METHOD FOR DETECTING A DEFECT USING ULTRASONIC TECHNOLOGY

Abstract

Method for detecting a defect in a region of interest within a part to be tested, comprising the following steps: a) for a reference part identical to the part to be tested but without a defect, a1) determination of a set of resonance modes, each defining: - a resonance frequency of the reference part, considering that the modulus of elasticity of the reference part is constant, and - a field of mechanical stresses on and / or in the reference part generated when the reference part is in resonance at said resonance frequency; a2) selection of the resonance mode, called the "optimal resonance mode", generating, in the region of interest, a maximum mechanical stress;b) determination of an "optimal" excitation mode, primarily activating said optimal resonance mode, an excitation mode defining at least one excitation wave, an injection zone of the excitation wave in the reference part, and an output zone where an output wave resulting from the passage of the excitation wave from the injection zone to the output zone is captured; c) analysis by nonlinear resonance spectrometry from the optimal excitation mode, so as to determine a nonlinearity parameter for each of said test and reference parts; d) classification of the test part according to the difference between the nonlinearity parameters for the test part and for the reference part.

Description

Title of the invention: METHOD FOR DETECTING A DEFECT BY ULTRASOUND Technical field

[0001] The invention relates to a method for detecting a defect in a part, in particular a ceramic or glass-ceramic part. State of the art

[0002] The detection of internal defects in a part by analysis of acoustic waves is known.

[0003] The oldest method consists of measuring and analyzing the transmission speed of an acoustic wave in the room. Its scope of application is however limited to rooms of simple shape and made of a homogeneous and uniform material. It is also known to analyze the attenuation of the injected acoustic wave, by linear analysis, that is to say by considering that, in a given environment, the resonance frequencies depend exclusively on the room, namely its shape and its constituent material (which determines the speed of the waves). This type of analysis has however also shown its limits for discriminating between rooms of complex structure or of a heterogeneous material.

[0004] Nonlinear resonant ultrasonic spectroscopy (NRUS) or, more generally, nonlinear resonant acoustic spectroscopy (NRAS) are more recent methods exploiting the fact that the modulus of elasticity of the material constituting the part is not constant, but varies according to the mechanical stresses generated in the part by the excitation wave. The resonance frequencies thus depend on the amplitude of the excitation wave. This method is based in particular on an analysis of the evolution of the resonance frequency spectrum as a function of the amplitude of the injected wave.

[0005] Typically, if, in a given stress mode, - fo is the first linear resonance frequency of the part, i.e. considering that the modulus of elasticity of the part is constant, the linear resonance frequency being conventionally evaluated by numerical simulation or measured by analysis of the response to the injection of a low amplitude wave in said stress mode, and - f is the resonance frequency measured for waves of greater amplitude than the excitation wave, the method evaluates the frequency shift lf-fol / fo as a function of said amplitude. It then exploits a difference in the evolution of the frequency shift depending on the presence or absence of damage, as described in US6330827B1.

[0006] Application FR2960061A1 describes a use of the NRUS technique to explore the field of nonlinear elasticity of materials such as rocks.

[0007] However, the inventors have discovered that the application of the NRUS technique described in the prior art is not always reliable, particularly if the part is of complex shape. Furthermore, in the simultaneous presence of several defects, for example physical defects (such as a crack) and chemical defects (such as a local change in elemental composition or crystallographic phase), the frequency shifts induced by these defects can compensate for each other.

[0008] There is therefore a need for a method for detecting a defect in a part having improved reliability, even when this part has a complex geometry and / or internal structure or when it has several defects.

[0009] One aim of the invention is to meet, at least partially, this need. Summary of the invention

[0010] More specifically, the present invention relates to a method for detecting a defect in a region of interest within a part to be tested, said method comprising the following successive steps: a) for a reference part identical to the part to be tested but free from defects, al) preferably by numerical simulation, determination of a set of resonance modes each defining: - a resonance frequency of the reference part considering that the modulus of elasticity of the reference part is constant, and - a field of mechanical stresses or deformations on and / or in the reference part generated when the reference part is in resonance at said resonance frequency; a2) selection of the resonance mode, called the “optimal resonance mode”, generating, in the region of interest, a maximum mechanical stress or deformation compared to the other resonance modes; b) determining a solicitation mode, called “optimal”, mainly activating said optimal resonance mode, a solicitation mode defining at least one excitation wave, an injection zone of the excitation wave into the reference part, and an output zone where an output wave resulting from the crossing of the excitation wave from the injection zone to the output zone is captured; c) analysis by non-linear resonance spectrometry from the optimal stress mode, so as to determine a non-linearity parameter for each of said test and reference parts; d) classification of the test part according to the difference between the non-linearity parameters for the test part and for the reference part.

[0011] As will be seen in more detail in the remainder of the description, the inventors have discovered that the selection of an optimal resonance frequency as a function of the region of interest considerably improves the reliability of the detection of a defect by nonlinear resonance spectroscopy when the latter is carried out from a stress mode mainly activating the corresponding resonance mode. In particular, the detection is reliable for a part of complex shape or internal structure.

[0012] Remarkably, focusing on the region of interest leads to determining an optimal resonance frequency which is not necessarily the resonance frequency which leads to the most mechanical stresses or deformations of the reference part considered as a whole.

[0013] The analysis by non-linear resonance spectrometry from the optimal stress mode can be conventionally carried out by applying to the part considered (to be tested or reference) the optimal stress mode and derived stress modes which differ from the optimal stress mode only by the amplitude of the injected excitation wave. The evolution of the resonance frequency is then examined, from the optimal resonance frequency, under the effect of the evolution of said amplitude, to determine the non-linearity parameter.

