Patent Publication Number: US-2022221355-A1

Title: Sensing fibers for structural health monitoring

Description:
FIELD 
     The present disclosure relates to structural health monitoring, and in particular, to sensors for monitoring structural strain. 
     BACKGROUND 
     Structural health monitoring involves the monitoring of indicators of structural fatigue, failure, and fracture, in large infrastructure assets. Structural health monitoring may involve the use of sensors, such as accelerometers, strain gauges, displacement transducers, and other sensors, to collect data about a structural body (e.g. support column, roof of a building, wing of an aircraft). Such data may be analyzed for indications of strain or damage in the structural body. 
     SUMMARY 
     According to an aspect of the disclosure, a system for structural strain monitoring is provided. The system includes a structural body and a sensing fiber that extends through the structural body. The sensing fiber exhibits an electrical resistance that varies with deformation of the sensing fiber. The system further includes a processing unit to monitor the electrical resistance of the sensing fiber, determine a structural strain experienced by the structural body based on the electrical resistance, and output an indication of the structural strain. 
     According to another aspect of the disclosure, a device for structural strain monitoring is provided. The device includes a sensing fiber that extends through a structural body. The sensing fiber exhibits an electrical resistance that varies with deformation of the sensing fiber. The device further includes a processing unit to monitor the electrical resistance of the sensing fiber, determine a structural strain experienced by the structural body based on the electrical resistance, and output an indication of the structural strain. 
     According to another aspect of the disclosure, a method for structural strain monitoring is provided. The method involves monitoring electrical resistance of a sensing fiber extending through the structural body. The sensing fiber exhibits an electrical resistance that varies with deformation of the sensing fiber. The method further involves determining a structural strain experienced by the structural body based on the electrical resistance and outputting an indication of the structural strain. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an example system for structural strain monitoring that includes a sensing fiber. 
         FIG. 2  is a flowchart of an example method for structural strain monitoring. 
         FIG. 3A  is a schematic diagram of an example structural body under different circumstances of structural strain. 
         FIG. 3B  is a deformation-resistance plot that shows a relationship between the deformation of a sensing fiber and the electrical resistance exhibited by the sensing fiber. 
         FIG. 4  is a schematic diagram of another example system for structural strain monitoring that includes a sensing fiber, the sensing fiber including a stretchable fiber core surrounded by electrically conductive mesh. 
         FIG. 5  is a schematic diagram of a cross-section of an example structural body with a sensing fiber. 
         FIG. 6A  illustrates an example structural body with several sensing fibers fixed to a surface of the structural body. 
         FIG. 6B  illustrates an example structural body with layers of composite material and sensing fibers embedded between the layers of composite material. 
         FIG. 7A  illustrates an example wing of an aircraft with sensing fibers embedded therein. 
         FIG. 7B  illustrates an example support column with sensing fibers embedded therein. 
     
    
    
     DETAILED DESCRIPTION 
     The structural strain that is experienced by a structural body may be monitored using one or more point sensors (e.g. strain gauges) placed at specific points within or along the structural body. A point sensor monitors the local the deformations (e.g. bending, buckling) that take place around the point at which the point sensor is placed. For monitoring strains that cause larger deformations that span across a broader area of a structural body, a group of several point sensors may be placed at several different points spread throughout the broader area. 
     Although such a group of point sensors may provide limited information about a broad structural strain experienced across a broad area of a structural body, the completeness and the resolution of the structural strain information obtained by such sensors may be limited by the physical coverage of the point sensors. For example, a group of point sensor may miss information about a structural strain that is experienced directly at a gap between point sensors. As another example, even in the case of a larger strain that spans a broader area of the structural body, areas of the structural body in the gaps between the point sensors may contain important structural strain information that describes a characteristic of the larger strain, and this information may be missed. 
     An additional drawback of the use of point sensors embedded into a structural body is that such point sensors may compromise the structural integrity of the structural body itself. Due to the large size of conventionally used point sensors, these point sensors may create a point of failure, or weakness, within the structural body. 
