AU617770B2 - Ultrasonic load indicating member, apparatus and method - Google Patents

Description

Declared at Newtown Penn. sylvania this... 24th day of 1989 S ig ned:. AUSTRiAZIA Form PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Short Title: 617770 Int. Cl Application Number: Lodged: Complete Specification Lodged: Accepted: Lapsed: Published: 1

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Priority: Related Art: Name of Applicant: Address of Applicant: Actual Inventors: Address for Service: TLO BE CC4LTDB PLCN SPS TECHNOLOGIES) :C1C.. C Newtown, Pennsylvania 18940.

IAN E. KIBBLEWHITE CALLINANS, Patent Attorneys, of 48-50 Bridge Road, Richmond 3121, Victoria, Australia.

Complete Specification for the invention entitled: "ULTRASONIC LOAD INDICATING MEL4BER, APPARATUS AND MEOD".

The following statement is a full description of this inventiM, including the best method of performing it known to us:- II_ __i >L l SULTRASONIC LOAD INDICATING MEMBER, APPARATUS AND METHOD BACKGROUND OF THE PRESENT INVENTION The present invention relates to load indicating members and load indicating fasteners and further relates to methods and apparatuses for making and using load indicating members and load indicating fastening members. More particularly, the present invention relates to a method of measuring a load in an ultrasonic load indicating member, a 0 0 load measuring device using the method of measuring the load, a fastener tightening tool for tightening an ultrasonic load o o 000'4* £0 0 0 0 0 0 0o 0 00 o 0 o O 0 La indicating fastener member using the method of measuring, an ultrasonic load indicating member and an ultrasonic load indicating fastener for use in conjunction with the method of measuring, a method making the ultrasonic load indicating fastener, a method of tightening the ultrasonic load indicating fastener, and a transducer for instrumenting the load indicating member.

In many operations, it is desirable to determine the amount of longitudinal load experienced by a longitudinally stressed member. This information is particularly useful when o the longitudinally stressed member is a fastener since the measurement of the longitudinal stress provides a verification S0 of the existence of a proper joint.

0 f Many prior techniques have been developed to indicate the amount of longitudinal stress experienced by a fastener providing a load indicating feature on the fastener itself.

This is usually done by interconnecting one end of an elongated aw o o member, such as a pin, to a portion of the fastener prior to the installation of the fastener.

An example of this type of load indicating member is depicted in Figure 1 of the drawing. The elongated member

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extends parallel to the fastener 12 and is unaffected by the elastic deformation of the fastener in response to longitudinal stress. The free end 14 of the elongated member therefore provides a reference for indicating the elongation of the fastener 12 in response to the longitudinal stress. Typically, the elongated member 10 is a pin received within an elongated bore 16 disposed longitudinally of the fastener 12 and extended from the head 18 of the fastener and partially through the shank 20 of the fastener. One end 22 of the pin 10 is interconnected with the shank 20 of the fastener 12 at the base of the bore 16 by means, for example, of adhesives, threads, or an interference fit. The various prior load indicating members and load indicating fasteners of this type differed greatly in structure as well as in the methods and apparatuses with which they were used to provide an indication of the elongation of the load indicating member or fastener. Examples of this type of fastener are disclosed in o d No. 3,812,758 issued May 28, 1974 to Robert B. Bessler, Jr.; 3,823,639 issued July 16, 1974 to Theodore Liber; 2,503,141 issued April 4, 1950 to Albert R. Stone; No. 3,943,819 issued March 16, 1976 to Charles S. Charron; No. 2,600,029 issued June 10, 1952 to Albert R. Stone; 3,908,508 issued September oo 30, 1975 to William J. Payne; 3,987,668 issued October 26, 1976 to Charles H. Popenoe; and 4,144,428 issued September 19, 25 1978 to Charles H. Popenoe; as 6 i (I well as in commonly assigned United States Letters Patent Application No. 670,260 filed 11/13/84, now U.S. Patent No.

4,676,109.

While each of the.various pin-type load indicating members, and load measuring devices described above provides its own advantages in terms of accuracy, ease of manufacture, or ease of reading, they are still expensive to manufacture, since they each require extensive modifications and the addition of a centrally located pin-member to the load indicating member. As a result, such load indicating members are only selectively used in practice, either where there is a specific immediate diagnostic need or a serious recognized safety hazard involved.

These members are simply too expensive for routine use in assemblies which may only occasionally benefit from such monitoring.

An alternate approach to measuring the elongation of a member or fastener is to use an ultrasonic measurement device.

Typically, this is done, as shown in Figure 2 of the drawing, which corresponds to Figures 1 and 3 of prior art U.S. Patent No. 3,759,090, by removably interconnecting an ultrasonic transducer 58 to one end of the member being measured, typically to the head 106 of a fastener 98. In order to obtain a reliable indication, the head of the bolt must be ground extremely flat and a reliable ultrasonic transmission media must be applied to the head of the bolt. The transducer must be properly positioned on the bolt and held in position while the measurements are being taken. Examples of techniques and apparatuses using this method are disclosed in United States Letters Patent No. 3,306,100 issued February 28, 1967 to Wilheim et al.; 3,307,393 issued March 7, 1967 to Kessler; 3,308,476 issued March 7, 1967 to Kleesattel; 3,759,090 issued September 0 18, 1973 to McFaul et al.; 3,812,709 issued May 28, 1974 to Benson et al.; 3,822,587 issued July 9, 1974 to Makino et al.;4,014,208 issued March 29, 1977 to Moore et al.; 4,062,227 issued December 13, 1977 to Heyman; 4,117,731 issued October 3, 1978 to Heyman; 4,363,242 issued December 14, 1982 to Heyman; 0 0 4,402,222 issued September 6, 1983 to Olson et al.; 4,413,518 issued November 8, 1983 to Jones and 4,471,657 issued September 18, 1984 to Voris et al.

The patents teach the notion of combining the measuring device with a tightening tool so that the information gained s from measuring the elongation of the bolt can be used o Ca determining when to shut off the tightening tool or, alternatively, monitor the tightening process to determine whether a proper joint has been formed. Examples of such tightening tools are disclosed in United States Letters Patents nr t 2 No. 3,969,960 issued July 20, 1976 to Pagano, 3,969,810 issued July 20, 1976 to Pagano.

