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Title:
WASHER UNIT AND STRAIN DETECTION SYSTEM FOR A FASTENED JOINT
Document Type and Number:
WIPO Patent Application WO/2019/154827
Kind Code:
A1
Abstract:
The present invention is in the field of strain detection and/or monitoring system techniques for a fastened joint. In particular, the present invention provides systems and methods for detecting and/ or monitoring of the tension in a fastened joint; said systems and methods comprising a washer unit (100) configured for doing the same.

Inventors:
VAN INGELGEM, Yves (Dorpstraat 46, 3404 Landen, 3404, BE)
Application Number:
EP2019/052854
Publication Date:
August 15, 2019
Filing Date:
February 06, 2019
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
ZENSOR (Witte Paterstraat 4, 1040 Brussel, 1040, BE)
International Classes:
F16B31/02
Domestic Patent References:
WO2013030567A12013-03-07
Foreign References:
DE2939096A11981-04-16
DE102009043267A12011-04-21
DE102011005371A12012-09-13
DE202013010397U12014-11-03
US20160245709A12016-08-25
DE102009043267A12011-04-21
DE102011005371A12012-09-13
DE202013010397U12014-11-03
US20160245709A12016-08-25
Attorney, Agent or Firm:
DE CLERCQ & PARTNERS (Edgard Gevaertdreef 10a, 9830 Sint-Martens-Latem, 9830, BE)
Download PDF:
Claims:
CLAIMS

1. A washer unit (100) for providing on a fastener (420) comprising:

- a washer (200), and

- a strain-detecting element (300),

wherein

the strain-detecting element (300) is disposed within a passage (240, 242) within a body (202) of the washer (200),

at least one end of the passage (240, 242) opens to an edge (215) of the washer (200) body (202), and

the strain-detecting element is configured for detection of strain in one direction that is a direction of the passage (240, 242).

2. The washer unit (100) according to claim 1 , wherein both ends of the passage (240, 242) open to an outer edge (215) of the washer (200) body (202).

3. The washer unit (100) according to claim 1 or 2, wherein the passage is cylindrical and straight.

4. The washer unit (100) according to any of claims 1 to 3, wherein strain-detecting element (300) exhibits a measurable resistance or optical property that varies responsive to deformation of the strain-detecting element (300) and/or washer unit (200).

5. The washer unit (100) according to any of claims 1 to 4, wherein strain-detecting element (300) comprises a foil strain gauge, a semiconductor strain gauge, a piezo- resistive strain gauge, Fiber Bragg Grating (FBG) strain gauge or Brillouin strain gauge.

6. The washer unit (100) according to any of claims 1 to 5, provided with at least two strain-detecting elements (300a, 300b), each disposed within a separate passage (240, 242) within the body (202) of the washer (200).

7. The washer unit (100) according to claim 6, wherein one passage (240, 242) is disposed in a radial direction and one disposed in a tangential direction.

8. The washer unit (100) according to any of claims 1 to 7, further comprising a filler material disposed in the passage (240, 242) binding the strain-detecting element (300) to the washer body (202).

9. The washer unit (100) according to any of claims 1 to 8, further comprising a temperature sensor; preferably at least one temperature sensor per strain-detecting element (300); more preferably wherein the temperature sensor is disposed in the same passage (240, 242) as the strain detecting element (300).

10. A fastener (420) comprising a threaded bolt or threaded bar disposed with the washer unit (100) according to any of claims 1 to 9.

1 1. A strain detection system comprising:

- the washer unit (100) according to any of claims 1 to 9 disposed with a strain- detecting element comprising an electrical resistance strain gauge,

- a current source,

- a determination unit configured to determine from voltage potential across and current flowing through the strain-detecting element (300), a degree of deformation of the strain-detecting element (300).

12. A strain detection system comprising:

- the washer unit (100) according to any of claims 1 to 9 disposed with a strain- detecting element comprising an optical strain gauge,

- an interrogator unit configured to interrogate optically the optical strain gauge, and determine from light returning from the optical strain gauge the degree of deformation of the strain-detecting element.

13. The strain detection system according to claim 11 or 12, wherein the determination or interrogator unit is further configured to determine from the degree of deformation of the strain-detecting element (300) a quantitative or qualitative force factor indicative of tension in the joint fastened by the fastener (420).

14. A method of adapting a washer (200) to monitor of a pre-tensioned fastened joint (400) comprising introducing a strain-detecting element (300) in a passage (240, 242) within a body (202) of the washer (200) thereby forming a washer unit (100) according to any of claims 1 to 9.

15. A use of a washer unit (100) according to any of claims 1 to 9 and/or a strain detection system according to any of claims 1 1 to 13 for monitoring of a pre-tensioned fastened joint (400).

Description:
WASHER UNIT AND STRAIN DETECTION SYSTEM FOR A FASTENED JOINT

TECHNICAL FIELD

The present invention is in the field of strain detection and/or monitoring system techniques for fastened joint, for instance a threaded or bolted joint. In particular, the present invention provides systems and methods for detecting and/or monitoring of the tension in a fastened joint; said systems and methods comprising a washer unit configured for doing the same.

BACKGROUND

Fastened joints are among the most commonly used joining techniques in construction, consisting of a fastener such as a bolt or threaded bar and complementary nut(s) as well as the parts to be joined together. They typically serve to join two metallic parts, or to connect a metallic object to a concrete base.