[0014] For the reference part, the application of a stress mode can be carried out on the part itself or on a digital model of the part.

[0015] Preferably, a method according to the invention has one or more of the following optional characteristics:

[0016] • in step a1), the resonance modes are determined by numerical simulation, the modeling of the reference part taking into account the dimensions and geometry of the reference part, the apparent density of the material constituting the reference part, the modulus of elasticity of said material, and the Poisson's ratio of said material; • in step a2), three-dimensional digital models of the reference part are compared, each representing a field of said stresses or said mechanical deformations generated when the reference part is in resonance at a respective resonance frequency; • the volume of the region of interest is less than 0.2 times and greater than 0.01 times the volume of the reference part; • steps a) and b) are carried out simultaneously, the search for the optimal stress mode being carried out in the following manner: - numerical simulation of a plurality of stress modes, the simulation numerical determining for each stress mode, a mechanical stress field in the reference part and a theoretical output wave (i.e. a simulation of the output wave in this stress mode); - analysis of the mechanical stress fields and theoretical output waves in order to select, as the optimal stress mode, the stress mode generating, in the region of interest, a maximum mechanical stress or deformation and putting the reference part into resonance; in step c), the non-linearity parameter is the slope of a straight line representing the evolution of a frequency shift as a function of the evolution of the output wave amplitude when said amplitude of the output wave is modified, preferably increased, from the optimal stress mode, the frequency shift, for an amplitude of the output wave, being the ratio of the absolute value of the difference between the optimal resonant frequency (f0) in the optimal resonant mode and the resonant frequency (f) determined for said amplitude of the output wave, divided by the optimal resonant frequency (f0); in step d), the non-linearity parameter of the part to be tested is compared to a threshold determined from the non-linearity parameter of the reference part, then the part to be tested is classified according to the difference between the non-linearity parameter of the part to be tested and the threshold, preferably according to the sign of said difference; the defect is an empty space within the test part, or a space filled by a material different from the rest of the test part; the part is made of an inorganic material; the part is made of a metal, preferably sintered, a ceramic material, a glass-ceramic material or a mixture of these materials, in particular a composite; in the optimal loading mode, the main peak of an output Fonde frequency spectrum is at a frequency between 1 Hz and 200 KHz, preferably less than 100 KHz, preferably greater than 20 KHz; Output Fonde is at an acoustic wave; said part to be tested is chosen from: - a groove block or lintel, - a tank block, - a brick or block of wall or side, - a corner or angle block, - a nose brick, - a block or a sole slab, - a brick or a vault base, - a brick or block surrounding the nozzle, - a hole brick or casting channel, - an electrode holder block, - a piece of refractory beak from a glass furnace, - an injector block, - a glass furnace throat, - a part of a heat exchanger of the furnace, - a refractory plate or tile of a boiler lining, - a protective shell of an incinerator heater tube, - a tile of an incinerator, - a ceramic part of a solar absorber, - a tile or a protective part of a combustion turbine chamber.

[0017] The deformation of a part is the result of the application of a corresponding mechanical stress. In the context of the present invention, mechanical stresses and mechanical deformations are therefore equivalent. For the sake of clarity, the remainder of the description only refers to mechanical stresses. This description is however applicable to mechanical deformations.

[0018] The invention also relates to a method for sorting externally identical parts manufactured on a production line, in which a detection method according to the invention is implemented for each part, considered as a part to be tested, steps a) and b) and the analysis by non-linear resonance spectrometry from the optimal stress mode carried out on the reference part being preferably common to all the parts. The set of parts may comprise, for example, more than 10, more than 100 or more than 1000 parts to be tested.

[0019] The invention finally relates to a detection device intended for detecting a defect in a part to be tested, the device comprising: - a resonator capable of injecting, into the part to be tested, an excitation wave through an injection zone of the part to be tested; - a receiver capable of capturing an output wave through an output zone of the part to be tested, the output wave resulting from the passage of the excitation wave through the part to be tested; - a computer connected to the receiver so as to receive the output wave, the computer having a memory in which a non-parameter is recorded linearity resulting from a non-linear resonance spectrometry analysis carried out, in accordance with step c), from an optimal stress mode determined in accordance with steps a) and b), for a reference part identical to the part to be tested but free from defects, the computer being programmed to - carrying out a said non-linear resonance spectrometry analysis for the part to be tested, from the optimal stress mode in accordance with step c), so as to determine the non-linearity parameter for said part to be tested, then - determine a difference between the non-linearity parameters for the part to be tested and for the reference part, then - classify the part to be tested according to said difference.

[0020] In a preferred embodiment, the non-linearity parameter for the reference part is determined with the computer, according to steps a) and b).

[0021] Of course, the optional characteristics of steps a) to d) described for a method according to the invention are applicable to the corresponding steps implemented by the computer.