While the above listed methods and apparatuses can provide reliable information about a fastener and a joint, they are in very limited use. This is mainly because the bolt must be carefully manufactured and must be easily accessible to the instrumentation. Thus, ultrasonic tension measurement is recognized as a highly accurate laboratory tightening method for calibration application testing and for tightening very critical .0 joints. It is replacing strain gage bolts in several calibration and critical quality control applications. However, the practical difficulties associated with taking ultrasonic Sotension measurements described above have prevented its 00 application as a general assembly tightening strategy.

000000i 0 0 0 0I 600 0J 00 00 C 0 00 0 0 0O 0 00 00 OS 0 0 0a 0 0 00 Some attempts have been made to combine the advantages of the pin-type load indicating members, described previously, with the ultrasonic elongation measurement device described above by incorporating a piezoelectric or other ultrasonic sensor into the member itself. Examples of such members are disclosed, for example, in U.S. Patent No. 4,127,788 issued November 28, 1978 to Daugherty and 4,294,122 issued October 13, 1981 to Couchman. Each of these disclosures provide an instrumented load bearing fastener which has been modified to incorporate a stress indicating feature. However, like the pin-type fasteners described previously, these instrumented fasteners are greatly modified in order to accept large and complicated ultrasonic sensing devices. They are therefore prohibitably expensive for wide spread use.

Examples of additional prior patents known to the applicant which teach or claim ultrasonic piezoelectric or alternate methods are: I I I Patent No.

3,201,977 3,306,100 3,307,393 p 3,308,476 3,541,844 3,650,016 3,759,090 1 3,810,385 3,812,709 S 3,822,587 3,918,294 Issue pate 8/24/65 2/28/67 3/7/67 3/7/67 11/24/70 3/21/72 9/18/73 5/14/74 5/28/74 7/9/74 11/11/75 Inventor Kutsay Wilhelm, Lyndhurst, Kliever Kessler Kleesattel Stover McMaster McFaul McFaul Benson Makino Makino W 'I 4 3,924,444 3,930,405 3,969,810 3,969,960 3,975,948 4,014,208 4,015,464 4,062,227 4,117,731 .L0 4,121,467 4,127,788 Re. 30,183 4,294,122 4,393,242 1524,402,222 -4413, 518 4,423,634 4,445,360 '04,471,657 4 t0 4,569,229 a 4,567,766 4,584,676 S4,I601,207 4,602,511 12/9/75 1/6/76 7/20/76 7/20/7 6 8/24/76 3/29/77 4/5/77 12/13/77 10/3/78 10/24/78 11/28/78 1/8/8 0 10/13/81 12/1,4/82 9/6/83 11/8/8 3 1/3/84 5/1/84 9/18/84 2/11/85 2/4/86 4/22/8 6 7/22/86 7/29/86 Heyman Renken Paganio Pagano Makino Moore Miller Heyman Heymnan Gerhart Daugherty Popenoe Couchman Heyman Olson Jones Audenard Treder Voris de Halleux Seiferling Newman Steblay Holt r r -r r 1-I 1 Very few actual products have resulted from these developments and their use has generally been limited to laboratory work and expensive and critical installations due to the difficulty in maintaining reliable coupling during tightening, the expense and complexity of the required equipment, and the strict control required over fastener materials and properties.

What is needed, therefore, is a low cost ultrasonic transducer permanently attached to a fastener in an inexpensive manner to provide accurate tightening information on a mass production basis. Such an ultrasonic load indicating member would permit easy interconnection with measuring or- assembly tool devices and avoid the problems encountered with prior ultrasonic measuring devices in attempting to obtain a reliable acoustical coupling.

0 U 4 3_ SUMMARY OF THE PRESENT INVENTION S" The present invention provides a load indicating member and a load indicating fastener, as well as methods and "o apparatus for the use of the load indicating member and the load indicating fastener, which combine many of the advantages, .0 heretofore only available separately in different load o o indicating members or fasteners, in a single member or fastener.

4 i -i -r I t I I 1 The present invention further provides additional features and advantages not previously available in load indicating members, load indicating fasteners, load measuring devices, and tightening tools.

The load indicating member of the present invention includes a shank 5ubjected to elastic deformation when stressed longitudinally, and first and a second generally flat surface, each formed adjacent to one longitudinal end of the shank, the first and second flat surface being coplanar and at approximately a preselected distance apart when the' shank is unstressed. A piezoelectric film means permanently, mechanically and electrically interconnected with first and second electrode means is disposed on the first flat surface of 6 1 the shank. The first electrode means is permanently, mechanically, electrically and acoustically interconnected with the first flat surface. The first electrode means may, for 0 0 example, consist of an adhesive, a metallic film, or the member itself.

*a o In a preferred embodiment, the piezoelectric film means is a thin flexible piezoelectric film disk, the first and second electrode means being metallic layers deposited on opposite 0-0° faces of the disk. Furthermore, in this preferred embodiment, A the load indicating member is a load indicating fastener having an enlarged head and the first flat surface is formed on the head of the fastener.

The method of making a load indicating fastener according to the present invention includes the steps of providing a flat surface at one longitudinal 'end of a fastener, disposing first and second electrode means on opposite faces of a flexible piezoelectric film element; and permanently, mechanically, electrically and acoustically interconnecting the first electrode means with the flat surface of the fastener such as to electrically isolate the second electrode means from the surface.

o *The load measuring device according to the present I a "goo"' invention provides a first contact means electrically engageable with the first electrode means, second contact means engageable 00 o6 0o with the second electrode means, and an electronic measurement o 0 device responsive to electronic differential signals between the first and second electrode means such as to provide a o measurement of the tensile load of the load indicating member 2D when stressed longitudinally.

In a preferred embodiment, the piezoelectric film J a element may also provide a driving means for producing an a 6 0 a a.

ultrasonic signal such as to generate the electronic differential signals. Furthermore, in a preferred embodiment.

the load indicating member is electrically conductive and the first contacts means is electrically engaged with the first electrode means indirectly by engagement of the first contact means with the load indicating member.

The tightening tool of the present invention includes first and second contact means electrically engageable, respectively, with the first and second electrode means, a load inducing means for imparting a tensile load in the load bearing member, and a load means for imparting a tensile load in the load bearing member, and a load measuring device responsive to S the electrical differential signal such as to provide an *0 accurate measurement of the tensile load.

