Test piece, manufacture method of test piece, and fatigue crack growth test method

Abstract

To provide a test piece on which a crack grows long without bending and an opening / closing port of the crack can be properly observed.SOLUTION: The test piece on which fatigue crack grows in a width direction orthogonal to a depth direction and a tensile direction by applying load in the tensile direction orthogonal to the dept direction comprises: a notch which is cut in a center part in the tensile direction from one end side in the width direction toward the other side in the width direction; a first side face facing the one side in the depth direction; and a second side face facing the other side in the depth direction. The center part of the first side face in the tensile direction is provided with a groove formed to be continuous at the tip of the notch and extend from the tip toward the other side in the width direction. When viewed from the depth direction, an area inside the groove of the second side face has a planar shape orthogonal to the depth direction.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a test piece, a method for producing a test piece, and a fatigue crack growth test method.

Background Art

[0002] As one of the tests for examining the fatigue characteristics of materials, a fatigue crack growth test can be mentioned. In a fatigue crack growth test, for example, a CT test piece is used. The CT test piece includes two hole portions and a notch formed between the two hole portions. When loads in opposite directions are applied to the two hole portions so that the distance between the two hole portions increases, a crack progresses from the notch bottom of the notch. By observing the crack generated on the side surface of the CT test piece, the opening and closing behavior can be grasped.

[0003] Citation Document 1 discloses a miniature fracture toughness test piece. FIG. 5 of Citation Document 1 discloses a test piece having a notch formed between two hole portions. Further, FIG. 7 of Citation Document 1 discloses a test piece having a notch formed between two hole portions and a side groove formed on the extension line of the notch. The side grooves are formed on both side surfaces of the test piece.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] When a fatigue crack growth test is performed using a test piece without side grooves as disclosed in FIG. 5 of Patent Document 1, on the side surface of the test piece, the crack may deviate from the extension line of the notch and grow. When attempting to examine the opening and closing behavior at a specific part of the material, if the crack does not progress to the target position and deviates, there is a risk that the opening and closing behavior as planned cannot be examined.

[0006] On the other hand, as disclosed in FIG. 7 of Patent Document 1, when a fatigue crack growth test is performed using a test piece with side grooves provided on both side surfaces of a CT test piece, the crack progresses from the notch along the side grooves. Therefore, compared with the case without side grooves, the crack can be made to progress straight from the notch. However, when observing the crack that has progressed through the side groove using a microscope, it is necessary to precisely adjust the focus of the microscope to match the recess of the side groove. At that time, even if the focus can be adjusted to the crack tip, there is a risk that the focus will not be achieved for a portion even slightly deviated from the crack tip. As a result, there is a risk that the crack cannot be observed appropriately.

[0007] Also, when stress is applied to a CT test piece as described above, stress concentrates at the notch, and a crack progresses from the notch. Here, when stress is applied to a CT test piece provided with side grooves, stress also concentrates on the side grooves in addition to the notch. As a result, excessive stress concentrates at the tip of the crack appearing on the side surface of the test piece compared with a test piece without side grooves, so it may be inappropriate for examining the original opening and closing behavior of the material.

[0008] Therefore, an object of the present invention is to solve the above problems and provide a test piece that allows a crack to grow long without bending and enables the crack to be observed appropriately.

Means for Solving the Problems

[0009] The gist of the present invention lies in the following test piece, method for producing a test piece, and fatigue crack growth test method.

[0010] (1) A test piece in which a fatigue crack propagates along the width direction orthogonal to both the thickness direction and the tensile direction by applying a load in the tensile direction orthogonal to the thickness direction, At the central portion in the tensile direction, a notch cut from one end in the width direction toward the other side in the width direction, A first side facing one side in the thickness direction, A second side facing the other side in the thickness direction, and having, At the central portion of the first side in the tensile direction, a groove is formed so as to be continuous with the tip of the notch and extend from the tip toward the other side in the width direction, When viewed from the thickness direction, the region inside the groove of the second side has a planar shape orthogonal to the thickness direction, Test piece.

[0011] (2) Having a rectangular parallelepiped shape or a disc shape, The test piece according to (1) above.

[0012] (3) Having a configuration in which the groove is formed in a CT test piece conforming to ASTM E-647-23A or JIS T 0310:2009, The test piece according to (1) above.

[0013] (4) Using a dummy test piece having the same configuration as the test piece described in (1) above except that the groove is not formed, performing a fatigue crack propagation test under preset test conditions, and measuring the length of the crack that has propagated without leaving a preset allowable distance or more in the tensile direction from the tip of the notch as the reference propagation amount. Assuming that the crack generated when performing a fatigue crack propagation test on the test piece described in (1) above propagates while being inclined with respect to the width direction of the test piece when viewed from the tensile direction, when the distance between the end on the groove side of the leading edge and the second side in the direction orthogonal to the leading edge of the crack when viewed from the tensile direction is defined as the assumed propagation amount, the assumed inclination angle of the leading edge with respect to the width direction is determined so that the assumed propagation amount is smaller than the reference propagation amount. When performing a fatigue crack propagation test on the test piece described in (1) above, assuming that the crack propagates while being inclined with respect to the width direction of the test piece when viewed from the tensile direction, when the distance between the end on the groove side of the leading edge and the second side in the direction orthogonal to the leading edge of the crack when viewed from the tensile direction is defined as the assumed propagation amount, the assumed inclination angle of the leading edge with respect to the width direction is determined so that the assumed propagation amount is smaller than the reference propagation amount. The depth of the groove corresponding to the assumed inclination angle obtained from the information showing the relationship between the thickness of the test piece, the inclination angle of the leading edge of the crack with respect to the width direction of the test piece, and the depth of the groove is defined as the depth of the groove of the test piece described in the above (1). Method for producing a test piece.

