Double Lap Shear Tests Research Articles

Quality of the bond between a Strain Hardening Cement Composite stay-in-place formwork and concrete: Comparison of two experimental set-ups

Strain Hardening Cement Composites (SHCC) have recently been investigated as a material for strengthening and retrofitting of existing concrete structures or as stay-in-place (SiP) formwork for new concrete structures. However, for efficient utilisation of SHCC in composite structures, the debonding mechanism between SHCC and concrete must be understood. This paper compares two different double-lap shear test set-ups to investigate the bond between SHCC-SiP formwork and concrete. The investigation concludes on the performance of these tests (symmetrical behaviour, repeatability), their critical analysis and the quality of the results. It validates digital image correlation as a full-field slip measuring technique which moreover allows to determine the effective bond length.

Mechanical behaviour of thermoplastic composites spot-welded and mechanically fastened joints: A preliminary comparison

The in-plane and out-of-plane mechanical behaviour of both ultrasonically spot-welded and mechanically fastened joints was investigated by double-lap shear and pull-through tests, respectively. Spot-welded specimens showed comparable onset failure load and significantly higher joint stiffness compared to mechanical fasteners when carrying shear load. The failure modes and the damage within specimens were analysed after mechanical tests. Intralaminar failure and very limited damage on the out-most ply were found for welded specimens, whereas catastrophic through-the-thickness failure was observed for mechanically fastened joints. Based on the experimental outcomes, the mechanical performance and failure mechanisms of spot-welded joints were critically assessed in comparison to the mechanical fasteners .

Fatigue properties of shear–peeling debonding between CFRP plates and concrete

This paper presents a study of the fatigue bond behaviour of the bonded interface between carbon fibre reinforced polymer (CFRP) plates and the concrete surface in double-lap shear tests. In total, 12 specimens were constructed and tested. The variables in the test were concrete compressive strength, initial angle of attachment at the specimen centre and different fatigue load levels. Strain gauges were bonded at 50 mm intervals along the CFRP plate surface, and linear variable displacement transducers were used to measure the longitudinal strains and displacements between the CFRP plate and the concrete substrate in longitudinal and perpendicular directions. The relative slip evolution curves of the CFRP plate–concrete bonded interface under fatigue load were calculated and the strain distributions and bond–slip behaviour were analysed. Fitting of the S–N curve showed no influence on fatigue life corresponding to the initial angle induced by fast peeling by peel and shear. The results predicted using the fatigue local bond–slip model were found to agree with the test results.

On Energy Release Rates and Configurational Forces for Interfacial Propagating Cracks: A Lattice Approach With a Brittle Erosion Technique

A numerical 2D lattice approach with an erosion algorithm is employed to analyze bimaterial interface fracture quantities in brittle heterogeneous materials in the context of linear elastic fracture mechanics (LEFM). The concept of configurational force is elucidated and the importance of nodal configurational changes in a mesh where no stress–strain analyses are needed is investigated. Three fracture problems, i.e., an infinite panel with a bi-material interface crack, a double-lap shear test, and a prenotched four-point bending masonry beam are then considered. Validated by analytical solutions, the lattice model uses two distinct postprocessing approaches to derive the energy release rates and configurational forces directly at bimaterial interface crack tips. While the first method takes advantage of the change of the lattice mesh's global stiffness matrix before and after crack growth without any stress–strain calculations to obtain crack tip driving forces, the second approach analyzes the configurational forces opposing the crack tip motion using the Eshelby stress tensor and local force balance law in cracked and heterogeneous domains. It is demonstrated that the discrete material forces at crack tips are closely equal to the tip driving forces for the three fracture problems, confirming that the lattice is an appropriate numerical tool to analyze fracture properties of evolving interface cracks. Satisfying C1 continuity by including rotational displacements for frame struts, there is also no need for the lattice to update interior computational points in the mesh to eliminate spurious material forces away from the tip.

