USE OF LOAD MEASUREMENTS FOR QUALITY CONTROL OF JOINTS DURING ASSEMBLY

Abstract: A method for detecting an error in measuring a load on a load-bearing member and detecting joint defects or irregularities. The method includes measuring a load on the load-bearing member with a first load measurement method and simultaneously measuring a load on the load-bearing member with a second load measurement method. The two load measurements are then compared and a relationship is determined between the two. Then, an error is identified in the load measurement made with the first or second method using the relationship. A second method includes an additional step of correcting the erroneous load measurement by compensating for the error. In one embodiment, the first method uses longitudinal and shear waves and the second method uses longitudinal waves. In another embodiment, the first method uses longitudinal and shear waves and the second method uses shear waves. In yet another embodiment, the error is determined using changes in load measured with longitudinal waves alone or load measured using longitudinal and shear waves versus time.

Description: USE OF LOAD MEASUREMENTS FOR QUALITY CONTROL OF JOINTS DURING ASSEMBLY BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a method for measuring the load applied to a fastener during assembly and, more specifically, to a method of using direct load measurement methods— such as those measuring load with ultrasonic waves or the rate of change in direct load measurement methods using ultrasonic waves with time--to provide reliable and accurate assembly of a joint. Description of the Related Art There are many methods and devices which use ultrasonic waves to measure the tensile load on a load-bearing member such as a fastener. Specifically, ultrasonic techniques can be used to provide a precise measure of the tensile load in a fastener (such as a bolt) during installation, or subsequently for purposes of inspection. These techniques are well documented in the prior art. In addition, techniques have been documented which use ultrasonic waves to detect flaws and for other nondestructive evaluation purposes in fasteners and other load-bearing members. The success of all of these techniques depends upon reliably detecting echo signals from within the load- bearing member under evaluation. Several references have described using time-of-flight measurements of longitudinal and shear waves to calculate tensile stress in load-bearing members. For example, Bobrenko et al., "Ultrasonic Method of Measuring Stresses in Parts of Threaded Joints," All Union Scientific Research Institute of Non-Destructive Testing, Kishinev, Translated from Defektoskpiya, No. 1, pp. 72-81 , January - February 1974, and Johnson et al. , "An Ultrasonic Method for Determining Axial Stress in Bolts, " Journal of Testing and Evaluation, Volume 14, No. 5, pp. 253-259, September, 1986, describe methods for determining stresses in load-bearing members including measuring the time-of-flight required for longitudinal and shear ultrasonic waves to travel up and down the length of load-bearing members. In both references, the user is not required to know the length of the load-bearing member to determine the stress on the load-bearing member. Further, in both references, the stress on the bolt can be measured where only one end of the bolt is accessible. U.S. Patent No. 4,602,511 (Holt) teaches a method of determining stress by using the times-of-flight of both longitudinal and shear waves without taking a time-of- flight measurement at zero load in a bolt. This method allows one to measure the tensile load in a previously installed fastener. The time-of-flight for each of the waves is measured and then used to calculate the tensile stress in the fastener using the formula: n _ t 'r V 10 - t ' 2 V* 20 tM - t 2 V 20 d 2 - 2k{d, -d 2 ) where S is the stress; V 10 and V 20 are the speeds of the longitudinal and shear waves in the unstressed bolt, respectively; / , and 1 2 are the times-of-flight of the longitudinal and shear waves, respectively; k is the length of the unstressed portion of the fastener; and d j and d 2 are acoustoelastic constants which are not equal to one another. The above formula is disclosed in Johnson et al. "An Ultrasonic Method for Determining Axial Stress in Bolts, " A Journal of Testing and Evaluation, Volume 14, No. 5, pp. 253-259, September, 1986. This formula is a simplified version of the formula disclosed in the '511 patent. U.S. Patent No. 4,846,001 (issued to Kibblewhite and assigned to SPS Technologies, Inc.) teaches the use of a thin piezoelectric polymer film which is bonded to the upper surface of a load-bearing member, fasteners incorporating such films, and fastener tightening apparatuses for use with such fasteners. The method determines the length, tensile load, stress, and other tensile load-dependent characteristics of the member by ultrasonic techniques. The '001 patent discloses using a contact pin engaged with the piezoelectric polymer film and disposed in a fastener tightening tool which is electrically connected to an electronic control device. The electronic control device transmits and receives electronic signals from the piezoelectric polymer sensor to provide ultrasonic measurement of the tensile load, stress, or elongation of the shank of the fastener. U.S. Patent No. 5,131,276 (issued to Kibblewhite