Metal sheet bent portion fatigue crack growth suppressing method and automotive part

Abstract

A method for suppressing the progression of fatigue cracks in a bending portion of a metal plate related to the present invention is to suppress the progression of fatigue cracks in a bending portion (3) of a metal plate (1) that has been bent, by applying plastic deformation in a direction perpendicular to the curve direction along the bending portion (3) at least within the range from the bending point to the bending end point in the inner side of the bending portion (3) at a distance greater than the plate thickness of the metal plate (1), thereby generating compressive residual stress.

Description

Technology Field

[0001] The present invention relates to a method for suppressing fatigue crack growth in a bent portion of a metal sheet, which suppresses the growth of a fatigue crack in a bent portion of a metal sheet, and an automotive part having a bent portion of a metal sheet and suppressing the growth of a fatigue crack in the bent portion. Background Technology

[0002] It is known that in a bent portion formed by press forming of a metal plate, localized strong compressive stress is applied, causing cracks to form on the inner surface of the bend, and when a cyclic load is applied to a part having such a bent portion, the cracks propagate, thereby reducing the fatigue life. Therefore, several techniques have been proposed to improve the fatigue life of parts having a bent portion formed by bending a metal plate.

[0003] Patent Documents 1 and 2 disclose a technique for suppressing cracks on the inner side of a local bend that constitutes the source of a burring portion during coining of a metal plate by performing a bending process with a radius of curvature different from the final shape (including a radius of curvature of infinity, i.e., a straight line) in the first process, and performing a bending process with the radius of curvature of the final shape in the second process so as to introduce tensile stress into the inner side of the bend. In addition, Patent Document 3 proposes a technique for improving fatigue strength after shot peening by increasing the yield stress by performing a surface hardening treatment by shot peening on the surface of a metal plate subjected to bending stress. In addition, Patent Document 4 discloses a method for repairing a fatigue crack that occurs in a curved plate portion of steel, wherein at least one of the two sides of the fatigue crack on the surface of the steel is peened parallel to the fatigue crack to close the opening of the fatigue crack, and then additionally peened immediately above the fatigue crack. Prior art literature

[0004] Japanese Published Patent Application No. 2018-51608 Japanese Published Patent Application No. 2018-51609 Japanese Published Patent Application No. 2000-225567 Japanese Patent Application No. 4441641 The problem to be solved

[0005] The technology disclosed in Patent Document 1 had the problem of incurring manufacturing costs because it required preparing two types of molds since the bending process involved two steps. The technology disclosed in Patent Document 2 had the problem that the applicable points were limited to the burring area. The technology disclosed in Patent Document 3 used shot peening, so it was necessary to perform the processing in a sealed container to prevent scattering of the projection material. The technology disclosed in Patent Document 4 is a method of performing peening on a flat plate or a weld toe using an air-type hammer peening device, but when applied to a part having a bent portion formed by bending a metal plate, there were problems such as low productivity because the entire inner side of the bent portion, which is a risk area for cracking, is struck, the entire part is deformed by the impact load of the impact pin used for peening, and the bending angle of the bent portion expands if the tip radius of the impact pin is larger than the radius of curvature of the bent portion.

[0006] The present invention aims to solve the above-mentioned problems and to provide a method for suppressing the progression of fatigue cracks in a bent portion of a metal plate, which is applicable to the bent portion after bending without increasing the manufacturing cost of the metal plate bending process, does not reduce productivity, and suppresses the progression of fatigue cracks in the bent portion without changing the bending angle of the entire part or the bent portion, and an automobile part having a bent portion formed by bending a metal plate and suppressing the progression of fatigue cracks in the bent portion. means of solving the problem

[0007] A method for suppressing the progression of fatigue cracks in a bending portion of a metal plate related to the present invention suppresses the progression of fatigue cracks in a bending portion of a metal plate that has been bent, by applying plastic strain in a direction perpendicular to the curved direction of the bending portion at a distance greater than the thickness of the metal plate, and within a range from a bending start point to a bending end point within the inner side of the bending portion, thereby generating compressive residual stress.

[0008] In addition, the above compressive residual stress is generated by forming an indentation band, which is a series of indentations, on the inner side of the bend of the bending processed part by a needle peening treatment using a striking pin, and as the striking pin, one is used in which the radius of curvature in the cross-section perpendicular to the curve direction of the tip is less than or equal to the radius of curvature of the bending processed part.

