Search

In this study, infrared thermography is used for vibration-based structural health monitoring (SHM). Heat sources are employed as sensors. An acrylic frame structure was experimentally investigated using the heat sources as structural marker points to record the vibration response. The effectiveness of the infrared thermography measurement system was verified by comparing the results obtained using an infrared thermal imager with those obtained using accelerometers. The average error in natural frequency was between only 0.64% and 3.84%. To guarantee the applicability of the system, this study employed the mode shape curvature method to locate damage on a structure under harsh environments, for instance, in dark, hindered, and hazy conditions. Moreover, we propose the mode shape recombination method (MSRM) to realize large-scale structural measurement. The partial mode shapes of the 3D frame structure are combined using the MSRM to obtain the entire mode shape with a satisfactory model assurance criterion. Experimental results confirmed the feasibility of using heat sources as sensors and indicated that the proposed methods are suitable for overcoming the numerous inherent limitations associated with SHM in harsh or remote environments as well as the limitations associated with the SHM of large-scale structures.

Modal parameter monitoring is a widely used structural health monitoring method. However, among other limitations, this method cannot effectively identify slight damage under ambient conditions. This study proposed a novel strain expansion&ndash, reduction approach for identifying damage. To verify the feasibility of the proposed method, we numerically and experimentally tested the method using a rigid acrylic frame. The frame was artificially damaged at various depths to reflect various damage scenarios. The increase in the damage index provided an accurate estimation of damage severity. For the case with merely 0.5% damage zone in one slat, the index is increased by 259% of the intact case. When the damage zone was doubled, the index increases significantly by 467% of the intact case, demonstrating excellent sensitivity of the proposed method. To guarantee practical use, the numerical model of the proposed method was applied to an offshore wind turbine jacket substructure and successfully identified multiple damage sites and the damage severity with extremely high (&gt, 10) damage index.

This paper presents a new method for the inversion of elastic properties of an isotropic thin plate by line-focus acoustic microscopy. Over 10 modes of the leaky Lamb waves of a 380 $$\upmu $$ m thick aluminum plate were extracted by the $$V(f$$ , $$z)$$ measurement. The inversion method of hybrid particle swarm-based-simulated annealing (PS-B-SA) optimization induces an objective function dependent on the determinant of the coefficient matrix of the dispersive characteristic equation. PS-B-SA allows considerable flexibility in parametric inversion problem and seeks the global rather than the local minimum. An alternative image display method combined with the objective function will be used to show the dispersion curves and to demonstrate the principles of the PS-B-SA optimization algorithm. The elastic properties (Young’s modulus $$E$$ , shear modulus $$G$$ , Poisson’s ratio $$\nu $$ ) and thickness (2 $$h)$$ of the specimen are determined by the inversion of the dispersion curves. Inversed parameters by particle swarm optimization algorithm are compared with the PS-B-SA results to show the validity and stability of the hybrid method. Agreement between the inversed material parameters and the reported data is shown to be excellent.

This paper investigates a new method for fabrication of broadband line-focus ultrasonic transducers by sol–gel spin-coating the poly(vinylidene difluoride-trifluroethylene) [P(VDF-TrFE)] copolymer film on a concave fine-polished beryllium copper backing. The ferroelectric hysteresis loops of the P(VDF-TrFE) films spin-coated from different molar ratios of VDF/TrFE, 77/23 and 55/45, were measured to select the better mixture. Owing to the better acoustic matching to water, compared with lead zirconate titanate (PZT), the fabricated transducers show relatively wide bandwidth of approximately 50 MHz with high central frequency of 60 MHz obtained at the focal plane when a fused-quartz acts as a reflecting target. Each one of the two finished transducers has a focal length of 5 mm and a full aperture angle of 90°. After applying the specially developed digital signal processing algorithm to the defocusing experiment data, which is called V ( f , z ) analysis method based on two-dimensional fast Fourier transform (2-D FFT), the operating frequency can extend from several MHz to over 90 MHz. Surface acoustic wave (SAW) velocities of a typical (1 0 0) silicon wafer was measured along various directions between [1 0 0] and [0 1 0] to represent the anisotropic features.

