Ultrasonic load indicating member, apparatus and method

Abstract: A method of measuring the load in a member subjected to longitudinal stress, a load measuring device and a fastener tightening device using the method of measuring, a load indicating member and a load indicating fastener for use in conjunction with the method of measuring, a method of making the load indicating fastener, a method of tightening the load indicating fastener and a transducer for instrumenting a load bearing member are each disclosed and claimed. A thin piezoelectric sensor consisting of a piezoelectric film sandwiched between two thin electrodes is permanently mechanically and acoustically coupled to the upper surface of a member and is used to determine the length, tensile load, stress, or other tensile load dependant characteristic of the member by ultrasonic techniques.

Description: The present invention relates to a load indicating member according to claim 1 and further relates to a method according to claim 24 for making a 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 the longitudinally stressed member is a fastener since the measurement of the longitudinal stress provides a verification of the existence of a proper joint. One approach to measure the elongation of a member or fastener is to use an ultrasonic measurement device. According to US-A-3 759 090, this is done by removably interconnecting an ultrasonic transducer to one end of the member being measured. 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. According to US-A-4 294 122, ultrasonic load measurements are made by incorporating an ultrasonic transducer into the member itself, i.e. into the head or an end of a fastener. A thick piezoelectric layer made from a material such as PbZrTi is provided with thin layers of electrically conducting material on both opposite surfaces. Electrodes are secured to both layers, respectively. The so formed ultrasonic transducer is secured in place with adhesive or epoxy. One of the electrodes is in contact with the fastener. This provides an instrumented load bearing fastener which has been modified to incorporate a stress indicating feature. However, these instrumented fasteners are greatly modified due to the large and complicated ultrasonic transducer. They are therefore prohibitably expensive for wide spread use. 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 material and properties. Furthermore, document EP-A-0 233 057 discloses the use of polymer-based piezoelectric films in vibration sensors. This document focuses on the bending properties of such sheet-like sensing elements, which are particularly suitable for use on curved surfaces. Document EP-A-0 193 048, similarly, details the use of polymer-based piezoelectric layers in ultrasonic transducers for use underwater or for diagnosis of living bodies. What is needed, therefore, is a low cost ultrasonic transducer permanently attached to e.g. a fastener in an inexpensive manner to provide accurate tightening information on a mass production basis. The present invention provides - according to claims 1 and 24 - a load indicating member as well as a method which combines many of the advantages in a single member, heretofore only available separately in different load indicating members. The echo signal levels of the transducer adhesively coupled with the member and containing the thin layer of piezoelectric film material with a thickness of 5 to 110 microns was compared with the echo signal of prior art thick ceramic transducers removably coupled with a member. Not only were the signal levels of the new transducer considerably higher, but also the signal to noise ratio 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 thick transducer. Reflected acoustic waves within the film transducer may decay more quickly since the lower acoustic impedance of a very thin layer of piezoelectric film material, like a polymeric piezoelectric film 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 thin film transducer therefore is a significant transducer performance improvement over the prior art. In a preferred embodiment, the thin piezoelectric film element may also provide a driving means for producing an 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 contact means is electrically engaged with the first electrode means indirectly by engagement of the first contact means with the load indicating member. The output signals thus gained 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 idication of the load in a previously tightened fastener. When the load indicating member is a fastener, a load measuring device may be used simultaneously with a fastener tightening tool or, alternatively, may be incorporated directly into the fastener tightening tool. When the fastener tighening tool incorporating the load measuring device is of an automatic tightening type, the tensile load indication in the load measuring device may be combined with other parameters, monitored by the fastener tightening tool, such as angle and torque, to determine when the tightening cycle is complete and to detect irregularities in the joint. An object of the present invention is to provide an inexpensive load indicating member which may be easily installed with conventional tools. Another object of the present invention is to provide an inexpensive and accurate method of making the load indicating member or a load indicating fastener. Another object of