[0009] Furthermore, the automobile part related to the present invention is an automobile part having a bending portion formed by bending a metal plate, and which suppresses the progression of fatigue cracks in the bending portion, and has a series of impact marks formed by needle peening treatment using an impact pin within a range from the bending point to the bending end point in a direction perpendicular to the curve direction, spaced apart by a thickness greater than or equal to the plate thickness of the metal plate along the curve direction of the bending portion. Effects of the invention

[0010] In the present invention, the progression of cracks occurring on the inner side of the bend of a bent portion formed by bending a metal plate can be suppressed in the curved direction, thereby effectively improving the fatigue life of an automobile part having said bent portion. Brief explanation of the drawing

[0011] FIG. 1 is a drawing illustrating a method for suppressing fatigue crack propagation in a bending portion of a metal plate related to Embodiment 1 of the present invention and an automobile part related to Embodiment 2 of the present invention ((a) a plan view facing the bending portion, (b) a cross-sectional view, (c) an explanatory diagram of crack fracture surfaces and compressive residual stress occurring inside the bending portion). FIG. 2 is a diagram illustrating the progression of fatigue cracks in a bending portion and a concept for suppressing said fatigue crack progression as the basis for the present invention ((a) crack progression in a conventional (general) metal plate, (b) crack progression in a bending portion). FIG. 3 is a diagram illustrating the stress distribution in a bending process section in the course leading to the present invention ((a) immediately after bending process, (b) after springback). FIG. 4 is a diagram showing the results of examining the stress intensity factor in the bending process section in the course leading to the present invention ((a) a schematic diagram of a crack fracture surface, (b) stress intensity factors in the depth direction and transverse direction at the crack fracture surface, (c) a diagram showing the correspondence between the transverse stress intensity factor K (0) at the crack tip and the crack fracture surface). FIG. 5 is a drawing showing a fatigue test specimen used in a fatigue test in an embodiment of the present invention ((a) front view, (b) cross-sectional view). FIG. 6 is a drawing illustrating a fatigue test method in an embodiment of the present invention. Specific details for implementing the invention

[0012] <Circumstances Leading to the Present Invention>

[0013] Fatigue cracks occurring on the surface of a metal plate generally progress as semi-elliptical crack fracture surfaces expand in both the transverse direction (direction along the surface of the metal plate (1)) and the depth direction (thickness direction of the metal plate (1)) from the point of occurrence of the crack (5), as shown in FIG. 2(a), and soon the crack fracture surfaces reach the back side of the metal plate (1) and lead to fatigue fracture.

[0014] Thus, the inventor observed in detail the progression of a fatigue crack occurring on the inner side of the bending process of the metal plate (1) that was bent, as shown in FIG. 2(b). As a result, it was found that the crack (5) occurring on the inner side of the bending process of the metal plate (1) progresses in both the depth direction (the thickness direction of the metal plate (1)) and the transverse direction (the curve direction on the inner side of the bend) immediately after it occurs, but the progression of the crack (5) in the thickness direction stops when it reaches a depth of about 1 / 4 of the thickness (t), as shown in FIG. 2(b), and thereafter, the crack (5) progresses only in the transverse direction.

[0015] In examining the cause of this, the inventor focused on the stress distribution in the bending portion (3) as shown in FIG. 3. Immediately after bending the metal plate (1) in the bending portion (3), as shown in FIG. 3(a), compressive stress acts on the area inside the bend relative to the neutral axis, and tensile stress acts on the area outside the bend relative to the neutral axis. However, after the bending moment of the bent portion (3) is unloaded and springed back, as shown in FIG. 3(b), the surface layer inside the bend is reversed to tensile stress, and compressive stress remains at a position approximately 1 / 4 of the plate thickness (t) from the surface inside the bend in the bending portion (3).

[0016] In this regard, it was found that the progression of fatigue cracks occurring in the inner side of the bending process (3) in the depth direction (plate thickness direction) stops at a depth of about 1 / 4 of the plate thickness from the inner side of the bending process surface, and that this is due to compressive residual stress remaining inside the bending process (3) even after springback.

