JP7814616B2 - Stress measurement device - Google Patents

Description

This disclosure relates to a stress measuring device that measures stress in an object using a stress-luminescent material.

Stress is sometimes measured to predict or detect damage to an object. Conventional methods for measuring stress include attaching a strain gauge to the object to measure strain and determine stress, using an infrared camera to measure heat generation or endothermic effects to determine stress distribution, and digital image correlation, which applies a random pattern to the surface of the object, captures an image with a camera, and determines stress from changes in strain.

However, strain gauges require time-consuming attachment and are limited in the areas that can be measured. Furthermore, methods using infrared cameras require a camera capable of measuring temperature with high accuracy, resulting in high measurement system costs. Digital image correlation methods require the preparation of surface patterns in advance and have significant limitations on the positioning of the measurement system. For this reason, a technology that measures stress distribution in a measurement object without contact by imaging mesoluminescence from a measurement object with a mesoluminescent material applied to its surface has attracted attention for non-contact, wide-area stress measurement. To induce mesoluminescence in a mesoluminescent object, the mesoluminescent object must first be irradiated with excitation light to saturate its excitation state. For example, Patent Document 1 describes a technology that irradiates the mesoluminescent object with excitation light to saturate its excitation state, and then causes the mesoluminescent object to emit light under certain conditions to image the object's surface.

Patent No. 6499363

Stress acting on an object includes static components, which do not change, and dynamic components, which change. Even when an object is stationary, static stress continues to act as a static component. For this reason, there is a need for technology that can measure static stress, for example, to improve the accuracy of predicting object destruction. The method described in Patent Document 1 measures stress by utilizing the properties of mechanoluminescent materials, which emit light when stress changes. While this method is capable of measuring stress that includes static and dynamic components, it has the problem of being unable to measure static stress, which is a static component.

This disclosure has been made in consideration of the above and aims to provide a stress measuring device capable of measuring static stress occurring in a measurement object.

In order to solve the above-mentioned problems and achieve the objectives, the stress measuring device according to the present disclosure comprises a measurement unit that measures the luminescence intensity obtained by continuously irradiating excitation light onto a measurement object coated with a stress-luminescent material and in which an unknown fluctuating stress occurs, a memory unit that stores the luminescence intensity measured by the measurement unit, and a signal analysis unit that calculates static stress, which is the static component of the unknown fluctuating stress, based on the time-varying waveform of the luminescence intensity stored in the memory unit, wherein the fluctuating stress is a stress that fluctuates periodically, and the signal analysis unit calculates the static stress based on the ratio of the intensities of the peaks of two consecutive waves in the time-varying waveform.

The present disclosure has the effect of providing a stress measuring device capable of measuring static stress occurring in a measurement object.

FIG. 1 is a diagram showing a configuration example of a stress measuring device according to a first embodiment; FIG. 10 is a diagram showing a first comparative example of the results of calculating the luminescence intensity using the luminescence intensity formulas (1) and (2), respectively. FIG. 10 is a diagram showing a second comparative example of the results of calculating the luminescence intensity using the luminescence intensity formulas (1) and (2), respectively. FIG. 10 is a diagram showing a first comparative example of the results of calculating the luminescence intensity using the luminescence intensity formulas (1) and (3), respectively. FIG. 10 is a diagram showing a second comparative example of the results of calculating the luminescence intensity using the luminescence intensity formulas (1) and (3), respectively. 1 is a flowchart showing an example of an operation of the stress measuring apparatus according to the first embodiment to evaluate a waveform of a stress; FIG. 10 is a diagram showing an example of stress measured by the stress measuring device according to the second embodiment; FIG. 1 is a first diagram showing the relationship between the stress generated in the measurement object and the mechanoluminescence intensity of the mechanoluminescent material. FIG. 2 is a second diagram showing the relationship between the stress generated in the measurement object and the mechanoluminescence intensity of the mechanoluminescent material. FIG. 3 is a third diagram showing the relationship between the stress generated in the measurement object and the mechanoluminescence intensity of the mechanoluminescent material. A diagram showing the relationship between the dynamic component of stress generated in the measurement target and the luminous intensity of the mechanoluminescent material. A diagram showing the relationship between the static component of stress generated in the measurement target and the luminous intensity of the mechanoluminescent material. FIG. 10 is a diagram showing a configuration example of a stress measuring device according to a third embodiment; 10 is a flowchart showing an example of an operation of the stress measuring device according to the fourth embodiment to evaluate a waveform of a stress. FIG. 10 is a diagram showing a configuration example of a stress measuring device according to a fifth embodiment. FIG. 13 is a diagram for explaining a stress measurement method using a stress measurement device according to a sixth embodiment. 13 is a flowchart showing an example of an operation of the stress measuring device according to the sixth embodiment to evaluate a waveform of a stress. FIG. 1 is a diagram showing an example of the configuration of a processing circuit when an analysis device operating as a signal analysis unit is realized by a processor and a memory.

The stress measuring device according to the embodiments of the present disclosure is described in detail below with reference to the drawings. The stress measuring device according to each embodiment can be used for stress measurement in a variety of applications, such as measuring the stress of test specimens in laboratories and inspecting the mechanical load conditions of infrastructure facilities. Note that in each embodiment, time-varying stress may be referred to as fluctuating stress. Furthermore, the dynamic component of fluctuating stress may be referred to as dynamic stress, and the static component of fluctuating stress may be referred to as static stress.

In each of the embodiments shown below, a measurement method is described in which the stress acting on a measurement object is evaluated by analyzing the intensity of mechanoluminescence, which is light emitted by a mechanoluminescent material when an unknown fluctuating stress acts on the measurement object to which the mechanoluminescent material is applied.