Shear Behavior of 3D Woven Hollow Integrated Sandwich Composites: Experimental, Theoretical and Numerical Study

An experimental, theoretical and numerical investigation on the shear behavior of 3D woven hollow integrated sandwich composites was presented in this paper. The microstructure of the composites was studied, then the shear modulus and load-deflection curves were obtained by double lap shear tests on the specimens in two principal directions of the sandwich panels, called warp and weft. The experimental results showed that the shear modulus of the warp was higher than that of the weft and the failure occurred in the roots of piles. A finite element model was established to predict the shear behavior of the composites. The simulated results agreed well with the experimental data. Simultaneously, a theoretical method was developed to predict the shear modulus. By comparing with the experimental data, the accuracy of the theoretical method was verified. The influence of structural parameters on shear modulus was also discussed. The higher yarn number, yarn density and dip angle of the piles could all improve the shear modulus of 3D woven hollow integrated sandwich composites at different levels, while the increasing height would decrease the shear modulus.

Flexural Performance of HFRP-RC Composite T-Beams with Different Interfaces

AbstractA method of improving the capacity of reinforced concrete T-beams using profiles made of hybrid basalt-carbon fiber sheet–reinforced polymer (HFRP) bonded to concrete using three different methods was researched. Various basalt-carbon ratios were considered, along with three types of different bonding interfaces. The flexural performance of the HFRP-RC (reinforced concrete) composite T-beams with different interfaces was studied by both experimental and theoretical analysis methods, the main areas of focus including the feasibility of the interface type, failure mode of these beams, load capacity, flexural stiffness, and ductility, among others. Three types of interface were studied by both the double-lap shear test and the four-point bending test of beam specimens. Good bond behavior can be achieved with the three methods and the additional anchorage system. The hybrid effect of hybrid fiber sheets made of basalt fiber (BFRP) and carbon fiber (CFRP) was studied both by a tensile test of hybrid fi...

Recycling of carbon fiber-reinforced thermoplastic composite wastes from the aerospace industry

Post-industrial trimmings and off-cuts of carbon fiber/polyether ether ketone composite were successfully recycled into new composite products. The original composites were thermally characterized by dynamic thermomechanical analysis, differential scanning calorimetry, and thermogravimetric analysis. Melt-bonding and thermoset adhesives were used to bond the carbon fiber/polyether ether ketone. Performance of the bond was evaluated through double lap-shear tests. The carbon fiber/polyether ether ketone scraps were mechanically refined to a variety of elemental sizes, subsequently subjected to high-temperature hot pressing to form panel composites. The influences of element size and processing temperature were evaluated through mechanical testing.

Investigation of the bond behavior of flax FRP strengthened RC structures through double lap shear testing

Abstract This paper focuses on the characterization of the bond behavior of adhesively bonded Flax Fiber Reinforced polymers (FRP) to concrete substrate. An experimental investigation has been done comparing flax and glass FRP bonded to concrete using a double shear test configuration. Flax composites appear to show the same interface behavior as the usual synthetic fiber reinforcement. Analyses of the strain and shear stress distribution along the interface reveal behavior differences. To investigate if the same models could be accurately used in the case of flax FRP or if a new model is needed, experimental results of debonding load and efficient bonded length have been compared to the values of four different prediction models. Coefficients calibrated for the flax composite lead to an underestimation of the efficient bonded length for the glass composite.

Effect of the inherent eccentricity in single-lap direct-shear tests of PBO FRCM-concrete joints

Investigation of the bond between fiber reinforced composites and the substrates onto which they are applied is of critical importance to understand their failure mechanisms. The bond behavior can be studied using different experimental test set-ups; the most commonly used are the single-lap and double-lap direct-shear tests. Although single-lap shear tests are simpler to carry out than double-lap shear tests, the presence of an eccentricity between the pulling and restraining forces leads to a mixed mode fracture process at the interface, which may influence the results. This study investigates the eccentricity effect on the bond behavior of fiber reinforced cementitious matrix (FRCM) composite–concrete joints. FRCM composite strips with the same bonded length were applied to concrete blocks of different lengths and tested using the single-lap shear test. The use of digital image correlation (DIC) allowed for studying the strain field on the surface of the bonded composite. Results were compared with those from double-lap shear tests of the same composite. The results obtained confirm that the eccentricity effect is negligible for bonded lengths longer than the effective bond length.

