Horizontal-axis wind turbines (HAWTs) are growing in size and popularity for the generation of renewable energy to meet the world’s ever increasing demand. Long-term safety and stability are major concerns related to the construction and use-phase of these structures. Braking and active pitch control are important tools to help maintain safe and stable operation, however variable cross-section control represents another possible tool as well. To properly evaluate the usefulness of this approach, modeling tools capable of representing the dynamic behavior of blades with conformable cross sections are necessary. In this study, a modeling method for representing turbine blades as a series of interconnected spinning finite elements (SPEs) is presented where the aerodynamic properties of individual elements may be altered to represent changes in the cross section due to conformability (e.g., use of a mechanical flap or a “smart” conformable surface). Such a model is expected to be highly valuable in design of control rules for HAWT blades with conformable elements. Sensitivity and stability of the modeling approach are explored.
This study investigates the internal-frost damage due to ice crystallization pressure in capillary pores of concrete. The pore structures have significant impact on freeze-thaw durability of cement/concrete samples. The scanning electron microscope (SEM) techniques were applied to characterize freeze-thaw damage within pore structure. The digital sample was generated from SEM imaging processing. In the microscale pore system, the crystallization pressures at subcooling temperatures were calculated using interface energy balance with thermodynamic analysis. The largest crystallization pressure on the pore wall was used for the fracture simulation with the developed Extended Finite Element Model (XFEM). The largest crystallization pressure on the pore wall was used for the fracture simulation with the developed Extended Finite Element Model (XFEM). One comparison study between model simulation and test results indicates that internalfrost damage model can reasonably predict the crack nucleation and propagation within multiphase cement microstructure.
This study applied the AE techniques with statistical analysis to investigate the damage process of singleedge notched beam (SEB) tests with normal strength concrete specimens. The SEB tests with the labprepared NSC specimens were conducted by employing a clip-gauge controlled servo-hydraulic testing system and an AE damage detection system. It was found that the cumulative AE events with respect to the crack mouth opening displacement (CMOD) or the crack tip opening displacement (CTOD) correlate to the mechanical loading of the specimens. A Weibull rupture probability distribution was proposed to quantitatively describe the mechanical damage behavior under the SEB test. A bi-logarithmic regression analysis was conducted to calibrate the Weibull damage distribution with detected AE signals and to predict the damage process as a function of the crack opening displacements. The calibrated Weibull damage functions were compared among concrete specimens. The comparison results indicate the Weibull function calibrated with AE signals can describe the damage behavior of concrete beam specimens.
This paper presents the combined micromechanics analysis and finite element modeling of the electromechanical properties of piezoelectric structural fiber (PSF) composites. The active piezoelectric materials are widely used due to their high stiffness, voltage-dependent actuation capability, and broadband electro-mechanical interactions. However, the fragile nature of piezoceramics limits their sensing and actuating applications. In this study, the active PSF composites were made by deploying the longitudinally poled PSFs into a polymer matrix. The PSF itself consists a silicon carbide (SiC) or carbon core fiber as reinforcement to the fragile piezoceramic shell. To predict the electromechanical properties of PSF composites, the micromechanics analysis was firstly conducted with the dilute approximation model and the Mori-Tanaka approach. The extended Rule of Mixtures was also applied to accurately predict the transverse properties by considering the effects of microstructure including inclusion sizes and geometries. The piezoelectric finite element (FE) modeling was developed with the ABAQUS software to predict the detailed mechanical and electrical field distribution within a representative volume element (RVE) of PSF composites. The simulated energy or deformation under imposed specific boundary conditions was used to calculate each individual property with constitutive laws. The comparison between micromechanical analysis and finite element modeling indicates the combination of the dilute approximation model, the Mori-Tanaka approach and the extended Rule of Mixtures can favorably predict the electromechanical properties of three-phase PSF composites.