Embodiment 1.
1 is a diagram showing an example of the configuration of a stress measuring apparatus 1 according to a first embodiment. Also shown in FIG. 1 is a measurement object 111, the object of which stress is measured by the stress measuring apparatus 1. A stress-luminescent material 112 is applied to the measurement object 111. Furthermore, a time-varying stress 121 is generated in the measurement object 111.

The stress measurement device 1 comprises a light irradiation unit 11 that continuously irradiates excitation light 101 toward the measurement object 111, a measurement unit 12 that measures the emission intensity of the stress-emitting material 112, a memory unit 13 that stores the emission intensity measured by the measurement unit 12, and a signal analysis unit 14 that analyzes the time-varying waveform of the emission intensity stored in the memory unit 13.

The stress-luminescent material 112 applied to the measurement target 111 is a material that emits light when irradiated with excitation light and then subjected to mechanical stimuli such as friction, impact, compression, tension, or shear. Examples include europium-doped strontium aluminate (SrAl2O4:Eu) and zinc sulfide doped with transition metals or rare earths (ZnS:Mn), but the material is not limited to these, as long as it emits light with a brightness that corresponds to the stress.

The excitation light 101 is irradiated continuously over time and has a wavelength sufficient to excite the stress-luminescent material 112. The measurement unit 12 sequentially detects the intensity of stress-luminescence (hereinafter sometimes referred to as stress-luminescence intensity) of the stress-luminescent material 112. The measurement unit 12 is only required to detect stress-luminescence, and is realized, for example, using an optical sensor sensitive to the emission wavelength of stress-luminescence. The optical sensor can be, for example, a photodiode, or a general-purpose digital camera or video camera using a two-dimensional image sensor such as a CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor). When a two-dimensional image sensor is used, not only can the stress-luminescence intensity be measured, but also the stress-luminescence at each position of the measurement target can be measured as an image. The measurement unit 12 outputs the stress-luminescence intensity over time, a series of still images, or a video. The memory unit 13 is a storage device that stores the output of the measurement unit 12. The signal analysis unit 14 is an analysis device that extracts the mechanoluminescence intensity from the data stored in the storage unit 13 and analyzes the time variation of the mechanoluminescence intensity.

It is already known that the luminous intensity of a stress-stimulated luminescent material depends on both the stress σ acting on the material and the rate of change of the stress σ, dσ/dt. It is also known that the luminous intensity of a stress-stimulated luminescent material can be approximately expressed by the following equation (1) using coefficients C, D, and E:

However, because Equation (1) does not take into account the static and dynamic components of stress, i.e., static and dynamic stresses, it is not possible to calculate the static and dynamic stresses from the mechanoluminescence intensity. To calculate the static and dynamic stresses from the mechanoluminescence intensity of a mechanoluminescent material, it is necessary to use an equation that takes into account these relationships, i.e., the correspondence between mechanoluminescence intensity, static stress, and dynamic stress. For example, if the static stress is σ _s and the dynamic stress is σ _d , then σ in Equation (1) can be replaced with σ _s + σ _d , and individual coefficients can be set for each term. In this case, the luminescence intensity equation takes the form of Equation (2).

Furthermore, by replacing the second and third terms on the right side of equation (2), which are the terms for static stress σ _s and dynamic stress σ _d , with the product term of static stress σ _s and dynamic stress σ _d , the luminescence intensity equation takes the form of equation (3).

Equation (3) can approximate the luminescence intensity when static stress varies phenomenologically with the same accuracy, even with fewer parameters than equation (2). Using equation (3), which has fewer parameters, reduces the computational load, allowing for shorter measurement times and the use of cheaper computers.

Figures 2 and 3 compare the results of adjusting the coefficients in equations (1) and (2) to match the actual stress-luminescence intensity I(t). Figure 2 shows a comparison when the mean stress is 130 MPa and the stress amplitude is 110 MPa, while Figure 3 shows a comparison when the mean stress is 400 MPa and the stress amplitude is 110 MPa. Figures 4 and 5 also compare the results of adjusting the coefficients in equations (1) and (3) to match the actual stress-luminescence intensity I(t). Figure 4 shows a comparison when the mean stress is 130 MPa and the stress amplitude is 110 MPa, while Figure 5 shows a comparison when the mean stress is 400 MPa and the stress amplitude is 110 MPa. In this way, the luminescence intensity for different mean stresses that could not be reproduced using equation (1) can be approximated using equation (2) or equation (3).

Below, we will explain the procedure for evaluating stress waveforms using equation (3) as an example of a stress-luminescence equation. In order to evaluate the stress of the measurement object 111, the coefficients A" to F" in equation (3) must be determined in advance. Therefore, before evaluating the stress of the measurement object 111, the coefficients A" to F" are calibrated in advance by measuring the luminescence intensity of a test specimen to which a known fluctuating stress σ has been applied. Below, we will explain in detail the procedure for evaluating a general stress waveform that changes irregularly.

Figure 6 is a flowchart showing an example of the operation of the stress measuring device 1 according to the first embodiment to evaluate a stress waveform. This flowchart includes a calibration load application step (step S11) for applying a known fluctuating load to a calibration specimen, a recording step (step S12) for recording the mechanoluminescence intensity I(t) of the mechanoluminescent material when the fluctuating load is applied, a calibration step (step S13) for calibrating the coefficients of equation (3), a measurement step (step S14) for recording the mechanoluminescence intensity I'(t) when an unknown fluctuating stress σ'(t) is applied to the measurement object 111, a calculation step (step S15) for substituting I'(t) into equation (3) to obtain σ'(t), and an output step (step S16) for calculating static stress. Each of these steps is described in detail below.

Steps S11 to S13 are performed, for example, in a configuration in which the measurement object 111 shown in Figure 1 is replaced with a calibration test piece.

