JP2018155542A - Four point bending corrosion test method, four point bending corrosion test device, and test piece - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a strain of a load surface of a test piece to be made close to uniform in a 4 point bending corrosion test.SOLUTION: A four point bending corrosion test method includes: a step of supporting a test piece SJ via a pair of first contact tools and a pair of second contact tools; a step of attaching a restraining tool 7 for limiting the replacement in a width direction of a both side surface SJc of a test piece SJ which contacts a portion lower than a center in a thickness direction of the test piece SJ which is a both side surface SJc in a width direction of the test piece SJ between the pair of second contact tools; and a step of allowing the test piece SJ to be supported in a state that the pair of first contact tools contacts a first surface SJa and the pair of second contact tools contacts a second surface SJb and adding a load in a thickness direction of the test piece to the test piece SJ attached with the restraining tool 7.SELECTED DRAWING: Figure 9

Description

  The present invention relates to a four-point bending corrosion test method, a four-point bending corrosion test apparatus, and a test piece used for a four-point bending corrosion test.

  Conventionally, the four-point bending test has been used for a material strength test and a corrosion test. For example, JP-A-6-207895 (Patent Document 1) discloses a ceramic four-point bending test jig. The test jig includes a pair of support pins that support the test piece, a base that supports at least one support pin of the pair of support pins, a holding unit that holds a distance between the pair of support pins constant, A load pin for applying a load to the test piece supported by the pair of support pins. The base supports the at least one support pin in a swingable manner in a plane perpendicular to the longitudinal axis of the test piece. This eliminates measurement errors due to unbalanced loads caused by the jig shape and test piece shape.

  Japanese Patent Application Laid-Open No. 2004-279083 (Patent Document 2) discloses a bending test method in which an axial compression load is applied to a test piece of FRP to cause bending deformation and destruction. In this bending test method, a test piece is used in which a value obtained by multiplying the ratio of the length and thickness of the test piece by a coefficient is within a predetermined range represented by a bending elastic modulus. The test piece is supported at both ends by a jig that is rotatable in the direction of bending deformation of the test piece. The jig supports one end of the test piece at the axial center of the axial compression load and supports the other end at a position offset from the axial center of the axial compression load. According to this test method, the bending strength and the like of the FRP plate can be measured with high accuracy.

JP-A-6-207895 JP 2004-279083 A

  In the above Japanese Patent Application Laid-Open No. 2004-279083, although there is a disclosure about the elongated ratio of a test piece for obtaining high measurement accuracy in a test in which an axial compression load is applied to an FRP test piece, it is more appropriate in a four-point bending corrosion test. There is no disclosure of the configuration of the test piece for evaluation. On the other hand, in order to perform an appropriate evaluation at a low cost in a four-point bending corrosion test, it is preferable that the strain on the load surface of the test piece be uniform while suppressing the magnitude of the load. Has been recognized by. JP-A-6-207895 does not disclose means for making the strain on the load surface of the test piece uniform.

  Therefore, the present application discloses a four-point bending test method, a four-point bending test apparatus, and a test piece that can make the strain on the load surface of the test piece uniform in a four-point bending corrosion test.

  In the four-point bending corrosion test method according to the first aspect of the present invention, a pair of first contact tools separated by a first distance in the longitudinal direction of the test piece are in contact with the first surface of the test piece, and the test is performed. A second distance that is shorter than the first distance in the longitudinal direction of the test piece is spaced apart in a region between the pair of first contact tools on the second surface opposite to the first surface of the piece. The longitudinal direction of the test piece between the step of supporting the test piece in a state of contacting a pair of second contact tools and the pair of second contact tools spaced apart by a second distance in the longitudinal direction. And both sides in the width direction that are perpendicular to both the thickness direction between the first surface and the second surface, and closer to the second surface than the center in the thickness direction of the test piece And attaching a restraining jig for restricting the displacement in the width direction of the both side surfaces of the test piece. The pair of first contact tools are supported in a state where the first contact tools are in contact with the first surface and the pair of second contact tools are in contact with the second surface, and the restraint jig is attached to the test piece. And applying a load in the thickness direction via the pair of first contact tools in contact with the first surface and the pair of second contact tools in contact with the second surface.

  According to the present disclosure, in the four-point bending corrosion test, the strain on the load surface of the test piece can be made close to uniform.

FIG. 1 is a side view showing a configuration example of a four-point bending corrosion test apparatus. FIG. 2 is a diagram illustrating a state in which a strain gauge is attached to a test piece. FIG. 3 is a view showing a state in which a strain gauge is attached to a test piece. FIG. 4 is a diagram for explaining the shape of the test piece. FIG. 5 is a diagram for explaining the shape of the test piece. It is a top view which shows the structure which looked at the test piece, the 1st contact tool, the 2nd contact tool, and a restraint jig which are shown in FIG. FIG. 7 is a side view of the configuration shown in FIG. 6 viewed from the lateral direction (x direction). FIG. 8 is a cross-sectional view showing a cross section taken along line AA in FIGS. 6 and 7. FIG. 9 is a cross-sectional view showing a modification of the cross-sectional shape of the test piece SJ. FIG. 10 is a diagram illustrating an analysis model for simulation. FIG. 11 is a graph showing a stress-strain curve of the material used in the simulation. FIG. 12 shows a mesh (finite element) in the xy plane of the inner surface As test piece. FIG. 13 is a graph showing a strain distribution (solid line) and an actual measurement value (triangular plot) obtained by FEM analysis of the inner surface As test piece. FIG. 14 is a diagram showing a mesh (finite element) and constraint conditions in the xy plane of a smooth test piece. FIG. 15 is a diagram illustrating a mesh (finite element) and constraint conditions in the xy plane of the inner surface As test piece. FIG. 16 is a graph showing the results of FEM analysis using the analysis model of the smooth test piece shown in FIG. FIG. 17 is a graph showing the results of FEM analysis using the analysis model of the inner surface As test piece shown in FIG. FIG. 18 is a graph showing the calculation result of the strain distribution when the leg length of the test piece is changed. FIG. 19 is a graph showing the analysis result of the strain distribution when the side surface of the smooth test piece without a leg is constrained. FIG. 20 is a graph showing the analysis result of the strain distribution when the side surface of the inner surface As test piece without a leg is constrained.

