Method and system for analyzing a test piece using a vibration response signal

Abstract

The present invention relates to a method for acoustically measuring material properties of a test piece at high temperatures, comprising the following steps: a. heating the test piece to a test temperature range; b. performing background measurements within the test temperature range by collecting vibration signals of the test piece during a calibration period, thereby obtaining a noise signal; c. performing acoustic measurements on the test piece within the test temperature range and during a test period by: c. applying a vibration excitation to the test piece; c. collecting vibration signals of the test piece during the test period, thereby obtaining a vibration response signal to the vibration excitation; and d. obtaining material properties of the test piece by analyzing the vibration response signal, thereby taking the noise signal into account. The present invention also relates to a system for acoustically measuring material properties of a test piece at high temperatures.

Description

Technical Field

[0001] The present invention relates to a method and system for analyzing a test piece using sound waves or ultrasonic waves. The method is particularly suitable for detecting defects and anomalies in solids and for characterizing the E modulus, G modulus and Poisson's constant.

[0002] The present invention also relates to a method and a system for obtaining thermal expansion parameters of a test piece. Background Art

[0003] Solids can be tested in a variety of ways. Initial testing often involves visual inspection to ensure the solid is the correct size and shape, and to check for surface defects. However, in many cases, the properties of a solid depend significantly on its internal structure, which may not be visible. For example, the strength of metals and alloys depends significantly on the type and number of certain macroscopic defects. These defects can be small or large and may result from natural causes, production methods, wear, accidents, and other factors.

[0004] Regardless of how the defect occurs, in some cases it is important to understand the state of the solid with respect to its defect. To analyze solid defects, either invasive or non-invasive methods can be used on a test piece of the solid. Non-invasive methods thus allow analysis without destroying or altering the test piece. Therefore, non-invasive methods are often used for test pieces that will be used again later or that require further testing (which may itself be invasive or non-invasive).

[0005] A non-invasive testing method for solids uses vibrations to analyze the test piece. The test piece is therefore subjected to controlled vibrations that can propagate through the solid and thereby be transmitted, reflected or absorbed. The controlled vibrations can be induced by means of an impact excitation technique (IET), whereby the test piece is positioned in such a way that it vibrates essentially unhindered when impacted. In the impact excitation technique, the material properties of the test piece are determined by impacting the sample using a special tool or a projectile and analyzing the resulting vibrations when these are picked up by a vibration signal measuring sensor (such as a piezoelectric sensor, a microphone, a laser vibrometer or an accelerometer). Vibrations passing through solids are also referred to as sounds or sound waves, and the measurement is also referred to as acoustic measurement. Therefore, in the context of the present application, a vibration signal measuring sensor is also referred to as an "acoustic sensor."

[0006] International application WO 2019 / 020825 Al has described an apparatus for testing solids using sound. This document discloses an apparatus for analyzing the mechanical vibration response of a solid material sample, the apparatus comprising: an array of impactors arranged to apply impacts to well-defined points on the surface of the solid material sample; a sensor configured to capture the mechanical vibration response as a time-varying signal after the impact of at least one impactor; and a processing device configured to analyze the time-varying signal to determine the frequency and attenuation constant of the sinusoidal waves constituting the time-varying signal. The present invention also relates to a corresponding method for characterizing a solid material sample.

[0007] Analysis of the response of a solid test piece to a vibration excitation typically involves extracting one or more, and preferably all, of the following parameters:

[0008] - Young's modulus (E), which indicates the tensile elasticity of the test piece;

[0009] - shear modulus (G), which indicates the response of the test piece to shear stress;

[0010] - Poisson's ratio (v), which indicates the deformation of the test piece in a direction orthogonal to the applied uniaxial stress;

[0011] These properties often depend on the frequency or frequency range of the vibration excitation. Therefore, a test piece may have multiple resonant frequencies, depending on its different vibration modes. Important modes are bending and torsional modes. The parameter values ​​at these resonant frequencies are crucial numbers used to describe the test piece's behavior.

[0012] In many applications, solid components (for example in machinery) are used at different temperatures. As a result, the operating temperature can fluctuate rapidly and / or over a wide temperature range. For example, brake discs in vehicles heat up significantly and rapidly during operation (from basic ambient temperature to over 500°C in a few seconds). Another example is the kerosene injection nozzle of an aircraft nozzle, which can heat up to over 1500°C during operation. High temperature ranges can significantly affect the material properties of the component, and in particular the elastic properties, such as the above-mentioned Young's modulus, shear modulus and Poisson's ratio. For example, it can generally be expected that solids become more flexible at higher temperatures due to the weakening of the average internal bonds, which basically means that one expects E and G to gradually decrease with increasing temperature.

