• Structural Engineering and Mechanics
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• v.48 no.5
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• pp.737-755
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• 2013

This paper aims to develop a practical approach to modeling of fiber reinforced polymers (FRP) strengthened masonry panels. The main objective is to provide suitable relations for the material characterization of the masonry constituents so that the finite element applications of elasto-plastic theory achieves a close fit to the experimental load-displacement diagrams of the walls subjected to in-plane shear and compression. Two relations proposed for masonry columns confined with FRP are adjusted for the cohesion and the internal friction angle of both units and mortar. Relating the mechanical parameters to the uniaxial compression strength and the hydrostatic pressure acting over the wall surface, the effects of major and intermediate principal stresses ${\sigma}_1$ and ${\sigma}_2$ on the yielding and the shape of the deviatoric section are then reflected into the analyses. Performing nonlinear finite element analyses (NLFEA) for the three walls tested in two different studies, their stress-strain response and failure modes are eventually evaluated through the comparisons with the experimental behavior.

• Steel and Composite Structures
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• v.31 no.5
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• pp.517-527
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• 2019

To move forward in large steps rather than in small increments, the community would benefit from a systematic and comprehensive database of multi-scale composites and measured properties, driven by comprehensive studies with a full range of types of fiber-reinforced polymers. The multi-scale hierarchy is a promising chemical approach that provides superior performance in synergistically integrated microstructured fibers and nanostructured materials in composite applications. Achieving high-efficiency thermal conductivity and mechanical properties with a simple surface treatment on single-walled carbon nanotubes (SWCNTs) is important for multi-scale composites. The main purpose of the project is to introduce ozone-treated SWCNTs between an epoxy matrix and basalt fibers to improve mechanical properties and thermal conductivity by enhancing dispersion and interfacial adhesion. The obvious advantage of this approach is that it is much more effective than the conventional approach at improving the thermal conductivity and mechanical properties of materials under an equivalent load, and shows particularly significant improvement for high loads. Such an effort could accelerate the conversion of multi-scale composites into high performance materials and provide more rational guidance and fundamental understanding towards realizing the theoretical limits of thermal and mechanical properties.

• Structural Engineering and Mechanics
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• v.34 no.4
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• pp.525-539
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• 2010

Polymer matrix composites are widely used in many engineering applications as they can be customized to meet a desired performance while not only maintaining low cost but also reducing weight. Polymers can experience viscoelastic-viscoplastic response when subjected to external loadings. Various reinforcements and fillers are added to polymers which bring out more complexity in analyzing the timedependent response. This study formulates an integrated micromechanical model and finite element (FE) analysis for predicting effective viscoelastic-viscoplastic response of polymer based hybrid composites. The studied hybrid system consists of unidirectional short-fiber reinforcements and a matrix system which is composed of solid spherical particle fillers dispersed in a homogeneous polymer constituent. The goal is to predict effective performance of hybrid systems having different compositions and properties of the fiber, particle, and matrix constituents. A combined Schapery's viscoelastic integral model and Valanis's endochronic viscoplastic model is used for the polymer constituent. The particle and fiber constituents are assumed linear elastic. A previously developed micromechanical model of particle reinforced composite is first used to obtain effective mechanical properties of the matrix systems. The effective properties of the matrix are then integrated to a unit-cell model of short-fiber reinforced composites, which is generated using the FE. The effective properties of the matrix are implemented using a user material subroutine in the FE framework. Limited experimental data and analytical solutions available in the literatures are used for comparisons.

• Journal of the Korea institute for structural maintenance and inspection
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• v.13 no.6 s.58
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• pp.115-125
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• 2009

In these days, carbon fiber-reinforced polymers (CFRP) have been widely used for retrofitting and/or strengthening structural elements. However, there are not enough test data to predict the long-term performance of the retrofitted structures exposed to freeze-thawing cycles. This paper presents the results of experimental program undertaken to investigate the effects of freeze-thawing cycling (from-18 to $4^{\circ}C) on the behavior and failure characteristics of reinforced concrete (RC) beams strengthened in shear with CFRP sheet and plate using acoustic emission (AE) technique.

• Fibers and Polymers
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• v.2 no.1
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• pp.163-172
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• 2001

To determine three-dimensional fiber orientation states in injection-molded short fiber composites a CLSM (Confocal Laser Scanning Microscope) is used. Since the CLSM optically sections the composites, more than two cross-sections either on or below the surface of the composite can be obtained. Three dimensional fiber orientation states can be determined with geometric parameters of fibers on two parallel cross-sections. For experiment, carbon fiber reinforced polystyrene is examined by the CLSM. Geometric parameters of fibers are measured by image analysis. In order to compactly describe fiber orientation states, orientation tensors are used. Orientation tensors are determined at different positions of the prepared specimen. Three dimensional orientation states are obtained without the difficulty in determining the out-of-plane angles by utilizing images on two parallel planes acquired by the CLSM. Orientation states are different at different positions and show the shell-core structure along the thickness of the specimen.

