Abstract:
A bolt shear force sensor for in-service monitoring is provided in which the sensor includes a deformable ring with channel-like cross-sections and a wire egress hole. The ring surrounds a bolt or fastener to be tested. The channel-like cross sections face exterior to a center of the ring. Strain sensing elements are mounted within the channel cross-sections as an outer circumference of the ring. The sensing elements capture hoop strain from surrounding plates of the operating system which holds the fastener. A signal conditioner operationally connected via wire conductors is used to convert strain sensing signals from the sensing elements to strain output. The strain outputs are then used in conjunction with a calibration curve of shear force versus strain to determine the magnitudes and directions of the resultant shear forces.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a bolt shear force sensor for use in design testing as well as in-service monitoring in order to provide a structural assessment of a bolt or fastener while attached in an operating system. 
     (2) Description of the Prior Art 
     The design of submarine and ship systems required to survive the effects of mechanical shock must consider not only the system and foundation to which the system is attached but also the mechanical fasteners connecting them. While it is well known that structures typically fail at mechanical joints and other discontinuities; there are significant difficulties in assessing and predicting localized distribution of forces within the mechanical fasteners used at the joints. 
     These difficulties increase when loading forces are dynamic as in the case of shock, blast, vibration and seismic events. As such, making a determination of proper sizing, spacing, pre-tensioning and quality of fasteners becomes increasingly critical. Therefore, achieving the required level of structural integrity in mechanically fastened joints has further implications on the design methods used and the need for experimental testing. 
     In commercial applications, qualification by analysis is becoming more prevalent; especially when providing design guidance and establishing performance criteria. Since the need remains to understand how mechanically-fastened joints behave; a need also exists to identify individual fastener behavior from structural tests and to improve the knowledge base and design methods for joints in load-carrying structures. 
     Multiple devices and methods for measuring axial forces in fasteners are described in the art. However, these devices and methods do not adequately measure fastener shear forces in which those forces transversely align with a longitudinal axis of a bolt. In a properly designed joint, axial and shear forces must be considered because of their cumulative effect on fastener stress. 
     While handbook calculations and numerical methods are typically employed for estimation purposes; both types of methods require kinematic and constitutive (stiffness) assumptions in regard to the transfer of external loads and the resulting distribution of internal reaction forces among participating joint components. In mechanically fastened joints subjected to shearing loads; factors (such as elasticity of the joined members, hole tolerances, interfacial friction, creep, thermal expansion, etc) affect the ability of the joint to resist shearing motions among joined members. 
     While fastener shear forces directly create shear stresses; the forces also create a secondary effect that generates fastener bending stresses. These bending stresses, which are difficult to quantify because the bending moment arm in each fastener is often unknown, superimpose with pretension stresses. As such, it is apparent that for joints subject to shearing loads; fastener tensions stresses comprise the direct axial tension (the sum of initial pretension and externally applied axial joint loads) and the bending stress due to shearing forces. 
     In the known art, Slack (U.S. Pat. No. 4,870,866) describes an ultrasonic method for measuring contact pressures on mating interfaces of solids undergoing compression. In the reference, an interfacial region contains an entrapped film of liquid. By monitoring the change in acoustic pulses; the contact pressure can be determined. However, to determine the shear force, additional processing would require an integration of the contact pressure over the contact area. Hence, to obtain the shear force; the contact area must also be monitored. This method would be difficult to employ with existing in-service monitoring applications. 
     Hay (U.S. Pat. No. 5,945,665) describes a fiber optic strain gage-based transducer which measures only the axial force (including pretension) in the bolt (See  FIG. 1 ). In the figure, a Bragg grating sensor  1  is inserted and held by epoxy  2  in a mechanical fastener  10 . Optical fiber  3  connects the grating sensor  1  to an external connector  4 . Other devices use washers instrumented with wire (foil) strain gages in which the gages only measure the axial forces in bolts. 
