Dynamic Bond Strength Research Articles

Study on the effect of strain rate on the bond performance between GFRP bars and ECC after seawater freeze-thaw cycles

With the increase in marine and coastal infrastructure in cold regions, freeze-thaw damage and dynamic loads will reduce structural performance. However, the dynamic bonding performance of seawater sea sand ECC and glass fiber-reinforced polymer (GFRP) bars after freeze-thaw cycles is still unclear. In this paper, the mass loss rate, dynamic elastic modulus, and compressive strength of seawater sea sand ECC were studied through freeze-thaw cycle tests. The bonding properties of seawater sea sand ECC and GFRP bars were examined under two freeze-thaw environments (Seawater environment, Freshwater environment), four freeze-thaw cycles (0, 50, 100, and 150), and four strain rates (2.63 × 10−5s−1, 2.63 × 10−4s−1, 2.63 × 10−3s−1, and1.32 × 10−2s−1) using pull-out tests. Additionally, the microstructure was analyzed using SEM. The results show that compared with the freshwater freeze-thaw cycle, the seawater freeze-thaw cycle deteriorates the surface peeling resistance and mechanical properties of seawater sea sand ECC. Compared with the non-freeze-thaw specimens, the failure mechanism after freeze-thaw cycles changed from the gradual increase of bond strength with the increase of strain rate to the gradual decrease. Compared with the dynamic elastic modulus, compressive strength, and bond strength freeze-thaw damage model based on the Weibull distribution, it was observed that the predicted life of compressive strength is overestimated, while the predicted life of bond strength is underestimated. The bond strength and slip calculation models for freeze-thaw cycles and strain rates have been established. This provides a theoretical foundation for predicting the structural service life and conducting durability research on ECC and GFRP bars in seawater and sea sand.

Effect of initial pretension on dynamic bond behavior of an ultra-high performance concrete-filled anchorage for carbon fiber-reinforced polymer tendons

This paper is focused on the effect of initial pretension on the dynamic bond performance of the anchorage under impact loads. Drop-weight impact tests were conducted on 15 groups of specimens with four embedded lengths and various pretension ratios; the effects of pretension and embedded length on the bond strength and slip of the anchorage are discussed in the paper. The test results show that a higher pretension ratio led to severer surface damage on the CFRP tendons. With the increase of the pretension ratio, the mechanical interlocking between CFRP ribs and UHPC was improved, leading to an increased average bond strength. By contrast, the relatively large shear deformation and severe damage of CFRP ribs induced by pretension formed “initial defects”, which resulted in a lower residual bond performance during impact loading. Based on the test results, a pretension ratio-dependent strength retention function was proposed to quantify the shear damage effect of CFRP ribs caused by pretension; by incorporating this function, accurate formulas were developed for determining the dynamic bond strength of a pretensioned anchorage.

Investigation on the dynamic bond-slip behaviour between steel bar and concrete

The bond behaviour between steel bar and concrete affects the response of reinforced concrete (RC) structures subjected to static and dynamic loads. Many studies have investigated the influencing factors on static bond behaviour between steel bar and concrete, e.g., cover depth to rebar diameter ratio (c/d) and concrete strength and various empirical formulae have been proposed to quantify the static bond strength, but the study of dynamic bond strength is very limited. As a result, in the analysis of dynamic structural responses, either perfect bond is assumed or static bond strength is adopted, which may lead to inaccurate structural response predictions. In this study, dynamic bond-slip behaviour between steel bar and concrete is numerically investigated by using the finite element (FE) model developed in LS-DYNA and verified against testing results. The effects of the concrete specimen shape, c/d ratio and concrete strength on the dynamic pull-out test are examined. It is found that the splitting of concrete dominates the failure mode of dynamic bond-slip behaviour. The ultimate bonding load is enhanced with the increase of loading rate. It is observed that the increase of bond strength relates to the dynamic increase factor (DIF) of concrete strength. The parametric studies show that among all considered factors, the shape of the pull-out test specimens has a minor effect on the ultimate bonding load, which on the other hand is highly dependent on the concrete strength and c/d ratio owing to the strain rate effect. An empirical relation of DIF of bond strength as a function of c/d ratio, concrete strength and strain rate is proposed based on the numerical results. A case study of drop weight test on an RC beam is then carried out to demonstrate the influence of considering the dynamic bond strength in numerical prediction of structural responses under dynamic loading.

