• Advances in nano research
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• v.14 no.4
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• pp.363-373
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• 2023

A novel type of steel fiber with a rounded-end shape is presented to improve the bonding behavior of fibers with Carbon Nanotubes (CNT)-reinforced Ultra-High Performance Concrete (UHPC) matrix. For this purpose, by performing a parametric study and using the nonlinear finite element method, the impact of geometric characteristics of the fiber end on its bonding behavior with UHPC has been studied. The cohesive zone model investigates the interface between the fibers and the cement matrix. The mechanical properties of the cohesive zone model are determined by calibrating the finite element results and the experimental fiber pull-out test. Also, the results are evaluated with the straight steel fibers outcomes. Using the novel presented fibers, the bond strength has significantly improved compared to the straight steel fibers. The new proposed fibers increase bond strength by 1.1 times for the same diameter of fibers. By creating fillet at the contact area between the rounded end and the fiber, bond strength is significantly improved, the maximum fiber capacity is reachable, and the pull-out occurs in the form of fracture and tearing of the fibers, which is the most desirable bonding mode for fibers. This also improves the energy absorbed by the fibers and is 4.4 times more than the corresponding straight fibers.

• Computers and Concrete
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• v.26 no.6
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• pp.467-482
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• 2020

This research focused on analyzing the post-fire behavior of high-performance concrete-filled steel tube (CFST) columns, with the concrete containing tire rubber and steel fibers, under axial compressive loading. The finite element (FE) modeling of such heated columns containing recycled aggregate is a branch of this field which has not received the proper attention of researchers. Better understanding the post-fire behavior of these columns by measuring their residual strength and deformation is critical for achieving the minimum repair level required for structures damaged in the fire. Therefore, to develop this model, 19 groups of confined and unconfined specimens with the variables including the volume ratio of steel fibers, tire rubber content, diameter-to-thickness (D/t) ratio of the steel tube, and exposure temperature were considered. The ABAQUS software was employed to model the tested specimens so that the accurate behavior of the FE-modeled specimens could be examined under test conditions. To achieve desirable results for the modeling of the specimens, in addition to the novel procedure described in this research, the modified versions of models presented by previous researchers were also utilized. After the completion of modeling, the load-axial strain and load-lateral strain relationships, ultimate strength, and failure mode of the modeled CFST specimens were evaluated against the test data, through which the satisfactory accuracy of this modeling procedure was established. Afterward, using a parametric study, the effect of factors such as the concrete core strength at different temperatures and the D/t ratio on the behavior of the CFST columns was explored. Finally, the compressive strength values obtained from the FE model were compared with the corresponding values predicted by various codes, the results of which indicated that most codes were conservative in terms of these predictions.

• Journal of the Korean Society for Nondestructive Testing
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• v.31 no.6
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• pp.601-608
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• 2011

This paper presents the damage assessment of a building structure by using a novel optical fiber accelerometer system. Especially, a sub-scaled building model is designed and manufactured to check up the feasibility of the optical fiber accelerometer for structural health monitoring. The novel accelerometer exploits the moir$\acute{e}$ fringe optical phenomenon and two pairs of optical fibers to measure the displacement with a high accuracy, and furthermore a pendulum to convert the displacement into acceleration. A prototype of optical fiber accelerometer system has been successfully developed that consists of a sensor head, a control unit and a signal processing unit. The building model is also designed as a 4-story building with a rectangular shape of $200{\times}300$ mm of edges. Each floor is connected to the next ones by 6 steel columns which are threaded rods. Basically, a random vibration test of the building model is done with a shaker and all of acceleration data is successfully measured at the assigned points by the optical fiber accelerometer. The experiments are repeated in the undamaged state and the damaged state. The comparison of dynamic parameters including the natural frequencies and the eigenvectors is successfully carried out. Finally, the optical fiber accelerometer is proven to be prospective to evaluate dynamic characteristics of a building structure for the damage assessment.

