METHOD FOR MEASURING STRESS IN A WORKING STRUCTURE OR IN A COMPONENT PART OF A WORKING STRUCTURE

TECHNICAL FIELD The present invention relates to a method for measuring stress in a working structure.

The present invention may be used to advantage, though not exclusively, for measuring stress in a working structure or in a component part of a working structure, such as a rail or part of a rail of a working railroad line, to which the following description refers purely by way of example. BACKGROUND ART

Various methods are known for measuring or monitoring stress in a working structure, such as a metal structure in use, or in a particular mechanical part of the structure .

A first typology of such measuring methods employs sensors, such as strain gauges, piezoelectric or Hall sensors, which call for extensive electric wiring for power supply and signal transmission to a processing unit. To monitor the rails of a railroad line, in particular, using sensors installed along the rails, the wiring extends over such long distances as to be far more expensive than the actual sensors as a whole. Moreover, fixing the sensors to the rails is often fairly invasive and expensive, and often involves shutting down traffic along the monitored railroad

sections. For example, the rails must be cut and rewelded at the sensor installation points.

Another typology of such measuring methods utilizes brittle- lacquer sensors or fibre sensors. The first call for a sophisticated reading technique that can only be performed in a laboratory and is therefore practically impossible to carry out on a working structure; whereas the second involve highly critical preparation, and are only suitable for detecting crossing of a predetermined stress threshold, i.e. the fracture resistance of the fibre. Fibre sensors can also be used by measuring variations in their electric resistance, but this technique also has the same wiring drawbacks described above . A third typology of such measuring methods uses the magnetostrictive properties of ferromagnetic materials. Methods of this sort, however, can obviously only be applied to objects made of such materials, and have the drawback of introducing serious measuring errors and spread, due to variations in magnetic permeability, and of being fairly slow.

Another method for measuring structural stress measures variations in the propagation time of a sound wave (ultrasounds) along a predetermined path in the monitored structure. Methods of this sort call for the use of equipment that is critical to operate, and involve complex, slow measuring procedures.

Mechanical elongation or contraction measuring

systems also exist, but are extremely complex, bulky, and inaccurate, and, what is more, call for shutting down operation of the monitored structure.

DISCLOSURE OF INVENTION It is an object of the present invention to provide a method for measuring stress in a working structure or in a component part of a working structure, designed to eliminate the aforementioned drawbacks, and which in particular provides for precise non-destructive measurements, while at the same time being cheap and easy to implement .

According to the present invention, there are provided a method and system for measuring stress in a working structure, and a stress sensor for determining stress in a working structure, as claimed in the accompanying Claims .

BRIEF DESCRIPTION OF THE DRAWINGS

A non- limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows schematically a fluorescence spectrum of a stress sensor in accordance with the present invention;

Figure 2 shows a principle by which to determine a physical characteristic of a stress sensor in accordance with the present invention;

Figure 3 shows schematically a system for measuring stress in a working structure, in accordance with the

present invention and comprising sensors with the characteristics shown in Figures 1 and 2.

BEST MODE FOR CARRYING OUT THE INVENTION

The method for measuring stress in a working structure according to the present invention comprises forming a stress sensor from a respective element of piezospectroscopically active material with a given piezospectroscopic coefficient IT, e.g. a ceramic material such as alumina (Al ₂ O ₃ ) , an alumina- zirconia compound (Al ₂ O ₃ - ZrO ₂ ) containing 60% alumina and 40% zirconia, or any other piezospectroscopic-effect material, i.e. capable of emitting a characteristic fluorescence or Raman spectrum when excited with appropriate electromagnetic radiation in the visible range .

The fluorescence or Raman spectrum of a piezospectroscopically active material - hereinafter referred to simply as emission spectrum - has at least one characteristic peak, i.e. an extremely high- intensity peak for a characteristic wave number of the material. When the material is subjected to mechanical stress, the position of the peak in the emission spectrum, i.e. the corresponding characteristic wave number, shifts with respect to the unstressed value. By way of example, Figure 1 shows the fluorescence spectrum of alumina in terms of emitted-radiation intensity I, expressed in arbitrary units (a.u.), as a function of emitted-radiation wave number, and which shows the

characteristic peaks in the absence of stress, corresponding to wave numbers v _A and v _B , and the same characteristic peaks in the presence of compression and tensile stress of the material capable of producing a shift δv in the peak in terms of wave number. Generally speaking, the position of characteristic peaks also depends on the temperature of the material .

