Farid Taheri

Experimental and analytical investigation of fatigue characteristics of 350WT steel under constant and variable amplitude loadings

Abstract Fatigue analysis is a complex and uncertain process. Various models have been proposed; however, no universal or all-encompassing model exists. A state-of-the-art literature review of available fatigue crack propagation models, both for constant and variable amplitude loadings, was conducted to identify their advantages and limitations. Emphasis was placed on models that were simple to evaluate and required few (if any) empirical curve-fitting parameters. An experimental fatigue program was conducted, consisting of constant and semi-random (variable) amplitude cyclic loadings on 350WT steel 40J at −40°C. The fatigue models examined in the literature review were then compared to results obtained from the experiments. Findings for the constant amplitude loading (CAL) fatigue baseline data obtained experimentally for 350WT steel are reported and evaluated against the predictions by various models. Results of an experimental investigation into fatigue crack propagation (FCP) response of the steel under variable amplitude loading (VAL), consisting of CAL with random overloads (OL) is also presented. This includes a summary of theoretical models applicable to VAL fatigue crack propagation, and comparison of the experimental results to the models.

Delamination buckling analysis of general laminated composite beams by differential quadrature method

This article focuses on the application of the differential quadrature method (DQM) for the buckling analysis of one-dimensional (1D) composite laminated beam-plates. The DQM formulation of the problem is presented. The formulation accounts for the effects of shear deformation and bending-extension coupling. The governing 1D differential equations were transferred into a system of linear algebraic eigenvalue equations. A standard eigen-solver was used to obtain the delamination buckling loads and the associated mode shapes. Several case studies were used to investigate some of the parameters that affect the buckling response of such composite beams. The study demonstrates the excellent accuracy and efficiency that can be obtained by applying the DQM to treat delamination buckling of composites.

Damage identification in beams using empirical mode decomposition

Damage detection of beam-type components, which are often vital elements in many structures, is crucial for the prevention of failure of the entire structure and potential catastrophic consequences. In this article, the effectiveness of a damage index, referred to as the EMD energy damage index, for damage detection of beams is demonstrated through a set of numerical and experimental investigations. The proposed damage index utilizes the empirical mode decomposition for health assessment of the system based on its vibrational data. In the numerical study, finite element simulation of a cantilevered steel beam with a transverse notch was analyzed and various notch sizes, located at different locations along the beam, were investigated. In the experimental investigation, which used the same beam as in the numerical study, five notch sizes at the mid-span of the beam were examined. In both the numerical and experimental studies, the free vibration of the beam was acquired via piezoceramic sensors adjacent to the notch and then processed by the proposed methodology for evaluating the EMD energy damage index. This was motivated as the preliminary stage of our investigation with the notion of detecting the presence of a crack in a welded joint. The results were encouraging and proved the capability of the EMD energy damage index for detection and quantification of notches in beams and therefore can be regarded as an effective tool for structural health monitoring purposes. The results were also compared with a method based on changes in the beam natural frequencies. The effect of the boundary conditions on the EMD energy damage index was also experimentally studied.

Improvement of a Vibration-Based Damage Detection Approach for Health Monitoring of Bolted Flange Joints in Pipelines

Early detection of bolt loosening is a major concern in the oil and gas industry. In this study, a vibration-based health monitoring strategy has been developed for detecting the loosening of bolts in a pipeline’s bolted flange joint. Both numerical and experimental studies are conducted to verify the integrity of our implementation as well as of an enhancement developed along with it. Several damage scenarios are simulated by the loosening of the bolts through varying the applied torque on each bolt. An electric impact hammer is used to vibrate (excite) the system in a consistent manner. The induced vibration signals are collected via piezoceramic sensors bonded onto the pipe and flange. These signals are transferred remotely by a wireless data acquisition module and then processed with a code developed in-house in the MATLAB environment. After normalization and filtering of the signals, the empirical mode decomposition is applied to establish an effective energy-based damage index. The assessment of the damage indices thus obtained for the various scenarios verifies the integrity of the proposed methodology for identifying the damage and its progression in bolted joints as well as the major enhancements applied onto the methodology.

Treatment of Unsymmetric Adhesively Bonded Composite Sandwich Panels-To-Flange Joints

ABSTRACT An analytical solution for the determination of stresses in unsymmetric adhesively bonded joints is presented. The classical sandwich plate theory and an adhesive interface constitutive model are employed for this development. In addition, an analytical solution for the consideration of the effect of fracture in the joints, using the modified crack closure also is developed. Joints with various configurations (symmetrical and/or unsymmetrical, single skin and/or sandwich composites), and joints having different thickness, length, and lay up, including the effect of crack tip position, can be treated with these solutions. FEA is used to confirm the integrity of these solutions. The results from the proposed solution agree well with the results obtained from finite element analyses (FEA).