In step S11 (calibration load application step), a known fluctuating load is applied to the calibration specimen. The calibration specimen may be a resin material containing a stress-stimulated luminescent material, or a material not containing a stress-stimulated luminescent material, such as a metal, resin material, ceramic, glass, or concrete, with a stress-stimulated luminescent material applied to its surface. Furthermore, the calibration specimen may have any shape that allows for predicting the distribution of stress generated in the stress-stimulated luminescent material when a load is applied, and preferably has a simple shape such as a flat plate or column. The known fluctuating load may be any shape that allows for predicting the stress generated in the stress-stimulated luminescent material of the calibration specimen when applied. A sinusoidal uniaxial load is preferred. Applying multiple patterns of known fluctuating loads with different average loads, load amplitudes, and load frequencies to the specimen enables highly accurate calibration.

In step S12 (recording step), the stress measuring device 1 records the mechanoluminescence intensity I(t) while applying a fluctuating load to the calibration specimen. That is, the light irradiating unit 11 of the stress measuring device 1 irradiates the mechanoluminescent material of the specimen to which the fluctuating load is applied with excitation light 101, and the mechanoluminescence intensity of the mechanoluminescent material measured by the measuring unit 12 is recorded in the memory unit 13. At the same time, the stress σ(t) applied to the mechanoluminescent material of the specimen is recorded. The stress σ(t) applied to the mechanoluminescent material of the specimen may be calculated based on the fluctuating load applied to the specimen, or it may be obtained by measuring the strain of the specimen and dividing the measured strain by the elastic modulus of the mechanoluminescent material. It is preferable to perform similar measurements for multiple known fluctuating loads with different applied average loads, load amplitudes, and load frequencies, and record the stress σ(t) and mechanoluminescence intensity I(t) for each.

In step S13 (calibration step), equation (3), which is the conversion equation between the stress σ(t) generated in the calibration specimen when a fluctuating load is applied and the mechanoluminescence intensity I(t), is calibrated. Specifically, the signal analysis unit 14 reads out the mechanoluminescence intensity I(t) and stress σ(t) recorded in step S12 from the memory unit 13, and adjusts the coefficients of equation (3) so that the mechanoluminescence intensity I(t) is reproduced when the stress σ(t) is substituted into equation (3). The coefficients are adjusted, for example, so that the sum of squares of the difference between I(t) and the value obtained by substituting σ(t) into equation (3) is minimized.

By performing the above steps S11 to S13, calibration for adjusting the coefficients A" to F" in equation (3) is completed, and it becomes possible to determine the unknown fluctuating stress using equation (3) and the mechanoluminescence intensity of the mechanoluminescent material.

In step S14 (measurement step), the stress-emission intensity I'(t) of the measurement object 111 to which an unknown fluctuating stress 121 is applied is recorded. Examples of measurement objects 111 to which an unknown fluctuating stress 121 is applied include a pipe carrying a fluid, a power transmission shaft, a bridge beam, and a tightened bolt. The fluctuating stress 121 is generated by repeated loading from a stress application unit. The stress application unit is either included in the measurement object 111 itself or is an inseparable component of the measurement object 111. As an example, if the measurement object 111 is a pipe carrying a fluid, the stress application unit is the fluid within the pipe, and the stress is caused by fluctuations in the pressure of the fluid. As another example, if the measurement object 111 is a rotating power transmission shaft, the stress may be the transmitted power or vibrations generated in the process of transmitting the power. In step S14, specifically, the light irradiating unit 11 of the stress measuring device 1 irradiates the excitation light 101 to the stress-luminescent material 112 of the measurement object 111 to which the fluctuating stress 121 is applied, and the measuring unit 12 measures the stress-luminescent intensity of the stress-luminescent material 112. The memory unit 13 records the stress-luminescent intensity measured by the measuring unit 12. The measurement and recording of the stress-luminescent intensity I'(t) are performed multiple times.

In step S15 (calculation step), equation (3) with the coefficients adjusted in step S13 and the mechanoluminescence intensity I'(t) recorded in step S14 are used to calculate the fluctuating stress σ'(t) such that I'(t) is the value obtained by substituting σ'(t) into equation (3), and the calculated σ'(t) is used as an estimate of the unknown fluctuating stress. Here, when calculating σ'(t) using I'(t), an inverse problem must be solved; σ'(t) cannot be calculated by simply substituting the recorded I'(t) into equation (3). Instead, commonly known methods for solving inverse problems can be used. For example, if σ'(t) is assumed to be quasi-periodic, σ'(t) can be calculated in wavenumber space by Fourier transforming σ'(t). Alternatively, as a numerical solution, σ'(t) may be estimated by sequentially substituting σ'(t) into equation (3) to minimize the difference between the value obtained by substituting σ'(t) into equation (3) and I'(t). Furthermore, σ'(t) may be estimated using a machine learning model that has previously trained a data set in which σ'(t) having a random time waveform is input and the value obtained by substituting σ'(t) into equation (3) is output.

As described above, by adjusting the coefficients contained in equation (3) in advance, the unknown fluctuating stress σ'(t) can be estimated with high accuracy. Note that while equation (3) was used as a representative equation for approximately expressing the luminous intensity, the form of the equation is not limited to equations (2) and (3), as long as it contains terms for the static component, dynamic component, and time derivative of the dynamic component of the stress. For example, by adding higher-order terms, the luminous intensity can be approximated with higher accuracy.

Depending on the form of the emission intensity formula, σ'(t) may not be uniquely determined for I'(t). In this case, a step of selecting one of multiple σ'(t) is added as necessary. In this selection step, the most plausible solution predicted physically may be selected, or a solution may be selected based on other criteria. As an example of other criteria, we will explain the case of selecting a solution that is on the safer side. For example, if the measurement object 111 is a structural material and you want to verify that the measurement object 111 will not break under conditions where continuously fluctuating stress is applied, you can make a safe evaluation by selecting the solution that results in the greatest degree of fatigue damage from among multiple σ'(t).

