JP2015158400A - stress measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stress measuring method, even in a case of applying a single load to a measurement object, capable of easily and highly accurately measuring stress by excluding influence of thermal diffusion.SOLUTION: The method for measuring the stress generated in a measurement object using thermoelastic effects on the basis of temperature fluctuation of a surface of the measurement object when a single load is applied to the measurement object is disclosed. The stress measuring method includes: calculating first energy based on a difference between a reference temperature as a temperature of the measurement object without any application of the load, and a measurement temperature as a temperature when the load is applied thereto; regulates a virtual temperature fluctuation without any thermal diffusion by a virtual temperature fluctuation history associated to a load history representing time fluctuation of magnitude of the single load; calculates second energy based on the virtual temperature fluctuation history and the reference temperature; on the condition that the first energy is equal to the second energy, calculates an extremal value in the virtual temperature fluctuation history; and multiplies a difference between the calculated extremal value and the reference temperature by a thermoelastic coefficient to calculate the stress generated in the measurement object.

Description

  The present invention relates to a stress measurement method. More specifically, the present invention relates to a stress measurement method using a thermoelastic effect, in which the influence of thermal diffusion is corrected.

  In order to optimally design a member, it is important to grasp what kind of stress is generated in the member. For this purpose, for example, a method for obtaining a stress distribution by numerical analysis such as a finite element method (FEM) or a boundary element method (BEM), a method for measuring stress applied to a member using a strain gauge, and the like are known.

However, when the shape of the member is complicated or when the member is composed of a plurality of parts, the stress may not be accurately obtained by the above-described method.
Therefore, various infrared stress measurement methods have been proposed in which the temperature distribution of a member subjected to a load is measured and the stress distribution of the member is estimated from the change in the temperature distribution using the thermoelastic effect (for example, Patent Documents). 1-2).

  The infrared stress measurement method is a method in which a change in heat (temperature) generated in a member in accordance with a change in stress acting on the member is measured by an infrared camera. Spread. For this reason, for example, in the case of a single load (a load that is applied in a single shot like a hammering test, unlike a load that is repeatedly applied continuously), a temperature difference that is measured as described below. Becomes smaller than the original value, and as a result, the measured stress may also be smaller than the actual stress.

  FIG. 3 is an explanatory diagram of a load load test in which a tensile load is applied to the member 1, and FIG. 4 is a diagram illustrating a time variation of the applied load. FIG. 5 is a diagram showing temperature fluctuations in the vicinity of the notch 1a of the member 1 in the load test shown in FIG. The lower end of the strip-shaped member 1 is fixed, and a tensile force (see the white arrow) is applied to the upper end of the member 1 by a load loading mechanism (not shown). The member 1 is given a single parabolic variation load as shown in FIG. The tensile force is not instantaneously applied and released, but is a tensile force that requires a time of about 0.2 to 0.5 seconds from the start of loading to the end of unloading. By applying a tensile force, stress concentrates in the vicinity of the notch 1a of the member 1, and the temperature change in this part becomes larger than in other parts. And the influence by thermal diffusion also becomes large due to a large temperature fluctuation. A temperature change occurring in the member 1 is detected by an infrared camera 2 arranged opposite to the member 1.

In the test example shown in FIG. 3, when a tensile force is applied, the temperature in the vicinity of the notch 1a where stress concentrates starts to decrease from the reference temperature (initial temperature) T _ref as shown in FIG. And, if there is a state where the applied tensile force is maintained, the temperature rises due to thermal diffusion in the meantime, and a temperature rise due to unloading occurs from the raised state. As shown, the phenomenon in which the temperature of the member 1 rises above the reference temperature T _ref (hereinafter, this phenomenon, and conversely, the phenomenon in which the temperature of the member falls below the reference temperature is also referred to as “temperature overshoot”). Happens. In an ideal state where no thermal diffusion occurs, the amount of overshoot described above is zero, and the temperature variation pattern is the inverse of the load variation pattern. That is, both patterns have a substantially line-symmetric shape with a line segment indicating the reference temperature T _ref as an axis. However, in reality, since “temperature overshoot” occurs due to thermal diffusion, the temperature difference obtained by measurement (the difference between the reference temperature and the minimum measurement temperature or maximum measurement temperature) becomes smaller than the original value.