and assigned to Ultrafast, Inc.) teaches a load-indicating fastener having an ultrasonic transducer, including an acoustoelectric film, grown directly on the fastener surface. By growing the acoustoelectric film directly on the fastener, the film is mechanically, electrically, and acoustically interconnected to the surface. This advance not only allows a precise pulse- echo load measurement technique to be used during a production assembly operation but also significantly improves load measurement accuracy by eliminating errors that would result from axial and radial movement of the transducer relative to the bolt and from variations in the coupling media. In both the '001 and '276 patents, it is stated that where the ultrasonic load measurement fastener tightening tools described therein are used in an automatic tightening operation, the tensile load indication in the load measuring device may be combined with other parameters monitored by the fastener tightening tool, such as torque and angle, to determine when the tightening cycle is complete or to detect irregularities in the joint. One example of a joint irregularity is stick/slip. Stick/slip conditions occur where an increased friction between the fastener and the medium into which the fastener is being inserted causes an erroneous load measurement due to the increased torque that must be applied to overcome the friction and to the sudden decrease in torque required to tighten the fastener after the friction has been overcome. Each upward spike and downward spike in the graph of torque versus angle, shown in Figure 1 , indicates an increased or decreased, respectively, friction condition and the increase or decrease in torque associated with that condition. It is known that by measuring both torque and angle, joint irregularities or irregular friction conditions such as stick/slip can be detected. Other examples of joint irregularities occur due to damaged or dirty threads, incorrectly tapped holes, out-of-tolerance joint components or incorrect joint materials or surface treatments. Although the load measurement accuracy of ultrasonic techniques is generally recognized as superior to other techniques, further improvements in accuracy and reliability are desirable. To further enhance the accuracy and reliability of ultrasonic load determination, the present invention provides a method which uses a combination of ultrasonic load measurements, or the rate of change of an ultrasonic load measurement with time, to accurately and reliably measure the load applied to a load-bearing member. The present invention may be used to detect material yield, to detect errors in ultrasonic load measurements, and to detect irregularities in fasteners or joints during assembly and quality control operations. One object of the present invention is to detect joint and assembly irregularities using ultrasonic measurement alone without the need for expensive torque and angle transducers. SUMMARY OF THE INVENTION To achieve this and other objects, and in view of its purposes, the present invention provides a method for detecting an error in measuring a load on a load-bearing member comprising measuring a load on the load-bearing member with a first load measurement method and simultaneously measuring a load on the load-bearing member with a second load measurement method. The two load measurements are then compared and a relationship is determined between the two. Then, an error is identified in the load measurement made with the first or second method using the relationship. The present invention also provides a method for correcting an erroneous load measurement by compensating for the error. In one embodiment, the first method uses longitudinal and shear waves, and the second method uses longitudinal waves. In yet another embodiment, the error is determined using changes in load measured with longitudinal waves alone, load measured with shear waves alone, or load measured using longitudinal and shear waves, versus time. In another embodiment, the first method uses longitudinal and shear waves and the second method uses shear waves. It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing torque versus angle and illustrating a slip/stick condition; Fig. 2 is a graph, for a first error condition, showing load measured using longitudinal and shear waves (L LS ) versus load measured with longitudinal waves (LJ only or load measured with shear waves (Ls) only; Fig. 3 is a graph, for a second error condition, showing load measured with longitudinal waves (LjJ only or load measured with shear waves (Lj) only versus load measured with longitudinal and shear waves (L LS ); Fig. 4 is a graph, for a third error condition, showing load measured with longitudinal and shear waves (L LS ) versus load measured with longitudinal waves (LJ only or load measured with shear waves (Lj) only; Fig. 5 is a graph, for a fourth error condition, showing load measured with longitudinal and shear waves (L LS ) versus load measured with longitudinal waves (LJ only or load measured with shear waves (Lg) only; Fig. 6 is a graph, for the third error condition, showing load measured with longitudinal waves (LJ only versus time measured during tightening; Fig. 7 is a graph, for the fourth error condition, showing load measured with longitudinal waves (L L ) only versus time measured during tightening; Fig. 8 is a graph, for the third