Structural analysis of FRP wind turbine blades must take into account phenomena associated with aerodynamics as well as fluid–structure coupling, because aerodynamic loading causes blades to bend mostly in the flapwise direction, and simultaneously causes foil sections to rotate to create new fluid fields around the foils. This study developed an analytical process for calculating fluid–structure interaction, while considering the effects of aerodynamic pressure and finite element analysis in the design of wind turbine blades. In addition, we calculated turbine power efficiency to evaluate the results of fluid–structure interaction displaying approximately power capacity loss of 17% at a wind speed of 25 m/s, and proposed three feasible improvements to enhance the performance of wind turbines. The presented study provided a comprehensible means by which to interpret changes in the aeroelastic response of blades, and was helpful to modify the original wind turbine model.

A sandwich assembly with core material enhances the bending stiffness of a structure and achieves lightweight requirements. Resin flowing behavior inside a sandwich assembly consisting of fiber layers and a segmented core is more complex than that for a fibrous assembly consisting of only fiber layers. This study presents infusion experiments on sandwich assemblies in the VARTM process, and compares the results to simulations from a 3D sandwich model including circular pipe elements to substitute for core grooves. Using the concepts of equivalent thickness to define the porosity of a sandwich assembly and porous space helps derive permeability trends and predictive equations to simplify permeability calculation. The deck infusing experiment in this study not only proves the permeability rationality of a sandwich assembly, but also provides concepts and suggestions for infusion arrangements. Consequently, the permeant characteristics in the sandwich assembly are applicable to manufacturing sandwich structures with good saturation.

Utilizing a sandwich structure enables FRP assembly to increase bending stiffness and achieve lightweight requirements. Core surfaces are often cut and infused together with fiber laminates in the VARTM process. Nevertheless, resin infusion in the sandwich structure differs from pure fibrous laminates because resin flows rapidly in the grooves and then saturates into the laminates. This research performed sandwich structure infusion experiments under the VARTM process, and defined four resin saturation stages inside the sandwich assembly by observing and explaining the nonlinear experimental flowing fronts. This study also discussed the race-tracking phenomenon of the sandwich structure in detail. This research executed infusion simulations in the 3D sandwich model, including fiber layers and grooves replaced by the circular pipe elements, to compare with the experimental results. Realization of the permeant characteristics in the sandwich structure establishes a base to ensure complete saturation and to analyze the manufacture of large sandwich structures.

Recently, the vacuum-injection method was considered the best approach for making the hulls of yachts. However, the resins show a print-through phenomenon (PTP) on the yacht surface when using unsaturated polyester resins (UP)/glass fiber and vinyl ester (VE)/glass fiber composites. The reason is the different degrees of curing within the UP or VE resin which leads to different thermal contractions of the composite. When the copolymer is prepared by the free-radical polymerization reactions of UP/ST, the variation in volume shrinkage is about 7–10%, thereby causing twists and wrinkles on the surface. To reduce these problems, organo-montmorillonite was added as filler to improve the dimension stability of the UP/ST/LPA matrix. Styrene (ST) is a bifunctional material, acting as a solvent and as a crossing agent in UP. LPA is a low-profile additive, which is applied for dimensional stability to improve the shrinkage. Flame-resistant improvement was also tested after adding organo-montmorillonite. Wide-angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM) techniques were applied to explain the morphology of the UP/ST/organo-montmorillonite nanocomposites. The volume shrinkage of the resin was determined by density measurements.

This paper investigates a modified method for fabricating focusing ultrasound transducers based on spin-coated poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer films. The ferroelectric hysteresis loops of the spin-coated P(VDF-TrFE) films are measured to establish the best fabrication parameters. The fabricated transducers are used for high-frequency wave velocity measurements and for non-destructive determination of elastic constants of thin isotropic plates. Both through-thickness longitudinal wave velocities and Rayleigh surface wave velocities of thin-plate samples with thicknesses ranging from 125 to 250 μm are measured, and hence their elastic constants can be determined in a straightforward and accurate way. The obtained results are in good agreement with the data reported by the manufacturers. Limitation of the proposed fabrication method and conventional method is compared and discussed in terms of signal intensity and frequency characteristics.

