The ultrasonic method for determining the preload force is based on the measurement of the relative change of the ultrasonic transit time in the bolt. The ultrasonic wave is introduced into the joint with an ultrasonic transducer at the bolt head or bolt end and passes through the bolt . The ultrasonic wave is reflected at the opposite end and returns to the point of introduction. The time between the transmission of the pulse and the reception of the echo is measured (Fig. 1). The ultrasonic transducer acts as both transmitter and receiver.
The ultrasonic time of flight (TOF) along the bolt axis shows a clear dependence on the increase in tensile stress associated with tightening. The elongation caused by the tensile stress reduces the sonic transit time. Both effects act in the same direction and are sufficiently linear that the changes in transit time between a no-load and loaded condition can be used as a direct measure of the applied load. The influence of a temperature that remains constant over the length of the bolt must be taken into account.
To initiate and receive the ultrasonic signal, a thin-film ultrasonic transducer is connected to the bolt head or the face of the bolt shank (direct mechanical coupling). The transducer is applied in a vacuum process (sputtering) and becomes a permanent element of the bolt (Fig. 2). Compared with the use of a mobile ultrasonic sensor, this has a positive effect on the accuracy and reproducibility of the measurement results, since no additional coupling medium is required.
Correct determination of the assembly preload FM requires sufficient quality of the echo signal. The size and shape of the received echo depend to a large extent on the bolt geometry. As in optics, the entrance angle corresponds to the exit angle for reflection from each interface. As an integrating element, the transducer detects all the different echo fractions received simultaneously with an echo signal.
The accuracy of the measurement result depends on several factors. It is reduced, for example, by scattering of the mechanical properties of the bolts within their batch (Young's modulus, transverse strain), by temperature gradients in the bolt material and by interferences during the application of the ultrasonic signal.
A flat bolt reflection end is required to avoid disturbing interference during echo formation. The bolt head surface with the transducers applied and the reflection end of the bolt should be parallel. This supports the direct return of the ultrasonic waves from the reflection end to the transducer. The measurement error with the method is approx. ± 3 %. Because of the nonlinear relationship between strain and ultrasonic transit time in the overelastic range, the method is not very suitable for overelastic tightening processes, since a considerable calibration effort is required. The ultrasonic method is generally used for bolt sizes from M4 to M100.
In recent years, more than 15,000 bolts on steel and machine components have been fitted with ultrasonic transducers and the assembly preload force FM determined (Fig. 3). The bolts used were IHF bolt sets consisting of cap bolts or stud bolts and IHF round nuts. The required assembly pretensioning force FM min was applied hydraulically, without friction or torsion, using ITH Bolt Tensioning Cylinders.
The measurement results prove that all bolted joints could be pretensioned very precisely and that a maintenance-free bolted joint can be realized (Fig. 4).
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