• Advances in concrete construction
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• v.2 no.4
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• pp.281-300
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• 2014

This study presents the experimental results of twenty three reinforced concrete beams with rectangular web openings externally strengthened with Fiber Reinforced Polymers (FRP) composites bonded around openings. All tested beams had the same geometry and reinforcement details. At openings locations, the stirrups intercepted the openings were cut during fabrication of reinforcement cage to simulate the condition of inclusion of an opening in an existing beam. Several design parameters are considered including the opening dimensions and location in the shear zone, the wrapping configurations, and the amount and the type of the FRP composites in the vicinity of the openings. The wrapping configurations of FRP included: sheets, strips, U-shape strips, and U-shape strips with bundles of FRP strands placed at the top and sides of the beam forming a fan under the strips to achieve closed wrapping. The effect of these parameters on the failure modes, the ultimate load, and the beam stiffness were investigated. The shear contribution of FRP on the shear capacity of tested beams with web openings was estimated according to ACI Committee 440-08, Canadian Standards S6-06, and Khalifa et al. model and examined against the test results. A modification factor to account for the dimensions of opening chords was applied to the predicted gain in the shear capacity according to ACI 440-08 and CSA S6-06 for bonded Glass Fiber Reinforced Polymers (GFRP) around openings. The analytical results after incorporating the modification factor into the codes guidelines showed good agreement with the test results.

• Steel and Composite Structures
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• v.14 no.1
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• pp.41-55
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• 2013

Fiber reinforced polymers (FRP) materials are increasingly being used for strengthening and repair of steel structures. An issue that concerns engineers in steel members which are retrofitted with FRP is stress experienced due to temperature changes. The changing temperature affects the interface bond between the FRP and Steel. This research focused on the effects of cyclical thermal loadings on the interface properties of FRP bounded to steel members. Over fifty tests were conducted to investigate the thermal effects on bonding between FRP and steel, which were cycled from temperature of $-11^{\circ}C$ ($12^{\circ}F$) to $60^{\circ}C$ ($140^{\circ}F$) for 21-36 days. This investigation consisted of two test protocols, 1) the tensile test of epoxy resin, tack coat, FRP and FRP-steel plate, 2) tensile test of each FRP compound and FRP with steel after going through thermal cyclic loading. This investigation reveals an extensive reduction in the composite's strength.

• Steel and Composite Structures
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• v.15 no.2
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• pp.131-150
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• 2013

This paper presents the experimental results of axially loaded stub columns of slender steel hollow square section (SHS) strengthened with carbon fiber reinforced polymers (CFRP) sheets. 9 specimens were fabricated and the main parameters were: width-thickness ratio (b/t), the number of CFRP ply, and the CFRP sheet orientation. From the tests, it was observed that two sides would typically buckle outward and the other two sides would buckle inward. A maximum increase of 33% was achieved in axial-load capacity when 3 layers of CFRP were used to wrap HSS columns of b/t = 100 transversely. Also, stiffness and ductility index (DI) were compared between un-retrofitted specimens and retrofitted specimens. Finally, it was shown that the application of CFRP to slender sections delays local buckling and subsequently results in significant increases in elastic buckling stress. In the last section, a prediction formula of the ultimate strength developed using the experimental results is presented.

• Proceedings of the Korea Concrete Institute Conference
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• 2003.05a
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• pp.531-536
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• 2003

It is generally known that RC flexural members strengthened by GFRP(Glass Fiber Reinforced Polymers) tend to be failed by premature bond failure near the flexural-shear cracks happened at the mid-span of beams. It is therefore strongly recommended that premature bond failure must be avoided to insure the intended strengthening effects sufficiently. The various methodologies such as increasing bonded length of GFRP and bonding details including U-shape wrappings and epoxy shear-keys are examined in this study. The bonded length of GFRP are calculated based on the assumed bond strengths of epoxy saturating resin. Total six half scale RC beam specimens were constructed and tested to investigate the effectiveness of each methodologies to prevent the bond failure of GFRP. Test results of each specimens are discussed in this paper.

• Steel and Composite Structures
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• v.11 no.6
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• pp.455-468
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• 2011

Although retrofitting and strengthening reinforced concrete (RC) columns by wrapping fiber reinforced polymer (FRP) composites have become a popular technique in civil engineering, the study on reinforced high-strength concrete (HSC) columns is still not sufficient. The objective of these companion papers is to investigate the mechanical properties of reinforced HSC square columns confined by aramid FRP (AFRP) jackets under concentric compressive loading. In the part I of these companion papers, an experiment was conducted on 54 confined RC specimens and nine unconfined plain specimens, the considered parameters were the concrete strength, the thickness of AFRP jackets, and the form of AFRP wrapping. The experimental process and results are presented in detail. Subsequently, some discussions on the confinement effect, failure modes, strength, and ductility of the columns are carried out.