     The present disclosure provides sensing fibers that may be used to more comprehensively monitor strains experienced by a structural body than a group of point sensors. As described herein, a structural body may be fitted with a sensing fiber that extends through a section of the structural body. The sensing fiber exhibits an electrical resistance that varies with deformation of the sensing fiber. Structural strain is determined by monitoring the electrical resistance across the sensing fiber as the structural body is deformed. 
     The sensing fiber reacts to strain experienced anywhere along its length, and thus structural strains experienced by the structural body may be monitored with fewer and/or smaller unmonitored gaps between sensors. The sensing fiber is a continuous sensor, and thus has no unmonitored gaps along its length. Several sensing fibers (e.g., in a mesh) may be placed throughout the structural body (e.g., in layers) to provide for broader (i.e., “global”) monitoring of the structural body. Further, the sensing fibers may be of sufficiently small size (i.e., thin), on a similar order as the size of fibers used in composite materials, and thus the sensing fibers may have a reduced impact on the structural integrity of the structural body when embedded in composite materials. 
       FIG. 1  is a schematic diagram of an example system  100  for structural strain monitoring. The system  100  includes a structural body  102 , such as a wall, support column, structural cable, hull of a ship, body of a vehicle, wing of an aircraft, blade of a wind turbine, or any other structural body that may experience structural strain. 
     The system  100  further includes a sensing fiber  110  that extends through the structural body  102 . That is, the sensing fiber  110  extends through at least a portion of the structural body  102 , which may be referred to as the monitored section. The sensing fiber  110  may be embedded into the structural body  102  (i.e., embedded into the material of the structural body  102  during manufacture of the structural body  102 ). The structural body  102  may be made of a composite material of structural fibers into which the sensing fiber  110  is embedded alongside the other structural fibers. The sensing fiber  110  may be softer (i.e., of a low Young&#39;s modulus) than the materials into which it is embedded, and thus may act as a passive reporter of its surrounding structural environment while having little structural impact on its surroundings. The sensing fiber  110  may include a polymer-based fiber core, and thus may be sufficiently flexible to deform along with any deformations experienced by the structural body  102 . 
     The sensing fiber  110  may span a majority of a dimension of interest (e.g., the entire length or width) of the structural body  102 . For example, where the structural body  102  includes a wing of an aircraft, the sensing fiber  110  may span from the base of the wing to the tip of the wing, and therefore may monitor structural strains that would appear anywhere along the length of the wing. 
     The sensing fiber  110  is to exhibit an electrical resistance that varies with deformation of the sensing fiber  110 . The sensing fiber  110  is electrically conductive along its entire length, and thus, the sensing fiber  110  may detect deformations that take place at any point along its length. 
     As the structural body  102  experiences a strain, the structural body  102  deforms, thereby causing the sensing fiber  110  to deform, and the electrical resistance exhibited by the sensing fiber  110  changes. Deformation of the sensing fiber  110  may refer to compression or elongation of the length of the sensing fiber  110 . 
     The property that the sensing fiber  110  exhibits variable electrical resistance with deformation of the sensing fiber  110  may be referred to as its piezo-resistive property. The change in electrical resistance that is exhibited by the sensing fiber  110  may be sufficiently great to produce a signal that may be detected by a resistance measuring device. For example, elongation of the sensing fiber  110  by about 10% may result in a change in electrical resistance of about 40%. 
     As described in greater detail with reference to  FIGS. 3A-3B  below, the change in electrical resistance may depend on the position of the sensing fiber  110  in the structural body  102  and the type of strain experienced by the structural body  102 . 
     The system  100  further includes a processing unit  120 . The processing unit is to monitor the electrical resistance of the sensing fiber  110 , determine a structural strain experienced by the structural body  102  based on the electrical resistance, and output an indication of the structural strain. 
     In some examples, the processing unit  120  may include a data acquisition unit that monitors the electrical resistance, and may further include one or more computing devices (e.g. remote servers) which determine the structural strain and output the indication of structural strain. Thus, the processing unit  120  may include any quantity and combination of a processor, a central processing unit (CPU), a microprocessor, a microcontroller, a field-programmable gate array (FPGA), and similar, and may further include a memory (volatile and non-volatile storage) and/or communication interface (e.g. network interface), to perform the functionality as described herein. 