0 04 0 a 04400 4 04 0 0004 4 4Y4 4) 04 04 In the preferred embodiment, the tightening tool of the present invention includes a electrically conductive fastener engagement means engageable with a load indicating fastener, a contact member engageable with a second electrode means of the load indicating fastener, a drive means imparting a torque on the fastener engagement means so as to rotatably drive the load indicating fastener, and a load measuring device responsive to the electrical differential signal received from the socket and the contact member such as to provide an accurate measurement of the tensile load of the shank of the fastener when stressed longitudinally as a result of the tightening process.

The output of the load measuring device may be used to provide a continuous reading of the instantaneous tensile load of the fastener or, alternatively, may be used to determine when the fastening operation is complete or to provide an indication of the load in a previously tightened fastener. When the load indicating member is a fastener, the load measuring device may ID be used simultaneously with a fastener tightening tool or, alternatively, may be incorporated directly into the fastener tightening tool. When the fastener tightening tool Q 0 incorporating the load measuring device is of an automatic tightening type, the tensile load indication in the load p measuring device may be combined with other parameters, oo 00 S monitored by the fastener tightening tool, such as angle and torque, to determine when the tightening cycle is complete and o to detect irregularities in the joint.

pop The method of measuring the load in a load indicating member according to the present invention includes the steps of Sinterconnecting a load measuring device of the present invention with a load indicating member of the present invention and 13 j_ -L -L calculating the tensile load of the shank of the load indicating member from ultrasonic measurements.

The method of tightening a load indicating fastener according to the present invention includes the steps of interconnecting a load measuring device according to the present invention to the head of a load indicating fastener according to the present invention, tightening the load indicating fastener while continuously monitoring the load measuring device to determine when a preselected load is reached, and ceasing the ID tightening of the load indicating fastener when the preselected load is reached, as indicated by the load measuring device.

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a? P rimar" oh~-v~tt of th rtodent I :rr rnitrn i~ to rid an inexpensive load indicating member or fastener which m be easily installed with conventional tools. Another o ect of the o present invention is to provide an inexpensive nd accurate method of measuring the load in the load ndicating member or a Sload indicating fastener, a load i dicating device and a fastener driving tool using e method of measuring of the present invention, a loa indicating member for use in conjunction with the method of measuring of the present invention, method of making the load indicating member of the l prese invention, and a method of tightening the load dicating fastener of the prcsnt invention.

In accordance with a first aspect of the present invention, therefore, there is provided a load indicating member including a shank and a head and wherein said shank which, when stressed longitudinally, is subject to elastic deformation such that one portion thereof moves relative to another portion thereof; a first generally flat surface formed adjacent one longitudinal end of said shank; first electrode means permanently, mechanically, electrically, and acoustically interconnected with said shank; piezoelectric film means permanently, mechanically, and electrically interconnected with said first electrode means; second electrode means permanently, mechanically, and electrically interconnected with said piezoelectric film means and electrically isolated from said head and said first electrode means, such that said electrode means and said piezoelectric film means cooperate to form an ultrasonic transducer; and a second generally flat surface formed on said shank at a location remote from said one longitudinal end of said shank, said second flat surface being generally coplanar with said first flat surface and spaced a predetermined distance therefrom.

In accordance with a further aspect of the present invention, there is provided a fastener adapted for measuring longitudinal strain in the fastener, said t fastener including: a shank, having a longitudinal axis and a predetermined longitudinal length, said shank being adapted to be subjected to longitudinal strain along said longitudinal axis; a head formed at one extreme and of said shank said fastener having a generally flat surface disposed on said head normal to said longitudinal axis; first electrode means permanently, mechanically, electrically, and acoustically interconnected with said head; piezoelectric film means permanently, mechanically, and electrically interconnected with said first electrode means; and 0..0 second electrode means permanently, mechanically and electrically interconnected with said piezoelectric film means and electrically isolated from said head and said first electrode means, such that said electrode and said piezoelectric film means cooperate to form an ultrasonic transducer.

0 In accordance with yet another aspect of the present invention, there is 1 5 provided a fastener adapted for measuring longitudinal strain in the fastener, said fastener including: a shank having a longitudinal axis and a predetermined longitudinal length, said shank being adapted to be subjected to longitudinal strain along said longitudinal axis; a head formed at one extreme of said shank, said fastener having a generally flat surface disposed on said head normal to said longitudinal axis; a first electrode means permanently, mechanically, electrically, and acoustically interconnected with said head by means of an adhesive; a thin piezoelectric film disc permanently, mechanically, and electrically interconnected with said first electrode means; second electrode means permanently, mechanically, and electrically interconnected with said piezoelectric film disc and electrically isolated from said head and said first electrode means, such that said electrode means and said piezoelectric film disc cooperate to form an ultrasonic transducer, said first and second electrode means comprising metallic films disposed on opposite faces of said piezoelectric film disc.

In accordance with yet another aspect of the present invention, there is provided a method of making a load indicating fastener from a fastener having a head and a shank extending from the head, said shank being subjected to elastic deformation when stressed longitudinally such that one portion thereof moves relative to another portion thereof, said method including the steps of: forming a first flat surface on said head and a location remote from said shank, said first flat surface being normal to the longitudinal axis of said shank; forming a piezoelectric sensor by applying a first and a second electrode means to opposite faces of a piezoelectric film disc and permanently, acoustically, and mechanically interconnecting said piezoelectric sensor to said head of said fastener such as to electrically interconnect a first electrode of said piezoelectric sensor with said head 20 and, further, such as to isolate said second electrode from said head.

In accordance with yet another aspect of the present invention, there is provided a method of tightening a load indicating fastener of the type having a shank subject to elastic deformation when stressed longitudinally, a head formed at one longitudinal end of said load indicating fastener, a first flat surface formed 25 adjacent said head, tool engagement means formed on said head adjacent said first flat surface, piezoelectric film sensor means permanently mounted on said first flat surface, and a second flat surface formed at the other longitudinal end of said shank, said method including the steps of: engaging a tightening tool with said tool engagement means; engaging contact means with said piezoelectric film sensor means; operating said tightening tool to tighten said load indicating fastener; and measuring with an electronic control device a signal representative of the time of flight of an ultrasonic wave along said shank by monitoring the differential electronic signal between said tool engagement means and said contact means such as to provide a continuous measurement of the tensile load of said shank during the operation of said tightening tool.

