Pull-out Mechanism Research Articles

Pull-out behavior and failure mechanism of steel expansion anchors in high-performance steel fiber reinforced concrete(HPSFRC)

Past research on the pull-out performance of anchors mainly focuses on normal concrete(NC) concretes. New design methods are needed for the pull-out performance of anchors in the concretes of new cementitious materials with high strength and toughness. Accordingly, the pull-out behavior of torque-controlled expansion anchor(TCE) in high-performance steel fiber reinforced concrete(HPSFRC) concrete with different fiber content and installation conditions(embedment depth and anchor diameter) was investigated. The focus was on the failure modes, crack morphology, and load-displacement behavior(including pull-out load-bearing capacity, anchorage stiffness, and anchorage toughness). The addition of fibers leads to increased load-bearing capacity and toughness, reduced stiffness, and all failure modes exhibited ductile cracking and sufficient tensile ductility. Both diameter and depth had a positive impact on load-bearing capacity. A broader database based on literature and this study was analyzed for further exploration of the effects of fibers. The accuracy of predicting load-bearing capacity was compared between two empirical models(concrete capacity design-CCD and Toth models) and the proposed modified model. The CCD model greatly underestimates capacity, and the Toth model is unsuitable for high fiber content(exceeds 2.0 %) HPSFRC scenarios. The modified model showed the best bearing capacity prediction for anchors on HPSFRC concretes, with a mean ratio of predicted values to experimental values reaching 1.01–1.08 and a coefficient of dispersion below 15.0 %.

Non-destructive AC impedance spectroscopy (ACIS) analysis to characterize the evolution of impact damage in UHPC

Ultra-high performance concrete (UHPC) widely serves as protection material owing to superior impact resistance. Rapid and effective assessment of impact damage level is crucial for ensuring structural safety. The non-destructive AC impedance responds sensitively to impact-induced microstructural changes, providing important information for understanding the damage development mechanism. Therefore, this paper intends to investigate the feasibility of AC impedance spectroscopy (ACIS) for UHPC impact damage detection. UHPC specimens with different types and volume fractions of steel fibers were prepared, subjected to multiple repetitive low-velocity hammer impacts to induce cumulative damage, and the impedance responses were simultaneously tested in the frequency range of 10 Hz-5 MHz. Additionally, the effects of steel fiber distribution characteristics and pull-out mechanism on the conductive path and impact resistance were explained through microstructural analysis and 3D reconstructed model. The results show that ACIS exhibits a strong correlation with impact damage, where the increment of the imaginary part can be used as an effective and sensitive indicator to achieve whole-process damage assessment. Compared to straight steel fibers, hooked-end and corrugated fibers greatly enhance the impact resistance of UHPC but cause a nonlinear response in impedance due to extracted deformations and orifice complexities. These findings demonstrate the potential application of ACIS for detecting impact damage in UHPC and warrant further investigation.

The interface mechanical and ablative evolution behaviors under different heat flux of multiphase HfC-SiC matrix composites

Multiphase HfC-SiC ceramic matrix composites were prepared by a combination process. The interface mechanics and ablation properties behaviors were investigated. Introducing the PyC-SiC interface, the composites showed a second-order pull-out mechanism. The mechanical failure model showed that fibers and matrix have different failure strain under load depending on the component that fails first. Moreover, the ablative airflow in the center area will spread to the edge area through the thermal shock microcracks generated. Increasing of the heat flux, the crack width gradually increased to be ditches. It accelerated the evolution of surface morphology, which showed HfC-SiC substrate was first transformed into Hf-Si-O solid solution on the process of phase transformation of oxidation products from 1680 ℃ to 2150 ℃, and eventually sintered into HfO2. The change in the morphology of oxidation product consumed more heat and protected the substrate from oxidation, resulting in improving the ablation resistance of composites.