[0013] Solid components can have different material properties at different temperatures. This can affect the component's functionality. It's important to identify the temperature or temperature range where the component's normal or intended function is impeded. This could be due to bulk properties of the material, defects that may arise due to changing temperatures, defects that become more significant at higher temperatures, variability in the expansion of different parts of the component, and so on. Specific examples of temperature dependence of component material properties include:

[0014] - general changes in elastic properties which make the component less rigid and thus prevent it from working properly, e.g. a brake disc at high temperatures may become too elastic to brake properly,

[0015] - Phase transitions at a specific temperature or within a specific temperature range may significantly alter the material properties of a component,

[0016] - Chemical reactions can also change the material properties of components. Stoichiometrically different phases may precipitate, for example in alloys, due to, for example, incorrect production methods, and lead to different temperature-dependent expansions at internal material boundaries between materials. This can obviously lead to serious problems.

[0017] - Laminates or other layered assemblies whereby the adhesion of one layer to the next degrades with increasing temperature.

[0018] To test the proper functionality of solid-state components at various temperatures and / or across the entire temperature range of expected operation, it is necessary to measure the material properties of the test piece at various temperatures. The test piece can be a complete component, a portion of a component, or a piece of the same material. Testing is best performed in a controlled environment that allows for at least temperature control. The material properties are then measured at various temperatures.

[0019] The inventors have found that measuring the material properties of a test piece over a wide temperature range (which may span from less than -50°C to over 2000°C) can be very difficult in practice. Creating a temperature-controlled environment for such a temperature range typically requires an oven, which contains many components that generate vibrations. Obviously, the background vibrations generated by the oven may interfere with the measurement process. These background vibrations typically worsen at higher temperatures. In addition, since one might expect certain material properties (e.g., E and G) to decrease with increasing temperature, the signal will also decrease. At high temperatures, both of these effects lead to a lower signal-to-noise ratio (STN). The present invention aims to address the problem of a worsened STN ratio for acoustic measurements of test pieces at high temperatures.

[0020] Furthermore, the present invention provides a relatively inexpensive and accurate method for determining one or more thermal expansion parameters of a material by means of acoustic measurement techniques. Summary of the Invention

[0021] In a first aspect, the present invention relates to a method for acoustically measuring material properties of a test piece at high temperature, comprising the following steps:

[0022] a. Heat the test piece to the test temperature range;

[0023] b. By collecting the vibration signal of the test piece during the calibration period, performing background measurement within the test temperature range to obtain a noise signal;

[0024] c. Perform acoustic measurements on the test piece within the test temperature range and test period by:

[0025] cl. applying vibration excitation to the test piece;

[0026] c2. Collecting vibration signals of the test piece during the test period to obtain a vibration response signal of the vibration excitation; and

[0027] d. Obtain the material properties of the test piece by analyzing the vibration response signal, thereby taking the noise signal into account.

[0028] First, the ambient background noise is taken into account when analyzing the vibration response, leading to a better determination of the material properties. Second, by taking the background noise into account in the same test temperature range as the acoustic measurement, the inventors have found that a better analysis of the vibration response can be performed. This is because the noise may be highly dependent on the temperature range due to, for example, heating elements, ventilators, or any other equipment or equipment components used to perform the measurement, each of which may exhibit different behavior in different temperature ranges.

[0029] The inventors have further discovered that optimal results are achieved if the background measurement is performed by performing all the steps of performing the acoustic measurement, except for applying the vibration excitation to the test piece. Therefore, in a preferred embodiment, step b) of performing the background measurement includes all the steps of step c) of performing the acoustic measurement, except for step c) of applying the vibration excitation to the test piece. This applies to the method described above and to all embodiments of the method described later in this document and in the claims.

[0030] In addition to the method, the present invention also relates to a system for acoustically measuring material properties of a test piece at high temperature, comprising:

[0031] - a heating chamber comprising a heating element for bringing the test piece within a test temperature range;

[0032] - a sensor configured to acquire a vibration signal from the test piece over a period of time;

[0033] an impact system comprising an impactor and an impactor actuator for applying a vibration excitation to a test piece located within the heating chamber;

[0034] - A control system connected to the heating chamber, the sensor and the impactor, the control system being configured to:

[0035] Instruct the heating chamber to place the test piece within the test temperature range;

[0036] During a test cycle, instructing the impact system to actuate the impactor using the impactor actuator so that a vibration excitation is mechanically applied by the impactor to the test piece located in the heating chamber, and acquiring a vibration response signal to the vibration excitation from the sensor,

[0037] During the calibration cycle, instructing the impact system to actuate the impactor using the impactor actuator so that the impactor does not mechanically apply a vibration excitation to the test piece located in the heating chamber and acquiring a noise signal from the sensor, and

[0038] Analyze the vibration response signal to obtain the material properties of the test piece, thereby taking the noise signal into account.