An analytical approach of design for recycling of laminate structures by the use of magnetic pulse disassembling

Composite materials in association with metal sheets or ceramic coatings have found a variety of applications, especially as load-bearing components in composite structures. Their disassembly for recycling or maintenance has therefore become more complex. The magnetic pulse technique, usually dedicated to dynamic forming and welding processes, can be used to separate composite materials of a laminate structure of their recyclable adjacent materials. This technology profits from Lorentz forces appearing in electrically conductive materials placed in the vicinity of conductors in which high-intensity electrical discharges are performed. These body forces act locally during the fast energy discharge, from which a stress wave propagates in the structure. The propagation of this stress wave can be used to generate interfacial tensile stress in laminate structures or assemblies. Especially, this technique can be used for evaluating the dynamic bond strength of an interface. This works aims at determining the required conditions for disassembling a laminate structure using a magnetic pulse imposed on one side of the assembly, without significantly damaging the debonded layers. Ideally, the generated interfacial tensile stress should be greater than the interfacial bond strength. In this paper, a one-dimensional analytical analysis is performed in linear elastodynamics on a tri-layered laminate structure, infinite in transverse directions, based on the method of characteristics. An optimisation problem is defined, maximising the interfacial stress between the first two layers, whose solution requires to study various configurations of the assembly involving different sets of characteristics in the laminate, associated with different areas of the domain of feasibility spanned by the unknown vector. Some assumptions and introduced simplifications of the optimisation problem allows to follow a simple two-stage solution procedure, first with respect to the thickness of the first layer for a given stress pulse, then with respect to material impedances of all layers. Several configurations of the assembly are shown to create a sufficiently large interfacial tensile stress to reach the crack initiation and propagation, and a maximum tensile interface stress of twice the maximum applied pressure is obtained in the asymptotic limit.

Bond behavior of an ultra-high performance concrete-filled anchorage for carbon fiber-reinforced polymer tendons under static and impact loads

To investigate the bond behavior of an ultra-high performance concrete (UHPC)-filled bond-type anchorage for carbon fiber-reinforced polymer (CFRP) tendons, both static tensile tests and impact tests were performed on 20 groups of specimens. The anchorages with embedded lengths of 50, 100, 150 and 200 mm were subjected to the static tension and longitudinal impact loads with drop heights of 150, 300, 600 and 900 mm. During the tests, the longitudinal tension and slip of the anchorage were obtained, and the effect of anchorage length and loading rate on the bond behavior was discussed. The results show that in the static tests, slip failure occurred for the anchorages with lengths from 50 to 150 mm, and CFRP bar ruptured as the length raised to 200 mm. When subjected to impact loads, slip failure was observed for all specimens. The damages on the CFRP bar to UHPC interface under impact loads were slighter than those under static tension, and correspondingly the dynamic bond strength was approximately half of the static one owing to the weakened mechanical interlocking. With increasing the embedded length, the bond strength and slip increased due to the radial constraining effect of outside steel tube. Based on the experimental results, a strain rate-dependent function was proposed to quantify the retention of bond strength under impact loads, and prediction formulas were established for determining the dynamic bond strength and critical embedded length of the bond-type anchorage.

