It is highly desirable to be able to tighten bolts and similar fasteners to the correct load level. Underloading will result in leaks through joints when fastened apparatus is in operation, whereas overloading increases the likelihood of fatigue failure of the fastener, again causing leaks. Incorrect loading therefore severely impairs the performance of a fastener.

While extremely accurate torque loading equipment is readily available to tighten fasteners, the actual load present can vary by as much as 50% due to friction, bending, etc. within a structure. Consequently a number of fastener systems have been developed which measure directly the strain developed within the fastener itself.

PCT publication no WO 99/09327 describes a fastener system which provides an indication of the strain to which it is subjected. The system is essentially a bolt with an axial bore hole in the shaft into which a pin-like measuring element is located anchored at the base and ground flush with the top surface of the shaft. As the bolt is loaded the shaft will expand whereas the pin will not and the formerly flush pin is displaced relative to the level of the top of the shaft. A portable measuring head can be attached to the top of the bolt in order to measure the size of this displacement. In this way the extension of the bolt, and hence the strain it is experiencing can be deduced.

US Re. 30,183 also describes a microdisplacement transducer which is suitable for use in, among other things, mine-roof bolts. In this system a pin is again located internal to the bolt in such a way that extension of the bolt is not communicated to the pin. A mechanical extension of the bolt effects a change in separation of two plates of a capacitor by means of this

pin. The capacitor is part of an L-C circuit and measurement is made, using an external oscillator, of the resonant frequency of this circuit. From this measurement, capacitance and hence plate separation and finally strain can be deduced.

However neither of these fasteners are suitable for strain measurement in hostile environments. Both require attachment of a hand-held device-the portable head in one instance and the oscillator in the other-which precludes operation in uninhabitable environments. Moreover the device of WO 99/09327 leaves the measuring pin exposed at the surface and susceptible to corrosion and contamination. Since displacements measured are extremely small (of the order 50, um), small particles of dust or dirt between the portable head and pin or portable head and bolt can cause large errors. Similarly, scratching or indentation of the exposed measurement surface (s) may also give rise to significant errors. If the hostile operating environment is at an extreme of temperature then the device of US Re. 30,180 is further unsuitable. This device makes use of coils which themselves have a temperature-dependent impedance.

Although materials are available that minimise some of the effects of temperature-dependence, a number of the more common coil arrangements are unsuitable for use in varying temperatures.

An inability to measure strain in a hostile environment gives rise to numerous difficulties. Although it may be possible to infer the loading of a fastener by the torque applied to fasten it, this is not a direct measurement of strain, and approximations are inevitably made. Moreover the loading of the fastener and strain experienced will change over time. This change, arising as the fastener ages in situ, cannot be followed once the fastener is in a hostile environment. In some cases a plant intended for operation in a hostile environment is assembled under ambient conditions and then moved for operation. This may ensure correct loading initially, but the effect of the changed environment itself on loading cannot be measured.

Incorrect loading may therefore have been provided even at the start of the

fastener's operating life.

Temperature has a particularly significant effect on a loaded fastener.

Unless the fastener and flange materials have identical coefficients of thermal expansion, the fastener will expand or contract in proportion to the differential between the coefficients. This results in a temperature- dependent variation in the load experienced by the fastener. In addition a time-dependent, usually permanent, deformation known as creep deformation will occur. Creep deformation results in an unloading, or stress relaxation, of the fastener. It is important to be able to re-tighten the fastener when appropriate in order to counter the effects of stress relaxation. Thus it is highly desirable to be able to monitor the loading of a fastener if operating at extremes of temperature. There could be potentially disastrous consequences if a joint fails in a critical application such as a nuclear reactor, oil rig, crane, etc.

There is thus a perceived need to provide a fastener which has the flexibility to admit of direct strain measurement regardless of the hostility of the environment. It is an object of this invention to provide such a fastener.

The present invention provides a probe attachment with cable arranged to carry electrical signals to and from a sensor with sensing surface, the probe being locatable within a bore hole made within a fastening device, wherein the probe attachment is securely attachable at a region remote from its sensing surface to the fastening device so as to seal a pocket within the bore hole and to leave a gap between its sensing surface and a surface of the bore hole and is adapted to measure a parameter indicative of capacitive reactance of the two surfaces separated by the gap.

