WO1993004350A1

WO1993004350A1 - Optical fiber strain transducer having a radius of curvature equal to or less than a critical radius of curvature

Info

Publication number: WO1993004350A1
Application number: PCT/US1992/004275
Authority: WO
Inventors: James Richard Houghton; Dale Alan Wilson; Robert Lee Wood; Andrew Khung-Chue Tay; Abdullah Caner Demirdogen
Original assignee: Avco Corporation
Priority date: 1991-08-25
Filing date: 1992-05-20
Publication date: 1993-03-04

Abstract

A strain transducer (14) is responsive to a deflection thereof for providing a detectable output signal having a characteristic that is expressive of the deflection. The transducer includes an optical fiber having an input port (16) for coupling to a source (18) of optical radiation and an output port (20) for coupling to a receiver (22) of the optical radiation. The optical fiber has at least one region (14') that has a radius of curvature that is equal to or less than a critical radius of curvature for the optical fiber. The at least one region includes a detectable loss in the optical signal in response to being deflected. The optical fiber is disposed within a plane and the detectable loss is shown to be greater for an out-of-plane deflection than for an in-plane deflection. The strain transducer is shown to be especially useful for inclusion within a laminated composite structure, such as a component of an aircraft wing.

Description

OPTICAL FIBER STRAIN TRANSDUCER HAVING A RADIUS OF CURVATURE EQUAL TO OR LESS THAN A CRITICAL RADIUS OF CURVATURE

FIELD OF THE INVENTION:

This invention relates generally to strain transducers and, in particular, to a fiber optic strain transducer having a specific geometry. The invention also relates to structures, including composite structures, having one or more of the strain transducers disposed upon or within the structure.

BACKGROUND OF THE INVENTION;

A significant amount of effort has been directed towards developing integral instrumentation for structures, including structures employed as external surfaces for manned and unmanned vehicles. Such structures are known in the art as "smart structures" and "smart skins". Examples of vehicles benefitting from such technology include, but are not limited to, aircraft, space platforms, off shore oil drilling platforms, bridges, buildings, and submarines. Structures of particular interest are composite structures comprised of a number of plies or layers of material that are bonded together into a strong yet light-weight integral member. By example, modern aircraft wing sections may be comprised of composite materials. The use of integral strain transducers embedded between the plies of the structure, or bonded on the surface of the structure, is of particular interest in that such transducers allow for in-flight damage assessment, routine maintenance checks of composite structure internal integrity, and also enable a dynamic determination of wing stress to be made during flight.

Fiber optic-based transducers are particularly attractive for such applications for a number of reasons. Firstly, optical fibers typically have diameters that are compatible with the inter-ply spacing of many composite structures. Secondly, optical fibers are inexpensive, lightweight and, when properly supported, are sufficiently rugged to withstand the stresses experienced during use. A third important consideration is that transducer systems that employ optical wavelengths do not radiate any significant amount of detectable electromagnetic or thermal radiation into the environment. Nor are such transducer systems subject to false readings or damage due to external electrical or thermal radiation.

However, several problems have heretofore impeded the successful integration of optical fiber strain transducers into structures. A first problem relates to generating an adequate signal from the embedded transducer so as to enable an accurate determination of a strain induced within the structure. A second problem relates to generating a signal that accurately indicates a magnitude of a strain at a desired localized region of the structure, as opposed to a total signal indicating the accumulated strain applied to the entire structure.

It is thus an object of the invention to provide an optical fiber strain transducer having at least one region that has a radius of curvature equal to or less than a critical radius of curvature.

It is another object of the invention to provide an optical fiber strain transducer having a planar lay-out geometry that is readily incorporated within or upon a structure and that is sensitive to an out-of-plane bending.

It is a further object of the invention to provide an optical fiber strain transducer disposed in a serpentine pattern that includes regions having a radius of curvature selected to be a function of a predetermined critical radius of curvature.

It is another object of the invention to provide a structure that includes one or more integral optical fiber strain transducers, each of the transducers having a radius of curvature that is equal to or less than a critical radius of curvature. A still further object of the invention is to provide a component that is comprised of a composite structure including one or more integral optical fiber strain transducer(s) , each of the transducers including at least one curved region having a curvature that is equal to or less than a critical radius of curvature for the optical fiber.