[0039] In a second aspect, the present invention relates to a thermal expansion measurement method for obtaining thermal expansion parameters of a test piece. The thermal expansion measurement method comprises the following steps:

[0040] - extracting at least two resonant frequencies from the experimental data at the first temperature,

[0041] - obtaining a first value of a first dimensional parameter from said at least two resonant frequencies at said first temperature,

[0042] - comparing said first value of said first dimensional parameter at said first measurement temperature with a second value of said first dimensional parameter at said second temperature,

[0043] - calculating a thermal expansion parameter based on said comparison.

[0044] Therefore, the at least two resonant frequencies at the first temperature are material properties of the test piece, which are preferably obtained via the acoustic measurement method described above and further herein. A system for acoustically measuring material properties of a test piece at high temperature, as described above and further herein, can preferably be used to obtain the at least two resonant frequencies.

[0045] BRIEF DESCRIPTION OF THE DRAWINGS

[0046] These and other features and advantages of various embodiments of the present invention will now be described in more detail with reference to the accompanying drawings, in which:

[0047] Figure 1schematically illustrates the vibrations induced in a beam-shaped test piece when excited in a flexural mode;

[0048] Figure 2 schematically illustrates the vibrations induced in a beam-shaped test piece when excited in a torsional mode;

[0049] Figure 3 An embodiment of a measuring system according to the invention is schematically illustrated; and

[0050] Figure 4 An embodiment of the measuring method according to the invention is schematically illustrated.

[0051] Detailed Description of the Invention

[0052] In the known Impact Excitation Technique (IET) procedure, the test piece is positioned so that it can vibrate essentially unhindered when impacted. This can be achieved by placing the test piece on a piece of lightweight foam or by placing it on a linear support (e.g., a wire or a narrow rod) that is strategically positioned below the zero point (node) of the vibration mode to be excited. Excitation is performed by impacting the sample at the anti-node, i.e., the point where the local amplitude of the induced motion for the mode of interest is maximum. Figure 1 The vibrations induced in a beam-shaped test piece when excited in a flexural (particularly out-of-plane) mode are schematically illustrated. The eigenfrequency ff of this vibration mode indicates the dynamic Young's modulus E of the sample. For the illustrated beam with mass m, length L, width b, and thickness t, the following relationship can be used:

[0053]

[0054] The correction factor T is defined as:

[0055]

[0056] Figure 2 The vibrations induced in a beam-shaped sample to be tested when excited in a torsional mode are schematically illustrated. The eigenfrequency ff of this vibration mode indicates the shear modulus of the sample. For the illustrated beam with mass m, length L, width b, and thickness t, the following relationship can be used:

[0057]

[0058] The correction factor R is defined as:

[0059]

[0060] These known methods of determining E and G rely solely on peak frequencies in the sample's response spectrum. The inventors have discovered that while a shift in E and / or G from expected values ​​may in itself indicate the presence of a defect in the sample, an abnormally rapid decay of certain frequency components is a more reliable indicator of such a defect.

[0061] The inventors have discovered that the response of a solid test piece to an impact excitation is physically approximated by a sum of exponentially decaying sinusoids. Based on the underlying physics of this phenomenon, by assuming this particular mathematical form of the excitation response, greater accuracy can be achieved than simply extracting peaks from a (fast) Fourier transform. The decay constant provides more insight into the presence of defects than simply identifying the resonant frequency of the test piece. Therefore, identifying the decay constant constitutes a harmonic inversion problem, which can be solved using known mathematical methods.

[0062] Furthermore, the inventors have discovered that the material properties E and G may vary depending on the temperature of the test piece. This can be observed in acoustic measurements, whereby the material structure of the test piece may vary with temperature and therefore the resonant frequency may shift with temperature. Due to the slow weakening of internal boundaries or the slow growth of volume and surface defects, a slow change in the resonant frequency with temperature may be expected, which would tend to result in a decrease in E and G. However, sudden or unexpected changes in the material properties within a certain temperature range tend to indicate the emergence of new types of defects at that temperature range. Furthermore, even if the material properties vary slowly, the present method and system allow determining at what point the material properties will change to such an extent that the test piece or any component made from the same material or in the same production process as the test piece will not meet the desired specifications.