High-strength TiO2/TPU composite fiber based textiles for organic pollutant removal

Effective integration of nanostructured photocatalysts into polymer matrices is crucial for the broad practical application of fiber-based photocatalytic composite textiles. Herein, highly durable TiO2/TPU composite fibers with high TiO2 content (>30 wt. %) were prepared through wet spinning using 1D electrospun TiO2 nanofibers (TNFs). Due to the fiber reinforcement principle, the TNF/TPU composite fibers exhibited superior mechanical properties compared with pure TPU fibers. Furthermore, the TNF/TPU composite fibers with a ratio of 1:2 process impressive degradation efficiency for rhodamine B (RhB). The orientation of 1D TNFs and stronger interface between TNF and TPU matrices not only enable the excellent load sharing and transfer effects following fiber pullout and crack bridging mechanisms, but also improve photogenerated charge transfer efficiency, resulting in high strength and high photocatalytic activity. In addition, enhanced TNF exposure on the fiber is due to the hollow and porous structures of composite fibers, which is advantageous for photocatalytic degradation. Notably, when the RhB concentration exceeded 15 ppm, the TNF/TPU composite fibers exhibited higher degradation efficiency than the TNF powder. Therefore, TNF/TPU composite fiber–based textiles with high photocatalytic activity and strength are promising for overcoming the separation and recovery issues of nanostructured catalysts in practical wastewater treatment applications.

An efficient method for evaluating the wind resistance performance of high-vertical standing seam metal cladding systems

Uplift wind-induced responses and failure criteria of high-vertical standing seam metal cladding systems (SSMCSs) are of paramount importance for evaluating their wind resistance performance. This paper proposes an efficient evaluation method that combines a highly efficient finite element (FE) model for structural response analysis with reasonable failure criteria, based on experimental and numerical investigation. The FE model incorporating the developed equivalent spring model is established with the calibrated stiffness from tensile tests and validated through the wind uplift resistance experiments conducted on large-scale prototype structures. The recorded structural responses from the experiments validate the feasibility of the FE model in simulating the geometric and material nonlinearities of the systems. To evaluate the wind resistance performance, the failure criteria are developed based on the observed pullout mechanisms and failure modes from the experiments. These criteria take the relative deformation of standing webs and the pullout resistance strength of seam connections as critical parameters, and the relationship between them is calibrated through a series of tensile tests using a self-designed fixture. Tensile test results show notable variability in the pullout resistance strength of seam connections, with a mean value decreasing as the relative rotational angle of the standing webs increases. According to the developed failure criteria, the ultimate uplift pressures evaluated from the FE analysis exhibit minimal positive deviations of 5.3 % and 7.3 % from the experimental results for different failure modes, respectively. This highlights the effectiveness of the proposed method in evaluating the wind resistance performance of high-vertical SSMCSs.

Investigation of recycled carbon fiber-reinforced ultrafine-grain carbon-matrix composites

To broaden the usefulness of recycled carbon fibers and develop the high value-added product, the recycled carbon fiber-reinforced carbon-matrix composites were prepared using ultrafine-grain coke as a filler and coal tar pitch as a binder via a liquid mixing process. A comprehensive study and investigation of the microstructures and properties of recycled carbon fibers and composites were conducted. It was found that the recycled PAN-based carbon fiber (rPCF) outperformed the recycled rayon-based carbon fiber (rRCF) in terms of fiber integrity and pitch-coated effect in the recycling and forming processes. By relieving thermal stress, lowering stacking pores, and inhibiting the growth of shrinkage pores, the rCF can promote the sintering of the green body. The flexural strength of rPCF-reinforced carbon-matrix composite (30.70 MPa) and rRCF-reinforced carbon-matrix composite (20.75 MPa) increased by 60.6% and 8.6% than that of pristine carbon-matrix composite (19.11 MPa), respectively. The difference in mechanical properties between rPCF-reinforced carbon-matrix composite and rRCF-reinforced carbon-matrix composite is attributed to the mechanical interlock mechanism and fiber pull-out mechanism. This work provides a propagable, affordable, and environment-friendly idea for recycling waste carbon fiber and producing recycled carbon fiber reinforced composites.

