A General Purpose Fiber-Optic Hydrophone Made of Castable Epoxy

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Since the normalized sensitivity is proportional to AL/L, the sensitivity of a hydrophone element based on the integrated strain on the surfaces of a flexural plate will also be proportional to the ratio of yield strength to Young's modulus. It is possible to use the size of the air gap in the dual-plate hydrophone element to control the maximum strain experienced by the glass fiber and to provide support at depths which are greater than those given be equation (5)23. This is accomplished, as shown in Figure 3, by choosing the plate spacing provided by the air gap to be twice the displacement of a single plate at the maximum operating pressure. The displacement of the center of a thin clamped plate, z, is given26 as 3(1 - zc= 128Eh3 (8) With the plates in contact, the hydrophone element will be protected from catastrophic failure at the expense of becoming acoustically insensitive until the pressure is once again reduced below 1max 3.1 Material Selection 3. HYDROPHONE ELEMENT CONSTRUCTION A castable epoxy was chosen as the capsule material so that the fiber coils could be cast directly into the sensing plates of the hydrophone. This also facilitated the fabrication of plates which were of uniform thickness. Casting the sensor capsule as a single unit also ensures that the boundary conditions at the wall were well matched and reproducible for both plates. TABLE 1. Measured Material Properties of E-CAST F-28 Property Mass density Dynamic Young's modulus (E) Dynamic shear modulus (G) Thermal coefficient of dynamic elasticity (lnE/aT) Thermal coefficient of dynamic elasticity ()lnG/aT) Static Young's modulus (Estat) Tensile strength (Tyjeld) Poisson's ration () Strength-to-modulusratio (TyjeidE) Value 1.15 3.4 0.2 1.15 0.09 0.41 0.2 0.39 0.2 3.0 0.2 6.8 0.4 0.39 0.02 Units gm/cm3 iO Pa 10 Pa %/°C %/°C 10 Pa 10 Pa The process which led to the selection of E-CAST F-28 with 215 hardner27 as the hydrophone element body and plate material involved the testing of seventeen different room temperature castable epoxy compounds and composites to determine their static Young's modulus, yield strength, and dynamic shear and Young's moduli as a function of temperature in the range from 0°C to 25°C and for frequencies 18 / SPIE Vol. 1367 Fiber Optic and Laser Sensors VIII (1990)
on the order of one kilohertz. The results of these measurements, on all seventeen materials, will be reported separately28 as well as the method29 and apparatus3° which was developed to automate the measurements of the dynamic shear and Young's moduli as a function of temperature. The static Young's modulus and tensile strength were determined by conventional techniques31. The measured properties of the E-CAST F-28 are listed in Table 1. This was the only epoxy material used in the construction of the capsule so in all subsequent discussion the term epoxy will be understood to refer to E-CAST F-28. 3 . 2 Resonance Frequency The gravest resonance frequency of the capsule sets an upper limit on the hydrophone bandwidth and also provides insight into the actual boundary conditions experienced by the plates. The theoretical resonance frequency of the sensing plates can be calculated from equation (3). Assuming ideally clamped plate boundary conditions, a resonance frequency of 1 1 .0 kHz was predicted for each plate. If the boundary was simply supported, rather than clamped, the resonance frequency would be 5.3 kHz. The actual resonance frequencies for both plates were measured by tapping on the plates and recording the resulting free decay response with a microphone and digital storage oscilloscope. Based on the period of twenty cycles, the resonance frequency of the top and bottom plates was determined to be 7.79 0.03 kHz and 7.77 0.01 kHz respectively. Since the measured resonance frequencies fall between the two ideal cases, the boundary conditions were considered to be a compromise between ideally clamped and simply supported. To further investigate the modes of vibration, a finite element model of the capsule was generated. That model predicted a resonance frequency of 7. 14 kHz which was within eight percent of the measured result. That small difference may be due to the perturbations induced by fiber coils and the attachment of the stem which housed the coupler, neither of which were included in the model. The modal shape is shown in the upper portion of Figure 4. It is interesting to note that a slight deformation of the outer wall occurs and hence the boundary condition is truly elastic33. The resonant mode for an acceleration induced response occurs at 8.95 kHz and is shown in the lower portion of Figure 5. If the plates are identical, this mode produces no net change in the interferometer's optical path length and therefore, it can be employed to evaluate the effectiveness of the acceleration canceling design. 3 . 3 Interferometer Fabrication The objective of this sensor design was to provide a means of fabrication that was relatively simple, yet integrates delicate fiber optic components into a single, rugged capsule that is suitable for sustained underwater use. The first step in the fabrication process was the construction of the all-fiber, Michelson interferometer. The flat spiral optical fiber coils were wound by hand between two lucite plates separated by a washer with a thickness of approximately 300 .tm. This allowed the optical fiber which had an acrylate coating with an outer diameter of 250 m to pass between the plates but prevented the individual turns from sliding over each other during the winding process. Each coil consisted of 4.50 meters of fiber with approximately one centimeter of fiber between the coil pairs. The coils were bonded with thin strips of rubber adhesive before they were removed from the winding jig to prevent unwinding prior to their attachment to the sensing plates. SPIE Vol. 1367 Fiber Optic and Laser Sensors VIII (1990) / 19
Page 1 and 2: A General Purpose Fiber-Optic Hydro
Page 3 and 4: ATMOSPHERIC PRESSURE Sensing Plate
Page 5: The reader is reminded that M is th
Page 9 and 10: 3 .4 Capsule Fabrication The task o
Page 11 and 12: Acoustic calibrations in water were
Page 13 and 14: 4 . 5 Acceleration Sensitivity and
Page 15 and 16: 5. DISCUSSIONOF RESULTS The results
Page 17: 23. D. Ricketts, "Model for a piezo

A General Purpose Fiber-Optic Hydrophone Made of Castable Epoxy

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