     In other commercially-available measurement devices, pressure sensitive films are mounted between the fasteners. The bearing surface of the mating holes for the fasteners can be used to measure shear forces from patterns of contact pressures. These films contain microcapsules that release color at prescribed pressure ranges. A scanning device is then used to measure the variations in color intensity of the contact pressure pattern. However, the films do not allow continuous, real-time monitoring of forces. Disassembly of the joint is required to quantify the magnitudes of the shear forces. 
     Also in the prior art, Shah (U.S. Pat. No. 8,433,160) includes optical sensing elements positioned through a continuous aperture collinearly with a longitudinal axis of a fastener in order to sense strain and temperature. The sensing elements sense localized fastener deformations that occur along the longitudinal axis. The fiber optic sensing elements can be used to capture axial, transverse and torsional strains as measured from the localized deformations of the drilled hole region of the fastener. However, the Shah reference cannot distinguish directly between transverse compression strains due to Poisson&#39;s effect and bolt bearing strains. 
     Using  FIG. 2 , consider an element of material  11  of a fastener  12  loaded in pure axial tension. By Poisson&#39;s effect, a compressive strain would develop along an axis orthogonally transverse to the longitudinal axis of the fastener (See  FIG. 3 ). The Shah reference senses this transverse strain which is not a result of any bolt bearing induced strains. 
     Using  FIG. 4 , now consider the combined presence of Poisson&#39;s strains from bolt tension and transverse strains of a bolt bearing. For example in the figure, the fastener  12  with the identified element  11  is loaded in double shear by forces “P” and “P/ 2 ” with the fastener also under axial tensile force as indicated by direction arrows A and B. In  FIG. 5 , the identified element  11  is under compressive and axial strain. 
     These transverse strains are sensed rather than the individual transverse strain components. With the use of signal processing, the Poisson&#39;s effect transverse strains (i.e., the transverse compressive strains resulting from axial tensile loads in the fastener) must be separated from the transverse bolt bearing strains. This is an indirect approach. 
     Based on the existing art, a need still exists for a bolt shear force sensor for design validation testing as well as in-service monitoring for structural health assessment. The sensor should be available for use in both static and dynamic applications. 
     SUMMARY OF THE INVENTION 
     It is therefore a general purpose and primary object of the present invention to provide a bolt shear force sensor for use in design validation as well as in-service monitoring. 
     In order to attain the object described, the present invention includes a bolt shear force sensor having a deformable ring with channel-like cross-sections as a circumference and a wire egress extending thru one of the flanges (arms) of the channel. Strain sensing elements are mounted to an outer face of the circumference in a central area (mid-section) of the channel region. The deformable ring surrounds a bolt or mechanical fastener positioned or attached in an operating system. 
     Once the bolt is subjected to an external load from the surrounding system; any frictional resistance to slip is overcome to the extent that a bolt bearing develops within the system. The bolt then is locked in position and resists further motion. Also at this time, the bolt compresses the sensors or strain gages of the surrounding deformable ring and the ring deforms. 
     The strain sensing elements of the deformable ring capture hoop strain and then produce signals based on the strain. A signal conditioner accepts the signals from the strain sensing elements via wire conductors or optical fibers. The signal conditioner calculates and converts the strain sensing signals to strain output. The strain outputs are then used in conjunction with a calibration curve of shear force versus strain to determine the magnitudes and directions of the resultant shear forces. 
