GB2542475A

GB2542475A - Methods and apparatus for measuring deformation

Info

Publication number: GB2542475A
Application number: GB1612453.9A
Authority: GB
Inventors: Xia Qingfeng
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-07-24
Filing date: 2016-07-19
Publication date: 2017-03-22
Also published as: GB201612453D0; GB201513104D0

Abstract

Method and apparatus for measuring separation or strain at the surface of a structure are disclosed. In one arrangement, the apparatus 2 comprises a bridging element 16 attached at first and second connection areas 8, 9, on the surface of the structure 1 to be measured. The bridging element provides a mechanical bridge between the first and second connection areas. The bridging element has two support members 5, 7, and a spanning member 6. Deformation in the bridging element is measured by strain sensors 12, 14, mounted respectively on surfaces 17, 19, of the spanning member, and calculated using the processing unit 4. The relationship between the measured strain in the bridging element and the separation between the first and second connection areas is insensitive to load type applied on the structure. The measured strain in the bridging element is used to determine the separation between the first and second connection areas of the structure; surface strain is determined from the changed separation in case of the continuous surface between the first and second connection areas.

Description

METHODS AND APPARATUS FOR MEASURING DEFORMATION

The present invention relates to methods and apparatus for measuring deformation of a structure, particularly in the context of structural health monitoring and other applications where reliability over extended periods is a priority.

Longevity and reliability in conventional strain sensors can be limited by fatigue failure of components of the strain gauges where strains are relatively large and where they vary dynamically. US 20070279180 A1 discloses a foil strain gauge that is capable of measuring relatively large static strains, for example up to 15%. However, the disclosed arrangements are not configured to deal with dynamically varying strains for extended periods. Clip gauge is widely used to measure large strain in the material tensile load test, but it is not designed for large dynamic strain under compound or alternative load. Clip gauge can scale down static strain, but scaling ratio is not stable for bending or tensile load induced strain. The defect of significant difference in the scaling ratios makes it not suitable for local surface deformation or strain measurement under compound load of different load types. Fibre optic strain gauges offer an alternative which may be able to deal with dynamically varying strains for extended periods, but the optical signal acquisition equipment needed to operate such devices is expensive, cumbersome and has a large power consumption.

Furthermore, variating separation of two components, e.g. transient change of crack/seam width under dynamic load, is of interest to structural health monitoring. Crack development can be monitored by photography and image processing which is not capable of monitoring crack width under highly dynamic load. Displacement sensors like eddy current sensor, LVDT and capacitance displacement sensor can be used to measure the varying crack width, while it is cost-effective to monitor small displacement by existent strain measurement system.

It is also an object of the invention to provide methods and apparatus to measure deformation between connection areas with insensitivity to load type applied on the structure, which allow strains to be measured for extended periods in a reliable manner even where the level of the strains is large and/or dynamic, without requiring complex and expensive apparatus and/or high power consumption; allow varying spatial separation between two surface areas on a structure to be measured by strain sensors in a reliable and cost-effective manner.

According to an aspect of the invention, there is provided a method of measuring deformation of a structure, comprising: attaching a bridging element at first and second connection areas on the surface of the structure, the bridging element providing a mechanical bridge between the first and second connection areas; measuring a deformation by strain sensors in the bridging element; and using the measured deformation in the bridging element to determine the separation and a strain at the surface between the first and second connection areas wherein the relationship between the measured strain in the bridging element and the separation between the first and second connection areas is insensitive to load type applied on the structure.

Thus, a method is provided in which a separation between two connections and/or the strain between two connections at the continuous surface of a structure is measured indirectly via a bridging element. The deformation of the bridging element can be measured by measuring a strain which is linear with the separation between two connections and thus the strain at the surface of the structure that is being measured. This means that measurement challenges caused by the nature of the strain in the structure, for example the level/magnitude or dynamic nature of the strain, can be circumvented, providing the basis for example for measuring the strain in a more reliable manner over extended periods.

In particular embodiments the bridging member is configured so that a strain in a portion of the bridging member is measured and used to determine the strain at the surface of the structure. In such embodiments the bridging member can be configured so that the strain in the portion of the bridging member is smaller, optionally many times smaller, than the strain at the surface of the structure. Thus, a method is provided in which a strain at the surface of a structure can be determined by measuring a much smaller strain. Measuring a smaller strain is easier to achieve over extended periods (e.g. facilitating use of cheap “off-the-shelf’ metal foil strain gauges) in comparison with measuring larger strains. Fatigue failure rate of any strain gauge used for the strain measurement will be lower, meaning that fatigue failure will occur later and the strain gauge will remain operational longer.