Another selection method is to measure stress at a single point on the measurement object 111 without using the stress-luminescent element 112, and select and output the solution that matches the measured stress. A stress measurement method that does not use the stress-luminescent element 112 can use a method that measures strain using a strain gauge or extensometer, for example, and converts it into stress. This makes it possible to evaluate the stress distribution over the entire measurement object 111 using the stress-luminescent element 112 while referring to the stress measurement value at only one point.

In step S16 (output step), the static stress included in the fluctuating stress σ'(t) obtained in step S15 is calculated and output to the outside of the stress measuring device 1. For example, the static stress is notified to the outside by displaying it on a display device. The fluctuating stress σ'(t) may also be output to the outside. The static stress is calculated using the multiple emission intensities recorded in step S14, for example, σ'( _t1 ) and σ'( _t1 ) corresponding to I'( _t1 ) and I'( _t2 ), respectively, and an emission intensity formula.

As described above, the stress measuring device 1 according to this embodiment calculates the stress and static stress acting on the measurement object 111 using the stress σ acting on the stress-luminescent material 112 possessed by the measurement object 111 and the emission intensity equation showing the relationship between the emission intensity I of the stress-luminescent material. The stress measuring device 1 according to this embodiment can continuously measure the stress acting on the measurement object 111, and can also determine the static stress from the measured stress.

Furthermore, when excitation light is intermittently irradiated onto an object whose stress is to be measured, the excitation state of the excitation light also becomes intermittent, posing the problem of not being able to measure the stress continuously. However, the stress measuring device 1 according to this embodiment continuously irradiates the measurement object 111 with excitation light 101, so it is possible to continuously measure the stress acting on the measurement object 111. This makes it possible to evaluate with high accuracy the stress of, for example, an object that requires continuous stress evaluation over time, or an object to which unexpected stress may be suddenly applied.

Embodiment 2.
Next, a stress measuring device according to embodiment 2 will be described. The configuration of the stress measuring device according to embodiment 2 is the same as that of embodiment 1. In this embodiment, a measurement method will be described in which the stress 121 generated in the measurement object 111 fluctuates periodically.

For example, if the measurement object 111 is a pipe through which a fluid passes and stress is generated due to pulsations in the fluid's pressure, the stress will fluctuate periodically. Furthermore, if the measurement object 111 is a rotating power transmission shaft and stress is generated due to pulsations in the transmitted power, the stress is also considered to fluctuate periodically. In such cases, as shown in Figure 7, stress 121 can be divided into a static component 122 that does not fluctuate over time or whose fluctuation time scale is sufficiently long, and a dynamic component 123, which is stress 121 minus static component 122, and the dynamic component 123 fluctuates periodically. Furthermore, static component 122 corresponds to the time average of stress 121. Figure 7 shows an example of stress 121 measured by the stress measuring device according to the second embodiment. While we have described periodically fluctuating stress 121, even in the case of stress that fluctuates randomly rather than periodically, the static component of stress, or static stress, corresponds to the time average of stress.

The stress measuring device 1 of this embodiment differs from that of embodiment 1 in the operation of the signal analysis unit 14. The light irradiation unit 11 and the measurement unit 12 perform the same processing as step S14 of FIG. 6 described in embodiment 1, and record the luminous intensity of the stress-luminescent material 112 in the memory unit 13.

8 to 12, the operation of the signal analysis unit 14 to analyze the stress-luminescence intensity recorded in the memory unit 13 to determine the static component 122 and dynamic component 123 of the stress 121 will be described. FIG. 8 is a first diagram showing the relationship between the stress 121 generated in the measurement object 111 and the stress-luminescence intensity of the stress-luminescent material 112. FIG. 9 is a second diagram showing the relationship between the stress 121 generated in the measurement object 111 and the stress-luminescence intensity of the stress-luminescent material 112. FIG. 10 is a third diagram showing the relationship between the stress 121 generated in the measurement object 111 and the stress-luminescence intensity of the stress-luminescent material 112. FIG. 11 is a diagram showing the relationship between the dynamic component 123 of the stress 121 generated in the measurement object 111 and the emission intensity of the stress-luminescent material 112. FIG. 12 is a diagram showing the relationship between the static component 122 of the stress 121 generated in the measurement object 111 and the emission intensity of the stress-luminescent material 112.

As an example, a case where the waveform of the dynamic component of the stress 121 is a sine wave will be described. As described above, the stress 121 applied to the measurement object 111 can be divided into a static component 122, which is a time average, and a dynamic component 123, which is the stress 121 minus the static component 122. In this case, the mechanoluminescence intensity of the mechanoluminescent material 112 exhibits two peaks per cycle of the dynamic component 123 of the stress 121, as shown in FIGS. 8 to 10 . This is because the mechanoluminescence intensity depends on the fluctuation speed of the stress 121. Hereinafter, the peak of the mechanoluminescence intensity occurring when the stress 121 decreases will be referred to as a first peak 131, and the peak of the mechanoluminescence intensity occurring when the stress 121 increases will be referred to as a second peak 132. The mechanoluminescence intensity of the first peak 131 will be referred to as P ₁ , and the mechanoluminescence intensity of the second peak 132 will be referred to as P ₂ .

The ratio of _P1 to _P2 varies depending on the static component 122. Specifically, when the static component 122 is positive, as in the example shown in Fig. 8, _P1 is higher than _P2 . When the static component 122 is zero, as in the example shown in Fig. 9, _P1 and _P2 are equal. When the static component 122 is negative, as in the example shown in Fig. 10, _P2 is higher than _P1 . Furthermore, as shown in Fig. 11, the average value of _P1 and _P2 is proportional to the square of the amplitude of the dynamic component 123, and as shown in Fig. 12, the value obtained by dividing _P1 by _P2 is proportional to the exponential function of the static component 122.

From these relationships, the static component 122 and dynamic component 123 of stress 121 can be calculated using the following equation (4). In equation (4), A and B are coefficients depending on the measurement system.

