Note: Descriptions are shown in the official language in which they were submitted.
73~
Ultrason;c process for measur;ng stress in a bolt or
similar part adapted to this method.
The present invention relates to a process for
measùr;ng stress in a medium, particularly a high-
strength bolt, a threaded rod or a t;e, by reflection
of acoustic waves.
It also relates to parts so adapted as to be able
to make the measurement of stress by means of the pro-
ce3s accord;ng to the invention.
By stress in the sense of the invention should be
understood the cause which g;ves rise to the variation
of the transit time of the useful acoust;c ray over a
path of determined length, which cause may act d;rectly
or indirectly.
In the case of direct act;on, th;s may be a var;-
ation of the state of internal stress ;n the usual
sense of the term, along the path of the acoustic ray
~hile local temperatures remain constant, or a varia-
t;on of the temperature state along the path of the
acoust;c ray wh;le the state of ;nternal stress, in the
usual sense of the term, rema;ns constant therein.
In the case of indirect act;on, there ;s a system
of external forces appl;ed to the sol;d, produc;ng a
variation of the state of ;nternal stress, in the usual
sense of the term, along the acoustic path.
8y prestress of an element we understand a par-
ticular technique of utilization of the element, con-
s;st;ng of ;ntroducing into the latter an initial ten-
sion or compression which will continue to ex;st,
73~
possibly w;th variable amplitude, whatever the system
of loads applied.
The invention is in par~icular applicable to the
measurement of the stress occurring in bolts, threaded
rods~ and ties~ but is not limited to these applica-
tions.
The medium in which the measurement of stress is
made will here;nafter be referred to as medium, body,
mechan;cal component, part, connect;on elements~ bolt,
tie, threaded rod, and this will be done by way of ex
ample and without any desire for Limitat;on.
The particular feature of high-strength bolts re-
sides 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
jo;ned together a local pressure capable of opposing
the;r relative movement through the action of external
stresses.
Th;s explains the importance of being able to
control and measure the prestress introduced ;nto the
body of the bolt when it is tightened.
~ Since the techniques of mechanical measurement, such
as for example the torque wren`ch, particularly for measur-
~ng prestress during tightening, have been found too in-
accurate and capable of permitting only with difficulty
measurements of res;dual prestress which are spaced out at
;ntervals of time, other solutions have been sought.
Alsor certain authors have proposed measurement
of stress by means of acoustic waves.
~Z~73~
It is in fact known that the speeds of propaga-
t;on of acoustic waves, and in part;cular of longitud-
inal 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 isotrop-
;c medium, the speeds of the L waves, the T waves and
the R waves are dependent on the longitudinal modulus
of elastic;ty E, the density p , and the Poisson's
rat;o h~. ~;th regard to the Lamb waves~ their speed
of propagation ;s in addition dependen~ on frequency.
The speeds of the waves are ;nfluenced by all the
factors wh;ch ;nfluence these phys;cal parameters, and
;n part;cular by stress and temperature.
The longitudinal and transverse acoust;c waves
emitted by a plane crystal exh;bit, for frequencies de-
pendent on the material med;um and the equ;valent d;a-
meter of the em;tt;ng crystal, a property of direct-
;v;ty wh;ch permits select;ve location of anomalies re-
flecting all or part of the energy of the ;ncident
~ave~ and th;s ;s achieved ;n dependence on the re-
spective pos;t;ons of the transm;tter and also of the
receiver and the angle of penetrat;on of the ax;s of
the beam into the material.
In known processes, stress ;s measured either by
measur;ng the trans;t 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
7~
-- 4
sustained ~ave. The resonance frequencies constitute
an arithmetic progression in which the common differ-
ence ;s the fundamental frequency. The inverse of the
latter is the minimum transit time of the useful
acoustic ray.
Nevertheless, the measurement of stress by ultra-
sound generally makes use of direct measurement of the
transit t;me of a longitudinal wave or of a transverse
wave~
For this purpose a measurement may be made by
transm;ss;on, by plac;ng an ultrasonic transmitter on
one s;de of the part and a receiver on the opposite
side. It ;s however also poss;ble to make a measure-
ment by reflect;on by us;ng one or two sensors, ~h;ch
cons;st of a transm;tter and a rece;ver, most usually
comb;ned or separate ;n one and the same sensor,
the measurement then be;ng made by detect;on of echos
resulting from reflect;ons of the ~ave on the ends of
the part and determ;nat;on of the transit t;me of the
useful acoust;c ray by determ;ning the t;me separat;ng
the appearance of the feet of the echo peaks of ends on
the representation of type A, each reflection giving
rise in fact, ;f the reflected energy returns to the
rece;ver, to an anomaly echo characterized by a peak in
the representat;on of type A used in usual conventional
ultrasonic control apparatus.