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In order that the invention may be more dearly understood and put into practical effect they shall now described in detail preferred embodiments of an ultrasonic load indicating member, a method of making such a member and a method of utilising such a member in accordance with the invention. The ensuing description is given by way of non-limitative example only and is with reference to the accompanying drawings, wherein: .o a 0 a 0 0 16a

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t

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In the drawings, wherein like rcfrcnoc numcrals rcfer to the like elaments throughout: Fig. 1 is a partially cut-away side view of the load indicating member of the prior art; Fig. 2. is a partially cut-away side view of an ultrasonic load measuring device of the prior art; Fig. 3 is a perspective view depicting an example of a load indicating member according to the present invention; *o o ,2 Fig. 4 is an enlarged and partially sectional view 4 illustrating the indicating fastener of Fig. 1; Figs. 4a through 4c are partial views similar to Fig. 4 but illustrating alternate examples of load indicating members according to the present invention; Fig. 5 is a partially schematic and partially cut-away side view depicting a load indicating fastener according to the present invention as well as a fastener tightening tool engaged therewith, the fastener tightening tool incorporating a load measuring device according to the present invention; and Figs. 6 through 10 are graphical views illustrating the acoustical operation of the present invention in its various embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and more particularly to Figures 3 and 4 thereof, a first example of a load indicating member, and more particularly, a load indicating fastener 110 is illustrated. The load indicating fastener 110 is formed from a ao 0 conventional bolt which has been modified to provide an o indication of the tensile load, stress, elongation or other characteristics of the bolt during a tightening operation as :0 well as at various other times during the life of a a* joint. The bolt has a shank 112 with threads 114 formed at one o.Oo end and with a head 116 formed at the other end. A shoulder 118 o, is formed between the head 116 and the shank 112. The l oad 116 has a generally flat upper surface 120 disposed normal t the o* a0 longitudinal axis 122 of the shank 112. A lower surface 124 is formed at the opposite end of the shank 112 and is also disposed 0 N* r* 0o oU Og4 *4 Q *r *0 #0 o *0r *o C *u 00 0 0 normal to the longitudinal axis 122. The head is also provided with a wrenching or tool engagement surface 126, such as a hexagonal wrenching surface, disposed about the periphery chereof.

Alternatively, as shown in Fig. 4b, a flat surface 120a may be formed in a recess 121 in a head 116a. The recess 121 may be a tool engagement socket or a lightening hole or a shallow recess created for the purpose of protecting the piezoelectric film sensor 128 from environmental hazards.

The piezoelectric ultrasonic film sensor 128 is permanently or semi-permanently mounted to the upper surface 120 or 120a of the head 116. As illustrated in the drawing, the piezoelectric film sensor preferably consists of a disk 130 of piezoelectric film material having a first and second electrode 132 and 134, respectively, applied to each of its opposite flat surfaces.

Preferably, the piezoelectric film sensor is formed of a thin layer of flexible inexpensive piezoelectric film material, such as a polymeric material, and is approximately 5-110 microns thick. In the preferred embodiment, polyvinylidene fluoride and derivatives thereof are chosen since 1 'they are very resistant to corrosive substances.. However, other materials or copolymers may exist or be developed having satisfactory properties and laminated bimorphs and multimorphs are contemplated as alternatives. The electrodes may be formed of metallic layers vacuumed deposited on the piezoelectric disk, a conductive ink or paint, or an adhesive. Alternatively, one or both may be a conductive foil permanently bonded to the disk.

The first electrode is electrically and acoustically coupled to the head 116 while the piezoelectric disk 130 and the second 0D electrode 134 are electrically isolated from the head 116. For some embodiments, the head 116 will perform the function of the first electrode 132.

op 0* o For some installations, the first electrode 132 may consist of either a conductive or non-conductive adhesive.

Preferably, the adhesive is non-conductive and relies on o capacitive coupling for the electrical communication between the piezoelectric film sensor 128 and the head 116.

o a 000 In experimental work, the echo signal levels of a thin *0 90 film polymeric piezoelectric transducer adhesively coupled with .0 2o a member was compared with prior art thick ceramic transducers removably mechanically coupled with a member. Not only were the signal levels considerably higher, but also the signal to noise i. i

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ratio was considerably higher with the film transducer, thus demonstrating, a greater ease and reliability of echo detection.

The higher noise levels of the ceramic transducer may be attributable to echoes resulting from ultrasonic waves reflected internally within the transducer and transducer housing. Reflected acoustic waves within the film transducer may decay more quickly since the lower acoustic impedance of polymeric piezoelectric films such as polyvinylidene fluoride and derivatives thereof permit more efficient transfer of energy to the air or conductive rubber contact.

It should be noted that the use of the piezoelectric film transducer therefore is a significant transducer 0 performance improvement over the prior art.

0 0 Referring to Figure 5, an example of a load indicating fastener 110 described above is shown with a fastener tightening tool 140 according to the present invention engaged therewith.

0 0. The fastener tightening tool 140 includes a conventional power tool 142, only the rotating drive part of which is shown in the drawing. The conventional power tool 142 has a rotary output driver 144 engageable with a socket member 146. The fastener oo.o tightening tool 140 and the rotary output driver 144 rotate mi~m~~i i together. The socket member 146 engages the head 116 of the fastener 110 both electrically and mechanically.

A contact pin 148 is reciprocally mounted to the fastener tightening tool 140 to reciprocate relative to the socket member 146 into engagement with the second electrode 134 of the piezoelectric film sensor 128 and the head 116 of the fastener 110. The contact pin 148 is preferably provided with a conductive rubber tip 148a such as to provide a low acoustic impedance interface while refraining from damaging the transducer.

An electronic control device 150, shown only 4 a o o schematically in the drawing, is electrically interconnected .with the contact pin 148 and the socket member 146 by means of oelectrical lines 152 and 154, respectively, by way of slip ring 4 0 156 and 158, as is well-known in the art. Alternatively, the signal may be transferred by a non-contact means, such as by capacitive coupling, and other techniques well-known in the art.

.",.oThe electronic control device 150 supplies and measures electronic differential signals between the first and second electrodes 132 and 134 of the piezoelectric film sensor 128 such as to provide an ultrasonic measurement of the tensile load, *stress or elongation of the shank 112 of the fastener 110.