Experimental study of anisotropic behavior of PU foam used in sandwich panels

Polyurethane foam with low density used in sandwich panels is examined in the paper. A series of experiments was carried out to identify mechanical parameters of the foam. Various experimental methods were used for determining the shear modulus, namely a four and three point bending tests (the most common in engineering practice), a double-lap shear test and a torsion test. The behavior of PU in axial compression and tension was also studied. The experiments revealed pronounced anisotropy of the PU foam. An orthotropic model is proposed. Limitations of application of isotropic model of PU in engineering practice is also discussed.

Numerical modelling of the bond between concrete and CFRP laminates at elevated temperatures

This paper presents numerical investigations on the bond between concrete and CFRP strengthening systems at elevated temperatures. The numerical study was developed using the commercial package Abaqus and comprised the simulation of double-lap shear tests previously performed by the authors, in which the influence of the strengthening technique – externally bonding reinforcement (EBR) or near surface mounted (NSM) – on the bond behaviour was evaluated along with the temperature increase. In those tests, the two types of specimens (EBR and NSM) were first heated up to elevated temperatures (20°C, 55°C, 90°C, 120°C and 150°C), and then loaded up to failure. Three-dimensional finite element (FE) models of the specimens were developed, in which the global behaviour of the CFRP-concrete interaction was simulated by means of bi-linear bond–slip laws. These laws, which can be used for design purposes, were derived for each of the tested temperatures based on an inverse analysis, in order to maximize the agreement between experimental and numerical results in terms of (i) failure loads, (ii) load vs. slip responses, and (iii) strain distributions. Compared to the EBR specimens, the bond–slip relationships derived for the NSM specimens at ambient temperature presented (i) similar stiffness and (ii) higher values of maximum shear stress. With increasing temperatures, although both strengthening techniques exhibited comparable stiffness decrease, the maximum shear stresses for the NSM technique were higher than the EBR counterparts, attesting the better performance of the former technique.

A comparison of the bond behavior of PBO-FRCM composites determined by double-lap and single-lap shear tests

This paper presents the results of an investigation on the bond behavior and stress-transfer mechanism of polyparaphenylene benzobisoxazole (PBO) fiber-reinforced cementitious matrix (FRCM) composites bonded to a concrete substrate using double-lap and single-lap shear tests. Results of double-lap shear tests with different composite bonded lengths and widths are presented and compared with those from single-lap shear tests previously reported by the authors. The idealized load response developed from single-lap shear tests is found to characterize the response of the composite in double-lap shear tests, although with a few key differences. With the double-lap shear test, load redistribution among the composite strips influences the post-peak response if debonding does not occur equally in both strips. Values of the ultimate (peak) stress determined using double-lap shear tests are generally consistent with, although slightly lower than, those determined by single-lap shear tests when the bonded length is longer than the effective bond length.

Bond-slip model for FRP-to-concrete bonded joints under external compression

The influence of compressive stresses exerted on FRP-concrete joints created by external strengthening of structural members on the performance of the system requires better understanding especially when mechanical devices are used to anchor the externally bonded reinforcement (EBR). The numerical modelling of those systems is a tool that permits insight into the performance of the corresponding interfaces and was used in the present study, essentially directed to analyse the effectiveness of EBR systems under compressive stresses normal to the composite surface applied to GFRP-to-concrete interfaces. The compressive stresses imposed on the GFRP-to-concrete interface model the effect produced by a mechanical anchorage system applied to the EBR system. An experimental program is described on which double-lap shear tests were performed that created normal stresses externally applied on the GFRP plates. A corresponding bond-slip model is proposed and the results of its introduction in the numerical analysis based in an available 3D finite element code are displayed, showing satisfactory agreement with the experimental data. The results also showed that lateral compressive stresses tend to increase the maximum bond stress of the interface and also originate a residual bond stress which has significant influence on the interface strength. Also, the strength of the interface increases with the increase of the bonded length which have consequences on the definition of the effective bond length.

