Encyclopedia of Smart Materials (Vols 1 and 2) - M. Schwartz (2002) WW Part 15 pps

Electrostrictive materials Example: Lead-magnesium niobate (PMN) Magnetostrictive materials Example: Terfenol-D Shape-memory alloys (SMA) Example: NITINOL Optical fibers Examples: Bragg grating, Fabry-Perot Electrorheological fluids (ER) Example: Alumino-silicate in paraffin oil r Used as sensor and actuators r Lower hysteresis and creep r More sensitive to temperature variations than piezoelectrics compared to piezoelectric r Potentially larger recoverable strain than piezoelectric r Higher force and strain capability than piezoceramics (typically, 1000 microstrain deformation) r Suited for high-precision applications r Suited for compressive load carrying components r Very durable r Low recoverable strain (0.15%) r Only for compression components r Nonlinear behavior r Large recoverable strain (8%) r Suited for low-frequency (0–10 Hz) used largely for actuation due to large force generation r Low voltage requirements r Slow response time r Complex constitutive behavior and low-precision application with large hysteresis r Suited for remote sensing of structures r Corrosion resistant r Immune to electric interference r Small, light, and compatible with r Used for sensing alone r Behavior is complicated by thermal stra advanced composite r Simple and quiet devices r Suitable for vibration control r Offers significant capability and flexibility for altering structural response r Low density r Low-frequency applications r Nonlinear behavior r Cannot tolerate impurities r Fluid and solid phases tend to separate r Not suitable for low temperature applic r High-voltage requirements (2–10 kV) r Higher η p/τ ratio than MR* y Magnetorheological fluids (MR) r Simple and quiet devices r Quick response time r Suitable for vibration control r Offers significant capability and r Nonlinear behavior r Higher density than ER flexibility for altering structural response r Low voltage requirements r Behavior not affected by impurities r Suitable for wide range of temperatures r Lower η p/τ ratio than ER* y Microphones r Low cost r Large dynamic range r Excellent linearity r Sensitive to turbulent flow r Need to achieve directionality in some active control systems (e.g., ducts r Need protection to dust, moisture, high temperature amplifiers (piezoelectric accelerometers) Loudspeakers r Low cost r Nonlinear behavior if driven close to maxim r Space requirement (backing enclosure) r Need protection to dust, moisture, high temperature, corrosive environment Electrodynamic and electromagnetic actuators Hydraulic and pneumatic actuators r Relatively large force/large r May need a large reaction mass to displacement capability r Excellent linearity r Extended frequency range r Space requirement r Large force/large r Low-frequency range (0–10 Hz for displacement capability in an annular armature When the coil is activated, the TERFENOL rod expands and produces a displacement The TERFENOL-D bar, coil, and armature are assembled between two steel washers and put inside a protective wrapping to form the basic magnetoactive induced strain actuator unit (7) The main advantage of terfenol is its high-force capability at relatively low cost (21) It also has the advantage of small size and light weight, which makes it suitable for situations where no reactive mass is required such as in stiffened structures of aircraft and submarine hulls The disadvantages of TERFENOL include its brittleness and low tensile strength (100 MPa) compared to compressive strength (780 MPa) Its low displacement capability is also a major disadvantage especially in the low-frequency range (less than 100 Hz) In addition, it also exhibits large hysteresis resulting in a highly nonlinear behavior that is difficult to model in practical applications (20,21) Tani et al (20) have reviewed of studies on modeling the nonlinear behavior of TERFENOL-D as well as its application in smart structures Ackermann et al (22) developed a transduction model for magnetostrictive actuators through an impedance analysis of the electromagneto-mechanical coupling of the actuator device This model provided a tool for in-depth investigation of the frequency-dependent behavior of the magnetostrictive actuator, such as energy conversion, output stroke, and force The feasibility of using embedded magnetostrictive mini actuators (MMA) for vibration suppression has been investigated by (20) transmit large forces pneumatic; 0–150 Hz for hydraulic) r Need for hydraulic or compressed air powe r Nonlinear behavior r Space requirement Shape-Memory Alloys (SMAs) Shape-memory alloys (SMAs) are materials tha shape