Ayman M. Okeil

Novel Technique for Inhibiting Buckling of Thin-Walled Steel Structures Using Pultruded Glass FRP Sections

The use of composite materials for strengthening the ailing infrastructure has been steadily gaining acceptance and market share. It can even be stated that this strengthening technique has become main stream in some applications such as strengthening concrete structures. The same cannot be said about steel structures; for which research on composite material strengthening is relatively new. Several challenges face strengthening steel structures using composite materials such as the need for high-modulus composites to improve the effectiveness of the strengthening system. This paper explores a new approach for strengthening steel structures by introducing additional stiffness to buckling-prone regions. The proposed technique relies on improving the out-of-plane stiffness of buckling-prone members by bonding pultruded fiber-reinforced polymer (FRP) sections as opposed to the commonly used approach that relies on in-plane FRP contribution. The paper presents results from an experimental investigation where ...

Overview of Potential and Existing Applications of Shape Memory Alloys in Bridges

Bridges are the backbones of transportation lines for modern cities. Damage to bridges could disrupt the flow of traffic and be disastrous for the communities they serve, especially when reconstruction and recovery activities are needed, such as after strong earthquakes and hurricanes. Recent earthquake and hurricane damage has exposed the vulnerability of existing bridges under strong ground motions and unexpected wave loads. In recent decades, several kinds of smart materials have been investigated to improve the performance of bridge structures during extreme events such as earthquakes and strong winds. Among these materials, shape memory alloys (SMAs) have exhibited great potential in enhancing the performance of bridge structures because of their unique properties, such as the shape memory effect and superelasticity effect. This paper, for the first time, systematically reviews and summarizes the applications of SMAs in bridge structures. The unique properties of SMAs are presented first, and several simplified one-dimensional constitutive material models of superelastic SMAs are introduced. Finally, applications of SMAs in five areas of bridge engineering are discussed. DOI: 10.1061/(ASCE)BE.1943-5592.0000145.

LRFD FLEXURAL PROVISIONS FOR PRESTRESSED CONCRETE BRIDGE GIRDERS STRENGTHENED WITH CARBON FIBER-REINFORCED POLYMER LAMINATES

This paper discusses the behavior and design of prestressed concrete (PSC) bridge girders flexurally strengthened with carbon fiber-reinforced polymer (CFRP) laminates. A fiber section model that accounts for inelastic material behavior as well as the construction sequence including transfer, composite action between the cast-in-place deck and girder, and bonding of CFRP laminates, is developed. The model is verified and is then used to conduct thousands of Monte Carlo simulations of a number of bridges designed according to the 1998 AASHTO LRFD. The bridge designs address a broad range of design parameters. The numerical simulations are used to develop cross-sectional resistance models from which the flexural reliability of the designed bridges is calculated using the first-order reliability method. An equation for the flexural strength reduction factor for PSC bridge girders strengthened with CFRP laminates is proposed.

SHORT-TERM TENSILE STRENGTH OF CARBON-FIBER-REINFORCED POLYMER LAMINATES FOR FLEXURAL STRENGTHENING OF CONCRETE GIRDERS

This paper presents an analytical technique based on the Weibull theory for brittle materials that can be used to estimate the strength of unidirectional carbon fiber-reinforced polymer (CFRP) laminates from fiber properties reported by the manufacturer. Good agreement is found between theoretical and experimental results of concrete T-beams strengthened with varying amounts of CFRP laminates. The design implications of the developed methodology are discussed, and a chart is provided for calculating CFRP laminate strength from fiber data reported by the manufacturer.

Impact of Friction Stir Welding (FSW) Process Parameters on Thermal Modeling and Heat Generation of Aluminum Alloy Joints

Friction stir welding (FSW) is a solid-state joining process, where joint properties largely depend on the amount of heat generation during the welding process. The objective of this paper was to develop a numerical thermomechanical model for FSW of aluminum–copper alloy AA2219 and analyze heat generation during the welding process. The thermomechanical model has been developed utilizing ANSYS® APDL. The model was verified by comparing simulated temperature profile of three different weld schedules (i.e., different combinations of weld parameters in real weld situations) from simulation with experimental results. Furthermore, the verified model was used to analyze the effect of different weld parameters on heat generation. Among all the weld parameters, the effect of rotational speed on heat generation is the highest.

Reliability Assessment of FRP-Strengthened Concrete Bridge Girders in Shear

AbstractThis paper presents the results from an investigation into the reliability of reinforced concrete bridge girders strengthened in shear using fiber-reinforced polymers (FRP). Two expressions for the shear design of FRP-strengthened girders were developed as part of a National Cooperative Highway Research Program project. The expressions were developed for two distinct cases, namely, bonded and anchored FRP reinforcement. Uncertainties inherent in the new design models were first assessed using an extensive database of hundreds of experimentally tested specimens. Variabilities in material, fabrication tolerances, dead and live loads, and distribution factors obtained from the literature were then included in a limit state function for the shear strength mode of failure. The reliability of a representative design space comprising 18 bridge girders that cover different span lengths, shear deficiency levels, and girder location (exterior versus interior) was calculated for both bonded and anchored stre...

