GB2193318A

GB2193318A - Monitoring the integrity of mechanical structures

Info

Publication number: GB2193318A
Application number: GB08618678A
Authority: GB
Inventors: Ian Robert Fothergill
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1986-07-31
Filing date: 1986-07-31
Publication date: 1988-02-03
Also published as: GB8618678D0

Abstract

A method of monitoring the integrity of a mechanical structure such as a nuclear reactor primary vessel comprises locating therein at each of a number of sites a noise generator (16, 18, 20) and mechanically exciting the structure with vibrations at predetermined resonant frequencies so as to induce noise production by different ones or combinations of the noise generators. The noise produced by the noise generators is monitored by external accelerometers (24-34) so that changes in the excitation characteristics can be detected. The noise generators may be spring- mounted masses, Fig. 3 (not shown). <IMAGE>

Description

SPECIFICATION Monitoring of the integrity of mechanical structures This invention relates to the monitoring of the integrity of mechanical structures in order to detect the development of possible failure conditions, for example the development of crack growth in critical regions.

The invention is particularly applicable to structures in which location of sensors at such critical regions is not feasible or practicable owing to difficulty of access or hostility of the environment. One example is the core support structure within the primary vessel of a nuclear reactor, eg. a liquid metal cooled fast reactor. In such a reactor, access to internal structural members is limited and it is often not practicable to rout signal leads from sensors located at critical internal sites through tortuous paths to a non-hostile region at which signal processing can take place. Even where access is possible, there may be substantial degradation of the signals due to the long signal conduction paths involved and, moreover, the long term stability of the sensors may be questionable when subject to the hostile environment within the reactor.

The object of the present invention is to provide a method of monitoring mechanical structures, which method overcomes at least some of the difficulties referred to above.

According to the present invention there is provided a method of monitoring a plurality of sites associated with a mechanical structure, said method comprising locating at each such site a mechanically-responsive noise generator, mechanically exciting the structure so as to induce noise production by different ones or combinations of said noise generators and monitoring said noise generators to determine changes in the excitation characteristics of said sites.

The noise generators may be monitored by suitable sensors such as a plurality of accelerometers which can be located at more easily accessible locations than the sites to be monitored since the acoustic signals will be transmitted to the accelerometers via parts of the mechanical structure. The accelerometers can therefore be remote from the sites of interest and triangulation-type techniques can be applied to the outputs of the sensors in order to identify a site at which a change in excitation characteristics occurs.

Each noise generator conveniently comprises a spring-mounted mass so arranged that, when the respective site is subject to large amplitude standing wave conditions as a result of mechanical excitation of the structure as a whole, the mass is set into oscillation and repeatedly strikes the adjacent surface of the structure. In this way, the noise generator produces a periodic "chatter" giving rise to corresponding broad band pulsed noise signals which will be transmitted through the structure and thereby be detectable at locations remote from the site at which the noise generator is located.

In one embodiment, the mass of the noise generators is constrained to move in one degree of freedom and the arrangement may be such that the direction of movement of the mass substantially coincides with the direction of polarisation of the vibration excited at the respective site.

One example of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of the primary vessel of a sodium cooled fast reactor; Figure 2 is a schematic view of the primary vessel taken in the direction 2-2 in Figure and Figure 3 is a sectional view of a parasitic noise generator which may be employed in the method of the invention.

Figures 1 and 2 illustrate purely schematically the primary vessel 10, reactor core 12 and support structure 14 for the core. parasitic noise generators 16, 18 and 20 are secured to ribs 22 of the support structure within the hostile liquid metal environment of the reactor. A number of accelerometers 24, 26, 28, 30, 32, 34 are secured to the primary vessel externally thereof thus making the accelerometers relatively accessible and isolating them from the hostile environment within the primary vessel 10. The accelerometers are acoustically coupled to the noise generators via the mechanical structure comprising the primary vessel and core support structure. A mechanical excitation source 36 such as a vibrator is connected to the primary vessel 10 at a convenient location.

The vibrator 36 may have variable frequency and amplitude controls to enable the mechanical structure to be excited through some of its resonant modes at one or more sites of interest, eg. the ribs 14. The noise generators are located at these sites and as will be explained below each genrator is arranged to produce "chatter" when the respective site is subject to large amplitude standing wave conditions as a result of vibrations induced by the vibrator 36. Thus, for example, each noise generator may produce "chatter" in response to a different frequency of vibration applied to the structure by the vibrator 36.

Initially, the system may be calibrated by experiment so as to determine the frequencies necessary to induce resonant conditions at each site. Certain frequencies may induce resonant conditions simultaneously at more than one site so that "chatter" of different ones or combinations of the noise genrators corresponds to different resonant modes of the structure. The accelerometers may be employed to sense the noise generated by the generators during the calibration procedure and the outputs of the accelerometers for resonances at the different sites may be recorded for use as references. The particular noise generator creating chatter may be identified from the accelerometer outputs by triangulation-type techniques.

In use, the vibrational characteristics at each site when excited by the resonant frequency may change significantly if any potential fault, such as crack growth, develops at or adjacent such site. Thus, by for example periodically exciting the structure, by means of the vibrator 36, with vibrations at the previously determined resonant frequencies for the different sites and monitoring the noise generators with the accelerometers, fault conditions can be detected and located.

Figure 3 illustrates one form of noise generator comprising a mass 40 carried at the free end of a blade spring 42 which constrains the mass to move in one degree of freedom, ie.

angularly about the fixed end 44. The mass 40 and spring 42 are encapsulated within a housing 46 which may be filled with an inert gas such as Argon. The spring 42 is designed so as to press the mass 40 against the wall 48 and hence the surface of the structural part to which the housing is secured. If the mass 40 is subjected to suitably large amplitude standing wave conditions, with the direction of polarisation predominantly in the same directon as that in which the mass 40 is free to move, then the inertial forces acting on the mass will exceed the opposing spring forces (and any gravitational forces) and the mass 40 will lift off the wall 48 and then spring back.

In this way, the mass 40 will repeatedly impact against the wall 48 and produce "chatter" which will be transmitted through the mechanical structure for detection by the accelerometers. Such "chatter" will provide corresponding broad band noise signaiswhose amplitude will fall off with distance from the generator, providing the possibility of identification of the "chattering" generator using triangulation-type techniques applied to the outputs of the accelerometers or other sensors.

Although the invention is described herein with reference to monitoring mechanical structure of nuclear reactors, it is also applicable to other mechanical structures, such as oil rigs.

Claims

1. A method of monitoring a plurality of sites associated with a mechanical structure, said method comprising locating at each such site a mechanically-responsive noise generator, mechanically exciting the structure so as to induce noise production by different ones or combinations of said noise generators and monitoring said noise generators to determine changes in the excitation characteristics of said sites.

2. A method as claimed in Claim 1 in which each noise generator comprises a springmounted mass so arranged that, when the respective site is subject to large amplitude standing wave conditions as a result of mechanical excitation of the structure as a whole, the mass is set into oscillation and repeatedly strikes the adjacent surface of the structure.

3. A method as claimed in Claim 2 in which the mass of the noise generators is constrained to move in one degree of freedom and the arrangement is such that the direction of movement of the mass substantially coincides with the direction of polarisation of the vibration excited at the respective site.

4. A method as claimed in Claim 1 substantially as hereinbefore described with reference to the accompanying drawings.