The conventional method for determining stress
in a bolt consists in suitably disposing a sensor on
one end of a bolt and measuring the time required for
, . . . . . ....... ....... . . . .... .. . .. . . ... .. . . . .
~4~734!
. - 5 -
the useful acoustic ray to travel twice, or a multiple
of twice, the length of the bolt~ the acoustic ray be-
ing partially or completely reflected at the two ends
of the bolt.
The transit time in the bolt is dependent on the
stress on the run of the useful acoustic ray, because
this stress influences the speed of transit and the
length of transit.
Nevertheless, the conventional method has numer-
ous d;sadvantages:
- the mechanical component, for example a bolt, does
not exh;bit uniform stress over the length travelled by
the acoustic wave. In the case of the prestressed
bo~t, the stem is subjected to tension, the head is
subjected to a complex force including flexion, the
threaded ~one is subjected to variable tension, and any
free portion of the threaded rod ;s without stress.
Th;s results ;n ;naccurate knowledge of the
length of the tensioned portion of the bolt ;n which
the spèed of propagat;on of the acoust;c waves ;s ;n-
fluenced by the stress, and of the total length of the
bolt one part only of wh;ch ;s subjected to stress. A
systematic error ;s therefore comm;tted ;n the deter-
mination of the stress by acoustic waves, except w;th
calibration in an ;dent;cal configuration. This there-
fore requ;res cal;brat;on for each position of the nut
on the threaded rod and each length of bolt;
- ;n a jo;nt ;t ;s rare for the support surfaces of the
bolt head and of the nut to be str;ctly plane and
~Z~'73~
-- 6 --
parallel. There is conse~uently a flexion of the
stem of the bolt when it is tightened, and this changes
the useful acoustic ray for the measurement and, be-
cause of the flexion alone, modifies the length of the
characteristic run~ 8ecause of the foregoing, this
disadvantage ;ntroduces a first systema~ic error into
the measurement of stress by acoustic waves. In additi-
on, the local stress introduced by the flexion on the
path of the useful acoustic ray modifies the speed of
propagation of the usefuL acoustic ray and consequently
falsifies the measurement of the prestressing of the
bolt. The lack of parallelism of the ends of the
bolt or of the threaded rod, due to flexion, may have
the consequence that the reflected acoustic energy does
not reach the receiver and makes all measurement impos-
sible when the measurement is made by reflection.
- If the two ends of a narrow part, which has plane,
parallel surfaces, are not sufficiently perpendicular
to the ax;s, although it is possible to propagate an
acoustic wave in the material, if the narrow part is of
cons;derable length it may be difficult, or even impos-
sible, to ensure that a useful acoustic ray will reach
the receiver by reflection on one end, so that measure-
ment of stress is impossible.
- The exact positioning of the sensor on the end of the
bolt is of importance if the reflecting surfaces are
not strictly parallel or are not perfectly plane. The
information supplied by the acoustic ray of the reflec-
tion wave which reaches the receiver and which advan-
. ~ .. .. .. ,.. ......... , . . . . .. ... ., .. . . .. .. ~ . . ... ..... ..
L73fl~
- 7
tageously has the shortest transit time in the part is
in fact ut;lized. This m;n;mum trans;t t;me ;s then
dependent on the position of the sensor on the ends of
the part The width and the opening of the beam are
of importance because acoustic rays can be reflected by
parasit;c reflectors, for example the edges and faces
of the screwthreads, before the useful acoustic ray is
reflected by the second end of the part. The height of
the peaks of the paras;t;c echos may hamper the selec-
t;on of the peaks of the useful echoes. In addition,
if peaks of parasitic echoes are superposed on peaks of
useful echoes, it ;s no longer possible to determine
w;th great accuracy the m;n;mum trans;t t;me of the
useful ray, part;cularly if use ;s made of a run or a
group of rùns of the acoustic ray other than the f;rst
run. The minimum travel time is in fact ;deally de~
term;ned by the determ;nat;on of the t;me separat;ng
the feet of the peaks of echoes retained~
- ~he concav;ty or conve~;ty of the end of the bolt
fals;f;es the measurement of the min;mum transit time
of the useful acoustic ray for different positions of
the sensor on the head, for the same reasons as above.
