Shape memory alloys (SMAs) are a relatively new class of functional materials, exhibiting unique
thermo-mechanical behaviors, such as shape memory effect and superelasticity, which enable their great potentials in
seismic engineering as energy dissipation devices. This paper presents a study of the mechanical behaviors of
superelastic SMAs, specially emphasizing on the influence of strain rate under various strain amplitudes. Cyclic
tensile tests on superelastic NiTi SMA wires with different diameters under quasi-static and dynamic loadings were
carried out to assess their dynamic behaviors. An internal temperature variable which indicates the influence of
loading frequency under various strain amplitudes and different temperatures was introduced to the Liang's
constitutive equation of SMA. Numerical simulation results based on the proposed constitutive equations and
experimental results are in good agreement. The findings in this paper will assist the future design of superelatic
SMA-based energy dissipation devices for seismic protection of structures.
Concrete piles are widely used in the construction of civil infrastructures and it is important to perform the health
monitoring of concrete piles for safety purposes. In this paper, a piezoceramic-based innovative approach is proposed for
the damage detection and health monitoring of concrete piles. A multi-functional piezoceramic-based transducer device,
the smart aggregate, is developed for health monitoring purposes. An active-sensing network is formed by embedding
the proposed smart aggregates at the pre-determined locations in the concrete piles before casting. In the proposed
approach, one smart aggregate is used as an actuator to excite the desired waves and the other distributed smart
aggregates are used as sensors to detect the wave responses. An energy distribution vector is formed based on the
wavelet-packet analysis results of sensor signals. A damage index is formed by comparing the difference between the
energy distribution vectors of the health concrete pile and that of the damaged concrete pile. To verify the effectiveness
of the proposed approach, two concrete piles instrumented with smart aggregates are used as testing objects. One
concrete pile is intact and the other has a man-made crack in the middle of the pile. Experimental results show that the
there are differences between the energy distribution vectors of the damaged pile and that of the intact pile due to the existence of the crack. The proposed method has the potential to be applied to perform automated integrity inspections for new piles and for the long-term health monitoring of piles in services.
An improved two-dimensional constitutive model for Shape memory alloys (SMAs), which can describe both the shape
memory effect (SME) and super elasticity effect (SE) of the SMAs, is developed based on the previous work of Boyd
and Lagoudas, who used the thermodynamics theories of free energy and dissipation energy to derive the constitutive
law of SMAs. The improved model, which will combine the ideas of Brinsion's one-dimensional constitutive law and
the concepts of Boyd and Lagoudas' two-dimensional one, has a simple but accurate expression. The results of the
simulations show that the developed constitutive model can qualitatively describe the thermo-mechanical behaviors of
two-dimensional SMAs and can be used in the analysis of structures actuated by SMAs.
This paper presents studies of seismic response control of a frame structure braced with SMA (Shape Memory Alloy)
tendons through both numerical and experimental approaches. Based on the Brinson one-dimensional constitutive law
for SMAs, a two-story frame structure braced diagonally with SMA tendons is used as an example to simulate
numerically the vibration control process. By considering the temperature, different initial states and thermal properties
of the SMA tendon, and the variable intensity and frequency of earthquake input, the parameters of the system were
analyzed during the numerically simulation. The time histories of the displacement and hysteretic loops of the SMA
tendons were simulated under earthquake ground motion by using finite element method (FEM). To validate the
efficiency of the simulation, a shaking table test for the frame structure was conducted. Both numerical simulation and
experimental results show that the actively controlled martensite SMA tendons can effectively suppress the vibration of
the multi-story frame structure during an earthquake.
Vacuum-Assisted Resin Transfer Molding (VARTM) process was used to fabricate the nanocomposites through integrating carbon nanofiber paper into traditional glass fiber reinforced composites. The carbon nanofiber paper had a porous structure with highly entangled carbon nanofibers and short glass fibers. In this study, the carbon nanofiber paper was employed as an inter-layer and surface layer of composite laminates to enhance the damping properties. Experiments conducted using the nanocomposite beam indicated up to 200-700% increase of the damping ratios at higher frequencies. The scanning electron microscopy (SEM) characterization of the carbon nanofiber paper and the nanocomposites was also conducted to investigate the impregnation of carbon nanofiber paper by the resin during the VARTM process and the mechanics of damping augmentation. The study showed a complete penetration of the resin through the carbon nanofiber paper. The connectivities between carbon nanofibers and short glass fibers within the carbon nanofiber paper were responsible for the significant energy dissipation in the nanocomposites during the damping tests.