changes due to phase transformations a with the application of a thermal field Whe material is plastically deformed in its marten temperature) condition, and the stress is remo gains (memory) its original shape by phase tra tion to its austenite (high-temperature) condit heated SMAs are considered as functional mat cause of their ability to sense temperature a loading to produce large recovery deformations generation TiNi (nitinol), which is an alloy c approximately 50% nickel and 50% titanium, is commonly used SMA material Other SMA ma cluding FeMnSi, CuZnAl, and CuAlNi alloys have investigated (20,23) Typically, plastic strains of 6% to 8% can be c recovered by heating nitinol beyond its transitio ature (of 45–55◦ C) According to Liang and Roge straining the material from regaining its mem can yield stresses of up to 500 MPa for 8% plas and a temperature of 180◦ C By transformation martensite to austenite phase, the elastic modul nol increases threefold from 25 to 75 GPa, and stress increases eightfold from 80 to 600 MPa (2 SMAs can be used for sensing or actuation, they are largely used for actuation due to th force generation capabilities They have very lo Because of the numerous advantages they offer, several investigations on the application of SMAs have been carried out within the present decade Reviews of these applications, focusing on fabrication of SMA hybrid composites, analytical and computational modeling, active shape control, and vibration control, are presented in (20,23) Optical Fibers For many applications, ideal sensors would have such attributes as low weight, small size, low power, environmental ruggedness, immunity to electromagnetic interference, good performance specifications, and low cost The emergence of fiber-optic technology, which was largely driven by the telecommunication industry in the 1970s and 1980s, in combination with low-cost optoelectronic components, has enabled fiber-optic sensor technology to realize its potential for many applications (28–30) A wide variety of fiber-optic sensors are now being developed to measure strain, temperature, electric/magnetic fields, pressure, and other measurable quantities Many physical principles are involved in these measurements, ranging from the Pockel, Kerr, and Raman effects to the photoelastic effect (31) These sensors use intensity, phase, frequency, or polarization modulation (32) In addition, multiplexing is largely used for manysensor systems Fiber-optic sensors can also be divided in discrete sensors and distributed sensors to perform spatial integration or differentiation (33) Three types of fiberoptic strain sensors are reviewed in the following: extrinsic interferometric sensors, Bragg gratings, and sensors based on the photoelastic effect The most widely used phase modulating fiber-optic sensors are the extrinsic interferometric sensors Two fibers and directional couplers are generally used for these sensors One of the fibers acts as a reference arm, not affected by the strain, while the other fiber acts as the sensing arm measuring the strain field By combining the signals from both arms, an interference pattern is obtained from the optical path length difference This interference pattern is used to evaluate the strain affecting the sensing arm (e.g., by fringe counting) These sensors have a high sensitivity and can simultaneously measure strain and temperature One interferometer now being used in industrial applications is the Fabry-Perot interferometer, where a sensing cavity is used to measure the strain (34) This sensor uses a white-light source and a single multiple mode encapsulated version fiber, and provides absolute measurements This interferometer sensor is shown in Fig Bragg grating reflectors can be written on a fiber using a holographic system or a phase mask ate a periodic intensity profile (35) These senso used as point or quasi-distributed sensors The signal from these sensors consist of frequency com directly related to the number of lines per mill each grating reflector and, thus, to the strain ex by the sensor Fiber-optic sensors based on Bragg are used to measure strain and temperature, multaneously or individually (36) The Bragg gra traditionally interrogated using a tunable Fabry a Mach-Zender interferometer Recently, long-pe ings have been used to interrogate Bragg sensing (37) Bragg gratings have been used to measure v either directly or through the development of celerometers A typical fiber Bragg grating (FBG is illustrated in Fig The principle of operation of the sensors bas photoelastic effect is a phase variation of the ligh through a material (fiber) that is undergoing variation This