JP5000895B2 - Position correction method in infrared thermoelastic stress measurement - Google Patents

Description

  The present invention relates to a position correction method in infrared thermoelastic stress measurement, and in particular, position correction in infrared thermoelastic stress measurement that can accurately correct displacement / deformation of an object to be measured within an image field in an infrared thermoelastic stress measurement device. Regarding the law.

  Infrared thermography is a device that measures an infrared energy distribution radiated from the surface of an object with an infrared sensor, converts the image into a temperature distribution, images it, and displays it. FIG. 1 is a diagram showing an example of stress measurement using an infrared thermography. A variable load is repeatedly applied as a load to the object to be measured, and only temperature fluctuations that vary in synchronization with the load signal are output from the output of the infrared sensor. By taking out and integrating and averaging for each load cycle, it is possible to measure temperature with high resolution and high accuracy.

  2. Description of the Related Art In recent years, thermography that is superior in resolution, accuracy, and spatial resolution of temperature measurement and that can measure a transient phenomenon of temperature fluctuation at high speed has been developed due to advances in infrared sensor and output signal processing technology from the sensor. Against the backdrop of such advances in infrared image measurement technology, infrared that measures fluctuations in stress acting on an object based on the measurement of minute temperature fluctuations of the object that occur during elastic deformation, that is, thermoelastic temperature fluctuations Thermoelastic stress measurement technology has made progress (for example, see Non-Patent Document 1).

  In addition to infrared thermoelastic stress measurement technology, numerical analysis methods such as the finite element method (FEM) and boundary element method (BEM) have made significant progress, and it is possible to obtain stress distribution with high accuracy even for members with complex shapes. It became possible. However, when the load condition and boundary condition acting on the structural member cannot be easily determined, the stress / strain distribution cannot often be correctly evaluated by numerical analysis. In such a case, an experimental mechanical stress measurement technique such as an infrared thermoelastic stress measurement method is effective.

  In stress measurement using an infrared thermoelastic stress measurement device, attention must be paid to the movement and deformation of the object to be measured within the measurement field of view when a variable load is applied. If the object to be measured is greatly displaced or deformed in the measurement field of view and the stress is greatly changed locally, an error occurs in the measured value of the thermoelastic temperature fluctuation in each pixel. When measuring the stress distribution in a small area using the magnifying optical system, the influence of the displacement in the measurement field becomes larger.

  Explaining the current status of position correction technology in thermoelastic stress measurement, thermoelastic stress measurement generally realizes a reversible state in which the thermoelastic effect of temperature drop under the action of tensile stress and temperature rise under the action of compressive stress is established. Measurement is possible under the conditions. Therefore, at the time of measurement, it is necessary to measure the temperature fluctuation of the measurement object in a state in which the load is dynamically applied to the measurement object and the stress varies. However, if a load acts on the object to be measured, the object to be measured naturally undergoes displacement / deformation. When the measured object is greatly displaced and deformed, and the infrared measurement value changes greatly in that portion, an error occurs in the measurement value in each pixel.

  Now, as shown in FIG. 2, the object to be measured is displaced from the state shown in FIG. 2 (a) when the minimum load is applied to the state shown in FIG. 2 (b) when the maximum load is applied. -Consider the case of deformation. In the figure, a to c and 1 to 5 indicate horizontal and vertical pixels, respectively. When the object to be measured is displaced / deformed within the field of view, each pixel thus measures the thermoelastic temperature fluctuation at different locations of the object to be measured.

  Therefore, when the displacement / deformation is small and the movement within the pixel remains, no large measurement error occurs. However, when the displacement / deformation is large and occurs beyond the pixel, the measurement value at each pixel has an error. Occurs. In particular, as shown in the pixels (b, 2), (c, 2), (b, 4) and (c, 4) in FIG. When the difference is measured, the measurement error becomes very large. Since such an error becomes prominent in the contour portion of the object to be measured, it is called an edge effect.

In addition, in order to easily perform position correction, an infrared stress image measuring apparatus having a function capable of measuring a visible image having the same optical axis and the same field of view as a temperature image is used in combination with the temperature image and the visible image. A stress distribution image measurement method for performing position correction is disclosed (for example, see Patent Document 1).
Takahide Sakagami: Thermoelastic stress measurement by infrared thermography, Journal of the Japan Welding Society, 72-6 (2003), 511-515. JP 2001-41831 A

  However, in order to fundamentally solve the above-described problems, it is necessary to correct the measurement position shift due to the displacement / deformation of the object to be measured by some means. In view of this, an infrared thermoelastic stress measurement apparatus using an infrared thermography currently on the market is provided with the following position correction function.