The signal analysis unit 14 uses equation (4) to calculate and output the static component 122 and dynamic component 123 of stress 121 from the time variation of the stress-luminescence intensity.

The stress measuring device 1 of this embodiment can continuously measure the periodically fluctuating stress 121 and can also measure the static component 122 of the stress 121.

Embodiment 3.
In the first and second embodiments, we have described the case where the stress acting on the measurement object 111 varies over time. However, in this embodiment, we will describe a stress measuring device that can measure unknown static stress even when the stress acting on the measurement object 111 does not vary over time.

Figure 13 is a diagram showing an example configuration of a stress measuring device 1a according to embodiment 3. The stress measuring device 1a has a configuration in which a stress application unit 15 is added to the stress measuring device 1 according to embodiment 2. The stress application unit 15 applies a periodically fluctuating stress 121 to the measurement object 111. When the stress 121 is not being applied by the stress application unit 15, it is assumed that only an unknown static stress is generated in the measurement object 111.

In this embodiment, unknown static stress refers to stress acting on the object to be measured that does not fluctuate over time or whose fluctuation time scale is sufficiently large.For example, this is stress that can be expected in various situations, such as the stress that occurs in the bridge beam when it is deflected due to its own weight, the tensile stress that occurs in a tightened bolt, the thermal stress that occurs when a temperature change occurs in a component that combines materials with different linear expansion coefficients, and the residual stress that occurs during the molding of a resin molded product.

As an example, measuring the tensile stress on a bolt can be used to inspect for abnormal assembly conditions during a production process. Conventionally, one method for inspecting assembly conditions is a hammering test. This involves striking the object to be inspected with a hammer or similar tool, and using the sound produced to determine whether the condition is normal or abnormal. However, it is difficult to determine, for example, situations in which the bolt is not loose but has insufficient axial tension. In contrast, the method of this embodiment uses a bolt with a mechanoluminescent material applied to the head, and by recording and analyzing the mechanoluminescence intensity when struck with a hammer or similar tool, the axial tension of the bolt can be estimated, allowing for more detailed inspections than before.

The stress application unit 15 of the stress measurement device 1a applies a known fluctuating stress, preferably a sine wave, to the measurement object 111. The stress application unit 15 can be realized using, for example, a fatigue testing machine or a vibration exciter. The fluctuating stress 121 applied to the measurement object 111 may be any time-varying stress, and may be a wave other than a sine wave, such as a triangular wave or a rectangular wave.

The stress measuring device 1a analyzes the waveform of the stress luminescence intensity of the stress luminescent material 112 when the stress application unit 15 applies a known fluctuating stress 121 to the measurement object 111 in a manner similar to that of embodiment 1, thereby determining and outputting the static component 122 of the stress occurring in the measurement object 111.

As such, the stress measuring device 1a of this embodiment is equipped with a stress application unit 15 that applies a periodically fluctuating fluctuating stress 121 to the measurement object 111, and therefore the static component 122 of the stress can be obtained even if the stress generated in the measurement object 111 does not fluctuate over time.

Embodiment 4.
The mechanoluminescence intensity of a stress-stimulated luminescent material depends not only on the applied stress but also on the concentration distribution of the mechanoluminescent material. Furthermore, if the mechanoluminescent material is in the form of a coating film, it depends on the film thickness and also on the intensity of the excitation light. When the excitation state of the mechanoluminescent material is not saturated, the mechanoluminescence intensity is proportional to the concentration of the mechanoluminescent material, the film thickness, and the intensity of the excitation light. Such instability in the mechanoluminescence intensity can be a source of errors when evaluating stress. Therefore, in this embodiment, a method for correcting the instability in the mechanoluminescence intensity, which can be a source of errors in stress evaluation, will be described.

In a method for correcting the above-mentioned uncertainty, the luminescence intensity of the mechanoluminescent material is measured before or after measuring the fluctuating stress of the measurement object using any of the methods described in embodiments 1 to 3, when the measurement object is not subjected to fluctuating stress and is irradiated with excitation light. At this time, the mechanoluminescent material does not emit mechanoluminescence, but emits fluorescence due to the excitation light. This luminescence intensity is hereinafter referred to as the correction fluorescence intensity. Because the excitation light is irradiated continuously, the correction fluorescence intensity is also continuous. When the excitation state of the mechanoluminescent material is not saturated, the correction fluorescence intensity is proportional to the concentration of the mechanoluminescent material, film thickness, and excitation light intensity. Therefore, by correcting the mechanoluminescent intensity measured using the methods described in embodiments 1 to 3 by the correction fluorescence intensity, the effects of the concentration of the mechanoluminescent material, film thickness, and excitation light intensity can be eliminated.

When the measurement unit 12 of the stress measurement device 1, 1a described in embodiments 1 to 3 is an imaging system, the distribution of correction fluorescence intensity can be obtained as an image. By correcting the image of mechanoluminescence intensity measured using the method described in embodiments 1 to 3 by dividing it by the image of correction fluorescence intensity, the effects of the concentration of the mechanoluminescent material, film thickness, and excitation light intensity distribution can be removed.

Figure 14 is a flowchart showing an example of the operation of the stress measuring device according to embodiment 4 to evaluate a stress waveform. The configuration of the stress measuring device according to embodiment 4 is the same as that of embodiments 1 and 2 (see Figure 1) or that of embodiment 3 (see Figure 13). In Figure 14, the same step numbers are assigned to processes that are common to those included in the flowchart shown in Figure 6. Explanation of processes that are common to those included in the flowchart shown in Figure 6 will be omitted.