Glass beams reinforced with GFRP laminates: Experimental tests and numerical modelling using a discrete strong discontinuity approach

This paper presents an experimental and numerical study about the bond behaviour between glass panes and glass fibre reinforced polymer (GFRP) laminates and the structural response of glass–GFRP beams. For this purpose, tests on glass–GFRP double-lap joints and tests on glass–GFRP hybrid beams were carried out. The finite element method was used and a discrete crack approach based on non-linear fracture mechanics was adopted. The bond between glass and GFRP is modelled by means of zero thickness interface elements. The material properties that characterize the interface, namely the shear stiffness, the cohesion and the mode-II fracture energy, are evaluated with the objective of simulating the experimental results obtained from double lap shear tests on glass–GFRP bonded joints. The obtained parameters, describing the bond-slip law between glass and GFRP, are then used to model the structural response of glass beams reinforced with GFRP. The numerical model is assessed by comparing the simulated structural responses and corresponding crack patterns with the experimental counterparts. This work contributes for a better understanding of the stress transfer mechanisms between glass and GFRP and the failure mechanisms found in the above-mentioned experimental tests, as well as to clarify the interpretation of the results obtained from those tests.

Experimental characterization of the bond between externally bonded reinforcement (EBR) CFRP strips and concrete at elevated temperatures

This paper presents an experimental study about the bond between concrete and CFRP strengthening systems at elevated temperatures. The experimental campaign consisted of double-lap shear tests on concrete blocks strengthened with CFRP strips externally bonded with epoxy adhesive, conducted in both steady state (series S1) and transient (series S2) conditions. In series S1, two types of specimens were tested, with or without mechanical anchorage on the extremities of the CFRP strips. Specimens were first heated up to temperatures of 20°C, 55°C, 90°C and 120°C, and then loaded up to failure. In series S2, specimens were first loaded up to 25%, 50% or 75% of their ambient temperature strength and then heated up to failure. The obtained results in specimens without mechanical anchorage show that with increased temperature (i) the strain distributions along the bonded length change significantly, becoming closer to linear due to the epoxy adhesive softening; (ii) the effective bond length increases; (iii) the stiffness and the maximum shear stress of the bond–slip relationships suffer considerable reductions; (iv) the failure mode changes from cohesive (at ambient temperature) to adhesive (at elevated temperature); and (v) the overall stiffness and strength of CFRP–concrete interface decrease significantly, with the bond strength reduction in the transient tests being similar to that observed on the steady-state conditions. The incorporation of the mechanical anchorage, for all temperatures tested (i) led to more uniform axial strain distributions; (ii) presented bond–slip relationships with lower maximum shear stresses; and (iii) provided significantly higher bond strength.

Assessment of experimental methods for calibrating rate-dependent cohesive zone models for predicting failure in adhesively bonded metallic structures

Calibration of rate-dependent cohesive zone models for predicting damage and failure in adhesively bonded structures requires several parameters including rate dependent strength values in mode I and shear loading. This paper presents a total of six different combinations of parameter sets for mode I and shear strength which can all be derived from available experimental data. A finite element model of a T-joint subjected to six different loading configurations is calibrated with the six different parameter sets. Based on the available experimental data, recommendations are given for the type of test to choose for calibration of rate dependent strength parameters. It is recommended to use data from thick walled tube specimens for the mode I strength and a combination of torsion tests on thin-walled tubes and compressive double lap shear tests for the calibration of rate-dependent shear strength.

