycles (FTCs). Significant effects of the FTCs on soil structure include a reduction in macropores and an increase in mesopores. The addition of PP fibers reduces this change in the number of macropores from 28% to 18% following 10 FTCs. Unreinforced samples also show more localized propagation of shear/tensile cracks during tests than reinforced samples as a result of having a higher failure strength and ductility. The bridging effect of fibers, deviation of the failure path, and formation of microcracks around fibers are clearly illustrated in the CT images. This study provides significant insights relevant to engineering design in cold regions.

Mode I fracture toughness with fiber bridging of unidirectional composite laminates under cryogenic temperature

Carbon fiber reinforced polymer (CFRP) composites have widely been used in the aerospace field owing to their excellent mechanical properties. However, composite structures often fail under complex space environmental conditions. In particular, the fiber bridging and cryogenic temperature may greatly change the fracture toughness of CFRP composites. Herein, the effects of cryogenic temperature and fiber bridging on the fracture toughness of unidirectional composite laminates were experimentally examined by double cantilever beam (DCB) testing at different temperatures (room temperature, −50 °C, −70 °C, −100 °C, −150 °C, and −180 °C). Different calculation methods of fracture toughness were used and the results were compared. A method without measuring the delamination length at low temperatures was used and the results were consistent with other methods. The average difference in fracture toughness values was less than 5 %. The fiber-bridging microstructures of composites under cryogenic temperatures were examined by confocal microscopy along with the geometric morphology and parameters of surface roughness. The results suggested changes in the matrix failure mechanism at −50 °C, resulting in maximum fracture toughness, which is 26.45 % higher than that at room temperature. A semi-analytic approach was then proposed to express the bridge stress, and a bilinear cohesive zone model was established to simulate the delamination of composite laminates at low temperatures. Overall, the numerical data agreed well with the experimental results (peak force maximum error of 17 %), suggesting the usefulness of the proposed method for the prediction of interlamination properties and structural damage design of composite structures at cryogenic temperature.

Bond and Cracking Characteristics of PVA-Fiber-Reinforced Cementitious Composite Reinforced with Braided AFRP Bars

Easy maintenance and high durability are expected in structures made with fiber-reinforced cementitious composite (FRCC) reinforced with fiber-reinforced polymer (FRP) bars. In this study, we focused on the bond and cracking characteristics of polyvinyl alcohol (PVA)-FRCC reinforced with braided AFRP bars (AFRP/PVA-FRCC). Pullout tests on specimens with varying bond lengths were conducted. Beam specimens were also subjected to four-point bending tests. In the pullout tests, experimental parameters included the cross-sectional dimensions and the fiber volume fractions of PVA-FRCC. A trilinear model for the bond constitutive law (bond stress–loaded-end slip relationship) was proposed. In the pullout bond test with specimens of long bond length, bond strength was found to increase with increases in both the fiber volume fraction and the cross-sectional dimension of the specimens. Bond behavior in specimens of long bond length was analyzed numerically using the proposed bond constitutive law. The calculated average bond stress–loaded-end slip relationships favorably fitted the test results. In bending tests with AFRP/PVA-FRCC beam specimens, high ductility was indicated by the bridging effect of fibers. The number of cracks increased with increases in the fiber volume fraction of PVA-FRCC. In specimens with a fiber volume fraction of 2%, the load reached its maximum value due to compression fracture of the FRCC. The crack width in PVA-FRCC calculated by the predicted formula, considering the bond constitutive law and the fiber bridging law, showed good agreement with the reinforcement strain–crack width relationship obtained from the tests.

Novel Calculation Method for the Shear Capacity of a UHPC Beam with and without Web Reinforcement.

To accurately predict the shear-bearing capacity of UHPC beams, it is crucial to quantify the shear contribution of the fiber bridging effect and UHPC compression zone. Nevertheless, it should be noted that the shear contribution of UHPC in the compression zone is not fully considered in most existing calculation methods, and the probability distribution of fibers within the matrix is also not taken into full account, which reduces the calculation accuracy of the shear bearing capacity of UHPC beams. In this paper, a UHPC beam shear test database containing 247 samples was created, and the influencing factors on the shear capacity of UHPC beams, such as the shear span ratio, the web reinforcement ratio, and the volume fraction of steel fiber, were analyzed. It was found that the ratio of cracking load to ultimate load ranges from 0.2 to 0.6, and the failure in the compression zone of UHPC beams can be divided into diagonal tension failure and shear compression failure. Based on the failure mechanism of the compression zone, considering the contribution of fiber micro tensile strength, a formula for calculating the shear-bearing capacity of UHPC beams with and without web reinforcement was proposed. Verified by experimental data, the proposed formula accurately predicts the shear-bearing capacity of UHPC beams. In comparison with other shear capacity formulas in current design codes, the proposed formula in this paper provides a higher prediction accuracy.