[0022] Preferably, the operator proceeds as follows: - carrying out steps a) and b) and, for the reference part, step c); - positioning of the resonator and receiver on the injection and output zones, respectively; - injection of the excitation wave so as to stress the part to be tested according to the optimal stress mode; - launching a computer program to carry out step c) for the part to be tested, then step d). Brief description of the figures

[0023] Other characteristics and advantages of the invention will become apparent upon reading the detailed description which follows and upon examining the attached drawing in which: - [Fig.l] [Fig.l] is a photo of a ceramic piece of complex shape, in this case a photo of a piece of the front bowl of a glass furnace; - [Fig.2] [Fig.2] represents, for a rectangular parallelepiped part, the enriched models for six resonance modes; - [Fig.3] [Fig.3] schematically illustrates an optimal method of stressing said rectangular parallelepiped part, as well as a detection device according to the invention; - [Fig.4] [Fig.4] represents a part of a frequency spectrum for the output wave received in the output zone in the optimal stress mode of [Fig.3]; - [Fig.5] [Fig.5] illustrates the evolution of the frequency shift, in ordinates, as a function of the amplitude of the output wave, for eight corundum pieces with bonding nitride of the same shape, starting, to carry out the analysis by non-linear resonance spectrometry, from a non-optimal stress mode, according to the prior art, (left graph) and from an optimal stress mode, according to the invention (right graph, the two ovals in broken lines grouping the parts presenting a defect (“crack”) and the parts not presenting a defect (“sound”), respectively; - [Fig.6] [Fig.6] provides the non-linear parameters defined from the left graphs (left graph of [Fig.6]) and right graphs (right graph of [Fig.6]) of [Fig.5], respectively; - [Fig.7] [Fig.7] represents, for the part object of the first example, the enriched models for four resonance modes; - [Fig.8] [Fig.8] illustrates the selectivity that a method according to the invention allows, as described for the second example.

[0024] In the various figures, identical references are used to designate identical or similar members. Definitions

[0025] The amplitude of a wave is classically the height of the main peak of a frequency spectrum of said wave, the main peak being the highest peak.

[0026] Unless otherwise indicated, all percentages relating to compositions are percentages by mass.

[0027] Total porosity is typically equal to 100 x (absolute density - apparent density) / absolute density.

[0028] Apparent density measurements are carried out according to ISO5017 on a bar taken from the heart of the part, in the healthy zone. Absolute density is conventionally measured on ground powder, using a helium pycnometer.

[0029] “Behave”, “present” or “understand” must be interpreted in a manner broad, not restrictive. Detailed description

[0030] A method according to the invention can be implemented to detect a defect in any part to be tested. It is particularly well suited to a part to be tested comprising, preferably made of an inorganic material, preferably a sintered metal, a ceramic or a glass-ceramic.

[0031] Preferably, the part to be tested is made of a refractory material, that is to say a non-metallic inorganic material, for example molten or sintered.

[0032] Preferably, the modulus of elasticity (MOE) of the part to be tested is between 1 and 500 GPa, preferably between 5 and 100 GPa, at room temperature.

[0033] Preferably, the apparent density of the part to be tested is between 0.5 and 10 g / cm3, preferably between 1 and 6 g / cm3.

[0034] The part to be tested may be a single piece or be a rigid assembly of blocks.

[0035] It may in particular weigh more than 0.5 kg, more than 1 kg, more than 5 kg and / or less than 1000 kg, less than 100 kg, less than 50 kg or less than 20 kg.

[0036] The part to be tested is preferably chosen from: - a groove block or lintel, - a tank block, - a brick or block of wall or side, - a corner or angle block, - a nose brick, - a block or a sole slab, - a brick or a vault base, - a brick or block surrounding the nozzle, - a hole brick or casting channel, - an electrode holder block, - a piece of refractory beak from a glass furnace, - an injector block, - a glass furnace throat, - a part of a heat exchanger of the furnace, - a refractory plate or tile of a boiler lining, - a protective shell for an incinerator heater tube, - a tile from an incinerator, - a ceramic part of a solar absorber, - a tile or a protective part of a combustion turbine chamber.

[0037] The defect is a local modification, most often unwanted, of: - the geometry of the part to be tested; or - the chemical composition of the part to be tested; or - the microstructure of the part to be tested.

[0038] The defect may be, in particular, an empty space or a space filled with a material different from the rest of the part. It may be, for example, a crack, a shrinkage, or an inclusion.

[0039] The defect can also be a chemical or structural variation, in particular crystallographic.

[0040] The dimensions of the defect can be any. In particular, its largest dimension (length) can be between 1 μm and 100 mm.

[0041] Before step a), the method may include determining the region of interest, i.e. a region internal to the part to be tested and in which the probability of occurrence of a defect is high.

[0042] In particular, the experiment may highlight a region of the parts in which defects appear repeatedly. Such a region may be chosen as a region of interest. Determining the presence of a defect in a part may require destruction of the part, for example by cutting and observation along several section planes. A non-destructive method, such as radiography or tomography, may also be used to determine the presence of a defect, if the dimensions of the part allow it.

[0043] It is also possible to carry out a local analysis, for example a local elementary chemical measurement on a sample taken from the part, for example by means of a microscope equipped with an EDS or EDX probe or by means of a phase analysis, for example by X-ray diffraction.

[0044] The region of interest can have any shape.

[0045] When the general shape of the defect to be detected is known, the region of interest is preferably determined so as to encompass the defect, preferably as precisely as possible.

[0046] The volume of the region of interest is preferably greater than that of the defect. It is preferably less than 5 times, 3 times, 2 times or 1.5 times the volume of the defect.

[0047] When the shape of the defect to be detected is known, the region of interest preferably has a similar shape, in which the defect extends.

[0048] The volume of the region of interest is preferably less than 1000 cm3 and / or greater than 0.1 cm3.

[0049] The volume of the region of interest is preferably less than 0.5 times, preferably less than 0.3 times, preferably less than 0.2 times, preferably less than 0.1 times, preferably less than 0.05 times, and / or greater than 0.01 times the volume of the part to be tested.