It will be appreciated by those skilled in the art that the fastener tightening tool 140 may be provided with a display device, not shown in the drawing, for displaying ultrasonic measurement of the tensile load, stress or elongation obtained during the fastener operation. Alternately, the fastener tightening tool 140 may be adapted to use the information continuously supplied by the electronic control device to determine when a predetermined amount of tensile load or elongation has occurred and therefore when a tightening iD operation should be stopped.

It will further be appreciated by those skilled in the art that the power tool chosen may, in a manner well-known in 0 the art, monitor other characteristics of the joint being o 0 ~formed, such as the torque and the instantaneous angle of the o load indicating fastener. An example of such a power tool may 0 0 be found in the United States Letters Patent 4,344,216 issued 0 0 August 17, 1982 to Robert J. Finkleston. This other information available from the power tool may be combined with the tensile 00, load, stress or elongation information supplied by electronic control device 150 to provide a precisely controlled tightening 0" operation wherein the various measured parameters are used directly to control the tightening sequence or to monitor the results of the tightening operation. For example the socket as P1 ii I- i member 146 may be used in conjunction with the power tool using what is known in the art as a "turn of the nut tightening sequence" while the elongation information is used subsequently to determine whether the joint formed by the tightening sequence meets certain specifications.

In use, the load indicating fastener 110 for the present invention may be used to secure panels 136 and 138 together by being passed progressively through suitable bores therein and being fastened therebehind by a nut 139. The fastener tightening tool 140 or, if desired, any standard tightening tool, engages the tool engagement surface 126 of the load indicating fastener 110 and is rotated to tighten to joint.

As the panels 136 and 138 engage, respectively, the shoulder 118 and the nut 139, the shank 112 of the load indicating fastener 110 experiences longitudinal stress causing longitudinal elastic 0 deformation of the shank 112. The amount of tensile load, stress or elongation of the shank 112 can be measured by the fastener tightening tool 140.

While Figure 5 illustrates a fastener tightening tool 140 incorporating a convention power tool 142 and electronic control device 150, it will be appreciated by those skilled in 0000'the art that a fastener measuring tool may be made incorporating 24 all the components of the fastener tightening tool 140 except the power tool 142. Such a device may be used to measure the elongation of a bolt independently of the tightening tool.

It should be further noted that the design of the load indicating fastener 110 described above facilitates rapid modification of existing bolts. No special surface treatment is required except the provision of a generally flat upper surface 120. In practice, it was found to be desirable with prior art methods to have a surface finish on the order of 125 micro inches while 250 micro inches has been determined to be acceptable for the purpose of acoustical coupling. However, for the present invention, a surface finish of 16-200 microinches has been found to be preferable because of the use of an adhesive coupling. A piezoelectric film sensor 128 may be formed independently of the bolt by applying a film of metallic material to each of opposite faces of a piezoelectric disk 130.

4 A suitable adhesive for acoustically and mechanically coupling the piezoelectric film sensor 128 to the upper surface 120 of the head 116 may be applied to the piezoelectric sensor in 0 0 advance together with a peel-off sheet of inert material, not o shown in the drawing but well-known in the art for storing the piezoelectric film sensor until the time for installing the o 4sensor on the fastener 110.

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The apparatus of the present invention permits a direct measurement of the time of flight of ultrasonic waves along the axial length of the member. The time of flight will vary with the length of the bolt and with the stress in the bolt.

Many different electronic techniques for the measurement of time of flight are well-known in the art as a result of ultrasonic developments in the field of non-destructive testing. Most are capable of providing the required resolution and accuracy. However some offer advantages in terms of the number of pulses for accurate measurement, circuit complexity and power consumption.

a 0.

A significant factor is that the affect of axial stress 0 on the speed of transverse ultrasonic waves is much less than 0 that of longitudinal waves. Therefore, measurement of the time 050« of flight of both waves can be used to determine axial stress 0 0 without knowledge of the length of the bolt. This therefore allows measurement of the axial load in pre-installed fasteners.

0 o A brief comparison of the information available from using transverse and longitudinal waves is provided below.

o a All ultrasonic equipment currently available uses the measurement of the time of flight of a longitudinal ultrasonic iA wave generated with a piezoelectric transducer mounted on one end of the bolt (usually the head). The ultrasonic wave travels to the opposite end where it is reflected back and detected by the same transducer. The numerous approaches to measuring this time of flight are briefly described below, but all produce a measurement of the change in time from the zero tension condition from which the tension is the calculated.

It is common practice to grind the ends of the fastener parallel and to a surface finish of better than 250 micro inches. A good surface finish is a requirement for adequate acoustic coupling to the bolt.

The ultrasonic wave used for this time of flight measurement may be a longitudinal wave. The particle motion in a longitudinal wave is in the direction of propagation forming 4 4 0 moving zones of compression and tension. The time of flight of a longitudinal wave is dependent on the length of the bolt and the speed of the ultrasonic wave. The variations in the length of the bolt result from thermal expansion, elongation due to axial load as a result of tightening, and plastic deformation if tightened to yield. The speed of the ultrasonic wave is dependent on the properties of the material, that is, the o, composition, heat treat, and temperature, for example, and the 27

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axial stress in the fastener induced by tightening. Parameters associated with material properties and their variations with temperature are determined experimentally and normally entered into the ultrasonic tension measurement equipment along with an ambient temperature measurement.

As the fastener is tightened, the bolt extends under axial load and the speed of propagation of the ultrasonic wave is reduced due to the axial stress. The latter accounts for approximately two-thirds of the increase in the measured time of I0 flight.

Grip length, or a preselected relevant compensating stress factor, must also be entered as a parameter when measuring tension with this technique since it affects not only the bolt elongation under load but also the average stress and hence the average speed of propagation over the length of the bolt.

There are therefore several limiting characteristics of 1 using the time of flight of longitudinal waves over the entire a 4 length of the fastener. The measurements are dependent on grip length which must be entered as a control parameter.

A.0° Additionally, variations in local stress distributions in the 28 -i region of the nut affect accuracy. It is not practical to measure the tension in the fastener after it is installed unless bolts are all ground to an exact length, typically to plus or minus one ten thousandth of an inch, or the exact length is measured and entered into the measurement system as a parameter.