the present invention is to provide an inexpensive method of making a reliable, accurate and compact load indicating member or fastener from a conventional member or fastener. A primary object of the present invention is to provide a transducer having improved performance including a higher signal to noise ratio than prior designed. Still another object of the present invention is to provide a load indicating member providing a means for monitoring the quality of the member by detecting imperfections in the member and further inspecting members which were stressed at an earlier time to determine the current condition, and particularly, to determine if prestressed fasteners have loosened. Yet another object of the present invention is to provde a load indicating fastener which may be tightened by conventional tightening tools and, more particularly, to provide a load indicating fastener which may be monitored by a load measuring device during a tightening operation. Preferred embodiments will be described with the help of the drawings. In the drawings is: Figure 1 a perpective view depicting an example of a load indicating member; Figure 2 an enlarged and partially sectional view illustrating the indicating fastener of Figure 1; Figure 2a through 2c partial views similar to Figure 2 but illustrating alternate examples of load indicating members; Figure 3 a partially schematic and partially cut-away side view depicting a load indicating fastener as well as a fastener tightening tool; Figure 4 through 8 are graphical views. In Figures 1 and 2, 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 conventional bolt which has been modified to provide an indication of the tensile load, stress, elongation or other characteristics of the bolt during a tightening operation as well as at various other times during the life of a joint. The bolt has a shank 112 with threads 114 formed at one end and with a head 116 formed at the other end. A shoulder 118 is formed between the head 116 and the shank 112. The head 116 has a generally flat upper surface 120 disposed normal to the 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 normal to the longitudinal axis 122. The head 116 is provided with a wrenching or tool engagement surface 126, such as a hexagonal wrenching surface, disposed about the periphery thereof. Alternatively, as shown in Fig. 2b, a flat surface 120a bay 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. 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 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 electrode 134 are electrically isolated from the head 116. For some embodiments, the head 116 will perform the function of the first electrode 132. 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 capacitive coupling for the electrical communication between the piezoelectric film sensor 128 and the head 116. In Figure 3 a load indicating fastener 110 is shown with a fastener tightening tool 140. The fastener tightening tool 140 includes a conventional power tool 142 and a rotary output driver 144 engageable with a socket member 146. The socket member 146 engages the head 116 of the fastener 110 both electrically and mechanically. A contact pin 148 is mounted to reciprocate 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, (Fig 3), is electrically interconnected with the contact pin 148 and the socket member 146 by means of electrical lines 152 and 154, respectively, by way of slip ring wipers 156 and 158. Alternatively, the signal may be transferred by a non-contact means, such as by capacitive coupling, and other techniques. 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. The fastener tightening tool 140 may be provided with a display device, (not shown) 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 operation should be stopped. The power tool may monitor other characteristics of the joint being formed, such as the torque and the instantaneous angle of the load indicating fastener. Such a power tool may be found in US-A-4,344,216. This other information available from the power tool may be combined with the tensile load, stress or elongation information supplied by electronic control device 150 to provide a precisely controlled tightening 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 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. The load indicating fastener 110 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, any standard tightening tool, 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 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. A fastener measuring tool may be made incorporating all the components of the fastener tightening tool 140 of Fig 3 except the power tool 142. Such a device may be used to measure the elongation of a bolt independently of the tightening tool. The design of the load indicating fastener 110 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 to have a surface finish in the order of 125 micro inches while 250 micro inches has been determined to be acceptable for the purpose of acoustical coupling. However, a surface finish of 16-200 micro-inches has been found to be preferable because of the use of an adhesive coupling. The 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. 