JP7096962B2 - Optimal method for measuring residual stress - Google Patents

Description

In the present invention, a reference hole and an annular trepanning hole are formed concentrically outside the reference hole in a measurement object such as a welded structure, and a change in the hole diameter of the reference hole is measured to measure residual stress on the surface and inside of the measurement object. The present invention relates to an optimum method for measuring residual stress in which the pore diameter ratio of the trepanning hole to the optimum reference hole is numerically ranged in order to be standardized as a residual stress measurement evaluation of various measurement objects.

Conventionally, the residual stress evaluation method by the deep hole drilling method (DHD) measures the hole diameter before and after stress release by four procedures as shown in FIG. 7, and the residual stress value inside the plate thickness is measured from the amount of change in the hole diameter. Is calculated. First, a front bush is attached to the drilled part of the object to be measured, and a through or non-penetrating hole is drilled using a gun drill (Reference hole (hereinafter referred to as "reference hole")). ) (See Step 1 in FIG. 7 (a)). Next, the hole diameter is measured with respect to this reference hole at one or more points in the hole depth direction and at three or more points in the circumferential direction using an air probe (see Step 2 in FIG. 7B). Next, the reference hole is subjected to a removal process such as a coaxial cylindrical hollowing process (trepanning process) using an electrode (Electrode) to release the restraint around the reference hole and release the residual stress (Fig.). 7 (c), see Step 3). Then, the hole diameter is measured again at one or more points in the hole depth direction and at three or more points in the circumferential direction with respect to the reference hole after the peripheral removal by the trepanning process (see Step 4 in FIG. 7 (d)). From these measured values, the in-plane stress (σx, σy, σxy) with respect to the hole diameter can be calculated by assuming that it is an elastic material, that it is a hole in an infinite flat plate, and that it is in a plane stress state.

Further, in order to eliminate the influence of plastic deformation that occurs in the stress release process in which the trepanning process is performed coaxially in a cylindrical shape, the trepanning process and the hole diameter measurement are sequentially performed by the sequential deep hole drilling method (iDHD method: incremental deep hole drilling). The hole axial component (σz) that can be calculated by the above-mentioned deep hole drilling method can also be calculated.

However, both the deep hole drilling method and the sequential deep hole drilling method do not incorporate the effects of the three-dimensional stress state and plastic deformation on the hole diameter, and there is a problem that the actual value deviates from the theoretical value and the accuracy does not improve. rice field.

Further, in the residual stress measuring method by the conventional deep hole drilling method and the sequential deep hole drilling method (hereinafter, collectively referred to as "DHD method"), the measurement target is an elastic material as described above, and the infinite flat plate is used. This is a method of calculating the residual stress (σx, σy, σxy) of the component in the direction perpendicular to the hole diameter on the assumption that the hole is in a plane stress state. Therefore, since the accuracy of residual stress decreases depending on the assumption conditions, the DHD method is not widely used as a practical measurement method for residual stress measurement methods, and the effects of three-dimensional stress states and plastic deformation that bring the assumption conditions closer to the actual phenomenon are affected. A highly accurate method for measuring internal residual stress in plate thickness that can be taken into consideration has been eagerly desired.

In order to solve the above-mentioned problems of the DHD method, the present inventors have a three-dimensional stress state that brings the assumed conditions closer to the actual phenomenon, and an improved deep hole drilling method that can consider the influence of plastic deformation (hereinafter, "MIRS method"). ) Is provided in Patent Document 2.

Japanese Unexamined Patent Publication No. 2007-167937 Japanese Unexamined Patent Publication No. 2015-184118

"Proposal of on-site measurement method of residual stress for primary system actual machine piping" INSS JOURNAL Vol. 21.1024, 61-74 Maekawa et al. "Measurement technique of residual stress confined inside structural members" IHI Technique Vol. 53, 2013, 54-58 Middle generation et al.

In the deep hole drilling method (DHD method, MIRS method (hereinafter, also referred to as "MIRS method")), it is assumed that the stress in the same plane of the object to be measured is uniform. However, it is known that the stress field in the same plane of the actual measurement object is not uniform. For example, FIGS. 5 and 6 exemplify a plan view in which the distribution of residual stress in the welding line direction at the butt weld joint is superimposed with the center of the reference hole and the trepanning hole as the zero point coordinates. In the example of FIG. 5, a region showing tension in the stress distribution on the x-axis is included inside the inner diameter of the trepanning hole. On the other hand, in the example of FIG. 6, it can be seen that the inside of the inner diameter of the trepanning hole includes a region showing not only tension but also compression in the stress distribution on the x-axis. As shown in FIG. 6, when tensile stress and compressive stress are included inside the inner diameter of the trepanning hole, even if the residual stress is released by trepanning, the compressive stress is also released, and the tensile stress (residual stress) at the reference hole position. Accurate measurement results cannot be obtained because the calculation based on the release of is not possible. From this, it was found that the wall thickness of the core portion sandwiched between the reference hole and the trepanning hole greatly affects the measured value of the residual stress value.