Experimental study of dynamic bond behaviour between corroded steel reinforcement and concrete

Corrosion damage of steel reinforcement is detrimental to the load-carrying capacity of reinforced concrete structures because it not only reduces the cross sectional area and strength of reinforcement bars, but also weakens the bond between reinforcement and concrete. Many studies have investigated the effects of corrosion on reinforcement loss and static bond strength between reinforcement and concrete, but no study of the influence of corrosion damage on dynamic bond strength has been reported yet. In this study, laboratory tests were conducted to investigate the effect of corrosion on the bond behaviour between steel reinforcement and concrete subjected to high rate dynamic load. Accelerated corrosion damage was induced to steel bars embedded in concrete specimens under laboratory condition. The bond parameters of steel reinforcement and concrete including the ultimate bonding load and bond-slip relationship corresponding to the various strain rates and corrosion degrees were obtained through dynamic pull-out tests. The empirical formulae of dynamic increase factors (DIFs) of the ultimate bonding strength corresponding to different corrosion levels and strain rates were proposed. The experimental results show that the strain rate effect on bonding behaviour is prominent irrespective of the corrosion level. The increase in the ultimate bonding strength with strain rate for the heavily corroded reinforcement (η = 5 %-10 %) is more prominent than that for the slightly corroded reinforcement (η = 0 %-5%). The decrease in the ultimate pull-out force caused by the corrosion damage at high strain rate is more pronounced than that at low strain rate. Corrosion-induced crack and the tensile strength of concrete have significant effect on both the static and dynamic bond behaviours. The results presented in the paper can be used for better modelling and prediction of the responses of corroded reinforced concrete structures subjected to dynamic loads.

Crystallization, morphology and mechanical property enhancement of block copolymer-based metallosupramolecular polymers by incorporating metal coordinating ligand into poly(L-lactic acid) block

We propose a new strategy to prepare robust films with unusual mechanical properties for low molecular weight block copolymers-based materials by utilizing metal−ligand (M−L) coordination to create physical crosslinking into the hard block. Using an amine-terminated triazole as an initiator, a poly(L-lactic acid) (PLLA) containing a triazole group was prepared by ring opening polymerization. The hydroxy group of this polymer was subsequently reacted with isocyanate-poly(butadiene) (PB), resulting for the first time in PLLA/PB block copolymers bearing metal-coordinating ligands on PLLA chain. The subsequent addition of Co2+, Fe3+, or Cu2+ metal ions resulted in formation of metallosupramolecular polymers (MSP) through M−L bonds, as could be shown by viscosimetry and UV–vis spectra. The influence of blend of different types of metal, L/M ratio, the chain length of PLLA blocks (1.6 or 3.0 kg/mol) of block copolymers on the properties has been investigated in detail. Compared to the mechanical properties of uncoordinated block copolymer, MSP showed significantly larger Young's modulus, higher elongation at break, and larger break strength. For uncoordinated oligomer with a toughness of 0.82 J/m3, the toughness increased by 101- and 348-fold with the incorporation of Co2+ and Cu2+, respectively. Because of the particularly weak bond strength associated with the Fe3+−triazole bond, MSP containing Fe3+ had lower Young's modulus, break strength than MSPs containing Co2+ and Cu2+. Incorporation of 50% Cu2+ and 50% Co2+ into block copolymers was found to be beneficial to prepare MSPs with high extensibility and large Young's modulus simultaneously. For all MSPs, M−L complex and polymer matrix were found to microphase separate into nano-domains. Crystallization rate of PLLA in MSP featuring Co2+ metal ion was lower than that of Cu2+ and Fe3+, demonstrating that chain mobility relied on both the dynamic character and bond strength.

Fatigue bond strength of dental adhesive systems: Historical background of test methodology, clinical considerations and future perspectives.

Numerous laboratory evaluations have been conducted since Dr. Rafael Bowen introduced a method for determining the bond strengths of adhesive systems to dental substrates in 1965. Most of the past studies have been conducted using static bond strength tests, such as shear and tensile bond strength testing with either macro or micro sized specimens. These static bond strength tests are conducted using a monotonically increasing load in which stress is applied continuously until failure occurs. Although the type of stress that develops in static bond strength tests is not typically encountered in clinical situations, over the years clinicians have based their choice of adhesive systems for use in daily practice on the results of such tests. However, some well-known researchers have reported that the results obtained from static bond strength testing may have limited clinical relevance and should not be used only by themselves to make recommendations for clinical use. In clinical situations, restorations undergo cyclic stress during mastication at stress levels well below the breaking stress used in static bond strength tests. Thus, dynamic bond strength tests, using cyclic loading, should be more clinically relevant than static bond strength tests. Over 15 years, a testing method designed to assess fatigue bond strengths of dental adhesive systems has been developed through inter-collegial and international collaborative efforts. This review discusses the development of fatigue bond strength testing methodology, provides both a historical perspective and current information regarding available testing data for all categories of adhesive systems to enamel and dentin and perspectives on the future development of both adhesive systems and testing methods.

