CA1241734A - Ultrasonic process for measuring stress in a bolt or similar part adapted to this method - Google Patents
Ultrasonic process for measuring stress in a bolt or similar part adapted to this methodInfo
- Publication number
- CA1241734A CA1241734A CA000490228A CA490228A CA1241734A CA 1241734 A CA1241734 A CA 1241734A CA 000490228 A CA000490228 A CA 000490228A CA 490228 A CA490228 A CA 490228A CA 1241734 A CA1241734 A CA 1241734A
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- reflectors
- stress
- acoustic
- measurement
- bores
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- Investigating Or Analyzing Materials By The Use Of Ultrasonic Waves (AREA)
Abstract
Abstract of the Disclosure The method for measuring strains in a part uses the reflection of an acoustic wave, while measuring a transit time of said wave by means of an apparatus which measures the fade-in time of an echo coming from an interface. The method comprises the steps of selecting within the medium one or a plurality of ends of rectilinear measuring runs, which ends are embodied by an inner artificial reflector, emitting of a beam of acoustic waves so that acoustic rays carrying sufficient energy strike the useful reflectors, selecting the echoes corresponding to the reflectors, determining by measurement the transit time which are characteristic of the useful acoustic rays up to the inner artificial reflectors and transposing the transit times for each inner reflector considered individually or the respective differences of transit times for each couple of reflectors into an outer strain value or into a strain value within the region delimited by each couple of reflectors. The part of implementing such method comprises reflectors consisting particularly of perforations or bores into the part.
Description
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~
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~
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~
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~
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~
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 73~
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,
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~
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~
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~
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~
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~
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 73~
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~
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~
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~
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
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~
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~
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~
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~
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~
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~
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~
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~
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 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~
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~
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~
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.
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~
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~
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~
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~
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~
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 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~
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~
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~
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.
Claims (25)
1. Process for the measurement of stress in a body by making use of the reflection of an acoustic wave and by measuring a transit time of the latter with the aid of an equipment measuring the time of appear-ance of an echo coming from an interface, wherein one or a plurality of ends of rectilinear measurement paths, having the material form of an in-ternal artificial reflector, is or are selected in the medium, wherein a beam of acoustic waves is transmitted in such a manner that acoustic rays carrying sufficient energy,touch the useful reflectors, wherein the echoes corresponding to the reflectors are selected, wherein the transit times characteristic of the useful acoustic rays as far as the internal artificial reflectors are determined by measurement, and wherein the transit times for each internal reflector considered individu-ally, or the respective differences of transit time of each pair of reflectors, are transposed into a value of external stress or into a value of stress in the zone delimited by each pair of reflectors.
2. Process as claimed in claim 1, wherein a rectilinear measurement path bounded by two internal artificial reflectors is selected in the medi-um, wherein a beam of acoustic waves, the axis of which is substantially identical with the selected rectiline-ar measurement path, is transmitted, wherein the echoes corresponding to the internal artificial reflectors are selected, wherein the transit time characteristic of an acoustic ray between the two reflectors is measured, this time being the difference between the transit times of the coincidence or substantially parallel acoustic rays as far as each of the two reflectors, and wherein the values of transit time are transposed into values of stress.
3. Process as claimes in claim 1 or 2, wherein a narrow ultrasonic beam is used.
4. Process as claimed in claim 1 or 2, wherein a focused ultrasonic beam is used.
5. Process as claimed in claim 1 or 2, wherein the measurement is made by continuous transmission and determination of the resonance frequencies of the wave which are due to the two internal reflectors.
6. Process as claimed in claim 1 or 2, wherein transverse ultrasonic waves are used.
7. Process as claimed in claim 1 or 2, wherein longitudinal ultrasonic waves are used.
8. Process as claimed in claim 1 or 2, wherein surface ultrasonic waves are used.
9. Process as claimed in claim 1 or 2, wherein a longitudinal wave and a transverse wave are used in combination, concomitantly or in succession.
10. Process as claimed in claim 1, wherein the artificial reflectors consist of transverse perforations or bores disposed in parallel planes and forming an angle in relation to one another.
11. Process as claimed in claim 10, wherein the perforations or bores form an angle of 90 degrees between them.
12. Process as claimed in claim 10, wherein the perforations or bores form an angle between 0 degrees and 90 degrees in a direction such that this angle increases with the angular torsional deformation on tightening.
13. Process as claimed in claim 1 or 2, wherein the reflectors consist of two coaxial bores of small diameter, but of different diameters and of different depths.
14. Process as claimed in claim 1 or 2, wherein the positioning of the transmitter/receiver sensor is effected by finding the position in which the amplitudes of the reflection echoes on the internal artificial reflectors are as far as possible simultaneously maximum or attain respective relative heights marked in advance during the calibration.
15. Process as claimed in claim 1 or 2, wherein the measurement is made by utilizing two different frequencies of acoustic waves.
16. Part for applying the process as claimed in claim 1, which has one or more artifical reflectors (8, 9) for the acoustic waves.
17. Part as claimed in claim 16, which consists of a bolt, a threaded rod, or a tie.
18. Part as claimed in claim 16, which is provided with artifical reflectors (8, 9) which consist of transverse perforations or bores disposed in parallel planes and forming an angle in relation to one another.
19. Part as claimed in claim 18, in which the perforations or bores (10, 11) form an angle of 90 degrees between them.
20. Part as claimed in claim 18, in which the perforations or bores (10, 11) form an angle between 0 degrees and 90 degrees in such a direction that this angle increases with the angular torsional deformation on tightening.
21. Part as claimed in claim 16, which is provided with positioning means such as a bush or a socket in the end of the part intended to receive a sensor.
22. Part as claimed in claim 17, which is provided with artificial reflectors (8, 9) which consist of transverse perforations or bores disposed in parallel planes and forming an angle in relation to one another.
23. Part as claimed in claim 22, in which the perforations or bores (10, 11) form an angle of 90 degrees between them.
24. Part as claimed in claim 22, in which the perforations or bores (10, 11) form an angle between 0 degrees and 90 degrees in such a direction that this angle increases with the angular torsional deformation on tightening.
25. Process for the measurement of stress in a body substantially as hereinbefore described with reference to the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000490228A CA1241734A (en) | 1985-09-09 | 1985-09-09 | Ultrasonic process for measuring stress in a bolt or similar part adapted to this method |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000490228A CA1241734A (en) | 1985-09-09 | 1985-09-09 | Ultrasonic process for measuring stress in a bolt or similar part adapted to this method |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1241734A true CA1241734A (en) | 1988-09-06 |
Family
ID=4131339
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000490228A Expired CA1241734A (en) | 1985-09-09 | 1985-09-09 | Ultrasonic process for measuring stress in a bolt or similar part adapted to this method |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1241734A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114674257A (en) * | 2022-03-31 | 2022-06-28 | 中国空气动力研究与发展中心计算空气动力研究所 | High-precision thickness measuring method and device based on ultrasonic transverse wave detection |
-
1985
- 1985-09-09 CA CA000490228A patent/CA1241734A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114674257A (en) * | 2022-03-31 | 2022-06-28 | 中国空气动力研究与发展中心计算空气动力研究所 | High-precision thickness measuring method and device based on ultrasonic transverse wave detection |
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