[0017] Therefore, based on the above-mentioned finding that the progression of a crack in the plate thickness direction is stopped by the compressive residual stress remaining inside the bending processing part, the inventor conceived that it might be possible to prevent or suppress the progression of a crack (5) in the bending processing part (3) in the curved direction (transverse direction) by generating compressive residual stress by applying plastic deformation at predetermined intervals along the curved direction (transverse direction).

[0018] The present invention was made through various reviews based on the above concept, and specifically comprises the following configuration.

[0019] [Embodiment 1]

[0020] <Method to Suppress Fatigue Crack Propagation in the Bending Part of a Metal Plate>

[0021] A method for suppressing the progression of fatigue cracks in a bending portion of a metal plate related to Embodiment 1 of the present invention is to suppress the progression of fatigue cracks occurring in a bending portion (3) of a metal plate (1) that has been bent, as shown in FIG. 1, by applying plastic deformation in a direction perpendicular to the curve direction of the inner side of the bending portion (3) at least within the range from the bending starting point (3a) to the bending end point (3b) in the inner side of the bending portion (3) at a distance greater than or equal to the plate thickness (t) of the metal plate (1), and generating compressive residual stress.

[0022] In addition, in the present embodiment 1, compressive residual stress is generated by forming a series of impact marks (7) indentation zones (9) (see FIG. 1(a)) within the range from the bending start point (3a) to the bending end point (3b) in the inner side of the bending process part (3) by needle peening treatment using an impact pin (11) as shown in FIG. 1(b). At this time, the spacing (d) in the curved direction of the indentation zones (9) is greater than or equal to the plate thickness (t) of the metal plate (1). And, the radius of curvature (r) in the cross-section perpendicular to the curved direction of the tip portion (13) of the impact pin (11) is less than or equal to the radius of curvature (R) in the inner side of the bending process part (3).

[0023] The reason why the curved direction progression of a crack occurring on the inner side of the bending of the metal plate (3) is suppressed by the method for suppressing fatigue crack progression in the bending portion of the metal plate related to the present embodiment 1 is explained based on the results of an examination of the stress intensity factor at the tip of the crack occurring on the inner side of the bending of the metal plate (1) shown in FIG. 4.

[0024] FIG. 4(a) is a schematic diagram showing the shape of a crack surface. It is assumed that a crack occurring on the inner surface of the bending portion (3) of the metal plate (1) progresses as the crack surface expands in a semi-elliptical shape in both the depth direction and the transverse direction, as shown in FIG. 4(a).

[0025] Figure 4(b) is the result of calculating the stress intensity factor K at the crack tip advancing in the transverse direction (curve direction on the inner side of the bend) and the depth direction (plate thickness direction), respectively, using the Newman-Raju equation shown in the following equation (publicly known literature: JC Newman Jr. and IS Raju: Eng. Fract. Mech., Vol.15, No.1-2 (1981), p.185).

[0026]

[0027] In the above formula, σt is tensile stress, σb is bending stress, a is the depth-direction length of the crack fracture surface (plate thickness direction), c is the transverse-direction length of the crack fracture surface (distance from the point of crack occurrence to the transverse crack tip), b is the plate width of the metal plate (1), t is the plate thickness of the bending processing part (3) (metal plate (1)), φ is the angle formed by the straight line (L) connecting the point of crack occurrence and the crack tip and the surface of the bending processing part (3), and H (a, c, t, φ), Q (a, c), and F (a, c, b, t, φ) are functions assigned to each variable (see the above known literature).

[0028] In FIG. 4(b), K (0) is a stress intensity factor that progresses in the transverse direction (curved direction on the inner side of the bend), i.e., in the direction of φ = 0°, and K (90) is a stress intensity factor that progresses in the depth direction (plate thickness direction), i.e., in the direction of φ = 90°. In calculating K (0) and K (90), the plate thickness (t) (= 2.8 mm), plate width 50 mm, and bending load 100 MPa were set, and the crack depth (a) was set to 0.7 mm, which is 1 / 4 of the plate thickness (t) 2.8 mm. FIG. 4(b) shows the relationship between K (0) and K (90) and the crack fracture surface ratio c / a, which is an indicator representing the shape of a semi-elliptical crack fracture surface. In addition, in the above equation, the influence of compressive residual stress at a depth of about 1 / 4 of the plate thickness (t) from the inner surface of the bend is not considered.