In another aspect this invention provides a component, such as a bolt, of a fastening system, the component including an internal air gap located intermediate an internal surface of the component and a sensing surface of a probe attachment, the probe attachment being secured to a part of the

fastening component remote from the internal surface and adapted to measure a parameter indicative of capacitive reactance of the two surfaces separated by the air gap.

This invention has numerous advantages over the prior art systems of strain measurement which make it suitable for use in extreme, particularly high temperature, environments. First it avoids the use of coils, moving parts and point contacts. Each of these depends on some mechanical or electrical material property, such as dielectric constant or thermal expansion coefficient, which varies with temperature rendering consistent measurement of strain impossible. Secondly it avoids the necessity of using any hand-held device to perform the measurement. Although such a hand-held device could be used if circumstances permit, it is not necessary to do so. The cable carries the output signal away from the locality of the measurement and its length can be adapted to suit the situation.

Specifically, the length of the cable merely has to be sufficient for any human observer making an attachment to the cable to take a measurement to be able to do so at a safe distance. Thirdly, the measuring surfaces are all enclosed within the fastening system. This significantly reduces the chances of contamination or corrosion of these surfaces, which would upset the accuracy and reproducibility of measurements made. Fourthly, it enables allowance to be made for high-temperature stress-induced permanent displacements. At elevated temperatures an applied load may result in a time-dependent permanent displacement of the fastener. Such displacement is generally associated with the first four threads of a threaded fastener, which are expected to experience the highest stress levels, being tightened. The consequences of this deformation (stress relaxation) with regard to the indicated load must be fully understood. Prior art load-indicating bolts cannot provide for long-term monitoring of this effect as they are unsuitable for use in high-temperature, corrosive or otherwise hostile environments.

The gap, which is most preferably an air gap, may be between the sensing

surface of the probe and the surface at an end of the bore hole. Generally fasteners such as bolts are strained lengthwise rather than in any other direction when loaded. By placing the sensing surface at the end of an axial bore hole and securing the probe nearer the top, maximum displacement of the air gap will result from anticipated strain, and the sensitivity of the measurement will be increased.

When the probe is secured within a fastening device, the cable may extend only a short distance outside of the device and, in such an embodiment, this cable is attachable either directly or indirectly to a signal processing means. To facilitate indirect connection, the cable is preferably attachable to a second cable which in turn may be attached to the signal processing means. This provides the probe with the advantage of flexibility.

Measurements can be taken through the short cable, if circumstances permit, but also through the longer length of cabling when the second cable is attached. Alternatively the probe may include a socket located at the region which is attachable to the fastening device and the cable is internal to the probe and connected to the socket.

Thus the probe attachment of this invention is suitable for measuring strain regardless of the hostility of the environment, provided the length of cable used is sufficient to extend from a comfortably-housed processor to the fastener location. In room or similar-temperature environments therefore, the cable need not extend outside of the fastener. It can terminate within, leaving no cable length whatsoever to interfere with tightening equipment, nor indeed to offer any more difficulty in loading than is offered by a standard bolt. All that is required in this embodiment therefore is that the processor, which for example may be a hand-held device, is able to connect with the cable termination inside the fastener. In the other alternative, the shortness of the permanently-attached cable means that it does not provide significant hindrance to fastening the device and the length is sufficient to extend the cable termination to a cooler zone outside of a moderately heated structure. It can therefore also be attached to a

hand-held processing means during loading in a slightly-or non-hostile environment. Whenever loading in a hostile environment, or if operating in a hostile environment after loading in a less severe situation, continuous monitoring may be provided by remote measurements taken via attached additional cabling. The physical shape of this embodiment of the invention is such that, when not connected to additional cabling, it avoids impediments such as cable connectors which both operate inefficiently at high temperatures and could interfere with torquing equipment as the fastener is loaded.

Preferably in the embodiment of the invention in which the permanently- attached cable extends outside of the fastener, the cable has length within the range 150-300 mm. In many applications this is sufficient to reach a cooler zone, for example outside thermal lagging placed upon hot pipework. This will generally therefore avoid the need for the connection between the permanently-attached cable and selected attachment (hand- held device or the second cable) and the second cable itself to be specifically adapted for high-temperature operation.

One arrangement to measure capacitive reactance may be provided by current supply means arranged to supply an a. c. current to the sensing surface and the probe adapted to measure potential drop across the gap between the sensing surface and the surface within the bore hole.