     The processing unit  120  may monitor the electrical resistance of the sensing fiber  110  through any suitable resistance-measuring device and electrical lead lines in contact with the sensing fiber  110 . The electrical resistance may be monitored continuously or periodically by such a resistance-measuring device. Electrical resistance readings may be temporarily stored on the processing unit  120 , either for processing, or for temporary storage prior to transmission to a remote device for processing. 
     The processing unit  120  may determine the structural strain based on one or more known relationships between measured electrical resistances of the sensing fiber  110  and corresponding deformations of the sensing fiber  110 . That is, the sensing fiber  110  may have been tested to determine how an amount of deformation causes an amount of change in electrical resistance, or the sensing fiber  110  may have been manufactured to particular a specification which defines how the electrical resistance is to vary with deformation. Thus, the sensitivity of electrical resistance to deformation may be tuned to suit various applications. Such relationships may be expressed in a mathematical function (i.e., a mathematical function by which electrical resistance of the sensing fiber  110  may be calculated as a function of deformation), in a look-up table, or in other ways. 
     Further, the processing unit  120  may determine the structural strain based on one or more known relationships between deformations of the sensing fiber  110  and corresponding strains of the structural body  102 . That is, the structural body  102  may have been tested to determine how an amount of strain causes an amount of deformation in the sensing fiber  110 , or the structural body  102  may have been manufactured to a particular specification which defines how the deformation is to vary with strain. Again, such relationships may be expressed in a mathematical function (i.e., a mathematical function by strain experienced by the structural body  102  may be calculated as a function of deformation of the sensing fiber  110 ), in a look-up table, or in other ways. 
     Thus, a change in electrical resistance of the sensing fiber  110  may be related to a deformation in the sensing fiber  110 , which in turn may be related to a strain on the structural body  102 . 
     Although the relationship between the electrical resistance of a sensing fiber  110  and an amount of deformation of the sensing fiber  110  may be well understood (i.e., lab-tested), the relationship between an amount of deformation (or change in electrical resistance) of the sensing fiber  110  and an amount of structural strain actually experienced by the structural body  102  may be unique to the particular structural body  102 , or, at least may be overly cumbersome to calculate directly for each structural body  102  that is to be monitored by a sensing fiber  110 . Thus, in some examples, the processing unit  120  may determine the structural strain experienced by the structural body  102  by applying a machine learning model that is trained to determine the structural strain experienced by the structural body  102  based on the electrical resistance of the sensing fiber  110 . 
     In practice, a structural body  102  having a sensing fiber  110  may undergo structural strain testing whereby the structural body  102  is placed under known strains (e.g. of different intensity and/or in different directions), and the resulting electrical resistances of the sensing fiber  110  may be measured. Thus, a relationship between electrical resistances of the sensing fiber  110  and strains on the structural body  102  may be determined. In the case of training a machine learning model, the strain data (that describes the structural strains that the structural body  102  was placed under) and the resistance data (that describes the electrical resistances of the sensing fiber  110  measured under those structural strains) may be fed into the machine learning model. The machine learning model may thereby be trained to predict structural strain of the structural body  102  based on electrical resistances of the sensing fiber  110 . The machine learning model may be stored at the processing unit  120  and thus may provide predictions of structural strain experienced by the structural body  102 . 
     Thus, a deformation of the structural body  102  may be reflected in deformation of the sensing fiber  110  and detected as a strain on the structural body  102 . Further, in cases where electrical conductivity through the sensing fiber  110  is lost, the loss of conductivity may be interpreted as an indication of a break, crack, or fracture of the structural body  102 . 
       FIG. 2  is a flowchart of an example method  200  for structural strain monitoring. For convenience, the method  200  is described with reference to the system  100  of  FIG. 1 , and for further description of the blocks of the method  200 , description of the system  100  of  FIG. 1  may be referenced. However, this is not limiting, and the method  200  may be performed with other systems. 