This paper develops broadband Poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] focusing transducers and its surface wave measurement system for determining the elastic constants of coating layers. The measurement is based on a defocussing measurement method and V(f,z) analysis of the focusing transducers. Few P(VDF-TrFE) focusing transducers are successfully fabricated in laboratory and the wave measurements can cover a wide frequency range of 4–120 MHz with great accuracy. Brass and glass substrates electroplated with nickel coating layer with thickness ranging from 15 to 60 μm are tested in this work. Dispersion curves sensitivity analysis is carried out to decide the best approach for inversely determination of coating elastic properties. A searching method based on the downhill simplex algorithm and numerical calculation on waves in a layered half-space model is used for inversely determining the elastic constants of coating layer. The results show good agreement with reported data. Measurement accuracy and potential applications for other types of nondestructive evaluation of the focusing transducers and measurement system are addressed.

When fiber-reinforced plastic (FRP) crafts sail at high speed, very large slamming forces impact the bottom structures of crafts. Therefore, the capacity to resist these forces is an important factor when designing crafts. In the past, the instability and complexity of material properties in failure analysis, in addition to the difficulties in conducting the laminate failure experiments due to water impact, resulted in few studies investigating the failure progress of FRP laminates subjected to water impact. This study presents a novel impulsive water-pressure failure experimental system that combines a hydraulic system with a water-pressure failure experimental fixture used in previous work, to generate impulsive water pressure. Using this system, the FRP laminates are deformed and damaged under water impact. In numerical analysis, the finite element software ABAQUS and user subroutine user-defined field (USDFLD) are utilized to calculate the strain field of laminates as a basis of failure evaluation and stiffness degradation of a structure after specifying failure criteria and stiffness modification curve obtained from material tests for different materials. By comparison with experimental data, the numerical model can be validated, regardless of whether the impulsive water loads are sufficiently small or large to cause structure failure. Consequently, based on the results of this study, the abilities of analyzing the dynamic failure progress under out-of-plane impulsive water pressure and the corresponding experimental systems can be well established. Furthermore, the capacity of FRP laminates in resisting water pressure can also be elucidated.

This study examines the progressive failure of marine glass fiberreinforced plastic (GFRP) laminates subjected to out-of-plane water pressure. First, experimental data concerning laminates that consist of a single type of fiber are used to establish the finite element model by separating the section into fiber layers and matrix layers. After a suitable failure criterion has been specified and the stiffness modification factor of different materials obtained experimentally, a numerical model is used to simulate the failure behavior of marine GFRP laminates. This numerical model is then applied to analyze the failure of GFRP ship bottom structures under water pressure. The water pressure failure experiment is designed to verify the numerical results. The experimental system includes a watertight cabin, which comprises the wall of the fixture, the specimen, and a movable piston, filled with water. Then, the water pressure is increased by moving the piston to compress the water-sealed zone, until the specimen reaches ultimate failure. After the numerical model has been validated, the abilities of testing and analyzing the failure behavior of laminates under static water pressure are established. More information is presented about the capacity to resist out-of-plane water pressure, the proper use of the material, and improving the efficiency of the design.

Marine glass fiber-reinforced plastic (GFRP) laminates, such as M-R laminates, have various tensile and flexural mechanical properties. This phenomenon holds not only for laminates composed of various fibers, but also for laminates composed of a single type fiber. Therefore, this paper will use both a theoretical and a numerical model to calculate the tensile and flexural Young's moduli, to elucidate the mechanism that explains the discrepancy. According to this, the numerical model of GFRP laminates made from a single type of fiber can be established from the data of material experiments, which are easily performed in the laboratory. The mechanical properties of laminates composed of different types of fibers can then be predicted precisely by this numerical model without further material experiments.

