GB2221301A

GB2221301A - An acoustic emission load cell

Info

Publication number: GB2221301A
Application number: GB8818210A
Authority: GB
Inventors: James Pears Angus; Mason Andrew Levy
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1988-07-30
Filing date: 1988-07-30
Publication date: 1990-01-31
Anticipated expiration: 2008-07-30
Also published as: GB2221301B; GB8818210D0

Abstract

An acoustic emission load cell (10) comprises a sheet of e.g. carbon fibre composite (12) positioned between two sheets of rubber (16, 18) and a conductor rod (14) is formed integrally with the carbon fibre composite (12). The carbon fibre composite (12) is an acoustically noisy material in that applied mechanical deformations generate acoustic emissions, or stress waves, which propagate there-through. An acoustic emission transducer (20) is acoustically coupled to the conductor rod (14), and the transducer (20) detects any acoustic emissions and converts them into an electrical signal. A signal processor (22) processes the electrical signals to give an electrical output signal which is a measure of the load applied. The electrical output signal is supplied to a recorder (24) and/or a display (26). The acoustic emission load cell is suitable for use in monitoring the load applied to railway line tracks by locomotives or rolling stock. Other suitable acoustically noisy materials may be used for the load cell. <IMAGE>

Description

AN ACOUSTIC EMISSION LOAD CELL The present invention relates to load cells and in particular to an acoustic emission load cell.

A problem exists in the quantitative measurement of loads imposed on railway lines during the passage of locomotives and rolling stock due to limitations of available space, non-uniform application of load and conditions of intense electrostatic and electromagnetic interference.

Prior attempts to measure the loads on railway lines have included resistance element strain gauge, piezoelectric load cells, capacitance load cells and LVDT displacement transducers. All these have experienced varying degrees of success when applied to non-electrified railway lines. However, they all suffer from electromagnetic interference from overhead power cables and electrical equipment on the locomotives when applied to electrified railway lines. In addition intense electrical field strength and current density in the vicinity of the railway lines for signalling purposes necessitates the use of extensive screening of the electrical leadouts in order that outputs from the various load measuring devices remain intact and uncorrupted. This is not possible because of space limitations and because of the need for the measuring device to be inconspicuous.

Not only are the measuring devices electronically vulnerable, the presence of these electric measuring devices in close proximity to the railway line may cause disruption of the signalling currents which is unacceptable for safety.

The use of track mounted optical interferometers, although non-electrical, requires a stable reference point remote from the track for an externally mounted transducer which is difficult to achieve.

The present invention seeks to provide a load cell which is compatible with use in relatively small spaces, which is suitable for use with non-uniform application of loads and which is not effected by intense electrostatic and electromagnetic fields.

Accordingly the present invention provides an acoustic emission load cell comprising a sheet of an acoustically noisy material, the acoustically noisy material generating acoustic emissions by friction or movement of defects within the acoustically noisy material when a load is applied to the acoustically noisy material, a transducer acoustically coupled to the acoustically noisy material, the transducer being arranged to detect and to convert any acoustic emissions generated in the acoustically noisy material by application of a load on the acoustically noisy material into an electrical signal, and a processor arranged to process the electrical signal to give an electrical output signal as a measure of the load applied on the acoustically noisy material.

The sheet of acoustically noisy material may be positioned between two sheets of resilient material.

The sheets of resilient material may be formed from rubber.

The acoustically noisy material may be positioned between a railway line track member and a railway sleeper member, one of the sheets of resilient material rests on the railway sleeper member and the railway track member rests on the other sheet of resilient material.

The acoustically noisy material may be a fibre composite.

The fibre composite may be formed from carbon fibres in a carbon matrix.

The fibre composite may be formed from glass fibres in a glass matrix.

The acoustically noisy material may have an electrically and acoustically insulating coating.

The orientation of the carbon fibres may be arranged to optimise the generation and transmission of acoustic emissions within the acoustically noisy material.

The transducer may be acoustically coupled to the acoustically noisy material by an acoustically conducting member.

The acoustically conducting member may be formed integral with the acoustically noisy material.

The present invention will be more fully described by way of example with reference to the accompanying drawings, in which: Figure 1 is a plan view of an acoustic emission load cell according to the present invention.

Figure 2 is a cross-sectional view in the direction of arrows A - A in Figure 1.

Figure 3 is a perspective view of the acoustic emission load cell in Figure 1.

Figure 4 shows a railway line track with the acoustic emission load cell of Figures 1 to 3.

An acoustic emission load cell 10 according to the present invention is shown in Figures 1 to 3. The acoustic emission load cell 10 comprises a sheet of an acoustically noisy material 12, ie a material which under mechanical deformation produces a characteristic patterns of sounds associated with the presence of internal features and defects, such as voids and dislocations, which migrate through the material as a load is applied or due to friction between features in the material or microcracking.

The sounds are high frequency acoustic emissions or stress waves, which propagate through the acoustically noisy material 12. An acoustic emission transducer 10 is acoustically coupled to the acoustically noisy material 12, and the acoustic emission transducer 20 detects any acoustic emissions generated in the acoustically noisy material by application of loads onto the acoustically noisy material and converts the acoustic emissions into an electrical signal. The transducer 20 is electrically connected to a signal processor 22, which processes the electrical signals produced by the transducer 20 and gives an electrical output signal which is a measure of the load applied on the acoustically noisy material 12. The electrical output signal of the signal processor 22 is electrically connected to a recorder 24 and a display 26.