SUMMARY OF THE INVENTION

The foregoing problems are overcome and other advantages are realized by a transducer that is responsive to a deflection thereof for providing a detectable output signal. The transducer includes an optical fiber having an input port for coupling to a source of optical radiation and an output port for coupling to a receiver of the optical radiation. During use, the transducer is coupled to an optical time domain reflectometer (OTDR) or equivalent signal processor. The optical fiber has at least one region that has a radius of curvature that is equal to or less than a critical radius of curvature for the optical fiber. This region induces a detectable loss in the optical signal in response to being deflected. The optical fiber is mounted in a plane and the detectable loss is shown to be greater for an out-of-plane bending than for an in-plane bending.

In accordance with one embodiment of the invention the optical fiber includes a plurality of regions each having a radius of curvature that is equal to or less than the critical radius of curvature for the optical fiber, each of the plurality of regions being separated by a segment of the optical fiber having a radius of curvature that is greater than the critical radius of curvature.

In accordance with another embodiment of the invention the optical fiber includes a plurality of contiguous regions each having a radius of curvature that is equal to or less than the critical radius of curvature.

BRIEF DESCRIPTION OF THE DRAWING

The above set forth and other features of the invention are made more apparent in the ensuing Detailed Description of the Invention when read in conjunction with the attached Drawing, wherein:

Fig. la is a block diagram showing a serpentine optical fiber strain transducer that is coupled to input and output devices in accordance with one embodiment of the invention;

Fig. lb is a block diagram showing a serpentine optical fiber strain transducer that is coupled to input and output devices in accordance with another embodiment of the invention; Fig. 2 is a graph showing a change in optical fiber transmission as a function of fiber bending radius and illustrates a critical radius where a slope of the transmittance curve changes abruptly;

Fig. 3a illustrates a plane containing an optical fiber strain transducer in an unstrained condition while Figs. 3b-3d illustrate the application of an out-of-plane strain causing a bending around a y-axis, the application of an out-of-plane strain causing a bending around an x-axis, and the application of an in-plane strain causing a bending around a z-axis, respectively;

Fig. 4a depicts a non-deformed strain transducer; Fig. 4b shows the transducer deformed due to bending; Fig. 4c shows the transducer deformed due to a uniform tensile stress field; and Fig. 4d shows the transducer deformed due to a uniform compressive stress field;

Fig. 5 illustrates the changes of radius of curvature on a circular loop due to bending around the y axis, Ryy, with the curvature direction being indicated in Fig. 3b;

Fig. 6 illustrates the changes of radius of curvature due to combined bending and a uniform compression stress field acting in the xy plane, with Fig. 4(d) illustrating an example of the changes caused by compression;

Fig. 7 illustrates the resultant light attenuation along the fiber due to the radius changes depicted in Fig. 6, resulting in an integration of the attenuation at each curved section due to co pressive stress bending;

Fig. 8 illustrates the changes of light loss at the exit of a 180 degree loop as the radius of curvature becomes larger, wherein the compressive stress field causes a significant loss of light; the tensile stress field causes an increase in light output; and the bending of the mounting surface causes a small change in light loss after the radius of curvature exceeds approximately four inches;

Fig. 9 illustrates that a transducer mounted orthogonal to the bending axis does not detect the tensile and compression stress fields due to cancellation, and that no stress sensitivity is indicated for the light output for curvatures below approximately four inches; and

Fig. 10 is a cut-away view of a composite structure showing several embodiments of an optical fiber strain transducer embedded within the composite structure. DETAILED DESCRIPTION OF THE INVENTION