[0014] (5) Using a test piece produced by the method for producing a test piece described in the above (4). Fatigue crack propagation test method.

Effect of the Invention

[0015] According to the present invention, it is possible to provide a test piece in which a crack propagates long without bending and the opening and closing of the crack can be appropriately observed.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Embodiments for Carrying out the Invention

[0017] Hereinafter, the test piece, the method for producing the test piece, and the fatigue crack growth test method according to the embodiments of the present invention will be described in detail.

[0018] 1. Test Piece FIG. 1 is a diagram showing the shape of the test piece 10. Specifically, (a) is a front view of the test piece 10, (b) is a right side view of the test piece 10, and (c) is a rear view of the test piece 10. In the fatigue crack growth test, the direction in which a load is applied to the test piece 10 is defined as the tensile direction D1. FIGS. 1(a) and (b) show the thickness direction D2 of the test piece 10. In the present embodiment, the thickness direction D2 is a direction perpendicular to the tensile direction D1. In FIG. 1(a), the direction perpendicular to the tensile direction D1 and the thickness direction D2 is defined as the width direction D3 of the test piece 10. In FIGS. 1(a) and (c), the two-dot chain line indicates the center line P passing through the center of the test piece 10 in the tensile direction D1. The test piece 10 is symmetric with respect to the tensile direction D1 with the center line P as the axis of symmetry. In the present embodiment, the test piece 10 has a plate shape (rectangular parallelepiped shape). In the present embodiment, the test piece 10 has the same configuration as a CT test piece conforming to ASTM E647-23A, except that a groove 16 described later is provided. Note that the test piece according to the embodiment of the present invention only needs to have a configuration in which a groove 16 described later is provided with respect to a known CT test piece. Therefore, the test piece according to the embodiment of the present invention may have the same configuration as a CT test piece conforming to JIS T 0310:2009, except that a groove 16 described later is provided.

[0019] The test piece 10 has two hole portions 12, a notch 14, and a groove 16. As shown in FIG. 1, the two hole portions 12 are formed at symmetric positions with the center line P of the test piece 10 as the axis of symmetry. Also, in the width direction D3, both of the two hole portions 12 are provided on one side of the center of the test piece 10. Both of the two hole portions 12 penetrate the test piece 10 in the thickness direction D2. In the fatigue crack growth test, jigs are attached to the two hole portions 12 respectively, and a load is applied to the test piece 10.

[0020] The notch 14 is provided at the central portion of the test piece 10 in the tensile direction D1. The notch 14 is formed so as to be cut from one end 15 on one side in the width direction D3 of the test piece 10 toward the other side in the width direction D3. The notch 14 is formed such that the tip 18 of the notch 14 is located at the center in the tensile direction D1 of the test piece 10. In FIG. 1(a), a virtual straight line P1 passing through the notch 14 side end of the hole portion 12 provided on one side in the tensile direction D1 as viewed from the thickness direction of the test piece 10 and parallel to the center line P, and a virtual straight line P2 passing through the notch 14 side end of the hole portion 12 provided on the other side in the tensile direction D1 and parallel to the center line P are shown. In the following, as viewed from the thickness direction D2, the portion of the test piece 10 between the straight line P1 and the straight line P2 is defined as the central portion 30 of the test piece 10.

[0021] In the following description, one side surface of the test piece 10 in the thickness direction D2 is referred to as the side surface 22. In the present embodiment, the side surface 22 corresponds to the first side surface. Also, the other side surface of the test piece 10 in the thickness direction D2 is referred to as the side surface 24. In the present embodiment, the side surface 24 corresponds to the second side surface. Note that the outer edge of the test piece 10 may be chamfered to be C-shaped or R-shaped. In such a case, among the side surface 22, the portion excluding the chamfered portion corresponds to the first side surface, and among the side surface 24, the portion excluding the chamfered portion corresponds to the second side surface.

[0022] The groove 16 is provided at the center in the tensile direction D1 of the test piece 10 on the side surface 22 of the test piece 10. The groove 16 is continuous with the tip 18 of the notch 14 and is formed so as to extend from the tip 18 of the notch 14 toward the other side in the width direction D3 of the test piece 10. In the present embodiment, the groove 16 is formed from the tip 18 of the notch 14 to the other end 20 (hereinafter, also simply referred to as "end 20") in the width direction D3 of the test piece 10. As shown in FIG. 1(b), in the present embodiment, the groove 16 is formed in a V shape so as to be recessed toward the other side in the thickness direction D2 when viewed from the width direction D3. The bottom 38 of the groove 16 is formed so as to be parallel to the center line P.