  The 4-point bending corrosion test is used, for example, for a test for evaluating the corrosion resistance performance of oil well pipes and line pipes. In the four-point bending corrosion test, the specimen is exposed to a corrosive environment for a predetermined period (for example, for one month) with a tensile load applied, and the presence or absence of cracks is examined. In this case, the applied load is set so that the longitudinal component of the strain at a predetermined position (for example, the center) of the surface to which the tensile stress is applied on the test piece, that is, the evaluation surface, becomes a predetermined value. Therefore, originally, it is preferable to evaluate the presence or absence of crack generation at a predetermined position of the test piece for which the strain value is set.

  However, the inventor has discovered that cracks may occur at sites other than the predetermined position where the strain value is set, particularly at the end of the test piece. As described above, when cracks occur in a portion other than the position where the strain value is set, it may be a problem whether the corrosion resistance performance of the steel pipe can be properly evaluated. Then, inventors examined the cause which a crack generate | occur | produces in a test piece edge part.

  In the examination, the inventors considered that the cause of the occurrence of cracks at the end of the test piece was that a uniform strain was not applied to the test piece. That is, it is considered that the strain at the end is larger than that at the center of the test piece. Then, the inventors examined the method of reducing the strain difference between the central portion and the end portion by FEM analysis. As a result, it was suggested that the strain difference could be reduced by increasing the specimen thickness.

  Based on the results of this study, the inventors predict the strain difference between the center and the end in accordance with the steel pipe size (inner diameter), and determine the thickness of the specimen that can be used to perform a four-point bending corrosion test without any strain difference. Invented a method to determine.

  Further, when the test piece thickness is increased, the four-point bending load for obtaining a predetermined strain also increases. Therefore, the inventors have invented a test piece in which a through hole is provided in the longitudinal direction while ensuring the thickness of the test piece as a test piece capable of reducing the load. Thereby, the thickness of a test piece is ensured and the distortion of a load surface can be closely approximated.

  Furthermore, methods other than the method of processing a through hole in the test piece longitudinal direction were examined. Inventors paid attention to the fact that the test piece is deformed so that the displacement in the width direction is different between the evaluation surface side and the opposite surface side of the test piece due to the four-point bending load, and the following examination was performed. .

  The stress and displacement when a four-point bending load was applied to the specimen were examined in more detail. In the four-point bending test, a pair of rollers spaced apart in the longitudinal direction of the test piece presses the evaluation surface of the test piece, and a pair of rollers forms a surface opposite to the evaluation surface in the region between the pair of rollers on the evaluation surface. A load is applied to the test piece by pushing. As a result, a tensile load in the longitudinal direction of the test piece is applied to the evaluation surface of the test piece. As an example, a case where the upper surface becomes an evaluation surface will be described.

  Tensile stress acts in the longitudinal direction of the test piece on the part above the center in the thickness direction of the test piece, that is, on the part closer to the evaluation surface than the center in the thickness direction. A compressive stress acts on a portion below the center in the thickness direction of the test piece, that is, a portion closer to the surface opposite to the evaluation surface than the center in the thickness direction of the test piece. For this reason, the part above the center of the thickness direction of a test piece shrinks in the width direction of a test piece by the Poisson effect. That is, displacement in the direction of compression occurs in the width direction. The portion below the center in the thickness direction of the test piece extends in the width direction of the test piece. That is, displacement in the direction of pulling in the width direction occurs.

  The inventors paid attention to stress and displacement in the width direction acting on the test piece in the four-point bending test. It was studied to suppress the difference between the displacement in the evaluation surface width direction of the test piece when a load was applied to the test piece and the displacement in the width direction of the surface opposite to the evaluation surface. Since it is difficult to constrain the displacement in the direction of compression in the width direction of the test piece, by constraining the displacement in the direction of pulling in the width direction, the displacement in the width direction of the evaluation surface of the test piece and the surface opposite to the evaluation surface It was investigated whether the difference with the displacement in the width direction could be reduced. As a result, it was found that restraining the displacement in the width direction of the test piece can reduce the difference between the displacement in the width direction of the evaluation surface of the test piece and the displacement in the width direction of the surface opposite to the evaluation surface.

  Furthermore, the strain distribution in the test piece was analyzed when a four-point bending load was applied while restraining the displacement in the width direction of the outer side surface in the width direction of the test piece. As a result, by reducing the difference between the displacement in the width direction of the evaluation surface and the displacement in the width direction of the surface opposite to the evaluation surface, the longitudinal strain at the end of the evaluation surface in the width direction and the evaluation surface It has been found that the difference from the strain in the longitudinal direction at the center in the width direction can be reduced. Based on this knowledge, the following embodiments of the present invention have been conceived.