Tension measurements are normally based on the increase in the time of flight relative to the zero load measurement made prior to commencement of tightening.

Longitudinal waves are the more commonly known sound waves in that they are the waves which are used to transmit the oscillations of a source of acoustic energy through the air to the ear. In contrast, the particles of a transverse wave do not oscillate in the direction of propagation but at right angles to 0 it. This wave is sometimes referred to as a shear wave since adjacent particles are subject to shear forces. Since gases and liquids are generally incapable of transmitting transverse waves, a special high viscosity coupling fluid is required by prior art equipment to acoustically couple from a transducer temporarily attached to the bolt.

A longitudinal wave travels almost twice as fast as a transverse wave in steel and the time of flight is affected to a 4, different degree by axial stress. The variation in speed of the transverse wave in response to stress is about one and one half orders of magnitude less than that of the longitudinal wave.

As described in the patent to Holt, cited in the background, longitudinal and transverse waves may be applied to one end of the fastener by a piezoelectric crystal producing a wave having both longitudinal and transverse components. The time of flight of each of the longitudinal and transverse waves is measured. The tensile stress can then be determined from these two measurements without prior knowledge of the exact length of the bolt.

The use of both longitudinal and transverse waves I provides, in theory, two significant advantages over the use of the longitudinal wave only. First, tensile load measurements o can be made without first making zero load measurement i.e. load can be measured in already installed fasteners. Second, the o effect of bolt to bolt variations from material properties, heat treatment etc, is reduced since they affect both longitudinal i and transverse waves.

4"J' The apparatus of the present invention may be used with either or both of these waves, as contemplated by the inventor.

a *4 It is further contemplated by the inventor that the apparatus of the present invention may be used with any of the various timing techniques, including direct timing, indirect timing, clock interpolation, double pulsing, resonant frequency detection, acoustic impedance detection, harmonic frequency detection, and phase detection, as briefly described below.

The direct timing technique involves measurement of the time interval from the transmission of the drive pulse to the receiving of the echo signal with a gated oscillator and counter as shown in Fig. 6. For example, one commercially available instrument sold by Raymond Engineering uses a 100 MHz clock and averages 160 measurements to achieve the required resolution.

0 a O Clock rates as high as 2 GHz are now possible with Gallium Arsenide technology.

The indirect timing techniques involves timing from the o 0 first echo to the second echo as shown in Fig. 7. Measurement 0 o0 of the time interval between the first and second echoes 00 0 o, eliminates errors due to trigger point variations with difference echo waveform shapes and circuit and wiring time delays. Bolt end finish is more critical since the second echo is attenuated at each of the three reflections.

In the clock interpolation method resolution is improved by using analog techniques to determine a fraction of a clock cycle in addition to the clock count. One approach illustrated in Fig. 8, uses two synchronized out of phase clocks which are each integrated over the same short period at the end of the gating interval.

In the double pulsing technique, two pulses A and B are transmitted, as depicted in Fig. 9. Pulse A produces first echo Al and second echo A2, and pulse B produces first echo B1 and second echo B2. The time interval between the paired pulses A and B is adjusted so that the second echo from A, i.e. A2. coincides with the first echo from pulse B, i.e. B1.

The result is rectified and integrated to produce a frequency 6r If control voltage for a voltage controlled oscillator which adjusts the pulse timing. The frequency of the voltage controlled oscillator is used to calculate tension.

In the fundamental frequency detection technique, the bolt is maintained in longitudinal wave resonance at its 4 fundamental frequency and the difference in its value before

IL

I and after tightening used to determine the tension.

4 4 32t- It -32i; In the acoustic impedance detection technique the bolt is driven near its fundamental frequency and the change in mechanical or acoustic impedance used to determine tension.

The harmonic resonance frequency detection technique is a variation of the resonant method in which a harmonic longitudinal wave resonance is maintained at a harmonic frequency in for example, the 5-10 MHz range. Bolt tension is calculated from frequency shift during tightening.

The phase detection technique uses a pulsed phase locked loop approach. The radio frequency output of a voltage controlled oscillator is periodically gated to the transducer.

The received signal produced by the reflected wave is mixed 015. with the output of the voltage controlled oscillator as shown in Fig. 10. If the oscillator and the received signal are in S phase, the summed signal will be a maximum, if one hundred and 0 eighty degrees out of phase, the two signals will cancel, 0 0 forming a minimum. This summed signal, therefore represents the difference in phase and is used to control the frequency o of the voltage controlled oscillator so that the phase remains 0 00 constant, that is, the summed signal is a minimum or maximum.

In this way, the frequency of the oscillator is adjusted so that the time of flight is maintained at an exact number of cycles.

-33- -33- The time of flight is determined from the frequency and the number of cycles counted between the start of transmitting to the start of receiving.

It will be appreciated that the above described techniques and the two types of waves may be used in various combinations to measure one or more parameters to the desired accuracy.

It should be appreciated by those skilled in the art that the load indicating fastener 110 of the present invention facilitates the rapid interconnection of a conventional fastening tool or an instrumented fastenin% tool such as the t T A A fastening tool 140. Both the fastener 110 and the fastener *'*'ttightening tool 140 are comparably inexpensive to manufacture $use. The present invention therefore would facilitate more 0 °,widespread use of ultrasonic fastening and monitoring techniques 0 0 in production.

The fastener 110 and the fastener tightening tool 140 facilitate rapid formation of reliable joints having repeatable oa 44 o°and predictable characteristics. They permit a means to detect flaws in the joint during actual installation process thus A Areducing the risk of catastrophic joint failure. They further 44I permit the monitoring, at a later time, of the characteristics of the joint.

It should be noted that the fastener, method, and tool of the present invention provide an inspection feature for monitoring the condition of the load indicating member. If the approximate length of the member is known, the approximate time of flight of an ultrasonic pulse is also known. If the measured result does not agree, or if the signal is in others unlike the expected signal, there may be imperfections, such as cracks, in the member. It should also be noted that the sensor 128 can be used on either end of any bolt or threaded rod and the tool can be used in engagement with other head engagement Yeans or with a nut engagement means.

The above detailed description includes the best mode E contemplated by the inventor at the time of filing of the present application for carrying out the present invention.

4 4 It will be appreciated by those skilled in the art that many modifications may be made to the load indicating fastener, the load measurement device, and the fastener tightening apparatus described above without departing from the spirit of the present invention. Such modifications are included within the iop intended scope of the claims appended hereto.