(a) Optical position correction This is a function for optically correcting the displacement of the measurement position in the measurement visual field due to the rigid body movement of the measurement object so that the measurement visual field follows the displacement of the measurement area of the measurement object. In an infrared thermoelastic stress measurement apparatus using a combination of an initial single infrared sensor and a reflecting mirror, a method of adjusting the swinging amount of the reflecting mirror according to the displacement was used. Infrared thermoelastic stress measurement equipment using a focal plane array infrared sensor has developed a special lens that can keep the measurement field of view constant by shifting a specific lens in the infrared optical system from the optical axis. It was done.

  However, in any of these cases, although there is an effect on the rigid body movement of the object to be measured, there is a problem that position correction cannot be performed when the object to be measured is deformed.

(b) Position correction method by software This is a method of performing position correction by software post-processing based on continuous image data related to the measured temperature distribution of the measured object. Most of infrared thermoelastic stress measuring devices currently on the market have a position correction function by this method. Select feature points that can represent the displacement / deformation of the object to be measured from the measured temperature distribution continuous images, and specify the position of each feature point at the maximum load and minimum load load on the software by the user Thus, information on the displacement / deformation of the object to be measured is obtained. Unlike the optical position correction described above, it is possible to cope with rigid body displacement, enlargement / reduction, and rotation of the measurement region.

  However, it is not easy for a user to accurately specify a feature point and track its movement in an infrared image with few features, and an error is inevitable in position correction. If the displacement or deformation is not uniform, there is a problem that it is impossible to perform position correction. Also, automation is impossible.

  Further, in the stress distribution image measurement method shown in Patent Document 1, since the visible image at the upper and lower peak values of the load load is simply used in combination with the infrared stress measurement image, for example, at an intermediate position of the load load. Accurate image correction cannot be performed, and a continuous corrected image cannot be obtained.

  In order to solve the above problems, the present invention proposes two position correction methods based on the digital image correlation method. One proposes a position correction method in infrared thermoelastic stress measurement that can apply the digital image correlation method only to the measurement image by infrared thermography, and can perform position correction more accurately and automatically.

  Another method is to measure the visible image from the digital video camera simultaneously with the infrared image, and to obtain displacement distribution information for position correction by applying the digital image correlation method to the visible image, thereby performing position correction more accurately. We propose a position correction method in infrared thermoelastic stress measurement.

In order to solve the above problems, the position correction method in the infrared thermoelastic stress measurement according to the present invention is a measurement data on the displacement distribution of the object to be measured in which a spot pattern having a different emissivity depending on the position of the surface is applied in advance A first measurement step of measuring time-series infrared intensity distribution data using infrared thermography, and applying a digital image correlation method to a frame constituting the acquired time-series infrared intensity distribution data, and using a load loading device Calculating the amount of displacement data related to the displacement distribution of the measured object with respect to the load load history, which is the load load history, and measuring time-series infrared intensity distribution data as stress data for thermoelastic stress measurement using infrared thermography And the time series infrared intensity distribution data obtained in the second measurement step. The distribution data for each data pixel, characterized in that it comprises a position correcting step of converting the distribution data for each coordinate obtained by correcting the displacement on the basis of the displacement amount data relating to the displacement distribution.

  With this configuration, the measurement data related to the displacement distribution is measured in the first measurement step using the object to be spotted with different emissivity, and the digital image correlation method is used in the calculation step using the measurement data. Since the displacement distribution data of the object to be measured is used, the accuracy of the displacement distribution data can be improved. Further, in the conversion correction step, the stress data of the object measured in the second measurement step can be corrected based on the displacement amount data. It is possible to achieve a position correction method based on a digital image correlation of a spot pattern having a different emissivity and realizing a high position correction.

  The position correction method according to the present invention further includes a third measurement step of measuring infrared intensity distribution data for thermoelastic temperature fluctuation measurement using the infrared thermography, and a thermoelastic temperature fluctuation in the three measurement steps. Simultaneously with the measurement, by applying a digital image correlation method to the displacement measurement step for obtaining the displacement measurement data of the object to be measured in the same visual field using a visible camera, and the displacement measurement data using the visible camera A second calculation step for obtaining time series displacement distribution information for all frames, and the infrared intensity distribution data acquired in the third measurement step based on the time series displacement distribution information obtained in the second calculation step. And a second position correction step for performing position correction.

  With this configuration, displacement measurement data can be obtained using a visible camera simultaneously with thermoelastic temperature fluctuation measurement, and time series displacement distribution information can be obtained by applying a digital image correlation method to the displacement measurement data. In the position correction step, the stress data can be corrected based on the time-series displacement distribution calculated in the second calculation step. A more accurate position correction method can be achieved.