(Method 1)
In the four-point bending corrosion test method according to the embodiment of the present invention, a pair of first contact tools that are separated from each other by a first distance in the longitudinal direction of the test piece are in contact with the first surface of the test piece. A pair of first surfaces spaced apart by a second distance shorter than the first distance in the longitudinal direction of the test piece in a region between the pair of first contact tools on the second surface opposite to the first surface. Between the step of supporting the test piece with the second contact tool in contact with the pair of second contact tools spaced apart by a second distance in the longitudinal direction, and the longitudinal direction of the test piece and the Both side surfaces in the width direction that are perpendicular to both the thickness directions between the first surface and the second surface, and are in contact with a portion closer to the second surface than the center in the thickness direction of the test piece. Attaching a restraining jig for restricting displacement in the width direction of the both side surfaces of the test piece; The pair of first contact tools are in contact with the first surface and the pair of second contact tools are supported in contact with the second surface, and the test piece to which the restraining jig is attached, Applying a load in the thickness direction via the pair of first contact tools in contact with the first surface and the pair of second contact tools in contact with the second surface (Method 1).

  In the method 1, when a load is applied to the test piece in the thickness direction via the pair of first contact tools and the pair of second contact tools, the test piece is closer to the first surface than the center in the thickness direction of the test piece. Tensile stress is generated in the longitudinal direction in the portion, and compressive stress is generated in the longitudinal direction in the portion closer to the second surface than the center in the thickness direction of the test piece. That is, the first surface is a surface to which tensile stress is applied, that is, an evaluation surface. Therefore, the first surface of the test piece tends to deform so as to extend in the longitudinal direction, and the second surface of the test piece tends to deform so as to shrink in the longitudinal direction. Therefore, the first surface of the test piece tends to deform so as to contract in the width direction, and the second surface tends to deform so as to extend in the width direction. Here, when a load is applied to the test piece, the restraining jig is in contact with a portion closer to the second surface than the center in the thickness direction of the test piece in the region between the pair of second contact tools, and the width of the test piece. The displacement in the width direction of both side surfaces outside the direction is limited. The restraining jig suppresses deformation between the pair of second contact tools, in which a portion near the surface second from the center in the thickness direction of the test piece extends in the width direction. Thereby, the difference between the displacement in the width direction of the first surface of the test piece and the displacement in the width direction of the second surface is reduced. As a result, in the region between the pair of second contact tools, the difference in the strain in the longitudinal direction between the center portion in the width direction and the end portion in the width direction is reduced. That is, in the four-point bending corrosion test, the strain on the load surface of the test piece can be made close to uniform.

(Method 2)
In the said method 2, the said test piece may have a thick part thicker than the said center part in the both sides of the center part in the said width direction. In this case, the restraining jig is attached to the test piece in a state in which the pair of second contact tools are in contact with the portion of the thick portion on the second surface. A load is applied to the test piece in the thickness direction of the test piece through the pair of first contact tools in contact with the first surface and the pair of second contact tools in contact with the second surface. .

  In this way, by making the thickness of the central part in the width direction of the test piece thinner than both end parts in the width direction, the strain on the load surface of the test piece is made uniform while suppressing the necessary load in the 4-point bending test. You can get closer.

(Method 3)
In the method 2, it is preferable that the difference between the thickness of the thick portion and the thickness of the central portion of the test piece is not less than one quarter of the thickness of the central portion. As a result, the strain on the load surface of the test piece can be made close to uniform while further suppressing the necessary load in the four-point bending test.

(Configuration 1)
A four-point bending corrosion test apparatus according to an embodiment of the present invention includes a pair of first contact tools that are brought into contact with an upper first surface of a test piece, a lower side of the test piece, and an opposite side of the first surface. A pair of second contact tools to be brought into contact with the second surface, and the pair of first contact tools are pressed against the first surface of the test piece from above to form a first distance in a vertical direction perpendicular to the vertical direction. The first support portion that is supported at a position spaced apart from the first contact tool and the pair of second contact tools are pressed from below the second surface of the test piece in the region between the pair of first contact tools. Between the second support portion that is supported at a position spaced apart by a second distance shorter than the first distance in the longitudinal direction, and the pair of second contactors that are separated by a second distance in the longitudinal direction, Both side surfaces in the width direction, which are directions perpendicular to both the vertical direction and the vertical direction of the test piece, Between the first support part and the second support part, which is in contact with the part below the center in the vertical direction of the test piece and restricts the displacement in the width direction of the both side surfaces of the test piece, The pair of first contact tools and the second support member provided and supported by the first support member by changing a relative distance between the first support member and the second support member in the vertical direction. And a load applying mechanism that applies the load in the vertical direction to the test piece to which the restraining jig is attached via the pair of second contact tools supported by the robot.

  According to the configuration 1, when a load is applied to the test piece by the load application mechanism, the restraining jig is in contact with the portion below the center in the thickness direction of the test piece in the region between the pair of second contact tools, The displacement in the width direction of both side surfaces on the outer side in the width direction of the test piece is limited. The restraining jig suppresses deformation between the pair of second contact tools in which the portion below the center in the thickness direction of the test piece tends to extend in the width direction. Thereby, the difference between the displacement in the width direction of the first surface of the test piece and the displacement in the width direction of the second surface is reduced. As a result, in the region between the pair of second contact tools, the difference in the strain in the longitudinal direction between the center portion in the width direction and the end portion in the width direction is reduced. That is, in the four-point bending corrosion test, the strain on the load surface of the test piece can be made close to uniform.

