Fiber Bridging Research Articles

Characterisation of near pure mode II quasi-static and fatigue delamination behaviour of a carbon-glass interface using a pure-moment-loaded DCB test rig

This work covers the characterisation of delamination growth in a carbon-glass interface, representative of the one in a wind turbine blade. A pure-moment-loaded Double Cantilever Beam (DCB) test rig is used under quasi-static and fatigue loading. A new fatigue characterisation procedure is adopted by real-time controlling the prescribed energy release rate G. This procedure enables sweeping any desired G range at a user-defined rate independently of the compliance of the specimen. A finite element simulation using a cohesive zone model confirms the testing was performed at near pure mode II conditions. From the quasi-static tests, the onset and steady-state fracture toughnesses are characterised. Fibre bridging is observed, which is believed to explain the characterised R-curve effect. From the fatigue tests, Paris’ law is characterised in a wide Gmax range in each test. The characterisation of the quasi-static and fatigue fracture behaviour of the carbon-glass interface is thus successfully achieved.

Comparison of mode II delamination behaviours in multidirectional and unidirectional composite laminates

Multidirectional (MD) composite laminates are extensively employed in structural applications owing to their superior mechanical characteristics. Nevertheless, the evaluation of the fracture toughness of composite laminates primarily relies on tests using unidirectional (UD) specimens. This study evaluates the reliability of characterizing mode II delamination behaviour in MD laminates by using UD specimens. The quantification of delamination area through Digital Image Correlation (DIC) analysis is integrated with a physical Energy Release Rate (ERR) method to ascertain the fracture resistance, which is compared with the ERR derived via a modified J-integral method and the standardized compliance methods. Fractographic analysis reveals similar fracture mechanisms in specimens with identical interfaces. The physical ERR increases notably due to large-scale fibre bridging induced by fibre nesting at 0∘//0∘ interfaces. Conversely, in 0∘//90∘ interfaces, large-area matrix cracking enhances the intrinsic fracture resistance, excluding the extrinsic toughening provided by fibre bridging.

Embedded optofluidic biosensing platform for enzyme-linked immunosorbent assay

With the growing demand for applications in disease diagnostics, bioanalysis, and health monitoring, developing efficient biosensing systems for fast detections and trace analysis of biomarkers is of great significance. In this work, an embedded optofluidic biosensing platform is proposed, which consists of asymmetrical core-offset optical fiber (ACOOF) structure, microfluidic chip and photoelectric detection system. In particular, by introducing the concept of optical fiber bridge, an ACOOF structure is designed to improve the optical coupling efficiency and reduce the limit of detection (LOD) of the developed sensor. On this basis, the human epidermal growth factor (EGF) was detected by constructing optofluidic chip and enzyme-linked immunosorbent assay (ELISA) kit. The LOD was 0.587 pg/mL. Leveraging the developed biosensing platform integrated with our chip, we achieved trace analysis with high specificity, as demonstrated by the measurement of interleukin 1α (IL-1α) with a low LOD of 43.3 fg/mL. Monitoring these two biomarkers is important for early cancer diagnosis. Beyond this, this novel platform has the potential for a range of applications, including disease diagnosis and management, bioanalysis, and health and environmental monitoring, with a focus on alternative biomarker targets.

A multi-linear constitutive relation considering the temperature effect on quasi-static mode I delamination in UD/MD laminates

In this study, a multi-linear constitutive relation taking into account temperature and fiber bridging is proposed for characterizing delamination behavior in composite laminates under various temperature conditions. An approach combining analytical solution and J-integral is also established for determining the cohesive parameters in the multi-linear constitutive relation. To validate the proposed constitutive relation, mode I quasi-static delamination experiments of unidirectional (UD) and multidirectional (MD) carbon/bismaleimide laminates are carried out at 25 ℃ (room temperature), 80 °C and 130 ℃. The experimental results show that the increasing temperature resulted in a monotonic increase in the fracture toughness of the UD laminates while affect the fracture toughness of MD laminates slightly. A FE model is established with the implementation of the proposed multi-linear constitutive relation using UMAT subroutine. Good agreements between the experimental and simulated results demonstrate the validity of the proposed constitutive relation, with the relative difference of peak load between predicted and experimental values less than 8.2 % and the relative difference of initial and steady-state fracture toughness between predicted and tested results less than 15 %. This study provides the possibility to numerically study the temperature effect on the delamination behavior of laminates and has promising applications in the damage tolerance design of composite structures.