In the example of operation shown in the flowchart of FIG. 14, first, the measurement unit 12 measures the emission intensity of the stress-luminescent material 112 to obtain a correction fluorescence intensity when the measurement object 111 is not subjected to fluctuating stress 121 and is irradiated with excitation light 101 (step S21). Next, the stress-luminescent intensity I'(t) obtained when an unknown fluctuating stress σ'(t) is applied to the measurement object 111 is recorded (step S14). Next, the stress-luminescent intensity I'(t) recorded in step S14 is divided by the correction fluorescence intensity obtained in step S21 to correct the stress-luminescent intensity I'(t) (step S22). Next, stress is calculated using the corrected stress-luminescent intensity I'(t) (step S23). In step S23, the unknown fluctuating stress σ'(t) of the measurement object 111 is calculated using one of the methods described in embodiments 1 to 3. Next, static stress is calculated (step S16). The static stress is calculated using one of the methods described in embodiments 1 to 3.

As described above, the stress measurement device according to this embodiment irradiates excitation light 101 onto the measurement object 111 in a state where no fluctuating stress is applied, measures the emission intensity of the stress-luminescent material 112, and uses this as a correction fluorescence intensity to correct the fluctuating stress measured by any of the methods described in embodiments 1 to 3. This makes it possible to remove the influence of the concentration and film thickness of the stress-luminescent material 112 and the intensity of the excitation light 101 irradiated onto the stress-luminescent material 112 from the measured value of the stress of the measurement object 111. In other words, the accuracy of stress measurement can be improved.

If the stress application unit that generates fluctuating stress in the measurement object 111 is included in the measurement object 111 or is an inseparable part of the measurement object 111, the correction fluorescence intensity can be acquired before the stress application unit starts applying stress. For example, if the measurement object 111 is a pipe through which a fluid passes, the correction fluorescence intensity is acquired before the fluid passes. If the measurement object 111 is a rotating power transmission shaft, the correction fluorescence intensity is acquired before the power transmission starts.

Embodiment 5.
In this embodiment, a method for reducing noise when measuring the stress-luminescence intensity and improving the stress measurement accuracy is shown by appropriately setting the wavelength of the excitation light that the stress measurement device irradiates onto the measurement object and the wavelength to which the measurement unit of the stress measurement device is sensitive. In the stress measurement devices 1 and 1a according to the above-described first to fourth embodiments, the excitation light 101 may have a wavelength that can excite the stress-luminescent material 112, and the measurement unit 12 may be sensitive to the emission wavelength of the stress-luminescent material 112. However, if the reflected light of the excitation light 101 is detected by the measurement unit 12, this will become noise when evaluating the stress-luminescence intensity, so the wavelength of the excitation light 101 is set outside the range of the wavelength band to which the measurement unit 12 is sensitive.

The emission wavelength of the stress-stimulated luminescent material 112 peaks in a narrow wavelength band. For example, in the case of strontium aluminate, the intensity is highest around 365 nm. Meanwhile, light that causes noise other than stress-stimulated luminescence, such as light from light sources such as sunlight, fluorescent lamps, and incandescent bulbs, and their scattered light, generally contains a large amount of light in wavelength bands other than stress-stimulated luminescence. Therefore, noise can be further reduced by adding a filter that transmits only stress-stimulated luminescence wavelengths to the front of the measurement unit 12. Figure 15 shows an example configuration of a stress measurement device 1b according to a fifth embodiment. In Figure 15, components common to the stress measurement devices according to the first to fourth embodiments are denoted by the same reference numerals. For example, as shown in Figure 15, a filter 16 that transmits stress-stimulated luminescence wavelengths but does not transmit other wavelengths, such as the wavelength of the excitation light 101, can be provided between the measurement unit 12 and the measurement object 111. This prevents detection of wavelengths that cause noise in stress-stimulated luminescence, such as reflected light from the excitation light 101, thereby improving the accuracy of stress measurement.

Embodiment 6.
In this embodiment, a stress measuring device capable of measuring stress by obtaining the load direction from an image to improve the evaluation accuracy of the stress tensor direction will be described. In the above-described first embodiment, the stress measuring device 1 capable of evaluating the static stress and the fluctuating stress of the irregularly fluctuating stress has been described. However, the stress-luminescent material 112 applied to the measurement object 111 may have feature points such as a random pattern drawn on it.

Figure 16 is a diagram for explaining a stress measurement method using a stress measurement device according to embodiment 6. The configuration of the stress measurement device according to this embodiment is the same as that of embodiment 1, but a random pattern is drawn on the stress-luminescent material 113 applied to the measurement object 111. In addition, the operation of the signal analysis unit 14 differs from that of embodiment 1.

An unknown time-varying load is applied to the measurement object 111, generating an unknown fluctuating stress 121. Therefore, as time elapses from _t1 to _t2 , the pattern depicted as the stress-luminescent body 113 is displaced in the load direction, which is the direction in which the load fluctuates. FIG. 16 shows a simple example in which the only unknown time-varying load is a load in the y-axis direction. However, even if an unknown load with an unknown load direction is applied, the displacement direction can be determined from the displacement amount of the feature point, and the tensor direction of the stress due to the time-varying load can be determined. That is, the signal analysis unit 14 of the stress measuring device 1 according to this embodiment can simultaneously determine the stress tensor direction from the displacement of the feature point in the image over time and the absolute value of the stress from the luminescence intensity of the stress-luminescent body.

Figure 17 is a flowchart showing an example of the operation of the stress measuring device 1 according to the sixth embodiment to evaluate a stress waveform. Note that in Figure 17, the same step numbers are used for processes that are common to those included in the flowchart shown in Figure 6. Explanation of processes that are common to those included in the flowchart shown in Figure 6 will be omitted.

The stress measuring device 1 first acquires an image under a first load (step S31) and then acquires an image under a second load (step S32). Specifically, the measuring unit 12 captures an image of the stress-luminescent material 113 at time _t1 and stores the image in the storage unit 13 as an image 151 under the first load, and then captures an image of the stress-luminescent material 113 at time _t2 and stores the image in the storage unit 13 as an image 152 under the second load.