  In order to eliminate the influence of such heat diffusion, the thermal stress in the measurement object is determined from the phase shift between the waveform of the load stress generated when the load is repeatedly applied and the measurement stress measured by the infrared imaging device. It has been proposed to calculate a measurement error due to conduction (see Patent Document 3).

  It is also conceivable to attach a strain gauge in the vicinity of the measurement target site and calibrate the stress value calculated by the infrared stress measurement method based on the stress value measured with the strain gauge.

Japanese Patent Laid-Open No. 2001-188028 JP2012-103124A JP 2006-153865 A

  However, the correction method described in Patent Document 3 is based on the premise that repeated loads are applied. For example, the correction method cannot be applied in the case of a single load that applies a load only once, and its application target is limited. is there.

  Further, since the strain gauge has a certain area, the stress value obtained by the strain gauge is an averaged value, and as a result, the accuracy of calibration is limited. In addition, the strain gauge is difficult to apply to a part where the stress concentration is predicted, a part whose shape changes suddenly, or a part having a complicated shape, and cannot be applied.

  The present invention has been made in view of such circumstances, and it is possible to easily and accurately measure stress by eliminating the influence of thermal diffusion even when a load is applied to an object to be measured only once. The object is to provide a stress measurement method capable of

(1) In the stress measuring method of the present invention, the stress generated in the object to be measured using the thermoelastic effect is based on the temperature fluctuation of the surface of the object to be measured when a single load is applied to the object to be measured. A method of measuring,
Calculating the first energy based on the difference between the reference temperature, which is the temperature when the object is not loaded, and the measured temperature, which is the temperature when the load is loaded,
A virtual temperature fluctuation when there is no thermal diffusion is defined by a virtual temperature fluctuation history corresponding to a load history indicating a time fluctuation of the magnitude of the one-shot load,
The second energy is calculated based on the virtual temperature fluctuation history and the reference temperature,
Assuming that the first energy and the second energy are equal, an extreme value in the virtual temperature fluctuation history is obtained,
The difference between the obtained extreme value and the reference temperature is multiplied by a thermoelastic coefficient to obtain the stress generated in the object to be measured.

  In the stress measurement method of the present invention, it is not necessary to repeatedly apply a load to the object to be measured, and the stress can be estimated by only one test. Therefore, the evaluation of the member (object to be measured) can be easily performed. It can be done in a short time. Further, even when a single load is applied to a member such as a hammering test or an impact test, the stress generated in the member can be estimated. In addition, the influence of thermal diffusion can be corrected by measuring the temperature of the object to be measured, and there is no place restriction (restriction of the place where the strain gauge is placed) as in the prior art using a strain gauge. Applications for high-precision stress measurement can be expanded.

In the present specification, the “object to be measured” is an object for measuring the stress distribution using the thermoelastic effect, and is not particularly limited in shape or size. An aluminum housing or the like with can be exemplified.
The “single load” is a load that is applied to the object to be measured, such as a hammering test and an impact test, unlike a load that is repeatedly and continuously applied.
The “load history” represents the time variation of the magnitude of the load applied to the object to be measured. When the horizontal axis represents time and the vertical axis represents the load magnitude, a curve or straight line or It can be shown in their mixture. Examples of curves include quadratic curves, cubic curves, and sine curves. Further, the load history may be drawn based on not only continuously acquired data but also discretely acquired data.

  According to the stress measurement method of the present invention, even when a load is applied to an object to be measured, the stress measurement can be performed easily and with high accuracy by eliminating the influence of thermal diffusion.

It is a figure explaining the principle of the stress measuring method of this invention, (a) is a figure which shows an example of the load fluctuation curve loaded on a to-be-measured object, (b) is a virtual temperature fluctuation | variation when there is no thermal diffusion It is a figure which shows an example of a curve, (c) is a figure which shows an example of the temperature fluctuation curve concerning the actual measurement with thermal diffusion. It is a figure which shows the other example of the load fluctuation | variation loaded on a member. It is explanatory drawing of the load load test which applies a tensile load to a member. It is a figure which shows the time fluctuation of the load loaded on a member by the load load test of FIG. It is a figure which shows the temperature fluctuation of the notch vicinity of the member in the load load test shown by FIG.

Hereinafter, embodiments of the stress measurement method of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a diagram for explaining the principle of the stress measurement method of the present invention, where (a) is a diagram showing an example of a load fluctuation curve loaded on an object to be measured, and (b) is a case where there is no thermal diffusion. It is a figure which shows an example of this virtual temperature fluctuation curve, (c) is a figure which shows an example of the temperature fluctuation curve which concerns on an actual measurement with thermal diffusion.

