JP5966204B2 - Stress distribution measuring device - Google Patents

Description

The present invention relates to a stress distribution measuring apparatus for measuring a stress distribution of a transparent material, a polymer material, or the like, which uses a color phase shift photoelastic method.

The photoelastic method can experimentally obtain the principal stress difference and principal stress direction in the birefringent object in all fields, and further obtain the stress component from those values by using the shear stress difference integration method. Is the method. The main stress difference and the main stress direction can be determined by assigning a fringe order and an angle called a photoelastic parameter to the equal color fringe and the isotropic line fringe obtained from the photoelastic method, respectively.

The full-field automatic measurement method of this photoelastic method is obtained by rotating an optical component such as a polarizing plate to a specified angle from a circular polarizer (including a semicircular polarizer) having a multi-wavelength light source or a plane polarizer. Several color phase shift photoelastic methods using a plurality of color images (referred to as color phase shift images) have been devised. In order to determine the stress component from the color phase shift photoelastic method, the absolute fringe order determined by some method such as phase connection from the relative fringe order of the isochromatic line fringes and the principal stress direction obtained from the isotropic fringes are required. It becomes.

K. Ramesh, “Digital Photoelasticity”, Springer, (2000) K.R. Madhu, R.G.R.Prasath and K. Ramesh, “Colour Adaptation in Three Fringe Photoelasticity”, Experimental Mechanics (2007), 47: 271-276 Umezaki / Koike / Watanabe: Automatic measurement of photoelastic strip order by generalized phase shift method, Proceedings of the Japan Society of Mechanical Engineers (A), 62-599 (1996), pp. 1690-1695 Ogasawara, Kodama, and Umezaki: Determination of principal stress direction using colored photoelastic stripes, Proceedings of the 34th Symposium on Stress and Strain Measurement, (2003), pp.3-6

Until now, in order to determine the absolute fringe order from the relative fringe order obtained using the arc tangent function or the inverse cosine function, find the known absolute fringe order such as the 0 (zero) order fringe, and from there It was necessary to scan in the horizontal and vertical directions of the image and add and subtract the phases so that the fringe order (phase distribution) is continuous. This method makes it difficult to determine the absolute fringe order when zero-order fringes do not appear or when there are discontinuous points such as holes. Further, since not only the information of the point for which the absolute fringe order is desired to be determined but also the peripheral information, there is a problem that a determination error is likely to occur.

As another method, a method of determining the absolute fringe order from a single color equal color line fringe obtained from a circular polarizer has been devised (Non-Patent Document 1). This is done by carrying out a calibration experiment in advance, creating a calibration table from the relationship between the red, green, and blue luminance values of the image and the absolute fringe order, and calculating the red, green, and blue luminance values of the measurement image. The calibration tables are compared, and the closest order is determined as the absolute fringe order. Basically, the absolute fringe order can be determined independently for each point. However, simply comparing the luminance values of Red, Green, and Blue gives the wrong result due to fluctuations in the light intensity of the light source and the effects of color unevenness. was there. A method for improving this misjudgment has been devised in Non-Patent Document 2, but information around the points is required, the absolute fringe order cannot be determined independently for each point, and white light (halogen) is used as the light source. Therefore, there is a problem that the fringes having a fringe order of 3rd or higher due to the influence of the crosstalk of the wavelengths of the red, green, and blue internal filters of the image sensor cannot be analyzed because the light intensity is attenuated.

As a countermeasure against wavelength crosstalk, it is necessary to make the relative spectral width of each wavelength narrow and make the intensity level uniform. Therefore, a combination of a multi-wavelength light source with a plurality of lasers and a beam expander or a combination of a white light source with a plurality of interference filters of Red, Green, and Blue and an ND filter has been proposed (Non-Patent Document 1). As a result, it is possible to cope with even higher-order stripes of about the sixth stripe, but there is a problem that the apparatus becomes expensive because there are many components.

