Appendix
Overview of Some Sensors and
Actuators Used for Smart Structure
Applications
A.1 Piezoelectric Materials
Piezoelectric materials have the ability to generate charge when subjected to mechanical stress, and conversely can elongate or contract when subjected to an electrical field. 1 Typical piezoelectric materials are: quartz, barium titanate, cadmium sulphide, lead zirconium titanate (PZT) and piezoelectric polymers polyvinyldene fluoride (PVDF), polymer films, and polyvinyl chloride (PVC).
A.1.1 Ceramics
Piezoelectric ceramic materials such as lead zirconate titanate [Pb(ZrTi)O3], known as PZT, exhibit the piezoelectric effect where the size of the deformation or voltage depends on crystal orientation. Properties of PZT are stable in the range - 22°C to + 155°C. PZT can be manufactured in thin plates, strips, or fibers suitable for embedding or surface bonding.
Piezoelectric properties are established by applying at elevated temperature a high electrical field in a direction known as the polling direction to align all the ferroelectric domains within the ceramic (i.e., to achieve constant polarization direction). Conventional PZT wafers or plates are generally polarized normal to the plane of the sheet. When a voltage is applied to the conducting layers on the surface of the sheet, the sheet deforms or develops in-plane forces.
Conventional PZT sheets are quite brittle (low strain to failure), limiting in actuation capability, and are intrusive if embedded in composite laminates. A new concept for the actuation and sensing of structures using PZT fibers and interdigitated electrodes has been developed (Fig. A.1). This material incorporates unidirectional PZT fibers (typically about 130 Ixm in diameter) into a matrix, producing a highly conformable and directional actuator material. The interdigitated electrode pattern provides the electric field in the direction of the fibers, and therefore the primary piezoelectric effect is also in the direction of the fibers. Thus, these actuators are 2 - 3 times more effective then piezoceramic wafers when used to excite in-plane motion.
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574 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES
generation of recovery stresses within the composites on heating have been shown to be related to the reversible martensitic transformation of the SMA wires,4 Studies have shown that there is little change to the transformation temperatures of the constrained SMA wires with increasing pre-strain, but the measurable transformation heats decrease significantly with increasing pre-strain.
A.3 Optical Fiber Sensors
In an optical fiber, the light travels down a central core that is surrounded by a cladding and a buffer layer. The core and cladding are made from silica, however the core is "doped" to increase its reflective index to create the waveguide for the light. There are two major classifications of optical fibers: viz. Singleand multimode fibers. Single-mode fibers typically have a core diameter of 10 Ixm, whereas the multi-mode fibers have core diameters between 50 and 100 Ixm. External diameters of the fiber range from 80 to 250 txm, depending on whether the fiber is singleor multi-mode and depending on the type and thickness of the buffer layer.
The fibers are capable of withstanding strains up to 5%. However, any microcracks in the cladding greatly degrade the fiber strain to failure. Moisture also significantly degrades the mechanical performance of the fiber. The polymer or metallic buffer layer provides the fibers with mechanical strength and durability (allows handling without damaging the fiber), and protects the fiber from environmental damage. The suitability of the optical fiber for embedding applications in composites will depend on the buffer materials. Common telecommunications fibers consist of a 62.5- ixm-thick ultraviolet (UV) cured acrylate layer. These coatings typically have an operational temperature range of - 40°C to + 85°C. A range of commercially available buffer coatings with their operating temperature ranges 1is tabulated in Table A. 1. Therefore for embedding applications in the most common high performance carbon/epoxy composite materials (with curing temperatures ranging from 120-180°C), fiber with polyimide buffer coatings are required.
Optical fibers can be used as sensors to detect a wide range of physical parameters (e.g., pressure, strain, temperature, chemical species, vibration) or
Table A.1 Temperature Ranges for Typical Optical Fiber Buffer CoatingsI
|
Minimum |
Maximum |
Buffer Coating Type |
Temperature (°C) |
Temperature (°C) |
Acrylate |
- 40 |
85 |
Tefzel |
- 40 |
150 |
Polyimide |
- 190 |
300 (375 short term) |
Aluminum |
- 269 |
400 |
Gold |
- 269 |
750 |
APPENDIX |
575 |
damage (e.g., inside composite structures or on the surface of metallic or composite structures). They have many advantages over conventional sensors including being light, small (intrusive when embedded in composites), low cost, very sensitive, and conformable. They have good spatial resolution, provide distributed or point sensing, have good fatigue/durability, are immune to electromagnetic interference, are safe in inflammable or explosive environments, operate over a wide temperature range, are capable of transmission over long distances, and are non-electrical and multifunctional. Sensing techniques may depend on modulation of the light in the fiber in amplitude, wavelength or frequency, phase, wavelength, polarization, optical backscatter, or modal distribution of the transmitted signal.5-8
The most common sensing technique used by many researchers incorporates the use of fiber Bragg grating sensors. A fiber Bragg grating sensor consists of a periodic modulation of the core refractive index of an optical fiber. Such a sensor is fabricated by forming defect sites in the glass matrix through exposure to intense UV light. 8 The effect of this periodic refractive index variation on propagating light is to reflect a narrow band of wavelengths back down the fiber, as is illustrated in Figure A.3. The peak wavelength of this reflected light is the
Bragg wavelength of the grating (An) and is given by equation (A.1): |
|
AB = 2nA |
(A. 1) |
where n is the effective core refractive index of the fiber and A is the period of the grating.
Temperature and strain both influence the Bragg wavelength because as they affect the physical properties of the fiber, resulting in changes to the refractive index and the period of the grating. The wavelength shift AABs, for an applied longitudinal strain Ae is given by:
AABs = As(1 - p ~ ) Ae |
(A.2) |
where p,~ is the photo elastic coefficient of the fiber and is given by:
Pa = n 2 / 2 [PiE - - "1) ( P l l - - P 1 2 ) ] |
(A.3) |
where Pll and Pl2 are the components of the fiber optic strain tensor and v is Poisson ratio. Similarly, for a temperature change of AT, the corresponding wavelength shift is given by:
Ahsr = An (1 + 0 AT |
(A.4) |
where ~ is the fiber thermo-optic coefficient. For silica fiber, the wavelength sensitivities of a fiber Bragg grating with AB of 1.55 ~m have been measured as 1.15 I~m/Ixe and 13 ~m/°C. 9 The various interrogation schemes are presented in Ref.A10
The reflected narrow spike central wavelength, AB, is linearly dependent on the grating period. Consequently, any extemal influences that act to alter the
576 COMPOSITEMATERIALS FOR AIRCRAFT STRUCTURES
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