(Configuration 2)
The test piece used for the four-point bending corrosion test in the embodiment of the present invention includes a first surface to which a load is applied in the four-point bending corrosion test, and a surface opposite to the first surface, And a second surface to which a load is applied in the point bending corrosion test. The thickness between the first surface and the second surface of the test piece is greater than the central portion of the test piece in the width direction perpendicular to both the longitudinal direction and the thickness direction of the test piece. The portions on both sides in the width direction are thicker.

  In this way, by reducing the thickness of the central part in the width direction of the test piece as compared with both end parts in the width direction, the load on the test piece is reduced while suppressing the load on the test piece necessary in the four-point bending test. Strain can be made uniform.

(Configuration 3)
In the test piece of Configuration 2, it is preferable that the difference between the thickness of the central portion and the thickness of both sides of the central portion in the width direction is equal to or more than a quarter of the thickness of the central portion. Thereby, the strain on the load surface of the test piece can be made close to uniform while further suppressing the load on the test piece necessary for the four-point bending test.

  In the present invention, the vertical direction may or may not coincide with the vertical direction, that is, the gravity direction. Further, the vertical direction may or may not coincide with the vertical direction. That is, the direction of gravity may be downward, the direction opposite to gravity may be upward, the direction of gravity may be upward, and the direction opposite to gravity may be downward. That is, in the four-point bending corrosion test, a load may be applied to the test piece with the first surface as the evaluation surface facing the ground and the second surface facing the top, or the first surface A load may be applied to the test piece in a state where is facing the top and the second surface is facing the ground.

[Embodiment]
(Configuration example of 4-point bending corrosion test equipment)
FIG. 1 is a side view illustrating a configuration example of a four-point bending corrosion test apparatus according to the present embodiment. The four-point bending corrosion test apparatus shown in FIG. 1 includes a pair of first contact tools 8a, a pair of second contact tools 8b, a first support portion 9a, a second support portion 9b, a restraining jig 7, and a load application mechanism 6. Is provided.

  The pair of first contact tools 8a is in contact with the first surface SJa on the upper side of the test piece SJ. The pair of first contact tools 8a are, for example, solid cylinders or rollers. The pair of second contact tools 8b are in contact with the lower second surface SJb of the test piece SJ. The pair of second contact tools 8b are, for example, solid cylinders or rollers. The second surface SJb is a surface on the opposite side of the first surface SJa in the test piece SJ.

  The first support portion 9a presses the pair of first contact tools 8a against the first surface SJa of the test piece SJ from above, and a first distance in the vertical direction (z direction) perpendicular to the vertical direction (y direction). Support at a position separated by only. The second support portion 9b presses against the second surface SJb of the test piece SJ from below and supports it at a position spaced apart by a second distance shorter than the first distance in the longitudinal direction (z direction). The second support portion 9b supports the pair of second contact tools 8b in the region between the pair of first contact tools 8a.

  The second surface SJb of the test piece SJ is a surface on the opposite side of the first surface SJa. The longitudinal direction of the test piece SJ corresponds to the longitudinal direction of the four-point bending corrosion test apparatus, and is the z direction in FIG. The thickness direction of the test piece SJ corresponds to the vertical direction of the four-point bending corrosion test apparatus, and is the y direction in FIG. The width direction of the test piece SJ corresponds to the lateral direction of the four-point bending corrosion test apparatus, and is the x direction in FIG.

  The restraining jig 7 is in contact with a portion below both sides of the test piece SJ in the width direction between the pair of second contact tools 8b and below the center in the thickness direction of the test piece SJ. The restraining jig 7 limits the displacement in the width direction of both side surfaces of the test piece SJ.

  The first support portion 9a includes an upper portion 9a1 that holds the pair of first contact tools 8a from above, and a lower portion 9a2 that extends from the upper portion 9a1 to the bottom of the second support portion 9b.

  The load application mechanism 6 is configured to be able to change the length between the lower portion 9a2 of the first support portion 9a and the second support portion 9b. In this example, the load application mechanism 6 is configured by a screw that penetrates the screw hole of the lower portion 9a2 of the first support portion 9a. When the screw rotates, the relative position of the second support portion 9b with respect to the lower portion 9a2 of the first support portion 9a changes.

  The first surface SJa of the test piece SJ is supported from above by the upper portion 9a1 of the first support portion 9a via the pair of first contact tools 8a. If the 2nd support part 9b moves up, the 2nd contact tool 8b will push the longitudinal direction center part of test piece SJ from the bottom. Therefore, a bending load in the thickness direction is applied to the test piece SJ.

  In addition, the structure of a 1st support part, a 2nd support part, and a load mechanism is not restricted to the example shown in FIG. In the above example, the load mechanism is configured to connect the first support part and the second support part to each other and to change the distance between the connected first support part and the second support part. The load mechanism may have a configuration different from the above configuration, and the relative position of the first support portion and the second support portion may be changed.

In the corrosion test, the test piece SJ loaded with the bending load in this way is exposed to a corrosion solution (for example, H ₂ S solution) for 720 hours, and the presence or absence of crack generation is examined. In order to generate a predetermined tensile stress on the test piece SJ, a load to be applied is controlled. As one method of setting the load, as shown in FIG. 2 or FIG. 3, a strain gauge 21 is attached to the test piece SJ, and the strain value is adjusted. The test piece SJ is bent under a load until the detected value of the strain gauge 21 reaches a predetermined strain (for example, a strain equivalent to 100% yield strength (YS) of the target material). In FIG. 2, the strain gauge 21 is stuck on the center of the test piece SJ in the longitudinal direction. This configuration is applied when the test piece SJ does not include a welded portion, for example, when the test piece SJ is a test piece taken from a base material portion of a steel pipe. In FIG. 3, the strain gauges 21 are affixed on both sides of the weld metal SJw at the center of the test piece SJ. This configuration is applied when the test piece SJ includes a weld metal, for example, when the test piece SJ is a test piece taken from a welded joint portion.