On the parameters influencing the performance of reinforced concrete beams strengthened with FRP plates

This paper presents a summary of our investigations that were aimed to assess the influence of various physical and mechanical parameters on the performance of reinforced concrete (RC) beams strengthened with fiber reinforced plastic (FRP) plates. The work was divided into two phases. The first phase investigated the influence of FRP plate length, fiber orientation, and surface preparation on the performance of FRP-reinforced RC beams. The FRP in this phase of the work was E-glass/epoxy. The second phase of the study was designed to confirm the validity of our hypothesis, which postulated that the delamination of the FRP plate would be a function of the Poisson ratio mismatch between concrete and FRP plate. Results from the second phase also suggested that less expensive glass FRP plates could adequately replace the more expensive carbon FRP plates by offering the beam more ductility, without sacrificing its expected performance. Moreover, results from both phases showed that the lateral stiffness of the FRP plate contributed to the overall flexural stiffness of the strengthened beam.

Effect of texture on acoustic emission produced by slip and twinning in AZ31B magnesium alloy—part II: clustering and neural network analysis

The mechanical behaviour of extruded AZ31B magnesium alloy with two different textures was studied. Acoustic emission (AE) was used to record the signals to detect the various deformation processes occurring during the mechanical test. Hierarchical clustering and Kohonens self-organising neural network map were used to process the data from AE and to develop a relationship in terms of the deformation modes. The signals produced from the two different deformation mechanisms (i.e. slip and twinning) were successfully identified. The developed method could be efficiently used for the classification of AE data.

An Engineering Approach for Design and Analysis of Metallic Pipe Joints Under Torsion by the Finite Element Method

Design and stress analysis of pipe joints are still matters of controversy with respect to a unified design approach, despite the fact that many exact and finite element solutions have been presented in the literature. Owing to the complicated and lengthy nature of most exact solutions, development of an applied method for optimized design and stress analysis of a strong and low-cost joint remains a pressing issue. In this work, a simple method was developed for assessing the behaviour of adhesively bonded tubular joints under torsion, based on a parametric study conducted by ABAQUS finite element software. Many case studies of typical metallic joints under torsion were considered for examining the interactions among the main parameters governing the joint performance (i.e. adhesive thickness, pipe and coupling thickness/diameter, joint length, and material properties). Finally, a prototypical joint was designed by using the developed design curves, and the stress distributions were verified by the same software.

Differential Quadrature Approach for Delamination Buckling Analysis of Composites with Shear Deformation

The differential quadrature method (DQM) is used to analyze the one-dimensional buckling of a laminated composite beam plate having an across-the-width delamination located at an arbitrary depth and an arbitrary location along its span. A beam theory with shear deformation is used in formulating the problem. Several case studies are conducted to examine the buckling response of laminates hosting such a delamination. Using DQM, the system of equilibrium equations and the boundary conditions are transformed into a system of linear algebraic eigenvalue equations that are solved by a standard eigensolver. The influences of several parameters that affect the buckling strength of such laminates are investigated. The investigated parameters are the shear deformation factor, the length of the delamination, and the through-the-thickness and longitudinal positions of the delamination. The results verify the accuracy and efficiency of DQM.

Computational simulation and experimental verification of a new vibration-based structural health monitoring approach using piezoelectric sensors

Detection of damage in vital and high-cost infrastructures has been one of the major concerns of operators of such structures in the past two decades. The growing demands in oil and gas have forced extraction of such vital fuels in deep waters. Bolted joints are widely used in pipes transporting oil and gas in deep waters. However, they do develop in-service problems, such as loosened bolts, which if remained undetected, could cause significant environmental damage and economical loss, especially in the case of pipes used in deep-water oil extraction. In this study, the energy damage index (EDI) calculated based on a novel vibration-based damage detection methodology using the empirical mode decomposition (EMD) is used to establish the existence of damage due to loosened bolts in common industrial bolted joints. A complete finite element (FE) model is established to simulate the whole process using the implicit dynamic solver of the commercial software ABAQUS©. Also, a comprehensive experimental program was designed and carried out to verify the FE results. Results show that the EDI based on the EMD method is a powerful tool for not only detecting the damage, but also the progression of the damage in bolted joints.