Method and device for determining the prestress force of a connection component

Abstract

A method for determining the prestress force of a connection component (10) is proposed. In the method, ultrasonic signals (22) are introduced into the connection component (10) and ultrasonic echoes (24) of the ultrasonic signals (22) are received again. The method comprises the following steps: a) introducing a longitudinal ultrasonic signal and determining a first signal time of flight FTOF.sub.L of the longitudinal ultrasonic signal until the reception of an echo of the longitudinal ultrasonic signal, b) introducing a transverse ultrasonic signal and determining a second signal time of flight FTOF.sub.T of the transverse ultrasonic signal until the reception of an echo of the transverse ultrasonic signal, and c) determining an effective temperature T.sub.eff and the prestress force of the connection component (10) on the basis of the first signal time of flight FTOF.sub.L, the second signal time of flight FTOF.sub.T, previously determined reference data and calibration factors using the assumption that a prestress force F.sub.L ascertained using the first signal time of flight FTOF.sub.L and a prestress force F.sub.T ascertained using the second signal time of flight FTOF.sub.T are equal in magnitude,
wherein steps a) and b) are carried out successively in any desired order or in parallel. A further aspect of the invention relates to a device for carrying out the method.

Claims

1. A method for determining the prestress force of a connection component, wherein ultrasonic signals are introduced into the connection component and ultrasonic echoes of the ultrasonic signals are received again, comprising the following steps: a) introducing a longitudinal ultrasonic signal and determining a first signal time of flight FTOF.sub.L of the longitudinal ultrasonic signal until the reception of an echo of the longitudinal ultrasonic signal, b) introducing a transverse ultrasonic signal and determining a second signal time of flight FTOF.sub.T of the transverse ultrasonic signal until the reception of an echo of the transverse ultrasonic signal, and c) determining an effective temperature T.sub.eff and the prestress force of the connection component on the basis of the first signal time of flight FTOF.sub.L, the second signal time of flight FTOF.sub.T, previously determined reference data, and calibration factors, using the assumption that a prestress force F.sub.L ascertained using the first signal time of flight FTOF.sub.L and a prestress force F.sub.T ascertained using the second signal time of flight FTOF.sub.T are equal in magnitude, wherein steps a) and b) are carried out successively in any desired order or in parallel, wherein the calibration factors describe an empirically ascertained relationship between prestress force and a change in a signal time of flight, and/or wherein the calibration factors describe an empirically ascertained temperature dependence of the time of flight of a non-loaded connection element, wherein a quadratic temperature compensation of a first and/or a second signal time of flight difference is performed, wherein a temperature-corrected first signal time of flight difference is given by $\Delta {\mathrm{TOF}}_{L_{\mathrm{corr}}}=\frac{{\mathrm{FTOF}}_L}{1+c_{1L}.Math.T_{\mathrm{eff}}+c_{2L}.Math.T_{\mathrm{eff}}^2}-\frac{{\mathrm{BTOF}}_L}{1+c_{1L}.Math.T_{\mathrm{eff}}+c_{2L}.Math.T_{\mathrm{eff}}^2}$ and a temperature-corrected second signal time of flight difference is given by $\Delta {\mathrm{TOF}}_{T_{\mathrm{corr}}}=\frac{{\mathrm{FTOF}}_T}{1+c_{1T}.Math.T_{\mathrm{eff}}+c_{2T}.Math.T_{\mathrm{eff}}^2}-\frac{{\mathrm{BTOF}}_T}{1+c_{1T}.Math.T_{\mathrm{eff}}+c_{2T}.Math.T_{\mathrm{eff}}^2},$ wherein C.sub.1L, C.sub.1T, C.sub.2L and C.sub.2T are empirically ascertained constants, wherein BTOF.sub.L is a first reference time of flight and BTOF.sub.T is a second reference time of flight.

2. The method as claimed in claim 1, wherein the previously determined reference data comprise a first reference time of flight BTOF.sub.L of the longitudinal ultrasonic signal and a second reference time of flight BTOF.sub.T of the transverse ultrasonic signal, which were ascertained at a reference temperature T.sub.BTOF.