It is important to conduct early age strength monitoring of concrete structures because it will help speed up construction.
Piezoelectric-based strength monitoring method provides an innovative experimental approach to conduct concrete
strength monitoring at early ages. In the presented paper, piezoelectric transducers were embedded into the concrete
specimen during casting. The development of strength of concrete structures was monitored by observing the
development of harmonic amplitude from the embedded piezoelectric sensor at early ages. From the experimental
results, the amplitude of the harmonic response decreased with the increment of the concrete strength. The amplitude of
the harmonic response from piezoelectric sensor dropped rapidly for the first week and continued to drop slowly as
hydration proceeded which matches the development of the concrete strength at early ages. Concrete is heterogeneous
and anisotropic which makes it difficult to be mathematically analyzed. Fuzzy logic has the advantage to conduct
analysis without requiring a mathematical model. In this paper, a fuzzy logic system was trained to correlate the
harmonic amplitude with the concrete strength based on the experimental data. The concrete strength estimated by the
trained fuzzy correlation system matches the experimental strength data. The proposed piezoelectric-based monitoring
method has the potential to be applied to strength monitoring of concrete structures at early ages.
Health monitoring for reinforced concrete bridges and other large-scale civil infrastructure has received considerable attention in recent years. Traditional inspection methods (x-ray, C-scan etc.) are expensive and sometimes ineffective for large-scale structures. Piezoceramic transducers have emerged as new tools to health monitoring of large size structures due to the advantages of active sensing, low cost, quick response, availability in different shapes, and simplicity for implementation. In this research, piezoceramic transducers in the form of patches are used to detect internal cracks of a 6.1-meter long reinforced concrete bridge bent-cap. Piezoceramic patches are embedded in the concrete structure at pre-determined spatial locations prior to casting. This research can be considered as a continuation of an early work, where four piezoceramic patches were embedded in planar locations near one end of the bent-cap. This research involves ten piezoceramic patches embedded at spatial locations in four different cross-sections. To induce cracks in the bent-cap, the structure is subjected to loads from four hydraulic actuators with capacities of 80-ton and 100-ton. In addition to the piezoceramic sensors, strain gages, LVDTs, and microscopes are used in the experiment. During the experiment, one embedded piezoceramic patch is used as an actuator to generate sweep sinusoidal waves, and the other piezoceramic patches are used as sensors to detect the propagating waves. With the increase of number of and severity of cracks, the magnitude of the sensor output decreases. Wavelet packet analysis is used to analyze the recorded sensor signals. A damage index is formed on the basis of the wavelet packet analysis. The experimental results show that the proposed methods using piezoceramic transducers along with the damage index based on wavelet packet analysis is effective in identifying the existence and severity of cracks inside the concrete structure. The experimental results also show that the proposed method has the ability to predict the failure of concrete as verified by results from conventional microscopes (MS) and LVDTs.
Composite materials are widely applied in aerospace, mechanical and civil structures. Delamination of composite material happens due to aging, chemical corruption and mechanical vibration, among other factors. It is important to detect the delamination in the incipient stage before the delamination reaches a notable level. Piezoelectric material can act as both actuators and sensors. In this research, two composite plates are fabricated as test specimen, of which one has a small delamination and the other is healthy. Four PZT patches are bonded at four corners of each composite plate, and one PZT patch is bonded in the middle of the composite plate. Wavelet packet analysis is applied as the signal-processing tool to analyze the sensor data. A damage index is formed based on the wavelet packet analysis to show the existence and the severity of damage. The experiment results show the proposed method can detect the delamination. This sensitive method is suited for delamination detection of inaccessible composite structures without using additional excitation facility.