  4 and 5 are diagrams for explaining the shape of the test piece SJ. The test piece SJ1 shown in FIG. 4 is a rectangular parallelepiped and is referred to as a smooth test piece. In the test piece SJ2 shown in FIG. 5, a part of the inner surface SJs of the steel pipe 80 that is a target material is directly one surface of the test piece, and is referred to as an inner surface As test piece.

  In the above four-point bending corrosion test, as an example, the presence or absence of crack generation near the center of the specimen SJ is evaluated. If the strain on the surface of the test piece SJ to which a load is applied is not uniform, a crack may occur at a position away from the center of the test piece SJ. When a crack occurs at a position away from the center, proper evaluation becomes difficult. Therefore, in this embodiment, with the restraining jig 7 attached to the test piece SJ, by applying a load to the test piece SJ, the strain on the surface is made closer to uniform without increasing the thickness of the test piece SJ. .

(Restraining jig)
FIG. 6 is a plan view showing the configuration of the test piece SJ, the first contact tool 8a, the second contact tool 8b, and the restraining jig 7 shown in FIG. FIG. 7 is a side view of the configuration shown in FIG. 6 viewed from the lateral direction (x direction). FIG. 8 is a cross-sectional view showing a cross section taken along line AA in FIGS. 6 and 7.

  As shown in FIG. 6, the restraining jig 7 includes a pair of both end portions 71 that are in contact with the side surfaces SJc at both ends in the width direction of the test piece SJ, and a bridging portion 72 that connects the pair of both end portions 71. The bridging portion 72 is formed to extend from one of the pair of both end portions 71 to the other in the width direction of the test piece between the pair of second contact tools 8b. The pair of both end portions 71 and the bridging portion 72 between them are fixed to each other. Therefore, in a state in which the pair of both end portions 71 are in contact with the side surfaces SJc at both ends in the width direction of the test piece SJ, the restraining jig 7 restricts the side surface SJc of the test piece SJ from being displaced outward in the width direction. Thereby, the deformation | transformation which 2nd surface SJb which is the lower surface of test piece SJ spreads in the width direction is suppressed.

  As shown in FIG. 7, both end portions 71 are in contact with portions below the center C1 in the thickness direction of the side surfaces SJc at both ends in the width direction of the test piece SJ. It is preferable that the both end portions 71 do not contact portions above the center C1 in the thickness direction of the side surfaces SJc at both ends in the width direction of the test piece SJ. When the first surface SJa that is the upper surface of the test piece SJ is an evaluation surface, if both end portions 71 are in contact with a portion above the center C1 in the thickness direction of the side surface SJc, the test result may be affected. It is.

  In the example shown in FIG. 7, the length in the longitudinal direction of the test piece SJ at both ends 71 of the restraining jig 7 is substantially the same as the distance Lb in the longitudinal direction between the pair of second contact tools 8b. As shown in FIGS. 7 and 8, the upper surfaces of both end portions 71 of the restraining jig 7 are second surfaces SJb from the center C1 in the thickness direction of the test piece SJ when viewed from the width direction, that is, in a side view. Close to the location. Both end portions 71 of the restraining jig 7 are in contact with a part of the region from the thickness direction center C1 to the second surface SJb on the side surface SJc of the test piece SJ between the pair of second contact jigs 8b.

  Further, as shown in FIG. 7, both end portions 71 are in contact with the side surfaces SJc at both ends in the width direction of the test piece SJ in the region between the pair of second contact tools 8b in the longitudinal direction (z direction) of the test piece. . Thereby, in the part of the test piece SJ between a pair of 2nd contact tools 8b which most influences a test result, distribution of the distortion of the width direction can be closely approximated.

  The longitudinal dimension of the bridging portion 72 is smaller than the longitudinal dimension of both end portions 71. Moreover, the dimension of the longitudinal direction of the bridge | crosslinking part 72 is smaller than the distance Lb between a pair of 2nd contact tools 8b. The vertical dimension or thickness of the bridging portion 72 is smaller than the vertical dimension of the second contact tool 8b (for example, the diameter of the roller constituting the second contact tool 8b). The bridging portion 72 is bridged between the pair of both end portions 71 under the test piece SJ and between the pair of second contact tools 8b.

  In the restraining jig 7, the pair of both end portions 71 and the bridging portion 72 may be integrally formed. Alternatively, at least one of the pair of both end portions 71 and the bridging portion 72 may be fastened by a fastening member such as a screw or a bolt. That is, at least one of the pair of both end portions 71 and the bridging portion 72 can be connected via the fastening member. By fastening with a fastening member, the both ends 71 and the test piece SJ can be more closely attached. Further, the dimensional accuracy between the pair of both end portions 71 is relaxed. As described above, the restraining jig 7 may be configured such that the distance between the pair of both end portions 71 can be adjusted.

  The restraining jig 7 is fitted on the outside of the test piece SJ as a frame material. That is, the test piece SJ is inserted between the pair of both end portions 71 of the restraining jig 7. The restraining jig 7 may be formed to be elastically deformed when the test piece SJ is inserted between the both end portions 71. Thereby, the restraining jig 7 can be attached to the test piece SJ in a state in which both end portions 71 are pressed against the test piece SJ by elastic force. The material of the restraining jig 7 is not particularly limited. For example, the restraining jig 7 can be formed of metal, resin, or ceramic.