[0063] In a preferred embodiment, the vibration excitation of step c1 is mechanically directed to the test piece, preferably by using an impactor that is configured to contact the test piece in step c1 and configured not to contact the test piece in step b.

[0064] In one embodiment of the present invention, the impact system includes a ballistic impactor, and the impact actuator is configured to provide a pulse to the ballistic impactor. Preferably, the method of the present invention is performed using the impact system, whereby in the acoustic measurement step, a measurement pulse is provided to the ballistic impactor, which applies a vibration excitation to the test piece, and whereby in the background measurement step, a background pulse is provided to the ballistic impactor, which background pulse does not apply a vibration excitation to the test piece. In this embodiment, the background pulse is preferably as close as possible to the measurement pulse. In one embodiment, the measurement pulse and the background pulse differ in magnitude by at most 2 times, more preferably at most 1.8 times, more preferably at most 1.6 times, but more preferably at most 1.4 times, even more preferably at most 1.2 times, and most preferably at most 1.1 times. Alternatively or additionally, the measurement pulse and the background pulse differ in direction and preferably differ by at most 60°, more preferably at most 45°, more preferably at most 35°, but more preferably at most 25°, even more preferably at most 15°, and most preferably at most 5°.

[0065] In a more preferred embodiment, the ballistic impactor is configured to approach the test piece from below in a vertical direction. Therefore, preferably, the measurement pulse provided to the ballistic impactor is configured to mechanically contact the test piece, and the background pulse provided to the ballistic impactor is configured to be unable to reach the test piece. Thus, in a preferred embodiment, the background pulse is at least 50% of the measurement pulse, more preferably at least 60%, even more preferably at least 70%, yet more preferably at least 80%, and even more preferably at least 90%. It should be noted that the closer the background pulse is to the measurement pulse, the better the noise signal obtained, and the more accurate the material properties obtained.

[0066] Figure 3 , an embodiment comprising a ballistic impactor configured to approach a test piece from below in a vertical direction is illustrated.

[0067] Figure 3 A heating chamber (301) is shown which includes a set of heating elements (302, 303, 304, 305, 306) capable of bringing the heating chamber and / or the test piece to a desired temperature or a desired temperature range. The heating elements (302, 303) can be attached to the walls of the heating chamber and provide heat thereto, for example by heat exchange with a high-pressure fluid at high temperature, by resistive heating, by magnetic induction, etc. The heat from these wall heating elements can be distributed into the heating chamber, for example, by ventilators (304, 305). The ventilators (304, 305) can also be arranged to provide a warm fluid, such as heated air or steam, to the heating chamber. The heating elements can also include radiant heating elements, such as microwave elements (306). The heating chamber preferably also includes one or more thermometers for measuring the actual temperature of the heating chamber and / or the test piece, and preferably also includes a control circuit arranged to control the temperature of the test piece and / or the heating chamber.

[0068] The test piece (307) is suspended by a support structure (308, 309) that allows the test piece to vibrate as freely as possible, for example by supporting it at the locations of its vibration nodes. An acoustic sensor (310), or a plurality of acoustic sensors (which may include microphones or laser interferometers), may preferably be in contact with the test piece (311) at well-defined locations, or a waveguide may be used to guide the acoustic response to the acoustic sensor, with one end of the waveguide preferably being close to or in direct contact with the test piece in the heating chamber, and the other end preferably being located outside the heating chamber. The latter embodiment allows the sensor to be placed outside the heating chamber.

[0069] In a preferred embodiment, the acoustic sensor comprises a laser interferometer. Laser interferometers are particularly suitable for measurements in a vacuum and allow contactless measurements. Alternatively or additionally, the acoustic sensor may comprise an ultrasonic measurement sensor and / or a time-of-flight sensor and / or a Doppler-based sensor, for example as described below:

[0070] -SR. Huang, RM Lerner, KJ Parker, "Time domain Doppler estimators of the amplitude of vibrating targets", J. Acous. Soc. Am., 91(2), 965-974 (1992);

[0071] - J. Tapson, "High precision, short range ultrasonic sensing by means of resonance mode-locking", Ultrasonics, 33, 6, 441-444 (1995), and

[0072] - R. Kazys, R. Sliteris, L. Mazeika, "Ultrasonic technique for Vibration Measurements", Proceedings of the 15th World Conference on Nondestructive Testing, Rome, October 15-21, 2000

[0073] https: / / www.ndt.net / article / wcndt00 / papers / idn246 / idn246.htm .

[0074] These types of sensors can also be used for contactless measurements.