Therefore, in the present invention, the light source is an LED three-wavelength light source, and a single RGB interference filter is attached to the image sensor, so that even higher-order fringes can be measured with an inexpensive apparatus configuration, and zero-order fringes are known. It is an object of the present invention to provide a stress distribution measuring apparatus that can accurately determine the absolute fringe order independently of each point without requiring the fringe order and that determines the stress component using the principal stress direction obtained at the same time.

Calculates the relative fringe order from a color phase shift image obtained by photographing an object image in a circular polarizer (including a semicircular polarizer) having a light source or a plane polarizer with an imaging device, and is a known object in advance. Calculate the absolute fringe order calibration curve from the multi-wavelength relative fringe order by, and refer to this to measure the stress distribution by determining the absolute fringe order from the multi-wavelength relative fringe order of the object The apparatus is characterized in that the light source is a three-wavelength LED light source, a single RGB interference filter is attached in front of the image sensor, and mask calculation is performed to determine the upper limit of the applied fringe order when calculating the absolute fringe order. A stress distribution measuring apparatus was used.

According to the present invention, the light source is an LED three-wavelength light source, and a single RGB interference filter is attached to the imaging device, so that even higher-order fringes can be measured with an inexpensive device configuration, and absolute values such as zero-order fringes are known. The fringe order is not required, and the absolute fringe order can be determined independently for each point. As a result, it is possible to provide a stress distribution measuring apparatus capable of accurately determining a stress component at a low cost.

  In the following, the method of determining the absolute fringe order in the color phase shift photoelastic method of the present invention, the method of enabling analysis up to higher fringes by using an inexpensive light source and one filter, and implementation of the stress distribution measuring device A form is demonstrated in detail with drawing.

In FIG. 1, the block diagram of the stress distribution measuring apparatus of this invention is shown. The stress distribution measuring apparatus includes a light source unit 1, a diffuser plate 2, a polarizer 3, an analyzer 5, a filter 6, an image sensor 7, a control signal processing unit 8, an input unit 9, and a display unit 10. Etc. The sample 4 is disposed between the polarizer 3 and the analyzer 5.

The light source unit 1 uses a red, green, and blue LED and an RGB interference filter that functions as a red, green, and blue interference filter in order to form multi-wavelength light. In the present embodiment, the relative fringe order of each wavelength is obtained from the light intensities of a plurality of color phase shift images, and the absolute fringe order is determined based on the relative fringe orders. Is stabilized so that it does not fluctuate over time, and in order to align the intensity level, one that can be adjusted independently for each wavelength is used.

The diffusing plate 2 has a function of diffusing the light emitted from the light source unit 1 and reducing luminance unevenness.

If the polarizer 3 and the analyzer 5 constitute a plane polarizer, and the sample 4 is placed between them to transmit light, the color photoelastic fringes in a state where the isochromatic stripes and the isoclinic stripes overlap each other. can get.

As the filter 6, an RGB interference filter having a function of an interference filter of Red, Green, and Blue is used. By attaching it in front of a color CCD camera or color CMOS camera, it has a function of reducing the influence of red, green, and blue wavelength crosstalk. A schematic diagram of the relative spectral response is shown in FIG.

The image sensor 7 is composed of a color CCD camera, a color CMOS camera, or the like, and has a function of converting the luminance distribution of the color photoelastic fringes obtained above into a digital signal.

The control / signal processing unit 8 is configured by a personal computer or the like, and controls the timing of imaging with the imaging device 6 while rotating the polarizer 3 and the analyzer 5 to a specified angle, and the obtained color phase shift. The digital signal of the image is stored in the memory, and the relative fringe orders and principal stress directions of the three wavelengths of the sample 4 are obtained from the signal by calculation. Further, the absolute signal is calculated by referring to the absolute fringe order calibration curve. The fringe order is determined independently for each point. Thereby, the stress component of the sample 4 can also be measured.