Analyzing the microstructure of cemented fills adding polypropylene-glass fibers with X-ray micro-computed tomography

Fiber reinforced cementitious tailings backfill (FRCTB) is progressively employed in mining industry due to its high strength/stiffness-to-weight ratio. However, the wide use of the FRCTB technique requires further study of the practice of many different types of fibers, either alone or in combination. In this regard, this paper explores the impact of composite (glass-polypropylene) fibers on microstructure of FRCTBs considering weak surface and pore characteristics. The fractures of backfills were quantitatively analyzed by a non-destructive X-ray micro-computed tomography system which permits imagining of the interior microstructures of backfills via the construction of 3D volumetric data in diverse spatial scales. Quantitative parameters cover percent weak surface, porosity, and pore sphericity. The spatial distribution of weak surfaces, pores, and fissures and the bridging effect of fibers were also analyzed based on the 3D reconstruction technique. Adding fibers increased the percentage of FRCTB's weak surface. The greater the glass fiber dose, the larger the percentage of the weak surface. Composite fibers can lessen FRCTB's porosity. The smallest FRCTB porosity was 0.07 % for glass and PP fiber contents of 0.4 wt% and 0.2 wt%, respectively. A rise in glass fiber dose causes a rise in volume of 10–100 mm3 pore content. Fibers have a greater impact on the volume of 0.1–100 mm3 pores, and the rise in glass fiber dose deteriorates shape of this type of pores, leading to a reduction in the sphericity of this type of pores. There is a clear correlation between the distribution patterns of weak surfaces, pores, and fissures along the height of the specimens. Weak surfaces and pores are prone to develop as fractures. Fibers can limit the crack expansion in FRCTB. Lastly, this study presents a time- and cost-saving method for analyzing FRCTB's microstructure.

Engineering and microstructural properties of carbon-fiber-reinforced fly-ash-based geopolymer composites

Alkali-activated fly ash and slag-based geopolymer composites offer sustainable cement alternatives to conventional cement materials. Incorporating fibers mitigates issues such as brittleness, insufficient toughness, and cracking in geopolymers. However, limited systematic research exists on the influence of carbon fiber characteristics on geopolymer performance. In this study, the effects of carbon fibers having varying lengths (6 mm and 12 mm) and volume fractions (0%, 0.2%, 0.4%, 0.6%, 0.8%, and 1.0%) on multiple properties of geopolymers, including workability, water absorption, porosity, mechanical strength, drying shrinkage, uniaxial tensile properties, fracture toughness, and microstructure, are investigated. Results indicate that the incorporation of carbon fibers diminishes the flowability of the geopolymer mix and accelerates its setting time. Parameters such as water absorption, porosity, and compressive and flexural strengths initially decrease and then increase with increasing fiber content. Carbon fibers positively affect drying shrinkage, particularly during the early stages. The fiber bridging effect enhances uniaxial tensile properties, reducing brittleness. Carbon fiber addition improves the fracture toughness of the composite, achieving an optimal volume fraction at 0.6%. At this volume fraction, carbon fibers of 6 mm length exhibit better crack initiation toughness and resistance to instability cracking than fibers of 12 mm length. Moreover, the presence of carbon fibers promotes the formation of hydration products, minimises the occurrence of microcracks, and interacts synergistically with the slurry matrix to enhance overall crack resistance and material toughness. This study provides a valuable reference for the development and application of fiber-reinforced geopolymer materials.

Feasibility of utilising waste tyre steel fibres to develop sustainable engineered cementitious composites: Engineering properties, impact resistance and environmental assessment

This study investigates the feasibility of developing sustainable engineered cementitious composite (ECC) with polyvinyl alcohol fibre (PVAF) and recycled tyre steel fibre (RTSF) to reduce the total cost of construction materials and ease the environmental pressure for disposing of waste tyres, putting an emphasis on the engineering properties, impact resistance and environmental assessment. Experimental results reveal that partially replacing PVAF with RTSF can reduce the loss in fluidity and promote flexural performance, while the fibre bridging stress of ECC can reach up to 4.41 MPa because of the enhanced fibre bridging effect. The dynamic compressive properties of ECC present a typical strain rate effect regardless of the fibre content, and the replacement of 0.25 vol% RTSF would cause a further 5.03%–25.55% increase in total dissipation energy within the strain rates of 58.2–124.5 s−1, relative to ECC reinforced by 2.0 vol% PVAF. The synergetic effects of hybrid fibres on dynamic compressive properties of ECC can be explained by the bridging and pull-out effects of PVAF at lower strain rates, as well as the enhanced fibre-to-matrix bonding of hydrophilic RTSF at relatively higher strain rates. Upcycling of waste steel fibres in ECC holds promise as a cost-effective and eco-friendly solution for defence engineering constructions.

