Measuring and testing – Specimen stress or strain – or testing by stress or strain... – Threaded fastener stress
Patent
1992-03-04
1994-01-25
Myracle, Jerry W.
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
Threaded fastener stress
73779, 324209, G01N 2772
Patent
active
052807258
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a process for the non-destructive determination of the stressing condition of ferromagnetic securing elements, in which process an alternating magnetic field is applied to said elements and the magneto-mechanical acoustic emission thereof is detected and evaluated.
Determining the stressing condition which occurs in a securing element at its site of use is a topical problem of vast significance. Its significance resides above all in the aspect of safety. However, the economic aspect of this problem is not to be underestimated, either. In this context, it may also be pointed out that all over the world, product liability has been playing an ever-increasing part.
What is especially significant is the determination of the stressing conditions prevailing in screwed connections. Whenever parts are to be detachably connected, this is as a rule done by means of screwed connections. In a motorvehicle, for example, about two to three thousand screwed connections can be found. Of these, about three hundred are crucial for safety. For this reason, these screws or bolts have to be tightened particularly carefully, i.e. nether too loosely nor too strongly (cf. "Industrieanzeiger" 13/1988, pp. 24 through 27).
When a screw or bolt is tightened, the ultimately decisive factor is the tension force. It is the tension force alone which decides on how tightly the parts are to be detachably connected. At present, the man skilled in the art does not disposed of any direct methods for measuring this force; he can merely conclude from indices or auxiliary magnitudes, namely the torque or the pivoting angle or a combination of the two. Furthermore, the yield point may also be considered to be an auxiliary magnitude in a certain sense.
When a screw or bolt is tightened under torque control, it must be taken into account that sixty to ninety per cent of the torque are required as friction under the screw or bolt head and in the thread. It is therefore the remaining ten to forty per cent of the torque alone which effect a strain on the screw or bolt. This strain on the screw or bolt alone causes the desired tension force. Under different friction conditions and with constant tightening moment, the tension force can vary up to a hundred per cent in the extreme case. For verifying the tension force of a screwed connection, detecting the torque thus remains merely a makeshift measuring solution. Deviations from the required tension force in screwed connections can be reduced by the use of a tightening process which is controlled by both the torque and the pivoting angle. In this process, however, the expenditure for electronic controlling and measuring equipment is far higher than with torque-controlled tightening. In a tightening process which is controlled by the torque/pivoting angle, the screw or bolt is turned in until a plastic deformation occurs, from which point onwards the screw or bolt is tightened by a predetermined angle. Since the strain of the screw or bolt can be determined from the pivoting angle and the pitch of the thread, the tension force of the screw or bolt may be calculated, taking into account the elastic modulus of the screw/bolt material. Irrespective of the friction conditions, this yield point-controlled tightening process provides the best indirect indications about the pre-stressing force of a screwed connection.
As a rule, it would be ideal for determining the prestressing force if the parameters stress and strain could be measured directly during the tightening of the screw or bolt. In production, however, when conventional tightening processes were used, it has so far been impossible to measure these parameters ("Industrieanzeiger" 16/1988, pp 21 through 24).
German patent application DE-AS 25 19 430 already discloses a measuring process for determining the axial force of screw bolts in which the screw bolts are excited to ultra-sonic resonance vibrations by means of transversal and longitudinal waves. The natural frequencies
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