Referring to Fig. la there is shown a strain measuring system 10 that includes a substrate 12 upon which an optical fiber strain transducer 14 is mounted. In accordance with an embodiment of the invention the transducer 14 has an undulating, serpentine shape comprised of a plurality of curved regions 14'. The transducer 14 has an input coupler 16 that is optically coupled to a source 18 of electromagnetic radiation. Typically the source 18 is comprised of a light emitting diode (LED) or a laser diode which may be operated in either a pulsed mode or a continuous wave mode. The transducer 14 also includes an output coupler 20 that is optically coupled to a receiver 22. The receiver 22 is typically a photodetector such as a silicon photodiode. Coupled to the receiver 22 is a processor 24 for converting the received optical radiation into an indication of an amount of bending loss experienced by the transducer 1 . The bending loss is a function of an amount of deflection experienced by the substrate 12 to which the transducer 14 is physically coupled. The deflection typically results from an externally applied force although, for composite structures, the deflection may also be a result of externally induced damage and /or internally generated stresses. In accordance with the invention each of the curved regions 14' has a radius of curvature that is a function of a critical radius of curvature of the fiber, and is selected to be equal to or less than the critical radius of curvature.

Fig. 2 is a graph showing a change in optical fiber transmittance, expressed as output power divided by input power, as a function of fiber bending radius. Fig. 2 also illustrates the critical radius of curvature (CR) where a slope of the transmittance curve changes abruptly. The critical radius of curvature has a value that is a function of material properties of the particular fiber type and the wavelength of the source 18. For the example illustrated the wavelength of the source 18 is approximately 850 nanometers and the critical radius of curvature is approximately 0.23 inches. By operating the transducer 14 at or near the critical radius of curvature, that is on the relatively steep portion of the transmittance curve, a controlled and known amount of optical attenuation is experienced by the radiation that passes through the transducer 14. Any change in the radius of curvature results in a significant change in the bending loss and, consequently, in a detectable change in the amount of radiation that passes through the transducer 14. The relationship of the radius of curvature of the transducer 14, relative to the critical radius of curvature, is described in greater detail below. In Fig. lb the components of Fig. la are arranged such that the input coupler 16 and the output coupler 20 are located at the same end of the transducer 14. The opposite end of the transducer 14 is made optically flat. This embodiment of the system 10 operates by detecting back-scattered radiation from a transducer probe pulse in accordance with an Optical Time Domain Reflectometry (OTDR) method. Other techniques, such as Optical Frequency Domain Reflectometry (OFDR) , may also be employed.

It is also within the scope of the invention to provide a correlation method such as that first introduced by M.J.E. Golay in "Complementary Series", IRE Trans., IT-7, 82, (1961) wherein a pair of signals having complementary codes are introduced as transducer probe pulses. The individual autocorrelations of the two codes contain side lobes. However, when the autocorrelations are added together, the sidelobes cancel exactly.

Whatever technique is employed to probe the transducer 14 , the regions 14 ' having the selected radius of curvature provide the transducer 14 with an enhanced sensitivity to deflection that is not found in the prior art.

Reference is now made to Figs. 3a-3d and Figs. 4a-4d in regard to the following discussion of the attenuation of light as a function of the curvature and stress upon the optical fiber transducer 14.

The strain sensing optical transducer 14 is provided as a plurality of half circles having a common initial radius. For simplification of this discussion the mathematical examples are presented for one circular loop and an independent variable (t) that is expressed in

10 degrees from zero to 360.

It is noted that as employed herein strain is considered to be a difference in an extended length (1.) from an unstrained gage length (1 ), -^> the difference being divided by the unstrained gage length.

Diagrams of the circular loop transducer 14 in three positions, which are referred to as a ²⁰ model A and a model B, are shown in Figs. 3a-3d. The circular loop transducer can also be provided with straight sections between two half circular loops or as a contiguous string of half circular loops disposed in a sinusoidal fashion.

*_ r ^> These additional embodiments are shown in Fig. 10.

The significant variables that are discussed below include: the radius (a) of the optical

30 fiber loop; the radius (R_vv) of the transducer 14 about the x-axis; and R , the radius of the transducer 14 about the y-axis. The bending with radius R about the z-axis is shown in Fig. 3d for transducer model B. One of the more significant observations is that transducer 14 model B (Fig. 3d) has a very low net change of attenuation due to the effects of in-plane bending of the structure. For a sinusoidal transducer the distortion effects tend to be equal and opposite across the length of the transducer and to effectively cancel one another. As a result, the transducer of the invention is most advantageously employed for detecting bending induced by a force applied from without, rather than within, the plane of the transducer.