Capacitive reactance is equal to voltage drop divided by applied current.

This therefore provides a straightforward way to measure capacitive reactance by taking only measurements of electrical current and voltage.

The voltage is preferably measured by means of electrical connections made between one element of a multi-axial cable and the fastener and between a second element and the sensor.

The current supply means is preferably arranged to supply a constant amplitude, constant frequency current. This avoids even the need to

measure the current, as it will already be known, and the voltage drop can be used as a direct measure of the value of the capacitative reactance.

Alternatively, changes in strain can be monitored simply by observing changes in the voltage drop.

The sensing surface is preferably a circular surface of a sensing cylinder.

A cylindrical sensor reduces potential cost of the system as this shape of hole will be most conveniently bored into the shaft of a fastener. More preferably, the sensing cylinder is part of a guard ring capacitance transducer. The capacitance transducer may therefore preferably comprise the cylindrical sensor mounted within a coaxial cylindrical guard ring. The guard ring component effectively isolates around the sensor the uniform section of an applied electric field, enabling reactance of a capacitor formed by the sensor and lower surface of the bore hole to be measured with improved linearity. To facilitate this isolation the guard ring should be driven at the same electric potential as the sensor, and this is preferably achieved by a connection made by a third element of the multi-axial cable between the guard ring and current supply means.

The guard ring, sensor and fastening device 12 are preferably fabricated from materials with the same or very similar thermal expansion coefficients.

This reduces the possibility of errors being introduced into the measurement of fastener loading by internal displacements arising from differential thermal expansion.

This probe attachment may be incorporated into a range of fastening systems, or indeed non-fastening systems which operate in a stressed environment, but is particularly suited for placement in the bolt part of a nut and bolt system.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings.

Figure 1 shows schematically an illustration of a load-indicating bolt

implementing one aspect the invention.

Figure 2 is a detailed illustration of the transducer component of Figure 1, which embodies a second aspect of the invention.

Figure 3a is an illustration of a second embodiment of the transducer in accordance with the invention.

Figure 3b is an illustration of the transducer of Figure 3a located within a bolt.

Figure 1 illustrates a nut 10 and bolt 12 fastening mechanism, the nut fitting onto a threaded portion 14 of the bolt so as to clamp a joint between its head 16 and the nut 10. In its top portion 18, near the head 16, the bolt 12 contains an axial bore hole 20 with a flat lower surface 22. A guard ring capacitance transducer 24 is positioned within the bore hole 20 and held in place by a plug 26 in a top surface of the bolt 12. When in position, an air pocket is trapped in the bore hole 20 by the transducer 24. In particular, an air gap 28 is left between a lower surface 30 of the transducer and the lower surface 22 of the bore hole. In this embodiment, cabling 32,34 extends from the transducer 24 through the plug 26 and a short way, typically 150 to 300 mm, outside the bolt. In this external portion cabling 34 is triaxial. One connection is made from the triaxial cabling to the bolt 16 in the region of the plug 26 and coaxial cabling 32 connects directly to the transducer 24. The triaxial cabling 34 is attachable at its free end either directly to a remote electronic conditioning unit (not shown) or via a second, longer, section of cabling 36. In the first instance the remote electronic unit will be a handset, whereas in the latter case it will be a less-portable signal processing unit. Circumstances in which each option is preferred will be described in more detail below.

When the bolt 12 is loaded an elastic displacement occurs between the head 16 of the bolt and the nut 10. The lower surface 22 of the bore hole 20 will thus be displaced relative to the lower surface 30 of the transducer,

which is attached at the bolt head 16. These two surfaces 22,30 surrounding the air gap 28 form a capacitor whose capacitance is dependent on the size of the air gap 28. As the fastened bolt is stressed, the displacement of head 16 and nut 10, and therefore also of bore hole lower surface 22 and transducer lower surface 30, is directly proportional to the strain experienced. As the two surfaces 22,30 move, the air gap 28 changes and therefore also the capacitance of this system. A measurement of this capacitance therefore provides an indication of loading strain. By means of the cabling 32,34 and transducer 24, capacitive reactance of the capacitor formed from lower surface 30 of the transducer, lower surface 22 of the bore hole and air gap 28 is measured.