     The present invention provides a device capable of continuous and real-time monitoring of shear forces in fasteners used in mechanical joints. The sensor can be used in both static and dynamic applications. The bolt shear sensor is particularly useful in joints containing multiple fasteners in which the distribution of fastener shear forces may be complicated or difficult to determine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the preferred embodiments and to the drawing, wherein: 
         FIG. 1  depicts a prior art axial bolt force sensor having fiber optic Bragg strain gages; 
         FIG. 2  depicts a fastener loaded in axial tension; 
         FIG. 3  depicts a material element of a fastener with transverse compressive strain and axial strain; 
         FIG. 4  depicts a fastener under axial tensile and double shearing forces; 
         FIG. 5  depicts a material element of a fastener subjected to multiple strains; 
         FIG. 6  depicts a cross-section view of a bolt shear force sensor of the present invention with a bolted joint under double shear stress with the sensor operationally connected to a signal conditioner; 
         FIG. 7  is an isometric view of a deformable ring of the bolt shear force sensor of the present invention with an optical fiber having Bragg gratings wound around the ring; 
         FIG. 8  is an isometric view of the deformable ring with wire strain gages wound around the ring; 
         FIG. 9  depicts the bolt shear force sensor of the present invention having a wire conductor egress path thru a bolt; 
         FIG. 10  depicts a shear force impacting a deformable ring of the shear force sensor of the present invention; 
         FIG. 11  depicts exterior strain areas on a deformable ring of the shear force sensor of the present invention; 
         FIG. 12  depicts interior strain areas on a deformed ring of the shear force sensor of the present invention; and 
         FIG. 13  depicts shear force reactions in a multi-fastener, eccentrically loaded bolted connection. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As depicted in  FIG. 6 , the present invention provides a bolt shear force sensor  100  having a nearly-rigid, yet deformable ring  102  with indented channel-like cross-sections. In the figure, a bolt  12  is positioned between plates  20  and secured with washers  22  and a nut  24 . Forces acting on the bolt  12  and the deformable ring  102  are depicted by direction arrows “F” and “F/ 2 ”. 
     As shown in the detail view of  FIG. 7 , the deformable ring  102  is a shear force sensor ring employing an optical fiber  110 . The optical fiber  110  is continuously wound around and glues (fastened) to a vertical section  103  positioned between an upper flange  104  and a lower flange  105  of the ring  102 . The optical fiber  110  includes Bragg gratings which are commonly used as measuring sensors. 
     The deformable ring  102  includes at least one egress hole or aperture  106  which provides a conduit for the optical fiber  110  to connect to a signal conditioner  200  (See  FIG. 6 ). The deformable ring  102  may be constructed from conventional materials (metal, composite, plastic etc.) or with piezoelectric ceramics. If the ring  102  is constructed with piezoelectric ceramics, the ring itself is the sensor; therefore, there is not a need for the optical fiber  110  or any other measuring sensor wound around the ring. 
     Alternatively, as shown in  FIG. 8 , one or more strain sensing elements  120  (such as wire strain gages) of sufficient size and quantity are fastened (using conventional adhesives) to the vertical section  103  of the ring  102 . The sensing elements  120  are centered on the vertical section  103  at mid-height and are aligned to measure hoop (circumferential) strains. The sensing elements  120  are fixedly bonded to the vertical section  103  using conventional adhesives such as epoxies or other suitable bonding agents. 
     Because these types of gages are discrete strain measuring devices; a sufficient number of strain gages  120  would be required for a given shear load. The number of strain gages  120  will depend on the level of accuracy desired for determining the direction of the resultant shear force vector and magnitude. For conventional electrical resistance-based strain gages, the resistance value and the length of the gage being used will increase sensor performance. The bolt shear force sensor  100  supplies a consistent strain regardless of the angular position (within a plane perpendicular to the longitudinal axis of the bolt or mechanical fastener  12 ) made by the shear force vector and the strain gages  120 . 
     Returning to  FIG. 6 , the signal conditioner  200  is operationally connected to the strain gages  120  via the optical fibers  110  or wire conductors  122  (shown in  FIG. 9 ). A grooved adapter plate  40  provides a conduit for the optical fibers  110  and the wire conductors  122  from the aperture  106  to the signal conditioner  200 . The signal conditioner  200  is used to convert the gage signals (i.e., change in resistance for the wire strain gages  120  and change in light frequency for Bragg strain gages of the optical fiber  110 ) to strain output. 