[0050] In one embodiment, the region of interest is delimited by a volume of revolution in which the defect is inscribed. Depending on the shape of the defect, the volume of revolution may be a cylinder (in the case of an elongated defect in a single dimension of space) or a sphere.

[0051] In step a), a reference part is analyzed to search for an optimal resonance mode.

[0052] The reference part is a part identical to the part to be tested, except, possibly, in the region in which the defect is sought, i.e. in the region of interest. By "identical" is meant "having substantially the same geometry (i.e. the same shape), to the nearest manufacturing tolerances, and made of the same material (same composition, same microstructure), to the nearest variations due to the raw materials and the conduct of the manufacturing process". The dimensional manufacturing tolerances are conventionally less than one millimeter in the refractories, and less than 100 microns in the field of technical ceramics. Composition tolerances are typically + / -5% for a component with a content greater than 30% by mass, and less than + / - 1% for another component.

[0053] The reference part being of the same general shape as the part to be tested, for the sake of clarity, the equivalent region of the reference part is also called "region of interest", that is to say the region which would be superimposed on the region of interest of the part to be tested if the latter could be "superimposed" with the reference part to be confused with it.

[0054] In step a1), preferably with suitable software and a computer, resonance modes of the reference part are determined, considering that the modulus of elasticity of the reference part is constant (linear acoustics). Each resonance mode defines a resonance frequency of the reference part and a mechanical stress field generated when the reference part is in resonance at said resonance frequency.

[0055] Preferably, conventional software is used, for example ComSol® or Abacus®.

[0056] Conventionally, a digital model of the reference part is first generated. Such a model is a three-dimensional representation, in space, of the reference part. The dimensions and geometry of the reference part are defined by a mesh of points, or "voxels".

[0057] Preferably, the mesh density or "volumetric density of the mesh points" is defined so that at least one point of the mesh is present in the region of interest.

[0058] Preferably, at least the apparent density of the material constituting the reference part, the modulus of elasticity of said material and its Poisson's ratio are entered into the software.

[0059] A person skilled in the art knows how to determine the apparent density. For example, for a material made of ceramic, metal or glass, it can be measured directly on the reference part or on a sample of the part according to the ISO 5017 standard. The modulus of elasticity can be measured by dynamic tests, preferably according to the ASTM C1259-01 standard, in particular for a ceramic reference part for which the Poisson's ratio is generally equal to 0.2.

[0060] Conventionally by a finite element calculation, the software determines, at a plurality of points of the reference part, preferably at least on the surface of the reference part, the mechanical stresses, in particular in traction and / or in tension, and / or in compression and / or in torsion and / or in shear, in different directions. Alternatively, or in addition, the software can determine an equivalent property to mechanical constraints, such as deformation.

[0061] The software can calculate the absolute value of a mechanical stress, or a relative value calculated with respect to the value at a predefined location on the reference part. Furthermore, at a point on the part, the mechanical stresses vary over time under the effect of the excitation wave. The software can, for example, provide the average value or the maximum value of a mechanical stress or the amplitude of its variation.

[0062] Preferably, the software presents the mechanical constraints in graphical form, preferably in the form of an "enriched" model, so the appearance, for example the color, depends locally on the local mechanical constraint. A view of an enriched model is therefore a map representing a stress field.

[0063] In one embodiment, the local mechanical constraints are only represented on the surface of the model. In one embodiment, the software makes it possible to generate section planes and to visualize the local mechanical constraints in the section plane.

[0064] The number of resonance modes determined may be greater than 3, 5, 10 and / or less than 30 or 20. Conventionally, it is possible to enter this number into the software so that it provides the main resonance modes.

[0065] In step a2), the “optimal” resonance mode is selected from among the resonance modes determined in step a1), which generates, in the region of interest, a maximum mechanical stress. In other words, in the region of interest, it is the optimal resonance mode which produces the highest mechanical stress (or mechanical deformation) compared to those produced in the other resonance modes in said region of interest.

[0066] Advantageously, a simple visual comparison of the different enriched models, obtained for the different resonance modes, makes it possible to immediately identify the resonance mode generating maximum stresses in the region of interest of the reference part, and therefore to determine the associated resonance frequency, called “optimal”.

[0067] Several resonance modes can generate maximum stresses in the region of interest of the reference part. In this exceptional situation, the optimal resonance mode is preferably the resonance mode which, in practice, is the easiest to implement.

[0068] The inventors have discovered that steps a1) to a2) make it possible to define an optimal resonance mode which, by means of known NRUS or NRAS type non-linear resonance spectrometry analyses, can detect a defect in the part to be tested in a particularly reliable manner.

[0069] [Fig.2] represents, for six resonance modes, the enriched models cor- corresponding obtained with the ComSol® software, for a rectangular parallelepiped block 2. Each enriched model represents, in shades of gray, the local deformation on the surface of the block, for a respective resonance mode. The darker the gray, the greater the deformation. In the same region, we immediately see that the local deformations, and therefore the local mechanical stresses, vary considerably depending on the resonance mode considered.

[0070] For a region of interest in the center of the block, it immediately appears that the first resonance mode (top left model) is the optimal resonance mode.

[0071] In step b), an optimal stress mode is first determined, mainly activating said optimal resonance mode.