- Attacks by general;zed corrosion of the head and end
of the bolt give rise to a variation of the reference
length, that is to say the run of the ray which has
the shortest transit time. In addi~ion, the surfaces
of the head and end of the bolt may deteriorate in use,
for example through ~ear, through bruising, through
generalized or localized corrosion, in such a manner
73~
-- 8
that the relative position of the sensor in relation to
the part has changed. In the usual methods of measur-
ing stress in bolts it is not permissible to grind the
ends in order to enable the measurements to be made~
Grinding entails in fact a non-quantifiable reduction
of the transit length.
- Coupling ;s usually necessary between the sensor or
sensors and the part, ;n order to transmit to the lat-
ter a part of the acousti~ energy of ~he transmitter
and to rece;ve at the rece;ver a part of the energy re-
flected. The thickness of the film of coupling medium
between the sensor or sensors and the part ;nfluences
the measurement of the trans;t time of the acoust;c
wave in cases where use is made of the first run
o~ the acoustic waves in the part. The reflection peak
of the bottom face of the sensor in fact covers the re-
flection peak at the entry into the part. The transit
time in the coupling medium is then counted as forming
part of the transit time in the part. In view of the
fact that for the usual dimensions of bolts the varia-
tions of the transit time which are utilized for measur-
ing stress are slight~ the error made in respect of ~he
transit time of the bolt because of the transit time ;n
the coupling medium may result ;n a large error in re-
spect of stress. The contact pressure between sensor
and part is consequently of importance, because it
mod;fies the thickness of the film of coupling medium.
- Some authors propose to eliminate the influences due
to the entry of the wave into the material by measuring
o 3~
the time requ;red for the wave to travel a plural;ty of
~;mes over the length of the bolt, the first run being
elimina~ed. This procedure can be applied in prac-
tise only ~ith sensors in which the transmitter and re-
ce;ver are combined, and not with a sensor or sensors
having separate transmitters and receivers. It may,
however, encoun~er difficulties due to parasitic echoes
ori~inat;ng, among other sources, from any reflect;ons
occurr;ng on the faces and the corners of screwthreads.
~he paras;t;c peaks due to reflections on scre~threads
make the measurement ;naccurate~ because it may not be
poss;ble to determ;ne w;th the necessary accuracy the
minimum transit time of the acoustic ray between the
characteristic reflecting sur~aces, In these cases,
the other disadvantages are not always elim;nated~
It has also been proposed in G~-A-1,369,858 to
measure an axial force, particularly in a bolt, by
using the natural resonance frequency of the object sub-
jected to the measurement, under the act;on of the d;f-
ferent forced osc;llations~
~ he present invention therefore seeks to prov;de
3 process for the measurement of stress in a medium by
mak;ng use of the reflection of an acoustic ray and by
measuring the transit time of the lat~er ~ith the aid
of an ad hoc equipment, for example an equipment deter-
mining the time between the feet of two character;stic
echo reflection peaks, wh;le eliminating the disadvan-
tages of known processes or at least reducing their im-
pact considerably.
3~
- 10 -
The process of the present invention is comprises
one or a plurality of ends of rectilinear measurement
paths, hav;ng the material form of an internal arti~
ficial reflector, being selected in the medium, a
beam of acoustic waves being transmit~ed in such a
manner that acoustic rays carrying sufficient energy
touch the useful reflectors, the echoes corresponding to
the re~lectors being selected, the transit times charac-
teristic of ~he useful acoustic rays as far as the
internal artific;al reflector being determ;ned by
measurement, and the trans;t times for each internal
reflector considered individually, or the respective
differences of transit time of each pair of reflectors,
be;ng transposed into a value of external stress or into
a value of stress ;n the zone del;m;ted by each pair of
reflecto rs .
One preferred pract;cal embod;ment of the ;nven-
tion is the embod;ment ~here;n a rectilinear measure-
ment path bounded by two ;nternal artificial reflectors
is selected ;n the med;um, wherein a beam of acoustic
waves, the ax;s of wh;ch ;s substant;ally ;dent;cal
w;th the selected rect;l;near measurement path, ;s trans-
mitted, ~herein the echoes corresponding to the reflec-
t~rs are selected, where;n the trans;t t;me character-
ist;c of an acoust;c ray between the two refLectors
;s measured, th;s t;me be;ng the d;fference between
the trans;t t;mes of the co;ncident or substantially
parallel acoustic rays as far as each of the two
L73~
reflectors, and wherein the vaiues of transit time are
transposed ;nto values of stress.