The transducer 14 is assumed to be originally provided upon a carrier or substrate. Because of a transducer 14 stress field environment, other significant variables are σ and σ The stress in the z direction is neglected in the present discussion.

The transducer 14 is also assumed to include a single mode optical fiber in that multi-mode fibers have many variables that contribute to attenuation and shifting of modes as the light passes around the curved path. A useful description of the physics of bend loss in a single mode optical fiber as a function of wavelength and bend radius can be found in an article by A. J. Harris and P.F. Castle, "Bend Loss Measurements on High Numerical Aperture Single-Mode Fibers as a Function of Wavelength and Bend Radius", IEEE Journal of Lightwave Tech., Vol. LT-4, Jan. 1986, pp. 34-40 and in a text on Single-Mode Fiber Optics by L. B. Jeunhomme Single-Mode Fiber Optics, Marcel Dekker, Inc., 1983, pp. 88-94.

Of course, multi-mode fibers may be employed if the multi-mode effects are properly compensated.

The transfer function relationship of the light attenuation to the radius of curvature is given by Equation (1) below. The following conditions and assumptions are employed:

(a) a transition loss component ln[P O/P_._.],_, is assumed to be zero; and

(b) the optical fiber is assumed to follow all displacements of the structure.

An important relationship that applies to the strain transducer 14 is the ratio of the output power, P , to the input power. P., as follows:

where [P /P-]_A is assumed to be symmetric at the input and output of the circular section. The pure bending loss coefficient is ._

( 2 ) 2 c = [ -__- ] exp ( -Bp ) vΦ where ,

p = radius of curvature of the optical fiber, and

A and B = wavelength of light and the optical fiber material property terms, as described by A. J. Harris et al. in the above-referenced article.

The radius of curvature of the optical fiber of the transducer 14 initially is (a) as shown in Fig. 4(a). An important aspect of the transducer 14 is the manner in which the radius (a) changes as a function of angle (t) as the circular loop is bent around a cylinder of radius R oriented along the y axis; as is shown in Fig. 4(b). The expression for the path of the transducer loop along the cylinder surface is given by the following expressions:

x__*(t) = R cos[a/R)cos(t) + C_L]; (3) y_c ^{(t ) = a sin} (^t) ^{? and} < ⁴ ) z (t) = R sin [a/R)cos(t) + C ] ; (5)

where the constant C. is a function of the reference axis. The reference axis may be selected such that C is zero. From vector analysis of the tangent to the curve and the curvature of the path (the radius of curvature is the reciprocal of the curvature of the path) there is found the relationship for the radius of curvature (a) as a function of angle (t) :

Modifications may be made to Equation (6) to express as a function of distance along the loop and to express those cases where there are two axes of bending (R and R_vv) • For the present discussion Equation (6) is used with (R) equal to (R ) .

Other significant parameters that may influence the optical path curvature, p(t) , are the stress field in the composite material. For this analysis it is assumed that the stress σx and σ y are uniform over the area of the loop. An example of the type of radius of curvature distortion that is caused by a tension stress in the x direction is shown in Fig. 4(c). A distortion due to a compression stress is shown in Fig. 4(d). Examining the equation of the loop without the presence of bending radius changes (R_vv) the loop equation is

^_(.)=ta(i₊(-^)-(J_ _:_))cost]iv

[a^(l ₊ ⁽^ⁱ⁾-⁽^^{) )}sin(t)]J The radius of curvature is found using the relationship from calculus that

(8)

\ VxA \

where

V = the velocity along the path, and A = the acceleration on the path.

The radius of curvature for the path of Equation (7) is given by:

The changes of the true radius of curvature (t) with respect to the angle p(t) , around one loop, are shown in Figs. 5 through 9 for an initial loop starting radius (a) of 0.21875 inches, a value selected to be less than the determined critical radius of the optical fiber. The contributions of Equation (6) for a transducer 14 bent around a radius R_vv, ranging from 1.0 to 10.0 inches, are shown in Fig. 5. The contribution of the stress acting in one direction (σ ) in compression is shown in Fig. 6. It can be seen from the scale on the left of Fig. 6 that the amount of contribution to the radius of curvature change from the stress field is significantly higher than the contribution from the bending of the surface.