[0023] In FIG. 1(c), the position corresponding to the outer edge of the groove 16 is indicated by a broken line. When viewed from the thickness direction, the inner region 28 of the side surface 24 of the groove 16 has a planar shape orthogonal to the thickness direction D2. In other words, on the side surface 24, a groove extending from the tip 18 of the notch 14 toward the other side in the width direction D3 of the test piece 10 is not formed.

[0024] Note that, at the central portion 30 of the test piece 10, it is preferable that all regions of the side surface 22 except the groove 16 have a planar shape orthogonal to the thickness direction D2. Also, at the central portion 30 of the test piece 10, it is preferable that all regions of the side surface 24 have a planar shape orthogonal to the thickness direction D2. In the present embodiment, the entire region of the side surface 22 of the test piece 10 other than the groove 16 and the entire side surface 24 have a planar shape orthogonal to the thickness direction D2.

[0025] Also, the size of the test piece 10 is not particularly limited, but the ratio of the depth (depth L2 in FIG. 6(c)) in the thickness direction D2 of the groove 16 of the test piece 10 to the thickness (thickness L1 in FIG. 6(c)) of the test piece 10 is preferably 0.05 to 0.25. Referring to FIG. 1(b), when the test piece 10 is viewed from the width direction D3, the angle (angle β in FIG. 6(c)) formed between the straight line passing through the bottom 38 of the groove 16 and parallel to the thickness direction D2 and the surface of the groove 16 is preferably 10° to 45°.

[0026] (Effect) The inventors conducted fatigue crack growth tests using test pieces of various materials and shapes, and examined the opening and closing behavior. In this process, a fatigue crack growth test was conducted using the test piece 10 provided with the groove 16 on the side surface 22 as described above, and a comparison was made with the fatigue crack growth test using a conventional CT test piece. Hereinafter, the examination contents by the inventors will be specifically described.

[0027] Fig. 2 is a view of the test piece after the fatigue crack growth test as seen from the other side in the thickness direction. In (a), the conventional CT test piece 11 is shown, and in (b), the test piece 10 according to the present embodiment is shown. Note that the conventional CT test piece 11 shown in Fig. 2(a) (hereinafter abbreviated as test piece 11) is different from the test piece 10 according to the present embodiment in that no groove 16 is formed on one side surface in the thickness direction D2 of the test piece 11.

[0028] As shown in Fig. 2(a), when a fatigue crack growth test is conducted using the conventional test piece 11, the crack 42 may progress along the width direction D3 from the tip 18 (notch bottom) of the notch 14 and then progress so as to deviate in the tensile direction D1. As described above, when attempting to examine the opening and closing behavior at a specific part of the material, if the crack does not progress to the target position and deviates, there is a possibility that the planned opening and closing behavior cannot be examined. Therefore, it is desired that the amount of deviation of the crack 42 in the tensile direction D1 be as small as possible. Note that the allowable deviation amount C1 of the crack 42 in the tensile direction D1 (distance from the center line Pa in the tensile direction D1 of the test piece 11: hereinafter referred to as allowable distance C1) varies depending on the purpose of the test and the like.

[0029] Here, if the progress length in the width direction D3 of the crack 42 that has progressed without leaving the allowable distance C1 preset in the tensile direction D1 from the center line Pa is defined as the reference progress amount L, the reference progress amount L varies depending on the material and dimensions of the test piece 11. On the other hand, when tests are conducted on a plurality of test pieces 11 made of the same material and having the same dimensions, the reference progress amount L does not change significantly.

[0030] Since the outer dimensions (thickness, width, length in the tensile direction) of the test piece are defined by standards and the like, the inventors have conducted various studies on configurations for increasing the reference progress amount L without changing the outer dimensions of the test piece. However, it was difficult to increase the reference progress amount L without changing the outer dimensions of the test piece.

[0031] Therefore, the inventors once stopped the study on increasing the reference progress amount L and conducted a study on how the stress distribution in the thickness direction of the test piece affects the propagation mode of the crack. In the course of this study, the inventors considered forming the groove 16 only on one side surface 22 as in the test piece 10 according to the present embodiment. In this case, due to stress concentration occurring in the groove 16, it is considered that the stress generated on the side surface 22 side becomes higher than the stress generated on the side surface 24 side around the tip of the crack 42. The inventors investigated how the crack propagates in the above stress state by observing the fracture surface of the test piece and performing FEM analysis described later. Hereinafter, the propagation mode of the crack in the test piece 10 according to the present embodiment will be described while comparing it with the propagation mode of the crack in the conventional test piece 11.

[0032] FIG. 3 and FIG. 4 are diagrams for explaining the propagation mode of the crack in the fatigue crack propagation test. Specifically, FIG. 3 is a diagram for explaining the propagation mode of the crack in the conventional test piece 11, and FIG. 4 is a diagram for explaining the propagation mode of the crack in the test piece 10 according to the present embodiment. In addition, in FIG. 3 and FIG. 4, a schematic cross section of the test piece perpendicular to the tensile direction D1 (a cross section corresponding to the A-A portion in FIG. 1(a)) is shown. Also, in FIG. 3 and FIG. 4, it is assumed that the crack 42 propagates in the order of (a), (b), and (c).