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 advance together with a peel-off sheet of inert material, for storing the piezoelectric film sensor until the time for installing the sensor on the fastener 110. The arrangement 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. A significant factor is that the affect of axial stress on the speed of transverse ultrasonic waves is much less than that of longitudinal waves. Therefore, measurement of the time of flight of both waves can be used to determine axial stress without knowledge of the length of the bolt. This therefore allows measurement of the axial load in pre-installed fasteners. Ultrasonic equipment uses the measurement of the time of flight of a longitudinal ultrasonic 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 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 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 composition, heat treat, and temperature, for example, and the 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 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 using the time of flight of longitudinal waves over the entire length of the fastener. The measurements are dependent on grip length which must be entered as a control parameter. Additionally, variations in local stress distributions in the 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, 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 it. This wave is sometimes referred to as a shear wave since adjacent particles are subject to shear forces. A longitudinal wave travels almost twice as fast as a transverse wave in steel and the time of flight is affected to a 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. Longitudinal and transverse waves may be applied to one end of the fastener by the piezoelectric transducer 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 provides, in theory, two significant advantages over the use of the longitudinal wave only. First, tensile load measurements can be made without first making zero load measurement i.e. load can be measured in already installed fasteners. Second, the effect of bolt to bolt variations from material properties, heat treatment etc, is reduced since they affect both longitudinal and transverse waves. The apparatus as disclosed may be used with either or both of these waves. 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. 4. For example, one commercially available instrument sold by Raymond Engineering uses a 100 MHz clock and averages 160 measurements to achieve the required resolution. Clock rates as high as 2 GHz are now possible with Gallium Arsenide technology. The indirect timing techniques involves timing from the first echo to the second echo as shown in Fig. 5. Measurement of the time interval between the first and second echoes 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. 6, 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. 7. Pulse A produces first echo A1 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 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 fundamental frequency and the difference in its value before and after tightening used to determine the tension. 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 with the output of the voltage controlled oscillator as shown in Fig. 8. If the oscillator and the received signal are in phase, the summed signal will be a maximum, if one hundred and eighty degrees out of phase, the two signals will cancel, forming a minimum. This summed signal, therefore represents the difference in phase and is used to control the frequency of the voltage controlled oscillator so that the phase remains 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. 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. The load indicating fastener 110 as disclosed facilitates the rapid interconnection of a conventional fastening tool or an instrumented fastening tool such as the fastening tool 140. Both the fastener 110 and the fastener tightening tool 140 are comparably inexpensive to manufacture. This facilitates more widespread use of ultrasonic fastening and monitoring techniques in production. The fastener 110 and the fastener tightening tool 140 facilitate rapid formation of reliable joints having repeatable and predictable characteristics. They permit a means to detect flaws in the joint during actual installation process thus reducing the risk of catastrophic joint failure. They further permit the monitoring, at a later time, of the characteristics of the joint. The fastener as disclosed provides an inspection feature for monitoring its condition. 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 means or with a nut engagement means.

Claims: A load indicating member comprising: a shank (112), 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 (120,120a) on a head (116,116a) formed adjacent one longitudinal end of said shank (112); first electrode means (132) permanently, mechanically, electrically, and acoustically interconnected with said shank (112); a piezoelectric material layer permanently, mechanically, and electrically interconnected with said first electrode means; second electrode means (134) permanently, mechanically, and electrically interconnected with the piezoelectric material layer and electrically isolated from the head (116,116a) and the first electrode means, such that the electrode means and the piezoelectric material layer cooperate to form an ultrasonic transducer; and a second generally flat surface (124,124a) formed on said shank (112) at a location remote from said one longitudinal end of said shank, said second flat surface (124,124a) being