The piezospectroscopic coefficient IT expresses the relationship between the mechanical stress exerted and the corresponding shift in the characteristic peak, and can be determined using known procedures which substantially comprise applying known stress on a massive piece of material, and determining the corresponding characteristic peak shifts with respect to the emission spectrum of the material in the absence of stress. Figure 2 shows a graph of the characteristic peak shift δv alongside variations in intensity of an induced stress S applied on a piece of alumina (top graph) and alumina- zirconia (bottom graph) . As shown in Figure 2, piezospectroscopic coefficient IT corresponds to the angular coefficient of the line approximating the pattern of shift δv alongside variations in induced stress S. The piezospectroscopic coefficient Il does not depend on the size of the piece of material. In Figure 3, number 1 indicates as a whole a system for measuring stress in a working structure according to the present invention; and number 2 indicates the monitored working structure which comprises a portion of

rail of a working railroad line (not shown) .

System 1 comprises one or more stress sensors 3, each formed as described above and fixed in a respective position along rail 2; a portable spectrometer 4 for acquiring the emission spectrum produced by each sensor 3; and a computer 5, e.g. a portable computer , (laptop) , connected to spectrometer 4 to memorize and process the acquired emission spectra. More specifically, computer 5 comprises a memory unit (not shown) equipped with a data bank for storing the acquired emission spectra and the piezospectroscopic coefficient IT, determined as described above, of the material of which sensors 3 are made .

System 1 also comprises one or more identifiers 6, each of which is defined, for example, by a bar code or transponder (RFID tag) , is fitted to a respective sensor 3, and comprises information by which to unmistakably identify sensor 3 and therefore the emission spectra produced by sensor 3 , and so determine the point on rail 2 corresponding to the acquired emission spectra; a code reader 7 connected to computer 5 to acquire the identification information from identifiers 6; and a temperature probe 8 connected to computer 5 to acquire temperature values of rail 2. In an alternative, equivalent embodiment (not shown), each identifier 6 is applied to rail 2, close to respective sensor 3.

Spectrometer 4 typically comprises a laser source 9

for directing onto sensor 3 a coherent- light beam 10 with a wave number in the visible range and such as to excite fluorescence of sensor 3; an optical probe 11 comprising an optical-fibre bundle for gathering the light 12 emitted by sensor 3 in response to excitation; a light dispersing device 13, also known as monochromator, for separating light 12 into its monochromatic parts; and an optical detector 14 defined, for example, by a CCD sensor or matrix of photomultipliers, to convert the separated light into digital electric signals containing information about the emission spectrum of sensor 3, and of such a format as to be easily processed by computer 5.

Laser source 9 is preferably, though not necessarily, equipped with an optical- fibre extension 15 to direct the laser beam more easily onto sensor 3. In the method for measuring stress in a working structure according to the present invention, sensors 3 are designed as a function of the working conditions of the structure, e.g. the way in which stress is thought to be exerted on the structure, and as a function of the physical characteristics of the structure, e.g. the material, height, length, and thickness of the structure. In the Figure 3 example, each sensor 3 fixed to rail 2 has a circular cylindrical shape having preferably, though not necessarily, 1 cm in diameter and 0.5 cm in height .

The method also comprises fixing each sensor 3 to

rail 2 so that the stress in rail 2 and the variations in temperature of rail 2 are transmitted to sensor 3. The stress status of sensor 3 , represented by its emission spectrum, thus substantially corresponds to the stress status of rail 2 at the point of the rail 2 where sensor 3 is attached. The forces that determine the fixing of the sensor 3 to the rail 2 are necessarily the result of a trade-off, that is they must be sufficient to secure sensor 3 firmly to rail 2 and ensure adequate stress transmission between the working rail 2 and sensor 3, while at the same time allowing the characteristics of sensor 3 to remain as far as possible unchanged over time. The sensor 3 is preferably, though not necessarily, attached when manufacturing rail 2. The sensor 3 may be attached to rail 2 , for example, by forming a circular seat (not shown) , slightly smaller in diameter than sensor 3, in rail 2, preferably when rail 2 is being manufactured and is still hot, or by heating rail 2 to temporarily expand the seat, inserting sensor 3 inside the seat and cooling rail 2 to contract the seat and lock sensor 3 rigidly to rail 2. Alternatively, the sensor 3 may be attached using other mechanical anchoring devices or appropriate adhesives . Before rail 2 is put into service, i.e. before trains are run along the relative line, the method according to the present invention comprises acquiring, by means of spectrometer 4 and for each sensor 3 fixed

to rail 2, a first emission spectrum from a precise point of sensor 3, e.g. a point at the centre of sensor 3 , and for a number of temperature values of rail 2. The first emission spectra of all the sensors 3 are then memorized in the data bank of computer 5, together with the corresponding identities of sensors 3, and the identities are read from respective identifiers 6 by reader 7. The first emission spectrum defines the reference stress status of rail 2 at a given temperature .