  By using the position correction method in the infrared thermoelastic stress measurement according to the present invention, there is an effect that the stress measurement accuracy by the infrared thermoelastic stress measurement device that is rapidly spread as an in-situ measurement method of the stress distribution can be improved. .

  Hereinafter, a position correction method in thermoelastic stress measurement according to the present invention will be described with reference to the drawings.

(Embodiment)
FIG. 3 shows a functional block diagram of the infrared stress measurement apparatus 100 used for thermoelastic stress measurement according to the present invention.

  An infrared stress measuring apparatus 100 with an automatic position correction function according to the present invention includes a load generating unit 101 that repeatedly applies a load to the object to be measured, an infrared thermography 102 that measures the temperature of the object to be measured while synchronizing the load timing, and an infrared ray A time-series intensity distribution acquisition unit 103 that acquires time-series intensity distribution data of an object to be measured from measured values using the thermography 102, and a displacement amount due to movement / deformation of the object due to a load generated in the load generator 101. The displacement data is calculated by the digital image correlation method using the time series intensity distribution data, and the stress data of the measured object is measured from the time series intensity distribution data measured using the infrared thermography 102. Stress measurement unit 105, position correction of stress data after displacement / deformation of measured object based on displacement amount data And an image display unit 107 for displaying an infrared image after position correction unit 106, and the position correction performed.

  In addition, as described above, in infrared measurement images, in general, only the distribution of infrared intensity (luminance value) is often poor in feature quantity. For this reason, when the digital image correlation method is applied to the infrared image of the object to be measured, the amount of information on the luminance value distribution that represents the displacement / deformation of the object to be measured is insufficient, so accurate displacement distribution measurement is possible. It will be impossible.

  Therefore, in the present invention, in order to improve the correlation in the digital image correlation method, the surface of the object to be measured is preliminarily provided with several kinds of speckle patterns between white to gray to black, and an infrared image is obtained. The difference in emissivity also affects the infrared intensity (luminance value) so that the speckled pattern can be clearly confirmed. Thus, the displacement distribution can be obtained with high accuracy even when the digital image correlation method is applied to the infrared image.

  The speckled pattern is applied using a black or gray spray paint or a transfer sheet on which a speckled pattern having a different emissivity is applied in advance. Alternatively, a spray nozzle that can spray paints having different emissivities at once to give an ideal spot pattern may be used.

  The operation procedure of the position correction method in the thermoelastic stress measurement according to the present embodiment will be described using the flowchart shown in FIG.

  First, time-series infrared intensity distribution data of an object to be measured with a speckle pattern is acquired under the same load condition in thermoelastic stress measurement (S401). A digital image correlation method, which will be described later, is applied to all frames of the acquired time-series infrared data to obtain a displacement distribution of the object to be measured with respect to the load load history (S402). Through the above process, the displacement amount data of the displacement distribution of the object to be measured is acquired.

  Next, after removing the speckle pattern of the object to be measured and applying a matte black paint, time series infrared intensity distribution data for thermoelastic stress measurement is acquired (S403). In this process, the stress data of the object to be measured is measured. It is also conceivable to apply a matte black paint on the spotted pattern.

  Then, based on the measurement data relating to the displacement distribution obtained previously, the acquired time-series infrared intensity distribution is converted from the distribution data for each pixel into distribution data for each coordinate whose displacement is corrected (S404).

  With the above position correction method, even if the object to be measured is displaced or deformed, it is possible to perform image correction so that infrared energy can always be measured at the same position, and therefore, under the same load load conditions with reproducibility, It is possible to perform position correction in the infrared thermoelastic stress measurement apparatus with high accuracy.

(Position correction method using visible image)
According to the above-described method, position correction with high accuracy can be performed using only infrared measurement data. However, in the displacement measurement for the object to be spotted, it is required that the same displacement / deformation as in the thermoelastic stress measurement occurs. For this reason, it is impossible to perform position correction under an impact load or random load with poor reproducibility. In order to solve this problem, we propose a method to perform the same visual field and simultaneous measurement of visible image and infrared image. In this case, an infrared thermography and a visible camera having the same measurement field of view and measurement speed are prepared.

  FIG. 5 is a flowchart showing an operation procedure in a method of performing the same field of view and simultaneous measurement of an infrared image using a visible camera.