According to an alternative aspect of the invention, there is provided an apparatus for measuring strain at a surface of a structure, comprising: a bridging element configured to be attached at first and second connection areas to the surface of the structure in order to provide a mechanical bridge between the first and second connection areas; and a measuring system configured to measure a deformation in the bridging element.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Figure 1 is a schematic side view of an apparatus for measuring separation of two components of a structure;

Figure 2 is a schematic side view of an apparatus for measuring strain at the continuous surface of a structure;

Figure 3 is a schematic top view of the apparatus as an assembly;

Figure 4 is a schematic side view of the apparatus of Figure 3 illustrating deformation of a bridging element in the case of increased separation the structure;

Figure 5 is a schematic side view of the arrangement of Figure 3 in the case of decreased separation;

Figure 6 is a schematic top view of the apparatus where bridging elements and supporting elements are of a single piece sheet;

Figure 7 is a schematic side view of the apparatus of Figure 6 illustrating deformation of a bridging element in the case of increased separation the structure;

Figure 8 is a schematic side view of the arrangement of Figure 6 in the case of decreased separation;

Figure 9 is a schematic side view of the arrangement of Figure 6 in the case where the increased separation is induced by purely bending deformation.

Figure 10 is a schematic side view of the arrangement of Figure 6 in the case where separation changes in two directions.

The term "separation" in this application is used in the geometry sense, corresponding the relative spatial position between two geometric bodies in to three dimensional space. The term "load" in this application is used in the engineering or physics sense, load such as force, displacement and velocity applied on a structure leads to deformation and strain on the surface of the structure. The term "linear" in this application is used in measurement and instrumentation sense, which suggests a reasonably constant ratio of strain in a portion of the bridging element and strain on the structure surface being measured in the designed strain measurement range. The term "strain" in this application is used in the engineering or physics sense, corresponding in particular to a normalized measure of deformation (e.g. a change in length divided by an original length). For example, the strain at the surface may be defined as the change in length of the portion of the structure forming the continuous surface (and therefore of the surface itself) relative to a reference length of the portion of the structure forming the surface. The reference length may be the length when the structure is in a reference state (e.g. at a reference temperature and subjected to reference stresses, for example steady state stresses, average stresses or no stresses). The skilled person is aware of various ways of measuring strains at a surface. For example, a foil strain gauge consisting of an insulated flexible backing supporting a metallic foil pattern may be used. When such a gauge is attached to a surface of a structure to be measured, subsequent deformation of the structure can cause a deformation of the foil pattern. The foil pattern is arranged so that the deformation of the foil pattern causes a change in the electrical resistance of the foil pattern that is characteristic of the deformation, thereby enabling a measure of strain at the surface to be derived by measuring the electrical resistance.

In an embodiment, examples of which are shown in Figures 1-10, an apparatus 2 for measuring the separation of the first connection area 8 and the second connection area 9 on a continuous surface or separated components of a structure 1 is provided. The apparatus 2 comprises a bridging element 16. The bridging element 16 is configured to be attached at first and second connection areas 8, 9 to the surface of the structure 1. The attachments are elastic. The bridging element 16 provides a mechanical bridge between the first and second connection areas 8, 9, thereby providing a mechanical connection between the first and second connection areas 8, 9 which does not pass through any part of the structure 1 itself. A measuring system is provided to measure a deformation in the bridging element 16. The bridging element 16 is configured such that a strain at the surface of the structure 1 will lead to a deformation in the bridging element 16 which is characteristic of the strain at the surface of the structure 1. Thus, by measuring the deformation of the bridging element 16 it is possible to obtain a measure of the strain at the continuous surface of the structure 1 as in Figure 2.

In an embodiment, a processing unit 4 is provided which is configured to determine a strain at the surface of the structure 1 between the first and second connection areas 8,9 using the measured deformation in the bridging element 16. A conversion between the measured deformation in the bridging element 16 and the strain at the surface of the structure 1 may be made for example by reference to pre-prepared calibration data, theoretical calculations or computer simulations. The processing unit 4 may comprise any standard hardware (e.g. CPU, memory, etc.) and software components programmed to carry out the required calculations.