     At block  202 , the processing unit  120  monitors the electrical resistance of the sensing fiber  110 . As discussed above, the sensing fiber  110  extends through the structural body  102  and exhibits an electrical resistance that varies with deformation of the sensing fiber  110 . At block  204 , the processing unit  120  determines the structural strain experienced by the structural body  102  based on the electrical resistance. At block  206 , the processing unit  120  outputs an indication of the structural strain. 
     One or more of the blocks of the method  200  may be embodied in instructions stored on a non-transitory machine-readable storage medium that when executed causes a processor of a computing device (e.g. the processing unit  120  of  FIG. 1 ) to perform the one or more blocks of the method  200 . Thus, the processing unit  120  may be configured to perform one or more blocks of the method  200 . 
     Turning to  FIGS. 3A-3B , as mentioned above, the change in electrical resistance of the sensing fiber  110  may depend on the position of the sensing fiber  110  in the structural body  102  and the type of strain experienced by the structural body  102 .  FIG. 3A  depicts schematic representations of a structural body  302  under three different circumstances of structural strain. The structural body  302  is similar to the structural body  102  of  FIG. 1  and includes a sensing fiber  310  (shown in dotted lines) that is similar to the sensing fiber  110  of  FIG. 1 . The structural body  302  includes a neutral plane or median line  301  (shown in dashed lines) that bisects the structural body  302  lengthwise into a first half and a second half. The sensing fiber  310  is positioned in the first half of the structural body  302  and runs substantially parallel with the median line  301  along the length of the structural body  302 . 
     In circumstance (A), the structural body  302  undergoes a strain that causes compression of the length of the first half of the structural body  302  (i.e., is deflected or bent upward), thereby causing compression of the length of the sensing fiber  310 . In circumstance (B), the structural body  302  is in a neutral state under no strain, leaving the sensing fiber  310  at its neutral length. In circumstance (C), the structural body  302  undergoes a strain that causes elongation of the length of the first half of the structural body  302  (or compression of the second half of the structural body  302 ) (i.e., is deflected or bent downward), thereby causing elongation of the length of the sensing fiber  310 . 
       FIG. 3B  is a deformation-resistance plot showing a deformation-resistance curve  350  that describes a relationship between the electrical resistance of the sensing fiber  310  and deformation of the sensing fiber  310  in each of the circumstances (A), (B), and (C) described in  FIG. 3A . The electrical resistance exhibited by the sensing fiber  310  increases as the length of the sensing fiber  310  increases (i.e., is elongated), and decreases as the length of the sensing fiber  310  decreases (i.e., is compressed). 
     At point (B), which corresponds to circumstance (B), the structural body  302  is under no strain, the sensing fiber  310  is at a neutral or initial length, and the sensing fiber  310  exhibits a neutral or initial electrical resistance. At point (A), which corresponds to circumstance (A), the structural body  302  is under a strain, the sensing fiber  310  is at a compressed length, and the sensing fiber  310  exhibits a compressed electrical resistance that is lower than the neutral or initial electrical resistance. At point (C), which corresponds to circumstance (C), the structural body  302  is under an opposite strain, the sensing fiber  310  is at an elongated length, and the sensing fiber  310  exhibits an elongated electrical resistance that is higher than the neutral or initial electrical resistance. 
     For each increment of length compression or elongation of the sensing fiber  310 , the electrical resistance exhibited by the sensing fiber  310  is also incremented, and this relationship is described by the deformation-resistance curve  350  shown in  FIG. 3B . The deformation-resistance curve  350  represents a known relationship between deformation of the sensing fiber  310  and electrical resistance of the sensing fiber  310  that may be used to determine the electrical resistance of the sensing fiber  310  at any given amount of deformation. The deformation-resistance curve  350  may be expressed in a mathematical function by which electrical resistance of the sensing fiber  310  may be calculated as a function of deformation. Although describes in terms of lengths of the structural body  302 , it is to be understood that the principle described above applies to any deformation of any dimension of the structural body  302  (i.e., length, width, depth), or indeed any path through the structural body  302  which when deformed causes compression or elongation of the sensing fiber  310 . In some examples, the sensing fiber  310  may be tested under circumstances similar to circumstances (A), (B), and (C), for the training of a machine learning model to determine the structural strain expensed by the structural body  302  based on electrical resistance of the sensing fiber  310 , as described above. 