error condition, showing load measured with longitudinal and shear waves (L LS ) versus time measured during tightening; Fig. 9 is a graph, for the fourth error condition, showing load measured with longitudinal and shear waves (L^) versus time measured during tightening; Fig. 10 is a graph, for a fifth error condition, showing load measured using longitudinal waves (LjJ only, load measured with shear waves only (Ls), or load measured with longitudinal and shear waves (L s) versus time measured during tightening; Fig. 11 is a graph, for the second error condition, showing load measured with longitudinal waves (L L ) only, load measured with shear waves only (Ls), or load measured with longitudinal and shear waves (L LS ) versus time measured during tightening; and Fig. 12 is a graph, for the first error condition, showing the use of load measured with longitudinal waves (L L ) only or load measured with shear waves (Lg) only to correct the erroneous load measurement made with longitudinal and shear waves (L LS ). DETAILED DESCRIPTION OF THE INVENTION Traditionally, load measurement techniques based on torque (rotational force applied to the bolt), angle (number of degrees the bolt is rotated), and time (period of time the bolt is rotated) have been used to measure and control the load applied to load- bearing members (such as fasteners) and to bolted joints. Load measurement accuracy with these techniques varies from ±10% to ±30% . Torque, independently, may also be used to measure load during tightening of a threaded fastener. Ultrasonic measurement of load is accurate and provides a way to inspect a tightened bolt but, until recently, was impractical for use in automated assembly operations. The technology developed by Ultrafast and described in U.S. Patent No. 5,131,276 (to Kibblewhite) allows the use of ultrasonic waves to measure and control load applied to a load-bearing member with power and hand tools. This technology is particularly useful in high volume production assembly operations in the automotive, aerospace, construction, and other industries. In addition to accurately measuring the load applied to a load-bearing member, more than one ultrasonic measurement method can be used simultaneously, or the rate of change with time of load measured with one or more ultrasonic load measurement methods may be used to improve the accuracy and reliability of ultrasonic load measurements and to detect undesirable joint irregularities during assembly operations. For example, load measured with longitudinal and shear ultrasonic waves can be used with load measured with longitudinal ultrasonic waves alone or shear ultrasonic waves alone—or rate of change of load measured with longitudinal and shear waves, longitudinal waves alone, or shear ultrasonic waves alone versus time may be used to improve the accuracy and reliability of load measurement. There are two main ultrasonic techniques for measuring load. The first uses pulse-echo time-of-flight measurements of longitudinal ultrasonic waves only. This method is very accurate (±3 %) but generally requires a zero load measurement and, furthermore, cannot be used to measure loads at yield. That is, ultrasonic techniques using longitudinal waves alone cannot be used to measure load where there is plastic deformation of the fastener material. The second technique, as described in Bobrenko, V M., Averbukh, LL, and Chichugov A. A. , M Ultrasonic Method of Measuring Stresses in Parts of Threaded Joints," Translation of the Soviet Journal of Nondestructive Testing, Volume 9, No. 1, pp. 59-66, Jan./Feb. 1974; Johnson G.C., Holt, A.C, and Cunningham, B., "An Ultrasonic Method for Determining Axial Stress in Bolts, " Journal of Testing and Evaluation, Volume 14, Sept. 1986, pp. 253-259; and U.S. Patent No. 4,602,511 (Holt), uses both longitudinal and shear ultrasonic waves. This method can be used to measure load at yield. The methods disclosed in these references, however, are less accurate because the load measurements are more susceptible to bolt-to-bolt variations. For example, bolts which have identical nominal measurements and are formed of the same materials often vary in length, diameter, and material makeup. These variations may cause inaccuracies in measuring load with the methods described in Bobrenko et al. , Johnson et al. , and Holt. The present invention is discussed using the reference variable L L , which refers to load measured using longitudinal ultrasonic waves only; Ls, which refers to load measured with shear ultrasonic waves only; and the reference variable L^, which refers to load measured using both longitudinal and shear ultrasonic waves. The present invention uses load measured using longitudinal waves or load measured with shear waves in combination with load measured using longitudinal and shear waves; or uses the rate of change of these load measurements with time to detect errors in load measurement. It will be appreciated by one skilled in the art that time-of-flight measurements of the longitudinal waves, shear waves, or both could be used, or the rate of change of these time-of-flight measurements with time could be used, to detect, for example, erroneous load measurements, material variations, or material yield as set forth below, rather than using the load calculated with these time-of-flight measurements to identify the errors or conditions. The present invention can detect the following errors and conditions: 1. Detect errors in load measured with longitudinal and shear waves (LL S ) using load measured at zero load with longitudinal waves (LjJ or shear waves (Ls) prior to or during tightening of the fastener. Errors in load caused from bolt-to-bolt variations (as discussed above) may be detected and removed in this way. 2. Detect ultrasonic load measurement errors resulting from a discontinuity in measuring the time-of-flight due to incorrectly identifying the correct cycle in an echo. This discontinuity is sometimes referred to as a trigger point shift in the echo waveform. The discontinuity may result in abrupt changes in load measurements. 