Experimental investigation of the bond strength between GFRP and steel bars and soil-cement

Steel rebars have been used in soil-cement mixtures to increase their flexural capacity in shoring projects. However, the interaction of the reinforcing rebars and soil-cement and their bond strength has been rarely considered. A practical formula for predicting the rebar-soil-cement bond strength considering the strength characteristics of both has not been developed. The current study performed 60 pullout tests of rebars embedded in soil-cement and analyzed the pullout mechanisms as well as the effective parameters on the pullout force. The test parameters were rebar type, size and embedded length. Smooth, ribbed steel and GFRP rebars in diameters of 8 and 12 mm were tested. A pullout frame was added to a universal testing machine and the load-displacement behavior of the rebars and induced cracking were analyzed. The results showed that the prevailing failure mechanism during pullout of the rebars from the soil-cement was slippage and not cone/splitting failure. During slippage, some the soil-cement adhered to the rebar between its ribs because of the low compressive strength of the soil-cement. With a 50 % increase in rebar diameter, the bond strength decreased about 18 % and 30 % for the ribbed and GFRP rebars, respectively. This indicates the importance of the rebar diameter on the bond strength. The steel rebars exhibited greater bond strength with soil-cement in comparison with the GFRP rebars. A new equation has been proposed to calculate the reinforcement-soil-cement bond strength by applying a reduction factor to the ACI equation.

A rate-dependent cohesive zone model for dynamic crack growth in carbon nanotube reinforced polymers

Carbon nanotubes (CNTs) enhance the fracture toughness of polymer-based matrix composites by dissipating the fracture energy through the pull-out deformation damage mechanism. The rate-dependent behavior of the matrix phase and the CNT/matrix interface affects the contribution of CNTs in enhancing the fracture toughness under dynamic loading and rapid crack growth. A continuum-based finite element (FE) model is utilized in this research to analyze the CNT pull-out damage mechanism. The influence of CNTs on the dynamic fracture behavior of polymer-based composites is studied taking into account the crack opening speed and loading rate effects. The matrix phase is treated as a viscoelastic-viscoplastic material and a new rate-dependent cohesive zone model (CZM) is proposed for modeling the behavior of interface between the CNTs and matrix. The rate-dependent traction-separation laws for the cohesive zone elements are established at different pull-out or crack opening speeds. The proposed rate-dependent FE model of pull-out mechanism facilitates the investigation of the effective factors of CNTs, including length, orientation, and waviness, on fracture energy dissipation at different pull-out speeds. Developed model is very suitable for very long CNTs where atomistic-based molecular dynamics and molecular mechanics methods are associated with difficulties and are more costly and time-consuming.

Effect of boron nitride content on mechanical、dielectric and thermal shock resistance properties of Si3N4-BN-MAS composites

Achieving an optimal balance between mechanical properties, dielectric properties, and thermal shock resistance presents a significant challenge in the development of advanced ceramics. This study investigates Si3N4-BN composite materials, prepared by hot-press sintering with magnesium aluminum silicate (MAS) as the sintering additive, to assess the impact of varying hexagonal boron nitride (h-BN) content on their mechanical and dielectric properties, as well as their thermal shock resistance. At an h-BN content of 20 wt%, the BN layers act as a toughening phase through a pull-out mechanism, minimally impacting the composite's density. This composition results in superior fracture toughness (7.4 MPa m1/2), high strength (598 MPa), low dielectric constants (∼6.8), and dielectric loss (∼7.4 × 10−3). Notably, after a thermal shock at 1000 °C, the composite exhibits a peak residual strength of 621 MPa, surpassing its initial strength. This enhanced performance is attributed to the formation of a dense Mg–Al–Si–B–O oxide glass layer on the surface during thermal shock, which not only repairs surface defects but also induces compressive stress. The development of the Si3N4-BN-MAS composite, with its outstanding comprehensive performance, marks a significant advancement for materials used in high-temperature wave transmission applications.

Large deformation of cable networks with fiber sliding as a second-order cone programming

A new model for irreversible large deformations of fiber networks is developed. The fibers are considered as inextensible cables that slide relative to each other in the frictional junctions. This sliding is constituted by a rate-independent flow rule. The nonsmooth dissipation potential for each sliding system is defined as a product of the yield strength and the absolute value of the fiber sliding. The response of the cable segments is nonsmooth as well, since it shows asymmetry with respect to tension and compression. A principle of minimum incremental potential and a pure complementary energy principle are derived for the equilibrium incremental loading of the network at large deformations. They form a pair of primal and dual second-order cone programming problems with matching sets of displacement-based and force-based variables. These problems can be effectively solved by interior point methods that have many advantages compared to the gradient-based methods or dynamic relaxation. The model is extended by a simple mechanism of fiber pull-out resulting from fiber sliding at the free unconstrained ends. This can be used for the microstructural analysis of failure of needle-punched nonwoven materials.