[0072] A stress mode defines conditions under which a part is made to vibrate, i.e. specifies, for a vibration regime, the excitation conditions necessary to obtain this vibration regime. A stress mode can thus specify - an injection zone, i.e. a zone on the surface of the part through which an excitation wave penetrates into the part, - an output zone, that is to say an area on the surface of the part by which the excitation wave is received after having passed, at least partially, through the part, this wave then being called an “output wave”, and - the characteristics of the excitation wave, which, when injected through the injection zone, lead to the vibration regime.

[0073] [Fig. 3] represents a detection device 3 according to the invention, in a service position. This device comprises a computer 4, a resonator 5, in the form of a hammer, and a receiver 6. This figure illustrates an optimal stress mode for the block 2 of [Fig. 2]. We distinguish the excitation zone 7 on which an impact, symbolized by the arrow, is produced, the output zone 8 and the region of interest 9.

[0074] The characteristics of the excitation wave are not limited. The excitation wave may in particular be a pulse, a succession of pulses, a periodic or non-periodic signal, or a succession of such signals.

[0075] It may be a sinusoidal wave, for example injected by means of a piezoelectric disk, as described in US6330827B1.

[0076] A conventional spectral analysis of the output wave generates a frequency spectrum comprising a set of peaks. The top of the highest peak, or "main peak" is located at a so-called "resonance" frequency.

[0077] A stress mode “activates” a resonance mode when the frequency spectrum of the output wave includes a peak at the resonance frequency of this mode. A stress mode can theoretically activate only one resonance mode. In practice, and in particular for a part of complex shape, a stress mode activates several resonance modes, more or less intensely. A mode of solicitation “primarily activates” a resonance mode when, on the frequency spectrum, the highest peak is at the resonance frequency of this resonance mode.

[0078] The excitation wave is preferably chosen to activate primarily the optimal resonance mode.

[0079] The optimal resonance frequency depends in particular on the dimensions and shape of the part. Preferably, the excitation wave is chosen so that its frequency spectrum has a main peak centered on a main frequency greater than 1 Hz, preferably greater than 50 Hz, preferably greater than 100 Hz, and / or less than 100 KHz, preferably less than 50 KHz.

[0080] Preferably, the excitation wave is chosen so that the main peak of the frequency spectrum of the output wave is centered on a main frequency greater than 1 Hz, preferably greater than 50 Hz, preferably greater than 100 Hz, and / or less than 100 KHz, preferably less than 50 KHz.

[0081] The resonator for injecting excitation waves may be, for example, a hammer, a vibrating pot, or a piezoelectric transducer. The resonator may also be spaced from the part, for example, an audio speaker. The resonator may be a heating device for injecting an excitation wave in the form of a thermal flash or a laser wave.

[0082] In practice, the excitation wave can be injected into the part by impact, for example by a series of impacts in the injection zone, or "impact zone". An impact advantageously makes it possible to inject an excitation wave having a wide frequency spectrum.

[0083] A hammer made of a hard material, for example silicon carbide, makes it possible to inject an excitation wave whose frequency spectrum has high peaks for high frequencies. Such a hammer is well suited for small parts.

[0084] A hammer made of a soft material, for example rubber, makes it possible to inject an excitation wave whose frequency spectrum has high peaks for low frequencies. Such a hammer is well suited for large parts.

[0085] Simple tests allow you to choose a suitable hammer.

[0086] The receiver arranged in the output zone to capture the output wave may be, for example, a detection or reception transducer, in particular a microphone, an accelerometer, a high-speed camera or a laser.

[0087] Preferably, the excitation wave is a periodic wave having the optimal resonant frequency, or a wave whose main frequency is the optimal resonant frequency.

[0088] Simple tests, optionally computer-simulated, make it possible to determine a stress mode which mainly activates the optimal resonance mode, i.e. the optimal stress mode.

[0089] To guide these tests, the exit zone can initially be moved as far away as possible from the injection zone, but these zones can be placed in zones in which the mechanical stresses are high. By moving the injection zone, the evolution of the size of the peaks on the successive frequency spectra can be observed. It is therefore sufficient to move the injection zone until the height of the peak centered on the optimal resonance frequency is greater than the height of the other peaks. Preferably, this movement is continued to a position in which this peak is as detached as possible from the other peaks, i.e. to a position activating the other resonance modes as little as possible.

[0090] Preferably, the injection zone is chosen to be as close as possible to the region of interest.

[0091] The direction of injection of the excitation wave, or "impact direction", is preferably substantially parallel to the main direction of deformation in the optimal resonance mode.

[0092] When the region of interest has a generally flattened shape, the impact direction is preferably substantially perpendicular to the general plane along which the region of interest extends. Preferably, it forms, with this general plane, an angle of less than 30°, preferably less than 20°, preferably less than 10°. For example, to detect a crack, the impact direction is preferably perpendicular to the plane of the crack.

[0093] Preferably, the part to which a stress mode is applied is arranged on a support configured so as to remain as close as possible to the free vibration conditions and to minimize the effects of weight and contact with the support. Preferably, the part is placed on foam. Preferably, the part rests on its most rigid parts. For example, if the part is a front body bowl (CAC), it is preferable for it to rest on its bottom. More preferably, the support points on the support are at vibration nodes.

[0094] [Fig.4] represents the frequency spectrum of the output wave in the optimal stress mode illustrated in [Fig.3]. In particular, the main peak 10 centered on the optimal resonance frequency of 6,496 kHz of the resonance mode represented in [Fig.3] can be distinguished.