3. Detect the onset of yield. Yield occurs when the load on the fastener causes the fastener material to make a transition from elastic deformation to plastic deformation. The onset of yield is detected as a gradual change in slope of the curve of load measured with one ultrasonic method versus load measured with another ultrasonic method, or rate of change of load measured using one ultrasonic method with time. Detection techniques can be used to measure change in slope of load measured with one method plotted against the load measured with a second method or one method plotted against time, which can indicate onset of yield. 4. Detect fastener material variations, such as the incorrect grade or hardness, for example, by detecting premature yield (as discussed above in 3). 5. Detect joint friction irregularities, such as slip/stick indicated by detecting discrete increases in load. Table 1 shows which of the five above-described conditions can be detected using load measured with ultrasonic longitudinal waves (L L ) or ultrasonic shear waves (Ls), load measured with ultrasonic longitudinal and shear waves (L LS ), and rate of change of load measured with these direct load measurements with time. The above load measurement methods can be combined to accurately measure and monitor the load on a load-bearing member during, for example, an assembly operation. Note that, in Table 1, the upper right hand diagonal of the table is a mirror image of the lower left hand diagonal of the table. Table 1 Table 1 describes load conditions that can be detected reliably during tightening and quahty control steps in an assembly operation. Figs. 2 through 12 show the relationship between possible load measuring methods during an assembly operation and how the different methods may be used to show the five above-listed conditions. Figs. 2-5 are graphs comparing load measured with longitudinal and shear waves against the load measured with longitudinal or shear waves alone. Fig. 2 is a graph showing that load measured with longitudinal or shear waves alone can be used to detect erroneous load measurements made with longitudinal and shear waves (Condition 1). The graph indicates that, with no load applied to the load-bearing member, the load measured with longitudinal and shear waves (L LS ) was not zero. This "error" is indicated by an offset load "a. " Because the load measured with longitudinal waves (L L ) or shear waves (Lg) alone requires an initial load measurement at zero load, the graph can be used to show that the load measured with longitudinal and shear waves is erroneously offset by load "a. " The L L or Ls measurement can be used, therefore, to detect and correct the erroneous L measurement. Fig. 3 is a graph showing that load measured with longitudinal and shear waves can be used to detect erroneous load measurements made with longitudinal or shear waves alone (Condition 2). The graph shows that, at load "b," the load measured with longitudinal or shear waves erroneously indicated that the load on the fastener was increasing. This "error" can be detected by using the load measured with longitudinal and shear waves which indicates that the load on the fastener is not increasing or increasing by load "b. " Fig. 4 is a graph showing that load measured with longitudinal and shear waves can be used to detect yield in a bolt. In the region of yield, the load measured with longitudinal and shear waves begins to increase at a lower rate than the load measured with longitudinal or shear waves alone (Condition 3). Yield occurs where the fastener material goes through a transition from elastic deformation to plastic deformation. The load measured with longitudinal or shear waves shows that the load continues to increase. Load measured with longitudinal and shear waves shows, however, that the load has not increased. Measuring load by longitudinal or shear waves alone erroneously shows increasing load because the fastener length is increasing due to plastic deformation which causes the time-of-flight of the longitudinal and shear waves to increase. If the load had been measured by longitudinal or shear waves alone, the onset of yield would not have been detected. Fig. 5 is also a graph showing that load measured with longitudinal and shear waves can be used to detect the constant load at yield in a bolt whereas load measured with longitudinal or shear waves alone indicates that load continues to increase with tightening. In this graph, though, the load measured with longitudinal and shear waves is used to detect variations in the fastener material such as grade or hardness (Condition 4). The load at which the fastener undergoes yield is indicated by the flattening of the curve. Fig. 5 