[0075] In one embodiment, the method of the present invention is performed such that the heating chamber comprises a pressure below atmospheric pressure, preferably 0.5 bar or less, more preferably 0.2 bar or less, and most preferably substantially vacuum pressure. Thus, preferably, a laser interferometer is used to allow non-contact vibration measurement down to zero pressure. Measurements down to zero pressure suppress ambient noise, thereby improving the signal-to-noise ratio.

[0076] The impact system includes a ballistic impactor (312) that can be pulsed by an impactor actuator (313). The ballistic impactor (312) is a ceramic rod that can withstand high temperatures and whose properties do not change significantly with temperature. It can be ejected (314) upwards towards the test piece using the impactor actuator (313), which includes:

[0077] - a guide tube (315) passing through the bottom of the heating chamber for guiding the ballistic impactor in a preferably vertical direction;

[0078] - an electromechanically operated hammer (316) arranged to apply a preferably vertical pulse (317) to the impactor (312). The hammer (316) may include an electrical coil (318) and a movable rod or bullet (319) that is movable according to the current flowing through the coil (318). An example of such a system is provided in U.S. Patent 6,782,970 B2, whereby in the present invention the bullet of the impactor actuator applies the pulse to the ceramic impactor (312) rather than directly to the test piece. Alternatively, a pressure-driven impactor or a pressure-driven impact actuator (380) for applying the pulse to the impactor may be used.

[0079] When performing step b), the impactor actuator is arranged to pulse the ballistic impactor so that it reaches within 2 cm of the test piece, but does not contact the test piece. When performing step c, and particularly step c1, the impactor actuator is arranged to pulse the ballistic impactor so that it contacts the test piece, thereby mechanically directing vibration excitation toward the test piece. During step b, background noise can be measured. This noise may originate, for example, from the heating element, but may also primarily originate from the impactor actuator and impactor, which also generate noise that can be picked up by the sensor.

[0080] In one embodiment of the present invention, steps a to d are performed more than once at the same temperature, different temperatures or different temperature ranges, and are preferably performed repeatedly. In a preferred embodiment, the test piece is heated continuously, thereby performing steps ad in subsequent temperature ranges.

[0081] For example, the test piece can be heated continuously at a rate of 1°C / s starting from room temperature (20°C). Steps ad can then be performed periodically every 20 seconds, meaning that the first set of steps ad are performed within a temperature range of 20 to 40°C, the second set of steps ad are performed within a temperature range of 40 to 60°C, and further up to a maximum temperature range of, for example, 1780 to 1800°C. Note, however, that the temperature ranges do not need to be equally large, but can, for example, be smaller for the temperature range of interest. For example, if it is known that the test piece or a component made of the same material as the test piece will primarily be used at temperatures between 700 and 800°C, then the material properties can be more accurately measured between these temperatures by employing a smaller temperature range (e.g., spanning 5°C).

[0082] It should also be noted that the measurements performed in steps b and / or c can be combined with measurements performed in steps b and / or c at other temperatures to obtain a more accurate noise signal and / or vibration response signal. For example, suppose one performs steps a and b in the temperature range of 80°C to 100°C and again in the temperature range of 100°C to 120°C. Thus, one could perform step b for the first time in the temperature sub-range of 82-87°C and for the second time in the temperature sub-range of 102-107°C, while step c could be performed for the first time in the sub-range of 92-97°C. In this case, the analysis in step d can take into account both noise signals obtained in the two executions of step b. Therefore, in a preferred embodiment, steps a, b, c, and / or d are performed two or more times within the temperature range. Alternatively or additionally, steps a, b, c, and / or d can be performed multiple times in different, possibly overlapping, temperature ranges.

[0083] In an embodiment, the control system comprises a processing device configured to analyze the vibration response signal by taking the noise signal into account to determine the material properties of the test piece.