generally coplanar with said first flat surface (120,120a) and spaced a predetermined distance therefrom, characterised in that the piezoelectric material layer is a thin layer of piezoelectric film material with a thickness of 5 to 110 microns. Member of claim 1, characterised in that the thickness of the thin layer of piezoelectric film material is 20 to 60 microns. Member of claim 1, characterised in that the thin layer of piezoelectric film material consists of polymeric material. Member of claim 3, characterised in that the polymeric material is polyvinylidene fluoride or derivatives thereof. Member of claim 1, characterised in that the first electrode means (132) comprises said shank (112). Member of claim 1, characterised in that the first electrode means (132) comprises an adhesive. Member of claim 1, characterised in that a recess (121) is formed in said head (116a) and said first flat surface (120a) being formed in the base of said recess (121). Member of claim 1, characterised in that the first electrode means (132) comprises a non-conductive adhesive capacitively coupling said thin layer of piezoelectric film material with said shank (112). Member of claim 1, characterised in that an engagement means (126) is formed at said one longitudinal end of said shank (112) for removably positioning a measurement device relative to said first and second flat surfaces (120,120a,124,124a) such as to be removably interconnected with said first and second electrodes (132,134) to provide a measurement of a tensile load dependent characteristic of said shank (112). Member of claim 1, characterised in that the thin layer of piezoelectric film material defines a flexible disk (130) which is disposed in axial alignment with the shank (112). Member of claim 10, characterised in that the disk (130) has a diameter approximately between the diameter of the shank (112) and half the diameter of the shank (112). Member of claim 1, characterised in that at least one of said first and second electrode means (132,134) comprises a thin conductive layer disposed on a surface of the thin layer of piezoelectric film material. Member of claim 1, characterised in that the first electrode means (132) is interconnected with said first generally flat surface (120,120a) by an adhesive providing both mechanical and acoustical interconnection. Member of claim 1, characterised in that at least one of said electrode means (132,134) is formed by vapor depositing, electrodepositing or painting a thin film of metal on a surface of said thin layer of piezoelectric film material. Member of claim 1, characterised in that at least one of said electrode means (132,134) comprises a metallic foil. Member of claim 1, characterised in that the first generally flat surface (120,120a) has a surface finish of approximately 16 to 200 micro-inches. Member according to claims 1 to 16 in which the member is a fastener (110). Member of claim 17, characterised in that the first electrode means (132) is interconnected with said head (116,116a) by means of an adhesive, and that the first and second electrode means (132,134) comprise metallic films disposed on opposite surfaces of a disk (130) of thin piezoelectric film material. Member of claim 17, characterised in that the electrode means are less than 30 microns thick. Member of claim 18, characterised in that at least one of said first and second electrode means (132,134) is disk shaped and has approximately the same diameter as the disk (130). Member of claim 17, characterised in that the fastener (110) further comprises a bolt having threads (114) disposed on said shank (112) remote from said head (116,116a). Member of claim 17, characterised in that the head (116,116a) is formed of an electrically conductive material and said first electrode means (132) is electrically or capacitively interconnected therewith. Member of claim 17, characterised in that the first electrode means comprises an adhesive, preferably a non-conductive adhesive, capacitively coupling said thin layer of piezoelectric film material with said shank (112). A method of making a load indicating member from a fastener (110) having a head (116,116a) and a shank (112) extending from the head, said shank (112) being subjected to elastic deformation when stressed longitudinally such that one portion thereof moves relative to another portion thereof, comprising the steps of: forming a first flat surface (120,120a) on said head (116,116a) and a location remote from said shank (112), said first flat surface being normal to the longitudinal axis of said shank; permanently, acoustically, and mechanically interconnecting an acoustic transducer (128) to said head (116,116a) of said fastener such as to electrically interconnect a first electrode (132) of the transducer with the head (116,116a) and, further, such as to isolate a second electrode (134) of the transducer from said head (116,116a) characterised in the step of forming said acoustic transducer (128) by applying first and second electrodes (132,134) to opposite faces of a thin piezoelectric film disk (130) having a thickness of 5 to 110 microns The method of claim 24, characterised by forming the electrodes (132,134) by vapor deposition, electrodeposition or by metallic painting.

Inventors: KIBBLEWHITE IAN E, KIBBLEWHITE, IAN E.
Applicants: SPS TECHNOLOGIES, SPS TECHNOLOGIES, INC.
Application year: 1989
Main classification: G01L5
IPC codes: F16B31, G01L5, B25B23