[0095] In one embodiment, steps a) and b) are carried out simultaneously, the search for the optimal stress mode being carried out in the following manner: - numerical simulation of a plurality of stress modes, the numerical simulation determining for each stress mode, a stress field mechanical in the reference part and a theoretical output wave; - analysis of mechanical stress fields and theoretical output waves in order to select, as the optimal stress mode, the stress mode generating, in the region of interest, maximum mechanical stress and putting the reference part into resonance.

[0096] The reference part is considered to be in resonance if the frequency spectrum of the output wave shows a main peak whose height (amplitude of the output wave) is at least 1.5 times, preferably at least twice, preferably at least three times greater than that of the other peaks.

[0097] It is possible to use software capable, from a model of the reference part, of calculating the mechanical stress fields and of simulating an output wave in an output zone under the effect of a simulation of an excitation wave in an excitation zone of said reference part.

[0098] In step c), an analysis is carried out of the output waves obtained in the optimal stress mode for the reference part and the part to be tested. The same analysis, by non-linear resonance spectrometry, is carried out for both parts. This analysis is based on the principles of the NRAS or NRUS methods.

[0099] This analysis is conventional and known to those skilled in the art. It makes it possible to determine a non-linearity parameter representative of the variation in the modulus of elasticity of the part when the amplitude of the excitation wave of the optimal stress mode determined in step b) is modified.

[0100] US6330827B1 provides details of an ul- resonance spectrometry analysis non-linear trasonic.

[0101] The amplitude of the output wave follows the evolution of the amplitude of the excitation wave. Varying the amplitude of the excitation wave is therefore equivalent to varying the amplitude of the output wave. The output wave is however easier to analyze and therefore used in practice.

[0102] In practice, we therefore choose a non-linearity parameter representative of a variation in the resonance frequency of the part when the amplitude of the output wave varies.

[0103] Preferably, the non-linearity parameter is representative of a variation of a frequency shift of the resonant frequency relative to the optimal resonant frequency f0, when the amplitude of the output wave varies from the optimal stress mode.

[0104] In practice, the amplitude of the excitation wave is preferably gradually increased to limit the negative influence of hysteresis phenomena. The initial amplitude of the excitation wave or the output wave is preferably chosen to be as low as possible, but sufficient so that the noise measured in the output wave is relatively low. tively negligible, for example to be greater than 2 or 3 times the noise amplitude. It is then increased, preferably to a final amplitude at least 2 times, preferably at least 3 times, or even at least 5 or 10 times greater than the initial amplitude.

[0105] The frequency shift is preferably, for an amplitude of the output wave, the ratio of the absolute value of the difference between the optimal resonant frequency fo and the resonant frequency f determined for said amplitude, divided by f0, i.e. lf0 - fl / f0. The resonant frequency f can be determined conventionally by spectral analysis, being the frequency of the highest peak.

[0106] The frequency shifts are preferably calculated as described by US6330827B1.

[0107] The non-linearity parameter is preferably the slope of a straight line representative of the evolution of the frequency shift when the amplitude of the output wave varies, in particular linearly or logarithmically. Said straight line is preferably the straight line which defines the general direction of the broken line which connects a plurality of points each giving an amplitude of the output wave and the corresponding frequency shift. It is preferably determined by linear regression.

[0108] The slope of this line is affected by the presence of a defect in the part, which is used to determine the presence of a defect.

[0109] The non-linearity parameter, in particular said slope, depends not only on the dimensions, shape and constituent material of the part analyzed, but also on the dimensions and nature of the defect.

[0110] A difference between the non-linearity parameters determined for the reference part and for the part to be tested thus makes it possible to detect the presence of a defect in the part to be tested. The inventors have discovered that a method according to the invention makes it possible to increase the reliability of this detection.

[0111] In step d), this difference is used to detect the possible presence of a defect, for example a crack.

[0112] The inventors have in fact noted that the non-linearity parameter is particularly different depending on whether or not a part has a defect in the region of interest.

[0113] The part to be tested can thus be classified into the category of parts having a defect in the region of interest, or into the category of parts not having a defect in the region of interest. In one embodiment, it is discarded if it has a defect.

[0114] According to the experience acquired by the inventors, the slope, in absolute value, determined for a part to be tested presenting a defect, is higher by more than 50%, or even more than 100%, than that measured on a part to be tested not presenting one.

[0115] In one embodiment, a maximum slope and / or a minimum slope are determined by testing to delimit an acceptance range of the part to be tested. Examples

[0116] The following examples are provided for illustrative purposes and do not limit the invention.

[0117] According to a first example, the part to be tested is a bowl of a glass furnace forebody 12, made of sintered refractory based on zirconia and silica alumina grains (see [Fig.l]). Tests have shown that this type of bowl can have radial cracks, not visible from the outside, extending from the tap hole 14.

[0118] The region of interest was therefore defined as a region extending to the periphery of the tap hole.

[0119] A model of the reference part was created using ComSol® software. The mesh of the model was determined to include at least one point in the region of interest.

[0120] The apparent density, the modulus of elasticity at 20°C and the Poisson's ratio of the refractory material constituting the reference part are respectively 2.6 Kg / m3, 38 GPa and 0.2. These data were entered into the software.

[0121] The software was then configured so that the surface of the enriched model represents the Von Mises stresses. This enrichment makes it possible to highlight local traction / compression phenomena as well as shear stresses.