phase variation can be produce effects on the fiber: (1) the variation of the length by the strain; (2) the photo-elastic effect and t dispersion caused by the variation of the diame fiber These sensors are classified in modal interf sensors and polarimetric sensors As it integ strain effect over its length, the modal interf sensor can act as a spatial filter if the propagation is given a spatial weighting (38) Reflected wave Incident wave Bragg grating Transmitte wave Figure Bragg grating on an optical fiber ER materials consist of a base fluid (usually a low viscosity liquid) mixed with nonconductive particles, typically in the range of to 10 m diameter These particles become polarized on the application of an electric field, leading to solidification of the material mixture Typical yield stresses in shear for ER materials are about to10 kPa The most common type of ER material is the class of dielectric oils doped with semiconductor particle suspensions, such as aluminosilicate in paraffin oil The material exhibits nonlinear behavior, which is still not completely understood by the research community This lack of understanding has hindered efforts in developing optimal applications of ER materials However, electrorheological fluids may be suitable for many devices, such as shock absobers and engine mounts (23,25) Magnetorheological Fluids (MR) Magnetorheological fluids (MR) are similar to ER materials in that they are also controllable fluids These materials respond to an applied magnetic field with a change in the rheological behavior MR fluids, which are less known than ER materials, are typically noncolloidal suspensions of micron-sized paramagnetic particles The key differences between MR and ER fluids are highlighted in Table In general, MR fluids have maximum yield stresses that are 20 to 50 times higher than those of ER fluids, and they may be operated directly from low-voltage power supplies compared to ER fluids which require high-voltage (2–5 kV) power supplies Furthermore, MR fluids are less sensitive to contaminants and temperature variations than are ER fluids MR fluids also have lower ratios of η p/τ y than ER materials, where η p is the plastic viscosity and τ y the maximum yield stress This ratio is an important parameter in the design of controllable fluid device design, in which minimization of the ratio is always a desired objective These factors make MR fluids the controllable choice for recent practical applications Several MR fluid devices developed by Lord Corporation in North Carolina under the Rheonetic trade name (23) Microphones Microphones are usually the preferred acoustic sensors in active noise control applications Relatively inexpensive microphones (electret or piezoelectric microphones) can be used in most active noise control systems because the frequency response flatness of the microphones is not critical in digital active control systems, as it is compensa identification of the control path The most comm of microphones are omni-directional, directional, microphones Whenever turbulent flow is present in the medium (e.g., a turbulent flow in a duct conveyin a fluid), turbulent random pressure fluctuations ated in the flow, adding to the disturbance pres The most common way of reducing the influence lent noise is to use a probe tube microphone a long, narrow tube with a standard microphone at the end The walls of the tube are porous o holes or an axial slit The probe tube microphon oriented with the microphone facing the flow P microphones are convenient as reference sensor control systems in ducts because they act as b tional sensors and turbulence filtering sensors D the principle of operation can be found in (39) microphone probes for hot corrosive industria ments are also available from Soft dB Inc Figure microphone adapted for such environments Displacement and Velocity Transducers Although their dynamic range is usually much that for accelerometers, displacement and veloc ducers are often more practical for very low fr (0–10 Hz) where vibration amplitudes can be of of a millimeter or more for heavy structures wh sponding accelerations are small Also, in lowactive control systems, displacement or veloci than acceleration can be the preferred quantitie imize The displacement and velocity transduce scribed below Proximity probes are the most common type o ment transducers There are two main types of probes, the capacitance probe and the Eddy curr Proximity probes allow noncontact measuremen tion displacements They are well suited to vibr placement measurements on rotating structure namic range of proximity probe is very small— 100 : for low-frequency applications (