Using molecular dynamics simulations to interrogate T cell receptor non-equilibrium kinetics

An atomic-scale mechanism of T Cell Receptor (TCR) mechanosensing of peptides in the binding groove of the peptide-major histocompatibility complex (pMHC) may inform the design of novel TCRs for immunotherapies. Using steered molecular dynamics simulations, our study demonstrates that mutations to peptides in the binding groove of the pMHC – which are known to discretely alter the T cell response to an antigen – alter the MHC conformation at equilibrium. This subsequently impacts the overall strength (duration and length) of the TCR-pMHC bond under constant load. Moreover, physiochemical features of the TCR-pMHC dynamic bond strength, such as hydrogen bonds and Lennard-Jones contacts, correlate with the immunogenic response elicited by the specific peptide in the MHC groove. Thus, formation of transient TCR-pMHC bonds is characteristic of immunogenic peptides, and steered molecular dynamics simulations can be used in the overall design strategy of TCRs for immunotherapies.

STUDY ON BOND BEHAVIOR OF BFRP BARS AND CONCRETE UNDER MEDIUM AND LOW LOADING RATES

BFRP bar is a new type of fiber composite material used in civil engineering field instead of steel bar. The bonding performance of BFRP bars and concrete under medium and low loading rates is an important premise to ensure the joint action of BFRP bars and concrete under dynamic loading. 16 groups of bonding specimens are designed according to the orthogonal test method, and the pull-out test is carried out on the BFRP bar-concrete specimens under different loading rates (0.005 mm/s-5 mm/s) by using the MTS test system, and the influence of loading rate, of concrete strength and of the diameter of BFRP bars on the bonding performance is studied. Based on the existing calculation model for bond strength, the bond characteristic parameters are modified, a formula is proposed for calculating the dynamic bond strength of BFRP bars and concrete under medium and low loading rates, and the bond-slip constitutive relationship model is further established for BFRP bars and concrete. The results show that the specimens fail due to the pull-out failure or the splitting failure, and bond stress-slip curves can be divided into a sliding stage, a declining stage and a residual stage. The bond strength increases with the increase of loading rate and of concrete strength, but decreases significantly with the increase of the diameter of BFRP bars. The theoretical calculation results are in good agreement with the experimental results, which provides an effective method for predicting the bonding properties of BFRP bars and concrete under medium and low loading rates.

Wi-Fi Live-Streaming Centrifuge Force Microscope for Benchtop Single-Molecule Experiments

The ability to apply controlled forces to individual molecules has been revolutionary in shaping our understanding of biophysics in areas as diverse as dynamic bond strength, biological motor operation, and DNA replication. However, the methodology to perform single-molecule experiments remains relatively inaccessible because of cost and complexity. In 2010, we introduced the centrifuge force microscope (CFM) as a platform for accessible and high-throughput single-molecule experimentation. The CFM consists of a rotating microscope with which prescribed centrifugal forces can be applied to microsphere-tethered biomolecules. In this work, we develop and demonstrate a next-generation Wi-Fi CFM that offers unprecedented ease of use and flexibility in design. The modular CFM unit fits within a standard benchtop centrifuge and connects by Wi-Fi to an external computer for live control and streaming at near gigabit speeds. The use of commercial wireless hardware allows for flexibility in programming and provides a streamlined upgrade path as Wi-Fi technology advances. To facilitate ease of use, detailed build and setup instructions, as well as LabVIEW-based control software and MATLAB-based analysis software, are provided. We demonstrate the instrument’s performance by analysis of force-dependent dissociation of short DNA duplexes of 7, 8, and 9 bp. We showcase the sensitivity of the approach by resolving distinct dissociation kinetic rates for a 7 bp duplex in which one G-C basepair is mutated to an A-T basepair.