[0033] As shown in FIGS. 3(a) to 3(c), in the fatigue crack growth test using the conventional test piece 11, the crack 42 progresses so as to go straight in the width direction D3. In other words, when viewed from the tensile direction D1, the crack 42 progresses in the width direction D3 while maintaining a state where its leading edge 40 is substantially parallel to the tip 18 of the notch 14. Note that although the leading edge 40 of the crack 42 does not become a straight line in an actual fatigue crack growth test, it is represented as a straight line in a simplified manner in FIG. 3. The same applies to FIGS. 4 and 5.

[0034] On the other hand, in the fatigue crack growth test using the test piece 10 according to the present embodiment, stress concentration occurs at the bottom 38 of the groove 16. For this reason, the progress speed of the crack 42 is faster on the side surface 22 where the groove 16 is formed than on the side surface 24 side. As a result, as shown in FIGS. 4(a) and 4(b), when viewed from the tensile direction D1, the crack 42 progresses in the width direction D3 while its leading edge 40 gradually inclines with respect to the thickness direction D2.

[0035] Further, as shown in FIGS. 4(b) and 4(c), when viewed from the tensile direction D1, after the inclination of the leading edge 40 of the crack 42 with respect to the thickness direction D2 reaches a certain angle α, the crack 42 progresses in the width direction D3 while progressing with the direction D4 perpendicular to the leading edge 40 as the main direction. As a result, the crack 42 progresses in the width direction D3 while maintaining the inclination angle α of the leading edge 40 with respect to the thickness direction D2. Hereinafter, the progress direction D4 of the crack after the inclination of the leading edge 40 of the crack 42 reaches a certain angle α is referred to as the main direction D4.

[0036] FIG. 5 is a schematic cross-sectional view of the test piece 10 in a state where the inclination of the leading edge 40 of the crack 42 with respect to the thickness direction D2 has reached a certain angle α. FIG. 5 shows the intersection G1 between the leading edge 40 of the crack 42 and the bottom 38 of the groove 16, the distance L' between the intersection G1 in the main direction D4 and the side surface 24, and the inclination angle θ1 of the leading edge 40 with respect to the width direction D3.

[0037] As a result of the research by the present inventors, it is considered that the progress amount of the crack 42 in the main direction D4 when the inclination of the leading edge 40 reaches a certain angle α greatly affects the deviation of the crack 42 in the tensile direction D1. Specifically, it is considered that the distance L' in the test piece 10 (the progress amount of the crack 42 from the end on the groove 16 side of the crack 42 (intersection point G1) to the main direction D4) corresponds to the progress amount of the crack in the width direction D3 in the conventional test piece 11 having the same outer dimensions as the test piece 10. Note that Fig. 5(a) shows the case where the distance L' of the test piece 10 is less than or equal to the reference progress amount L of the test piece 11, and Fig. 5(b) shows the case where the distance L' is greater than the reference progress amount L. Further, Fig. 5(b) shows a position G2 that is the reference progress amount L away from the intersection point G1 in the main direction D4. Furthermore, Fig. 5(b) shows an intersection point G3 between a straight line passing through the position G2 and parallel to the leading edge 40 of the crack 42 and the side surface 24.

[0038] As a result of the research by the present inventors, as shown in Fig. 5(a), when the distance L' of the test piece 10 is less than or equal to the reference progress amount L of the test piece 11, it was found that in the test piece 10, the crack 42 can progress in the width direction D3 without deviating from the tensile direction D1 by more than the above-mentioned allowable distance C1 (allowable deviation amount C1). That is, as shown in Fig. 2(b), the crack 42 observed on the side surface 24 progresses in the width direction D3 without deviating from the tensile direction D1 by more than the above-mentioned allowable distance C1 (allowable deviation amount C1).

[0039] On the other hand, as shown in Fig. 5(b), when the distance L' of the test piece 10 is greater than the reference progress amount L of the test piece 11, it was found that on the end 20 side of the position G2 in the width direction D3, the crack 42 may deviate from the tensile direction D1 by more than the above-mentioned allowable distance C1 (allowable deviation amount C1) and progress. However, even in this case, the distance L'' in the width direction D3 from the tip 18 of the notch 14 to the position G3 is longer than the distance L. Therefore, the crack 42 observed on the side surface 24 can progress in the width direction D3 beyond the reference progress amount L without deviating from the tensile direction D1 by more than the above-mentioned allowable distance C1 (allowable deviation amount C1).

[0040] Thus, in the fatigue crack growth test using the test piece 10 according to this embodiment, the crack 42 observed on the side surface 24 can progress beyond the reference progress amount L in the width direction D3 without departing from the above-described allowable distance C1 (allowable amount of deviation C1) or more in the tensile direction D1. As a result, the crack 42 can be appropriately observed on the side surface 24.

[0041] (Other embodiments) In the above-described embodiment, the case where the test piece 10 has a rectangular parallelepiped shape has been described. However, the shape of the test piece 10 may be a disc shape. Thereby, the test piece can be made to conform to the purpose or conditions of the fatigue crack growth test.