To prevent loosening of the joint, it has become common practice to place a washer on a shaft of the fastener usually between the head of the bolt and the joined parts. Washers may aid in the tightening of the joint, minimize embedment of the bolt’s head into the joined parts, and provide a flat and smooth surface for optimal torqueing. More importantly, they may serve to better distribute the forces applied on the joint over a larger surface area.

In various applications (e.g. with a dynamic or cyclically applied load) the fastened joints require an initial clamping force to be applied to the fastener shaft, also referred to as pre-tensioning or preloading. The pre-tension ensures that the joint members stay clamped together and the fastener shaft remains in a state of tension.

Preloading may for instance be achieved by torqueing the fastener to a certain value such that, when the external torque is removed, the bolt will experience a constant (predefined) tensile stress. Alternatively, the threaded shaft of the fastener may be tensioned using an external tensioning device during fastening of the joint. After removal of the device the fastener shaft will similarly remain under the effect of the imposed tension.

Properly preloading and subsequently maintaining a sufficiently large tension may reduce fatigue damage and in turn extend the life of the fastened joint. Moreover, by ensuring a reliable long-term mechanical connection the lifetime of the joined parts may be extended, and in turn also that of the structure comprising the one or more fastened joints. In fact, for certain tall or extensive structures a lack of proper preloading may cause serious safety hazards; for example, if a bolt is loosened it may fall off and hurt nearby passer-by’s, or it may also result in structural damages.

At present, several methods exist that are suitable for measuring and tracking the tensile stress present in a fastened joint. For instance, the tension may be tracked by means of an ultrasonic transducer. By transmitting ultrasound through a bolt shaft and recording the soundwave’s travel time throughout said bolt the elongation of the bolt may be monitored. However, this method requires specific set-ups and readout units, is laborious and time-consuming, may require additional calibration steps to accord for variables such as variations in bolt lengths, and is moreover dependent on the surface quality of the fastened elements.

An alternative method involves tracking the tensile stress by means of a strain gauge sensor. For example, an optical fibre or resistive element may be either connected to or inserted into the fastened joint. However, this method may require modifications of the fastener to provide for sufficient space for the sensor; for example a central hole may need to be drilled. Moreover, the materials and readout units may be very costly and not practical for day-to-day field-use. Alternatively, a load cell containing pre- calibrated strain gauges may be placed between the bolt’s head and the adjoining structure. During torqueing of the joint, the load cell will deform and the deformation can serve as an indicator of the degree of tension exerted on the fastener. However, similarly to the previous methods, it requires expensive apparatus for a prolonged follow-up and may also require the use of longer fasteners to allow for sufficient fitting of the load cell. Moreover, a load cell presents a minimum thickness that is unsuitable or at least undesirable in many applications.

The prior art describes various ways to monitor fastened joints which suffer from potential mechanical instability, for instance, DE 10 2009 043 267 A1 an arrangement of a washer provided with a groove connected to a face of washer. DE 10 201 1 005 371 describes pressure sensors covered by a rubber or plastic covering. DE 20 2013 010 397 describes an arrangement with an external circumferential (multi-directional) sensor. In US 2016/0245709 a pressure sensor is provided with an encapsulating material.

Accordingly, it is an aim of the invention to overcome the problems of the art. In particular, there is a need for improved systems, devices and/or methods that can improve the tracking of tensile stress, thereby potentially improving the accuracy of preloading and reliability of the fastened joints. SUMMARY

The devices and methods according to the present disclosure solve the aforementioned problems. Accordingly, provided herein are systems and methods for detecting and/or monitoring of the tension in a fastened joint; said systems and methods comprising a washer unit configured for doing the same.

An aspect of the invention provides a washer unit for providing on a bolt comprising:

- a washer, and

- a strain-detecting element,

optionally, a filler material;

wherein the strain-detecting element is disposed at least partially below a surface of a body of the washer.

In some embodiments the strain-detecting element is disposed within a passage within the body of the washer, and at least one end of the passage opens to an edge of the washer body, optionally wherein the passage is disposed in a radial direction or in a tangential direction.

Another aspect provides washer unit (100) for providing on a fastener (420) comprising:

- a washer (200), and

- a strain-detecting element (300),

wherein

the strain-detecting element (300) is disposed within a passage (240, 242) within a body (202) of the washer (200),

at least one end of the passage (240, 242) opens to an edge (215) of the washer (200) body (202), and

the strain-detecting element is configured for detection of strain in one direction that is a direction of the passage (240, 242).

Both ends of the passage (240, 242) may open to an outer edge (215) of the washer (200) body (202). The passage may be straight and cylindrical. At least two strain- detecting elements (300a, 300b) may be provided, each disposed within a separate passage (240, 242) within the body (202) of the washer (200). One passage (240, 242) is disposed in a radial direction and one disposed in a tangential direction. 8. The washer unit (100) may further comprising a filler material disposed in the passage (240, 242) binding the strain-detecting element (300) to the washer body (202).

In some embodiments the washer unit is provided with two strain-detecting elements, each disposed within a separate passage within the body of the washer, wherein at least one end of each passage opens to an edge of the washer body, optionally wherein a first passage is disposed in a radial direction and a longitudinal axis of a second passage is disposed in a tangential direction.

In some embodiments the strain-detecting element is disposed within a groove on a face of the body of the washer, and recessed or flush with the face.

In some embodiments the washer body is elastically deformable in a radial direction with respect to a central axis of the washer and/or in a tangential direction.

In some embodiments the strain-detecting element exhibits a measurable resistance that varies responsive to deformation of the strain-detecting element and/or washer unit.

In some embodiments the strain-detecting element comprises a foil strain gauge, a semiconductor strain gauge, a piezo-resistive strain gauge, Fiber Bragg Grating (FBG) strain gauge or Brillouin strain gauge.