The characteristic transit time is advantageously
the minimum transit t;me.
For the purpose of the measurement use will ad-
vantageously be made of an equipment making it possible
to select the feet of peaks due to the internal artifi-
cial reflectors, and to obtain the difference between
the two transit times of the wave from the transmitter
source to the corresponding reflector~
The artificial reflectors are advantageously ar-
ranged in such a manner that the run of the acoustic
wave is parallel and very close to the neutral line, or
even on that line. This arrangement makes it possible
for the ;nfluences of flex;on, which may falsify the
measurements in the usual process, to be practically
eliminated.
Use is preferably made of a sufficiently narrow
ultrasonic beam, which increases the accuracy of the
measurement~ if it is combined with practically puncti~
form reflectors or reflectors having a behavior prac-
tically similar to that of a punctiform reflector. The
beam used will advantageously be focused.
The measurement of the transit time may be made
by a continuous transmiss;on and determ;nation of the
resonance frequencies of the wave which are due to the
internal reflectors. The resonance frequencies con-
stitute an ar;thmetic progression in which the common
difference is the fundamental frequency. The inverse
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- 12 -
of the latter is the minimum transit time of ~he useful
acoustic ray between the reflectors concerned. The
end of the bolt w;ll advantageously be prof;led, for
example, convexly or concavely in order to eliminate
the resonance frequency related to the length of the
bolt.
In preference, the measurement of the transit
time of the acoustic wave between the two reflectors
will advantageously be made with the aid of a
th;ckness measurer making use of ultrasonic waves. The
ultrasonic thickness measur;ng apparatus can in fact be
considered as an apparatus which directly or indirectLy
measures a trans;t t;me of an ultrason;c ray ;n the
part.
It is therefore necessary to measure the trans;t
time to of the useful acoustic ray between the two
artif;c;al internal reflectors when the body ;s not
subjected to a stress.
~ he trans;t time t of the acoustic wave between
the two ;nternal reflectors when the body ;s subjected
to a stress w;ll then be measured.
rhe d;fference between t and to ;s due:
1. to the var;at;on of the speed of the acoustic wave
w;th the stress on the run of the useful acoustic ray,
2~ to the var;at;on of the character;stic d;stance be-
tween the ;nternal reflectors because of the stress.
It ;s necessary to establish a calibration curve
linking the stress to the transit time t between the
two internal reflectors and making it possible to
~2~ a~73~
- 13 -
transpose the transit time values into stress values.
Accord;ng to an alternative operating procedure,
it would be possible to use transverse ultrasonic waves
in cases where the stress applied to the body during
the calibration stage shows an ;nfluence on the charac-
terist;c trans;t time of the useful acoustic ray along
the line separating the artificial internal reflectors.
~owever, th;s g;ves rise to various difficulties known
to those versed ;n the art, part;cularly the d;fficulty
of generating this type of wave in the material in the
case of propagation normal to the end of the part~
According to an alternative operating procedure,
;t would be possible to use any type of acoustic wave,
particularly ultrasonic surface waves, provided that
the stress applied to the body has an influence on the
characteristic transit t;me of the useful acoustic wave
3S the result of the stress thus created on the line
separating the artificial ;nternal reflectors. In this
case the artific;al internal reflectors would have to
be relat;vely close to the surface, because the sur-
face ~ave affects only a depth of material of the order
of the wavelength.
Accord;ng to another particularly preferred oper-
ating procedure in the case of bolts, threaded rods and
t;es, use is made of longitudinal ultrason;c waves,
which have the advantage, in the case of normal ;nc;-
dence, of being eas;ly transmitted to the part.
It would also be possible to use in combinat;on,
and concomitantly or ;n succession, a longitudinal wave
73~
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and a transverse wave, which gives an additional datum
for determining the stress.
The ratio of the respective trans;t t;mes of an
L wave and a T wave ;n a suff;c;ently homogeneous and
isotropic medium between two reflectors is equal to the
inverse of the ratio of ~he respective speeds of these
two waves, and is solely dependent on the value of the
Poisson's ratio ~. It therefore does not depend on
the distance between the reflectors. It is thus suf-
fic;ent to calibrate th;s ratio of the transit times
for the material in dependence on the stress. It ;s
desirabLe to do this on the component having the re-
flectors.