3. The method as claimed in claim 1, wherein a linear temperature compensation of a first and a second signal time of flight difference is performed, wherein a temperature-corrected first signal time of flight difference is given by $\Delta {\mathrm{TOF}}_{L_{\mathrm{corr}}}=\frac{{\mathrm{FTOF}}_L}{1+c_{1L}.Math.T_{\mathrm{eff}}}-\frac{{\mathrm{BTOF}}_L}{1+c_{1L}.Math.T_{\mathrm{eff}}}$ and a temperature-corrected second signal time of flight difference is given by $\Delta {\mathrm{TOF}}_{T_{\mathrm{corr}}}=\frac{{\mathrm{FTOF}}_T}{1+c_{1T}.Math.T_{\mathrm{eff}}}-\frac{{\mathrm{BTOF}}_T}{1+c_{1T}.Math.T_{\mathrm{eff}}},$ wherein C.sub.L and C.sub.T are empirically ascertained constants.

4. The method as claimed in claim 1, wherein a linear relationship between prestress force and a signal time of flight difference is assumed, wherein the prestress force F.sub.L ascertained by way of the first signal time of flight is given by F.sub.L=k.sub.L.Math.ΔTOF.sub.L.sub.corr and the prestress force F.sub.T ascertained by way of the second signal time of flight is given by F.sub.T=k.sub.T.Math.ΔTOF.sub.T.sub.corr, wherein k.sub.L and k.sub.T are empirically determined material constants.

5. The method as claimed in claim 1, wherein the effective temperature T is given by equating F.sub.T and F.sub.L and solving the resulting quadratic equation with respect to T.sub.eff.

6. The method as claimed in claim 1, wherein a quadratic relationship between prestress force and a signal time of flight difference is assumed, wherein the prestress force F.sub.L ascertained by way of the first signal time of flight is given by F.sub.L=k.sub.1L.Math.ΔTOF.sub.L.sub.corr+k.sub.2L.Math.(ΔTOF.sub.L.sub.corr).sup.2 and the prestress force F.sub.T ascertained by way of the second signal time of flight is given by F.sub.T=k.sub.1T.Math.ΔTOF.sub.T.sub.corr+k.sub.2T.Math.(ΔTOF.sub.T.sub.corr).sub.2, wherein k.sub.1L, k.sub.2L, k.sub.1T and k.sub.2T are empirically determined constants.

7. The method as claimed in claim 1, wherein the effective temperature T.sub.eff is given by equating F.sub.T and F.sub.L and numerically seeking solutions to the equation for T.sub.eff.

8. A device for determining the prestress force of a connection component comprising means for introducing longitudinal ultrasonic signals and transverse ultrasonic signals into the connection component, means for receiving ultrasonic echoes of the ultrasonic signals, and a control unit, wherein the control unit is configured to carry out one of the methods as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Exemplary embodiments of the invention are illustrated in the drawings and are explained in greater detail in the description below. In the figures:

(2) FIG. 1 shows a schematic illustration of a connection component with an ultrasonic transducer,

(3) FIG. 2 shows a schematic illustration of an arrangement for measuring a prestress force of a connection component, and

(4) FIG. 3 shows a measurement series carried out using the experimental arrangement from FIG. 2.

(5) In the following description of the exemplary embodiments of the invention, identical component parts and elements are denoted by identical reference signs, a repeated description of these component parts or elements in individual cases being dispensed with. The figures merely schematically illustrate the subject matter of the invention.

(6) FIG. 1 schematically shows a connection component 10. In the illustration in FIG. 1, the connection component 10 is configured as a screw having a screw head 12 and a screw shaft 14. The screw shaft 14 has a thread 16.

(7) The connection component 10 has a first end 18, representing the side of the screw having the screw head 12. The connection component 10 furthermore has a second end 20 on the opposite side at the screw shaft 14.