Mode I fatigue delamination growth behavior and model for multidirectional laminates with the same overall stiffness

In order to isolate the influence of ply orientation on the mode I fatigue delamination propagation behavior of carbon fiber reinforced composite laminates, multidirectional laminates with the same overall stiffness are designed while three different interfaces (0//0, 45//45, and 90//90), which can avoid the coupling effect of remote plies. Test results show that the fatigue delamination behavior is obviously affected by the interface angle. In addition, a novel and simple method for determining the fatigue delamination resistance is proposed. The measured fatigue delamination resistance is lower than the quasi-static one for all interfaces. The specimen with 45//45 interface exhibits more significant delamination resistance, whereas the delamination resistances of specimens with 0/0 and 90//90 interfaces are similar, which is consistent with the phenomenon of more bridging fibers accompanying in the delamination process of 45//45 interface. In order to consider the effect of fiber bridging and reduce the dispersion of the data, experimental data are further processed using a normalized model considering the effect of fiber bridging. The normalized results can be characterized by a single curve, suggesting that the normalized model is effective in describing the fatigue delamination behavior in the presence of fiber bridging.

Fracture Toughness of Short Fibre-Reinforced Composites-In Vitro Study.

The development of dental materials needs to be supported with sound evidence. This in vitro study aimed to measure the fracture toughness of a short fibre-reinforced composite (sFRC), at differing thicknesses. In this study, 2 mm, 3 mm and 4 mm depths of sFRC were prepared. Using ISO4049, each preparation was tested to failure. A total of 60 samples were tested: 10 samples for each combination of sFRC and depth. Fractured samples were viewed, and outcomes were analysed. EXF showed greater toughness than EXP, with a mean of 2.49 (95%CI: 2.25, 2.73) MPa.m1/2 compared to a mean of 2.13 (95%CI: 1.95, 2.31) MPa.m1/2, (F(1,54) = 21.28; p < 0.001). This difference was particularly pronounced at 2 mm depths where the mean (95%CI) values were 2.72 (2.49, 2.95) for EXF and 1.90 (1.78, 2.02) for EXP (Interaction F(2,54) = 7.93; p < 0.001). Both materials performed similarly at the depths of 3 mm and 4 mm. The results for both materials were within the accepted fracture toughness values of dentine of 1.79-3.08 MPa.m1/2. Analysis showed crack deflection and bridging fibre behaviour. The optimal thickness at the cavity base for EXF was 2 mm and for EXP 4 mm. Crack deflection and bridging behaviour indicated that restorations incorporating sFRCs are not prone to catastrophic failure and confirmed that sFRCs have similar fracture toughness to dentine. sFRCs could be a suitable biomimetic material to replace dentine.

A cohesive fracture-enhanced phase-field approach for modeling the damage behavior of steel fiber-reinforced concrete

This study introduces a novel computational framework based on the phase-field fracture/damage model (PFM), providing rapid and accurate predictions of the damage behavior of Steel Fiber Reinforced Concrete (SFRC) material. The framework integrates the interfacial cohesive fracture model with the PFM to depict the interaction between the cementitious matrix and fibers, marking the first demonstration that this model adequately captures both local fracture phenomena between fibers and concrete (such as fiber bridging and interfacial debonding) and mesoscopic stress–strain behavior. Additionally, an enhanced geometric approximation is established based on the spatial distribution density function of fibers and particles, allowing for the transformation of the three-dimensions (3-D) fiber-reinforced material structure into an equivalent two-dimensions (2-D) material. This approach enables the reduction of computational time, a significant limitation of the phase-field method, thereby allowing an in-house code to perform various computations with complex material structures such as SFRC. The effectiveness of the approach is demonstrated through four examples involving variations in the microgeometry and material properties of SFRC structures, ranging from the simplest configurations to real experimental material structures. Extensive Monte Carlo simulations of the model are also employed to provide results closer to reality, which are then compared with recent experimental data and numerical models, demonstrating the efficacy of the proposed computational model.

