Abstract
There exists a need for a relatively inexpensive system for measuring strain in bolts. The torque wrench is one technique for straining bolts which has been widely applied. Unfortunately, friction in the bolt threads and between the nut and the work tend to make such a simple system inaccurate. In practice, a torque wrench is unacceptable for many situations where strain is critical. In this article, an ultrasonic technique is described which can indicate changes in bolt strain to better than one part in 104. The technique is based on the one-dimensional propagating-ultrasonic-wave model and uses a new ultrasonic instrument called a Reflection Oscillator Ultrasonic Spectrometer which is a closed-loop feedback marginal-oscillator system that frequency locks the device to the peak of a mechanical resonance in the bolt. The instrument indicates a shift in the bolt resonance frequency due to elongation and changes in velocity of sound due to strain. Data are presented comparing a standard torque wrench to the ultrasonic monitor for different measured stresses on the bolt as well as for different bolt conditions. The strain instrument can be used to monitor changing stresses, to measure material properties and may be applied as a strain gage or load cell.
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Abbreviations
- a :
-
twice the length of the bolt
- E :
-
Young's modulus
- F j :
-
jth harmonic frequency
- FΔ:
-
frequency separation between harmonics
- j :
-
harmonic integer
- l :
-
3rd-order elastic constant
- m :
-
3rd-order elastic constant
- n :
-
3rd-order elastic constant
- Q :
-
resonance-quality factor
- S :
-
applied stress
- v :
-
longitudinal velocity of sound
- λ:
-
Lamé constant
- ρ:
-
density at zero strain
- μ:
-
Lamé constant
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Heyman, J.S. A CW ultrasonic bolt-strain monitor. Experimental Mechanics 17, 183–187 (1977). https://doi.org/10.1007/BF02330995
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DOI: https://doi.org/10.1007/BF02330995