     In the example shown, the relationship between electrical resistance and deformation is non-linear. In particular, the electrical resistance of the sensing fiber  310  is more sensitive to deformations when the sensing fiber  310  is elongated than when the sensing fiber  310  is compressed. However, this is shown by way of example only, and the relationship between electrical resistance and deformation may be tuned to suit a given application. 
       FIG. 4  is a schematic diagram of another system  400  of structural strain monitoring. The system  400  is similar to the system  100  of  FIG. 1 , with like components numbered in the “ 400 ” series rather than the “ 100 ” series, and includes a structural body  402 , a sensing fiber  410 , and a processing unit  420 . For further description of these components, description of the like components in the system  100  of  FIG. 1  may be referenced. 
     In the system  400 , the sensing fiber  410  includes a stretchable fiber core  412  surrounded by an electrically conductive mesh  414 , as described for example in PCT/IB2019/051634, which is incorporated herein by reference. The electrically conductive mesh  414  includes a plurality of high aspect ratio nanomaterials  416  coated around the stretchable fiber core  412  to conduct electricity across the sensing fiber  410 . A portion of the sensing fiber  410  is magnified for clearer viewing of these components. Such sensing fibers  410  may be particularly thin (e.g., having diameter of 10 μm to 1 mm) so as to have minimal structural impact on the structural body  402  into which they are embedded. 
     The stretchable fiber core  412  is flexible and elastic, and thus the sensing fiber  410  reversibly deforms when the structural body  402  deforms. The electrically conductive mesh  414  increases in resistance as the high aspect ratio nanomaterials  416  are pulled apart from one another and decreases in resistance as the high aspect ratio nanomaterials  416  are brought closer together. Thus, the electrical resistance of the sensing fiber  410  decreases as the sensing fiber  410  is elongated and decreases as the sensing fiber  410  is compressed. 
     The stretchable fiber core  412  is stretchable in that it is flexible, bendable, deformable, and may be elongated or compressed to a substantial degree without breaking. The stretchable fiber core  412  may include a polymeric material, such as for example, one or a combination of polystyrene, poly(methyl methacrylate), poly(n-butyl methacrylate), polyamide, polyester, polyvinyl, polyolefin, acrylic polymer, nylon, polyurethane, and thermoplastic polyurethane (TPU). The stretchable fiber core  412  may be manufactured to have a radius of less than about 1 millimeter. 
     The high aspect ratio nanomaterials  416  may include slender nanomaterial deposits which are substantially greater in length than in width or diameter. When combined into the electrically conductive mesh  414 , the high aspect ratio nanomaterials  416  are more thoroughly electrically connected when compressed together (and thus of lower resistance), and are less thoroughly electrically connected when elongated apart (and thus of higher resistance). The high aspect ratio nanomaterials  416  may have an average length-to-diameter aspect ratio of at least about 50:1, or more preferably at about 500:1, more preferably still about 1000:1, more preferably still 10,000:1. High aspect ratio nanomaterials  416  having an average length-to-diameter aspect ratio of about 1,000,000:1, or greater, may be used. The high aspect ratio nanomaterials  416  may have an average diameter of less than about 50 nanometers. The high aspect ratio nanomaterials  416  are electrically conductive, and thus may include metallic compounds or elements such as copper, silver, gold, platinum, iron in nanowire form, carbon nanotubes, other high aspect-ratio nanoparticles, and other high aspect-ratio nanomaterials. 