[0029] From the graph shown in FIG. 4(b), it can be seen that the stress intensity factor K (0) in the transverse direction is approximately constant and high in the region where the fracture surface ratio c / a is |c / a| ≤ 2 (hereinafter referred to as "Region P"), that is, when the shape of the fracture surface is close to a semicircle, and the driving force for the propagation (advancement) of the crack in the transverse direction is large. On the other hand, in the region where the fracture surface ratio |c / a| > 2 (hereinafter referred to as "Region Q"), as the shape of the fracture surface expands in the transverse direction (increases the absolute value of c / a) and becomes a semi-ellipse, it can be seen that the stress intensity factor K (0) decreases and the driving force for the propagation of the crack in the transverse direction also decreases. In other words, the magnitude of the driving force for the propagation of the crack in the transverse direction is the fracture surface ratio |c / a| It changes with 2 as the boundary.

[0030] Here, based on the results of the above review, an idea to suppress the progression of cracks in the curved direction (transverse direction) occurring on the inner side of the bending process part (3) is explained. Assuming that the progression of the crack in the depth direction stops at a position of about 1 / 4 of the plate thickness (t), the depth direction length (a) of the crack fracture surface is expressed as a ≈ t / 4. Therefore, when the crack fracture surface ratio c / a = 2, the relationship 2c ≈ t holds. Also, assuming that the crack progresses symmetrically in the transverse direction, 2c is the transverse width of the crack fracture surface. Therefore, when the crack fracture surface ratio c / a = 2, it can be seen that the width of the transverse crack fracture surface (= 2c) is approximately the same size as the plate thickness (t) of the metal plate (1).

[0031] That is, when the purpose is to prevent or suppress fatigue failure caused by the progression of a crack in the curved direction (transverse direction) occurring on the inner side of the bending process part (3), the progression of the crack in the depth direction stops at a position 1 / 4 of the plate thickness (t), and the crack also progresses only in the transverse direction. During the time when the transverse width (= 2c) of the crack fracture surface is less than or equal to the plate thickness t, the stress intensity factor K (0) in the transverse direction is greater than the stress intensity factor K (90) in the depth direction, so the driving force for crack propagation in the transverse direction is considered to be large, and thus it is considered difficult to stop crack propagation in that area.

[0032] However, in the region (region Q) where the crack has grown to a certain extent and the transverse width (= 2c) of the crack fracture surface has expanded to be greater than the plate thickness t, the stress intensity factor K (0) is smaller than the stress intensity factor K (90) in the depth direction and gradually decreases with increasing absolute value of the crack fracture surface ratio c / a, so it is thought that the driving force for crack propagation is small in the region and it is easy to stop crack propagation.

[0033] Therefore, it is thought that rather than stopping the progression of the crack in region P where the stress intensity factor K (0) in the transverse direction is high compared to the depth direction, if the crack growth is allowed up to region Q where the stress intensity factor K (0) in the transverse direction is low, it is possible to stop or suppress the crack with a compressive residual stress lower than that in region P.

[0034] FIG. 4(c) shows a graph showing the relationship between the transverse stress intensity factor K (0) at the crack tip and the crack fracture surface ratio c / a, and a schematic diagram showing the crack fracture surface when compressive residual stress is applied by forming an indentation zone (9) in the crack region Q by needle peening treatment using a striking pin (11) as shown in FIG. 1. The example shown in FIG. 4(c) is one in which the spacing (d) of the indentation zone (9) shown in FIG. 1 is made larger than the plate thickness (t) (≈2c, the transverse width of the crack fracture surface when the crack fracture surface ratio c / a = 2), and the stress intensity factor K (0) at the location where compressive residual stress is applied by needle peening is 10 to 12% lower than the stress intensity factor K (0) in region P (-2 ≤ c / a ≤ 2).

[0035] In this way, by forming an indentation zone (9) with a spacing (d) greater than the plate thickness (t) along the curved direction of the bending processing part (3) and applying compressive residual stress, it becomes possible to suppress the progression of cracks in the curved direction.