Therefore, in order to obtain accurate measurement results by the MIRS method or the like, in the case of FIG. 6, the inner diameter of the trepanning hole contains only the tensile stress as shown in FIG. 5, and the maximum value of the tensile stress can be evaluated. , It can be seen that it is necessary to reduce the wall thickness of the core part. The present inventor considered that it is necessary to provide non-dimensional recommended measurement conditions numerically in order to standardize the measurement method regardless of the measurement target or processing conditions.

Therefore, an object of the present invention is to provide an optimum method for measuring residual stress, which shows a numerical range of the hole diameter ratio of the trepanning hole to the optimum reference hole for standardization as a residual stress measurement evaluation of various measurement objects. ..

In the present invention, a reference hole in the thickness direction and a trepanning hole hollowed out substantially concentrically to the outside of the reference hole are formed at a measurement point of the member to be measured, and the change in the hole diameter of the reference hole before and after the formation of the trepanning hole is observed. It is an optimum measurement method of residual stress that measures and calculates the residual stress value on the surface and inside of the member to be measured.
It is formed within the range of the inner diameter of the trepanning hole / the hole diameter of the reference hole corresponding to the predetermined measurement error range.

In the method for optimally measuring residual stress of the present invention,
The reference hole and the trepanning hole are formed so as to have 1.5 ≦ the inner diameter of the trepanning hole / the hole diameter of the reference hole.

Further, in the method for optimally measuring the residual stress, the reference hole and the trepanning hole are defined as the inner diameter of the trepanning hole / the inner diameter of the hole diameter of the reference hole <3.0.
It is preferable to form so as to be.

As described above, it was found that the maximum value of the tensile stress in the residual stress can be evaluated by reducing the pore size ratio by the MIRS method or the like. On the other hand, in order to make the MIRS method or the like a versatile measurement method, it is necessary to be within an acceptable measurement error range. The present invention provides an optimum measurement method corresponding to the pore size ratio as a reference for evaluating the tensile stress within the allowable range of measurement error. Therefore, even if there is a difference in the setting of the allowable measurement error, the technical idea of the present invention is followed when the hole diameter ratio is provided as the optimum measurement method.

Specifically, as will be described later, it was found that if the measurement error is 80% or more, the residual stress is evaluated and it is considered to be good. As will be described later, this is a hole diameter ratio of 1.5 ≦, and it is necessary to secure a hole diameter ratio of 1.5 or more even for a measurement target such as a corner.

Further, as described above, it is better to reduce the hole diameter ratio as an evaluation of residual stress, but there is no choice but to increase the inner diameter of the trepanning or reduce the reference hole, the former is not suitable for corners and the like, and the latter is processed. It is difficult and lacks the versatility of this measurement method. In that sense, it is preferable to set an upper limit value in order to provide a numerical range as a standardized measurement condition. In the present invention, as will be described later, 97%, which can be said to be highly accurate as the residual stress evaluation, is set as the upper limit, and it is excessively required as a standardized measurement condition to obtain an accuracy higher than that (rather, the pore size ratio becomes large and the compressive stress is also evaluated. Or, it is harmful in terms of the corners and processing). In the past, as a rule of thumb, three or more measurement examples were disclosed, but those skilled in the art have followed this without any particular verification. As a result, the present inventor can be said to have provided the optimum measurement conditions for the residual stress, which are measured in a range smaller than this.
In the past, only cases of 3.0 or more were found when calculated by the pore size ratio, and the present invention, which provided for the first time that a range smaller than this is the optimum measurement condition, is epoch-making and is less than 3.0. It can be said that the case of measuring under the conditions is an implementation of the measuring method of the present invention.

According to the optimum measurement method of residual stress of the present invention, a predetermined numerical range of the hole diameter ratio of the trepanning hole to the reference hole is provided as the optimum measurement condition in order to standardize as the residual stress measurement evaluation of various measurement objects. It is a great advantage in that.