[0042] Further, in the above-described embodiment, the case where the groove 16 has a V-shaped cross section has been described. However, the shape of the groove is not limited to the above example. The groove may be formed so that stress concentration occurs at the center of the bottom. Therefore, for example, the groove may have a U-shaped cross section.

[0043] 2. Method for manufacturing test piece (Studies by the present inventors) The present inventors created an analysis model corresponding to the test piece 10 and the jig, and studied preferable conditions for causing the crack to progress long without bending by elastic analysis by the finite element method simulating the fatigue crack growth test using the test piece 10. In the present invention, "progressing without bending" means progressing without departing from the allowable distance C1 or more in the tensile direction D1 from the tip 18 of the notch 14.

[0044] FIG. 6 is a diagram showing the analysis model 50. (a) is a perspective view of the analysis model 50, (b) is a front view of the analysis model 50, and (c) is an enlarged left side view of the analysis model 50. As shown in FIG. 1, the test piece 10 is symmetric about the center line P as the axis of symmetry in the tensile direction D1. Therefore, the analysis model 50 was made into a 1 / 2 model corresponding to one side in the tensile direction D1 from the center line P of the test piece 10. For simplicity of explanation, hereinafter, the element corresponding to the test piece 10 in the analysis model 50 will be referred to as the test piece 10. Further, elements corresponding to each part (the notch 14, the groove 16, etc.) of the test piece 10 in the analysis model 50 are given the same name and reference numeral as each part in the test piece 10. In FIG. 6, one side in the width direction D3, one side in the tensile direction D1, and one side in the thickness direction D2 are taken as the X-axis direction, the Y-axis direction, and the Z-axis direction, respectively.

[0045] In the analysis model 50, a jig 34 for applying a load is attached to the hole 12 (see FIG. 1) of the test piece 10. The jig 34 includes a pin 34a passed through the hole 12 (see FIG. 1) and two plates 34b fixed to both ends of the pin 34a. A plurality of analysis models 50 were created by changing the dimensions of each part of the test piece 10. The thickness L1 of the test piece 10 was set to 2 mm. The depths L2 of the groove 16 were set to 0.1 mm, 0.2 mm, and 0.5 mm. The angles β of the groove 16 were set to 15°, 30°, and 45°. The mechanical properties of the test piece 10 were set to a general value of a steel material, a Young's modulus of 205.8 GPa, and a Poisson's ratio of 0.3. The analysis conditions will be described in more detail with reference to FIG. 7.

[0046] FIG. 7 is an enlarged bottom view showing the B-B portion (see FIG. 6(b).) of the test piece 10 in the analysis model 50. In the following description, among the enlarged bottom views of the B-B portion of the test piece 10 shown in FIG. 7, the portion excluding the groove 16 is referred to as the bottom surface 36. In FIG. 7, for the purpose of combining with the description to be described later, the enlarged bottom view of the B-B portion is displayed such that the side surface 24 is on the left side of the paper surface and the side surface 22 is on the right side of the paper surface. First, as shown in FIG. 7, the analysis was performed assuming that the leading edge D of the crack advanced 4 mm parallel to the tip 18 of the notch 14 from the tip 18 of the notch 14. Specifically, for the region of the bottom surface 36 from the leading edge D of the crack to the end 20, the displacement in the Y-axis direction was constrained, the jig 34 was moved in the Y-axis direction to apply a load to the test piece 10, and the stress generated in the test piece 10 was obtained. The load to be applied was determined by simulating the load during the actual fatigue crack growth test. Then, the stress intensity factor (mode I) at the leading edge D of the crack was calculated from the obtained stress. Specifically, the stress intensity factors at a total of three locations, namely, the intersection E of the leading edge D of the crack and the side surface 24 (hereinafter simply referred to as "point E"), the point F at the center of the thickness, and the intersection G of the leading edge D of the crack and the bottom 38 of the groove 16 (hereinafter simply referred to as "point G") on the leading edge D of the crack were calculated. The stress intensity factor was calculated from the tensile stress generated in a certain region on the end 20 side from the leading edge D of the crack.

[0047] FIG. 8 is a graph showing the analysis results. (a) shows the case where the depth L2 of the groove 16 is 0.1 mm, (b) shows the case where the depth L2 of the groove 16 is 0.2 mm, and (c) shows the case where the depth L2 of the groove 16 is 0.5 mm. In FIG. 8, the horizontal axis represents the distance (mm) from the side surface 24, and the vertical axis represents the stress intensity factor (MPa·m 1 / 2 )). In FIG. 8, the position where the distance from the side surface 24 is 0 mm indicates point E, and the position where the distance from the side surface 24 is 1 mm indicates point F. Also, in FIG. 8, in (a), the position where the distance from the side surface 24 is 1.9 mm, in (b), the position where the distance from the side surface 24 is 1.8 mm, and in (c), the position where the distance from the side surface 24 is 1.5 mm indicate point G.

