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.
An improved multi-objective genetic algorithm for structural passive control system optimization is proposed. Based on
the two-branch tournament genetic algorithm, the selection operator is constructed by evaluating individuals according to
their dominance in one run. For a constrained problem, the dominance-based penalty function method is advanced,
containing information on an individual's status (feasible or infeasible), position in a search space, and distance from a
Pareto optimal set. The proposed approach is used for the optimal designs of a six-storey building with shape memory
alloy dampers subjected to earthquake. The number and position of dampers are chosen as the design variables. The
number of dampers and peak relative inter-storey drift are considered as the objective functions. Numerical results
generate a set of non-dominated solutions.
The problem of optimizing an absorber system for three-dimensional seismic structures is addressed. The objective is to
determine the number and position of absorbers to minimize the coupling effects of translation-torsion of structures at
minimum cost. A procedure for a multi-objective optimization problem is developed by integrating a dominance-based
selection operator and a dominance-based penalty function method. Based on the two-branch tournament genetic
algorithm, the selection operator is constructed by evaluating individuals according to their dominance in one run. The
technique guarantees the better performing individual winning its competition, provides a slight selection pressure
toward individuals and maintains diversity in the population. Moreover, due to the evaluation for individuals in each
generation being finished in one run, less computational effort is taken. Penalty function methods are generally used to
transform a constrained optimization problem into an unconstrained one. The dominance-based penalty function contains
necessary information on non-dominated character and infeasible position of an individual, essential for success in
seeking a Pareto optimal set. The proposed approach is used to obtain a set of non-dominated designs for a six-storey
three-dimensional building with shape memory alloy dampers subjected to earthquake.
Superelastic shape memory alloy (SMA) is a potential candidate for use in structural damping devices due to its unique
mechanical properties. An innovative re-centering SMA damper is presented. Being configurated simply, the device
comprises two functional groups of SMA strands, such as the un-pre-tensioned wires and the pre-tensioned wires,
resulting in a perfect energy dissipation compatible with a negligible residual displacement. Based on the cyclic loading
tests of the superelastic SMA wires, the Lagoudas simplified model is determined. Extensive experiments are carried out
to investigate the influence of cycles, frequency and displacement amplitude on the mechanical behaviors of the damper,
such as the secant stiffness, the dissipated energy per cycle and the equivalent viscous damping. By analyzing the
working mechanism, a model is set up to simulate the hysteretic curve of the damper, its feasible predictions being
validated by the experimental results. Furthermore, nonlinear time history analyses of a SDOF system are performed,
and the results show that the re-centering damper not only can decrease the vibration of the system under excitations, but
also can mitigate residual displacement after excitations.
Superelastic shape memory alloy (SMA) is a potential candidate for use in structure damping devices due to its unique
mechanical properties. In order to mitigate the vibration of a structure subjected to earthquake tremors from different
directions, an innovative, multi-directional SMA-based damper is advanced. The damper, with two movable cylinders
attached to four groups of SMA strands arranged in a radial symmetry, can not only function in a plane, but also can
work vertically and rotationally. Based on experimentation, the Graesser model of superelastic SMA is determined. By
analyzing the damper's mechanism working in different directions, the corresponding theoretical models are developed.
Numerical simulations are conducted to attain the damper's hysteresis. Working in a plane, the damper, with a 3%
initial strain, provides a rectangular hysteresis with the maximum amount of damping. A rectangular flag hysteresis can
be supplied in the absence of a pre-stress in the wires, going through the origin with a moderate amount of energy
dissipation and higher force capacity. Moreover, the damper has better working capacities (i.e. force, stroke and energy
dissipation) if the deflection is parallel to the internal bisectors of the tension axes. Working vertically or rotationally,
similar triangular flag hysteresis is generated with small energy dissipation and a self-centering capacity. For a given
deflection, the initial strain (3%) increases the force of the damper, but decreases its stroke.
Laser wavemeter with a Michelson interferometer is a high-precision instrument. The signal and data processing is
so important to this wavemeter. It includes an optoelectronic conversion module, an amplifying and comparator
circuit, a PLL unit, a counter, serial communications between the MCU and the computer. S1223 photodiodes are
selected as the detectors. PLL unit including NE564 and 74LS193 is used to multiply frequency. The timing
capacitor, loop gain coefficient, and the locking range of PLL are calculated. The counter is used to measure the
number of interference fringes of the reference laser and the number of interference fringes of an unknown laser.
Finally, the wavelength of an unknown laser is calculated and displayed on LCD. The signal and data processing of
the wavemeter meets its accuracy of seven-bit significant figure.
The superelastic shape memory alloys (SMAs) have received increasing interest attributed to their unique mechanical properties. Modeling of SMAs' thermomechanical behavior has been an active area of research; however the existing models are generally valid only for quasi-static loading conditions and extremely complex for practical use. In this research, one-dimensional cyclic loading tests of superelastic shape memory alloy wires are first performed to determine their hysteresis properties. The effects of the strain amplitude and the loading rate on the mechanical properties are studied and formulized by least-square method. Based on the Graesser's model, an improved model is developed. The improved model divides the full loop into three parts: the loading branch, the unloading branch before the completion of the reverse transformation and the elastic unloading branch after the completion of reverse transformation, each part adopts its respective parameters. The improved model not only has the same advantages as the Graesser's model, such as relative simple formulation with parameters that can be easily acquired and being valid for dynamic loading conditions, but also overcomes the deficiency of the Graesser's model, i.e. ignoring the effects of loading path on the model parameters. Numerical simulations are conducted. Comparisons indicate that the improved Graesser's model accurately reflects all the hysteresis characteristics and provides a better prediction of the SMA's actual hyteresis behavior than the Graesser's model at varying levels of strain and loading rate.
A wavemeter based on Michelson interferometer accurately measure static wavelength of a tunable laser. Its operation principle is formulated in details. Double longitudinal-mode He-Ne laser with frequency stabilization is used as the reference optical source of the wavemeter. Voice-coil motor using PID means can realize to move in uniform motion. Phase-locked loop circuit including NE564 and 74LS193 is used to enhance resolution of the wavemeter. Data processing is carried out by the counter unit including two 8254 programmable timer, a MCU, a LCD. The test shows that its measurement accuracy is 1×10-6 and is higher than those of other wavemeters such as Fizeau interference and Fabry-Perot wavemeter.