Recent studies showed that the active piezoelectric structural fiber (PSF) composites may achieve
significant and simultaneous improvements in sensing/actuating, stiffness, fracture toughness and vibration
damping. These characteristics can be very important in the application of civil, mechanical and aerospace
structures. The PSF is fabricated by coating the piezoceramic onto the silicon carbide core fiber with
electrophoretic deposition (EPD) process to overcome the fragile nature of the monolithic piezoelectric
materials. The PSF composite laminates are made of longitudinally poled PSFs that are unidirectionally
deployed in the polymer binding matrix. The PSF laminate transducer has electrical inputs/outputs that are
delivered through a separate etched interdigital electrode layer. This study analyzed the electromechanical
properties with the generalized dilute scheme for active PSF composite laminate by considering multiinclusions.
The well-known Mori-Tanaka approach was used to evaluate the concentration tensor in the
multi-inclusion micromechanics model. To accurately predict the transverse properties, the extended role of
mixtures were applied by considering the inclusions' geometry and shape. The micromechanical finite
element modeling was also conducted with representative volume element (RVE) to compare with the
micromechanics analysis on the electromechanical properties. The micromechanics analysis and finite
element micromechanical modeling were conducted with varied fiber geometry dimensions and volume
fractions. These comparison studies indicate the combined micromechanics models with the generalized
dilute scheme can effectively predict the electro-elastic properties of multi-inclusion PSF composites.
Internal-frost damage is one of the major problems affecting the durability of concrete in cold regions. This paper
presents micromechanics models and innovative sensor technologies to study the fundamental mechanisms of frost
damage in concrete. The crystallization pressure due to ice nucleation with capillary pores is the primary cause of
internal-frost damage of concrete. The crystallization pressure of a cylinder pore was formulated using interface energy
balance with thermodynamics equations. The obtained crystallization pressure on the pore wall was input for the fracture
simulation with the developed Extended Finite Element Model (XFEM). The XFEM fracture simulation on a
homogeneous beam sample with a vertical cylinder pore leads to a straight line. The XFEM simulation was also
conducted on the generated digital sample. The simulation results were favorable compared with the middle-notched
single edge beam bending specimen due to the open-mode fracture behavior in both cases. An innovative Time-Domain
Reflectometry (TDR) sensor was developed to nondestructively monitor the freezing process. The experimental data
shows that the TDR sensor signals can detect the freezing degree, an important input parameter to micromechanics
models. These studies indicate that the developed micromechanics models and TDR sensor techniques can be used by
the practitioners to evaluate internal-frost damage of concrete. Future work will incorporate the TDR sensor
measurements into micromechanics models to real-time predict the internal-frost damage process in concrete specimens.
The predicted freeze-thaw damage process will be verified with acoustic emission detection.
Recent studies showed that the active piezoelectric structural fiber (PSF) composites may achieve
significant and simultaneous improvements in sensing/actuating, stiffness, fracture toughness and vibration
damping. These characteristics can be of particular importance in various civil, mechanical and aerospace
structures. This study firstly conducted the micromechanical finite element analysis to predict the elastic
properties and piezoelectrical coupling parameters of a special type of an active PSF composite laminate.
The PSF composite laminates are made of longitudinally poled PSFs that are unidirectionally deployed in
the polymer binding matrix. The passive damping performance of these active composites was studied
under the cyclic force loadings with different frequencies. It was found that the passive electric-mechanical
coupling behavior can absorb limited dynamic energy and delay the structure responses with minimum
viscoelastic damping. The actuating function of piezoelectric materials was then applied to reduce the
dynamic mechanical deformation. The step voltage inputs were imposed to the interdigital electrodes of
PSF laminate transducer along the poled direction. The cyclic pressure loading was applied transversely to
the composite laminate. The electromechnical interaction with the 1-3 coupling parameter generated the
transverse expansion, which can reduce the cyclic deformation evenly by shifting the response waves. This
study shows the promise in using this type of active composites as actuators to improve stability of the
structure dynamic.
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