[0048] As shown in Fig. 8, regardless of the depth L2 of the groove 16 and the angle β of the groove 16, as going from point E to point G, the stress intensity factor tends to increase. The reason is that due to the stress concentration in the groove 16, the stress generated on the side 22 side is greater than that on the side 24 side. Thus, as the stress intensity factor on the point G side (side 22 side) increases, as described above, it is considered that the crack propagates faster on the side 22 side than on the side 24 side. As a result, as shown by the dashed line H in Fig. 7, it is considered that the crack gradually inclines and propagates with respect to the tip 18 of the notch 14.

[0049] Next, the present inventors changed the restraint condition of the bottom surface 36 of the analysis model 50 and performed analysis to examine the stress generated in the test piece 10 in a state where the crack front is inclined. As shown in Fig. 9, for the restraint condition of the bottom surface 36, the end on the side 22 side of the crack front I is fixed at a position 4 mm from the tip 18 of the notch 14, and the angle θ formed by the crack front I and the X-axis direction (the bottom 38 of the groove 16) is changed to 50°, 60°, and 70°. In the present embodiment, the angle θ is an acute angle. In the following description, the case where the angle β of the groove 16 is 30° will be described.

[0050] Similar to the analysis described with reference to Fig. 7, the depth L2 of the groove 16 was set to 0.1 mm, 0.2 mm, and 0.5 mm. From the stress obtained by the analysis, the stress intensity factors at a total of three locations, namely point E, the point F at the center of the thickness, and point G, on the crack front I were calculated.

[0051] Fig. 10 is a graph showing the analysis results. (a) shows the case where the depth L2 of the groove 16 is 0.1 mm, (b) shows the case where the depth L2 of the groove 16 is 0.2 mm, and (c) shows the case where the depth L2 of the groove 16 is 0.5 mm. In Fig. 10, the horizontal axis represents the distance (mm) from the side 24, and the vertical axis represents the stress intensity factor (MPa·m 1 / 2) is shown. In FIG. 10, the position where the distance from the side surface 24 is 0 mm is indicated by point E, and the position where the distance from the side surface 24 is 1 mm is indicated by point F. Also, in FIG. 10, in (a), the position where the distance from the side surface 24 is 1.9 mm, in (b), the position where the distance from the side surface 24 is 1.8 mm, and in (c), the position where the distance from the side surface 24 is 1.5 mm are indicated by point G.

[0052] As shown in FIGS. 10(a) to (c), when the depth L2 of the groove 16 is 0.1 mm, the angle θ is 60°; when the depth L2 of the groove 16 is 0.2 mm, the angle θ is 70°; when the depth L2 of the groove 16 is 0.5 mm, the angle θ is 50°. The difference in the stress intensity factor at points E, F, and G becomes the smallest. From these analysis results, it can be seen that the angle θ greatly affects the magnitude relationship of the stress intensity factor at each position (points E, F, and G) of the crack front I.

[0053] Therefore, based on the data shown in FIGS. 8 and 10, the relationship between the angle θ and the stress intensity factor at each position (points E, F, G) of the crack front I was sorted out.

[0054] FIG. 11 is a graph showing the relationship between the angle θ and the stress intensity factor. (a) is the case where the depth L2 of the groove 16 is 0.1 mm, (b) is the case where the depth L2 of the groove 16 is 0.2 mm, and (c) is the case where the depth L2 of the groove 16 is 0.5 mm. In FIG. 11, the horizontal axis represents the angle θ, and the vertical axis represents the stress intensity factor (MPa·m 1 / 2 ) is shown. As shown by the solid line in FIG. 11, the values of the stress intensity factor at the thickness center (point F), the side surface 24 side (point E), and the side surface 22 side (point G) were approximated by straight lines respectively.

[0055] In FIGS. 11(a), 11(b), and 11(c), the stress intensity factor at the thickness center is almost constant regardless of the angle θ. However, in FIGS. 11(a), 11(b), and 11(c), the stress intensity factor on the side 24 side decreases as the angle θ increases. Also, in FIGS. 11(a), 11(b), and 11(c), the stress intensity factor on the side 22 side increases as the angle θ increases. From these analyses, it can be seen that for each depth L2 of the groove 16, there is a case where the difference in the stress intensity factors at the thickness center (point F), the side 24 side (point E), and the side 22 side (point G) is minimized. At this time, since the stress intensity factor at the crack front I is almost the same value regardless of the position, as shown by the dashed line N in FIG. 9, it is considered that the crack propagates while maintaining the angle θ. That is, it can be considered that the angle θ when the difference in the stress intensity factors at the thickness center (point F), the side 24 side (point E), and the side 22 side (point G) is minimized corresponds to the inclination angle θ1 shown in FIG. 5.