In other cases - non-homogeneous or anisotrop;c
media - for example, when the calibrations with each
type of wave are applied to the component having the
two art;ficial internal reflectors, it is possible to
take into account any other factors influencing the
transit time of the wave in the material, such as the
temperature for example.
Measurement of stress by ultrasound in the plas-
tic range is possible only if it is possible to be
free of the irreversible ;ncrease of distances between
two reflectors. One way of do;ng th;s is to use in
combination, and concom;tantly or ;n success;on, an L
wave and a T wave, provided that the ratio of the
two transit times does not depend substantially on
plastic deformations, thus making it possible, within
sufficiently homogeneous and isotropic media to become
~2~ ~'73~
- 15 -
independent of the ;nstantaneous distance between the
reflectors. Th;s solution cou~d probably be extended
to the case of non-homogeneous and/or anisotropic
media.
It may also be expected to solve these problems
by using two waves, which may or may not be of the same
type and which have different frequencies, and to make
use of any influence that the frequency may have on the
speed of propagation ;n order to make it possible, in
the same way and w;th the same l;m;tat;on~ to measure
stress in the plastic range.
In this case, it would be possible, both for the
first solut;on and for the second, to determ;ne, ;nd;-
rectly w;th the a;d of ad hoc cal;brat;on curves made
on the material, the amplitude of plastic deformation,
working as ;n the case of a gage which is at one and
the same time a stress gage and a plast;c deformat;on
gage.
It would also be poss;ble to use in combination,
~nd concomitantly or in succession, two or more acous-
tic waves of different frequenc;es in cases where, ;n
3 real medium, the speed of an acoustic wave would be
suffic;ently inflùenced by the frequency of the wave,
whereby one or more add;t;onal data for the calibrat;on
for determ;ning the stress would be gained.
It would thus be possible to be ;ndependent of
knowledge of the d;stance between two internal reflec-
tors, or to take into account other factors ;nfluenc;ng
the trans;t t;me of the wave ;n the mater;al, such as
73~
~o --
temperature or, ~here applicable, plastic deformation,
for example.
By selecting a character;stic run subject to
un;form stress, zones wh;ch are of no ;nterest to the
measurement or which disturb the measurement, are
eLiminated. This process therefore makes i~ possible,
in cases where the cal;bration was not made in an
identical configuration of the threaded rod and of the
nut, to el;minate from the character;stic path of the
acoustic ray the ~ones which are free from stress andt
or are subjected to non-un;form stressr and which are a
cause of error in the measurement of stress because the
distr;bution of stresses is poorLy defined there and be-
cause they ;nfluence the transit time by affecting the
speed of the wave and the length of the character;stic
run.
The process of ~he ;nvention therefore no longer
requires calibration for each position of the nut in
the case of its appl;cation to a bolted structure. An
equ;valent remark may be made in the case of a thread`ed
rod or of a tie threaded or anchored in a solid mass~
Even ;f the support surfaces of the bolt head and
of the nut are not parallel because of flexion, the ef-
fect of flexion on the measurement is considerably re-
duced in comparison with known processes. In fact,
the reflectors are smaller and the path of the waves
is the path connecting by a straight line the two re-
flectors disposed on the slightly incurved neutral
line. The error due to the difference between the
7~
- 17 -
length of the chord and the length of the arc formed by
the neutral axis is not substantially modified, but the
useful acoustic ray is propagated in a medium close to
the axis of the part where the disturbing stress due to
flexion can usually be considered as negligible, where-
as in kno~n processes this ray is displaced into the
zone of compression introduced by the flexion. ~n ~he
process of the invention a close approach is therefore
made to measurement of stress along the axis of the
part i~ the artificial reflectors are situated on that
ax;s.
In view of the fact that use ;s made of the
transit time of the useful acoustic ray on the pa~h
separat;ng the two arti~ic;al reflectors, the disad-
vantages connected w;th the reflection peak on entry
into the part and also the d;sadvantages connected with
coup~ing, coupl;ng pressure, surface state, wear and
bruis;ng of the ends of the bolt, etc., are eliminated.
It ~ould also be poss;ble to become independent
of differences in the positioning of the sensor on the
bolt head by utilizing the transit times statistically,
for example by utilizing the mathematical expectation
or an estimation thereof, of the transit t;mes obtained
~or var;ous posit;ons and orientations of the sensor on
the bolt head.
ay utilizing small (quasi-punctiform or equival-
ent) artificial reflectors, the sensor will advantageously
always be situated in the same place on the head~ failing
which no signal will be picked up or signals of different
3~
- 18 -
energies will be picked up.