[0084] Therefore, the processing means may preferably be configured to subtract the noise signal from the vibration response signal in the time domain, or more preferably in the frequency domain, by using a Fourier transform or a fast Fourier transform or a harmonic decomposition of the acquired signal. The subtraction may also be performed in a combination of the time and frequency domains. The processing means is preferably configured to analyze the time-varying signal to determine the frequencies and decay constants of the sinusoids that constitute the time-varying signal, i.e., it solves the harmonic inversion problem. The harmonic inversion problem is well known in the literature, but has not heretofore been associated with IET, and more generally involves determining the frequencies, decay constants, amplitudes, and phases of the sinusoids that constitute a discrete-time, finite-length signal consisting of the sum of a finite number of such sinusoids within a given bandwidth. Vladimir A. Mandelstam and Howard S. Taylor, in their seminal paper "Harmonic inversion of time signals and its applications," Journal of Chemical Physics 1076756 (1997), described reformulating the harmonic inversion problem as one of small matrix diagonalization, and solving it using the general filter diagonalization method of Wall and Neuhauser. Computer-based implementations of this technique are known in the art, including the "Harminv" program by Steven G. Johnson of MIT. The results of the analysis can be output to screen 140 or any other suitable interface for storage or further processing by other equipment. The processing device may include one or more dedicated hardware components (e.g., ASICs), appropriately configured configurable hardware components (e.g., FPGAs), microprocessors equipped with appropriate software, or a combination of the above. The same components may also perform other functions. In a preferred embodiment of the present invention, the test piece is a workpiece, a device, or a component of a device. In another preferred embodiment of the invention, the test piece comprises a well-defined shape, preferably a beam shape, and is made of the same material or using the same production technology as the workpiece, device or device component. Figure 4An embodiment of a method according to the invention is illustrated. In a first step a, the temperature is brought to a first temperature range (1021). Step b (1002) is then performed, taking care not to cause an impact to the test piece (1001), whereby the impact system is preferably activated, but in a manner that avoids impact. Step c is then performed, an impact is applied to the test piece (step c1, 1010), and a vibration response signal is acquired (step c2, 1020). The signal is then analyzed (step d, 1050) to obtain E, G, v and / or other characteristics (in particular also the decay constant) within the temperature range of step a. The impact avoidance actuation step (1001), the noise acquisition step (1002), the impact step (1010) and the response acquisition step (1020) can be repeated before analyzing the signal (1030) (as illustrated, for example when the device has multiple impactors operating in series), or alternatively, each acquired signal can be analyzed separately. Therefore, in the analysis of step d, noise signals and / or vibration response signals at different temperatures or temperature ranges may be combined to obtain more accurate material property values ​​and / or determine the temperature dependencies of these material properties.

[0085] Depending on the selected excitation mode, the analysis 1030 may also include determining the dynamic Young's modulus (E) or the shear modulus (G) from the frequencies in the response spectrum, in particular by identifying the peak frequencies and applying formulas such as those provided above. Preferably, the analysis also includes comparing the decay constant with a reference value 1040. This step allows the method according to the present invention to be used for quality control purposes. In practice, a method for controlling the quality of an article comprises characterizing at least a portion of the article as the solid material sample using the method described above, signaling a "pass" condition 1040 / yes if the decay constant is within a predetermined range of the reference value; and declaring a "fail" condition 1040 / no if the decay constant is outside the predetermined range of the reference value. Thus, according to one aspect of the present invention, a method for controlling the quality of an article is provided, comprising: characterizing at least a portion of the article as the test piece using the method described above; signaling a "pass" condition if the decay constant is within a predetermined range of the reference value; and declaring a "fail" condition if the decay constant is outside the predetermined range of the reference value.

[0086] The invention also relates to a computer program product comprising coding means configured to cause a processor to perform the calculation steps of the above method.

[0087] As mentioned above, in a second aspect, the present invention relates to a thermal expansion measurement method for obtaining thermal expansion parameters of a test piece, comprising the following steps:

[0088] - extracting at least two resonant frequencies from the experimental data at the first temperature,

[0089] - obtaining a first value of a first dimensional parameter from said at least two resonant frequencies at said first temperature,

[0090] - comparing said first value of said first dimensional parameter at said first measurement temperature with a second value of said first dimensional parameter at said second temperature,

[0091] - calculating a thermal expansion parameter based on said comparison.

[0092] Therefore, the at least two resonant frequencies at the first temperature are material properties of the test piece, which are preferably obtained via the acoustic measurement method described above and further herein. A system for acoustically measuring material properties of a test piece at high temperature, as described above and further herein, can preferably be used to obtain the at least two resonant frequencies.

[0093] In the thermal expansion measurement method, the second value of the first dimensional parameter at the second temperature can be a known reference value, such as a known dimension of the test piece at room temperature or 0°C or other predefined temperature, or can also be obtained by extracting two resonant frequencies from experimental data at the second temperature. Preferably, the method and / or system for acoustically measuring material properties of a test piece described in this document is used to obtain the two resonant frequencies as material properties of the test piece at the second temperature.