(I

Claims (64)

1. A load indicating member including a shank and a head and wherein said shank which, when stressed longitudinally, is subject to elastic deformation such that one portion thereof moves relative to another portion thereof; a first generally flat surface formed adjacent one longitudinal end of said shank; first electrode means permanently, mechanically, electrically, and acoustically interconnected with said shank; piezoelectric film means permanently, mechanically, and electrically interconnected with said first electrode means; second electrode means permanently, mechanically, and electrically interconnected with said piezoelectric film means and electrically isolated from said head and said first electrode means, such that said electrode means and said piezoelectric film means cooperate to form an ultrasonic transducer; and a second generally flat surface formed on said shank at a location remote from said one longitudinal end of said shank, said second flat surface being generally coplanar with said first flat surface and spaced a predetermined distance therefrom.

2. The load indicating member as claimed in claim 1, wherein said first electrode means includes said shank.

3. The load indicating member as claimed in claim 1 or claim 2, wherein said first electrode means is an adhesive.

4. The load indicating member as claimed in any one of claims 1 to 3, wherein said head is formed integrally with said shank at said one longitudinal and thereof, said first flat surface being formed on said head, and including fastening means formed integrally with said shank at the other longitudinal end thereof remote from said first flat surface.

The load indicating member as claimed in claim 4, further including a recess formed in said head, said first flat surface being formed in the base of said recess.

6. The load indicating member as claimed in any one of the preceding claims, wherein said first electrode means comprises a non-conductive adhesive capacitively coupling said piezoelectric film means with said shank.

7. The load indicating member as claimed in any one of the preceding claims, further including engagement means formed at said one longitudinal end of said shank for removably positioning a measurement device relative to said first and second flat surfaces such as to be removably interconnected with said first and second electrodes to provide a measurement of a tensile load dependent characteristic of said shank. -36- _Ijl__

8. The load indicating member as claimed in any one of the preceding claims, wherein said piezoelectric film means includes a thin layer of piezoelectric film material.

9. The load indicating member as claimed in claim 8, wherein said piezoelectric film material is polyvinylidene fluoride and derivatives thereof.

The load indicating member as claimed in claim 8 or claim 9, wherein said layer of piezoelectric film material is from 5 to 110 microns thick.

11. The load indicating member as claimed in claim 10, wherein said layer is from 20 to 60 microns thick.

12. The load indicating member as claimed in any one of the preceding claims, wherein said piezoelectric film means is disc shaped and is disposed in axial alignment with said shank.

13. The load indicating member as claimed in any one of the preceding claims, wherein said piezoelectric film means has a diameter approximately between the diameter of said shank and half the diameter of said shank.

14. The load indicating member as claimed in any one of the preceding claims, wherein at least one of said first and second electrode means includes a thin conductive layer disposed on a surface of said piezoelectric film means. S

15. The load indicating member as claimed in any one of the preceding claims, wherein said first electrode means is interconnected with said first generally flat surface by an adhesive providing both mechanical and acoustical interconnection.

16. The load indicating member as claimed in any one of the preceding claims, wherein at least one of said electrode means is formed by vapour depositing, electrodepositing or painting a thin film of metal on a surface of said piezoelectric film means.

17. The load indicating member as claimed in claim 16, wherein said piezoelectric film means includes a thin layer of polymeric material.

18. The load indicating member as claimed in any one of the preceding claims, wherein at least one of said electrode means includes a metallic foil.

19. The load indicating member as claimed in any one of the preceding claims, wherein said first generally flat surface has a surface finish of fom 16 to 200 micro- inches.

A fastener adapted for measuring longitudinal strain in the fastener, said fastener including: a shank having a longitudinal axis and a predetermined longitudinal length, said shank being adapted to be subjected to longitudinal strain 37 i jj i;_ V t4 C along said longitudinal axis; a head formed at one extreme and of said shank said fastener having a generally flat surface disposed on said head normal to said longitudinal axis; first electrode means permanently, mechanically, electrically, and acoustically interconnected with said head; piezoelectric film means permanently, mechanically, and electrically interconnected with said first electrode means; and second electrode means permanently, mechanically and electrically interconnected with said piezoelectric film means and electrically isolated from said head and said first electrode means, such that said electrode and said piezoelectric film means cooperate to form an ultrasonic transducer.

21. The fastener as claimed in claim 20, wherein said piezoelectric film means includes a thin layer of piezoelectric film material.

22. The fastener as claimed in claim 21, wherein said piezoelectric film material is polyvinylidene fluoride and derivatives thereof.

23. The fastener as claimed in claim 21 or claim 22, wherein said piezoelectric film material is between 5 and 110 microns thick.

24. The fastener as claimed in any one of claims 20 to 23, wherein said electrode means are less than 30 microns thick.

25. The fastener as claimed in any one of claims 20 to 24, wherein said 4:o01 piezoelectric film means is disc shaped and is disposed in axial alignment with said S shank.

26. The fastener as claimed in claim 25, wherein said disc has approximately 0 between the same diameter as said shank and one-half the diameter of said shank.

27. The fastener as claimed in any one of claims 20 to 25, wherein at least one of said first and second electrode means is disc shaped and has approximately the same diameter as said piezoelectric film means. 4 o4 o4 0

28. The fastener as claimed in any one of claims 20 to 27, wherein at least one of said first and second electrode means includes a thin metallic layer disposed on a surface of said piezoelectric film means.

29. The fastener as claimed in claim 28, wherein said at least one electrode A o a means is formed by viapor deposition.

The fastener as claimed in any one of claims 20 to 29, wherein said piezoelectric film means includes polyvinylidene fluoride and derivatives thereof.

31. The fastener as claimed in any one of claims 20 to 30, wherein said first electrode means is interconnected with said surface of said head by an adhesive providing both mechanical and acoustical interconnection. -38-

32. The fastener as claimed in any one of claims 20 to 31, wherein said fastener further includes a bolt having threads disposed on said shank remote from said head.

33. The fastener as claimed in any one of claims 20 to 32, wherein said head is formed of an electrically conductive material and said first electrode means is electrically or capacitively interconnected therewith.