The stress measuring device 1 then calculates the fluctuating stress tensor using digital image correlation (step S33). Specifically, the signal analysis unit 14 reads the image 151 in the first load state and the image 152 in the second load state from the memory unit 13, analyzes the image 151 in the first load state and the image 152 in the second load state using digital image correlation, and calculates the fluctuating stress tensor.

The stress measuring device 1 then records the stress luminescence intensity I'(t) when an unknown fluctuating stress σ'(t) is applied to the measurement object 111 (step S14).

The stress measuring device 1 then calculates the static stress (step S35). Specifically, the signal analysis unit 14 calculates the static stress using one of the methods described in embodiments 1 to 5.

The stress measuring device 1 then adds the fluctuating stress tensor and the static stress (step S36). Specifically, the signal analysis unit 14 adds the fluctuating stress tensor calculated in step S33 and the static stress calculated in step S35.

Finally, the stress measuring device 1 outputs the evaluation result, i.e., the addition result performed in step S36, to the outside (step S37). The stress measuring device 1, for example, displays the evaluation result on a display device.

In addition, steps S31 to S33 and steps S14 and S35 above may be executed in parallel.

Digital image correlation evaluates stress by comparing images before and after deformation to determine strain, so it cannot evaluate stress that does not fluctuate over time. By simultaneously performing digital image correlation and stress evaluation using mechanoluminescent materials, as in this method, it is possible to evaluate static stress, which does not fluctuate over time.

When drawing feature points on the stress-luminescent material 113 applied to the measurement object 111, the feature points can be drawn by mixing powder such as carbon black into the stress-luminescent material 113 and applying it, or by painting the feature points on the surface of the stress-luminescent material 113.

Next, we will explain the hardware that realizes each part of the stress measurement device 1. The light irradiation unit 11 of the stress measurement device 1 can be realized, for example, by a semiconductor laser. As mentioned above, the measurement unit 12 can be realized by an optical sensor such as a photodiode or a two-dimensional image sensor. The memory unit 13 can be realized by a memory device such as a RAM (Random Access Memory), a ROM (Read Only Memory), a flash memory, an EPROM (Erasable Programmable Read Only Memory), or an EEPROM (registered trademark) (Electrically Erasable Programmable Read Only Memory).

As described above, the signal analysis unit 14 can be implemented using an analysis device. This analysis device may be implemented, for example, as a dedicated hardware processing circuit, specifically, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuit), an FPGA (Field Programmable Gate Array), or a combination of these. Alternatively, it may be implemented as a control circuit composed of a processor and memory that executes programs stored in memory. Figure 18 shows an example of the processing circuit configuration when an analysis device operating as the signal analysis unit 14 is implemented using a processor and memory. The processor 201 is a CPU (Central Processing Unit, also known as a central processing unit, processing device, arithmetic unit, microprocessor, microcomputer, processor, or DSP (Digital Signal Processor)). The memory 202 is, for example, RAM, ROM, flash memory, EPROM, EEPROM, etc. The memory 202 stores programs for operating each component of the signal analysis unit 14, and the processor 201 reads and executes these programs to implement the signal analysis unit 14. When the signal analysis unit 14 is realized by the processor 201 and the memory 202, the program stored in the memory 202 may be provided to a user or the like in a state written on a storage medium such as a CD (Compact Disc)-ROM or a DVD (Digital Versatile Disc)-ROM, or may be provided via a network. Note that the storage unit 13 may be realized by the memory 202.

The configurations shown in the above embodiments are merely examples, and may be combined with other known technologies, or different embodiments may be combined with each other. Parts of the configuration may also be omitted or modified without departing from the spirit of the invention.

The various aspects of this disclosure are summarized below as appendices.

(Appendix 1)
a measuring unit that continuously irradiates excitation light onto a measurement target to which a stress-stimulated luminescent material is applied and to which an unknown fluctuating stress is generated, and measures the luminescence intensity obtained by the irradiation;
a storage unit that stores the emission intensity measured by the measurement unit;
a signal analysis unit that calculates a static stress, which is a static component of the unknown fluctuating stress, based on the time-varying waveform of the emission intensity stored in the storage unit; and
A stress measuring device comprising:
(Appendix 2)
When the fluctuating stress is a periodically fluctuating stress,
the signal analysis unit calculates the static stress based on a ratio of intensities of two successive wave peaks of the time-varying waveform.
2. The stress measuring device according to claim 1,
(Appendix 3)
a light irradiation unit that continuously irradiates the measurement object with the excitation light;
a stress applying unit that applies a periodically varying stress to the measurement object;
Equipped with
the light irradiating unit continuously irradiates the measurement object with the excitation light while the stress is being applied by the stress applying unit;
the measuring unit measures the luminous intensity of the stress-stimulated luminescent material when the light irradiating unit irradiates the stress-stimulated luminescent material with the excitation light.
3. The stress measuring device according to claim 1 or 2.
(Appendix 4)
The signal analysis unit
applying a known fluctuating load to a test piece coated with the stress-luminescent material and irradiating the excitation light to the test piece, adjusting a coefficient of a conversion equation between the fluctuating stress and the emission intensity based on the emission intensity; and calculating the static stress based on the conversion equation after the coefficient adjustment and the emission intensity stored in the storage unit.
4. The stress measuring device according to claim 1, wherein:
(Appendix 5)
The transformation equation includes terms for a static component of stress, a dynamic component, and a time derivative of the dynamic component.
5. The stress measuring device according to claim 4,
(Appendix 6)
the signal analysis unit, when there are a plurality of fluctuating stresses obtained by converting the emission intensity using the conversion formula, selects a fluctuating stress that is physically predicted to be plausible;
6. The stress measuring device according to claim 4 or 5.
(Appendix 7)
the signal analysis unit, when there are a plurality of fluctuating stresses obtained by converting the emission intensity using the conversion formula, selects a fluctuating stress that puts the measurement object on the safer side;
6. The stress measuring device according to claim 4 or 5.
(Appendix 8)
the signal analysis unit, when there are a plurality of fluctuating stresses obtained by converting the emission intensity using the conversion formula, selects one of the fluctuating stresses based on a comparison result with a stress measured at a certain point of the measurement object by a method not using the mechano-luminescent material;
6. The stress measuring device according to claim 4 or 5.
(Appendix 9)
The signal analysis unit
acquiring, as a correction fluorescence intensity, a luminescence intensity obtained by irradiating the measurement object with the excitation light in a state in which the fluctuating stress is not applied;
correcting the luminescence intensity stored in the storage unit using the correction fluorescence intensity;
Calculating static stress using the corrected emission intensity.
9. The stress measuring device according to claim 1,
(Appendix 10)
the wavelength of the excitation light is different from the emission wavelength of the stress-luminescent material;
10. The stress measuring device according to any one of claims 1 to 9.
(Appendix 11)
a filter having a characteristic of transmitting the wavelength of light emitted by the stress-luminescent material in response to irradiation with the excitation light but not transmitting the wavelength of the excitation light is provided between the measurement object and the measurement unit;
11. The stress measuring device according to claim 10.
(Appendix 12)
an optically identifiable feature is formed on the surface of the stress-luminescent material;
The signal analysis unit further calculates the tensor direction of the fluctuating stress by analyzing a plurality of images obtained by photographing the characteristic points at different times using a digital image correlation method.
12. The stress measuring device according to any one of claims 1 to 11.