Offshore wind farm is a renewable energy that has been commercially operated in Europe over 3.8 GW capacities in 2011 and will be targeted upon 150 GW in 2030. Taiwan also plans to install 3 GW offshore wind farms in 2030. Thus, a number of condition assessments and engineering designs are being studied, such as the site selection, the wind turbines layout, the environmental impact assessment, the maritime engineering, the installation and maintenance fleets. In this paper, we carried out the winds and waves as the critical conditions of sea status for the installation of offshore wind turbines. The winds and waves data were collected by a data buoy which was deployed near the entrance of Taichung Harbor. The wind speeds were measured at 15 m above the sea surface. In the case study, we assumed a jack-up vessel will be used for installation of wind turbines. Then, some benchmarking conditions are filled into the assessment. For example, the conditions for operated criteria were suggested as wind speeds below 15 m/s and significant wave heights below 1.5 m in a water depth of 30 m. Moreover, a continuous working duration is considered as 15 hours for installation of a wind turbine from a tower, a nacelle, and blades. Eventually, the yearly available days for the wind turbine installation were assessed about 175 days. Different working modes, such as piling, cable laying, and maintenance, were discussed those yearly available days as well. From the operation and maintenance (O&M) supporting vessels working days (260 days), an offshore wind farm owned 50 wind turbines and each needs maintenance of 260 hours every year that was a benchmark scenario. The supporting vessels require 6 at least that had been estimated.

This paper summarizes the design and fabrication of several novel ultrasound transducers for measuring surface and Lamb waves. The experimental setup and measurement method for each developed transducer are also addressed.According to piezoelectric materials being used, two types of ultrasound transducers are discussed which are PVDF focusing transducers and micro-machined PZT transducers. For the PVDF transducers, a defocusing measurement along with a V(f,z) waveform processing technique are developed which allow accurately determination of dispersion curves of either surface waves of a layered medium or Lamb waves of a thin plate. Elastic properties of coating layers and thin plates are therefore non-destructive determined. For the micro-machined PZT transducers, a differential type wave velocity measurement is developed which is extremely sensitive to small wave velocity variation. Hence, the mechanical and dielectric loading effects of a fluid to the Lamb waves of a piezoelectric plate are experimentally and theoretically characterized, which can be used for acoustic sensing of fluids.

This research proposes a new type of focusing ultrasound transducer called Analytical Back Scattering Arrayed Ultrasound Transducer (ABSAUT). Different to the conventional focusing ultrasound transducer, an additional PVDF film with patterned electrodes is attached to the concave spherical surface for collecting back scattered ultrasound. It is designed for detecting and characterizing internal defects of a sample in an analytic and quantitative way via multiple back scattering ultrasound signals collection. Standard testing including pulse echo, transducer defocusing testing and sound field scanning are carried out and all the information is recorded for performance verification. Furthermore, a powerful angular spectrum algorithm is involved in the study for time-domain waveform prediction measured by the PVDF sensing elements. In this work, we have proved that ABSAUT can collect the reflected or back scattering sound wave information from the sample under testing. And the time-domain waveform predicted by angular spectrum algorithm also shows good agreement with the data measured by PVDF sensing elements. Finally, future improvements and applications of the ABSAUT will be addressed.

This research proposes a new type of focusing ultrasound transducer called Analytical Back Scattering Arrayed Ultrasound Transducer (ABSAUT). It is designed for detecting and characterizing internal defects of a sample in an analytic and quantitative way via collecting multiple back scattering ultrasound signals. Different from conventional focusing ultrasound transducers, a PVDF film with patterned electrodes is attached to the concave spherical surface for collecting back scattered ultrasounds. Standard testing including pulse-echo testing, transducer defocusing testing, tone burst source testing, and sound field scanning are carried out and all the information is recorded for performance verification. Furthermore, the angular spectrum analysis is adopted in predicting the time-domain waveforms measured by the PVDF sensing elements. In this work, we have proved that ABSAUT can collect the reflected or back scattering sound wave information from the sample under testing. And the time-domain waveforms predicted by angular spectrum algorithm also show good agreements with the data measured by PVDF sensing elements. Finally, future improvements and applications of the ABSAUT will be addressed