In some embodiments the washer unit comprises a temperature sensor; preferably at least one temperature sensor per strain-detecting element (300); more preferably wherein each different temperature sensor is disposed contiguous to or in the vicinity of each strain detecting element. The temperature sensor may be disposed in the same passage (240, 242) as the strain detecting element (300).

In some embodiments the strain-detecting element is provided in fixed relation to the washer such that deformations of the washer are transferred to the strain-detecting element.

In some embodiments the washer unit is compliant with one or more of DIN 93, DIN 267-26, DIN 432, DIN 433, DIN 433-1 , DIN 433-2, DIN 462, DIN 463, DIN 470, DIN 6319, DIN 6340, DIN 6797, DIN 6798, DIN 6799, DIN 7349, DIN 7989-1 , DIN 7989-2, and DIN 9021 ; and/or with one or more of ISO 7089, ISO 7091 , ISO 7092, ISO 7093- 1 , ISO 7093-2, ISO 7094.

A further aspect of the invention provides a threaded bolt disposed with the washer unit according to any of the embodiments as described herein.

A further aspect of the invention provides a strain detection system comprising:

- the washer unit according to any of the embodiments as described herein disposed with a strain-detecting element comprising an electrical resistance strain gauge;

- a current source,

- a determination unit configured to determine from voltage potential across and current flowing through the strain-detecting element, a degree of deformation of the strain-detecting element. A further aspect of the invention provides a strain detection system comprising:

- the washer unit according to any of the embodiments as described herein disposed with a strain-detecting element comprising an optical strain gauge;

- an interrogator unit configured to interrogate optically the optical strain gauge, and determine from light returning from the optical strain gauge the degree of deformation of the strain-detecting element.

In some embodiments the determination or interrogator unit is further configured to determine from the degree of deformation of the strain-detecting element a quantitative or qualitative force factor indicative of tension in the fastened joint.

In some embodiments the strain detection system further comprises a communication unit configured to transmit data to a remote location, the data comprising one or more of the quantitative, the qualitative force factor, the degree of deformation of the strain- detecting element.

A further aspect of the invention provides a method of adapting a washer to monitor of a pre-tensioned fastened joint comprising introducing a strain-detecting element at least partially below a surface of a body of the washer thereby forming a washer unit according to any of the embodiments as described herein. A further aspect provides a method of adapting a washer to monitor of a pre-tensioned fastened joint comprising introducing a strain-detecting element in a passage (240, 242) within a body (202) of the washer (200) thereby forming a washer unit (100) as described herein.

A further aspect of the invention provides a use of a washer unit according to any of the embodiments as described herein for monitoring of a pre-tensioned joint fastened by the fastener (420).

A further aspect of the invention provides a use of strain detection system according to any of the embodiments as described herein for monitoring of a pre-tensioned fastened joint.

DESCRIPTION OF THE FIGURES

The following description of the figures of specific embodiments of the invention is only given by way of example and is not intended to limit the present explanation, its application or use. In the drawings, identical reference numerals refer to the same or similar parts and features.

FIG. 1A shows an exemplary washer unit (100) from cross-sectional perspective.

FIG. 1 B shows an exemplary washer unit (100) from cross-sectional perspective.

FIG. 1C shows an exemplary washer unit (100) from top perspective.

FIG. 1 D shows an exemplary washer unit (100) from top perspective. FIG. 1 E shows an exemplary washer unit (100) from top perspective.

FIG. 2A shows an exemplary washer unit (100) from a cross-sectional perspective.

FIG. 2B shows an exemplary washer unit (100) from a cross-sectional perspective.

FIG. 2C shows an exemplary washer unit (100) from top perspective.

FIG. 3 shows exemplary directions of strain detection directions on a washer unit.

FIG. 4 shows an exemplary fastened joint (400) from a cross-sectional perspective. FIG. 5 shows an exemplary strain detection system.

FIG. 6 shows an exemplary application of a strain detection system in a wind turbine. FIG. 7 shows a graph of a strain value read-out for a washer unit when pressure is applied

FIG. 8 shows a graph of a washer unit readout that is qualitative when pressure is applied

FIG. 9 shows strain value read-out of a washer unit, and an effect of the read-out due to temperature.

DESCRIPTION OF THE INVENTION

As used below in this text, the singular forms“a”,“an”,“the” include both the singular and the plural, unless the context clearly indicates otherwise.

The terms“comprise”,“comprises” as used below are synonymous with“including”, “include” or “contain”, “contains” and are inclusive or open and do not exclude additional unmentioned parts, elements or method steps. Where this description refers to a product or process which“comprises” specific features, parts or steps, this refers to the possibility that other features, parts or steps may also be present, but may also refer to embodiments which only contain the listed features, parts or steps.

The enumeration of numeric values by means of ranges of figures comprises all values and fractions in these ranges, as well as the cited end points.

The term“approximately” as used when referring to a measurable value, such as a parameter, an amount, a time period, and the like, is intended to include variations of +/- 10% or less, preferably +/-5% or less, more preferably +/-1% or less, and still more preferably +/-0.1% or less, of and from the specified value, in so far as the variations apply to the invention disclosed herein. It should be understood that the value to which the term“approximately” refers per se has also been disclosed.

All references cited in this description are hereby deemed to be incorporated in their entirety by way of reference.

Unless defined otherwise, all terms disclosed in the invention, including technical and scientific terms, have the meaning which a person skilled in the art usually gives them. For further guidance, definitions are included to further explain terms which are used in the description of the invention.