This positioning is made possible by the presence
of the art;ficial internal reflectors. It w;ll, how-
ever, possibly necessitate the use of a conventional
apparatus making it possible to display the amplitude
of reflect;on echo peaks relating to the internal arti-
f;cial reflectors in a type A representation or equiv-
3lent, in such a manner as to position the sensor at
the point where the reflection echo peaks of the t~o
artific;al reflectors are, if possible, respectively
identical in amplitude to the peaks obtained during the
calibration.
As it is known that the acoustic energy trans-
mitted depends, among other factors~ on the thickness
of the couplinq liquid and the pressure applied to the
sensor, it is at least necessary that the relative
heights of the peaks should be substantially in the
same ratio as during the calibration, which is equiv-
alent to saying that the proportions of the energy
transmitted into the bolt and reflected by each reflec-
tor to the receiver should be substantially identical.
For certain configurations of artificial reflec-
t~rs, namely for example concentric axial bores of
d;fferent diameters, as a rule the echo reflect;on
peaks of the two artif;cial reflectors will as far as
possible be simultaneously at their maximum when the
axis of the beam substantially coincides with the axis
of the bolt.
7~
- 19 -
With regard to this problem of the positioning of
the sensor on the end of the bolt, the following con-
siderations may be set down:
The measurement of stress poses no problem when
the sensor remains in a fixed position on the end of
the bolt.
When for each measurement it is necessary to re-
moYe and replace the sensor, ;f the ar~ificial reflec-
tors are for example at a distance of about one meter
from the sensor, no substantial random variatlons of
transit t;me attributable to the positioning of the
sensor will be observed. For certain types of reflec-
tor it does however occur that two or more families of
characteristic transit t;mes are shown.
In cases where the reflectors are bores orthogon-
al to the stem of the bolt and form a certain angle to
one another, this is due to the reflection by the
second reflector on each side of the shadow created by
the first.
When the reflectors are brought close to the end
~here the sensor is situated, a rando~ variation of the
transit time relating to each family ;s observed, de-
pending on the position of the sensor. These differ-
ences are due, among other factors, to a parallax ef-
fect, to the imperfect geometry of the reflectors, to
faulty alignment of the cylindrical reflectors, to de-
fects in the sensor, for example non-parallelism of the
crystal to the contact face, and non-uniformity of the
acoustic energy in the beam.
- 20 ~
It may therefore be advantageous to have avail-
able a geometrical positioning of the sensor on the
contact face, such as for example a positioning bush or
a socket in the end of the part.
In addition, the presence of the internal reflec-
tors no longer makes it necessary to generate and re-
ce;ve the acoust;c wave by contact. It would be
poss;ble to generate the wave w;thout contact with the
bolt by tak;ng advantage of magnetostr;ct;on ;n the
case of ferromagnet;c materials or of the mechanical
shak;ng produced by the impact of a laser ray.
It is therefore found that the process of the in-
vention is more accurate than known processes, and that
it enables the disadvantages mentioned to be elimin-
ated.
It should also be noted that the process of the
invention is extremely simple and that the invention
resides precisely in this extremely simple solut;on,
whereas all previous attempts to f;nd solutions seek to
achieve a part;al improvement of the processes or to
elim;nate only some d;sadvantages, for example by ;m-
posing str;ct requ;rements ;n respect of the qual;ty of
the end surfaces of bolts, etc., w;thout ever el;m;nat-
ing all the disadvantages.
In the course of the description of the present
invention reference ;s made ;n general, by way of ;l-
lustrat;on, to bolts. It should be noted that the in-
vent;on prov;des a much awaited solut;on in th;s field
of mechanical engineering. The application of the
7 3~
present invention is however~ not Limited to bolts; it
is possible to quote any connection means, such as
threaded rods, studs, r;vets, etc., or even any struc-
tural component or element.
The field of application is not limited to steel.
Stress may be analysed or measured in any material,
even ;n so-called plastic materials provided that they
are permeable to acoustic waves at the useful fre-
quenc i es .