     The signal conditioner  200  may include a monitor that indicates a magnitude of resistance force (F S ) compared to a measured strain in the sensor ring (E). The strain outputs are then used in conjunction with a calibration curve of shear force versus strain to determine the magnitudes and directions of the resultant shear forces. 
     In the case of a sensor ring  102  made with piezoelectric ceramics, no strain gages are necessary since the ceramics produce a voltage as the ring deforms and becomes stressed. A wire conductor  120  would attach directly to the sensor ring  102  and would transmit voltages to the signal conditioner  200 . In this case, the signal conditioner  200  converts the voltage readings to strain. Using the shear force versus strain calibration curve; shear forces are then calculated. 
     Sensitivity of the deformable ring  102  can be easily increased by incorporating additional strain sensing elements and/or through shape changes of the channel cross-section such as crowning or curving the vertical section  103 . Crowning the vertical section  103  maximizes the bending moment and ensures that contact between the bolt  12  and the strain gages  120  (or the optical fiber  110 ) occurs at the vertical section rather than at the flanges  104 ,  105  to produce greater hoop strain and therefore increased sensitivity to ensure that the strain gages  120  are not subjected to strains along an axial direction of the bolt  12 ; the thickness of the strain gages must be less than the thicknesses of the plates that the strain gages are installed with. 
     For the double shear lap joint shown in  FIG. 6  and  FIG. 9 ; multiple fasteners can be used to clamp the plates together. In  FIG. 9 , the wire conductors  122  or the optical fibers  108  are placed into holes of a bolt  160  after which the plates  20  are stacked accordingly. The bolt  160  is placed into position by passing between the deformable rings  102  and the strain gages  120  or the optical fiber  110 . The diameters of the flanges  104 ,  105  should be minimized as much as possible to allow an increased hole or aperture diameter in the plates  20  and mid-plate  30 . Doing so will ensure that the presence of the bolt shear force sensor  100  does not adversely affect joint strength. 
     Once the joint is subjected to an external load (see force arrows “F” and “F/ 2 ”); any frictional resistance to slip is overcome and relative motions of the plates  20 ,  30  occur to the extent that a bolt bearing has developed. At this time, contact between the crown of the deformable ring  102  and the bolt  12  (or  160 ) occurs. The bolt  12  (or  160 ) is then locked in position and resists further plate motion. The bolt  12  (or  160 ) compresses the sensors or strain gages  106  against the middle plate  30  and the ring  102  deforms. 
     Similarly, contact between outer radial edges of the deformable ring  102  (180 degrees away from the contact developed at the crown) and the bolt hole surface occurs. Once these contact conditions are generated; a radial compressive force is produced in the bolt shear force sensor  100 . This compressive force produces a measurable mechanical deformation (and strain) in the shear force sensor  100 . 
     Depending upon the type of sensing elements used (i.e., resistance strain gages, fiber optic Bragg grating or piezoelectric material), an output is generated (i.e., resistance change, light frequency shift, or voltage) and is transmitted from the shear force sensor through either egress (aperture) shown in  FIG. 6  and  FIG. 9 . The output is processed using the signal conditioner  200 . Using calibration curves, the signal is converted to a shearing force and if sufficient sensing elements are incorporated within the shear force sensor  100 , the vector of the shearing force is determined. 
       FIGS. 10-12  depict results of a modeling simulation that demonstrates the performance of the bolt shear force sensor  100 . Referring to  FIG. 10 , the bolt  12  (or bolt  160 ) is fixed and a quasi-static horizontal displacement is applied to the plate  30 . A reaction force occurs and is represented by the arrow designated as the bolt shear force vector, F s . 
       FIG. 11  depicts exterior strain areas on the deformed ring  102  of the shear force sensor  100 .  FIG. 12  depicts interior strain areas on the deformed ring  102  of the shear force sensor  100 . Real compressive principal strains are developed along the inner surface of the sensor at the vertical section  103  where direct contact occurs with the bolt  12  or  160  (See contour region “K” of  FIG. 12 ). Strain values decrease as points of interest become further away from region K. 