The present invention provides systems and methods for monitoring of a pre- tensioned fastened joint such as a threaded joint or a bolted joint. The terms fastened joint or connection, or simply joint are used interchangeably herein and generally refer to a structure comprising a fastener, in particular a threaded fastener. A threaded fastener comprises a threaded shaft (e.g. threaded bolt shaft, threaded bar), and a complementary nut. A fastened threaded joint comprises a threaded fastener, a washer, and the elements or parts that are connected or clamped by said fastener. The terms pre-tension, pre-stress and preload are also used interchangeably herein. The threaded bolt comprising a bolt head and a threaded bolt shaft is disposed with the washer unit. The threaded bar comprising a threaded bolt shaft is disposed with the washer unit. The fastened joint is pre-tensioned.

The primary advantage of the present invention is that industry standard components can be used for the joint. In particular the key components (e.g. fastener and washer) need not undergo substantial in-field or factory modifications, which may improve their reliability and reduce structural failure; moreover, no additional components need to be added to the fastened connection, which reduces the complexity. The former advantages may in turn improve the reliability of the fastened connection itself and the structures it is part of. Production of the present invention and its components is not resource intensive and may be suitable for larger scale production.

Furthermore, the readout and monitoring of the pre-tensioning may be more user- friendly, fast, efficient, accurate and/or reliable. As a result thereof the invention can also be used on larger bolt sizes and more complex installations or constructions, which would otherwise be too costly or labour intensive for individual monitoring.

An aspect of the invention provides a washer unit for providing on a fastener in particular a threaded fastener (e.g. threaded bolt or bar) of a pre-tensioned fastened joint, the washer unit comprising: a washer, and a strain-detecting element, wherein the strain-detecting element is disposed at least partially, preferably entirely below a surface of a body of the washer.

The washer has a body having a first face and an opposing second face and is disposed with an opening usually disposed in a centre of the faces connecting both first and second faces for passage of the fastener (e.g. threaded bolt or bar). The washer has an outer edge joining the first and second faces on an outer edge of the washer. The washer has an inner edge joining the first and second faces around the opening. The washer may be one of any type (e.g. plain, spring or locking) or form (e.g. form A to G). The washer shape may be varied depending on the application (e.g. annular, spherical, triangular, etc.) and may comprise one or more curvatures. Preferably the washer has a flat annular form for improved ease of use. Typically the washer is made of a metal, such as steel, or an alloy; however, other materials such as polymers or carbon may also be suitable provided they have sufficient material strength. The thickness of the washer may be varied depending on the application, although it is preferably of sufficient thickness to at least partially envelop the strain- detecting element for protective purposes. It is noted that the present invention may allow for production of washers with a thickness thinner than that of load cells or the like.

The strain-detecting element may be a sensor of any type that is configured for detection of strain. Preferably the strain-detecting element is configured for detection of strain in one direction. Preferably the strain-detecting element is configured for detection of strain in only one direction. Preferably the strain-detecting element is configured for detection of strain in a radial direction. The term radial direction is known in the art. It refers to a direction projecting outwards from a geometric centre of the washer and preferably parallel to a first or second face, in particular a direction running perpendicular to the central axis of the washer through (as indicated by A-A on Fig. 1A). Additionally or alternatively, the strain-detecting element may be configured for detection of strain in a tangential direction. The term tangential direction is known in the art. It refers to a direction tangential to a circle centred around a geometric centre, the circle centred and perpendicular to a central axis of the washer (as indicated by A-A’ on Fig. 1 A)).

Additionally or alternatively, the strain-detecting element may be configured for detection of strain in a direction divergent from the radial direction. The term divergent direction refers to a direction that crosses a radial direction at angle (e.g. 1 to 89 degrees) and is preferably parallel to a first or second face.

The strain-detecting element is preferably disposed in a way suitable for detecting strain exerted on the washer in a plane perpendicular to the axial (A-A’) direction or in a plane perpendicular to a central axis (A-A’) of the washer. The shape and volume of the sensor may be varied according to the requirements of its application. For example, the sensor may be a foil or a strand, or any shape in-between. The strain-detecting element is disposed at least partially, preferably entirely below a surface of the body of the washer; it may be disposed in a groove below said surface thereby essentially extending the surface area, or be disposed deeper within the body, for example running centrally through said washer body. Sufficient space may be provided within the washer’s body during production, or alternatively, the washer’s body may be modified post-production. For example, a groove may be machined out from the first or second face of the washer, holes may be drilled through the outer or inner side edge of the washer, or a recess may be introduced by means of a stamping process.

In some preferred embodiments the strain-detecting element is provided in fixed relation to the washer such that deformations of the washer are transferred to the strain-detecting element.

In some embodiments the strain-detecting element is disposed within a passage within the body of the washer, and at least one end of the passage opens to the outer edge of the washer body. The other end of the passage may open to the inner edge of the washer body. Both ends of the passage may open to the outer edge of the washer body. The passage may be disposed parallel to the first and/or second face. The passage may be disposed in a radial direction with respect to a central axis (A-A’) of the washer, or in a direction divergent to said radial direction, or in a tangential direction. The passage is disposed between the first and second washer faces. The passage is disposed at least partially preferably entirely below the first and second washer faces. The passage is typically a void space enclosed within the body of the washer, open at one or both ends. A wall of the passage within the body of the washer may be closed to the first and second faces.