Experimental and theoretical studies on the shear resistance of reinforced engineered cementitious composite beams with GFRP rebars and natural fibers

In this work, an attempt is made to produce a ductile construction system using natural fiber-based engineered cementitious composite (ECC) beams where the conventional longitudinal steel reinforcements are replaced using glass fiber-reinforced polymer (GFRP) bars. In total, sixteen medium-scale reinforced ECC beams were tested under an un-symmetric three-point bending configuration to generate shear failure. The parameters considered as a part of the experimental program include (a) type of longitudinal reinforcement (steel or GFRP) and (b) type of natural fiber (flax, hemp, kenaf and pineapple). In addition, a detailed analytical study was performed using the Architectural Institute of Japan (AIJ) method, and a modified equation was proposed for predicting the shear behavior of FRP reinforcement using ECC beams. Test results reveal that the use of GFRP reinforcements resulted in the reduction of peak load from 33.6% to 65.4% when compared to similar steel-reinforced R-ECC beams. However, the failure mode was highly ductile due to the excellent fiber bridging mechanism, multi-crack formation and effective stress redistribution at the crack tip. The predictions obtained from the modified AIJ method improved the accuracy of shear capacity FRP-reinforced ECC sections when compared to the experiments.

Effect of fiber orientation of adjacent plies on the mode I delamination fracture of carbon fiber reinforced polymer multidirectional laminates

AbstractThe extensive use of multidirectional composite laminates requires to understand the delamination behavior at interfaces between plies with different fiber orientations. In this study, double cantilever beam testing was performed to investigate the mode I interlaminar fracture of carbon fiber reinforced polymer laminates with different central interfaces (0//0, 0//+45 and +45//−45). X‐ray microtomography revealed the curved crack front shape that was incorporated to calculate fracture toughness. The fracture toughness varies with the crack length in a typical delamination resistance curve, which can be quantified by an empirical equation. Incorporation of variable fracture toughness into a cohesive zone model can better predict the delamination fracture of the laminate, compared to constant toughness. It was found that the delamination mechanism is independent of central interfaces. However, compared to unidirectional laminates, multidirectional laminates are less resistant to crack initiation, but more resistant to propagation with higher toughness and shorter fiber bridging zone.Highlights Mode I interlaminar fracture of CFRP laminates with different central interfaces was investigated experimentally and numerically. Curved crack front shape was incorporated to calculate fracture toughness. Incorporation of variable fracture toughness into a cohesive zone model can better predict the delamination fracture of the laminate. Multidirectional laminates are less resistant to crack initiation but more resistant to propagation with higher toughness and shorter fiber bridging zone.

Mode II fracture properties of parallel neosinocalamus affinis bamboo strand lumber

This study investigates the Mode II fracture properties in the RL and TL planes of parallel neosinocalamus affinis bamboo strand lumber. Eighty End-Notched Flexure (ENF) specimens were designed, varying pre-crack lengths and specimen widths. Digital Image Correlation (DIC) was employed to analyze strain variations during the development of the fracture process zone (FPZ) and crack propagation stages. The Compliance Based Beam Method (CBBM) and equivalent crack length calculations were used to determine the strain energy release rate (GIIC) of PNABSL. R curves and crack growth rate curves revealed that crack propagation in the RL and TL planes. The results indicated that Mode II fracture propagation in both the RL and TL planes of PNABSL predominantly exhibited self-similar cracking, though the fracture surfaces were rough and uneven, with noticeable fiber bridging. The study found that Mode II fracture propagation was less stable in the RL plane compared to the TL plane, which is attributed to the easier fracturing of thick-walled fiber cells in the RL plane. The critical strain energy release rate for PNABSL was determined, offering valuable insights into its fracture behavior.