34. The fastener as claimed in any one of claims 20 to 33, wherein at least one of said electrode means is formed by vapor depositing, electrodepositing or painting a thin film of metal on a surface of said piezoelectric film means.

The fastener as claimed in claim 34, wherein piezoelectric film means includes a thin layer of polymeric film material.

36. The fastener as claimed in any one of claims 20 to 35, wherein said first electrode means comprises a non-conductive adhesive capacitively coupling said piezoelectric film means with said shank.

37. The fastener as claimed in any one of claim 23, wherein said piezoelectric film material is between 20 and 60 microns thick.

38. The fastener as claimed in any one of claims 20 to 37, wherein said first generally flat surface has a suface finish from 16 to 200 micro-inches.

39. A fastener adapted for measuring longitudinal strain in the fastener, said fastener including: a shank having a longitudinal axis and a predetermined longitudinal length, said shank being adapted to be subjected to longitudinal strain S along said longitudinal axis; a head formed at one extreme of said shank, said fastener having a generally flat surface disposed on said head normal to said longitudinal axis; a first electrode means permanently, mechanically, electrically, and acoustically interconnected with said head by means of an adhesive; a thin piezoelectric film disc permanently, mechanically, and electrically interconnected with said first electrode means; second electrode means permanently, mechanically, and electrically interconnected with said piezoelectric film disc and electrically isolated from said head and said first electrode means, such that said S electrode means and said piezoelectric film disc cooperate to form an ultrasonic transducer, said first and second electrode means comprising metallic films disposed on opposite faces of said piezoelectric film disc.

The fastener as claimed in claim 39, wherein said piezoelectric film disc is polyvinylidene fluoride.

41. The fastener as claimed in claim 39 or claim 40, wherein said piezoelectric -39- la q, film disc is between 5 and 110 microns thick.

42. The fastener as claimed in claim 41, wherein said piezoelectric film disc is from 20 to 60 microns thick.

43. The fastener as claimed in any one of claims 39 to 42, wherein said piezoelectric film disc has a diameter approximately between the diameter of said shank and one-half the diameter of said shank.

44. The fastener as claimed in any one of claims 39 to 43, wherein at least one of said electrode means is formed by vapor deposition, electrodeposition, or metallic paint.

The fastener as claimed in any one of claims 39 to 44, wherein said first electrode means is an adhesive.

46. The fastener as claimed in any one of claims 39 to 45, wherein said piezoelectric film disc is disc shaped and is disposed in axial alignment with said shank.

47. The fastener as claimed in any one of claims 39 to 46, wherein said piezoelectric film disc includes a polymeric material.

48. The fastener as claimed in any one of claims 39 to 47, wherein said first o generally flat surface has a surface finish of from 16 to 200 micro-inches.

49. A method of making a load indicating fastener from a fastener having a head and a shank extending from the head, said shank being subjected to elastic deformation when stressed longitudinally such that one portion thereof moves relative to another portion thereof, said method including the steps of: forming a first flat surface on said head and a location remote from said shank, said first flat surface being normal to the longitudinal axis of said shank; forming a piezoelectric sensor by applying a first and a second electrode means to opposite faces of a piezoelectric film disc and permanently, acoustically, and mechanically interconnecting said piezoelectric sensor to said head of said fastener such as to electrically interconnect a first electrode of said piezoelectric sensor with said head and, further, such as to isolate said second electrode from said head.

The method as claimed in claim 49, wherein said flat surface has a surface finish of from 16 to 200 micro inches.

51. The load indicating fastener as claimed in claim 50, made according to the method as claimed in claim 51.

52. The method as claimed in claim 49 or claim 50, wherein said electrodes are formed by vapor deposition, electrodeposition or metallic painting. 4, J

53. The method as claimed in any one of claims 49, 50 or 52, wherein said first electrode means is an electrically and acoustically conductive adhesive.

54. A method of tightening a load indicating fastener of the type having a shank subject to elastic deformation when stressed longitudinally, a head formed at one longitudinal end of said load indicating fastener, a first flat surface formed adjacent said head, tool engagement means formed on said head adjacent said first flat surface, piezoelectric film sensor means permanently mounted on said first flat surface, and a second flat surface formed at the other longitudinal end of said shank, said method including the steps of: engaging a tightening tool with said tool engagement means; engaging contact means with said piezoelectric film sensor means; operating said tightening tool to tighten said load indicating fastener; and measuring with an electronic control device a signal representative of the time of flight of an ultrasonic wave along said shank by monitoring the differential electronic signal between said tool engagement means and said contact means such as to provide a continuous measurement of the tensile load of said shank during the operation of said tightening tool.

The method as claimed in claim 54, wherein said step of operating said tightening tool further includes rotating said tightening tool until the occurrence of a condition measured by said electronic control device indicative of a proper joint, said method further includes the step of: verifying the existence of a proper joint using said measurement of said tensile load.

56. The method as claimed in claim 54 or claim 55, further including the step of: ceasing operation of said tightening tool when said measuring step indicates that a predetermined level of said tensile load is measured. S

57. The method as claimed in any one of claims 54 to 56, wherein said two engaging steps occur simultaneously, said tool engagement means being fixedly interconnected with said tightening tool and said contact means being reciprocally 4 o interconnected with said tool engagement means.

58. The method as claimed in any one of claims 54 to 57, wherein said tensile load includes stress, elongation, strain or tensile force.

59. The method as claimed in any onrie of claims 54 to 58, wherein said step of measuring a signal further includes detecting a first signal indicative of a transverse wave and a second signal indicative of a longitudinal wave and combining said first and second signal to determine said tensile load.

The method as claimed in any one of claims 54 to 59, wherein said step of -41- t- measuring a signal applies the direct timing, indirect timing, double pulsing, clock interpolation, fundamental frequency detection, acoustic impedance, harmonic resonance detection, or phase detection technique to determine said tensile load.

61. A load indicating member, substantially as described herein with reference to the accompanying drawings.

62. A fastener adapted for measuring longitudinal strain, substantially as described herein with reference to the accompanying drawings.

63. A method of making a load indicating fastener, substantially as described herein with reference to the accompanying drawings.

64. A method of tightening a load indicating fastener, substantially as described herein with reference to the accompanying drawings. DATED this 17th day of September 1991. SPS TECHNOLOGIES, INC. By their Patent Attorneys: CALLINAN LAWRIE I 4 i 0 4 a -42-