The passage may be of any shape, curvature and volume suitable to at least partially, preferably entirely contain the strain-detecting element below a surface of a body of the washer. Preferably the passage is a straight and cylindrical. Preferably, the passage provides space to fully contain the strain-detecting element below a surface of a body of the washer. The latter allows for better protection of the strain-detecting element.

The outer edge of the washer body is preferably for providing a means of connecting the strain-detecting element to a readout unit; for example a conductive materials such as a cable.

In some embodiments the passage forms a linear shape, such as at least one line crossing the body of the washer. Optionally multiple passages may connect or intersect throughout the washer body. This may allow for strain detection focusing on one or more area of interest, particularly well-suited for strand of fibre shaped strain- detecting element.

Two or more passages may be provided each for a separate strain-detecting element, wherein at least one end, preferably both ends, of each passage opens to an edge of the washer body. At least one passage may be disposed in a radial direction with respect to a central axis of the washer, and one other passage disposed in a tangential direction, both passages preferably being parallel to the first and/or second face of the washer.

The at least one passage may be introduced using a drilling process, for instance, drilling from an outer edge of the washing. The passage may be a borehole. The washer unit may be prepared starting from a standard or manufactured washer.

The strain-detecting element may be configured for detection of strain in a direction of the passage. The strain-detecting element may be configured for detection of strain in one direction that is a direction of the passage.

Deformation of the washer due to fastening of the bolted connection can be indirectly measured using a sensor in the passage from within a body of the washer in a qualitative manner sufficient to report a status of the fastened joint. In particular, when the strain-detecting element is configured for detection of strain in a direction of the passage a surface groove can be relinquished. The passageway disposed within the washer body protects the sensor from environmental stresses (e.g. sea water, rain), and eliminates or reduces possibility for sensor detachment that can lead to false readings in remote monitoring. Detachment of a groove-mounted sensor caused by washer deformation is problematic; the loss of contact between the sensor and the washer causes a change in readings, triggering an investigation of the remote joint that is costly in an off-shore setting.

The passage facilitates recording of measurements inside the washer, and not on a surface, reducing a possibility of distortions limited to one or other surface being undetected. As such the present arrangement is more sensitive to the overall deformation, and further, torsional distortions which do not affect the washer diameter are fully detectable.

With a passage open at both ends, there is a reduction in void spaces that can be formed within the passage when the sensor is set in position, for instance, using filler. Reduction or elimination of void spaces increases transmission of compression forces to the sensor, and reduces or eliminates corrosion of the washer and/or sensor from within due to trapped humidity.

In some embodiments the strain-detecting element is disposed within a groove on a first and/or second face of the body of the washer, and recessed or flush with the face. The strain-detecting element and hence groove may be provided on the face of the washer that points away from the object being fixed by the bolt. Preferably the strain- detecting element and hence groove is provided on the face of the washer that points towards the object being fixed by the bolt. The groove may have a rectangular, triangular or rounded profile when viewed in cross-section. The groove may form a closed loop path, such as an elliptical or annular shape when viewed on a face of the washer. This may allow for improved sensitivity allowing detection of strain over the whole washer body, particularly well-suited for foil shaped strain-detecting element. The groove may have an open path i.e. the ends do not close to form a loop, such as an annular segment when viewed on a face of the washer. The groove may continue as a path to the outer edge of the washer body; this provides a means of connecting the strain-detecting element to a readout unit; for example a conductive materials such as a cable. The groove may be connected to the outer edge of the washer body via a passage within the body of the washer; this provides a means of connecting the strain-detecting element to a readout unit; for example a conductive materials such as a cable. Disposing the strain-detecting element as a groove on the washer’s face allows for reduced complexity of production and moreover improved ease-of-access for maintenance (e.g. malfunction of the element) and connection (e.g. readout).

The groove may be introduced using a milling process, for instance, milling the form of the groove onto a first and/or second face of the washer. Alternatively, the groove may be introduced using a stamping process, for instance, stamping the form of the groove into a first and/or second face of the washer. The washer unit may be prepared starting from a standard or manufactured washer.

The strain-detecting element may be permanently or non-permanently disposed within the body of the washer; it is preferably permanently disposed. For example, the element may be mechanically clamped by the washer body, or it may be bound by means of an adhesive, such as glue, paste, silicon, and the like. In some embodiments the washer comprises a filler material. Preferably the filler material is disposed in a passage and/or groove of the washer unit. The filler material may be provided at least in a void space around the strain detection element(s).

The filler material may serve as an adhesive to bind the strain-detecting element to the washer body. The filler material may serve to occlude any voids in the introduced passage and/or groove. This may improve the reliability of the sensed strain by removing the presence of empty voids in the washer unit, which could otherwise impact the washer’s deformation resistance. Alternatively the passage and/or groove may be adapted to fit the shape of the strain-detecting element, in which case less to no filler material may be required.

The filler material may be disposed over and under the strain-detecting element to form a protective cover or shell surrounding the element. This may protect the element from mechanical shocks, for instance during transporting or handling of the washer unit, or environmental influences, such as humidity.

In some preferred embodiments the filler material is a resin; more preferably an epoxy resin, polyuria, vinyl ester, or the like.

One or more strain-detecting elements may be disposed in the groove of the washer body. In some preferred embodiments two or more strain-detecting elements may be disposed in the groove of the washer body. A first strain detecting element may be disposed in the groove for detection of strain in a radial direction and a second strain detecting element may be disposed in the groove for detection of strain in a tangential direction.

A first strain detecting element may be disposed in the groove for detection of strain in a radial direction or in a direction divergent to said radial direction, and a second strain detecting element may be disposed in the groove for detection of strain in a direction perpendicular to the first strain-detecting element.