  The restraining jig 7 in the above example is configured to restrain the displacement in the width direction of the side surface of the test piece in the region between the second contact tools 8b. On the other hand, the restraining jig 7 may be formed to extend outward from the region between the pair of second contact tools 8b, and the upper and lower sides of the side surface of the test piece may extend over the entire length of the test piece. A portion below the center in the direction may be constrained.

  Moreover, in the said example, although the bridge | crosslinking part 72 is between the pair of 2nd contact tools 8b and is arrange | positioned under the test piece 1, the position of the bridge | crosslinking part 72 is not restricted to this example. For example, the bridge portion 72 may be configured to extend in the width direction and connect the pair of both end portions 71 on the outer side in the longitudinal direction of the pair of second contact tools 8b.

  In the example shown in FIG.6 and FIG.7, the length of the longitudinal direction of the both ends 71 may be longer than the distance Lb of the longitudinal direction between a pair of 2nd contact tools 8b. In this case, both end portions 71 are formed to extend outward from the region between the pair of second contact tools 8b in the longitudinal direction. Further, both end portions 71 of the restraining jig 7 are in contact with each other in the entire region from the center C1 in the thickness direction to the second surface SJb on the side surface SJc of the test piece SJ between the pair of second contact jigs 8b. Also good.

(Test pieces)
In the example shown in FIG. 8, the cross section of the test piece SJ is rectangular. That is, the thickness of the test piece SJ is uniform in the width direction of the test piece SJ. FIG. 9 is a cross-sectional view showing a modification of the cross-sectional shape of the test piece SJ. In the example shown in FIG. 9, the test piece SJ has thick portions SJt having a thickness larger than that of the central portion SJm on both sides of the central portion SJm in the width direction. That is, the thick portions SJt on both sides in the width direction of the central portion SJm are thicker than the central portion SJm in the width direction of the test piece SJ.

  In other words, the test piece SJ shown in FIG. 9 has a pair of legs protruding downward at both ends in the width direction. In other words, a recess is formed on the second surface SJb of the central portion SJm in the width direction of the test piece SJ.

  Thus, by providing the thick portions SJt at both ends in the width direction of the test piece SJ, the strain in the center in the width direction of the first surface SJa and the strain in the end in the width direction when the test piece SJ is loaded. And the strain difference can be further reduced.

  The difference Δt between the thickness ta of the thick part SJt and the thickness tc of the central part SJm is preferably, for example, not less than ¼ of the thickness tc of the central part SJm (ta−tc = Δt ≧ tc / 4). ). Thereby, the distribution of the strain in the width direction of the first surface SJa, which is the evaluation surface when the test piece SJ is loaded, can be made more uniform. Note that this difference Δt can also be referred to as the leg height or leg length of the test piece SJ, or the depth of the recess formed in the central portion SJm.

  Further, as the thickness of the test piece SJ increases, the required load applied to the test piece SJ increases. Therefore, it is preferable that the thickness of the test piece SJ is not too thick while increasing the thickness of the first surface SJa so that the strain of the first surface SJa can be made uniform. From this viewpoint, the difference Δt between the thickness ta of the thick portion SJt and the thickness tc of the central portion SJm is preferably, for example, equal to or less than the thickness tc of the central portion SJm (tc ≧ Δt). It is more preferable to set it to 1/2 or less ((tc / 2) ≧ Δt) of the thickness tc of the portion SJm.

  The dimension w2 in the width direction of the thick portion SJt of the test piece SJ is preferably 1/10 or more (w2 ≧ w1 / 10) of the dimension w1 in the width direction of the entire test piece SJ (1/5 or more). More preferably (w2 ≧ w1 / 5). This is because if the width of the thick part SJt is too small, the thick part SJ4 (leg) is likely to be deformed by a load. The width w2 of the thick portion SJt of the test piece SJ is preferably 2/5 or less (w2 ≦ w1 × 2/5) of the width w1 of the entire test piece SJ. More preferably, it is 3 or less (w2 ≦ w1 × 3/10). This is because the necessary load increases if the width of the thick portion SJt is too large.

  In the example shown in FIG. 9, both ends of the test piece SJ in the width direction are thick portions SJt. The thick part SJt having a thickness larger than that of the central part SJm may be provided in a region that enters inside from both ends in the width direction of the test piece SJ. In addition, even when the test piece SJ has a thick portion SJt, the restraining jig 7 is in contact with the side surface SJc of the test piece SJ and closer to the second surface than the center in the thickness direction of the test piece SJ. It can be. In the example shown in FIG. 9, in the thickness direction of the test piece SJ, the position of one half (ta / 2) of the thickness ta of the thick portion SJt from the lower end of the test piece SJ, that is, the second surface SJb, It becomes the center in the thickness direction of the piece SJ.

  The inventors measured the difference in the longitudinal component of the strain between the vicinity of the center in the width direction of the upper surface of the test piece loaded and the vicinity of the end. The measurement was performed by actual measurement and simulation. In the simulation, an FEM analysis for a four-point bending corrosion test was performed using the analysis model shown in FIG.