A demonstration of the light attenuation due to the radius of curvature changes from bending and compressive stress is shown in Fig. 7. This figure represents a uniform stress field in a full loop (360 degrees) of fiber optics, as stressed in Fig. 3b, and shows the amount of attenuation increasing as the bending moment increases, i.e. the radius of curvature decreases. The representation in Fig. 7 is the distributed attenuation performance in its best position.

Another illustration of the net attenuation measured at the end of a 180° loop of optical fiber is shown in Fig. 8. This figure shows pure bending effects, uniform compression effects, and uniform tension effects, and how these effects change as the radius of curvature associated with the bending moment becomes larger.

The corresponding stress and bending on a 180° loop of optical fiber mounted in a plane perpendicular to that of Fig. 8 is shown in Fig. 9. This configuration is the Fig. 3d curvature and is orthogonal to the Fig. 3b curvature. It is pointed out that Fig. 9 shows insignificant sensitivity to tensile and compressive stress fields acting on the optical fiber loop. The difference in stress sensitivity between Fig. 8 and Fig. 9 is the direction to stress vector indication, and is an important aspect of the invention.

A function of the processor 24 of Fig. 1 is to determine the magnitude of the applied strain or stress from a change in the transducer 14 output signal. This determination is accomplished in accordance with the principles and relationships described above.

Referring now to Fig. 10 there is shown a cut-away view of a composite structure 30 comprised of a plurality of individual plies or layers 32. Such laminated structures may be comprised of, by example, graphite-epoxy or fiberglass-resin and may include from tens to hundreds of individual layers. Structure 30 may be a portion of an aircraft wing or may be any structural component where it is desirable to measure strain. Between two of the layers 32 are shown several embodiments of the optical fiber transducer 1 . Transducer 14a has a continuous serpentine shape that is approximately sinusoidal. A wavelength of the sinusoidal pattern is equal to or greater than approximately four times (CR) , where CR is the critical radius of the optical fiber at the wavelength of the source 18. Transducer 14b has sinusoidal regions interspersed with linear regions. Transducer 14c has a plurality of regions that describe half-circles interspersed with linear regions. For region 14c it should be 19

realized that as few as one such half-circular region could be provided. These various transducer embodiments (14a-14c) may be employed separately or in combination with one another. Transducers may also be provided between several pairs of adjacent layers 32. Also, these various transducer embodiments may be employed with other transducer types, such as temperature sensors, within a given structure.

For each of these embodiments (14a-14c) the strain transducer 14 is most sensitive, as described in detail above, to bending induced by a force applied out of the plane of the fiber and is less sensitive to a force applied in the plane of the fiber. Furthermore, and as is indicated in Fig. 2, for the embodiments of 14b and 14c a localized strain is more readily detected in that a greater optical attenuation is obtained for deflections imposed at the curved fiber regions as opposed to the linear regions. That is, the curved region(s) may be provided at known locations such that any change in the optical signal that is extracted from the transducer indicates that a deflection has occurred at the known location.

For each of the embodiments (14a-14c) it is advantageous to initially provide the transducer 14 preformed upon the substrate 12. In this regard the substrate 12 is comprised of a flexible material that is compatible with being incorporated within, or upon a surface of, the composite structure 30.

It should also be realized that the use of the strain transducer of the invention is not limited to only composite structures but may instead be employed with, for example, metallic and masonry structures.

Thus, v/hile the invention has been particularly shown and described with respect to specific embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention.

Claims

CLAIMSWhat is claimed is:

1. A transducer responsive to a deflection thereof for providing a detectable output signal having a characteristic that is expressive of the deflection, the transducer including an optical fiber having an input port for coupling to a source of optical radiation and an output port for coupling to a receiver of the optical radiation, the optical fiber having at least one region that has a radius of curvature that is equal to or less than a critical radius of curvature for the optical fiber, the at least one region inducing a detectable loss in the optical signal in response to being deflected.

