Won-Bae Na

Hybrid health monitoring of structural joints using modal parameters and EMI signatures

To develop a promising hybrid structural health monitoring system, which enables to detect damage by the dynamic response of the entire structure and more accurately locate damage with denser sensor array, a combined use of mechanical vibration and electro-mechanical impedance is proposed. For the verification of the proposed healthmonitoring scheme, a series of damage scenarios are designed to simulate various situations at which the connection joints can experience during their service life. The obtained experimental results, modal parameters and electro-magnetic impedance signatures, are carefully analyzed to recognize the connecting states and the target damage locations. From the analysis, it is shown that the proposed hybrid health monitoring system is successful for acquiring global and local damage information on the structural joints; hence, its effectiveness is verified.

Drag Coefficients of Stock and Stockless Anchors

In recent years, collisions between anchors and submarine power cables have attracted increasing research interest because of the rapid growth of offshore wind farms. Since the drag coefficients have not been calculated, conservative values have been assumed and used to simulate collisions. This conservative estimation is likely to result in unnecessarily heavy designs for the protective structures. Therefore, reliable calculations of the drag coefficients are required to reduce the design, manufacture, and installation costs of the protective structures. Here, the authors describe calculations of the drag coefficients of 2,000-kg stock and stockless anchors using numerical flow analyses. The authors show that the drag coefficients of the anchors converge to 1.0 and 0.8, respectively, regardless of the initial velocity and the Reynolds number.

Flow and Structural Response Characteristics of a Box-type Artificial Reef

We carried out flow and structural response analysis of a box-type artificial reef (AR), which is made of concrete and structural steel. From the flow analysis, the wake region and drag coefficient were evaluated and accordingly, the structural analysis was performed to evaluate the stress and deformation of the target reef by considering the pressure field obtained from the flow analysis. The concept of wake volume was presented to quantitatively estimate the wake region and its variation according to flow direction and velocity. From the results, it is shown that the flow responses are only sensitive to the flow direction; the structural responses are sensitive to both of the flow velocity and direction although the magnitudes are negligible; and the wake volume became 3.52 times the AR volume with an optimum installation condition (30 o , flow direction) of the target unit.

Anchor Drop Tests for a Submarine Power-Cable Protector

Submarine power cables are widely used for power transmission, such as between mainlands and offshore islands and from offshore wind farms to on-land substations. There are several ways to protect power cables from accidental loads. Protection includes concrete blankets, sand bags, bundles, tunnel-type protectors, and trenching. However, no design standard for power-cable protectors is currently available because of the varieties of cable protection solutions and man-made or natural hazards to submarine power cables. Thus, this paper presents anchor drop tests for a newly designed, matrix-type submarine power-cable protector assembled with reinforced concrete blocks, to make a safety assessment. Marine environments were surveyed at the target site and simulated in the test set-up. A 2-ton stock anchor was selected as the colliding object, and a 25-ton crane was prepared to drop the anchor. Preliminary tests were performed to investigate the effect of soil composition and protector arrangements on the test results. Finally, four field anchor drop test scenarios were designed, carried out, and analyzed, and a safety assessment was made for the submarine power cable. From the tests, it was found that, in addition to falling distances, the soil composition and saturation were significant factors for the settlement depth and damaged areas. Considering the settlement depth of soils, the damaged areas of the concrete blocks, and the damaged state of the pipes (safety zone), all of the test results showed that the mattress failed to protect the power cable from the anchor collision. The deformation, damage, and breakage of the pipe, which simulated the safety zone of the power cable, gave clues as to the reasons for the failure.

Finite element simulation of two-point elastic wave excitation method for damage detection in concrete structures

Two-point elastic wave excitation method is proposed to detect the damages (crack and deterioration). This method does not require any baseline signal to distinguish the damages. For verifying the proposed method, finite element simulations were carried out. Two models including 2-D plane and 3-D solid models were constructed, and non-reflecting boundary conditions were applied to the 3-D model to simulate a massive concrete structure. It is shown that cracks and deteriorations were successfully detected and their severities were well quantified through the finite element simulations. This baseline- free elastic wave method is useful for damage detection of concrete structures with or without nonreflecting boundaries.