     The present invention provides continuous and real-time monitoring of shear forces in fasteners used in mechanical joints. The present invention is particularly useful in joints containing multiple fasteners in which the distribution of fastener shear forces may be complicated (see the example of  FIG. 13 ) or cannot be determined. 
     The example of  FIG. 13  demonstrates the applied loading vector P and shear force vectors F′ i  and F″ i  and resultant shear force vector F s  for a ith bolt. The equations shown are used to perform the vector analysis calculations that are used to compute the magnitude of the resultant shear force vector for each bolt. F′ i  is the direct shear force reaction for the bolt i. F″ i  is the rotational shear force reaction for the bolt i. The sum of all F i ×r icg  is the total reaction moment of the joint for which a static equilibrium must equal P×L cg  (See Mechanical Engineering Design, J. E. Shigley, 3 rd  ed. pp. 256-263, 1977). 
     Returning to the prior art for a comparison of results, the Shah reference senses localized fastener deformations that occur along a longitudinal axis of a fastener. Alternatively, the present invention senses bolt bearing deformations by using a separate, deformable ring positioned externally to the fastener. The fiber optic sensing elements of the Shah reference capture an axial, total transverse and torsional strains as measured from localized deformation of drilled hole regions of the fastener. However, the cited reference cannot distinguish directly between transverse compression strains due to Poisson&#39;s effect and bolt bearing strains. 
     Consider a fastener loaded in pure axial tension as shown in  FIG. 2 . By Poisson&#39;s effect, a compressive strain would develop along an axis transverse to the longitudinal axis of the fastener. Now, consider the combined presence of Poisson&#39;s strains from bolt tension and transverse strains from a bolting bearing as shown in  FIG. 5 . The Shah reference senses the sum of these transverse strains rather than the individual strain components. The cited reference, through the use of signal processing, must separate the Poisson&#39;s effect transverse strains (i.e., the transverse compressive strains resulting from axial tensile loads in the fastener) from the transverse bolt bearing strains. This is an indirect approach of measurement. 
     The bolt shear sensor  100  of the present invention describes a direct approach and is not sensitive to fastener axial loads and the transverse strains resulting from the Poisson&#39;s effect. Furthermore, the invention describes sensing ring geometry to have a thickness “H” that is slightly less than the plate thickness “T” such that the deformable ring vertically “floats” in the hole of the plate (See  FIGS. 6 and 9 ). Therefore, the sensing ring  102  is not subject to through-thickness compressive joint forces and hence the transverse strains from the Poisson&#39;s effect. 
     Furthermore, the Shah reference requires drilling a hole through along a longitudinal axis of the fastener. This drilling decreases the axial, bending, torsional and direct shear strengths and stiffnesses of the fastener. The bolt shear force sensor of the present invention optionally discloses a similar technique used solely for enabling the egress of conductive wires from the ring sensor through one partially-drilled lateral hole and one partially drilled central hole in the fastener. Both the reference and the present invention have the potential to degrade bolt strength and stiffness but through holes provide significantly greater degradation. 
     Alternatively, the present invention also provides an egress method that avoids having to drill any holes in the fastener by using an adaptor plate with a wire conductor egress. This latter option preserves a complete range of strengths and stiffnesses of the fastener with no degradation. 
     The deformable ring  102  of the bolt shear force sensor  100  of the present invention can be made of conventional and non-conventional structural materials. The sensors can be instrumental with one or more wire strain gages, one or more Bragg gratings for fiber optic strain gages or piezoelectric materials, etc. 
     For optical strain gages that utilize Bragg gratings; a single optical fiber containing multiple, closely spaced Bragg gratings would be desirable to provide sufficient resolution and sensitivity. The cross section of the bolt force sensor  100  is not restricted to the channel configuration described herein; other strain sensitive geometries could be used but would require an axisymmetric (ring-like) shape. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description only. It is not intended to be exhaustive nor to limit the invention to the precise form disclosed; and obviously many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.