     The sensing fiber  410  may be coated with an electrically insulative material. The electrically insulative material may inhibit interference of the sensing fiber  410  with other components in the structural body  402 . The electrically insulative material may further inhibit the sensing fiber  410  from short-circuiting with other sensing fibers  410  in the structural body  402 , which may otherwise interfere with structural strain monitoring. The electrically insulative material may also be chemically resistant. Such an insulative material may include polystyrene, poly(methyl methacrylate), poly(n-butyl methacrylate), polyamides, polyesters, polyvinyls, polyolefins, acrylic polymers, polyurethanes or thermoplastic polyurethanes (TPU). 
     In some examples, the sensing fiber  410  may be wound into a yarn with several other sensing fibers  410 , and similarly disposed on or in the structural body  402 . A yarn of sensing fibers  410  may be more robust and reliable, especially when under strain, than a single sensing fiber  410 . 
       FIG. 5  is a schematic diagram of a cross-section of an example structural body  502  with a sensing fiber  510 . These components may be similar to the structural body  102  and sensing fiber  110  of  FIG. 1 , and thus for further description of these components, the description of the like components of  FIG. 1  may be referenced. 
     However, in the structural body  502 , the sensing fiber  510  follows a path through the structural body  502  that passes through a section of the structural body  502  at least twice. As shown, the sensing fiber  510  passes through three enhanced-sensing sections  506  each at least twice. That is, the sensing fiber  510  doubles back or backtracks through these sections  506  more than once to provide “overlapping” sensing of these enhanced-sensing sections  506 . During manufacture, when the sensing fiber  510  is embedded in the structural body  502 , the sensing fiber  510  may be lain in a pathway that passes through such an enhanced-sensing section  506  at least twice (i.e., back and forth). 
     If there is a deformation in the structural body  502  in these enhanced-sensing sections  506 , the sensing fiber  510  will undergo more extreme deformation than it would otherwise undergo in a section that it passes through only once, and thus the sensing fiber  510  will exhibit an increase or decrease (as the case may be) in electrical resistance to a greater degree in response to deformations in the structural body  502  in these enhanced-sensing sections  506  than it would otherwise exhibit in a section that it passes through only once. The sensing fiber  510  is therefore more sensitive to deformations in these enhanced-sensing sections  506  than it is to deformations elsewhere in the structural body  502 . 
     In application, the sensing fiber  510  may be disposed in the structural body  502  as described above in sections that are of particularly high importance where it would be particularly useful to receive especially sensitive information about structural strain. Thus, the sensitivity of structural strain monitoring may be tuned by establishing such enhanced-sensing sections  506  in which sensing fibers  510  are particularly sensitive. 
     The sensing fibers discussed herein may be applied to a structural body in several ways. As shown for example in  FIG. 6A , one or more sensing fibers  610 A may be fixed (e.g., adhered) to a surface of a structural body  602 A. 
     As another example as shown in  FIG. 6B , a structural body  602 B may include several layers of composite material in each of which several sensing fibers  610 B are embedded. The sensing fibers  610 B may be of similar size and flexibility as the fibers used in the composite material. Laying sensing fibers  610 B may be incorporated into a layup procedure for preparing the composite materials. 
     As yet another example, as shown in  FIG. 7A , one or more sensing fibers  710 A may be placed within a structural body  702 A which is shown as a wing of an aircraft. 
     In yet another example, as shown in  FIG. 7B , one or more sensing fibers  710 B may be embedded within a structural body  702 B which is shown as a support column or pillar. 
     Thus, sensing fibers may be provided for structural health monitoring applications. A sensing fiber may vary in electrical resistance when deformed by structural strains placed on a structural body, thereby providing a measure of structural strain. The sensing fibers may be made of low modulus materials which do not adversely affect the integrity of composite materials in which they are embedded, and thus can be safely embedded within structures while providing continuous structural monitoring of a large structural body. Sensing fibers may also be laid across a surface of a structural body, and may be adhered to the surface, to similarly monitor deformation of the material. 
     It should be recognized that features and aspects of the various examples provided above can be combined into further examples that also fall within the scope of the present disclosure. The scope of the claims should not be limited by the above examples but should be given the broadest interpretation consistent with the description as a whole.