The signal processor 22 may be connected to only one of the recorder 24 or display 26 if desired.

In operation of the acoustic emission load cell 10 the application of a load onto the acoustically noisy material 12 produces deformation of the internal structure of the acoustically noisy material generating acoustic emissions or friction between features in the material which generates acoustic emissions. The acoustic emissions are detected by the transducer 20 and an electrical signal is produced which is processed to give a measure of the load acting on the acoustically noisy material 12. It is believed that the application of load onto the acoustically noisy material will produce an acoustic emission activity, and corresponding electrical signal, proportional to the magnitude of the load.

The acoustically noisy material 12 may for example be a carbon fibre composite ie carbon fibres in a carbon matrix, or a glass fibre composite i.e. glass fibres in a glass matrix. The acoustically noisy material may also be a monolithic ceramic/glass i.e. a borosilicate glass such as alumina borosilicate, or an epoxy composite i.e. carbon fibre, glass fibre or Kevlar fibre reinforced epoxy resin.

If the acoustically noisy material 12 is a carbon fibre composite the performance of the load cell may be optimised by altering the orientation of the laying of the fibres in the acoustically noisy material, or by altering the orientation of the material. Carbon fibres in a carbon matrix are particularly suitable because of their unique internal structure making them particularly acoustically active when deformed due to the friction generated between the fibres and matrix, but because carbon fibres are electrically conducting this requires the use of electrical insulation, and also noise insulation to prevent or reduce the detection of extraneous noise.

The acoustically noisy material 12 as shown in the Figures is made with an integral conductor rod 14, also made of the same acoustically noisy material, which collects and transmits acoustic emissions generated in the acoustically noisy material 12 to the transducer 20. The transducer 20 is acoustically coupled to the free end of the conductor rod 14.

The acoustically noisy material 12 is sandwiched between two sheets of resilient material 16 and 18, the resilient material may for example be rubber.

The acoustic emission load cell 10 is shown, as an example, in position for use on a railway line in Figure 4.

The acoustic emission load cell 10 is positioned between a sleeper 32, which rests on the ground 30, and a railway line track 34. The lower sheet of rubber 18 rests on and abuts the sleeper 32, and the railway line track 34 rests on an abuts the upper sheet of rubber 16. The application of a load onto the railway line track 34, by the passage of a railway locomotive or rolling stock over the railway line track, is transmitted through the upper sheet of rubber 18 to the acoustically noisy material 12 of the load cell 10 and the acoustic emission activity and corresponding electrical signal are proportional to the magnitude of the load.

The acoustic emission load cell has several advantages for use with the railway line. The acoustic emission load cell is electronically passive, it is unobtrusive because it lies underneath the track, it provides an integrated response from the track because it averages out the load and it is independent of strain characteristics, no preparation of the track or sleeper is necessary, it is relatively cheap to make and replacement of the load cell is simple.

The acoustic emission load cells are suitable for short term measurement of loads on railway line tracks, continued useage of the acoustic emission load cell eventually leads to a loss of generation of the acoustic emissions. However, because these devices are relatively simple, cheap and easy to install replacement acoustic emission load cells can be inserted periodically.

Claims

Claims:

1. An acoustic emission load cell comprising a sheet of an acoustically noisy material, a transducer acoustically coupled to the acoustically noisy material, the acoustically noisy material generating acoustic emissions by friction or movement of defects within the acoustically noisy material when a load is applied to the acoustically noisy material, the transducer being arranged to detect and to convert acoustic emissions generated in the acoustically noisy material by applications of a load on the acoustically noisy material into an electrical signal, and a processor arranged to process the electrical signal to give an electrical output signal as a measure of the load applied on the acoustically noisy material.

2. An acoustic emission load cell as claimed in Claim 1 in which the sheet of acoustically noisy material is positioned between two sheets of resilient material.

3. An acoustic emission load cell as claimed in Claim 2 in which the sheets of resilient material are formed from rubber.

4. An acoustic emission load cell as claimed in Claim 2 or Claim 3 in which the acoustically noisy material is positioned between a railway line track member and a railway sleeper member, one of the sheets of resilient material resting on the railway sleeper member and the railway track member resting on the other sheet of resilient material.

5. An acoustic emission load cell as claimed in any of Claims 1 to 4 in which the acoustically noisy material is a fibre composite.

6. An acoustic emission load cell as claimed in Claim 5 in which the fibre composite is formed from carbon fibres in a carbon matrix.

7. An acoustic emission load cell as claimed in Claim 5 in which the fibre composite is formed from glass fibres in a glass matrix.

8. An acoustic emission load cell as claimed in any of Claims 1 to 7 in which the acoustically noisy material has an electrically and acoustically insulating coating.

9. An acoustic emission load cell as claimed in Claim 6 in which the orientation of the carbon fibres is arranged to optimise the generation and transmission of acoustic emissions within the acoustically noisy material.

10. An acoustic emission load cell as claimed in any of Claims 1 to 9 in which the transducer is acoustically coupled to the acoustically noisy material by an acoustically conducting member.

11. An acoustic emission load cell as claimed in Claim 10 in which the acoustically conducting member is formed integral with the acoustically noisy material.

12. An acoustic emission load cell substantially as hereinbefore described with reference to and as shown in the accompanying drawings.