(8) An ultrasonic transducer 30 is arranged in the center of the surface of the screw head 12, that is to say at the first end 18. The ultrasonic transducer 30, which is embodied for example as a piezoelectric element, is configured to couple ultrasonic signals 22 into the connection component 10 and to receive again ultrasonic echoes 24 reflected at the second end 20 of the connection component 10. In this case, the ultrasonic transducer 30 is arranged and configured such that it can couple in both longitudinal ultrasonic waves 28 and transverse ultrasonic waves 26. The ultrasonic transducer 30 can be embodied using thin-film technology, or else as an interconnection of at least one longitudinal and one transverse adhesively bonded piezo element, which are preferably connected in parallel or else in series (as a stack).

(9) Handheld transducers operated in parallel and embodied respectively as longitudinal and transverse versions are usable as an alternative to a connection component 10 with an integrated ultrasonic transducer 30.

(10) Furthermore, as an alternative to the solution shown in FIG. 1, it is possible to arrange an additional receiver for ultrasonic echoes at the second end 20 of the connection component 10, such that measurement can be effected using transmission geometry.

(11) FIG. 2 schematically shows an arrangement for measuring a prestress force of a connection component 10. In the exemplary arrangement, a connection component 10 configured as a screw has been screwed into a screw block 62 having an internal thread 64. The screw block 62 is positioned in the interior of a load cell 70. The load cell 70 serves as a reference for the prestress force of the connection component 10. The desired clamping length is set by way of a spacer 66 and a head plate 67.

(12) The measurement of the prestress force by means of ultrasound is coordinated by a control unit 52. The latter drives a signal generator 44, which generates an excitation signal. Said excitation signal is amplified by way of an amplifier 46 and guided via a splitter 40 and a connecting line 42 to the ultrasonic transducer 30 situated at the first end 18 of the connection component 10. On account of the excitation by the electrical signals obtained, the ultrasonic transducer 30 introduces ultrasonic waves into the connection component 10, with the result that longitudinal and transverse ultrasonic waves 26, 28, cf. FIG. 1, reach the second end 20 through the screw shaft 14. At the second end 20, the ultrasonic waves are reflected and return as ultrasonic echoes 24, cf. FIG. 1, to the ultrasonic transducer 30 once again through the screw shaft 14. The ultrasonic transducer 30 receives the ultrasonic echoes 24 and converts them into an electrical signal, which passes back to the splitter 40 via the connecting line 42. The splitter 40 guides the received signals via a reception amplifier 48 to a receiver 50, where the signals are digitized and transmitted to the control unit 52 for evaluation.

(13) The control unit 52 determines in each case the signal times of flight for the longitudinal and transverse ultrasonic signals. This is done by evaluating in each case how much time has elapsed between the generation of an excitation signal and the reception of the digitized echo signal.

(14) In the non-loaded state of the connection component 10, it is necessary to measure the signal times of flight for the longitudinal and the transverse ultrasonic waves as reference time of flight and the associated temperature once and to store them in the control unit 52. These three constants and the signal times of flight for the longitudinal and the transverse ultrasonic waves under load make it possible to calculate the prestress force without direct measurement of the temperature by means of a temperature sensor 68. The prestress force and the associated “effective” temperature of the connection component 10 can be calculated using Equation 11.

(15) In order to be able to test the ascertainment of the prestress force of the connection component 10 at various temperatures, a temperature chamber 60 is provided, which accommodates the connection component 10, screwed into a load cell 70 with screw block 62, spacer 66 and head plate 67. Furthermore, for comparison purposes, a temperature sensor 68 is arranged at the screw head 12 of the connection component 10.

(16) The load cell 70 allows an accurate determination of the prestress force for comparison purposes by means of direct measurement of the force acting on the load cell 70. The temperature sensor 68 is used to make it possible to compare a conventional temperature compensation with the effective temperature determined according to the invention.

(17) The diagram in FIG. 3 shows a measurement series carried out using the experimental arrangement from FIG. 2, which measurement series involved investigating a measurement of the prestress force of a screw as securing element under changing temperature conditions. Time is plotted on the X-axis of the diagram. The determined prestress force in kN is plotted on the left-hand Y-axis and the temperature T in ° C. measured by the temperature sensor 68 at the securing element 10 is plotted on the right-hand Y-axis. The time profile of the prestress force in the case of temperature changes from −10° C. to +10° C. was investigated in the measurement series illustrated.