Finite element modelling of dynamic bonding behaviours between fibre reinforced polymer sheet and concrete

In this study, dynamic debonding behaviour between fibre-reinforced polymer (FRP) and concrete is numerically investigated by using finite element code LS-DYNA. A three-dimensional (3D) finite element (FE) model is built and a bond-slip model is incorporated into the simulation of interfacial bond between FRP and concrete. To validate the accuracy of the numerical model, experimental results from 42 dynamic single shear tests on FRP-concrete joints are employed. The debonding load and shear slip responses, FRP strain distributions, and interfacial bond-slip responses are compared between the numerical and experimental results. It is found that the numerical model is capable of predicting the debonding behaviours under various loading rates. In addition, an analytical dynamic bond strength model is proposed to predict the ultimate debonding strain and debonding load by incorporating the dynamic increase factor and strain rate. In addition, a semi-empirical model is proposed to predict the ultimate debonding strain and debonding load by incorporating the dynamic increase factor and strain rate. The semi-empirical model is validated against the numerical results and 119 experimental tests with a good correlation.

Spall strength of steel-fiber-reinforced concrete under one-dimensional stress state

A specially designed large-scale Hopkinson bar (φ100 mm) with hollow aluminum alloy bar as transmitted bar was used to measure the dynamic spall strength of steel-fiber-reinforced concrete (SFRC). The experimental results indicate that the steel fibers significantly enhance the spall strength of the SFRC. The enhancement of the spall strength from the steel fibers is linear with the product of the fiber influencing coefficient (α), the slenderness ratio (l/d), and the fiber volume fraction (Vf). Under dynamic loading, the value of α is higher than that observed under quasi-static loading, which is due to the enhancement of the dynamic bond strength between the fiber and the cement matrix. Furthermore, undulated fibers bring about a higher α than hooked fibers. Finally, an empirical formula is built to estimate the dynamic spall strength of SFRC under one-dimensional stress. The prediction was shown to be consistent with the experimental results. The steel fiber enhanced spall strength can be regarded as an important guideline in the design of high-performance concrete for improving the penetration resistance of protective structures.

Laser adhesion test for thermal sprayed coatings on textured surface by laser

The laser shock adhesion test (LASAT) is a technique allowing the generation of high tensile stresses in materials. The LASAT consists in focusing a pulsed laser beam on a water-confined target. The laser pulse crosses the water transparent layer and is absorbed by the target. High energetic plasma is created at the surface of the sample. As a response to the expansion of the plasma, a shock wave is generated and propagates through the sample. This shock wave leads to the generation of high tensile stresses in the sample. These stresses allow the interface solicitation in order to evaluate the dynamic adhesive bond strength of coated systems. In order to determine interface strengths, this technique has already proven its feasibility. In this paper, the adhesion strength of coated system was evaluated using LASAT for two surface pretreatments of substrates obtained by grit-blasting and laser surface texturing techniques. The generation of the high-intensity shock wave by laser plasma in the water-confinement regime has been performed at 7.1 ns at 532 nm with the new Nd:YAG laser facility HEPHAISTOS. This paper shows that surface treatments have a great influence on the adherence results of the coated systems obtained with laser adhesion test. However, the LASAT is efficient on thin coating. In that sense, thicker industrial coatings are not adapted for the conventional LASAT anymore. Therefore, a new technique was designed to improve and extend the conventional technique. This technique consists of varying the delay Δt between two incident pulses to adjust the location of the maximum tensile stresses near the interface. Some preliminary results on the improved configuration are presented in this paper and the problematic of the laser-matter interaction with two time-delayed laser pulses which has arisen is discussed.