[0094] The thermal expansion parameter can be preferably

[0095] - a size dimension, such as length, width, thickness, surface, volume; in this case, the thermal expansion parameter is preferably the difference between the size dimension at the first temperature and the second temperature;

[0096] - an absolute expansion ratio, for example a linear expansion ratio, which is preferably obtained by taking the ratio of the size at the first temperature to the size at the second temperature;

[0097] - a relative expansion ratio, which is preferably obtained by taking the ratio of the difference between the size at the first temperature and the second temperature to a reference value of the size. This reference value can be any well-defined reference value, and is preferably the value of the size at the first temperature or the second temperature or a predefined reference temperature;

[0098] - a coefficient of thermal expansion or an average coefficient of thermal expansion in the interval between the first temperature and the second temperature, which is preferably obtained by taking the ratio between the relative expansion rate and the temperature difference between the first temperature and the second temperature;

[0099] An anisotropy ratio, preferably obtained by taking the ratio of a first dimension to a second dimension different from the first dimension at a first temperature and comparing this value with the same ratio at a second temperature. This can result in, for example, the anisotropy ratio of the coefficient of thermal expansion, i.e. the ratio of the coefficient of thermal expansion in the first direction to the coefficient of thermal expansion in the second direction, or its average value in the interval between the first and second temperatures.

[0100] The thermal expansion parameter can also be any combination and / or derivative of the above parameters. In a preferred embodiment, the thermal expansion parameter can be calculated analytically. The preferred method of analytical calculation is explained below using the standard ASTM C215-02 for calculating the E modulus based on pulse excitation of a specific test piece:

[0101] - Obtain the bending mode resonance frequency at the first temperature

[0102] - Obtain the longitudinal compression mode resonant frequency at the first temperature

[0103] - The bending mode resonance frequency ff1 is related to the E modulus, e.g. via

[0104]

[0105] For a beam-shaped test piece with mass m, length L, thickness t, and width b, the correction factor T is defined as

[0106]

[0107] - The longitudinal compression mode resonant frequency ff2 is related to the E modulus, e.g. via

[0108]

[0109] For a beam-shaped test piece with mass m, length L, thickness t, and width b,

[0110] - a value equal to the E modulus, allowing to obtain the size dimensions at the first temperature T1:

[0111]

[0112] - a comparison with the values ​​of the size dimensions at the second temperature T2, which then allows calculation of, for example, the anisotropy ratio of the coefficient of thermal expansion averaged between the first and second temperatures

[0113]

[0114] in

[0115]

[0116] here, is the coefficient of thermal expansion in the longitudinal direction averaged over the temperature interval [T1, T2], and is the coefficient of thermal expansion in the thickness direction, averaged over the same temperature interval [T1, T2]. For certain types of materials, understanding this ratio itself can be important, particularly to determine and quantify the anisotropy of the material, and variations in this ratio over a specific temperature interval can indicate the presence of phase transitions or fractures in the test piece. Thus, the dimensions of a test piece at room temperature (20°C) might, for example, be (10 cm x 2 cm x 5 mm), so this value can be used as the size dimension at the second temperature.

[0117] It is noted here that the resonant frequency value at the first temperature can preferably be obtained in a single acoustic measurement at the first temperature, and optionally, the resonant frequency value at the second temperature can also preferably be obtained in a single acoustic measurement at the second temperature. Preferably, these are obtained using the acoustic measurement method and / or system of the present invention. Therefore, by identifying the peaks in the acoustic response in the frequency domain, the resonant frequencies of different modes can be easily measured. In some cases, uncertainty may arise when identifying which resonant frequencies belong to which modes. In this case, the identification of the resonant frequencies of the modes can be obtained via modeling and / or simulation calculations, or via a set of additional acoustic measurements (selectively, whereby the test piece is impacted in a way that excites a specific mode).

[0118] As described above, the comparison may be performed using an analytical method, such as an equation-based method, whereby the value of the dimensional parameter is analytically extracted. Alternatively or additionally, the comparison may be performed using a simulation method, for example, whereby the simulation method includes modeling the test piece using a set of model parameters related to the dimensional parameter, and whereby the simulation method includes a resonance calculation algorithm to obtain the at least two resonant frequencies, whereby the model parameters are varied to best reproduce the at least two resonant frequencies, thereby obtaining a set of optimal model parameters, and whereby the value of the dimensional parameter is calculated from the set of optimal model parameters. The simulation method may preferably include a finite element method.

[0119] From the two measured resonant frequencies, for example, the E modulus and the size dimension values ​​can be derived using the following equations.

[0120] More resonant frequencies can be used, e.g.

[0121] - torsional resonance frequency,

[0122] -First harmonic of the bending mode

[0123] - Second harmonic of bending mode

[0124] -First harmonic of the longitudinal compression mode

[0125] -Second harmonic of the longitudinal compression mode

[0126] or any combination or derivative thereof.