[0056] To specifically obtain the angle θ when the difference in the stress intensity factors at the thickness center, the side 24 side, and the side 22 side is minimized, the following calculations were performed. For example, the stress intensity factors at the thickness center, the side 24 side, and the side 22 side when the angle θ is 50° are calculated from the approximate line. Here, among the calculated stress intensity factors, the value at the thickness center is defined as the stress intensity factor K0, the value at the side 24 side is defined as the stress intensity factor K 24 , and the value at the side 22 side is defined as the stress intensity factor K 22 . The difference Δ between the maximum value and the minimum value among the stress intensity factors K0, K 24 , and K 22 is obtained. The difference Δ is calculated for each angle θ, the angle θ at which the difference Δ is minimized is obtained, and the angle θ at that time is defined as the angle θs. As described above, the angle θs when the difference in the stress intensity factors at the thickness center, the side 24 side, and the side 22 side is minimized corresponds to the inclination angle θ1 shown in FIG. 5. In FIG. 11(a), the angle θs was 66.4°, in FIG. 11(b), the angle θs was 60.9°, and in FIG. 11(c), the angle θs was 50.0°. The relationship between these angles θs and the ratio of the depth L2 to the thickness L1 was plotted on the graph of FIG. 12.

[0057] FIG. 12 is a graph summarizing the analysis results. In FIG. 12, the horizontal axis represents the ratio of the depth L2 of the groove 16 to the thickness L1, and the vertical axis represents the angle θs when the difference in stress concentration factors at the thickness center, the side surface 24 side, and the side surface 22 side is minimized. Here, the thickness L1 greatly affects the calculation of the stress concentration factor. For example, even if the depth L2 of the groove 16 is 0.1 mm, the magnitude of the stress generated at the bottom 38 of the groove 16 differs depending on whether the thickness L1 is 1 mm or 10 mm. To enable the determination of the angle θs regardless of the thickness L1, the value obtained by dividing the depth L2 of the groove 16 by the thickness L1 is displayed on the horizontal axis. As shown by the solid line in FIG. 12, the angle θs obtained from the calculation using the data of FIGS. 11(a), (b), and (c) described above was approximated by a straight line. From the approximate line shown in FIG. 12, it can be seen that the angle θs is determined by the thickness L1 of the test piece 10 and the depth L2 of the groove 16.

[0058] Here, as described with reference to FIG. 5(a), when a fatigue crack growth test is performed, if the crack growth amount L' (the crack growth amount of the crack 42 from the end (intersection point G1) on the groove 16 side of the crack 42 in the main direction D4) in the test piece 10 is less than or equal to the reference growth amount L of the conventional test piece 11, the crack 42 can be advanced in the width direction D3 without deviating from the tensile direction D1 by more than the allowable distance C1 (the allowable deviation amount C1). As can be seen from FIG. 5(a), the growth amount L' can be calculated from the inclination angle θ1 of the leading edge 40 with respect to the width direction D3 and the thickness L1 of the test piece 10. Also, as described above, the angle θs corresponding to the inclination angle θ1 is determined by the thickness L1 of the test piece 10 and the depth L2 of the groove 16. Therefore, for example, assuming an inclination angle θ1 at which the growth amount L' becomes less than or equal to the reference growth amount L according to the thickness L1 of the test piece 10, and further, based on the assumed inclination angle θ1 and thickness L1, the depth L2 of the groove 16 can be determined according to the relationship shown in FIG. 12. Thereby, the crack 42 can be advanced in the width direction D3 without deviating from the tensile direction D1 by more than the allowable distance C1 (the allowable deviation amount C1).

[0059] (Method for manufacturing a test piece) The method for manufacturing a test piece according to the present invention has been completed based on the above findings. Hereinafter, taking the case of manufacturing the above-mentioned test piece 10 as an example, the manufacturing of the test piece according to the embodiment of the present invention will be described.

[0060] In the method for manufacturing a test piece according to the present embodiment, first, a fatigue crack propagation test is performed using a provisional test piece, and as described with reference to FIG. 2, a reference propagation amount L based on the allowable distance C1 is measured. At this time, the provisional test piece has the same configuration as the above-described test piece 10 except that the groove 16 is not formed. As described above, the allowable distance can be set in advance according to the purpose of the test. For example, when it is desired to measure the fatigue characteristics of specific crystal grains, the "distance of one crystal grain diameter in the tensile direction D1 from the center line P (see FIG. 2)" may be set as the allowable distance. The reference propagation amount L may be an average value when the fatigue crack propagation test is performed a plurality of times.

[0061] Next, an assumed value of the inclination angle θ1 (see FIG. 5) of the leading edge 40 of the crack 42 (hereinafter referred to as the assumed inclination angle θi) is determined so that the propagation amount in the main direction D4 (see FIG. 5) of the crack assumed in the test using the test piece 10 (the propagation amount corresponding to the propagation amount L' in FIG. 5; hereinafter referred to as the assumed propagation amount) is smaller than the reference propagation amount L. Empirically, the assumed inclination angle θi is preferably 50° to 60°. The assumed propagation amount is calculated as the distance between the end (intersection point G1) on the groove 16 side of the leading edge 40 of the crack 42 and the second side surface 24 in the direction orthogonal to the leading edge 40 of the crack 42 (main direction D4).

[0062] Finally, using the information showing the relationship among the thickness of the test piece, the inclination angle of the crack front with respect to the width direction of the test piece, and the depth of the groove (in this embodiment, the approximate line shown in FIG. 12), the depth L2 of the groove 16 of the test piece 10 is determined from the assumed inclination angle θi and the thickness L1 of the test piece 10. The test piece 10 is obtained by forming the groove 16 with the dimensions thus determined in a test piece having the same configuration as a known CT test piece. When changing the angle β of the groove 16 to an angle other than 30°, for example, 15°, 45°, etc., it is preferable to obtain the approximate line shown in FIG. 12 for each set angle β of the groove 16 to be set. Similarly, when changing the shape of the groove, it is also preferable to obtain the approximate line shown in FIG. 12 for each shape of the groove.