[0036] In addition, in the present embodiment 1, the compressive residual stress of the bending processing part (3) is generated by forming a series of impact marks (7) and indentation zones (9) on the inner side of the bending processing part (3) by needle peening treatment using an impact pin as described above, thereby imparting plastic deformation, and it is preferable that the radius of curvature (r) in the cross-section perpendicular to the curve direction of the tip portion (13) of the impact pin (11) is less than or equal to the radius of curvature (R) of the bending processing part (3).

[0037] This is because if the radius of curvature (r) of the tip (13) of the striking pin (11) is greater than the radius of curvature (R) of the bending processing part (3), the bending processing part (3) cannot be struck directly by the tip (13) of the striking pin (11), and thus compressive residual stress cannot be properly generated in the bending processing part (3).

[0038] In addition, in a part of the inner side of the bending process of the bending process (3), buckling deformation occurs during the bending process, and the radius of curvature becomes very small locally. The radius of curvature (R) of the bending process (3) can be the macroscopic radius of curvature, except for cases where the radius of curvature of the bending process (3) changes globally due to buckling.

[0039] In addition, regarding the problem in the technology described in the aforementioned patent document 4, that the bending portion (3) or its surroundings in the metal plate (1) are deformed by the impact load of the impact pin, and that the bending angle of the bending portion (3) is expanded when the radius of curvature of the impact pin (11) is larger than the radius of curvature of the bending portion (3), in the present embodiment 1, by striking with an impact pin (11) having a radius of curvature (r) smaller than the radius of curvature of the inner side of the bending portion (3), the deformation of the metal plate (1) and the change in the bending angle of the bending portion (3) can be suppressed, and at the same time, compressive residual stress can be appropriately introduced into the bending portion (3).

[0040] Additionally, regarding the range in which the indentation zone (9) is formed in a direction perpendicular to the curve direction on the inner side of the bending processing part (3), it may be either all or part of the range from the bending point on the inner side of the bending to the bending end point. Here, the bending point and the bending end point refer to the bending R contact point on the inner side of the bending processing part (3).

[0041] At this time, if the location of the crack is clear due to cracks, wrinkles, or stress conditions applied to the metal plate in the width direction of the bending processing part (3), it is not necessary to process the entire range of the bending processing part (3) along the curve direction inside the bend, and the compressive residual stress is generated by forming an indentation zone (9) only in the periphery of the crack location and applying plastic deformation.

[0042] The upper limit of the spacing (d) of the indentation zone (9) in the curved direction on the inner side of the bending part (3) is not specifically defined, but can be determined based on the allowable crack length, such as the life required for the part made by bending the metal plate, the crack length allowed as a part, or the lower limit of the crack length that can be detected by periodic inspection.

[0043] In addition, the present invention may be performed by simultaneously performing needle peening with a plurality of impact pins, and further improvement in productivity can be expected when combined with an automatic construction method using a robot arm, etc. Furthermore, in the case of parts having many bending parts, such as automotive parts, applying this treatment to the entire bending part reduces productivity, so it may be performed only at points where the radius of curvature of the bending part is small, or at places where fatigue failure inside the bend is a concern based on fatigue testing or stress analysis in advance.

[0044] In addition, since the present invention only requires generating compressive residual stress in the bending portion to suppress the propagation of cracks in the curved direction of the bending portion, the method of generating compressive residual stress is not limited to needle peening, and the same treatment may be performed by methods such as laser peening, shot peening, and cold spray. In the case of shot peening, an equivalent effect can be obtained by masking other than the location where the indentation is to be introduced to prevent unnecessary plastic deformation. These methods generate compressive residual stress by imparting plastic deformation by applying impact to the material, and laser peening imparts plastic deformation by impacting the bending portion through a shock wave during laser irradiation, while shot peening and cold spray impart plastic deformation by projecting a projectile material.

[0045] In addition, when the bending process of the metal plate is not pure bending but is carried out with axial tension and compression, the location in the plate thickness direction where compressive residual stress is introduced shifts, and in that case, the crack stops at the location where compressive residual stress exists in the same way. Therefore, if the location of the compressive residual stress is not significantly different from 1 / 4 of the plate thickness (t), construction can be carried out under the condition that the relationship between the depth of the compressive residual stress (a) and the transverse length (c) of the crack surface satisfies 2c > 4a.