This in turn provides an indication of capacitance and hence size of air gap 28.

Clearly in loading the bolt 12 excessive cabling will provide a hindrance and so, in the embodiment shown in Figure 1, for which the cabling 32,34 protrudes from the bolt 12, then only a short piece is permanently connected to the transducer. Once the bolt is loaded there is often no need to minimise wiring and so the second, longer, piece of cabling 36 can be connected. Once the apparatus is ready for operation in its hostile environment therefore, the longer cabling 36 is attached and data from the strain measurements are output to a signal processor. The processor is housed remote from the environment of the assembled apparatus. Once the longer cabling 36 is attached to the bolt 12, it can be left in place and the load experienced by the bolt continuously monitored.

Clearly, once installed, all the wiring from one structure (one triaxial cable per fastener) may be connected, if desired, to a single piece of electronic processing equipment.

From Hooke's law, the modulus of elasticity (E) of a bolt material can be written

E stress (a) strain (E) Stress and strain are further defined as load L displacement Ad cross sectional area a gauge length g From which the measured displacement can be seen to be where c is a constant characteristic of the bolt, namely Young's modulus of its material multiplied by its cross sectional area (Ex a). Strictly speaking c is not a constant but varies slightly with temperature: Young's modulus, cross sectional area and thermal coefficient of expansion are all temperature dependent. However, as is well known in the field, with an appropriate choice of materials, transducer characteristics may be matched to those of the bolt and any errors introduced by the variation of this constant c with temperature can be limited to around 5%.

The size of bore hole required to accommodate the transducer is only of the order 4.5% of the cross sectional area of the bolt. This is to be contrasted with threading a standard bolt in which typically 23% of cross sectional area is lost. Thus a small hole on the axis of the bolt, as required to accommodate the transducer, will not induce its permanent failure.

Referring to Figure 2, the operation of the transducer 24 in measuring the capacitance across the air gap 28 will now be further explained. The transducer 24 consists of a cylindrical sensor 52 contained within a cylindrical guard ring 54. Insulating layers 56a, 56b separate sensor 52 from guard ring 54 and guard ring 54 from the bore hole 20. The transducer lower surface 30 thus comprises lower surfaces 58,60 of the sensor 52 and guard ring 54.

An electric current from a constant current a. c. source (not shown) is applied to the capacitor formed by the sensor lower surface 58 and the lower surface 22 of the bore hole. The guard ring 54 is at the same time driven to the same electric potential as the sensor 52. An electric field therefore develops between the sensor 58/ring 60 surface and the lower surface 22 of the bore hole. The guard ring 54 serves to ensure that the field generated between the sensor 52 and lower surface 22 is uniform.

The required measurements are taken with respect to the capacitor formed between the sensor lower surface 58 and bore hole surface 22 only. The guard ring 54, driven at the same electrical potential as the sensor 52, effectively isolates the uniform section of the electric field, improving the linearity of the device. Once the current is applied a potential drop develops between the lower surface 58 of the sensor 52 and the lower surface 22 of the bore hole.

The output cabling 34 is triaxial enabling three electrical connections to be made to the system. A central signal cable 62 is connected to the sensor 52, a coaxial section 32 is connected to the guard ring 54 and an outer sheath of the triaxial section 34 is connected to the bolt 12. In this way a constant current is applied via the signal cable 58 to the sensor 52 and the same electric potential is applied via the coaxial section 32 to the guard ring 54. The voltage drop over the sensor surface 58-bore hole surface 22 capacitor is measured over signal cable 32 and outer sheath of the triaxial section 34. This measurement is made by a low capacitance voltage preamplifier.

The voltage drop (as measured) divided by current (which is constant) is known as capacitive reactance (Xc). Further, 1<BR> (t) c, where c3 is the angular frequency of the a. c. current and Cthe capacitance across the air gap. The capacitance is given by

£A<BR> d where A is the cross section area of the lower surface 58 of the sensor 52, £ the dielectric constant (of air) and d the width of the air gap 28.

From which, i. e. the capacitive reactance is directly proportional to the separation gap.

Thus monitoring of the change in capacitive reactance over time provides an indication of the extension (or contraction) displacement Ad resulting directly from the changing strain experienced by the bolt.