In an embodiment, the bridging element 16 is configured such that the deformation in the bridging element 16 is measureable by measuring strain in a portion of the bridging element 16. In such an embodiment, the bridging element 16 may be configured such that a magnitude of the strain in the portion of the bridging element 16 is optionally less than a magnitude of a strain at the continuous surface of the structure 1 which is causing the deformation of the bridging element 16. In this way, the bridging element 16 effectively provides a way to measure a strain at the surface of the structure 1 by measuring a smaller strain in the bridging element 16. In an embodiment, the strain measured in the bridging element 16 is at least 5 times smaller than the strain in the structure 1, optionally at least 10 times smaller, optionally at least 100 times smaller, and optionally at least 1000 times smaller. At least 100 times smaller is preferred for the case of the separation of two components la and lb for the structure.

In an embodiment, the bridging element 16 comprises a spanning member 6 configured to be connected via respective first and second support members 5,7 to the first and second connection areas 8, 9. The first and second support members 5,7 may be provided initially as components separate from the spanning member 6, as in the embodiment shown in Figure 3-5. Alternatively, the first and second support members 5,7 (represented as generic blocks in Figure 2) may be formed integrally with the spanning member 6, as in the embodiment of Figure 6-10. Further alternatively, the first and second support members 5,7 (represented as generic blocks in Figure 2) may be formed by lower line elasticity material than the spanning member 6, thus the magnitude of deformation on the spanning member is less that between first and second connection areas 8, 9. Optionally the first and second support members 5,7 are each attached to the spanning member 6 so as to extend relative thereto within 45 degrees, optionally within 20 degrees, of perpendicularly, optionally substantially perpendicularly. In this way the spanning member 6 is attached to the first and second connection areas 8,9 in the manner of a cantilever, which facilitates achieving of a small shunt factor (ratio of the magnitude of strain in the spanning member 6 to strain in the structure 1). Further optionally, multiple cantilevers can be inter-connected to form the spring-like supporting element which accommodates most of the deformation between the first and second connection areas; whereas a portion of the spanning member is kept in close proximity with the surface of the structure 1 (Figure 1).

In embodiments having a spanning member 6, the measurement system may be configured to measure a deformation in (e.g. via measurement of a strain at one or more surfaces of) the spanning member 6. The deformation of the spanning member 6 may be configured to be linear with the deformation of the structure surface where the bridging element 16 is attached, i.e. in a period between installation of the bridging element and any change in the strain at the surface of the structure between the first and second connection areas in response to applied load. Optionally, the spanning member 6 may also be configured to be parallel to the surface of the structure 1 in the period between attachment of the bridging element 16 and any change in the strain at the surface of the structure 1 between the first and second connection areas. Attachment may typically be implemented when the strain in the structure 1 is “zero” (not pre-stressed) or when the strain in the structure 1 is in a steady state condition or condition of average strain. Further optionally, thin metal sheet of 0.2 to 2 mm thickness is used to form a portion or the whole spanning member 6; thinner spanning member can reduce the intrusion effect (changed separation between the first and second connection areas after the connection of the bridging element 16) to the strain or separation to be measured on the structure 1.

In embodiments, a portion of the spanning member 6 should be in close proximity with the surface of structure 1, resulting in the shunt ratio insensitive to the load type applied on the structure. Since similar local surface strain of structure 1 (changed separation between the first and second connection areas along the straight line between the first and second connection areas) can be a result of different load types, strain measured on the bridging element 16 should be insensitive to load type. For example of a beam structure with one end fixed and the other end under load, bending load (Figure 9) and tensile load (Figure 7) lead to different strain magnitude on the spanning member 6 for the same strain magnitude on the surface where bridging element 16 is connected on the beam. The scaling ratio of the averaged strain on upper and lower surface centres of the spanning member 6 to the strain on the structure surface under purely bending load r* .··.< , can be predicted as: b ~ * h J‘s, where rs is the scaling ratio purely tensile load; H is the distance between the central plane of the spanning member 6 and surface of structure 1; h is the thickness of the structure 1 in case of simple rectangular beam ( h/2 is distance between the neutral plane and the surface of iff, f | --j the structure 1. Compared with the purely tensile load, a larger scaling ratio is expected ^ ' k for the purely tensile load. It is essential that a portion of the spanning member 6 should be in close proximity with the surface of structure 1, where the distance between the portion of the surface 17 with strain sensor of the spanning member 6 and the surface of structure 1 is preferably less and at most 3 times of the thickness of the spanning element 6, providing no mechanical interference between the spanning member 6 and the surface of structure 1. Given a small ratio of thickness of the spanning member 6 to the distance from the surface to the neutral plane of the structure 1, e.g. preferably less than 0.05 or no more than 0.15, the shunt ratio of strain in a portion of the bridging element and structure surface being measured is regarded as insensitive to the different load type.