3D mesoscale model of steel fiber reinforced concrete based on equivalent failure of steel fibers

Steel fiber reinforced concrete (SFRC) is widely used in civil engineering. Investigating the mechanical properties and failure mechanisms of SFRC materials using mesoscopic models holds significant theoretical and practical importance. The bond-slip interaction between steel fibers and the cementitious matrix is crucial, yet current computational capabilities struggle to simulate the interactions among the numerous components within concrete containing a large number of steel fibers. This paper establishes an equivalent failure model to replace the bond-slip interaction. Based on this model, a multiphase mesoscopic model (including aggregates, the interfacial transition zone (ITZ), mortar, and steel fibers) is developed to predict the mechanical properties and failure modes of SFRC. The accuracy and validity of the model are verified by comparing the simulation results with the experimental results from four-point bending tests on SFRC beams, focusing on load-displacement curves, failure modes, and crack propagation. The results indicate that under load, the ITZ at the sharp edges of large aggregates in the SFRC beam is the first to be damaged, forming microcracks. Subsequently, these cracks bypass the large particles and propagate towards weaker regions with fewer steel fibers, forming macrocracks. At this stage, the steel fibers take over the load from the cementitious matrix and limit crack propagation. The simulated crack propagation trend and crack width during the loading process of the SFRC multiphase mesoscale model match well with the experimental observations. A higher fiber content can mitigate stress concentration within SFRC beams and enhance the fiber bridging effect, significantly improving the flexural load-bearing capacity and toughness of the beams. A smaller steel fiber inclination angle can effectively inhibit the propagation of vertical cracks, thereby increasing the load-bearing capacity and toughness of SFRC beams and extending the service life of the structure.

Traction-separation law for glass/epoxy composite laminates under mixed mode I/II loading condition considering the effects of fiber bridging

This study analyzed the effects of fiber bridging on fracture behavior in glass/epoxy composite laminates under mixed mode I/II loading conditions and proposed new traction-separation laws (TSLs) for this loading condition. To this end, double cantilever beam (DCB), end notch flexure (ENF), and mixed mode I/II bending (MMB) tests were performed under mode I, mode II, and mixed mode I/II loading conditions. Also, finite element analysis (FEA) was employed to simulate these tests. Under mode I loading condition, the presence of fiber bridging increased fracture toughness significantly, which was modeled using a tri-linear TSL. In contrast, under mode II loading conditions, the effect of fiber bridging was negligible, and a bi-linear TSL was sufficient. On this basis, a novel TSL was proposed for mixed modes that considers the effect of fiber bridging under mode I loading conditions and neglects it under mode II loading conditions. This TSL was defined using a tri-linear TSL for mode I and a bi-linear TSL for mode II conditions, along with the B-K fracture criterion. In the bi-linear TSL, the point τb was defined corresponding to the fiber bridging strength (σb) of the tri-linear TSL, with the assumption that τb cannot exceed σb. To analyze the variation in the FEA results with respect to τb, three cases were defined: τb = 0.0001 σb, 0.5 σb, and σb, respectively. FEA results in the first case were observed to be consistent with load–displacement curves obtained from experimental results.

An overview of tensile and shear failure mechanisms of silicon carbide-based ceramic matrix composites

The successful applications of silicon carbide-ceramic matrix composites (SiC-CMCs) in aeronautical and aerospace industries require deeper understanding of their mechanical behaviors and failure mechanisms. The overview on tension and shear is then presented from meso-mechanics perspective, including matrix cracking, interface de-bonding and sliding, matrix crack propagation as well as fiber bridging. The results reveal that the matrix cracking stress is proportional to the interface sliding stress and the matrix fracture energy. The ratio of tensile to shear stress of the matrix cracking remains constant both in tension and in shear. In addition, the deflection of matrix cracks in the interface depends on the elastic modulus matching parameter, the angle between matrix cracks and interface, as well as the residual thermal stress. The matrix cracks propagate in a random or periodic pattern, which are proportional to the sliding length at the matrix cracking stress. The fiber bridging mechanism, related to the Weibull distribution of the fiber strength, determines the tensile and shear behaviors after the saturation of matrix cracks.