The presence of two or more strain-detecting elements may allow for more reliable and accurate monitoring of different areas within the washer body, or in different directions, such as the radial direction with respect to a central axis of the washer and in a tangential direction with respect to said radial direction.

In some preferred embodiments one or more, preferably two strain-detecting elements are provided, each strain-detecting element disposed within a separate passage within the body of the washer, wherein at least one end, preferably both ends, of each passage opens to an edge of the washer body.

The presence of two strain-detecting elements may allow for more reliable and accurate monitoring of different areas within the washer body, or in different directions, such as different radial directions, different tangential directions, or a combination of radial and tangential directions.

In some further embodiments a longitudinal axis of a first passage is disposed in a radial direction or in a direction divergent to said radial direction, and a longitudinal axis of a second passage is disposed in a tangential direction.

Simultaneous monitoring of different strain directions, such as the radial and the tangential direction, may further improve the reliability and accuracy of the washer unit.

In some embodiments at least two strain-detecting elements are provided, each strain-detecting element disposed within a separate passage within the body of the washer, wherein at least one end, preferably each end, of each passage opens to an edge of the washer body. A greater number of strain-detecting elements may provide for accurate monitoring of the pre-tension. It is thus noted that the present system can be adapted according to the requirements of its application.

The strain detecting element may comprise an electrical resistance strain gauge, where the strain gage's electrical resistance varies in proportion to the amount of strain. The electrical resistance strain gauge may be a foil strain gauge, a semiconductor strain gauge, and/or a piezo-resistive strain gauge. The strain- detecting element may comprise a curable strain sensitive polymer whose shape prior to curing can be adapted according to the form of the groove.

The strain -detecting element may comprise an optical strain gauge wherein the strain gage's optical properties, typically a wave-interference property, varies in proportion to the amount of strain; preferably a Fiber Bragg Grating (FBG) strain gauge or Brillouin strain gauge.

Preferably the strain-detecting element is configured for detection of strain in one direction. Preferably the strain-detecting element is configured for detection of strain in only one direction. The strain gauge is preferably configured for measuring deformation of the washer body in a radial or tangential direction.

Where two or more strain-detecting elements are provided, both strain detecting elements may be electrical resistance strain gauges, or both strain detecting elements be electrical resistance strain gauges, or both strain detecting elements be optical strain gauges, or a first strain-detecting element may be an electrical resistance strain gauge and a second strain-detecting element may be an optical strain gauge. Wherein two or more strain-detecting elements are provided one strain-detecting element may be a foil strain gauge and the second strain-detecting element is a piezo-resistive strain gauge. This embodiment is particularly well suited for highly accurate and reliable monitoring.

In some embodiments the strain-detecting element has width of at least 1 mm to at most 10 mm; preferably 2 to 8 mm; more preferably 3 to 6 mm; most preferably 4 to 5 mm.

In some embodiments the strain-detecting element has length of at least 1 mm to at most 10 mm; preferably 2 to 8 mm; more preferably 3 to 6 mm; most preferably 4 to 5 mm.

In some embodiments the strain-detecting element has a thickness of at least 0.01 mm to at most 1.0 mm; preferably 0.05 to 0.5 mm; more preferably 0.07 to 0.3 mm; most preferably 0.1 to 0.2 mm.

In some embodiments the washer unit is compliant with one or more of DIN 93, DIN 267-26, DIN 432, DIN 433, DIN 433-1 , DIN 433-2, DIN 462, DIN 463, DIN 470, DIN 6319, DIN 6340, DIN 6797, DIN 6798, DIN 6799, DIN 7349, DIN 7989-1 , DIN 7989-2, and DIN 9021 ; and of ISO 7089, ISO 7091 , ISO 7092, ISO 7093-1 , ISO 7093-2, ISO 7094.

In some embodiments the washer unit comprises at least one temperature sensor. The presence of a temperature sensor allows compensating for temperature variations in the washer body. Any changes in temperature may result in shrinking or expanding of the washer body, the degree depending on the washer body material, and may thus result in a‘faulty’ strain measurement of the strain. Significant thermal environmental changes are present in off-shore applications (e.g. wind turbine, oil rig) and on-shore applications (e.g. chemical plant). The temperature sensor may be disposed in a groove or passage, similarly to a strain detecting element; or it may be fixated onto the washer body surface.

In some embodiments the washer unit comprises at least one temperature sensor per strain detecting element; preferably wherein every temperature sensor is disposed contiguous to or in the vicinity of a different strain detecting element. By measuring the temperature variations locally the reliability of the washer unit may be further improved. In some embodiments the temperature sensor is a resistance thermometer; for example Pt100, Pt1000, and the like.

The temperature sensor may be disposed in a groove or within a passage. The groove or passage may be shared with the strain detecting element. The groove or passage may be separate from the groove or passage accommodating the strain detecting element. There may be one temperature sensor per strain-detecting element. A passage may contain the same number of temperature sensors as strain- detecting elements. A passage may contain one temperature sensor and one strain- detecting element.

A further aspect of the invention provides a threaded fastener disposed with the washer unit according to any of the embodiments described herein. The washer unit is thus configured for measuring the degree of pre-tension on said fastener. A further aspect of the invention provides a threaded bolt or bar and nut disposed with the washer unit according to any of the embodiments described herein. The washer unit is thus configured for measuring the degree of pre-tension on said threaded bolt.