BOND STRENGTH OF STEEL-CONCRETE COMPOSITE ELEMENTS USING A CEMENTITIOUS ADHESIVE

Steel-concrete composite slabs have been used for bridge deck construction in Japan because of several advantages, e.g., safety construction and high fatigue durability. On the other hand, these slabs may cause negative influences such as overweight, increasing costs and low constructability, due to many mechanical shear connectors. The quantities of shear connectors may be reduced by gluing steel-plate and concrete with a cementitious adhesive. The present study aims at examining shear bond strength and quantifying the dispersion of the strength. To investigate the dispersion, a double-lap shear test is conducted, in addition to a simple direct shear test, to examine surface treatment. A Monte Carlo Simulation using the Weibull distribution of the strength is performed to evaluate the effect of dispersion. The simulation implies that the shear stress due to the traffic load may be negligible, indicating the applicability of the composite system for highway bridge decks.

Bond Behavior between Near-Surface-Mounted CFRP Strips and Concrete at High Temperatures

AbstractThis paper presents an experimental study concerning the bond behavior at high temperatures between concrete and carbon fiber–reinforced polymer (CFRP) strips installed according to the near-surface-mounted (NSM) technique. Double-lap shear tests were performed on concrete blocks strengthened with CFRP strips installed into slits and bonded with either an epoxy adhesive (series EP) or a mixed grout composed of epoxy and cement binders (series MG). Specimens were first heated up to predefined temperatures (from 20 to 150°C, measured in the adhesive) and then were loaded up to failure. The obtained results allow drawing the following conclusions: (1) the axial strains along the bonded length became closer to linear (at high temperature) because of the softening (glass transition) of the bonding materials; (2) the effective bond length increased for elevated temperature; (3) the bond-slip curves exhibited considerable and consistent stiffness and maximum shear stress reduction with temperature; (4) t...

Creep of epoxy–clay nanocomposite adhesive at the FRP interface: A multi-scale investigation

Externally bonded FRP laminates are widely used for strengthening structures due to their high strength-to-weight ratio and extended durability. However, creep of epoxy adhesives at the FRP interface has been reported, creating a concern about the efficiency of the FRP strengthening with time. Nanoclay platelets have been suggested to control creep of epoxy adhesives at the FRP interface. A multi-scale experimental investigation of creep of the epoxy–clay nanocomposite was performed. This investigation included examining macrocreep of epoxy at the FRP–steel interface using double lap shear test set-up, followed by investigating nanocreep of the epoxy–clay nanocomposite using nanoindentation. Microstructural characterizations including X-ray diffraction (XRD), transmission electron microscope (TEM), differential scanning calorimetry (DSC), 29Si nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy of the epoxy–clay nanocomposite were performed. Nanoclay platelets dispersion, intercalation or exfoliation was found to significantly affect creep of epoxy. Exfoliated nanoclay was found to reduce epoxy cross-linking and thus increase creep. However, agglomeration of intercalated nanoclay platelets allowed them to work as micro-particulate reinforcement in the epoxy matrix and therefore reduce epoxy-chain mobility and limit creep. Such complexity in behavior does not allow upscaling creep behavior of epoxy–clay nanocomposites from nano to macroscale.

Mechanical Performance of the Wet-Bond Interface between FRP Plates and Cast-in-Place Concrete

AbstractThe mechanical performance of the bond between fiber-reinforced polymer (FRP) plates and cast-in-place concrete (referred to as the wet-bond interface) is investigated. The wet-bond interface is named with respect to the dry-bond interface. The dry-bond interface is a chemical bond that is developed to adhere FRP sheets or plates to cured concrete, whereas the wet-bond interface is a chemical-bond interface that is developed to connect stay-in-place FRP formwork and cast-in-place concrete. The double-lap shear test method is adopted to evaluate the effect of the bond. The mechanical properties of the wet-bond interface are compared with that of the conventional dry-bond interface using the same epoxy adhesive, including the load-slip curves, strain distributions, load capacity, and interfacial fracture energy. The results show that the failure modes of the wet-bond and dry-bond interfaces are different. In the dry-bond specimen, debonding occurs in the thin concrete layer and is accompanied by an ...