[0046] If the depth-direction position where compressive residual stress is introduced by springback deviates significantly from 1 / 4 of the plate thickness, the above method is recalculated, and the interval (d) for performing needle peening treatment is determined based on the value of c / a where K (0) decreases.

[0047] In summary, the present invention can effectively improve the fatigue life of a part having a bent portion formed by bending a metal plate. Furthermore, according to the present invention, since peening treatment is not required for the entire inner surface of the bend in the bent portion, an improvement in productivity can be expected, and an effect of improving fatigue life due to compressive residual stress or work hardening caused by localized plastic deformation on the inner side of the bend can also be expected.

[0048] In addition, according to the present invention, even when no cracks occur on the inner side of the bending portion of the original bending portion, the effect of reducing microscopic stress concentration can be expected in order to smooth out microscopic irregularities such as wrinkles. Furthermore, by using a striking pin having a tip portion with a small radius of curvature, the striking load required to deform the metal plate and form an indentation can be reduced. As a result, stable construction is possible by suppressing the reaction force, and not only is macroscopic deformation of the entire part, which is a concern when applying the method described in Patent Document 4 to a part having a bending portion of a metal plate, suppressed, but it is also possible to apply it to a production line using a robot arm, etc.

[0049] In addition, in the present invention, the possibility that the impact pin does not come into contact with the inner surface of the bending part and thus the impact is not sufficiently achieved can be eliminated. Furthermore, when applying the present invention to the manufacture of automobile parts, an additional press mold for performing bending processing as described in Patent Document 1 is not required, and it can be applied to a burring part as described in Patent Document 2, and furthermore, a scatter-prevention container required for shot peening treatment as described in Patent Document 3 is unnecessary.

[0050] [Embodiment 2]

[0051] <Automotive Parts>

[0052] The automobile part related to Embodiment 2 of the present invention has a bending portion (3) formed by bending a metal plate (1) as shown in FIG. 1, and the progression of fatigue cracks in the bending portion is suppressed. In this case, along the curved direction of the bending portion (3), at intervals greater than the thickness of the metal plate, and in a direction perpendicular to the curved direction, there are indentation zones which are a series of impact marks formed by needle peening treatment using an impact pin within the range from the bending point to the bending end point in the inner side of the bending portion (3).

[0053] In the automotive part related to the present embodiment 2, the fatigue life is improved because the progression of the crack in the curved direction is suppressed by the compressive residual stress generated by the indentation zone created by needle peening treatment at a distance greater than the thickness of the metal plate along the curved direction of the inner side of the bending portion, in the same way as the method for suppressing fatigue crack progression in the bending portion of the metal plate related to the above-described embodiment 1 of the present invention.

[0054] In addition, the plastic deformation applied to the bending portion of the automobile part related to the present embodiment 2 is by needle peening treatment, but the plastic deformation applied to the bending portion of the automobile part related to the present invention is not limited to needle peening treatment and may be by laser peening, shot peening, cold spraying, etc.

[0055] Examples

[0056] Experiments were conducted to verify the effects of the present invention, and these are described below.

[0057] In this embodiment, a fatigue test was performed on the fatigue test specimen (21) shown in FIG. 5 and the fatigue strength was evaluated.

[0058] A hot-rolled steel sheet with a plate thickness of 2.9 mm and a yield strength of 850 MPa is cut into 200 mm × 50 mm pieces to serve as a test specimen, and a bending process is performed using a press brake (bending machine) equipped with a bending tool (punch) having a radius of curvature of 3 mm at the tip, thereby forming a bending processed section (23) with an inner bending radius (R) = 2 mm and a plate thickness (t) = 2.8 mm, and flat sections (25a, 25b) extending from both ends of the bending processed section (23), and a hole section (27a, 27b) with a diameter of φ8.5 mm formed in each of the flat sections (25a, 25b). Manufactured.

[0059] Next, as shown in FIG. 1(a) and FIG. 1(b), needle peening treatment was performed on the bending portion (23) of the fatigue test specimen (21) to form indentation zones (29) at predetermined intervals (d) along the curved direction inside the bend. Here, the radius of curvature (r) of the tip of the impact pin (11) used for needle peening was set to r = 0.2 mm, which is smaller than the radius of curvature (R) = 2 mm in the cross-section of the curved direction of the bending portion (23).