Interlaminar Response of LSI-Produced C/SiC Ceramic Matrix Composites: Experiments and Modelling

This work investigates the interlaminar properties of a C/SiC composite produced by Liquid Silicon Infiltration, combining experiments based on Double Cantilever Beam tests and numerical analyses. Experimentally, a method to obtain pre-cracks with sharp tips at precise locations is proposed, and specimens with different thickness are used to investigate the effects of bending stress states in the delamination process. The properties of tri-linear Cohesive Zone Models for the modelling of delamination are identified numerically, by using automatic regression techniques without requiring additional assumptions or testing. Fiber bridging effects were observed and modelled, including the evaluation of the process zone lengths with different experimental, analytical, and numerical methods. Overall, the work provides a qualitative insight in the delamination process of long fiber reinforced C/SiC laminates produced with a cost-affordable technique and proposes an experimental and numerical protocol to characterize and model delamination phenomena, taking into account the scattering of material properties.

Size Effect of Synthetic Fibre Reinforced Concrete – Investigation using a Semi-Discrete Analytical Beam Model

Abstract The size effect is a well-known characteristic of concrete structures. However, in the case of fibre-reinforced concrete (FRC), this issue is not thoroughly explored. Most design recommendations of FRC neglect the size effect or handle the behaviour of FRC structures in case of different structural sizes similar to plain concrete structures (assuming FRC is a homogeneous material). The aim of this paper is to show that the size effect of FRC can be divided, the share of the concrete matrix and the fibres in the size-dependent properties is separable. For the size effect research fifteen synthetic macro fibre reinforced concrete and six plain concrete beam specimens were prepared and tested in three different sizes and then evaluated with the semi-discrete analytical (SDA) model. The analysis of the experimental specimens has shown that the size effect significantly influences the concrete material in the case of FRC with softening material behaviour, but the residual loadbearing capacity which mainly arise from the local bridging effect of fibres is essentially independent of the structural size. It is also shown in this paper that the two defining parameters of the SDA model is independent of the structural size, so the model provides an excellent tool in case of the design of real-sized FRC structures.

Improving the dynamic splitting properties of engineered cementitious composites using hybrid synthetic polymer and recycled steel fibres

To improve the dynamic mechanical behaviour and make full use of recycled waste tyres, this study investigates the feasibleness of partial replacement of polyvinyl alcohol (PVA) fibres with recycled tyre steel (RTS) fibres in engineered cementitious composites (ECC). A series of laboratory tests were carried out to evaluate the workability, elastic modulus and uniaxial tensile behaviour, as well as the quasi-static and dynamic splitting tensile behaviour of ECC containing PVA and RTS fibres. The experimental results indicate that the replacement of 0.25% PVA fibres with RTS fibres results in a 0.85 MPa drop in tensile strength, while a nearly one-tenth increase in first-crack strength appears as the stronger RTS fibre-to-matrix bonding is effective in controlling the initiation of the first crack. Under dynamic loading, the mono PVA fibre reinforced ECC gradually loses the specimen integrity due to the rupture of PVA fibres at higher strain rates, while the bridging effect of RTS fibres contributes to maintaining the specimen integrity even at the highest measured strain rates of 7.3–7.5 s−1. Moreover, the replacement of 0.5% RTS fibres can lead to up to 15.54% and 27.38% increase in dynamic splitting strength and dissipation energy, respectively, relative to that with 2.0% PVA fibres. In addition, the evolution of dynamic increase factor with strain rate is modelled by theoretical equations considering the content and length-to-diameter ratio of fibres, the predictions of which match well with the experimental results.

Effects of Polyethylene Fiber Dosage and Length on the Properties of High-Tensile-Strength Engineered Geopolymer Composite

This paper presents a high-tensile-strength engineered geopolymer composite (EGC) reinforced by polyethylene (PE) fibers. The influences of fiber dosage (1.5%, 1.75%, and 2.0%) and length (6, 12, and 18 mm) on the mechanical properties and strain-hardening performance of EGCs were examined. The results indicated that increasing either fiber dosage or length decreases the flowability of EGC due to the skeleton formed by fibers. The increase of fiber dosage from 1.5% to 2.0% enhanced the fiber bridging effect in the EGCs with 12-mm PE fibers and subsequently enhanced their compressive and tensile strengths by 9.0% and 12.7%, respectively. Differently, the increase of 18-mm fiber dosage from 1.5% to 2.0% introduced more voids inside the EGCs, which decreased their compressive and tensile strengths by 3.8% and 3.6%, respectively. Fiber clusters were more likely to occur in EGC with a higher dosage of longer fibers, which reduced its tensile strength. A higher fiber dosage improved both tensile strain capacity and crack control capacity of EGC. On the other hand, increasing the fiber length from 6 to 18 mm increased the tensile strength by 42.0%, strain capacity by 148.0%, and crack control ability of EGC by enhancing the fiber-bridging effect, although it was detrimental to the compressive strength of the EGCs with 18-mm fibers due to the magnified air-entrapping effect. In addition, a prediction model modified based on the test results can accurately predict the tensile strength of PE fiber–reinforced EGCs. The environmental assessment indicated that the developed EGCs exhibit dramatically lower environmental impacts than the conventional engineered cementitious composite.

