Measuring and testing – Specimen stress or strain – or testing by stress or strain... – Threaded fastener stress
Reexamination Certificate
2000-11-29
2001-12-11
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
Threaded fastener stress
C073S597000
Reexamination Certificate
active
06327912
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for measuring in situ the residual tension F
1
of a pre-stressed bar braced between two anchorages, by determining the two-way propagation time T
1
of an ultra-sonic wave between the two longitudinal ends of the bar and by evaluating the residual tension F
1
by a formula of the form F
1
=F
0
+k
b
. (T
1
−T
0
) where F
0
, T
0
and k
b
are constants determined during a preliminary calibration carried out without demounting the bar.
2. Description of Related Art
A process of this type is described in the French patent 2 750 498. According to this patent, the preliminary calibration consists in:
applying to one end of the pre-stressed bar a traction effort F which progressively increases;
measuring simultaneously the longitudinal displacement A of the end of the pre-stressed bar as well as the above-mentioned traction effort whereby these measurements correspond to a curve of the traction effort F as a function of the displacement A which presents an initial portion substantially following a first straight line having a high slope, then a curved portion and finally a final portion substantially following a second straight line having a lower slope;
determining the residual tension as being the value of the traction effort F which corresponds to the intersection of the two straight lines;
applying successively to the pre-stressed bar at least two known and distinct tension values higher than the residual tension, by traction on the end of said pre-stressed bar;
measuring for each of these tension values the two-way propagation time of an ultra-sonic wave between the two longitudinal ends of the pre-stressed bar;
and deducing from these measurements the constants of the evaluation formula of the residual tension.
Owing to these measures, it is necessary to apply traction efforts to the end of the pre-stressed bar only during calibration, which is carried out once for all.
Once the calibration has been carried out, one can subsequently check as often as necessary the residual tension of the pre-stressed bar solely by measuring the two-way propagation time of an ultra-sonic wave between the two ends of this bar. These subsequent checks are therefore very simple and very quick. This process is well adapted to cases where there are restrictions to the access to the structure, for example due to ionising rays.
An object of the present invention is to improve the accuracy of the processes for measuring residual tension by ultra-sound.
SUMMARY OF THE INVENTION
In a method according to the invention, of the type set out in the introduction, the preliminary calibration comprises the steps of:
subjecting the bar to different values of a traction effort F, applied in a zone situated outside of the interval between the two anchorages;
for each value of the applied traction effort F, measuring the two-way propagation time T of an ultra-sonic wave between the two longitudinal ends of the bar, in such a way as to memorize points of a curve associating the traction effort F with the two-way propagation time T;
determining the constant T
0
as being the two-way propagation time T measured for a traction effort F=0;
approaching said curve, in the neighborhood of the traction effort value F=0, by a first straight line having a slope k
a
;
approaching an upper portion of said curve by a second straight line of inclination k
L
;
determining the constant F
0
as being the traction effort F corresponding to the point of intersection of said first and second straight lines; and
determining the constant k
b
according to the equation 1/k
b
=(1/k
L
)−(1/k
a
).
This enables in particular an improvement in the determination of the coefficient k
b
by rigorously taking into account parasite effects that the calibration system can induce. The first straight line approaching the curve (T, F) in the neighborhood of the point (T
0
, 0) takes into account the elongation of the bar in the region situated between the point of application of the traction effort F and the adjacent anchorage, whereas the second straight line takes into account the increase of the length L between the point of application of the traction effort and the opposite anchorage. In service, and in particular during subsequent verifications, it is only the length b=L−a between the two anchorages which is subjected to the residual tension F
1
. It is thus judicious for the slope coefficient k
b
to be corrected in relation to the slope k
L
of the second straight line. This correction will be all the more important if the pre-stressed bar is relatively short.
In a preferred embodiment of the method, the approximation of said curve in the neighborhood of the traction effort value F=0 by a first straight line of slope k
a
comprises a fit of a parabola on a lower portion of the curve, and the determination of the first straight line as being the tangent of said parabola for the traction effort value F=0.
In an advantageous embodiment, the preliminary calibration involves a measurement of the temperature of the material of the bar and a memorization of the temperature measured. This enables the temperature effects upon the measured propagation times to be subsequently taken into consideration. Thus, during a measuring phase subsequent to the calibration, the temperature of the material of the bar and the two-way propagation time of an ultra-sonic wave between the two longitudinal ends of the bar are measured, and the time T
1
for the application of the above-mentioned formula is determined by correcting the measured two-way propagation time as a function of the deviation between the temperatures measured at the time of the preliminary calibration and the subsequent measuring phase.
REFERENCES:
patent: 4333351 (1982-06-01), Bickford
patent: 4413518 (1983-11-01), Jones
patent: 4485677 (1984-12-01), Amelot et al.
patent: 4899591 (1990-02-01), Kibblewhite
patent: 5220839 (1993-06-01), Kibblewhite
patent: 5461923 (1995-10-01), Meisterling
patent: 2 507 232 (1982-12-01), None
patent: 2 750 498 (1998-01-01), None
patent: WO 97/11343 (1997-03-01), None
Basile Bernard
Lancia Bruno
Cantor & Colburn LLP
Freyssinet International (Stup)
Larkin Daniel S.
Saint-Surin Jacques M.
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