A further aspect of the invention provides a strain detection system comprising:

- the washer unit according to any of the embodiments described herein disposed with a strain-detecting element that comprises an electrical resistance strain gauge;

- a current source; and,

- a determination unit configured to determine from voltage potential across and current flowing through the strain-detecting element, a degree of deformation of the strain-detecting element.

A further aspect of the invention provides a strain detection system comprising:

- the washer unit as described herein disposed with a strain-detecting element comprising an optical strain gauge;

and,

- an interrogator unit configured to interrogate optically the optical strain gauge, and determine from light returning from the optical strain gauge the degree of deformation of the strain-detecting element. The strain detection system is particularly well-suited for monitoring of a pre-tensioned fastened joint by detecting the degree of deformation of the washer unit, which can be related to the strain disposed in a radial or tangential direction.

The strain detection system may further determine a status of the fastener. The status may be binary (e.g. pass/fail). The status may be determined from a

non-linear relationship between strain value read-out and applied pressure, and whether the strain value read-out falls above or below a threshold range. The non-linear relationship and threshold range may be embedded within the system as part of a calibration during production or fabrication.

The washer unit may be connected to the determination unit with a connection means; preferably a cable. The cable may be any type of networking hardware; for example a standard networking cable, coaxial or fibre optic cable. Alternatively, the cable made be a purpose-made cable; for example a 4- , 5- or 6- wire electrical cable, with or without electromagnetic shielding. The connection means may also be partially or fully wireless provided that the washer unit is connected with a transmitter and current source.

The current source provides electrical energy to activate the strain-detecting element and transmit data to the determination unit. The current source may be an external power supply (e.g. power grid), or may be local unit (e.g. battery). In some embodiments the current source is provided by a same connection means; preferably the connection means connecting the washer unit and the determination unit. This allows the determination unit to the source of electrical energy, reducing complexity of the system.

In some embodiments the determination unit or interrogator unit is further configured to determine from the degree of deformation of the strain-detecting element a quantitative or qualitative force factor indicative of tension in the fastened joint. The force factor may be suitable for determining the pre-tension in the thread bolt.

In some embodiments the qualitative force factor indicates one or more of sufficient tension, insufficient tension of no danger, dangerously insufficient tension, or error. The qualitative force factors may be further limited or expanded upon depending on the specific application. Alternatively, the force factor may also be expressed numerically, such as percentages.

In some embodiments the strain detection system further comprises a communication unit configured to transmit data to a remote location, the data comprising one or more of the quantitative, the qualitative force factor, and the degree of deformation of the strain-detecting element. The communication unit may thus allow remote monitoring of the fastened joint. Additional units related to monitoring, such as an auditory (e.g. alarm) or visual indication system (e.g. LED) may be added to improve user- friendliness of the monitoring; for example by allowing a maintenance crew to more quickly and easily identify the fastened joint in need of maintenance, thereby reducing or altogether preventing the occurrence of safety hazards (e.g. loose bolts falling from a high structure such as a turbine tower).

A further aspect of the invention provides for a method of adapting a washer to monitor of a pre-tensioned fastened joint comprising introducing a strain-detecting element at least partially, preferably entirely below a surface of a body of the washer, thereby forming a washer unit according to any of the embodiments described herein. The at least one passage may be introduced using a drilling process, for instance, drilling from an outer edge of the washing. The washer unit may be prepared starting from a standard or manufactured washer.

The system may be configured for determining from data outputted by the washer unit, in particular the strain-detecting element, a local tensile force upon the fastener. The measurement may be performed by applying a low-amplitude current over the resistor and subsequently reading out the resulting potential value. By applying Ohm’s law the resistance can be calculated. The resistance value is proportional to the deformation experienced by the strain gauge. As the strain gauge is fixed to or in the washer, the deformation of the strain gauge is proportional to the deformation of the washer, which in turn is proportional to the tension of the fastener: the higher the tension, the higher the pressure on the washer, the higher the deformation of the washer.

A further aspect of the invention provides a use of a washer unit according to one or more embodiments as described herein. A further aspect of the invention provides a use of a strain detection system according to one or more embodiments as described herein.

In some embodiments the use of the washer unit and/or strain detection system is for monitoring of the deformation of the washer unit.

In some embodiments the use of the washer unit and/or strain detection system is for monitoring of the pre-tensioning of a fastened joint; for example a fastened joint clamping together at least two metal parts, such as in machinery, or a fastened joint connecting a metallic object to a concrete base, such as in construction.

Suitable examples may include, but are not limited to, monitoring of one or more fastened joint in a wind turbine (on- and offshore), gantry crane, general crane, architectural building (e.g. landmark buildings); general machinery (e.g. machine bases and mounting in production plants), piping and plumbing fitting (e.g. flanges) and others.

EXAMPLES

Example 1

Reference is made to FIG. 1A-E and FIG. 2A-C. The figures illustrate different views of a washer (200) or washer unit (100); according to an exemplary embodiment, the washer unit (100) comprising a washer (200) and a strain-detecting element (300), wherein the strain-detecting element (300) is disposed at least partially below a surface of a body (202) of the washer (200). The central axis (A-A’) of the washer (200) passing through opening (210) is illustrated with a dashed line. In FIGs. 1A-E the strain-detecting element (300) is disposed within a groove. In FIG. 2A-C the strain- detecting element (300) is disposed within a passage.