It is a figure for demonstrating the measuring method of the length change (elongation amount ΔZ) of the original hole 10 before and after the trepanning process in the MIRS method, and (a) is the measurement which formed the original hole 10 and the hollow hole 11. It is a perspective view which showed the target member 1. (B) is a top view of the member 1 to be measured. (C) is a diagram showing a state in which the length of the cylindrical portion 12 in the axial direction (Z direction) has changed after the trepanning process. (A) to (e) are explanatory views showing each step of the residual stress measuring method which concerns on one Embodiment of this invention. (A) and (b) are diagrams for explaining a method of measuring the inclination (tilt amount Δθ) of the axis of the original hole before and after the trepanning process. (C) is a diagram showing an example of a state in which the cylindrical portion is tilted after the trepanning process. It is a graph which shows the result of arranging the estimated value / input value of the residual stress at the time of drilling a hole of a reference hole and a trepanning hole 11. It is a top view which superposed the distribution of the residual stress in the welding line direction in the butt weld joint with the center of a reference hole and a trepanning hole as zero point coordinates. It is a top view which superposed the distribution of the residual stress in the welding line direction in the butt weld joint with the center of a reference hole and a trepanning hole as zero point coordinates. It is explanatory drawing which showed each process of the residual stress evaluation method by the conventional deep hole drilling method.

Hereinafter, an optimum method for measuring residual stress according to an embodiment of the present invention will be described with reference to FIGS. 1 to 7.

<< Residual stress measurement method (MIRS method) >>
First, as an explanation of the embodiment of the residual stress measuring method of the present invention, the measurement of the residual stress value at one point of the member to be measured will be described in the improved deep hole drilling method. 2 (a) to 2 (e) show each step of the residual stress measuring method at one place. In this residual stress evaluation method, the residual stress value inside the plate thickness is calculated by the five procedures as shown in FIGS. 2 (a) to 2 (e). Here, reference numeral 1 in the drawing is a member to be measured such as a welded structure, and reference numeral 2 is a drill capable of forming a reference hole 10 in the member 1 to be measured. Further, reference numeral 3 is an air probe (hole diameter measuring unit and hole diameter re-measuring unit) capable of measuring the inner diameter of the reference hole 10 formed in the measurement target member 1, and reference numeral 4 is around the reference hole 10 by electric discharge. It is a discharge processing machine capable of forming a hollow hole (trepanning hole) 11 by performing a hollowing process (trepanning process). Reference numeral 5 is a touch probe capable of measuring each of the axial extension amount ΔZ and the tilt amount Δθ of the cylindrical portion 12. Here, in this residual stress measuring method, at least the residual stress values on the surface and inside of the member 1 to be measured are calculated based on the shape change of the reference hole 10 before and after the trepanning process (hollowing process) using the air probe 3. do. In calculating the residual stress value, the hole diameter of the reference hole 10, the change in the length of the reference hole 10 in the longitudinal direction (elongation amount ΔZ), and the inclination of the axis of the reference hole 10 (tilt amount Δθ) are taken into consideration.

In this embodiment, for convenience of explanation, an XYZ three-dimensional coordinate system is defined, and a method for measuring residual stress will be described with reference to this coordinate system. In FIG. 2, the center position of the reference hole 10 is the origin O, the right direction of the paper surface is the X axis, the back direction perpendicular to the paper surface is the Y axis, and the upper vertical direction is the Z axis. That is, the upper surface of the member 1 to be measured defines an XY plane, the Z axis passes along the longitudinal direction (depth direction) of the reference hole 10, and the axial direction of the cylindrical portion 12 is the Z axis direction.

First, in FIG. 2A, a metal fitting (not shown) is attached to a drilling portion of the member 1 to be measured, and a reference hole 10 is formed by drilling using a drill 2. The reference hole 10 may be a through hole or a non-through hole. Next, in FIG. 2B, the hole diameter of the reference hole 10 is measured at one or more locations in the longitudinal direction (Z direction) and at three or more locations in the circumferential direction using the air probe 3. Next, in FIG. 2C, a hollowing process (trepanning process) is performed on the periphery of the reference hole 10, the restraint of the peripheral portion of the reference hole 10 is released (residual stress is released), and the coaxial shape is cylindrical. Form the cylindrical portion 12. Then, in FIG. 2D, once again, with respect to the reference hole 10 after the peripheral removal processing, the hole diameter is measured (remeasured) at one or more points in the longitudinal direction (Z direction) and three or more points in the circumferential direction using the air probe 3. )I do. Then, in FIG. 2E, the touch probe 5 is used to measure the amount of elongation (ΔZ) in the axial direction (Z direction) of the cylindrical portion 12 and the amount of tilt (Δθ) in the XY direction. By measuring the amount of elongation (ΔZ) and the amount of tilt (Δθ), the residual stress measurement by the DHD method considers only the residual stress component consisting of (σx, σy, σxy). In the improved deep hole drilling method in Patent Document 2, it is possible to measure the residual stress in consideration of the residual stress component consisting of 6 components (σx, σy, σz, σxy, σyz, σzx). The three-dimensional residual stress component, which was omitted under the conditions, can be measured with high accuracy.