2. A transducer as set forth in Claim 1 wherein the optical fiber is disposed upon a substrate.

3. A transducer as set orth in Claim 1 wherein the region is disposed within a plane and wherein the detectable loss is greater for an out-of-plane bending than for an in-plane bending.

4. A transducer as set forth in Claim 1 wherein the optical fiber includes a plurality of regions each having a radius of curvature that is equal to or less than the critical radius of curvature for the optical fiber, each of the plurality of regions being separated by a segment of the optical fiber having a radius of curvature that is greater than the critical radius of curvature.

5. A transducer as set forth in Claim 1 wherein the optical fiber includes a plurality of contiguous regions each having a radius of curvature that is equal to or less than the critical radius of curvature.

6. A transducer as set forth in Claim 1 wherein the source includes a pulsed source or a continuous source.

7. A transducer as set forth in Claim 1 wherein the input port and the output port are disposed at opposite ends of the optical fiber.

8. A transducer as set forth in Claim 1 wherein the input port and the output port are disposed at the same end of the optical fiber.

9. A transducer as set forth in Claim 1 wherein the optical fiber is a single mode optical fiber or a multimode optical fiber.

10. A transducer as set forth in Claim 1 wherein the transducer is interposed between two adjacent layers of a multi-layered laminated structure.

11. A transducer as set forth in Claim 1 wherein the transducer is disposed upon a surface of a structure.

12. A structure having at least one strain transducer, the at least one strain transducer including an optical fiber having an input port for coupling to a source of optical radiation and an output port for coupling to a receiver of the optical radiation, the optical fiber having at least one region that has a radius of curvature that is equal to or less than a critical radius of curvature for the optical fiber, the at least one region inducing a detectable loss in the optical signal in response to an externally induced or an internally induced deflection of the structure.

13. A structure as set forth in Claim 12 wherein the optical fiber is disposed within a plane and wherein the detectable loss is greater for an out-of-plane deflection than for an in-plane deflection.

14. A structure as set forth in Claim 12 wherein the optical fiber includes a plurality of regions each having a radius of curvature that is equal to or less than the critical radius of curvature for the optical fiber, each of the plurality of regions being separated by a segment of the optical fiber having a radius of curvature that is greater than the critical radius of curvature.

15. A structure as set forth in Claim 12 wherein the optical fiber includes a plurality of contiguous regions each having a radius of curvature that is equal to or less than the critical radius of curvature.

16. A structure as set forth in Claim 12 wherein the structure is comprised of a plurality of layers, and wherein the at least one region is disposed between two adjacent layers.

17. A structure as set forth in Claim 12 wherein the at least one region is disposed upon a surface of the structure.

18. A method of determining a deflection of an object, comprising the steps of:

coupling an optical fiber to the object, the optical fiber having at least one region that has a radius of curvature that is equal to or less than a critical radius of curvature for the optical fiber;

introducing an optical signal into the optical fiber, the curvature of the at least one region causing a predetermined loss to the optical signal;

extracting the optical signal from the optical fiber; determining a magnitude of the loss of the extracted optical signal; and

detecting a deflection of the object if the determined magnitude of the loss exceeds the predetermined loss.

19. A method as set forth in Claim 18 wherein the step of introducing includes a step of pulsing an optical source.

20. A method as set forth in Claim 18 wherein the step of coupling includes a step of providing the optical fiber with a plurality of regions each having a radius of curvature that is equal to or less than the critical radius of curvature for the optical fiber, each of the plurality of regions being separated by a segment of the optical fiber having a radius of curvature that is greater than the critical radius of curvature.

21. A method as set forth in Claim 18 wherein the step of coupling includes a step of providing the optical fiber with a plurality of contiguous regions each having a radius of curvature that is equal to or less than the critical radius of curvature.

22. A method as set forth in Claim 18 wherein the step of coupling includes a step of positioning the at least one region at a known location upon the object.

23. A method as set forth in Claim 18 wherein the step of determining includes a step of operating an optical time domain reflectometer means.