Experimental and numerical study of mode I interlaminar behavior of carbon nanotube reinforced glass–epoxy composite: A multiscale approach

In this paper, the effect of adding carbon nanotubes (CNTs) between glass–epoxy composite laminate in the mode I interlaminar behavior was investigated by experimental and numerical methods. Mode I interlaminar fracture toughness of epoxy matrix modified by 0.5 wt%, and 1 wt%, CNTs were compared with an unmodified epoxy matrix using the double cantilever beam (DCB) specimen. By adding a 1% weight ratio of CNT, the mode I interlayer fracture toughness increases by about 50%. A multiscale approach was used for modeling the DCB test. At the micro-scale, toughness mechanisms, including CNT pull-out, its breaking and bridging, and the separation of the nanotube from the surrounding resin were modeled. The separation of the nanotube from the surrounding resin was modeled with a cohesive element. The cohesive element around the nanotube was embedded in the three-dimensional elements of the resin and according to the weight percentage of the nanotube to the resin, a representative volume element (RVE) was created. The orientation, length, and diameter of CNT were considered randomly in the RVE. In the mesoscale, according to the percentage of glass fibers to resin, an RVE was created randomly. By using the damage model whose parameters were extracted from the micro-scale, crack growth was simulated. According to the Hill-Mendel conditions, the relationship between the meso and macro scale was established and the amount of effective quantities of traction and separation from the micro-scale was used as the parameter of the cohesive element in the macro-scale. To consider the bridging effect of glass fibers and its effect on interlayer fracture toughness, random distribution of interlayer beam elements was created. At the macro-scale, the DCB test simulated and cohesive elements were considered for modeling lower-scale mechanisms and beam elements for fiber bridging simultaneously. A favorable agreement between the experimental and simulation results was obtained.

Influence of curing pressure on the mode I static and fatigue delamination growth behavior of CFRP laminates

Curing conditions affect the mechanical properties of manufactured composite structures. However, the effect of curing pressure on the delamination behavior of laminates is still lack of investigation. This work studies the mode I static and fatigue delamination of carbon/epoxy laminates prepared at curing pressures of 1 MPa and 3 MPa. Delamination tests are conducted using double cantilever beam set-up. Test results show that more bridging fibers present in the laminates cured at 3 MPa, with more significant R-curve behavior, higher values of steady-state fracture toughness and maximum bridging stress than those in the laminates cured at 1 MPa. SEM images show that the laminates cured at 3 MPa mainly occur interlaminar damage while the major failure mechanism in the laminates cured at 1 MPa is the fiber/matrix debonding. A normalized model is proposed to describe the fatigue delamination behavior of laminates with fiber bridging effect. It shows that all fatigue data can be characterized by a single linear curve and curing pressure affects the slope of the linear curve. Furthermore, simulation for the delamination growth is conducted based on a tri-linear cohesive zone model. Good agreements between experimental and predicted load–displacement responses are achieved, illustrating the applicability of the established FE model.

Experimental study and evaluation of bonding properties between fiber and cement matrix under sulfate attack

The toughening effect of fibers on cementitious composites mainly comes from the bridging effect of fibers, and the core of the bridging effect depends on the bonding performance of fibers to the cement matrix. To study the fiber-cement matrix bonding performance under the action of sulfate-dry and wet cycles of erosion, fiber pull-out tests with different Na2SO4 solution concentrations (0–15%) and fly ash admixtures (0–30%) were carried out in this paper. Based on the principle of energy dissipation, the mathematical relationship between the toughness index and energy absorption index of fiber-cement-based mortar was established, and a comprehensive evaluation method of fiber-cement matrix bonding performance based on the weight determined by the CRITIC method is proposed. The results show that 15% Na2SO4 solution has a great influence on the erosion rate and erosion degree of interface bonding performance, and the peak load corrosion resistance coefficient is reduced by 43.7%. When the content of fly ash is 20%, the interface transition zone has the best resistance to sulfate attack, and the maximum value of bonding performance index B is 0.59 when the dry-wet cycle is 60 times. The findings can provide a theoretical basis for the interfacial adhesion resistance to sulfate attack and its degradation law under sulfate attack.