[0122] The software then determined four resonance modes and the corresponding resonance frequencies, namely 280 Hz, 366 Hz, 876 Hz, and 1040 Hz. The corresponding enriched models are shown in [Fig.7]. The surface of the model is all the darker as the local mechanical stresses (or equivalently the local deformation) are high. The enriched model of resonance mode No. 3 is the one that shows the greatest amplitude of deformation around the tap hole, in the region of interest. Resonance mode No. 3 is therefore optimal.

[0123] Among the other resonance modes, resonance mode No. 4 is the one with the highest resonance frequency and leads to the highest mechanical stresses. However, it is not optimal according to the invention, because these mechanical stresses are not localized in the region of interest. It is chosen as a comparative example.

[0124] In [Fig.7], reference 7 indicates the injection zone and reference 8 indicates the output zone. The excitation waves were injected by striking the injection zone using an acoustician's hammer. The output wave was captured with an accelerometer in the output zone 8.

[0125] The stress mode for resonance mode No. 3 was applied in 8 parts of the same production.

[0126] For each part, several tests were carried out, each time modifying the amplitude of the excitation wave, and therefore the amplitude of the output wave. A frequency spectrum (amplitudes according to the frequencies) was produced for each output wave received. The resonance frequency was determined from this spectrum.

[0127] We then calculated, for each part and for each output wave amplitude, the frequency shift and plotted a point giving the frequency shift as a function of the output wave amplitude ([Fig.5]). Then, by linear regression with the points thus determined for the same part, we determined a straight line and the slope of this straight line.

[0128] This slope is the non-linearity parameter NL of the resonance frequency shift (f-fo) / fo as a function of the amplitude of the output wave. It is shown in [Fig.6].

[0129] The stress mode for resonance mode No. 4, i.e. mainly activating said resonance mode No. 4, was then carried out by striking the lateral part 7 of the part using the acoustician's hammer. The output wave was captured in the lower left part 8 of the part.

[0130] The nonlinearity parameter was then determined for each part, as previously described for optimal resonance mode #3.

[0131] Each piece was then cut in order to observe its internal structure and check for the presence or absence of cracks.

[0132] Figures 5 and 6 illustrate the results obtained, the left graph corresponding to resonance mode No. 4 and the right graph corresponding to optimal resonance mode No. 3. In [Fig. 5], the amplitudes of the output waves are provided on the abscissa and the frequency shifts on the ordinate. In [Fig. 6], the sample numbers are provided on the abscissa and the non-linearity parameters on the ordinate.

[0133] These figures show that the invention considerably improves the discrimination between parts containing cracks and those without them.

[0134] According to a second example, E1-E5 refractory blocks, substantially rectangular parallelepipeds, 95 mm thick, approximately 135 mm wide and 500 mm long, made of Corundum bonded by a silicon nitride phase, were manufactured.

[0135] Samples taken allowed the following observations to be made:

[0136] • Block El had an internal structure without any visible internal defects. • Blocks E2, E3 and E4 had the same external appearance as block El, but an internal structure with a gray or black core, which does not pose a problem in application. • Block E5 had the same external appearance as block El, but a internal structure with a black or gray core and internal cracks likely to weaken the product in service.

[0137] The region of interest is therefore the heart of the block, likely to be cracked.

[0138] The characterization of the 5 blocks shows that they have an apparent density similar of 3.2+ / -0.1 g / cm3, a modulus of elasticity of approximately 50 GPa for a Poisson's ratio of 0.2. Blocks E2, E3 and E4 have a silicon metal content (chemical analysis from samples taken from the core of the block) of approximately 1%. Blocks El and E5 have a residual Si content of less than 0.5% by mass.

[0139] A numerical simulation using ComSol® software carried out as described in the previous example made it possible to identify the optimal resonance mode, in this case a flexural resonance mode.

[0140] Each block was then stressed according to an optimal stress mode, the excitation wave being injected by the impact of a hammer at one end of the block, on the edge of a large face, the receiver, in this case a microphone, being positioned at the opposite end. As for the previous example, the amplitude of the excitation wave was then increased to determine the non-linearity parameters.

[0141] The results have been reported in Table 1 below.

[0142] Each block has its own acoustic signature, and in particular a main resonance frequency (frequency of the highest peak on the frequency spectrum of the output wave) which is specific to it. It is observed that the main resonance frequency alone, even in the optimal stress mode, does not allow the E5 block to be specifically detected.

[0143] On the other hand, we observe that the non-linearity parameter NL makes it possible to specifically detect the E5 block because the analysis by non-linear resonance spectrometry was carried out from the optimal resonance mode, focused on the core of the blocks.

[0144] [Tables 1] Block El E2 to E4 ES First- resonance frequency (Hz) 1.050 moâuso 1150 Non-linearity parameter NL 5 6 to 10 18 Observation No visible defect “Black heart” “Black heart” with cracks

[0145] Samples were then taken and showed that this monitoring makes it possible to very selectively exclude cracked blocks, which all have a non-linearity parameter greater than the threshold of 15. On the other hand, the uncracked blocks all have a non-linearity parameter lower than the threshold of 15.

[0146] As is now clearly apparent, thanks to the invention, no complex and expensive tests, destructive or not, are necessary to reliably determine the presence of a defect in the region of interest.

[0147] Advantageously, unlike methods using radiography or X-ray tomography in particular, a method according to the invention can be implemented on a production line in a simple manner and at a reduced cost.

[0148] Of course, the invention is not limited to the embodiments described in detail above. In particular, it can be implemented to detect several defects in the same part to be tested.