[0060] Next, a fatigue test was performed on a fatigue test specimen (21) that had needle peening treatment applied to the bending portion (23) under the boundary conditions shown in FIG. 6. As shown in FIG. 6, the fatigue test was performed by fixing the hole portion (27a) (Fig. 6) of one portion (25a) of the fatigue test specimen (21) as a fixed point, and applying a repetitive load (load 2.3 kN, repetition frequency 10 Hz) with the hole portion (27b) of the other portion (25b) as a load input point.

[0061] Then, a penetrant inspection test was performed on the bending part (23) every 10,000 cycles of load input to measure the progression of cracks occurring in the bending part (23). Then, for fatigue test specimens in which the progressed crack passed through two or more points of the indentation zone (29), it was determined to be a failure because the crack had progressed, and the fatigue test was terminated. Also, the fatigue limit was set at 500,000 cycles of repeated load, and for fatigue test specimens (21) that reached the fatigue limit, it was determined to be a pass and the fatigue test was terminated.

[0062] In this embodiment, a fatigue test was performed on a fatigue test specimen (21) with a change in the spacing (d) of the indentation band (29), and fatigue test specimens (21) with spacing (d) of 3 mm, 4 mm, and 5 mm, which are greater than the plate thickness of the metal plate (1) within the range of the present invention, were designated as Invention Example 1, Invention Example 2, and Invention Example 3. Additionally, as a comparison subject, a specimen with a spacing (d) of the indentation band (29) of d = 2 mm, which is outside the range of the present invention, was designated as Comparative Example 1. Table 1 shows the spacing (d) of the indentation band (29) formed on the fatigue test specimen (21) and the fatigue test results.

[0063]

[0064] Comparative Example 1 was deemed unsuccessful because, at 200,000 cycles, crack propagation passed through two or more points of the indentation zone (29). In contrast, Invention Examples 1 to 3 were all deemed acceptable because, even at 500,000 cycles, no crack propagation passing through two or more points of the indentation zone (29) was observed, and they reached the fatigue limit.

[0065] As described above, according to the present invention, it is possible to improve fatigue life by suppressing the progression of fatigue cracks on the inner side of the bend of a bent portion of a metal plate that has been bent.

[0066] Industrial applicability

[0067] According to the present invention, a method for suppressing the progression of fatigue cracks in a bent portion of a metal plate can be applied to a bent portion after bending without increasing the manufacturing cost of the bending process of the metal plate, without reducing productivity, and without changing the bending angle of the entire part or the bent portion, and an automobile part having a bent portion formed by bending a metal plate and suppressing the progression of fatigue cracks in the bent portion. Explanation of the symbols

[0068] 1 : Metal plate 3 : Bending part 3a: Bending point 3b : Bending endpoint 5 : Crack 7: Impact marks 9: Indentation band 11: Striking pin 13: Tip 21: Fatigue test specimen 23 : Bending process part 25a, 25b: Single-sided 27a, 27b: Hole part 29 : Indentation band

Claims

Claim 1 A method for suppressing the progression of fatigue cracks in a bent portion of a metal plate, wherein the method comprises: a spacing greater than or equal to the thickness of the metal plate along the curved direction of the bent portion; and a plastic deformation applied in a direction perpendicular to the curved direction, within a range from the bending point to the bending end point within the inner side of the bend of the bent portion, thereby generating compressive residual stress. Claim 2 A method for suppressing fatigue crack propagation in a bending portion of a metal plate, wherein the compressive residual stress is generated by forming a series of impact marks, which are indentation zones, on the inner side of the bend of the bending portion by needle peening treatment using an impact pin, and wherein the impact pin used is one in which the radius of curvature in the cross-section perpendicular to the curve direction of the tip portion is less than or equal to the radius of curvature of the bending portion. Claim 3 An automobile part having a bending portion formed by bending a metal plate, and suppressing the progression of fatigue cracks in the bending portion, wherein the bending portion has a series of impact marks formed by needle peening treatment using an impact pin, spaced apart by a thickness greater than that of the metal plate along the curve direction of the bending portion and in a direction perpendicular to the curve direction, within a range from the bending point to the bending end point within the inner side of the bending portion, and the impact pin is used such that the radius of curvature in the cross-section perpendicular to the curve direction of the tip portion is less than or equal to the radius of curvature of the bending portion.

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