Investigating interphase bond behavior of waste badminton string fiber in composite matrix

ABSTRACT Recently moving towards the usage of sustainable and renewable construction materials, the badminton strings are identified as alternate material for commercial fibers. These recycled badminton strings once discarded from the racquets become a waste and cannot be reused. Hence investigation of the interphase bond behavior of these fibers with the cement and epoxy polymer matrix plays a major role. Therefore, the pullout mechanism between the fibers with the epoxy matrix is examined using the photoelasticity technique with a 40 mm embedment length. Whereas the pullout mechanism between the fibers with the cementitious matrix is examined for a single string of fiber using a pullout test with varied embedment lengths of 10, 20, 30, 40, 50, 60, 70, and 80 mm. From the study, the minimum and maximum bond stress between the fiber and epoxy interface is 3.23 MPa and 11.59 MPa respectively. Whereas between the fiber and cementitious interface, the minimum and maximum bond stress is 1.02 MPa and 2.15 MPa respectively. The experimental results are validated by the Weibull modulus approach which shows less variability. Thus, the results of the pullout test in composite matrix recommend the usage of waste badminton strings in fiber-reinforced concrete applications.

Numerical and semi-empirical models for the stress analysis of flexible pipes tensile armors terminations within end-fittings

The tensile armors of flexible pipes are typically terminated in two configurations, i.e., hook and pigtail, essential to guarantee the tensile armors’ anchorage in the pipe's end-fitting. However, despite its importance for the secure operation of flexible pipes, few works have dealt with the mechanical analysis of these terminations. Hence, this work proposes numerical and semi-empirical models to study the stresses induced on them by operational and extreme loading. Two- and three-dimensional finite element (FE) models are initially proposed, and their advantages and limitations in estimating local stiffness, stresses, and associated failure modes are highlighted. The initial analyses indicated that the two-dimensional model underestimated the terminations’ stiffness, while the detailed three-dimensional model required significant computational effort. However, a simplified three-dimensional model achieved the best compromise between accuracy and computational effort. In common, all models indicated that the hook termination failure is due to a pullout mechanism, while the pigtail induces the tensile armor steel rupture. After that, the simplified three-dimensional model was employed to conduct a parametric study involving several physical and geometric characteristics of the terminations. The importance of these parameters was evaluated using the Design of Experiment (DoE) technique, leading to semi-empirical equations to predict the maximum stresses induced in the terminations due to an axial load imposed on the tensile armor. Finally, the semi-empirical equation obtained for the hook termination was verified using previously presented full-scale experimental test results, showing good agreement.

Failure mechanism of steel fiber pullout in UHPC affected by alternating cryogenic and elevated variation

This paper investigates the pullout response of straight and hooked-end steel fiber embedded in Ultra-high Performance Concrete (UHPC) subjected to alternating cryogenic (−170 °C) and elevated (200 °C) temperature variation. The temporal evolution of Acoustic Emission (AE) parameters and failure behavior during the pullout process were monitored in situ using the AE test. Additionally, changes in the UHPC microstructure were also evaluated. Results showed that in addition to the UHPC matrices exposed to elevated temperature, the compressive and flexural strength of UHPC matrices subjected to single or multiple cycles were generally higher than those at ambient temperature. Whereas the bond strength/pullout energy of straight fiber increased with a single cycling exposure of UHPC specimens to either cryogenic or elevated temperature, the improvement in the pullout performance of the hooked-end fiber was obtained only after the post-cryogenic thawing of specimens. Moreover, the exposure to an increased number of temperature-variation cycles caused the pullout strength and energy of both fiber groups to gradually decrease. Nonetheless, the pullout performance of the hooked-end fiber was significantly and consistently superior to that of the straight fiber, regardless of the exposure temperature or cycles. The AE test confirmed that the pullout process of both fibers was mainly characterized by tensile failure, with a marginal occurrence of shear failure for the hooked-end fiber. When the UHPC was exposed to various temperatures, the fiber pullout behavior was predominantly influenced by the physical structure of the fibers, thermal deformation proclivity of the matrix, and the competition between mechanisms such as pore moisture phase change, secondary hydration, and the composition of hydrates.