In an embodiment the bridging element 16 is configured so that the spanning member 6 bends in response to the changed separation between the first and second connection areas of the structure, as a result of applied load. In such an embodiment the measurement system may be configured to measure a deformation in apparatus 2 associated with the changed separation (e.g. one or more strains at surfaces of the spanning member caused by the load) in order to obtain a measure of the strain at the continuous surface of the structure 1 or separation between components la and lb. The deformation associated with the changed separation between the first and second connection areas may be measured by one or more strain gauges positioned at one or more surfaces of the spanning member 6.

In an embodiment, the spanning member 6 comprises a first surface 17 and a second surface 19.

The first surface 17 is configured to face towards the surface of the structure 1 when the bridging element 16 is attached to the structure 1. The second surface 19 is configured to face away from the surface of the structure 1 when the bridging element 16 is attached to the structure 1. In embodiments of this type, and in other embodiments, the measurement system may comprise a first strain gauge 12 attached to the first surface 17 and a second strain gauge 14 attached to the second surface 19. In other embodiments, fewer than two strain gauges or more than two strain gauges are provided. In an embodiment, a strain gauge is provided to the first surface 17 but not to the second surface 19. In another embodiment, a strain gauge is provided to the second surface 19 but not to the first surface 17.

Providing at least one strain gauge on each of the first and second surfaces 17, 19 facilitates obtaining an accurate measure of the deformation of the bridging element 16. For example, by averaging or otherwise combining outputs from strain gauges on different sides of the spanning member 6 it is possible to remove or reduce effects on the strain gauge readings which arise from deformations in the structure 1 that are not simply strains along a straight line between the first and second connection areas 8,9. For example, the effects of strains in directions other than the straight line between the first and second connection areas 8,9, which might induce twisting in the spanning member 6, can be reduced or eliminated. Furthermore, averaging between the outputs from multiple strain gauges will tend to improve accuracy generally relative to where a single strain gauge is used, due to the reduction of random errors and temperature induced deviation.

In an embodiment, the output from a first strain gauge 12 attached to the first surface 17 is combined with the output from a second strain gauge 14 attached to the second surface 19 in such a way as to obtain an average strain in the spanning member. Where one or more strain gauges are to be provided on only one of the first and second surfaces 17,19, it may be advantageous to choose the first surface 17. Providing the strain gauge or gauges on the first surface means that the spanning member 6 can provide physical sheltering of the strain gauge or gauges from the environment surrounding the structure 1. This configuration may thus further enhance the longevity and/or reliability of the apparatus 2, and may be particularly preferred where the apparatus 2 is to operate in harsh conditions, such as where the apparatus 2 is exposed to weather and/or corrosive materials such as salt water (e.g. in an offshore wind turbine installation).

The deformation measured by the bridging element 16 should be selective in direction, and the deformation of the structure 1 parallel with the straight line between the first and second connection areas 8,9 should be measured which is insensitive to deformation of the structure 1 in other directions. Further optionally, deformation of the structure 1 perpendicular to the structure surface between the first and second connection areas 8,9, can be measured with extra strain sensors on the bridging element 16. In embodiments of Figure 1 and 10, two extra strain gauges are utilised to measure the varying separation 28 perpendicular to a straight line between the first and second connection areas 8,9, in addition to the measurement of varying separation 18 parallel with the straight line between the first and second connection areas 8,9.

Figures 4-5 and 7-9 illustrate schematically the deformation on the bridging element 16 of Figures 1 and 2 caused respectively by a positive strain at the continuous surface of the structure 1 and an increased separation between components la and lb (Figure 4, 7) and a negative strain at the continuous surface of the structure 1 and a decreased separation between components la and lb (Figure 5,8). In the case where the strain at the continuous surface of the structure 1 is positive (extension), the first and second connection areas 8, 9 move away from each other, as shown in Figure 4 and Figure 7, which results in an extension and a flexing upwards of the spanning member 6. This flexing upwards results in a compression 20 in the first surface 17 and an extension 22 in the second surface 19. In the case where the strain at the surface of the structure 1 is negative (compression), the first and second connection areas 8, 9 are driven close together, as shown in Figure 5 and Figure 8, which results in a compression and a flexing downwards of the spanning member 6. This results in an extension 20 in the first surface 17 and a compression 22 in the second surface 19. The compressions and extensions 20, 22 can be measured by the first and second strain gauges 12, 14 and used to derive the separation 18 and/or strain 18 at the surface of the structure 1. Although a small footprint is preferred for local surface strain measurement, a geometry ratio of spanning member length to strain gauge length >5 is necessary so that the strain gauge at the centre of the shunt sheet is not subject to the risk of fatigue failure serious warping deformation.