  In the stress measurement method of the present invention, based on the surface temperature of the measurement object that is the measurement object, the stress generated in the measurement object is measured using the thermoelastic effect. Therefore, it is necessary to measure the surface temperature of the measurement object when stress is applied to the measurement object. This surface temperature can be measured using, for example, a thermocouple, but in this embodiment, an infrared camera is used in consideration of the ability to measure temperature as responsiveness and surface information (thermocouple is a point measurement). doing. This infrared camera detects infrared rays emitted from the surface of an object to be measured, converts the infrared rays into an electrical signal by an infrared sensor, and outputs the signal as an image signal. The infrared camera is arranged toward the object to be measured (see FIG. 3), and a reference temperature image that is a temperature distribution of the object to be measured when no load is applied and a temperature that is a temperature distribution of the object to be measured when a load is applied. Get a fluctuating image. For each pixel of the obtained temperature fluctuation image, correction processing described later is performed to remove the influence of thermal diffusion, and the difference between the corrected temperature and the reference temperature obtained from the reference temperature image is a value specific to the material. By multiplying a certain thermoelastic coefficient (proportional constant between temperature and stress), the stress generated at the location corresponding to the pixel can be obtained. Then, the stress distribution calculated based on the corrected temperature can be acquired by performing the same processing for all of the temperature variation images or desired pixels.

In this embodiment, the single load applied to the object to be measured changes (varies) as defined by the sine function shown in FIG. Time from the load start dividing up the load ends where the load is released is t _1. In the present embodiment, the time t ₁ until the end of unloading is a predetermined time.

In an ideal state where there is no thermal diffusion, as shown in FIG. 1B, the surface temperature of the object to be measured is a load fluctuation curve (sine function) which is a load history shown in FIG. In accordance with a virtual fluctuation curve (virtual fluctuation history) corresponding to In other words, the curve representing the temperature fluctuation has a shape opposite to that of the curve representing the load fluctuation, and is symmetrical with the curve representing the load fluctuation about the line segment indicating the reference temperature _Tref . In the present embodiment, since a tensile force is applied to the object to be measured, the surface temperature of the object to be measured changes according to a convex sine curve as the load increases, and the temperature decreases. Surface temperature T decreases from the reference temperature _{T ref,} the minimum value at time _{t 3} when the load becomes the maximum value (extreme _{value) T MIN ',} and the flow returns to the reference temperature _{T ref} in unloading end t1 the load is released .

During actual measurement, thermal diffusion occurs in the object to be measured. For this reason, the temperature fluctuation of the measurement object at the time of load causes a “temperature overshoot” phenomenon as schematically shown in FIG. That is, the temperature rises due to thermal diffusion, and a temperature rise due to unloading occurs from the raised state, so that the temperature of the object to be measured rises above the reference temperature _Tref . When there is thermal diffusion, the surface temperature of the object to be measured returns to the reference temperature T _ref at a time t ₀ that is earlier than the time t ₁ at which the load is completely released, and further increases and gradually reaches a peak. And returns to the reference temperature T _ref at a time t ₂ later than the time t ₁ .

In the present invention, the energy expressed by the area surrounded by the temperature variation curve and the reference temperature T _{ref in the} case where there is thermal diffusion and in the case where there is no thermal diffusion is said to be equal to each other because the input energy is equal. Based on assumptions.

The energy (first energy) S ₁ represented by the area surrounded by the temperature variation curve and the reference temperature when there is thermal diffusion is expressed by the following equation (T) where the measured surface temperature is T and the reference temperature is T _ref : 1). The first term of the right side of the expression (1) represents the area S _a, the second term represents the area S _b. That is, energy (first energy) S ₁ = S _a + S _b when there is thermal diffusion.

On the other hand, since the temperature fluctuation curve in the present embodiment when there is no thermal diffusion is defined by a sine function, the energy (second energy) S represented by the area surrounded by the temperature fluctuation curve and the reference temperature in this case. ₂ is expressed by the following equation (2), where A is the amplitude of the sine curve.

  Since S1 = S2, the amplitude A represented by the following equation (3) is obtained from the equations (1) and (2).