shows that load measured by longitudinal and shear waves falls below the minimum expected yield load, load "c" (dotted line). This condition would not have been detected if load had been measured by longitudinal or shear waves alone. Figs. 6-11 are graphs showing changes in load measured with longitudinal and/or shear waves versus time. Fig. 6 is a graph showing load measured with longitudinal waves versus time to detect erroneous load measurements due, for example, to the onset of yield (Condition 3). The load measurements show that, in the region of yield, the load measured with longitudinal waves is increasing but at a decreasing rate. Fig. 7 is also a graph showing that load measured with longitudinal waves versus time can be used to detect yield in a bolt. In this graph, the load measured with longitudinal waves is used to detect variations in fastener material such as grade or hardness (Condition 4). The load at which the fastener experiences yield is indicated by the change of slope of the curve. Fig. 7 shows that the load at which the fastener experienced yield falls below the minimum expected yield load, load "d" (dotted line). The premature occurrence of yield indicates that there is some fault condition related either to the fastener material or to other factors in the load-bearing member or the joint. Fig. 8 is a graph showing that load measured using longitudinal and shear waves versus time can be used to detect, for example, the onset of yield (Condition 3). The flattening of the curve indicates that, while the bolt is still mrning, the load measured with longitudinal and shear waves is not increasing. Fig. 9 is also a graph showing that load measured using longitudinal and shear waves versus time can be used to detect premature yield in a fastener when used in conjunction with time. In this graph, the load measured with longitudinal and shear waves is used to detect variations in the fastener material such as grade or hardness (Condition 4). Fig. 9 shows that load measured by longitudinal and shear waves versus time detects that the fastener falls below the minimum required yield load "e. " Fig. 10 is a graph showing that load measured with longitudinal waves (L L ) only, load measured with shear waves only (Lg), or load measured with longitudinal and shear waves (L LS ) versus time can be used to detect a stick/slip condition (condition 5 above). In Fig. 10, discrete increases in ultrasonic load measured using longitudinal or shear waves alone, or longitudinal and shear waves, can be detected. Fig. 11 is graph showing load measured with longitudinal and shear waves, or load measured with either longitudinal or shear waves alone, versus time. The time measurement will be identical to the angle measurement if the fastener is rotated at a constant speed. Fig. 11 is a graph showing that discrete changes in load measurements made with longitudinal and shear waves, or with either longitudinal or shear waves alone, versus time (Condition 2) can be used to detect erroneous load measurements. The graph shows that, at load "f," the load measured with longitudinal and shear waves, or with either longitudinal or shear waves alone, erroneously indicated that the load on the fastener was increasing. This "error" can be detected by using a time measurement which shows that the load on the fastener could not have increased instantaneously. The sudden increase in load indicates that there has been an error in the time-of-flight measurement of the longitudinal and shear waves or of either the longitudinal or shear waves alone. The use of direct load measurements in combination with one another may also be used to correct enoneous load measurements. For example, as shown in Fig. 12, load measured with longitudinal or shear waves may be used to correct an erroneous load measurement made with longitudinal and shear waves. Fig. 12 shows that, with zero load applied to a load-bearing member, the load measured with longitudinal and shear waves is not zero. Rather, the load is offset by load "g" which is identical to load offset "a" described above with reference to Fig. 2. Because the load measured with longitudinal or shear waves alone requires an initial load measurement at zero load, the graph can be used to show that the load measured with longitudinal and shear waves is erroneously offset by load "g. " In addition, because the offset is a known value (load "g"), the load measured with longitudinal and shear waves may be corrected by this offset value such that the load measured using longitudinal and shear waves coincides with the load measured using longitudinal or shear waves. Thus, the load measured with longitudinal or shear waves alone can be used to detect and correct the erroneous load measurement made with longitudinal and shear waves. It will be appreciated by one skilled in the art that, in the above described techniques, rates of change of load measurements, or the slope or gradient of the load versus time graph, can be measured simply by taking the difference in load measurement over an interval of time and dividing by that interval of time. The applicability of these techniques is independent of how each load measurement is made. They could be used with direct load measurement technology other than ultrasonic load measurement methods should such methods be developed in the future. Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.