In an embodiment in which the bridging element 16 comprises a first support member 5 and a second support member 7, and in which a spanning member 6 is connected to the first connection area 8 via the first support member 5 and to the second connection area 9 via the second support member 7, the first and second support members 5 and 7 may be configured to extend away from the surface of the structure 1 at an angle within 45 degrees, optionally within 20 degrees, of the perpendicular to the surface of the structure 1 in the region immediately surrounding the respective first and second connection areas 8, 9. This configuration provides a particularly effective reduction in the size of the strain appearing in the spanning member 6 for a given strain at the surface of the structure 1 between the first and second connection areas 8, 9.

There is no restriction on the surface condition, curvature, metal or non-metal material on the structure 1, since the connection between the bridging element 16 and structure 1 (continuous surface or separated components la and lb) can be made in a variety of different ways. For example, the bridging element 16 may be welded, bolted or adhered to the structure 1 with the shape of planar beam or circular column. Similarly, where the bridging element 16 comprises first and second support members 5 and 7 and a spanning member 6 that is not integral with the first and second support members 5 and 7, various known methods may be used to provide the connection between the support members 5,7 and the spanning member 6. In the embodiments shown schematically in Figures 3-5, the support members 5, 7 protrude through holes in a spanning member 6 that is formed from metal sheeting. A fixed relationship between the first and second support members 5, 7 and the spanning member 6 may be established using nuts and bolts, welding or adhering for example.

Typically, the bridging element 16 will be formed from a material that is able to deform elastically with less concern of yielding or fatigue failure over the required range of dynamic deformation. Various types of high strength stainless steel will be suitable in many applications. In an embodiment, the bridging element 16 comprises a material having a yield strength of at least 50MPa.

In the case where the bridging element 16 acts to reduce a strain relative to the strain occurring at the continuous surface of the structure 1, the ratio of the strain measured in the bridging element 16 to the strain at the surface of the structure 1 may be referred to as a shunt factor. The shunt factor can be adjusted by altering the dimensions of the bridging element 16. The inventors have found for example that it is possible for a suitable configuration of the invention of the bridging member 16 to measure strains at the surface of the structure 1 of more than 20000 με without fatigue failure. This is considerably more than is easily possible using typical resistive strain gauges attached directly to the structure 1. For example, a typical yield strength for high strength Maraging steel is 1030-2420MPa, while the Young's modulus is around 210GPa. This means that the maximum strain without plastic deformation would be about 5000 με. Furthermore, because the strains occurring in the bridging element 16 can be made so much smaller than the strains occurring at the surface of the structure 1, the effects of dynamically varying strains on the longevity of the strain gauge are much reduced. Dynamic variations of a relatively small strain will lead to fatigue failure more slowly than dynamic variations of a larger strain that are otherwise identical.

In arrangements of the type shown in Figures 1-10 in which the bridging element 16 comprises a spanning member 6 and support members 5,7, bending/tilting of the support members 5,7 (as shown in Figures 4-5 and Figures 7-10 for example) leads to a reduction in the strain measured in the first and second surfaces 17, 19 of the spanning member 6 relative to the strain in the structure 1. Adjustment of the dimensions and materials of the support members 5,7 and of the spanning member 6 can be used to change the shunt factor according to requirements. Increasing the shunt factor will tend to increase the sensitivity of the strain measurement while decreasing the shunt factor will tend to improve longevity and reliability.

Any of the measurements of strain discussed above can be performed using any of the various standard methods known to the skilled person in the art. For example, thin film strain gauges may be used, attached to the surfaces in which strain is to be measured.

The above-described embodiments allow strain to be measured in one region on the surface of the structure 1 (between the two connection areas). It will be appreciated that multiple instances of the disclosed apparatuses can be used to measure strain at a corresponding multiplicity of different regions on the structure in order to obtain strain magnitude on different directions of the structure as a whole.