Mechanism based four-linear cohesive zone model for mode I fracture of different stacking sequence CFRP laminates

Delamination, a prevalent failure mode observed in laminated composites, exerts a significant impact on structural integrity and performance. The occurrence of fiber bridging during the fracture process adds complexity and elevates the research challenges associated with this phenomenon. Existing models exhibit limitations in accurately capturing bridging behavior and discerning its underlying mechanical mechanisms. This study addresses these limitations by analyzing experimental results, employing the J-integral, and analyzing R-curve behavior, proposing a mechanism-based four-linear cohesive zone model along with a new finite element implementation method. Comprising three overlapping bi-linear CZMs, this model effectively simulates mode I fracture behavior in laminates with different stacking sequences. Moreover, it intuitively illustrates the mechanical mechanisms during crack propagation and offers simplicity in implementation. This research contributes to a deeper understanding of composite fracture mechanics and provides a practical model for predicting delamination behavior in laminated structures.

Study on the fracture behavior and toughening mechanisms of continuous fiber reinforced Wf/Y2O3/W composites fabricated via powder metallurgy

Tungsten (W) is a promising candidate material for the plasma facing components in fusion reactors. However, it has issues regarding the intrinsic brittleness. Tungsten fiber reinforced tungsten composites (Wf/W) have been developed based on the concept of extrinsic toughening mechanisms and they show a pseudo-ductile behavior during the fracture process. In the present work, continuous fiber reinforced Wf/Y2O3/W composites were fabricated via a powder metallurgy (PM) process, and the microstructure and mechanical properties were characterized. The fracture behavior and toughening mechanisms were analyzed in detail combining the results of experiments and numerical simulation. The Wf/Y2O3/W composites is toughened by multiple mechanisms such as fiber bridging, crack bending and deflection, interface de-bonding and plastic deformation of fiber. The energy dissipation by interface de-bonding can be neglected. However, it is a necessary factor to ensure any extrinsic toughening mechanisms. The main contribution of the energy dissipation while composite failure is the plastic deformation of fibers.

Mode II fracture behavior of glass fiber composite-steel bonded interface–experiments and CZM

The dominant failure mode was characterized as debonding in the novel non-welded wrapped composite joint made with GFRP composites wrapped around steel sections. Glass fiber composite-steel three-point end notched flexure (3ENF) and four-point end notched flexure (4ENF) specimens were utilized to experimentally investigate mode II fracture behavior of this composite-steel bonded interface. Two new methods were proposed with the help of digital image correlation (DIC) technique to quantify fracture data during the tests: 1) the “shear strain scaling method” to quantify the crack length a; 2) the asymptotic analysis method based on the longitudinal displacement distribution along the height of the specimen at the pre-crack tip to quantify the crack tip opening displacement (CTOD). To numerically simulate the mode II fracture behavior, a four-linear traction-separation law was proposed in the cohesive zone modeling (CZM) where the softening behavior with a plateau was defined by the authors between traditionally considered initiation and fiber bridging behavior. The experimental and numerical approaches were validated mutually through good matches between the test and FEA results. 3ENF test provided good insight into softening behavior while 4ENF contributed to quantification of fiber bridging. These findings contribute to a more comprehensive characterization and understanding of the ductile fracture behavior of bi-material bonded joints, especially in mode II failure scenarios.

Bridging Behavior of Palm Fiber in Cementitious Composite

This study addresses the growing need for sustainable construction materials by investigating the mechanical properties and behavior of palm fiber-reinforced cementitious composite (FRCC), a potential eco-friendly alternative to synthetic fiber reinforcements. Despite the promise of natural fibers in enhancing the mechanical performance of composites, challenges remain in optimizing fiber distribution, fiber–composite bonding mechanism, and its balance to matrix strength. To address these challenges, this study conducted extensive experimental programs using palm fiber as reinforcement, focusing on understanding the fiber–matrix interaction, determining the pullout load–slip relationship, and modeling fiber bridging behavior. The experimental program included density calculations and scanning electron microscope (SEM) analysis to examine the surface morphology and diameter of the fibers. Single fiber pullout tests were performed under varying conditions to assess the pullout load, slip behavior, and failure modes of the palm fiber, and a relationship between the pullout load and slip with the embedded length of the palm fiber was constructed. A trilinear model was developed to describe the pullout load–slip behavior of single fibers, and a corresponding palm-FRCC bridging model was constructed using the results from these tests. Section analysis was conducted to assess the adaptability of the modeled bridging law calculations, and the analysis result of the bending moment–curvature relationship shows a good agreement with the experimental results obtained from the four-point bending test of palm-FRCC. These findings demonstrate the potential of palm fibers in improving the mechanical performance of FRCC and contribute to the broader understanding of natural fiber reinforcement in cementitious composites.