To measure stress in the working rail 2, e.g. as part of a routine check of the condition of the line, spectrometer 4 acquires a second emission spectrum from said precise point of each sensor 3, and temperature probe 8 acquires an operating temperature T of rail 2 at each attachment point of sensors 3 on the structure 2. The second emission spectra are also memorized in the data bank of computer 5 , together with the corresponding identities of sensors 3 and measured operating temperatures T.

Computer 5 is configured to compare the second emission spectrum of each sensor 3 with the relative first emission spectrum, to determine a respective spectrum shift. More specifically, the comparison comprises selecting the first emission spectrum corresponding to a temperature value, of said number of temperature values, that best approximates operating temperature T; and comparing the second emission

spectrum with the selected first emission spectrum. More specifically, the comparison comprises determining a first wave number V ₁ corresponding to a first position of a characteristic peak of sensor 3 in the selected first emission spectrum; determining a second wave number V ₂ corresponding to a second position of the same characteristic peak in the second emission spectrum; and calculating a shift δv in the characteristic peak as the difference between the second and first wave number: δv = V ₂ - V ₁ .

The calculated shift δv of each sensor 3 is then processed with piezospectroscopic coefficient Il to obtain, by means of a straightforward mathematical equation, a respective measurement of the stress in rail 2 at the attachment point of sensor 3. In other words, because sensors 3 are identifiable, the method provides for obtaining spot measurements of the stress in the rail at the attachments points of sensors 3.

If the material the sensor 3 is made of has more than one characteristic peak, e.g. alumina (Figure 1), the comparison between first and second emission spectrum as described above is preferably, though not necessarily, performed relative to the highest-intensity characteristic peak. If the material the sensor 3 is made of has a number of characteristic peaks of similar intensity, the comparison between first and second emission spectrum is performed relative to all the characteristic peaks, and the resulting shift δv used to

calculate the stress in rail 2 is calculated as a function of the shifts of all of the peaks, e.g. by performing the average of the shifts, to obtain a more accurate stress measurement . According to a further embodiment, not shown, of the present invention, spectrometer 4 has a built-in processing unit for storing and processing the acquired emission spectra, so computer 5 is not required.

In a further embodiment of the present invention, the method also comprises acquiring a third emission spectrum of each isolated sensor 3, i.e. before it is attached to rail 2, in the absence of induced stress and for different temperature values of the material of sensors 3; and comparing the third and first emission spectrum of each sensor 3 to determine the presence and amount of stress induced in sensor 3 by assembly to rail 2 , and which could stress sensor 3 to such an extent as to result in rapid degradation of the sensor during operation of the structure. The induced stress so determined may also be used in processing shift δv in the characteristic peak, to determine the absolute stress in rail 2 measured by system 1.

In a further embodiment of the present invention, the method comprises acquiring a series of first emission spectra from a respective plurality of points on the outer surface of each sensor 3 and for a plurality of temperature values of rail 2 before rail 2 is put into service; and a series of second emission

spectra from the same points on each sensor 3 after rail 2 is put into service. Each second emission spectrum is compared with the respective first emission spectrum. The series of first emission spectra is selected as a function of operating temperature T as already- described, so as to determine, for each sensor 3, a corresponding series of characteristic peak shifts δv by which to form a stress measurement map of sensor 3. To determine stress induced in sensor 3 by assembly to rail 2 , the method comprises acquiring a number of third emission spectra from said number of points on each isolated sensor 3 ; and comparing the number of third emission spectra with the number of first emission spectra as described above. In a further, simpler, embodiment, not shown, of the present invention, system 1 has no identifiers 6 and no reader 7. Moreover, the computer 5 stores the emission spectra acquired from each sensor 3, both before and after rail 2 is put into service, with no information identifying sensor 3. The shifts δv obtained from all sensors 3 are averaged to obtain one approximate measurement relative to the whole of rail 2. Generally speaking, this embodiment is the most appropriate for determining stress in a single component part of a complex structure using only a few sensors 3 or only one sensor 3 fixed to the actual component part. The method described above of measuring stress in a working structure or in a component part of a working

structure has the main advantage of providing accurate, reliable, non-destructive measurements, with no need for electric wiring for powering and communicating with detecting devices fitted to the monitored structure. The method described, in fact, utilizes the piezospectroscopic characteristics of various materials to form and operate extremely straightforward passive stress sensors of stable long-term performance, and also provides for fairly fast measurements .