• Steel and Composite Structures
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• v.30 no.1
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• pp.1-12
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• 2019

In this research, an efficient mixed mode I/II fracture criterion is developed for fracture investigation of orthotropic materials wherein crack is placed along the fibers. This criterion is developed based on extension of well-known Maximum Tensile Stress (MTS) criterion in conjunction with a novel material model titled as Equivalent Reinforced Isotropic Model (ERIM). In this model, orthotropic material is replaced with an isotropic matrix reinforced with fibers. A comparison between available experimental observations and theoretical estimation implies on capability of developed criterion for predicting both crack propagation direction and fracture instance, wherein the achieved fracture limit curves are also compatible with fracture mechanism of orthotic materials. It is also shown that unlike isotropic materials, fracture toughness of orthotic materials in mode $I(K)_{IC}{\mid})$ cannot be introduced as the maximum load bearing capacity and thus new fracture mechanics property, named here as maximum orthotropic fracture toughness in mode $I(K_{IC}{\mid}^{ortho}_{max})$ is defined. Optimum angle between crack and fiber direction for maximum load bearing in orthotropic materials is also defined.

• Steel and Composite Structures
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• v.44 no.6
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• pp.817-828
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• 2022

In this study, considering dissipated energy in fracture process zone (FPZ), a novel criterion based on maximum strain energy release rate (SER) for orthotropic materials is presented. General case of in-plane loading for cracks along the fibers is assumed. According to the experimental observations, crack propagation is supposed along the fibers and the reinforcement isotropic solid (RIS) concept is employed as a superior model for orthotropic materials. SER in crack initiation and propagation phases is investigated. Elastic properties of FPZ are extracted as a function of undamaged matrix media and micro-crack density. This criterion meaningfully links between dissipated energy due to toughening mechanisms of FPZ and the macroscopic fracture by defining stress intensity factors of the damaged zone. These coefficients are used in equations of maximum SER criterion. The effect of crack initiation angle and the damaged zone is considered simultaneously in this criterion and mode II stress intensity factor is extracted in terms of stress intensity factors of damage zone and crack initiation angle. This criterion can evaluate the effects of FPZ on the fracture behavior of orthotropic material. Good agreement between extracted fracture limit curves (FLC's) and available experimental data proves the ability of the new proposed criterion.

• Advances in concrete construction
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• v.2 no.1
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• pp.57-75
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• 2014

This paper presents an analysis of the results of an experimental program on the performance of a novel configuration of a hybrid FRP-concrete beam. The beam section consists of a GFRP pultruded profile, a CFRP laminate, and a concrete block all wrapped up using filament winding. It was found that the thickness of the concrete block and the confinement by the filament-wound wrapping had a profound effect on the energy dissipation behaviour of the beam. Using a shear punching model, and comparing the predicted results with the experimental ones, it was found that beyond a given value of the concrete block thickness, the deformational behaviour of the beam shifts from brittle to ductile. It was also found that the filament-wound wrap had many benefits such as providing a composite action between the concrete block and the GFRP box, improving the stiffness of the beam, and most importantly, enhancing the load carrying ability through induced confinement of the concrete.

• Steel and Composite Structures
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• v.49 no.4
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• pp.393-405
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• 2023

In this paper, the energy absorption capability of a novel cruciform composite lattice structure was evaluated through the simulation of compression tests. For this purpose, several test samples of Polylactic acid cellular reinforced with continuous glass fibers were prepared for compression testing using the additive manufacturing method of material extrusion. Using a conventional path design for material extrusion, multiple debonding is probable to be occurred at the joint regions of adjacent cells. Therefore, an innovative printing path design was proposed for the cruciform lattice structure. Afterwards, quasistatic compression tests were performed to evaluate the energy absorption behaviour of this structure. A finite element model based on local material property degradation was then developed to verify the experimental test and extend the virtual test method. Accordingly, different combinations of unit cells' dimensions using the design of the experiment were numerically proposed to obtain the optimal configuration in terms of the total absorbed energy. Having brilliant energy absorption properties, the studied cruciform lattice with its optimized unit cell dimensions can be used as an energy absorber in crashworthiness applications. Finally, a cellular structure will be suitable with optimal behavior in crush load efficiency and high energy absorption.