Degenerative osteoarthritis is recognized as the consequences of mechanical injuries. The abnormal impact force applied to articular cartilage would result in bone fracture or surface fissuring, and would cause the osteoarthritis [1,2]. The relation among the injury and impact energy was well studied. However, how the external energy attenuated to the internal joint is not carefully studied yet. The porcine knee joint was used as a biomechanical model for the simulation of human knee joint during impact loading. The objective of current study was to find the variation of kinetic characteristics between human and porcine knee joint during axial impact loading. Eight fresh-frozen knee joints from 10 month-old swine and seven cadaver human knee joints were used in the experiment. The mechanical responses such as forces and bending moment of knee joint, and the accelerations of femur was quantitatively analyzed. The results showed that the axial force response between human and porcine joints was similar, however, the anteroposterior shear, flexion bening moment and accelerations of these two joints were different.Copyright © 2003 by ASME

The components that share the loading of motion segment include the facet joint and disc. Nachemson [1] reported the facet joint share 18% of vertical loading in a motion segment; while many other researchers reported the load sharing percentage of facet joint ranges from 1% to 57% [2,3]. The current study developed a unique apparatus using an in vitro porcine spine model to quantify the alteration of loading in the facet joint under impact compressive loading at different loading conditions. A drop tower type impact apparatus was used to produce the impact energy for the motion segment. A 6-D load cell was placed under the specimen to detect the force and moment responses. The pressure sensor was inserted into the facet joint to find the contact force. The pointed axial compresive forces were applied at 8 locations from anterior to posterior of upper vertebrae to mimic different impact loading conditions. The impact energy was fixed at 1.2 J. We found that; when the loading was applied anteriorly, the facet joint sustained very small percentages of the loading; while the location of the loading moved posteriorly, the facet joint sharing percentages increased. The largest sharing percentages of facet joint reached 30% in the current study.Copyright © 2003 by ASME

The cyclic loading encountered due to exposure to whole body vibration has been implicated as a risk factor for occupational low back disorders (OLBD, Wilder, 1996). The frequent bending and lifting has been identified as a major risk for acute disc prolapse. Wilder (1985) has shown that cyclic loading causes a decline in the stiffness properties of the motion segments and predisposes them to more risk of buckling injury. Numerous studies have also demonstrated that soft tissues subjected to repetitive loading show creep and stress relaxation behavior because of their viscoelastic properties. The cumulative trauma and residual strain in the soft tissues due to repetitive loading may predispose the motion segment to altered load distribution—hence accelerating the process of disc degeneration. Although the association of load and risk of OLBD has been established, the nature of dose-response is less than clear. The current study developed a unique apparatus using an in vitro porcine spine model to quantify the alteration in the load response under cyclic compression loading at different loading conditions. The purpose of current project is to understand the mechanical gross response of the spinal motion segment during repetitive loading. The porcine spine motion segments were used in the study. Two group of loading condition were applied; one is the compression force evenly distributed on the top of vertebrae, and one is compression force at the anterior cortex of vertebrae. Both loading conditions were loaded for 90,000 cycles at 5 Hz. The total loading period is 5 hours. The loading was set at 200 N compression and 50 N tension from peak to peak. The results showed the spine is not stabilized even after 90,000 cycles of loading, and the evenly distributed loading condition obtained higher deformation than the anterior flexed loading condition.

In this paper we present a nondestructive method for determining the elastic constants of thin isotropic plates using poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)] copolymer ultrasound focusing transducers and Lamb wave measurements. Lensless P(VDF–TrFE) line-focus transducers are fabricated, and Lamb wave defocusing measurements are performed to measure the multimode Lamb wave dispersion curves of stainless steel and glass thin-plate samples of thicknesses ranging from 100 to 250 µm. Dispersion-curve-sensitivity analysis is carried out to determine the best method of inverse determination of the elastic properties. A search method based on the downhill simplex algorithm is applied to inversely determine the elastic constants of isotropic plate samples. The obtained results are in good agreement with the data reported by the manufacturers. Measurement accuracy and potential applications of the P(VDF–TrFE) ultrasound focusing transducers and measurements for other types of nondestructive evaluation are addressed.