  First, under a dynamic load, thermoelastic temperature fluctuation measurement is performed by infrared thermography (S501), and at the same time, the displacement / deformation of the object to be measured in the same field of view is measured by a visible camera (S502). By applying the digital image correlation method to the measurement data of the visible camera, displacement distribution information for all frames is obtained (S503). At this time, it is conceivable that the characteristic amount is lost in the visible data due to the application of the matte black paint for infrared measurement. In such a case, image processing is performed so as to enhance the image correlation so that the surface roughness due to the matte paint is emphasized in an image having an appropriate contrast.

  Finally, based on the obtained time-series displacement distribution information, position correction of infrared intensity distribution data for thermoelastic stress measurement is performed (S504).

  With the above position correction method, even if the object to be measured is displaced or deformed, the displacement data can be calculated using the digital image correlation method for the measurement data using the visible camera, and the infrared rays at the same position can be calculated. Image correction can be performed so that energy can be measured. Therefore, position correction in a highly accurate infrared thermoelastic stress measurement apparatus can be performed even under dynamic load conditions with no reproducibility.

(Digital image correlation method)
Next, the digital image correlation method will be described.

  Displacement distribution measurement by the digital image correlation method uses a technique that finds the movement destination of a partial area (subset) in a digital image by the same point search based on image correlation, which is widely used in general. In the search, a rough search of the destination of the subset was performed by a coarse search, and then a precise search by the Newton-Raphson method was performed.

First, in the coarse search, only the translation of the subset is assumed.When the center coordinates (x, y) of the subset move to (x ^* = x + u, y ^* = y + v), The amount of movement that minimizes the correlation function represented by the sum of squared differences was determined using the following (Equation 1).

  Here, F represents the luminance value of the subset before movement, and G represents the luminance value of the subset after movement. In order to improve convergence, nine subpixels were provided between each pixel in the subset, and luminance values of the subpixels were obtained by linear interpolation, and these were used for calculating the residual sum of squares.

  Next, a search by the Newton-Raphson method was performed using the movement amount of the subset obtained by the coarse search as an initial value. The subset was set to a size of 35x35 pixels. As the correlation function R, the following (Equation 2) was used.

Here, x ^* and y ^* are coordinates after movement, and in addition to the parallel movement used in the coarse search, a constant distortion is assumed in the following (Equation 3). Here, constant strain is assumed, but it is also possible to consider deformation due to higher-order strain of the subset.

Δx and Δy indicate distances in the x and y directions from the subset center. When the Newton-Raphson method is applied, it is necessary to obtain the luminance value and the gradient of the luminance value at a certain coordinate of the subpixel. For this reason, the following linear interpolation was used. If the position of (x ^* , y ^* ) is surrounded by four grid points (i, j), (i + 1, j), (i, j + 1) and (i + 1, j + 1) (x ^*, y ^*) luminance value G (x ^*, y ^*) of is given by the following equation (4).

Here, x ′ and y ′ take values from 0 to 1 in the distance from (i, j) to (x ^* , y ^* ).

  After obtaining the movement amount of the subset center coordinates by Newton-Raphson method, the movement amount for all points in the image after deformation was obtained by linear interpolation.

(Experimental result)
Next, of the two proposed position correction methods, experimental results regarding position correction in which the digital image correlation method is applied only to the infrared measurement image will be shown.

(Correction of translation)
A flat plate with a circular hole as shown in FIG. 6 was used as a test piece. The test piece was spotted with black and gray spray paint as shown in the figure.

  It was experimentally examined whether or not position correction was possible when the test piece was placed on a moving stage and the test piece was translated. The experimental results are shown in FIG. An infrared image before movement is shown in FIG. 7A, and an infrared image after translation is shown in FIG. 7B. There is a shift of 19 pixels in the vertical direction before and after the movement. FIG. 7C shows the result of position correction applied to the images before and after movement by applying the digital image correlation method. The spot pattern in FIG. 7 (c) completely coincides with the spot pattern before movement shown in FIG. 7 (a), indicating that the position correction is complete.

(Displacement / deformation correction)
Next, the result of a position correction experiment in a tensile test on a flat plate will be shown. Measure the infrared image of the flat test piece when it is loaded in steps of 1,96kN from -9,8kN (compressed) to 9,8kN, and based on this, position by digital image correlation method Correction was performed. The test pieces were spotted with black and gray spray paints as in the case of translation.
The experimental results are shown in FIG. FIG. 8A shows an infrared image when the load is 0, and FIG. 8B shows an infrared image when the load is 9 or 8 kN. Further, FIG. 8 (c) shows the result of applying position correction to the image before and after deformation by applying the digital image correlation method. The spot pattern in Fig. 8 (c) is completely consistent with the spot pattern in the case of zero load shown in Fig. 8 (a), indicating that the position correction can be performed completely even when there is deformation. ing.