According to one embod;ment of the ;nvent;an theart;f;c;al reflectors consist of transverse perfora-
t;ons or bores. These are advantageously disposed in
parallel planes, forming an angle to one another. The
two reflectors are preferably at r;ght angles to one
another. In the case of parts subjected to torsion it
may be advantageous to dispose the perforations or
bores at an angle between 0 and 9G and in a di-
rection such that this angle increases w;th the angular
torsional deformation~
According to another embod;ment of the invention
th~ reflectors consist of two coaxial bores of small dia-
meter, but having different d;ameters and d;fferent
~pths, the difference ;n depth determining the ut;l-
;2ed zone of trans;t of the useful acoust;c ray. The
trans;t;on zone between the two bores advantageously
has a sharp edge and a plane connecting surface, per-
pendicular to the axis. It is advantageous for the
acoustic ray to penetrate into the part via the end op-
posite the bore. It is certainly possible to find
7~
- 22 -
other possibilities ~or reflectors and the invent;on is
not limited to the cases of application mentioned nor to
the particular form of construction indicated for the
reflectors.
~ he present invention also relates to parts char-
acterized in that they are provided with artificial re-
flectors.
sy "part" is understood in particular connection
elements, for e~ample high-strength bolts, threaded
smooth or ribbed rods~ rivets, studs, or else struc-
tural elements, part;cularly beams or other construc-
t;onal elements.
These parts are advantageously provided with a
means for the geometrical positioning of the sensor on
the contact face, particularly a posit;oning bush or a
socket in the end of the part.
With a view to better explaining the present in-
vention, the latter will be described below ~ith refer-
ence to the accompanying drawings, in which:
- Figure lA shows a vie~ in longitudinal section of a
high-strength bolt provided with two artificial reflec-
tors, and Figure 1~ is a sect;on on the l;ne A-A ;n
F;gure 1A;
- Figure 2A shows a view in long;tud;nal section and an-
other type of high-strength bolt provided with artifi-
cial reflectors, and Figure 2~ is a section on the line
A-A in Figure 2A;
- Figure 3 illustrates an application of the measuring
process according to the invent;on to the measurement
~2~
- 23 -
of stress in a bolt;
- Figure 4 illustrates the type A representation ob-
tained in the case of the application shown in F;gure
3;
- F;gure 5 is the graphical representation of the vari-
ations of the transit time of the useful acoustic ray
in the case of a threaded M30 rod of steel plotted
against the stress for different respective configura-
tions of the connection element constituted by the
threaded rod and its two nuts when the measurement of
stress is made in accordance w;th ~he known process;
- F;gure 6 ;s the graph;cal representation of the vari-
ations of the transit time of the useful acoustic ray
;n the case of a threaded M30 rod of steel plotted
aga;nst the stress for d;fferent respective configura-
t;ons of the connection element constituted by the
threaded rod and its two nuts when the measurement of
stress is made ;n accordance w;th the process of the
invent;on on a threaded rod ;n wh;ch artif;cial reflec-
tors have been formed, wh;ch consist of transverse
perforations d;sposed ;n parallel planes at a d;stance
1~ apart and form;ng an angle of 90 w;th one an-
other.
Figure 1 shows by way of example a high-strength
bolt 1 compr;sing a head 2 and a body 3 prov;ded with a
scre~thread 5 at the end 4 opposite to the head. From
the end 4 of the bolt 1 two coax;al bores 6, 7 of dif-
ferent diameters were mach;ned. The bore 6 advantage~
ousLy has a dianeter ldrger than that of the bo~e 7 in
- 24 -
order not to be masked by the "shadow" formed by the
first reflector 8 consisting of ~he bottom of the bore
7 when the sensor disposed on the head 2 transmits an
ultrasonic wave through the bolt. The second artifi
cial reflector is formed by the shoulder 9. The bores
are advantageously disposed on the ax;al l;ne of the
bolt.
It is therefore seen that ;t ;s possibLe to d;s-
pose the reflectors 8 and 9 in such a manner that the
zone of measurement ;s disposed solely ;n the selected
~one of stress. In the present case of application
the part of the bolt body lying between the support
surface of the head and the screwthread engaged ;n the
nut is the zone in which the stress ;s considerable.
For the measurements use was made of an apparatus
intended for ultrasonic thickness measurements~ of the
UTG 5A-II8-SONATEST type, wh;ch automatically calcu-
lates the difference between the appearances of the
feet of peaks of the two echoes result;ng from the re-
flect;on of the useful acoustic ray on the two artifi-
c;al reflectors, eliminating the other echoes, in par-
t;cular the echo from the beginning and the bottom of
parts and, where applicable, the parasitic echoes of
the part beyond the second artificial reflector, for
example those resulting from edges and inclined sur-
faces of the screwthreads.