The input unit 9 includes a keyboard, a pointing device, and the like, and is used by a user to input data and commands to the control / signal processing unit 8.

The display unit 10 is configured by a display, a printer, or the like, and displays a calculation result from the control / signal processing unit 8 to the user.

In this embodiment, a color phase shift photoelastic method using three wavelengths of Red, Green, and Blue is employed. In this method, the relative fringe order (0 to 0.5 order, 0 to 0.25 order, or 0 to 1.0 order) and the principal stress direction of each of the three wavelengths can be obtained. Such a phase shift photoelasticity method is not described here because it is a method commonly used in the field of photoelasticity.

Next, the principle of the present invention will be described in detail. First, in order to determine the calibration curve, an experiment is performed on a beam subjected to a four-point bending load as an example of a known object in the configuration shown in FIG. 1, and the three wavelengths Red, Green, and Blue obtained by the color phase shift photoelastic method are used. Get the relative fringe order. An example of the stripe is shown in FIG. (A) is an image showing the relative fringe order of Red, (b) is an image showing the relative fringe order of Green, and (c) is an image showing the relative fringe order of Blue. Black corresponds to the 0th order and white corresponds to the 0.5th order, and these intermediate colors are displayed in gray.

In the case of a beam subjected to this four-point bending load, the absolute fringe order of the central vertical section increases linearly from the black part at the center of the beam in the vertical direction, and takes the maximum value at the upper and lower ends. FIG. 4 shows an absolute fringe order calibration curve that can be created from the relationship between the relative fringe order of red, green, and blue in the upward direction and the absolute fringe order from the black portion zeroth order fringe at the center of the central vertical section of FIG. This is an example in which the absolute fringe order is up to the sixth fringe.

If this absolute fringe order calibration curve is created and stored in the memory of the control signal processing unit, the absolute fringe order can be determined independently for each point by finding a combination of relative fringe orders that minimizes E in the following relational expression. The order can be determined.

E = Wr (Rc-Re) ^ 2 + Wg (Gc-Ge) ^ 2 + Wb (Bc-Be) ^ 2

Here, Rc, Gc, and Bc are relative fringe orders of Red, Green, and Blue of the absolute fringe order calibration curve obtained by the above-mentioned beam experiment, and Re, Ge, and Be are obtained from the experiment of the sample to be analyzed. The relative fringe orders of Red, Green, and Blue, and Wr, Wg, and Wb are coefficients that weight the ratio of Red, Green, and Blue.

Note that the method of determining the absolute fringe order of the present invention can be applied to either the fringe order obtained by the inverse tangent function (sawtooth wave distribution) or the inverse cosine function (triangular wave distribution). Therefore, even if the used polarizer is a circular polarizer, a semicircular polarizer, and a plane polarizer, it can respond.

Next, an example of a fringe image in the case of a disk that receives a concentrated compressive load facing the upper and lower sides is shown. 5A is an image showing the relative fringe order of Red, FIG. 5B is an image showing the relative fringe order of Green, and FIG. 5C is an image showing the relative fringe order of Blue. Black corresponds to the 0th order and white corresponds to the 0.5th order, and these intermediate colors are displayed in gray.

FIG. 7A shows an example of an absolute fringe order image determined by the absolute fringe order calibration curve of FIG. 4 and the above-described equation. Black corresponds to the 0th order, white corresponds to the 6th order, and these intermediate colors are displayed in gray. It can be seen that high-order fringes are assigned to low-order fringe regions in the image, and there is a determination error.

This is because the fringe gradient and the density are different between the low-order fringes and the high-order fringes, and the relative fringe order ratio in the absolute fringe order calibration curve is close.

In order to avoid this, we decided to use the range of the absolute fringe order calibration curve for low-order fringes and high-order fringes, extract the fringe gradient of the sample to be analyzed, and create a mask image by extracting the parts with high density. To do.