  In FIG. 10, the longitudinal direction of the test piece SJv is the z direction, the vertical direction of the test piece SJv, that is, the load direction is the y direction, and the z direction and the direction perpendicular to the y direction are the x direction. In the example illustrated in FIG. 10, in the test piece SJv, a central point in the z direction and the x direction of the first surface (evaluation surface) to which a load is applied from above is defined as a central point C0. A plane passing through the symmetric boundary line TKx in the x direction and parallel to the xy plane is a symmetric boundary surface in the longitudinal direction (z direction) of the test piece SJv. A plane that passes through the symmetric boundary line TKz in the z direction and is parallel to the yz plane is a symmetric boundary surface in the x direction of the test piece SJv.

  The first contact tool 8av and the second contact tool 8bv were rigid roller models. The size of the test piece SJv was set such that the width SW = 10 mm, the height SH = 2 mm, and the length SL = 75 mm. The shape of the test piece SJv was analyzed in two ways: a smooth test piece and an inner surface As test piece. The target material of the test piece SJv was a martensitic stainless steel pipe. The stress-strain curve shown in FIG. 11 was used as data on the stress-strain characteristics of the target material.

  FIG. 12 shows a mesh (finite element) in the xy plane of the inner surface As test piece. FIG. 12 shows only the right half of the symmetric boundary surface TSz including the symmetric boundary line TKz.

  FIG. 13 is a graph showing the distribution (solid line) and measured values (triangular plot) of the longitudinal component of the strain obtained by FEM analysis of the inner surface As test piece. The horizontal axis of FIG. 13 shows the position on a line extending in the x direction through the center point C0 on the upper surface of the test piece SJv. As shown in FIG. 13, the analysis result and the actual measurement value are almost the same. Thereby, it is considered that the strain distribution can be accurately estimated by FEM analysis.

  From the results shown in FIG. 13, it was found that the strain near the end portion was larger than that near the center portion in the width direction of the upper surface of the test piece. As a cause of the occurrence of cracks in the vicinity of the widthwise end of the upper surface of the test piece, it is considered that no uniform strain is applied to the upper surface of the test piece. That is, since the strain near the end portion is larger than that in the vicinity of the center of the upper surface of the test piece in the width direction, it is considered that cracking occurs near the end portion away from the center of the upper surface of the test piece.

  The inventors further studied, by FEM analysis, a method for reducing the strain difference between the central portion and the end portion in the width direction of the upper surface of the test piece when a load was applied to the test piece in the four-point bending corrosion test. The inventors predicted that if the difference in displacement in the width direction between the upper surface side and the lower surface side of the test piece can be reduced, the difference in strain in the longitudinal direction between the center portion and the end portion can be reduced. Therefore, FEM analysis was carried out under the condition that the test piece has a thick portion thicker than the central portion at both ends in the width direction of the test piece, and the side surface outside the width direction of the test piece is restricted in the width direction of the test piece. The strain distribution was calculated.

  FIG. 14 shows a mesh (finite element) and constraint conditions in the xy plane of the smooth test piece. FIG. 15 shows a mesh (finite element) and constraint conditions in the xy plane of the inner surface As test piece. In the analysis model shown in FIGS. 14 and 15, the widthwise dimension wf = 2 mm of the thick portion SJvt, the thickness ts = 2 mm of the central portion SJvm, and the difference td between the thickness ts of the central portion SJvm and the thick portion SJvt. = 1 mm. The dimension wd in the width direction from the symmetric boundary surface TSz to the side surface was wd = 5 mm. The restraint condition is to restrain the displacement in the width direction of the outer side surface of the leg (the portion from the bottom of the test piece SJv to td) in the thick portion SJvt. For comparison, FEM analysis was performed on a test piece without a leg, that is, a test piece with td = 0 mm in FIGS. The stress-strain curve shown in FIG. 11 was used as data on the stress-strain characteristics of the target material.

  FIG. 16 is a graph obtained by FEM analysis using the analysis model of the smooth test piece shown in FIG. For comparison, an FEM analysis result with a smooth test piece without restraint (without legs) is also shown. FIG. 17 is a graph obtained by FEM analysis using the analysis model of the inner surface As test piece shown in FIG. For comparison, an FEM analysis result with an inner surface As test piece without restraint (without legs) is also shown. FIG.16 and FIG.17 is a graph which shows distribution in the width direction of the distortion of the longitudinal direction of a test piece. The vertical axis of the graph represents the value of the strain ratio at each position when the value of the strain in the longitudinal direction at the center in the width direction of the specimen is 1. In the analysis result of the smooth test piece shown in FIG. 16, the ratio of the strain at the center in the width direction of the test piece to the strain at each position is approximately 1 when there is a constraint. That is, when the displacement in the width direction is constrained, the difference in strain between the center portion in the width direction and the end portion is reduced as compared with the case where the displacement is not constrained. In the analysis result of the inner surface As test piece shown in FIG. 17, in the case of restraint, the ratio of the strain at the center in the width direction of the test piece and the strain at each position tends to increase toward the end, but there is no restraint. It is reduced compared to the case. That is, in both the smooth test piece and the inner surface As test piece, when the displacement in the width direction is constrained, the difference in strain between the center portion in the width direction and the end portion is reduced as compared with the case where the displacement is not constrained.