Temperature-dependent R-curve and traction-separation relation in mode-I fracture of GFRP laminates

The R-curve and fiber bridging phenomenon in mode-I fracture of glass-fiber reinforced laminates at different temperatures are investigated in this study, aiming to reveal their changes with temperature. The mode-I fracture experiments are carried out by adopting double cantilever beam (DCB) configuration at −55 ℃, 23 ℃ and 80 ℃. Fiber bridging is observed during the tests. The R-curve and bridging traction are quantitatively analyzed, from which the relationship between the R-curve and fiber bridging phenomenon, and temperature is obtained. It is found that fiber bridging effect is enhanced with the increase of temperature. The bridging traction of specimens tested at 80 ℃ is significantly higher than that at −55 ℃ and 23 ℃. An R-curve model considering both temperature and fiber bridging effects is proposed. In addition, bilinear and tri-linear traction-separation relations (TSLs) are utilized to establish a numerical model for the simulation of delamination growth behavior with the consideration of the temperature-dependent effect on the mechanical properties of composite materials. When using the bilinear TSL, the fiber bridging is considered by integrating the resulted R-curve into finite element model via a user-defined USDFLD subroutine. Effects of initial interface stiffness, interface strength and viscosity coefficient on simulated results are numerically investigated. Finally, applicability of the established numerical models is illustrated by comparisons between the simulations and the test results.

Mechanical properties and cracking strength estimation of high-strength engineered cementitious composites considering the fracture process zone

In the present study, the effect of flaws on the mechanical properties of high-strength engineered cementitious composites (HS-ECC) was investigated. Compressive tests, tensile tests, and electron microscope scanning microscopic tests were designed to explore the influence mechanism of flaws on mechanical properties. The results show that the compressive failure of HS-ECC was primarily characterized by a high density of vertical micro-cracks, and the specimen maintained exceptional structural integrity. All tested mix ratios demonstrated tensile strain hardening. Notably, the test groups with an 18 mm fiber length exhibited superior crack width control, with an average crack width of less than 30 μm. A new fiber bridging model was proposed, informed by microstructural analysis and crack opening displacement observations. A theoretical formula for the cracking strength of HS-ECC was developed based on the fracture process zone concept. The calculated initial crack strength closely matches the experimental values, with a discrepancy range of [0, 17 %]. This close agreement provides robust validation for the accuracy and reliability of the enhanced cracking strength calculation theory, thereby supporting its application in the design of ECC.

Constructing flexible fiber bridging claws of micro/nano short aramid fiber at interlayer of basalt fiber reinforced polymer for improving compressive strength with and without impact

The high-performance Basalt Fiber Reinforced Polymer (BFRP) composites have been prepared by guiding Micro/Nano Short Aramid Fiber (MNSAF) into the interlayer to improve the resin-rich region and the interfacial transition region, and the flexible fiber bridging claws of MNSAF were constructed to grasp the adjacent layers for stronger interlaminar bond. The low-velocity impact results show that the MNSAF could improve the impact resistance of BFRP composites. The compression test results demonstrate that the compressive strength and the residual compressive strength after impact of MNSAF-reinforced BFRP composites were greater than those of unreinforced one, exhibiting the greatest 56.2% and 73.3% increments respectively for BFRP composites improved by 4wt% MNSAF. X-ray micro-computed tomography scanning results indicate that the “fiber bridging claws” contributed to better mechanical interlocking to inhibit the crack generation and propagation under impact and compression load, and the original delamination-dominated failure of unreinforced BFRP composites was altered into shear-dominated failure of MNSAF-reinforced BFRP composites. Overall, the MNSAF interleaving might be an effective method in manufacturing high-performance laminated fiber in industrial production.