Claims

1. A method of measuring deformation of a structure, comprising: attaching a bridging element at first and second connection areas on the surface of the structure, the bridging element providing a mechanical bridge between the first and second connection areas; measuring strain in the bridging element; using the measured strain in the bridging element to determine the separation between the first and second connection areas wherein the relationship between the measured strain in the bridging element and the separation between the first and second connection areas is insensitive to load type applied on the structure.

2. The method of claim 1, wherein: the measuring of a deformation comprises measuring strain in a portion of the bridging element, the separation between the first and second connection areas is determined using the measured strain in the portion of the bridging element; strain at the surface of the structure is determined from a change in the separation of the first and second connection areas divided by a reference separation in case of the surface between the first and second connection areas is continuous; and the bridging element is configured such that a magnitude of the strain in the portion of the bridging element is linear and less than a magnitude of the strain at the surface between the first and second connection areas of the structure in a period between attachment of the bridging element and any change in the separation between the first and second connection areas of the structure.

3. The method of claims 2, wherein: the bridging element comprises a spanning member connected via respective first and second support members to the first and second connection areas; and the measuring of the deformation in the bridging element comprises measuring a deformation by strain sensors in the spanning member.

4. The method of claims 1, the insensitivity of the strain measured in a portion of the bridging structure to load type is achievable via: a portion of the spanning member is parallel to the surface of the structure in a period between attachment of the bridging element and any change in the strain at the surface of the structure between the first and second connection areas; the first and second support members each extend away from the surface; whereas a portion of the spanning member is in close proximity but not in contact to the surface of the structure; the bridging element comprises an elastic material having a yield strength of at least 50 MPa.

5. The method of any of claims 1-4, wherein: the parallel portion of the spanning member comprises a first surface facing towards the surface of the structure and a second surface facing away from the surface of the structure; the load applied on the structure causes the spanning member to deform and the measuring of the deformation in the bridging element comprises measuring a deformation associated with the load; the deformation along the straight line between the first and second connection areas in the bridging element is measured using a first strain gauge attached to the first surface of the spanning member, and further measured using a second strain gauge attached to the second surface of the spanning member; the deformation in the bridging element other than along the straight line between the first and second connection areas is further measured using a third and fourth strain gauges, optionally fifth and sixth strain gauges attached to the surface of the spanning member.

6. An apparatus for measuring deformation of a structure, comprising: a bridging element configured to be attached at first and second connection areas to the surface of the structure in order to provide a mechanical bridge between the first and second connection areas; a measuring system configured to measure a deformation by strain sensors in the bridging element; and a processing unit configured to determine the separation between the first and second connection areas of the structure using the measured deformation in the bridging element.

7. The apparatus of claim 6, wherein: the bridging element is configured such that a magnitude of the strain in the portion of the bridging element is less than a magnitude of the strain at the surface of the structure; the strain on in the portion of the bridging element is configured to be linear in a period between attachment of the bridging element and any change in the separation between the first and second connection areas of the structure between the first and second connection areas; the bridging element is configured so that the spanning member deforms in response to the load in the structure and the magnitude of deformation is insensitive to the load type; the measurement system being configured to measure a deformation associated with the load; and the processing unit is configured to determine the separation between the first and second connection areas of the structure using the measured strain in the portion of the bridging element.

8. The apparatus of any of claims 6 or 7, wherein: the bridging element comprises a spanning member configured to be connected via respective first and second supporting members to the first and second connection areas; a portion of the spanning member is parallel to the surface of the structure in a period between attachment of the bridging element and any change in the strain at the surface of the structure between the first and second connection areas; the first and second support members each extend away from the surface and a portion of the spanning member is in close proximity but not in contact to the surface of the structure; and the bridging element comprises an elastic material having a yield strength of at least 50 MPa.

9. The apparatus of claim 6-8, wherein the measurement system further comprises a second strain sensor, the extra third and fourth strain sensors, optionally fifth and sixth strain sensors attached to the surface of the spanning member, further optionally strain sensor can be made onto the spanning element.

10. The apparatus of any of claims 6-8, wherein the strain at the continuous surface of the structure comprises a change in the separation of the first and second connection areas divided by a reference separation.

11. A method of measuring separation between the first and second connecting areas in a structure substantially as hereinbefore described with reference to and/or as illustrated in the accompanying drawings.

12. An apparatus for measuring separation between the first and second connecting areas in a structure configured and/or arranged to operate substantially as hereinbefore described with reference to and/or as illustrated in the accompanying drawings.