Experimental evaluation of the dynamic bond strength between CFRP sheets and steel under direct tensile loads

The efficiency and the adequacy of bonding carbon fibre reinforced polymer (CFRP) composites to strengthen/repair steel elements largely depend on the bond between the steel elements and the CFRP composite. To ensure high performance and safer strengthening/repair procedures, it is important to provide sufficient bond between the patching system and the steel by choosing a suitable adhesive. Although strengthened/repaired structures are more likely to be subjected to dynamic loadings, the effect of different loading rates on the adhesion bond between CFRP sheets and steel has not been examined to date. This paper presents an experimental investigation of the assessment of the adhesive strength between CFRP and steel under various dynamic rates (3.34×10−5, 3.35, 4.43 and 5m/s) by utilising pull-off tests. The values from samples with one CFRP layer are higher than those with three layers for both Araldite 420 and MBrace saturant adhesives due to CFRP delamination. The loading rate affects the failure mode of samples with one CFRP layer for both Araldite 420 and MBrace saturant adhesives and remarkable changes are clearly shown at dynamic compared to static loadings. Conversely, the failure mode does not change for samples with three CFRP layers and remains CFRP delamination at all rates. Overall, the increase in pull-off strength is about 100% on average within the dynamic loading rates tested.

Dynamic bond strength between CFRP sheet and steel

The efficiency of the application of adhesively-bonded CFRP materials to rehabilitate and/or strengthen steel structures is completely dependent on the bond between the CFRP composites and the steel. As the repaired and/or upgraded structural elements are likely to be exposed to impact tensile loads during service, this necessitates the importance of understanding the bond strength between CFRP materials and steel under dynamic loadings. This paper presents the experimental procedure, the equipment and the test results for double strap joints at four speeds of loading to highlight the effect of loading rate on the bond strength. Two different CFRP layouts with various bond lengths were examined. It was found that the loading rate has significant influence on bond strength and failure modes, although it has little influence on the effective bond length.

Comparison of dynamic versus static bond strength testing

Comparison of dynamic versus static bond strength testing

Overview on Measuring Methods of Bond Strength of Cladding Material

Bond strength is a key property to evaluate quality of cladding layer. According to the thickness of the cladding, surface cladding layer can be classified into two categories: thick and thin layer. The later is usually called thin film. Bond strength includes static and dynamic bond strength. As to thick cladding layer, main measuring methods of static bond strength are sticking extension method, interfacial indentation method, shearing method, fracture mechanics method. Contact fatigue method is the main measuring method of dynamic bond strength. As to thin film, main measuring methods of static bond strength are extension method, scratching method, indentation method, scraping method, bulge blister method, laser spallation method and so on. The main measuring methods of dynamic bond strength are facing contact rolling method and ball contact rolling method. The paper made overview on all above methods.

A rolling ball test for establishing contact fatique behaviour of bulk materials, surface layers and coatings

This paper describes a new method for characterising contact fatique behaviour of bulk materials, surface layers and coatings. It is based on contact between a tungsten-carbide ball and a cylinder. In principle most ‘pin-on-ring’ type apparatus can be adapted to meet the test requirements. The method seems particularly attractive for charaterising the performance of surface layers and coatings under dynamic loading conditions. Examples relate to carburised steel surfaces and to a series of six hard chromium coatings with different bond strengths, obtained by applying different plating conditions. The test with the chromium coatings were performed under conditions of substantial initial plastic deformation of the steel substrate material. They were supposed to yield information on the ‘dynamic bond strength’ of the coatings. It was found to be possible to distinguish four quality classes, in agreement with expectations expressed by a plating expert.

Some features of the dynamic bond strength of rubbers in symmetric alternating shear

A power law is obtained for the endurance of the joints in laminated rubber systems, and the relation between the shear stressesf and the strains e at joints parallel to the shear direction is shown to be linear for symmetric alternating shear. One consequence of the linearity of the relation betweenf and e is the nondependence of the power law coefficient (coefficient of resistance of the joint to repeated loading) on the loading conditions.