(18) The temperature within the temperature chamber 60 was changed repeatedly. In a first step, the desired temperature was set to −10° C. The curve 80 shows the measurement values of the temperature sensor 68. In the diagram in FIG. 3 it is evident that the temperature T which was ascertained by the temperature sensor 68 at the screw head 12 initially falls rapidly and then slowly approaches the desired value. After a first waiting time had elapsed, the desired temperature was subsequently set to 0° C. The measured temperature T in turn initially rose rapidly and then slowly approached the desired value. After a second waiting time, the desired value was set to 10° C. The measured temperature T once again initially rose rapidly and then slowly approached the desired value. After a third waiting time, the temperature regulation of the temperature chamber 60 was switched off, with the result that the temperature rose to approach room temperature.

(19) The curve 86 in FIG. 3 illustrates the reference prestress force ascertained by the load cell 70. It is evident here that a temperature change led in each case to a temporary change in the prestress force. The initial cooling to −10° C. proceeding from room temperature led to a temporary reduction in the prestress force. For each of the subsequent temperature steps in which the temperature in the temperature chamber 60 was increased, a temporary increase in the prestress force is discernable. The temporary change in the prestress force is attributable to the fact that, in the event of temperature change, the mechanical component parts involved, such as the screw block 62, the spacer 66 and the connection component 10 itself, heat up and cool down at different rates. In this regard, all the component parts of the experiment set-up were at room temperature level at the beginning of the test. The subsequent cooling down to −10° C. encompassed firstly the outer component parts such as the spacer 66. The connection component 10 seated in the interior of the experimental arrangement required the most time to assume the temperature of −10° C. The momentary reduction of the prestress force in the experimental arrangement can be explained by the faster shrinkage of the spacer 66 compared with the connection component 10. Conversely, each further temperature increase by in each case 10K in the temperature chamber leads to a momentary increase in the prestress force since the spacer 66 heats up and expands more rapidly than the connection component 10 seated in the interior.

(20) The three further curves 81, 82 and 83 in the diagram in FIG. 3 show in each case the prestress force of the connection component 10 determined on the basis of ultrasound measurements. The first curve 81 shows a first prestress force, for which only the longitudinal ultrasonic signals were taken into account, and the second curve 82 shows a second prestress force, for which only the transverse ultrasonic signals were taken into account. In order to carry out a temperature compensation, recourse was had to the temperature ascertained by the temperature sensor 68.

(21) In the illustration in FIG. 3, it is clearly evident that the prestress force respectively determined only by way of the longitudinal or transverse ultrasonic signals, in the case of a variation of the temperature, indicates an opposite variation in comparison with the reference prestress force determined by way of the load cell.

(22) This stems from the fact that the temperature of the connection component, in particular shortly after the beginning of a temperature change, is not uniform and lags behind the temperature of the temperature chamber 60. The value of the temperature T ascertained by the temperature sensor 68 is thus not representative of the entire connection component 10. The effective temperature T.sub.eff of the connection component 10, which is a temperate of the connection component 10 which is averaged over the signal propagation distance of the ultrasonic signals, deviates here distinctly from the temperature T ascertained by way of the temperature sensor 68.

(23) A third curve 83 shows a third prestress force, which was determined taking account of the longitudinal and the transverse ultrasonic signals. In this case, the effective temperature T.sub.eff of the connection component was determined and taken into account for a temperature compensation. In the case of the temperature changes carried out, the third prestress force follows the reference prestress force determined by way of the load cell 70 with only slight deviations.

LIST OF REFERENCE SIGNS

(24) 10 Connection component 12 Screw head 14 Screw shaft 16 Thread 18 First end 20 Second end 22 Ultrasonic signal 24 Ultrasonic echo 26 Transverse wave 28 Longitudinal wave 30 Ultrasonic transducer 40 Splitter 42 Connecting line 44 Signal generator 46 Transmission amplifier 48 Reception amplifier 50 Receiver 52 Control unit 60 Temperature chamber 62 Screw block 64 Internal thread 66 Spacer 67 Head plate 68 Temperature sensor 70 Load cell 80 Temperature curve 81 First curve 82 Second curve 83 Third curve 86 Prestress force