  As described above, in the present invention, the movement and deformation of the object to be measured in the measurement visual field at the time of infrared thermoelastic stress measurement are automatically detected by the digital image correlation method, and the position in the thermoelastic stress measurement is based on this. A method for automatic correction was developed. Two position correction methods, one using infrared image and the other using visible image, are proposed.

  As a result of experiments on position correction using the digital image correlation method for infrared measurement images, it became clear that the proposed method and the developed displacement can be corrected with high accuracy.

  By using the position correction method in infrared thermoelastic stress measurement according to the present invention, a high-performance infrared thermoelastic stress measurement system using a combination of a high-performance infrared camera and a PC can be developed. Furthermore, by adding a visible high-definition camera to this, it is possible to develop a high-performance infrared thermoelastic stress measurement system that can handle dynamic loads without reproducibility.

Diagram showing an example of stress measurement using infrared thermography FIG. 2 (a), which is the state when the minimum load is applied within the measurement field of view, and FIG. 2 (b), which is the state when the maximum load is applied Functional block diagram of an infrared stress measurement device used for thermoelastic stress measurement according to the present invention. Free chart showing operation procedure of position correction method in thermoelastic stress measurement according to this embodiment Flow chart showing the operation procedure in the method of performing the same field of view and simultaneous measurement of infrared images for visible images The figure which shows an example of the speckled pattern provided to the test piece The figure which shows the experimental result of position correction when moving a test piece in parallel The figure which shows the experimental result of position correction when a flat specimen is loaded stepwise

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Infrared stress measuring device 101 Load generation part 102 Infrared thermography 103 Time series intensity distribution data acquisition part 104 Displacement calculation part 105 Stress measurement part 106 Position correction part 107 Image display part

Claims (9)

A first measurement step for measuring time-series infrared intensity distribution data using infrared thermography as measurement data relating to the displacement distribution of the object to be measured, which is preliminarily provided with a speckled pattern having a different emissivity depending on the position of the surface;
The digital image correlation method is applied to the frames constituting the acquired time-series infrared intensity distribution data, and the displacement amount data relating to the displacement distribution of the measured object with respect to the load load history that is the load load history by the load load device Calculating step for obtaining
A second measurement step of measuring time-series infrared intensity distribution data as stress data for thermoelastic stress measurement using an infrared thermography;
A position correction step for converting distribution data for each pixel of the time-series infrared intensity distribution data acquired in the second measurement step into distribution data for each coordinate in which displacement is corrected based on the displacement amount data related to the displacement distribution; A position correction method in infrared thermoelastic stress measurement, characterized by comprising: The speckled pattern applied to the object to be measured is composed of several kinds of shades of white to black in order to change the infrared emissivity stepwise depending on the position of the surface. Correction method for infrared thermoelastic stress measurement. The position in the infrared thermoelastic stress measurement according to claim 1 or 2, wherein the spot pattern is applied using a black or gray spray paint or a transfer sheet having a spot pattern having a different emissivity in advance. Correction method. The position correction method in infrared thermoelastic stress measurement according to claim 1, wherein the measurement in the second measurement step is performed after removing the speckle pattern of the object to be measured and applying a matte black paint. Time series infrared intensity distribution data as measurement data related to the displacement distribution of the object to be measured, which is preliminarily provided with speckled patterns with different emissivities depending on the position of the surface, and time series as stress data for thermoelastic stress measurement Infrared thermography to measure infrared intensity distribution data,
A calculation means for applying a digital image correlation method to a frame constituting the time-series infrared intensity distribution data as measurement data relating to the displacement distribution to obtain displacement amount data relating to the displacement distribution of the measured object with respect to the load / load history. When,
Position correction means for converting distribution data for each pixel of the time-series infrared intensity distribution data as the stress data into distribution data for each coordinate in which displacement is corrected based on the displacement amount data relating to the displacement distribution. Infrared thermoelastic stress measuring device characterized by. The spot pattern applied to the DUT, in order to stepwise change the infrared emissivity by the position of the surface, according to claim 5, characterized in that it consists of several different shades with black from white Infrared thermoelastic stress measuring device. The infrared thermoelastic stress measurement device according to claim 5 or 6 , wherein the spot pattern is applied using a black or gray spray paint or a transfer sheet having a spot pattern having a different emissivity. The infrared thermoelastic stress measurement apparatus according to claim 5 , wherein the infrared thermography performs the measurement of the stress data after removing a spotted pattern of the object to be measured and applying a matte black paint. A program characterized by executing the position correction method according to claim 1 Symbol mounting to the computer.