1, 1a, 1b Stress measurement device, 11 Light irradiation unit, 12 Measurement unit, 13 Memory unit, 14 Signal analysis unit, 15 Stress application unit, 16 Filter, 101 Excitation light, 111 Measurement object, 112, 113 Stress-luminescent body, 121 Stress, 122 Static component, 123 Dynamic component, 131 First peak, 132 Second peak, 151 Image of first load state, 152 Image of second load state.

Claims (11)

a measuring unit that continuously irradiates excitation light onto a measurement target to which a stress-stimulated luminescent material is applied and to which an unknown fluctuating stress is generated, and measures the luminescence intensity obtained by the irradiation;
a storage unit that stores the emission intensity measured by the measurement unit;
a signal analysis unit that calculates a static stress, which is a static component of the unknown fluctuating stress, based on the time-varying waveform of the emission intensity stored in the storage unit; and
Equipped with
The fluctuating stress is a periodically fluctuating stress,
the signal analysis unit calculates the static stress based on a ratio of intensities of two successive wave peaks of the time-varying waveform.
A stress measuring device characterized by: a measuring unit that continuously irradiates excitation light onto a measurement target to which a stress-stimulated luminescent material is applied and to which an unknown fluctuating stress is generated, and measures the luminescence intensity obtained by the irradiation;
a storage unit that stores the emission intensity measured by the measurement unit;
a signal analysis unit that calculates a static stress, which is a static component of the unknown fluctuating stress, based on the time-varying waveform of the emission intensity stored in the storage unit; and
Equipped with
The signal analysis unit
applying a known fluctuating load to a test piece coated with the stress-luminescent material and irradiating the excitation light to the test piece, adjusting a coefficient of a conversion equation between the fluctuating stress and the emission intensity based on the emission intensity; and calculating the static stress based on the conversion equation after the coefficient adjustment and the emission intensity stored in the storage unit.
A stress measuring device characterized by: a light irradiation unit that continuously irradiates the measurement object with the excitation light;
a stress applying unit that applies a periodically varying stress to the measurement object;
Equipped with
the light irradiating unit continuously irradiates the measurement object with the excitation light while the stress is being applied by the stress applying unit;
the measuring unit measures the luminous intensity of the stress-stimulated luminescent material when the light irradiating unit irradiates the stress-stimulated luminescent material with the excitation light.
3. The stress measuring device according to claim 1 or 2. The transformation equation includes terms for a static component of stress, a dynamic component, and a time derivative of the dynamic component.
3. The stress measuring device according to claim 2. The conversion formula is expressed in the form of formula (1) using static stress σ _s which is a static component of stress, dynamic stress σ _d which is a dynamic component of stress, and coefficients A″ to F″, and includes a product term of static stress σ _s and dynamic stress σ _d :
3. The stress measuring device according to claim 2.
The measurement object is a structural material,
the signal analysis unit, when there are a plurality of fluctuating stresses obtained by converting the emission intensity using the conversion formula, selects a fluctuating stress that results in a greater degree of fatigue damage to the measurement object due to the fluctuating stress;
3. The stress measuring device according to claim 2. the signal analysis unit, when there are a plurality of fluctuating stresses obtained by converting the emission intensity using the conversion formula, selects one of the fluctuating stresses based on a comparison result with a stress measured at a certain point of the measurement object by a method not using the mechano-luminescent material;
3. The stress measuring device according to claim 2. The signal analysis unit
acquiring, as a correction fluorescence intensity, a luminescence intensity obtained by irradiating the measurement object with the excitation light in a state in which the fluctuating stress is not applied;
correcting the luminescence intensity stored in the storage unit using the correction fluorescence intensity;
Calculating static stress using the corrected emission intensity.
3. The stress measuring device according to claim 1 or 2. the wavelength of the excitation light is different from the emission wavelength of the stress-luminescent material;
3. The stress measuring device according to claim 1 or 2. a filter having a characteristic of transmitting the wavelength of light emitted by the stress-luminescent material in response to irradiation with the excitation light but not transmitting the wavelength of the excitation light is provided between the measurement object and the measurement unit;
10. The stress measuring device according to claim 9 . an optically identifiable feature is formed on the surface of the stress-luminescent material;
The signal analysis unit further calculates the tensor direction of the fluctuating stress by analyzing a plurality of images obtained by photographing the characteristic points at different times using a digital image correlation method.
3. The stress measuring device according to claim 1 or 2.