Developing a Model to Estimate Pile Setup for Individual Soil Layers on the Basis of Piezocone Penetration Test Data

This paper analyzes the results of 15 instrumented prestressed concrete test piles that were driven in four sites in Louisiana as part of a pile setup study. Static and dynamic load tests were conducted at specific time intervals to quantify the amount of setup after the end of driving. Laboratory and in situ soil investigations, such as the Piezocone penetration test (PCPT), were conducted at each test pile location to evaluate the subsurface soil condition. Vibrating wire strain gauges were installed in pairs in all instrumented test piles to measure the load transfer and increase in side resistance of individual soil layers over time. The load test results showed that setup followed a linear logarithmic trend. The unit side resistance, instead of the total pile resistance, was used to measure the logarithmic setup parameter A for individual soil layers along the pile lengths. Observations from the test results showed that the average A values for clayey and sandy soil layers for the four tested sites were 0.31 and 0.15, respectively. Analyses of the setup of individual soil layers showed that the parameter A decreases with increased corrected cone tip resistance measured by PCPT. A simple linear regression model is proposed to predict the side resistance setup for individual soil layers by correlating the setup parameter A with the corrected cone tip resistance.

Phased Array Ultrasonic Testing for Post-Weld and OnLine Detection of Friction Stir Welding Defects

ABSTRACT Nondestructive evaluation (NDE) techniques of phased array ultrasonic testing (PAUT) and digital X-ray radiography were employed on friction stir (FS)-welded Aluminum Alloy (AA)-2219-T87 specimens. PAUT intricacies required for scanning of FS-welded specimens with a 10-MHz 32-element transducer are discussed. The time corrected gain (TCG) calibration is required for scanning with an increase in index offset to compensate for decrease in A-Scan signal peak amplitude. Calibration techniques to find small defects with appropriate size tolerances are also established. The NDE technique of digital X-ray radiography is compared to PAUT, where it was found that a calibrated PAUT system is able to discover defects less than 0.2 mm where X-ray radiography could not. Incomplete penetration (IP), wormhole (WH), surface cavity (SC), and internal void (IV) defects are analyzed. Furthermore, an online PAUT system for FSW has been developed and successfully tested. The work provided herein will provide a gateway for an ultimate goal of an automated PAUT online sensing system.

Ultrasonic Signal Characteristics for Nondestructive-Yield Detection in Steel Structures

Nondestructive testing (NDT) methods for identifying stress levels in materials mostly rely on the theory of acoustoelasticity. However, the sensitivity and the accuracy of acoustoelasticity are affected by several factors such as the (1) type, (2) propagation, and (3) polarization directions of the used signals. This paper presents the results of an experimental investigation of longitudinal waves propagating perpendicular to the applied uniaxial tensile stresses in structural steel specimens. The changes in four ultrasonic signal characteristics were investigated with increasing stress levels ranging from below to above the yield stress of steel. The considered signal characteristics were the peak amplitudes and signal energy in the time domain, and the fast Fourier transform (FFT) and chirp-Z transform (CZT) in the frequency domain. Even though the acoustoelastic effect on the type ultrasonic signal used is very small, clear distinctions between prior to and postyielding are observed for all investigated parameters. The results are presented with a detailed statistical and receiver operating characteristics (ROCs) analyses. The results show that identifying damage to steel structures due to local yielding is possible using the simple ultrasonic signal classification.

Influence of Weld Defects and Postweld Heat Treatment of Gas Tungsten Arc-Welded AA-6061-T651 Aluminum Alloy

Welding defects and the reduction of mechanical performances are the foremost problems for fusion welded aluminum alloys joints. The influences of weld defects and post-weld heat treatment (PWHT) on tensile properties of gas tungsten arc (GTA) welded aluminum alloy AA-6061-T651 joints are investigated in this current study. All welded specimens are non-destructively inspected with phased array ultrasonic testing (PAUT) to classify weld defect and measure the projected defects area-ratio (AR). Ultimate tensile strength (UTS) decreased linearly with the increase of the size of weld defect but tensile toughness behaved non-linearly with defect size. Depending on defect size, defective samples’ joint efficiency (JE) varied from 35 to 48% of base metal’s UTS. Defect-free as-welded (AW) specimens observed to have 53% and 34% JE based on UTS and yield strength (YS) of base metal, respectively. PWHT was applied on defect-free welded specimens to improve tensile properties by precipitation hardening, microstructures refining, and removal of post-weld residual stresses. Solution treatment (at 540 °C) followed by varying levels of artificial age-hardening time was investigated to obtain optimum tensile properties. For GTA welded AA-6061-T651, peak aging time was 5