Measuring and testing – Vibration – By mechanical waves
Patent
1984-08-24
1986-02-11
Ciarlante, Anthony V.
Measuring and testing
Vibration
By mechanical waves
73761, G01N 2900
Patent
active
045692297
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for measuring stress in a structural element, particularly a high-strength bolt, a threaded rod or a tie, by reflection of acoustic waves.
It also relates to structural elements so adapted as to be able to make the measurement of stress by means of the process according to the invention.
By stress in the sense of the invention should be understood the cause which gives rise to the variation of the transit time of the useful acoustic ray over a path of determined length, which cause may act directly or indirectly.
This may be a variation of the state of internal stress in the usual sense of the term, along the path of the acoustic ray while local temperatures remain constant, or a system of external forces applied to the solid, producing a variation of the state of internal stress, in the usual sense of the term, along the acoustic path.
By prestress of an element we understand a particular technique of utilization of the element, consisting of introducing into the latter an initial tension or compression which will continue to exist, possibly with variable amplitude, whatever the system of loads applied.
The invention is in particular applicable to the measurement of the stress occurring in bolts, threaded rods, and ties, but is not limited to these applications.
The particular feature of high-strength bolts resides in the fact that, when they are fitted, they are subjected to a prestressed state in such a manner as to develop on the surfaces of contact of the elements joined together a local pressure capable of opposing their relative movement through the action of external stresses.
This explains the importance of being able to control and measure the prestress introduced into the body of the bolt when it is tightened.
Since the techniques of mechanical measurement, such as for example the torque wrench, particularly for measuring prestress during tightening, have been found too inaccurate and capable of permitting only with difficulty measurements of residual prestress which are spaced out at intervals of time, other solutions have been sought.
Also, certain authors have proposed measurement of stress by means of acoustic waves.
It is in fact known that the speeds of propagation of acoustic waves, and in particular of longitudinal waves (L waves), transverse waves (T waves), and surface waves (R waves) are dependent on physical parameters of the material medium in which the wave is propagated. As an example, for a homogeneous isotropic medium, the speeds of the L waves, the T waves and the R waves are dependent on the longitudinal modulus of elasticity E, the density .rho., and the Poisson's ratio .mu.. With regard to the Lamb waves, their speed of propagation is in addition dependent on frequency. The speeds of the waves are influenced by all the factors which influence these physical parameters, and in particular by stress and temperature.
The longitudinal and transverse acoustic waves emitted by a plane crystal exhibit, for frequencies dependent on the material medium and the equivalent diameter of the emitting crystal, a property of directivity which permits selective location of anomalies reflecting all or part of the energy of the incident wave, and this is achieved in dependence on the respective positions of the transmitter and also of the receiver and the angle of penetration of the axis of the beam into the material.
In known processes, stress is measured either by measuring the transit time of an ultrasonic wave in the length of the body, for example the bolt, which measurement can be made by an impulse echograph method, or by determination of the resonance frequencies of a sustained wave. The resonance frequencies constitute an arithmetic progression in which the common difference is the fundamental frequency. The inverse of the latter is the minimum transit time of the useful acoustic ray.
Nevertheless, the measurement of stress by ultrasound generally makes use of direct measurement of the transit time of a longitudinal wave or of a transverse wav
REFERENCES:
patent: 3538750 (1970-11-01), Lynnworth
patent: 3636754 (1972-01-01), Lynnworth et al.
patent: 3969960 (1976-07-01), Pagano
patent: 4014208 (1977-03-01), Moore et al.
"New Sensors for Ultrasound: Measuring Temperature Profiles", Materials Research & Standards, Lynnworth et al., Aug. 1970.
"Panatherm 5010C Ultrasonic Profiler", Panametrics, Jun. 1979, pp. 1-4.
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