The corrected temperature Tmin ′ is T _ref + A. That is, the corrected temperature Tmin ′ can be obtained by the following equation (4).

  The stress is calculated by multiplying the difference between the corrected temperature Tmin ′ and the reference temperature, that is, the amplitude A represented by Equation (3) in this embodiment by a known thermoelastic coefficient specific to the material constituting the object to be measured. Can be estimated.

[Other variations]
It should be noted that the embodiment disclosed this time is merely an example in all respects and is not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the meanings described above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  For example, in the above-described embodiment, the load applied to the object to be measured fluctuates according to a curve defined by a sine function. The fluctuation pattern at the time of unloading is not limited to the same pattern. For example, as shown in FIG. 2, the load fluctuation history at the time of loading is different from that at the time of unloading so that the load fluctuation history at the time of loading is linear and the load fluctuation history at the time of unloading is curved. Also good. In the hammering test and the impact test, the load variation pattern during loading and the load variation pattern during unloading are not the same.

Further, in the embodiment described above, the time t ₁ to stress unloading ends were predefined time, the time t ₁ is not intended to be limited to the time specified, for example, accelerometer It may be the time measured using. That is, for example, to measure the acceleration by using the acceleration pickups, load during acceleration, by the deceleration and unloading, it is possible to measure the time t ₁ until unloading ends.

  In the above-described embodiment, the temperature correction is performed for a certain point of the object to be measured. However, the temperature correction range can be expanded to a linear shape or a planar shape. That is, when an infrared camera is used as a temperature measuring means for measuring the temperature of the object to be measured, the temperature correction is performed by performing the above-described temperature correction on all or a desired pixel of the temperature image obtained by photographing the object to be measured. A corrected temperature distribution of the object can be obtained.

  In the above-described embodiment, the temperature of the object to be measured changes according to the curve defined by the sine function. However, when the actual load changes according to another function, the temperature is also changed to the other function. Therefore, the accuracy of stress estimation can be improved.

  In the above-described embodiment, the load is applied only once to the object to be measured. However, even when the load is periodically applied, the present invention can be applied to perform temperature correction at the time of applying each load. it can.

  In the present invention, the maximum temperature change amount with and without thermal diffusion (the maximum change amount from the reference temperature and the amplitude A when there is no thermal diffusion) can be acquired. In other words, it is possible to estimate the energy that escapes due to thermal diffusion. Therefore, based on this amount of energy and the stress applied to the object to be measured (assuming that a predetermined stress is applied), the thermoelastic coefficient is used to determine the energy to be applied. It is also possible to estimate the thermal conductivity characteristics (thermal conductivity) of the measurement object and further estimate the material of the measurement object from this thermal conductivity.

1: Member 2: Infrared camera

Claims (4)

Based on the temperature variation of the surface of the object to be measured when a single load is applied to the object to be measured, a method of measuring stress generated in the object to be measured using a thermoelastic effect,
Calculating the first energy based on the difference between the reference temperature, which is the temperature when the object is not loaded, and the measured temperature, which is the temperature when the load is loaded,
A virtual temperature fluctuation when there is no thermal diffusion is defined by a virtual temperature fluctuation history corresponding to a load history indicating a time fluctuation of the magnitude of the one-shot load,
The second energy is calculated based on the virtual temperature fluctuation history and the reference temperature,
Assuming that the first energy and the second energy are equal, an extreme value in the virtual temperature fluctuation history is obtained,
A stress measurement method characterized in that a stress generated in an object to be measured is obtained by multiplying a difference between the obtained extreme value and the reference temperature by a thermoelastic coefficient. The virtual temperature fluctuation history is a curve defined by a sine function with an amplitude of A,
The first energy is calculated by integrating the difference between the reference temperature and the measured temperature,
The second energy is calculated by integrating the difference between the curve defined by the sine function and the reference temperature,
The stress measurement method according to claim 1, wherein the stress generated in the object to be measured is obtained by multiplying the amplitude A obtained by assuming that the first energy and the second energy are equal to each other by a thermoelastic coefficient.   The stress measurement method according to claim 1, wherein the temperature variation is obtained from an infrared image captured by an infrared camera arranged to face the object to be measured.   The stress measurement method according to claim 3, wherein the extreme value is obtained for each pixel of the infrared image, and a stress distribution of the measurement object is obtained by multiplying a difference between the extreme value and the reference temperature by a thermoelastic coefficient.