Figure 2 shows another way of forming artificial
reflectors. Two transverse holes have been made in
the body 3 of the bo~t 1, perpendicuLar tp the a~is
3~
- 25 -
of the latter. The hGles 10,11 are advantageously
situated in parallel planes, while forming an angle in
relation to one another by projection onto one of the
planes in such a manner as to intersect, for e~ample,
on the axial line. The two perforations 1û,11 are ad
vantageously perpendicular to one another.
It is also possible to imag;ne a bolt hav;ng a
bore s;milar to the bore 7 in Figure 1 and a transverse
perforation similar to the perforation 11 in Figure 2~
The ultrasonic waves will thus be reflected by the bot-
tom of the bore 7 and by the wall of the bore 11. In
addition, the reflectors may be positioned so that the
transit times of the disturbing echoes of the screw-
threads will be longer than the trans1t t;me of the use-
ful acoustic ray reflected on the most distant art;fi-
c;al reflector. In this way, they do not d;sturb
the measurement ;n the f;rst run of the wave.
It ;s also possible to imagine a truss system in
a civil engineer;ng structure, provided with transverse
perforat;ons s;milar to the perforat;ons 10 and 11 in
Figure 2 and reproduced in each ~one of the bar where
knowledge of the stress ;s useful, these perforations
being disposed ;n such a manner that they do not form
acoustic screens ;n relat;on to one another~ th;s be;ng
ac~;eved, for example, by jud;c;ous selection of the;r
respective angles.
It is clearly understood that in the application
of the stress measuring process according to the inven-
tion it may, depending on circumstances, be desirable
3~
- 26 -
to take into account disturbing effects of factors
other than stress, which may gi~e rise to variations of
the transit time of the useful acoustic ray, such as,
for example~ the temperature, and to make the necessary
correction.
The measurement of the temperature could be made
s;multaneously w;th the measurement of the stress by
suitably ;ncorporating in the sensor a temperature de-
tector, such as for example a thermocouple.
Figure 3 shows a bolt provided with two artifi-
c;al ;nternal reflectors, cons;st;ng in this part;cular
case of trans~erse bores 1 and 2 d;sposed diametr;cally
in two parallel planes perpendicular to the axis of the
bolt~ rhe bolt ;s provided w;th a nut 3. On the
head 4 of the bolt has been d;sposed an ultrason;c
transm;tter/receiver sensor 5 conta;n;ng a p;ezoelec-
tric crystal 8, a plex;glass delay l;ne 7, and absorb-
ent mater;al 6. Between the sensor 5 and the end of
the bolt 10 there ;s a coupling l;quid 9.
The length of the path amounts ~o p in the plexi-
glass, to e in the coupling liqu;d, to 11 ;n the bolt
as far as the first reflector, to L between the two re-
flectors, and to 12 between the second reflector and
the end~
The bolt ;s subjected to a force F wh;ch br;ngs
about a var;at;on of the characteristic transit ~ime of
the useful acoustic ray between the two reflectors.
F;gure 4 shows type A reoresentation of
the results obta;ned. On the ordinate 1 is shown the
73~
- 27 -
height of the transmi,ssion or reflection peaks and on
the absc;ssa 2 the transit time.
The peak 3 is connected to the excitation of the
crystal 8 in Figure 3.
The peak 4 is related to the reflection at the
interface between the plexiglass and the coupling
liquid.
The peak 5 is related to the reflection at the
interface between the coupling liqu;d and the steel.
The peak 6 is related to the reflection on the
reflector 1 in Figure 3.
The peak 7 is related to the reflection on the
reflector 2 in Figure 3.
The peaks 8 are related to the reflections on the
faces and edges of the screwthread.
The peak 9 is related to the refl~ction on the
end of the bolt.
On the abscissa, the values of the d;fferent
character;st;c times are indicated.
Cp ;s the speed of the ultrasonic wave ;n the
plex;glass.
Ce ;s the speed of the ultrasonic wave in the
oi l.
Ca is the speed of the ultrasonic order in the
steel.
In Figures 5 and 6 the variation of the transit
time between the two ends of the threaded M30 rod is
shown on the ordinate. On the abscissa is shown the
tensile load in the tensioned portion of the threaded
2 ~ r
M30 rod between the two,nuts with a length 12 equal to
to 231.5, 200 and 169.5 mm.
A straight line corresponds to each configura-
tion of the connection elementO
In the case of application shown in Figure 5, the
straight lines are appreciably different, whereas ;n
the case of application shown ;n Figure 6 the straight
lines substantially coincide; the differences in this
case originate no doubt fro~ the slightly different
positioning of the sensor for each configuration of the
connection.