The extraction of the mask area is performed in order to find the upper limit value of the absolute fringe order calibration curve in which the isolated point of the high order fringe does not appear in the low order fringe, and the range of the calibration curve. The absolute fringe order is calculated while changing. In this example, if the upper limit of the calibration curve is less than the fourth order, the number of isolated points of the higher order stripes is reduced in the low order stripe area. Therefore, the area up to the upper limit of the fourth order is black and the other areas are white. Priced. The image is shown in FIG. In this preprocessing, a stripe gradient image may be obtained and binarized with an appropriate threshold value. These pretreatments can be automatically performed.

The black portion of the mask image is a low-order fringe candidate region, the white portion is a high-order stripe candidate region, and the black portion of the mask image of FIG. 6 uses an absolute fringe order calibration curve from 0 to less than 4th order, as shown in FIG. FIG. 7B shows an example of the absolute fringe order in which the white portion of the mask image is determined independently for each point using the absolute fringe order calibration curve from 0th to 6th. Black corresponds to the 0th order, white corresponds to the 6th order, and these intermediate colors are displayed in gray. This is an example in which the absolute fringe order can be stably determined up to the 6th-order higher-order fringes by performing mask processing.

FIG. 8 shows the principal stress direction. Black corresponds to 0 ° and white corresponds to 45 °, and these intermediate colors are displayed in gray.

FIG. 9 shows an example of the distribution of stress components obtained by applying the shear stress difference integration method to the absolute fringe order obtained by the absolute fringe order determining method invented and the principal stress direction obtained at the same time. This is a distribution along the line y = 0.5R, where R is the radius of the disc. The theoretical value is a value obtained from the elasticity theory of a disk. From this example, it can be seen that the stress component can be stably determined according to the present invention.

The present invention can stably determine the absolute fringe order in the birefringent object, and can accurately determine the stress component from those results. In addition, the combination of the LED three-wavelength light source and one RGB interference filter as the light source is extremely useful industrially in that an inexpensive stress distribution measuring device can be provided.

It is a block diagram of the stress distribution measuring apparatus in the Example of this invention. It is a schematic diagram of the relative spectral response of the LED three-wavelength light source, the RGB interference filter, and the red, green, and blue internal filters of the image sensor. (A) is an image showing the relative fringe order of Red, (b) is an image showing the relative fringe order of Green, and (c) is an image showing the relative fringe order of Blue. It is an absolute fringe order calibration curve created from FIG. (A) is an image showing the relative fringe order of Red, (b) is an image showing the relative fringe order of Green, and (c) is an image showing the relative fringe order of Blue. This is a mask image obtained by binarizing and extracting the area where the absolute fringe order is less than the upper limit of the fourth order in black and the other area in white. (A) is an image which shows the absolute fringe order when the mask process by the image of FIG. 5 is not performed, and (b) is an image which shows the absolute fringe order when the mask process by the image of FIG. 5 is performed. It is an image which shows the principal stress direction. The distribution of stress components is shown.

1 Light Source 2 Diffuser 3 Polarizer 4 Sample 5 Analyzer

6 Filter 7 Image sensor 8 Control / signal processing section

9 Input section

10 Display section

Claims (1)

A relative fringe order is calculated from a color phase shift image obtained by photographing an object image in a circular polarizer (including a semicircular polarizer) or a plane polarizer with a light source with an imaging device, and is known in advance. Calculate the absolute fringe order calibration curve from the multi-wavelength relative fringe order by the object and refer to this to determine the absolute fringe order from the multi-wavelength relative fringe order of the object and measure the stress distribution. In order to determine the upper limit order of the absolute fringe order calibration curve to be applied when calculating the absolute fringe order by using a three-wavelength LED light source as the light source and attaching a single RGB interference filter in front of the image sensor Create a mask image according to the stripe gradient or density,
A stress distribution measuring apparatus, wherein a low-order fringe candidate region and a high-order fringe candidate region are set and calculated.

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