Pull-out capacities of screw connections in thin steel battens exposed to bushfire conditions

External building envelopes made of cold-formed steel (CFS) claddings are commonly used in bushfire prone areas due to their non-combustible properties. However, their designs should ensure safety under the combined action of enhanced wind suction loading and elevated temperatures experienced during bushfires. Although numerous studies have investigated the critical pull-out failures in CFS batten screw connections under wind suction loading at ambient temperature, the pull-out failure behaviour of batten screw connections under bushfire conditions has not been investigated. The main objective of this study is to examine the pull-out failure mechanisms and capacities of screw connections in CFS battens under wind suction loading at elevated temperatures. Two hundred and sixteen small-scale tests were conducted on CFS battens made of three commonly used thicknesses and both high (G550) and low (G300) strength steels. Tests were conducted at six different temperatures using three types of screw fasteners to investigate the critical localised pull-out failures of batten screw connections. The results of these tests were utilised to develop applicable design equations and capacity reduction factors for the pull-out capacities of batten screw connections under the combined action of wind suction loading and elevated temperatures. The findings of this study will help design and build structures safely in bushfire-prone regions.

Additive manufacturing of high-performance CCF/SiC composites under dual protection

In this work, chopped carbon fibre reinforced silicon carbide (CCF/SiC) composites were fabricated by selective laser sintering (SLS) combined with reaction melt infiltration (RMI), using the SiC interface and pyrolytic carbon layer as a dual protection to protect the CCF from the erosion of molten silicon. The pyrolytic carbon layer encapsulates the CCF@SiC and the outer carbon preferentially reacts with silicon to form a β-SiC layer, which hinders the reaction of liquid silicon to CCF. Moreover, the fine-crystal β-SiC generated by the Si–C reaction optimizes the microstructure and enhances the mechanical performance of CCF/SiC composites. The CCF under the dual protection maintains its original high strength and high modulus, and the toughness of the CCF/SiC composites is significantly improved through the fibre debonding and fibre pull-out mechanism. The bending strength and toughness of CCF/SiC composites are 265.2 MPa and 3.5 MPa m1/2, respectively. The insights gained from this study contribute to a better understanding of microstructural engineering in SLS&RMI-fabricated CCF/SiC composites. This paves the way for future breakthroughs in composite material design and low-cost manufacturing for extreme environments.

A large deformation finite element investigation of SEPLA: Failure mechanism and bearing capacities when loaded differently

The Suction Embedded Plate Anchor (SEPLA) is an innovative deep-sea anchoring measure with precise positioning, easy installation, and high anchoring capacity. Most current research on SEPLA revolves around vertical pullout mechanism and thus its vertical bearing capacity. There are relatively few studies concerning its bearing behavior and failure mechanism when SEPLA is subjected to horizontal and moment loading. The latter is practically relevant when SEPLA is exposed to extreme environmental conditions or keyed under preloading. A more in-depth understanding of its failure mechanism under horizontal and moment loading is thereby equally important. In this work, the Coupled Eulerian-Lagrange (CEL) method is used to conduct large-deformation finite element analyses of SEPLA under vertical, horizontal, and moment loading. Emphasis is placed on ascertaining the dissimilarity of soil flow and failure mechanisms when SEPLA is loaded in different directions. Parametric investigation is done to explore the effect of anchor shape and thickness, embedment depth, and soil-anchor interfacial roughness. Semi-theoretical closed form solutions are developed to estimate the uniaxial bearing capacity factor of plate anchors in three directions. These research outcomes are likely helpful in deepening the understanding of SEPLA failure mechanism when loaded in different directions and obtaining more rational estimates of anchor bearing capacity.

Characterization of bamboo extrinsic toughening mechanism in bending by X-ray micro-computed tomography(micro-CT)

Bamboo as a natural biomaterial with excellent strength and toughness is widely used in construction, furniture, decoration, handicrafts and other fields. However, the internal cracks that lead to the bending-fracture behavior of bamboo during its application are not yet fully understood. Herein, we used micro-computed tomography (micro-CT) to investigate the external toughening mechanism of bamboo during bending aiming to broaden the application areas of bamboo. These studies demonstrated that bamboo's outstanding strength and toughness were attributed to the synergistic extrinsic toughening mechanism dominated by fibers. Moreover, the fiber delamination and interfacial debonding affected crack deflection in the surrounding parenchyma and vessels. Additionally, the fiber content was positively correlated with the energy release rate of fiber pullout. When the fiber content was close, the fibers closed to the tension side had a better inhibition effect on the crack propagation. Furthermore, the effects of different microstructure arrangements on crack deflection, crack deflection inside fibers, crack deflection in parenchyma, and the toughening mechanism of fiber pullout were also discussed. Therefore, this work further advances the mechanism strategy for enhancing the flexibility of bamboo, which indicates the great potential in bending applications.