Claims

Claims

1. Method for detecting a defect in a region of interest (9) within a part to be tested, said method comprising the following successive steps: a) for a reference part (2) identical to the part to be tested but devoid of defects, a1) determining a set of resonance modes each defining: - a resonance frequency of the reference part considering that the modulus of elasticity of the reference part is constant, and - a field of mechanical stresses or deformations on and / or in the reference part generated when the reference part is in resonance at said resonance frequency; a2) selection of the resonance mode, referred to as the "optimal resonance mode", generating, in the region of interest, a maximum mechanical stress or deformation compared to the other resonance modes;b) determining a solicitation mode, called "optimal", mainly activating said optimal resonance mode, a solicitation mode defining at least one excitation wave, an injection zone of the excitation wave into the reference part, and an output zone where an output wave resulting from the crossing of the excitation wave from the injection zone to the output zone is captured; c) analysis by non-linear resonance spectrometry from the optimal solicitation mode, so as to determine a non-linearity parameter for each of said test and reference parts; d) classification of the test part according to the difference between the non-linearity parameters for the test part and for the reference part.;

2. Method according to the preceding claim, in which, in step a1), the resonance modes are determined by numerical simulation, the modeling of the reference part taking into account the dimensions and geometry of the reference part, the apparent density of the material constituting the reference part, the modulus of elasticity of said material, and the Poisson's ratio of said material.

3. Method according to any one of the preceding claims, in which, in step a2), three-dimensional digital models are compared- sionals of the reference part each representing a field of said mechanical stresses or deformations generated when the reference part is in resonance at a respective resonance frequency.

4. A method according to any preceding claim, wherein the volume of the region of interest is less than 0.2 times and greater than 0.01 times the volume of the reference part.

5. Method according to any one of the preceding claims, in which steps a) and b) are carried out simultaneously, the search for the optimal stress mode being carried out in the following manner: - numerical simulation of a plurality of stress modes, the numerical simulation determining for each stress mode, a mechanical stress field in the reference part and a theoretical output wave; - analysis of the mechanical stress fields and the theoretical output waves so as to select, as the optimal stress mode, the stress mode generating, in the region of interest, a maximum mechanical stress and putting the reference part into resonance.

6. Method according to any one of the preceding claims, wherein, in step c), the non-linearity parameter is the slope of a straight line representative of the evolution of a frequency shift as a function of the evolution of the amplitude of the output wave when said amplitude of the output wave is modified, preferably increased, from the optimal stress mode, the frequency shift, for an amplitude of the output wave, being the ratio of the absolute value of the difference between the optimal resonance frequency (f0) in the optimal resonance mode and the resonance frequency (f) determined for said amplitude of the output wave, divided by the optimal resonance frequency (f0).

7. Method according to the immediately preceding claim, in which, in step d), the non-linearity parameter of the part to be tested is compared to a threshold determined from the non-linearity parameter of the reference part, then the part to be tested is classified according to the difference between the non-linearity parameter of the part to be tested and the threshold, preferably according to the sign of said difference.

8. A method according to any preceding claim, wherein the defect is an empty space within the test piece, or a space filled with a material different from the rest of the test piece.

9. A method according to any preceding claim, wherein the part is made of an inorganic material.

10. A method according to any preceding claim, wherein the part is made of a metal, a ceramic material, a glass-ceramic material or a mixture of these materials.

11. A method according to any preceding claim, wherein in the optimal loading mode the main peak of a frequency spectrum of the output wave is at a frequency between 1 Hz and 100 KHz.

12. A method according to any preceding claim, wherein the output source is an acoustic wave.

13. Method according to any one of the preceding claims, in which said part to be tested is chosen from: - a throat block or lintel, - a tank block, - a wall or side brick or block, - a corner or angle block, - a nose brick, - a sole block or slab, - a vault brick or base, - a tuyere surrounding brick or block, - a tap hole or channel brick, - an electrode holder block, - a refractory nozzle part of a glass furnace, - an injector block, - a glass furnace throat, - a part of a heat exchanger of the furnace, - a refractory plate or tile of a boiler lining, - a protective shell of a heater tube of an incinerator, - a tile of an incinerator, - a ceramic part of a solar absorber, - a protective tile or part of a turbine chamber of combustion.

14. Method for sorting externally identical parts manufactured on a production line, in which a method according to any one of the preceding claims is implemented for each part, considered as a part to be tested, steps a) and b) and analysis by non-linear resonance spectrometry from the stress mode optimal carried out on the reference part being preferably common to all the parts.

15. A detection device for detecting a defect in a part to be tested, the device comprising: - a resonator (5) capable of injecting, into the part to be tested, an excitation wave through an injection zone (7) of the part to be tested; - a receiver (6) capable of capturing an output wave through an output zone of the part to be tested, the output wave resulting from the passage of the excitation wave through the part to be tested; - a computer (4) connected to the receiver so as to receive the output wave, the computer having a memory in which is recorded a non-linearity parameter resulting from an analysis by non-linear resonance spectrometry carried out, in accordance with step c) of a method according to any one of the preceding claims, from an optimal stress mode determined in accordance with steps a) and b) of said method according to any one of the preceding claims, for a reference part identical to the part to be tested but free from defects, the computer being programmed to - performing a said non-linear resonance spectrometry analysis for the part to be tested, from the optimal stress mode in accordance with step c), so as to determine the non-linearity parameter for said part to be tested, then, in accordance with a step d) of said method according to any one of the preceding claims, - determining a difference between the non-linearity parameters for the part to be tested and for the reference part, then - classify the part to be tested according to said difference.

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