<< Measurement method of length change (elongation amount ΔZ) of reference hole 10 before and after trepanning >>
Next, with reference to FIG. 1, a method for measuring the length change (elongation amount ΔZ) of the reference hole 10 before and after the trepanning process will be described. FIG. 1A is a perspective view showing a measurement target member 1 in which a reference hole 10 and a hollow hole 11 are formed. FIG. 1B is a top view of the member 1 to be measured. FIG. 1 (c) is a diagram showing a state in which the length of the cylindrical portion 12 in the axial direction (Z direction) has changed after the trepanning process. The symbol ■ indicated by the black square in FIG. 1B indicates the measurement point in the height direction (Z direction) of the member 1 to be measured before the trepanning process, and the symbol ▲ indicated by the black regular triangle is. The measurement points in the height direction (Z direction) of the member 1 to be measured after the trepanning process are shown. In the present embodiment, as shown in FIG. 1 (b), the measurement points ■ and ▲ are the upper surfaces of the measurement target member 1 and are equidistant around the central axis O of the reference hole 10 (in the present embodiment). It is provided at intervals of 90 °) and four at positions equidistant from the central axis O. In the present embodiment, the average value of the measured values measured at the measurement points ■ and ▲ before and after the trepanning process is the height change (ΔZ) of the member 1 to be measured, that is, the axial direction (Z direction) of the cylindrical portion 12. ) Is measured as the amount of elongation (ΔZ), and the measurement result is input to the stress value calculation unit (not shown). As a result, the stress value calculation unit can consider the length change (elongation amount ΔZ) in the longitudinal direction of the reference hole 10 in the calculation of the residual stress value before and after the trepanning process. The points of each measurement point ■ and ▲ are not limited to four points, and may be any number as long as they are two or more points.

<< Measurement method of the inclination (tilt amount Δθ) of the axis of the reference hole 10 before and after trepanning >>
Next, with reference to FIG. 3, a method for measuring the inclination (tilt amount Δθ) of the axis of the reference hole 10 before and after the trepanning process will be described. FIGS. 3A and 3B are diagrams for explaining a method of measuring the inclination (tilt amount Δθ) of the axis of the reference hole 10 before and after the trepanning process. FIG. 3C is a diagram showing an example of a state in which the cylindrical portion 12 is tilted after the trepanning process. Here, FIG. 3B shows the outer shape of the reference hole 10 seen from the height direction (Z direction) of the member 1 to be measured, and the circular shape shown by the thick line on the left side of the paper surface is the reference hole before the trepanning process. The circular shape shown by the broken line on the right side of the paper surface indicates the outer shape of the reference hole 10 after the trepanning process. Further, points a and b in FIG. 3A are points located at a depth h from the upper surface of the member 1 to be measured to the bottom of the paper surface (Z-axis direction) on the inner peripheral surface of the reference hole 10. The line segment ab in FIG. 3B indicates the diameter of the reference hole 10 in the X direction, and the line segment cd indicates the diameter of the reference hole 10 in the Y direction. The depth h [mm] is preferably set to about 2.5 mm. Although not shown in FIG. 3 (a), points c and d also fall from the upper surface of the member 1 to be measured in the downward direction (Z-axis direction) on the inner peripheral surface of the reference hole 10 in the same manner as the points a and b. ) Is located at the depth h. Further, points O and O'in FIG. 3B indicate the centers of the reference holes 10 before and after the trepanning process. The position coordinates of the centers O and O'can be obtained by averaging the position coordinates of the four points a to d. In the present embodiment, the inclination of the axis of the reference hole 10 (tilt amount Δθ) is measured by observing the positional deviation of the centers O and O'based on the position coordinates of the centers O and O'.