The device can be pre-calibrated for various known sizes of air gap, and the air gap sizes pre-calibrated for various known loads. This provides a very quick way to take a measurement of capacitive reactance from which the loading of the fastener can be deduced.

In its most straightforward application a constant amplitude, constant frequency a. c. current is applied to the sensor 52. In this way continuous monitoring of the voltage drop between sensor and bolt provides a direct measurement of the changing strain experienced by the bolt. Thus in order to ensure ideal loading strain is maintained the only absolute value which is needed is that of the voltage drop corresponding to this strain.

On initially loading the bolt in a moderate or ambient environment, torque is gradually applied and capacitive reactance monitored using a handset directly connected to triaxial cable 34. Sometimes, if the structure being bolted is intended for operation in harsh environments, then initial loading may still be carried out under ambient conditions. Moreover, if the cabling 34 protrudes from the bolt at all, then the preferred length of triaxial cable 34 permanently attached to the bolt 12 is 150-300 mm. This is sufficient

to ensure that its free end generally lies within a"cool zone"outside of, for example, lagged pipework. Thus loading conditions are generally amenable to load monitoring by means of a hand-held device. Loading may be carried out either by tightening the nut 10 or by tightening the bolt 12 with torque equipment adapted for the short length of cabling 34 to pass through the centre. Thus, in the initial tightening process, applied load can be directly measured locally by means of a hand-held device monitoring capacitive reactance. Ideal loading, under ambient conditions, can thus be ensured.

After the initial loading described above, the bolted system may be transferred to its operating environment. In harsh environments, local monitoring of strain is precluded and so the output triaxial cable 34 of each bolt is attached via respective extension cables 36 to the remote electrical conditioning unit. Thus, while set up in this manner, reactance measurements may be regularly taken in order to provide continuous in situ monitoring of the strain experienced by each bolt of the system.

Figures 3a and 3b illustrate an alternative embodiment of the transducer of Figures 1 and 2, which is particularly suitable for operation in thermally non-hostile environments. In this illustration, in which like components are referenced similarly to previous illustrations, an alternative structure of guard ring capacitance transducer 80 is shown connected to a triaxial socket 84 in place of the plug 26 of the first embodiment. This transducer 80 consists of the sensor 52 separated from a guard ring 82 by an insulation layer 56a. The guard ring 82 is again covered by an insulation layer 56b to separate it from the bolt 12 when the transducer is in place.

The guard ring 82 in this embodiment extends from the sensor surface to the triaxial socket 84. There it is connected to one 86a of three connectors for electrical connection with an external plug (not shown). A second connector 86b is connected by cable 62 to the sensor and a third connector is in electronic communication with the bolt. A socket with three such connectors (triaxial socket) is readily available commercially, for example,

as manufactured by Lemo.

This embodiment of the invention measures capacitive reactance between sensor and bore hole surfaces in much the same way as the first embodiment. The difference lies in the fact that the cabling 62 is (see Figure 3b) terminated within the bolt. Electrical connections to supply the constant amplitude constant frequency current and to measure the voltage drop are made via the external plug which can be connected to the triaxial socket. In this way, there is no need for any cabling to extend beyond the bolt either in fastening it or in taking reactance measurements. A bolt fitted with this transducer 80 and socket 84 therefore provides no more hindrance to fastening than is provided by a standard bolt. Moreover, the pocket inside the bolt remains sealed to the environment by means of the socket 84. For this reason, this embodiment is particularly suited for use in hostile, but ambient temperature, environments where corrosion or contamination could readily occur if the measuring surfaces were to be exposed. Such environments include, for example, operation in structures such as oil rigs which are exposed to sea water, or parts of nuclear reactors.

In accordance with this invention therefore, a bolt can be manufactured with one of two permanently-attached lengths of cabling, the longer of these lengths being attachable to a second piece of cabling to create a third, longest, length. Three cable lengths can therefore be arranged.

First, a very short length may be used which terminates within the fastening device 12. This can be very conveniently used in room-or close to room- temperature applications. Secondly, a short length of cable which extends a short distance outside of the bolt may be fitted. If this second length is attachable either to a processing means or to a further cable extension, then this embodiment may be used in two different temperature regimes. If the temperature is moderately elevated or dropped, then the short length is attached directly to the signal processing means and if it is extreme, or other environmental conditions are extreme, then it is attached to the longer length of cabling which is in turn attached to the signal processing means.