Papers by Keyword: Debonding

Abstract: The acoustic emission (AE) technique is used for investigating the interfacial fracture and damage propagation in GFRP-and SRG-strengthened bricks during debonding tests. The bond behavior is investigated through single-lap shear bond tests and the fracture progress during the tests is recorded by means of AE sensors. The fracture progress and active debonding mechanisms are characterized in both specimen types with the aim of AE outputs. Moreover, a clear distinction between the AE outputs of specimens with different failure modes, in both SRG-and GFRP-strengthened specimens, is found which allows characterizing the debonding failure mode based on acoustic emission data.

Abstract: The effects of mortars joints in masonry substrate reinforced with FRP is investigated from the numerical point of view. The analysis has been conducted by means of a new interface model specifically developed to reproduce the debonding process occurring between an elastic thin body in adhesion with a cohesive support material, as concrete or masonry. The model accounts for mode I and mode II of failure, considering the effect of the in-plane deformation of the interface, i.e. the possible elongation or confinement of the material constituting the interface. Numerical results are compared with experimental evidences showing the good performances of the proposed model in investigating the transferring phenomena in heterogeneous interfaces and, in particular, in studying the influence of the presence of mortar joints in the masonry texture in the debonding process.

Abstract: In the framework of the reinforcement of masonry structures by FRP sheets (Fiber Reinforced Polymer), the use of “Fiber Anchor” is an innovative performing solution to increase the peak load and the dissipation capability of the strengthened system. To analyze the performance of this new technology an experimental campaign composed by 36 tests has been conducted. Six series of CFRP to brick bonded joint fastened by different configurations of fiber anchor have been tested. To deeply describe the measure fields, the Digital Image Correlation (DIC) has been used. The strain fields obtained in the series not fastened by anchors have permitted to define the bond stress field and the advancement of the stress transfer zone. The DIC appears as a performing methodology to use in a lot of civil engineering experimental problems.

Abstract: To evaluate the interface strength of externally bonded fiber-reinforced polymer (FRP) composites to concrete structures, the method of the Linear Elastic Fracture Mechanic (LEFM) model is simply used. The parameters defining the material properties, describing bond action of the FRP-concrete interface is used , which was got recently by Obaidat from three-dimensional (3D) finite-element simulation results. Both the fracture energy and shear strength of the interface are determined by a function of concrete compressive strength and the adhesive shear stiffness, the maximum transferable load is predicted by LEFM interface bond-slip model. Comparison between the predicted and the experiment results shows good agreement and a certain degree of safe estimation.

Abstract: Repair of reinforced concrete beam with externally bonded steel plate or fibre reinforced polymer (FRP) laminate is becoming both environmentally and economically preferable rather than replacement of deficient beam. The well known advantages of external reinforcement over other methods include; low cost, ease of maintenance and the ability to strengthen part of the structure while it is still in use. The disadvantage of this method, however, is the premature debonding of the externally bonded strips which is brittle and undesired mode of failure. It is also known that debonding of the externally bonded steel plates prevents the reinforced concrete (RC) beam from reaching its full strengthening capacity. The aim of this study was to increase the scientific understanding on the behaviour of damaged reinforced concrete beams strengthened and/or retrofitted for shear using vertical steel plate fixed with adhesive and steel connectors to eliminate or delay debonding failure. Four reinforced concrete beam specimens were prepared to investigate the effects of connectors in preventing or delaying premature debonding of shear strips to restore the capacities of fully damaged beams. Three damaged beams have been repaired and strengthened with steel plates and loaded monotonically up to the maximum load capacities in order to define load–deflection relationship. It is concluded that the repairing of severely shear-damaged RC beams with steel plates by using steel and adhesive connectors can fully restore the original shear capacities of the beams.

Abstract: The performance of the interface between FRP and masonry is one of the key factors affecting the behavior of the strengthened structure. A challenge is to better understand the potential failure modes, which are generally directly related to the interfacial behavior between the FRP composite and the masonry in masonry walls. In this research, the experimental program was focused on the bond behavior of GFRP strips externally bonded (EB) to brick masonry.

Abstract: This paper deals with the mechanical behavior of bamboo fiber-reinforced biodegradable composites (green composites). A starch-based, dispersion type biodegradable resin was used as matrix polymer, and this matrix was reinforced by long bamboo fibers which were extracted by a steam-explosion method. The experimental results showed that the developed green composites possessed the flexural and tensile strength of 263 MPa and 270 MPa, respectively. The mechanical properties of the green composites were evaluated as a function of fiber content. It is found that the observed tensile strength was slightly lower than that of estimated values from the rule of the mixture. This discrepancy might be related to the misorientation of the bamboo fiber in the composites and to that the final fracture of composites is also governed by the presence of weak bamboo fiber.

Abstract: The interaction of the fundamental anti-symmetric Lamb wave (A₀) with debondings at structural features is investigated using experimental data and finite element (FE) simulations. In this study explicit three-dimensional (3D) FE simulations are employed, which allows the study of the scattered wave along different propagation directions. Good agreement between the FE predictions and the measurements are obtained that demonstrates that the 3D FE scattering model is able to accurately predict the Lamb wave scattering characteristics at debondings. The study show that the characteristics of Lamb wave reflected from the debondings at the structure feature is much more complicated than that from defects in flat composite laminates. Parameter studies show that the backward and forward scattering coefficient of Lamb wave is a function of debonding size to wavelength ratio and debonding location. This shows the potential of employing Lamb wave to identify the size and monitor the growth of the debondings. The findings of the study provide improved physical insights into the scattering phenomena, which are important to further advance damage detection techniques for complex structures made by composite laminates.

Abstract: Damage detection using guided waves in the inspection of tapered sandwich structures with high density foam core (Dyvinicell HP100) is investigated. Characterisation of the fundamental symmetric and anti-symmetric Lamb wave modes is carried out in terms of their velocity and magnitude variation as they encounter a change in the thickness of a composite sandwich plate, aiming at optimising the mode selection to improve the capability and increase the sensitivity of guided waves in inspection of tapered sandwich structures. In addition, an imaging algorithm based on time reversal is developed to detect multiple debonding and artificial damage in tapered sandwich panels based guided waves from an active sensor network. The correlation coefficients between the original and reconstructed time reversal signals are calculated to define a damage index for individual sensing paths, which are used later in the fusion process, identifying the presence of damage in the monitoring area enclosed by the active sensor network. The results confirm that the incident wave signals and their reconstructed time-reversed counterparts can be used to accurately detect the debonding/damage in tapered sandwich structures.

Abstract: The aim of the present research is to assess the dynamic performance of bond-slip between reinforcing steel bar and concrete. Precisely, the sensitivity of the bond stress-displacement relationship in the interface between reinforcing steel bar and concrete to strain rate is investigated. In this paper, the reinforcing steel bar/concrete specimens were tested in dynamic push-out experiments with modified split Hopkinson pressure bar (SHPB) system. The incident bar was used to apply compressive loading to single steel bar embedded in a surrounding concrete. Using this technique, the debonding progress was analyzed. Besides, the effect of loading rate and effective depth on the push-out tests was studied with the impact velocities in the range of 9 m/s - 29 m/s. It is seen that maximum push-out force increases with increasing loading rate. In addition, the dynamic interface bond force does not proportionally increase with the effective depth of steel bar.