Wave-attenuation estimation in fluid-filled steel pipes: The first longitudinal guided wave mode

Attenuations of the first (or fundamental) longitudinal guided wave modes propagating in liquid-filled steel pipes are numerically investigated. Several filling liquids transported by the steel pipe are considered in the investigation. In the numerical modeling stage, a sink is considered for abandoning standing wave modes caused by the internal liquids; hence, the attenuation dispersion curves become simpler. From the attenuation dispersion curves, two specific attenuation values corresponding to 1 MHz and 2.68 MHz are selected; then, the concept of parametric density is introduced to predict attenuation for a certain filling material. With this concept, it is possible to approximately calculate attenuation values without a complex numerical attenuation calculation. This investigation may provide fundamental data to inspectors using ultrasonic guided-wave techniques in the petrochemical industry and in the field of water supply, two branches of the economy that are always under pressure owing to the demand of increasing productivity and that are challenged owing to stricter environmental rules, thus necessitating promising, low-cost inspection techniques.

Structural health monitoring and risk alarming in plate-girder bridges under uncertain temperature condition

Even significant damage may cause very small changes in structural characteristics, particularly for large structures. Furthermore, these changes may go undetected due to changes in environmental and operational conditions. In this paper, the temperature-driven variability on a combined structural health monitoring (SHM) system is examined in a model plate-girder bridge. The combined SHM system consists of global vibration-based technique and local electro-mechanical impedance (EMI) based technique. First, dynamic modal parameters of the test structure are measured before and after the occurrence of flexural cracks at various temperatures. Also, EMI signatures are sensed before and after the changes in support systems at various temperatures. Next, the risk of damage-occurrence in the structure is alarmed by statistical pattern recognition of the signals. Damage-induced changes in the signals are distinguished from temperature-driven uncertainty. The effect of temperature variability is also assessed to estimate the accuracy of damage detection.

Anchor Dragging Analysis of Rock-Berm Using Smoothed Particle Hydrodynamics Method

This study presents dynamic responses of rock-berm structural system under anchor dragging and accordingly provides the characteristics of the stresses and displacements obtained. For the purpose, first, a rock-berm was modeled by the SPH (smoothed particle hydrodynamics) method and piecewise Drucker-Prager material model by facilitating the associated software package—ANSYS-AUTODYN. Second, 2-ton stockless anchor was modeled as a rigid body and eventually dragging external force was obtained. Then, the dragging velocity (1 and 2 m/s) was considered as a parameter to investigate the effect of its variation on the responses. Finally, the dragging tensile forces of the anchor cable were obtained and compared according to the dragging velocities. It is shown that the four-layer rock-berm gives the safety margin to the submarine power cable according to the unaffected gauge points near the cable. This safety is accomplished by the four layers (related to rock-berm height) and the number of rock particles at each layer (related to rock-berm widths).

Development of Geometrically Nonlinear Finite Element Analysis Examples for Computational Structural Analysis

An undergraduate course named computational structural analysis becomes more significant in recent years because of its important role in industries and the recent innovation in computer technology. Typically, the course consists of introduction to finite element method, utilization of general purpose finite element software, and examples focusing on static and linear analyses on various structural members such as a beam, truss, frame, arch, and cable. However, in addition to the static and linear analyses, current industries ask graduates to acquire basic knowledge on structural dynamics and nonlinear analysis, which are not listed in the conventional syllabus of the computational structural analysis. Therefore, this study develops geometrically nonlinear examples, which can help students to easily capture the fundamental nonlinear theory, software manipulation, and problem solving skills. For the purpose, five different examples are found, developed for the analyses of cables and cable nets, which naturally have strong geometrical non-linearity. In the paper, these examples are presented, discussed, and finally compared for a better subject development.

Output-only Modal Analysis Approach for Time-Unsynchronization Signals in Wireless Sensor Network

In this study, an output-only modal analysis approach for wireless sensor nodes is proposed on the basis of assumption that a target structure is a linear system. In order to achieve the objective, the following approaches are implemented. Firstly, an output-only modal analysis method is selected for the wireless sensor networks. Secondly, the effect of time unsynchronization on the modal analysis method is mathematically derived. Thirdly, a new modal analysis approach using complex mode-shapes is proposed to extract modal parameters from unsynchronized signals. Finally, the proposed approach is evaluated by numerical tests and experimental tests.