[0127] Note that harmonics can depend on the scale of the test piece, so for non-isotropic expansion, the harmonic resonant frequencies are not necessarily multiples of the Earth mode resonant frequencies. This allows for the acquisition of values ​​for more than one size parameter, which is very useful in the case of non-isotropic expansion. Furthermore, by using more than two resonant frequencies, the resulting system can be overdetermined, in which case a best-fit solution for the desired size can be obtained, for example, through analytical and / or simulation methods.

[0128] The present invention also relates to a computer program product comprising coding means configured to cause a processor to execute the calculation steps of the thermal expansion measurement method described above.

Claims

1. A method for acoustically measuring material properties of a test piece at an elevated temperature, wherein the elevated temperature depends on the type of the test piece, comprising the following steps: a. heating the test piece to a test temperature range; b. By collecting the vibration signal of the test piece during the calibration period, performing background measurement within the test temperature range to obtain a noise signal; c. Perform acoustic measurements on the test piece within the test temperature range and test period by: cl applying vibration excitation to the test piece; c2 collecting the vibration signal of the test piece during the test period, thereby obtaining a vibration response signal to the vibration excitation; as well as d. obtaining the material properties of the test piece by analyzing the vibration response signal, thereby taking the noise signal into account, wherein the vibration excitation of step c1 is mechanically guided to the test piece, whereby using an impact system comprising a ballistic impactor and an impactor actuator, wherein the impactor actuator is configured to provide pulses to the ballistic impactor and to apply a vibration excitation to the test piece using the ballistic impactor, Thus, in the acoustic measurement step c, the ballistic impactor is supplied with a measurement pulse, which in step c1 applies a vibration excitation to the test piece, and Thus, in the background measurement step b, a background pulse is provided to the ballistic impactor, wherein the background pulse does not apply vibration excitation to the test piece. The noise signal thus includes noise generated by the impactor actuator and the ballistic impactor.

2. The method according to claim 1, wherein The ballistic impactor is configured to approach the test piece from below in a vertical direction.

3. The method according to claim 2, wherein The measurement pulse provided to the ballistic impactor is configured to mechanically contact the test piece, while the background pulse provided to the ballistic impactor is configured to be unable to reach the test piece.

4. The method according to claim 3, wherein The steps a to d are performed more than once at the same temperature, different temperatures or different temperature ranges.

5. The method according to claim 4, wherein The test piece is continuously heated so that steps ad are performed within subsequent temperature ranges.

6. The method according to any one of claims 1 to 5, characterized in that In step d, the temperature dependence of the material property is determined.

7. The method according to any one of claims 1 to 6, characterized in that In step d, the Young's modulus (E), the shear modulus (G) and / or the Poisson's ratio (ν) are determined.

8. The method according to any one of claims 1 to 7, characterized in that In step d, the attenuation constant of the material characteristic is determined, where the attenuation constant is the attenuation constant of the sinusoidal curve constituting the time-varying vibration response signal.

9. A system for acoustically measuring material properties of a test piece at an elevated temperature, wherein the elevated temperature depends on the type of the test piece, comprising: - a heating chamber comprising a heating element for bringing the test piece within a test temperature range; - a sensor configured to acquire a vibration signal from the test piece over a period of time; - an impact system comprising an impactor and an impactor actuator for applying a vibration excitation to a test piece located in the heating chamber; - a control system connected to the heating chamber, the sensor and the impactor, the control system being configured to: Instructing the heating chamber to place the test piece within a test temperature range; during a test cycle, instructing the impact system to actuate the impactor using the impactor actuator so that a vibration excitation is mechanically applied by the impactor to the test piece located within the heating chamber, and acquiring a vibration response signal to the vibration excitation from the sensor, during a calibration cycle, instructing the impact system to actuate the impactor using the impactor actuator so that the impactor does not mechanically apply a vibration excitation to the test piece located in the heating chamber, and acquiring a noise signal from the sensor, and deriving material properties of the test piece by analyzing the vibration response signal, thereby taking the noise signal into account, whereby the noise signal comprises noise generated by the impactor actuator and the impactor, wherein the impact system comprises a ballistic impactor, and the impact actuator is configured to provide a pulse to the ballistic impactor, The impact actuator is thereby configured to provide a measurement pulse to the ballistic impactor for applying a vibration excitation to the test piece in an acoustic measurement step, and The impact actuator is thereby configured to provide a background pulse to the ballistic impactor for avoiding application of a vibration excitation to the test piece during a background measurement step.

10. The system according to claim 9, wherein: The control system comprises a processing device configured to analyze the vibration response signal by subtracting the noise signal from the vibration response signal in the time domain or in the frequency domain, or in a combination of the time domain and the frequency domain.

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