[0063] As described with reference to FIGS. 4(b) and (c), after the inclination of the front edge 40 of the crack 42 reaches a certain angle, the crack 42 propagates in the width direction D3 while maintaining the inclination angle of the front edge 40. Therefore, after the inclination of the front edge 40 reaches a certain angle, the propagation amount L' (see FIG. 5: the propagation amount of the crack 42 from the end portion (intersection point G1) on the groove 16 side of the crack 42 to the main direction D4) also becomes constant. Here, as described above, in the method for manufacturing a test piece according to this embodiment, the test piece 10 is manufactured such that the assumed propagation amount (the propagation amount corresponding to the propagation amount L') is smaller than the reference propagation amount L. That is, in the test piece 10 manufactured by the method for manufacturing a test piece according to this embodiment, it is possible to prevent the propagation amount L' from exceeding the reference propagation amount L during the test. Therefore, according to the method for manufacturing a test piece according to this embodiment, a test piece 10 can be obtained in which the crack observed on the side surface 24 can propagate without leaving the allowable distance C1 or more in the tensile direction D1.

[0064] 3. Test method Using the test piece 10 produced by the method described above, a fatigue crack growth test is performed. For example, tests such as measuring the crack length and measuring the fatigue crack growth rate can be performed. Usually, a pre-crack is introduced into the test piece 10 to perform a fatigue crack growth test. At that time, it is preferable to introduce the pre-crack so that the angle of the leading edge of the pre-crack with respect to the width direction D3 is within the allowable range from the assumed inclination angle θi. After performing these tests, observe the fracture surface of the test piece, and by checking whether the angle θ1 (see Fig. 5) of the leading edge of the crack is within the allowable error range from the assumed inclination angle θi, it can be confirmed whether the test was appropriate. In the case of a test where the crack length is measured by stopping the test halfway without breaking the test piece, first, after the test is completed, immerse it in liquid nitrogen or the like, and break the test piece by advancing the crack by brittle fracture. By observing the subsequent fracture surface, the angle of the crack can be confirmed from the boundary line between the fracture surface due to fatigue fracture and the fracture surface due to brittle fracture.

[0065] Depending on the purpose and accuracy of the test, it may be appropriate to adopt the crack length or crack growth rate of a test piece in which the angle θ1 is within the allowable range (for example, within the error range) from the angle θi.

Industrial Applicability

[0066] According to the present invention, it becomes possible to provide a test piece that allows a crack to grow long without bending and that enables appropriate observation of the opening and closing of the crack.

Explanation of Signs

[0067] 10 Test piece 12 Hole part 14 Notch 16 Groove 18 Tip of notch 14 34 Fixture 36 Bottom surface 38 Bottom of groove 16 40 Leading edge of crack 42 Crack 50 Analysis model

Claims

1. A test piece in which a fatigue crack propagates along the width direction orthogonal to both the thickness direction and the tensile direction when a load is applied in the tensile direction orthogonal to the thickness direction, at the central portion in the tensile direction, a notch cut from one end in the width direction toward the other side in the width direction, a first side facing one side in the thickness direction, a second side facing the other side in the thickness direction, and having, at the central portion in the tensile direction of the first side, a groove is formed continuously with the tip of the notch and extending from the tip toward the other side in the width direction, A test piece in which, when viewed from the thickness direction, the region inside the groove of the second side has a planar shape orthogonal to the thickness direction.

2. The test piece according to claim 1, having a rectangular parallelepiped shape or a disc shape.

3. The test piece according to claim 1, having a configuration in which the groove is formed in a CT test piece conforming to ASTM E-647-23A or JIS T 0310:2009.

4. Using a dummy test piece having the same configuration as the test piece described in claim 1 except that the groove is not formed, a fatigue crack propagation test is performed under preset test conditions, and the length at which the crack propagates without leaving a preset allowable distance or more in the tensile direction from the tip of the notch is measured as the reference propagation amount, Assuming that when a fatigue crack propagation test is performed on the test piece described in claim 1, the crack propagates while being inclined with respect to the width direction of the test piece when viewed from the tensile direction, and when the distance between the end on the groove side of the leading edge and the second side in the direction orthogonal to the leading edge of the crack when viewed from the tensile direction is defined as the assumed propagation amount, the assumed inclination angle of the leading edge with respect to the width direction is determined so that the assumed propagation amount is smaller than the reference propagation amount, A method for manufacturing a test piece, wherein the depth of the groove corresponding to the assumed inclination angle obtained from information indicating the relationship between the thickness of the test piece, the inclination angle of the leading edge of the crack with respect to the width direction of the test piece, and the depth of the groove is defined as the depth of the groove of the test piece described in claim 1.

5. A fatigue crack propagation test method using a test piece manufactured by the method for manufacturing a test piece according to claim 4.

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