Enhancing the fracture toughness of polycrystalline diamond by adjusting the transgranular fracture and intergranular fracture modes

Enhancing the fracture toughness of polycrystalline diamond (PCD) is challenging because of its inherent brittleness and the absence of a toughness mechanism. Elucidating the transgranular fracture (TF) and intergranular fracture (IF) modes in PCD is urgent for fabricating robust PCDs. In this study, the toughness of PCD was optimised by adjusting the IF and TF modes. The high pressure and high temperature method (HPHT) was used to fabricate PCDs with different grain sizes (nano- to micro-scale) to modulate the IF and TF modes. The results indicate that a mixture of the IF and TF modes yields an optimum toughness in PCD compared to the single fracture mode, which yielded a high hardness (114.6 GPa) and high toughness (13.0 MPa∙m0.5). The toughness was approximately three times that of single-crystal diamonds (SCDs) and comparable to that of pure nanotwinned diamonds (NtDs). The mixed modes induce a toughening mechanism of high frequency large crack deflection, crack bridging, pull-out, and crack branching, which increases the toughness of the PCD. This work provides new insights into optimising the toughness of PCD, which is significant for fabricating robust PCDs for future applications.

Mechanical performance assessment of sustainable coal plastic composite building materials

This paper presents a study for developing a high filler content coal plastic composite (CPC) for use in building and construction applications as a substitute for the state-of-the-art wood-plastic composites (WPCs). The influence of coal type (bituminous, sub-bituminous) and content (up to 70 wt%) on the mechanical performance was characterized and compared with predominant commercially available WPC products. SEM micrographs revealed good adhesion between coal and the polymer at the interface with less porosity than WPCs. Additionally, particle fracture and particle pull-out failure mechanisms were evident in the SEM micrographs, with the latter being more dominant. DSC thermal analyses revealed potential chemical reactions between the coal and HDPE interfaces, resulting in enhanced composite performance. Results indicate the tensile strength for CPC material had inverse proportionality with coal content. Despite the reduction, tensile strength for CPC with 60 wt% bituminous coal was not significantly different from a typical WPC formulation. Flexural strength increased to a maximum value, at 60 wt% for bituminous coal and 50 wt% for sub-bituminous coal, followed by a decrease. CPC with 60 wt% bituminous coal possessed higher flexural strength and, in some cases, comparable flexural modulus as compared to WPC products. Flexural properties indicated that a composite deck made from the CPC material could meet or exceed the loading and deflection requirements for decking boards. FEA models indicated that a hollow deck profile would potentially have a better load-deflection curve with less localized stresses compared to common decking board profiled sections.

Pull-out failure capacities of cold-formed steel batten screw connections at sub-zero temperatures

Cold-formed steel (CFS) roof and wall cladding systems are used worldwide, and are constructed using high-strength, thin steel battens and claddings. In North American countries, which are frequently subjected to blizzards (winter storms), these cladding systems are particularly vulnerable to pull-out failures during combined windstorm and snowy conditions. Despite extensive research studies investigating the pull-out failure mechanism and the critical parameters of screw connections between cladding and battens at ambient temperature, the behaviour of these connections under blizzard conditions has not been experimentally investigated. To fill this knowledge gap, an experimental study was undertaken that included 144 laboratory tests of screw connections in battens of varying grades and thicknesses, and three distinct types of screws. The pull-out failure capacities of screw connections in battens were evaluated under wind suction loading at four different temperatures, including ambient and sub-zero temperatures. The results of the study provided insights into the pull-out failure mechanisms of screw connections in battens and enabled the development of suitable design rules for combined wind action and sub-zero temperature conditions. The findings of this study are expected to be of significant value to engineers and building designers who are involved in the construction of lightweight cold-formed steel buildings.