In FIG. 1A the strain-detecting element (300a) is disposed within a groove (230) on a face (212, 214) of the body (202) of the washer (200), and recessed or flush with the face (212, 214). A filler material (310), such as an epoxy resin is disposed in the groove (230). This embodiment is particularly well-suited for a foil or rectangular shaped strain-detecting element; such as a foil strain gauge. For example, the element may have a surface area of 4mm x 4mm, which can be disposed within the open space formed by the groove. FIG. 1 B is similar to the cross sectional view of FIG. 1A without the strain-detecting element and filler material, and showing the empty groove (230). FIG. 1C is an exemplary plan view where the groove (230) is formed as an annulus and wherein two strain-detecting elements (300a, 300b) have been inserted; namely a first strain-detecting element (300a) for detecting strain in a radial direction, and a second strain-detecting element (300b) for detecting strain in a tangential direction. FIG. 1 D shows an adaptation of the washer unit of FIG. 1C where the annular groove (230) opens to an edge (215) of the washer (200) body (202). FIG. 1 E shows an adaptation of the washer unit of FIG. 1C where the groove (230) is formed as an annulus segment.

In FIG. 2A the washer unit (100) is provided with two strain-detecting elements (300a, 300b), each disposed within a separate passage (240, 242) within the body (202) of the washer (200). The passages (240, 242) comprising the two strain-detecting elements (300a, 300b) are running centrally throughout a height of the washer body. This exemplary embodiment is particularly well-suited for a strand of fibre shaped strain-detecting element; such as a semiconductor strain gauge or a piezo-resistive strain gauge. The two strain-detecting elements (300a, b) have been fixed with a filler material (310), such as an adhesive or epoxy resin. FIG. 2B is a same as the cross sectional view of FIG. 2A, without the strain-detecting elements and showing the passages (240, 242). FIG. 2C is a plan view where each strain-detecting element (300a, 300b) is seen to lie tangential to a radial direction of the washer.

Further reference is made to FIG. 3, which illustrates different radial directions (352a to d) and different tangential directions (356i, iia, iib) of the washer, and different directions for the measurement of strain (358a to d). One strain measurement direction is radial (358a), two strain measurement directions are tangential (358b, c), the remaining strain direction (358d) is divergent from the radial direction. Also shown are centred circles (354I, II) that give rise to tangents (356i, iia, iib).

Further reference is made to FIG. 4, which shows a fastened joint (400) that is a bolted joint comprising a washer unit (100) and a threaded bolt (420) engaging with a threaded nut (440), which bolted joint is clamping an exemplary object (460) together; for example two metallic parts. The threaded bolt (420) comprising a bolt head (422) and a bolt shaft (424) is disposed with the washer unit (100) running parallel with the central axis (A-A’) of the washer (200). Also shown are centred circles (354I, II) that give rise to tangents (356i, iia, iib).

Example 2

Reference is made to FIG. 5 and FIG. 6, which illustrate a strain detection system and an exemplary application thereof.

Firstly, FIG. 5 shows a washer unit (100) which is connected to a (conductive) cable (400) that may be connected to a readout unit. In this particular example the cable is a standard networking cable, but any type of networking hardware such as a coaxial or fibre optic cables may be used instead. The washer unit (100) comprises a washer (200), and a strain-detecting element (suggested by arrow 300) disposed inside inside a groove in the washer body below filler material (310). Once said washer unit is installed in a fastened joint (illustrated in Example 1 ) and connected to the readout unit, the pre-tensioning of the fastened joint may be monitored.

As illustrated in FIG. 6, a suitable and non-limiting example may include a wind turbine comprising a plurality of fastened joints that are clamping together an upper and a lower section of said turbine, wherein each fastened joint is individually monitored by means of the strain detection system.

In case a fastened joint has insufficient tension, the strain detection system may transmit data to a remote location by means of a communication unit, which data may in turn alert a maintenance crew that a fastened joint has slightly insufficient tension, thus requiring attention, or has significantly insufficient tension, thus requiring immediate maintenance. Additionally, the communication unit may also transmit an error in the readout, which may indicate that a washer unit or a strain-detecting element is malfunctioning; for instance as a result of poor connection or in need of washer unit replacement.

Example 3

Reference is made to FIG. 7 showing a graph of a strain value read-out (in Micro- strain units) derived from a unidirectional strain-detecting element disposed within a passage within a body of a washer unit present in a bolt assembly, wherein pressure is applied to the washer unit by the nut fastened using a hydraulic torqueing tool or wrench. The graph confirms that relationship between applied pressure and the strain- detecting element read-out is non-linear.

The non-linear relationship, which can vary from washer unit-to-washer unit, was used to determine a threshold strain value range for each washer unit (e.g. 135 to 150 microstrain at 300 bar). A strain value read-out above the threshold range indicates a tensioned joint; a strain value read-out below the threshold range indicates loss of tension in the joint. The non-linear relationship, and threshold range may be embedded within a system as part of a calibration during production.

Example 4

Reference is made to FIG. 8. A bolt assembly described herein containing a calibrated washer unit is tensioned over time (hours) to the required bolt tension. The washer unit readout was essentially a plateau (-1.2 to -4.85 hours) until the required tension was applied using a tensioning tool (arrow), after which the readout significantly changed. The 2-state washer unit readout illustrates a strong qualitative indicator of the status of the tensioned joint by the washer unit.

The strain value read-out of the strain-detecting element was transformed into the washer unit readout using the known non-linear relationship and threshold range of Example 3 determined during a calibration step.

Example 5

Reference is made to FIG. 9 showing a graph of a strain value read-out of a unidirectional strain-detecting element disposed within a passage within a body of a washer unit, and an effect of the read-out due to temperature. Over a range of 55°C the strain value read-out can vary by 150 microstrain and the relationship between the strain value read-out and the temperature is a nonlinear one.