≪Inner diameter of trepanning hole / hole diameter of reference hole≫
The residual stress measurement based on the shape change of the reference hole 10 in the MIRS method has been described above, but in the present invention, the residual stress is measured from the hole diameter change of the reference hole 10 before and after trepanning, which is the most basic of the shape changes of the reference hole. The optimum ratio of the reference hole / trepanning hole diameter in the case was found. Hereinafter, a specific description will be given.
First, refer to FIG. 4 here. FIG. 4 is a graph showing the results of arranging the estimated / input values of the residual stress when the reference hole 10 and the trepanning hole 11 are drilled. The vertical axis shows the estimated value / input value of the residual stress, and the horizontal axis shows the inner diameter of the trepanning hole 11 / the hole diameter of the reference hole 10. From this graph, it can be seen that the accuracy of estimating the residual stress decreases as the inner diameter of the trepanning hole 11 / the hole diameter of the reference hole 10 becomes smaller. On the other hand, as the inner diameter of the trepanning hole 11 / the hole diameter of the reference hole 10 increases, the estimation accuracy of the residual stress improves and converges to 1.0. In that sense, it is necessary to increase the trepanning hole 11, but it is highly reliable when there is no space such as a corner of the member 1 to be measured or when the change in in-plane stress is rapid and a plurality of measurements are desired at a short distance. It is sufficient if the measurement accuracy can be sufficiently guaranteed, and rather, considering the measurement skill, it is necessary to eliminate the waste of space by increasing the inner diameter of the trepanning hole 11 / the hole diameter of the reference hole 10 as recommended as a general measurement technique. There is also. Therefore, it is preferable to quantify the upper and lower limits of the inner diameter of the trepanning hole 11 / the hole diameter of the reference hole 10.

The above-mentioned Non-Patent Documents 1 and 2 present an error that is actually considered to be appropriate as a residual stress evaluation. Specifically, in Non-Patent Document 1, it is stated that if it is 80% or more, it can be adopted as a residual stress evaluation. When this is verified by the numerical analysis result of FIG. 4, the estimated value of the residual stress corresponding to 80% / the input value = 0.8, the inner diameter of the trepanning hole 11 / the hole diameter of the reference hole 10 = 1.5. If it is more than the value, it can be said that it can be sufficiently adopted for the residual stress evaluation.

Further, in Non-Patent Document 2, it is said that if the content is 3% or less, the residual stress evaluation is more accurate than usual. Therefore, it can be said that usually less than 97% can be adopted as a sufficient residual stress evaluation, and it cannot be said that a requirement of 97% or more is preferable in the MIRS method as an evaluation method expected to be widely used in the future. When this idea is verified by the numerical analysis result of FIG. 4, the estimated value of residual stress corresponding to 97% / input value = 0.97, the inner diameter of the trepanning hole 11 / the hole diameter of the reference hole 10 = 3.0. It is preferable that the value above this value is out of the recommended range because it requires unnecessarily high accuracy in consideration of the difference in measurement skill and waste of space. Therefore, it is considered that the inner diameter of the trepanning hole 11 / the hole diameter of the reference hole 10 <3.0 is appropriate.

..
Summing up the above, the relationship between the inner diameter of the trepanning hole 11 and the reference hole 10 is
The inner diameter of the trepanning hole / the hole diameter of the reference hole ≥ 1.5.
Preferably,
1.5 ≤ inner diameter of trepanning hole 11 / hole diameter of reference hole 10 <3.0
Is recommended.

Although the embodiments of the present invention have been described above with reference to the drawings, it goes without saying that the specific configuration is not limited to these embodiments. The scope of the present invention is shown by the scope of claims rather than the description of the embodiment described above, and further includes all modifications within the meaning and scope equivalent to the scope of claims.

1 Member to be measured 2 Drill 3 Air probe 4 Electric discharge machine 5 Touch probe 10 Original hole 11 Hollow hole 12 Cylindrical part

Claims (1)

A reference hole in the thickness direction and a trepanning hole hollowed out substantially concentrically to the outside of the reference hole are formed at the measurement point of the member to be measured, and the change in the hole diameter of the reference hole before and after the formation of the trepanning hole is measured. In the optimum measurement method of residual stress for calculating the residual stress value on the surface and inside of the member to be measured.
The reference hole and the trepanning hole are defined as the inner diameter of the trepanning hole / the hole diameter of the reference hole <3.0.
An optimum method for measuring residual stress, which is characterized in that it is formed so as to be.

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