  Further, in the analysis models shown in FIG. 14 and FIG. 15, the inventors set the leg length of the test piece SJv, that is, the difference td between the thickness ts of the central portion SJvm and the thick portion SJvt to 0.5 mm, 1 mm, The strain distribution when 2 mm was calculated. The constraint condition is to restrain the displacement in the width direction of the outer side surface of the leg in the thick portion SJvt. FIG. 18 shows the relationship between the length of the leg of the test piece and the ratio of the strain in the longitudinal direction between the center and the end in the width direction of the test piece. As a comparison, the FEM analysis results of the smooth test piece without constraint (without legs) and the inner surface As test piece shown in FIGS. 16 and 17 were plotted at the position of leg length td = 0 mm. FIG. 18 shows that the strain difference between the central portion and the end portion becomes smaller as the leg length becomes longer. In both the smooth test piece and the inner surface As test piece, the strain ratio is greatly reduced by attaching a leg of 0.5 mm, but the strain ratio value hardly changes at longer leg lengths. The longer the leg length, the greater the required load, so it will not be longer than necessary. In other words, it has been found that if a leg length of 0.5 mm, that is, a leg length that is ¼ of the thickness ts (= 2 mm) of the test piece central portion SJvm, is given, there is a sufficient effect for reducing the strain difference.

  The inventors calculated the strain distribution with and without restraint in the specimen width direction for the specimen without the legs (thick part), that is, the specimen with td = 0 mm in FIGS. 14 and 15. . The constraint condition was to restrain the displacement in the width direction from the second surface of the test piece to a height of 1 mm. For comparison, the calculation was also performed for td = 1 mm and restraint (constraint displacement in the width direction of the outer side surface of the leg).

  FIG. 19 is a graph showing the analysis result of the smooth test piece, and FIG. 20 is a graph showing the analysis result of the inner surface As test piece. Even if both the smooth test piece and the inner surface As test piece are not provided with a leg, if they are restricted in the width direction, compared to a test piece without restriction and without a leg, there is a difference between the central part and the end of the test piece. It can be seen that the strain difference is reduced. Moreover, it turns out that the strain difference of a test piece which has a leg further reduces a center part and an edge part. That is, it can be said that it is preferable that the test piece has a thick portion, that is, a leg.

  As mentioned above, although one Embodiment of this invention was described, embodiment mentioned above is only the illustration for implementing this invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

6: Load mechanism 7: Restraint jig 8a: 1st contact tool 8b: 2nd contact tool 9a: 1st support part 9b: 2nd support part SJ: Test piece

Claims (6)

The first surface of the test piece is in contact with a pair of first contact tools separated by a first distance in the longitudinal direction of the test piece, and the second surface on the opposite side to the first surface of the test piece In a state where a pair of second contact tools separated by a second distance shorter than the first distance is in contact with a region between the pair of first contact tools in a longitudinal direction of the test piece, A supporting process;
Perpendicular to both the longitudinal direction of the test piece and the thickness direction between the first surface and the second surface between the pair of second contactors spaced apart by a second distance in the longitudinal direction. A restraining treatment that is in contact with a portion closer to the second surface than the center in the thickness direction of the test piece and restricts displacement in the width direction of the both side surfaces of the test piece. Attaching the tool,
The pair of first contact tools are in contact with the first surface and the pair of second contact tools are supported in contact with the second surface, and the test piece to which the restraining jig is attached, And applying a load in the thickness direction via the pair of first contact tools in contact with the first surface and the pair of second contact tools in contact with the second surface. Test method. The test piece has thick portions on both sides of the central portion in the width direction, the thickness being greater than the central portion,
The pair of first contact tools are supported in contact with the first surface, the pair of second contact tools are supported in contact with the thick portion of the second surface, and the restraining jig is attached. A load is applied to the test piece in the thickness direction of the test piece through the pair of first contact tools in contact with the first surface and the pair of second contact tools in contact with the second surface. Item 4. The four-point bending corrosion test method according to Item 1.   3. The four-point bending corrosion test method according to claim 2, wherein a difference between the thickness of the thick part and the thickness of the central part of the test piece is equal to or more than ¼ of the thickness of the central part. A pair of first contactors that are brought into contact with the upper first surface of the test piece;
A pair of second contact tools for contacting the lower surface of the test piece and the second surface opposite to the first surface;
A first support portion for supporting the pair of first contact tools from above on the first surface of the test piece and supporting the pair of first contact tools at a position separated by a first distance in a vertical direction perpendicular to the vertical direction;
In the region between the pair of first contact tools, the pair of second contact tools is pressed against the second surface of the test piece from below, and the second distance is shorter than the first distance in the longitudinal direction. A second support part for supporting at a position separated by a distance;
Between the pair of second contactors spaced apart by a second distance in the longitudinal direction, both side surfaces in the width direction that are perpendicular to both the longitudinal direction and the vertical direction of the test piece, A restraining jig that is in contact with a portion below the center of the test piece in the vertical direction and restricts the displacement in the width direction of the both side surfaces of the test piece;
It is provided between the first support part and the second support part, and is supported by the first support part by changing the relative distance in the vertical direction between the first support part and the second support part. The load application that applies the load in the vertical direction to the test piece to which the restraining jig is attached is performed through the pair of first contact tools and the pair of second contact tools supported by the second support portion. A four-point bending corrosion test apparatus equipped with a mechanism. A test piece used for a four-point bending corrosion test,
A first surface to which a load is applied in the four-point bending corrosion test;
A surface opposite to the first surface, the second surface being loaded in the four-point bending corrosion test,
The thickness between the first surface and the second surface is greater than the central portion of the test piece in the width direction perpendicular to both the longitudinal direction and the thickness direction of the test piece. A specimen with thicker parts on both sides.   The test piece according to claim 5, wherein a difference between the thickness of the central portion and the thickness of both sides of the central portion in the width direction is equal to or more than one quarter of the thickness of the central portion.