Claims: What is Claimed: I. A method for detecting an error in measuring a load on a load- bearing member comprising: (a) measuring a load on the load-bearing member with a first load measurement method and simultaneously measuring a load on the load-bearing member with a second load measurement method, wherein said first method is selected from a group consisting of methods using longitudinal and shear waves, longitudinal waves alone, and shear waves alone; said second method is selected from the group consisting of methods using longitudinal and shear waves, longitudinal waves alone, and shear waves alone; and said first and second methods differ; (b) comparing the load measured with said first method to the load measured with said second method; (c) determining a relationship between the load measured with said first method and the load measured with said second method; and (d) identifying an error in a load measurement made with a load measurement method selected from the group consisting of said first method and said second method using said relationship. 2. The method according to claim 1, wherein in said step (a) said first load measurement method uses longitudinal waves alone, and said second load measurement method uses longitudinal and shear waves. 3. The method according to claim 1, wherein in said step (d) said error in load measurement method is made with said first method. 4. The method according to claim 1 , wherein in said step (d) said enor in load measurement method is made with said second method. 5. A method for detecting an error in measuring a load on a load- bearing member, or defects or irregularities in the load-bearing member or components adjacent the load-bearing member, said method comprising: (a) measuring a load on the load-bearing member with a load measurement method selected from a group consisting of methods using longitudinal ultrasonic waves alone, shear ultrasonic waves alone, and both longitudinal and shear ultrasonic waves during loading of the load-bearing member; (b) measuring changes in the load measured with said load measurement method over intervals of time and using said changes in load to detect a condition selected from the group of conditions consisting of an error in the load measured with said load measurement method, and defects and irregularities in one of the load-bearing member and components adjacent the load-bearing member. 6. The method according to claim 5, wherein in said step (a) said load measurement method uses longitudinal ultrasonic waves alone. 7. The method according to claim 5, wherein in said step (a) said load measurement method uses shear ultrasonic waves alone. 8. The method according to claim 5, wherein in said step (a) said load measurement method uses longitudinal and shear ultrasonic waves. 9. A method for detecting and correcting an error in measuring a load on a load-bearing member comprising: (a) measuring a load on the load-bearing member with a first load measurement method and simultaneously measuring a load on the load-bearing member with a second load measurement method, wherein said first method is selected from a group consisting of methods using longitudinal ultrasonic waves alone, shear ultrasonic waves alone, and both longitudinal and shear ultrasonic waves; said second method is selected from a group consisting of methods using longitudinal ultrasonic waves alone, shear ultrasonic waves alone, and both longitudinal and shear ultrasonic waves; and said first and said second methods differ; (b) comparing the load measured with said first method to the load measured with said second method; (c) determining the relationship between the load measured with said first method and the load measured with said second method; (d) identifying an error in a load measurement made with a load measurement method selected from the group consisting of said first method and said second method using said relationship; and (e) correcting an erroneous load measurement by compensating said erroneous load measurement for said error. 10. The method according to claim 9, wherein in said step (a) said first load measurement method uses longitudinal waves alone and said second load measurement method uses longitudinal and shear waves. 11. The method according to claim 9, wherein in said step (d) said error in load measurement method is made with said first method. 12. The method according to claim 9, wherein in said step (d) said error in load measurement method is made with said second method.

Inventors: KIBBLEWHITE IAN E, VECCHIO CHRISTOPHER J, KIBBLEWHITE, IAN, E., VECCHIO, CHRISTOPHER, J.
Applicants: ULTRAFAST INC, ULTRAFAST, INC.
Application year: 1997
Main classification: G01N29
IPC codes: G01L5, G01N29