Method for suppressing fatigue crack propagation of a bent processed portion of a metal sheet and automotive component

Abstract

The method for suppressing the progress of fatigue cracks in a bent portion of a metal sheet according to the present invention suppresses the progress of fatigue cracks in a bent portion 3 formed by bending a metal sheet 1, and generates a compressive residual stress by imparting plastic strain in a range from a bending start point to a bending end point on the inside of the bend of the bent portion 3 in a direction orthogonal to a valley line direction of the bent portion 3 at intervals of the thickness of the metal sheet 1 or more along the valley line direction.

Description

Technical Field

[0001] The present invention relates to a method for suppressing fatigue crack growth in a bent portion of a metal sheet, and to an automotive part having a bent portion formed by bending a metal sheet and suppressing the growth of fatigue cracks at that bent portion. Background Technology

[0002] It is known that in a bent section formed by press forming of a sheet metal, a strong compressive stress is locally applied, causing cracks to form on the inner surface of the bend. When a cyclic load is applied to a component having such a bent section, these cracks progress, reducing fatigue life. Therefore, several techniques for improving the fatigue life of components having bent sections formed by press forming of a sheet metal have been proposed previously.

[0003] Patent Documents 1 and 2 disclose the following techniques: To suppress cracking on the inner side of a localized bend forming the base of a burring portion during coining of a metal sheet, tensile stress is introduced into the inner side of the bend by performing a bending process with a radius of curvature different from the final shape (including an infinite radius of curvature, i.e., a straight line) in the first step, and then performing a bending process with a radius of curvature that corresponds to the final shape in the second step, thereby suppressing cracking on the inner side of the bend. Furthermore, Patent Document 3 proposes the following technique: By performing a surface hardening treatment using shot peening on the surface of a metal sheet subjected to bending stress, the yield stress and fatigue strength after shot peening are increased. Furthermore, Patent Document 4 discloses the following technique: as a method for repairing fatigue cracks generated in the curved portion of steel, the opening of the fatigue crack is closed by peening at least one side of the two sides of the steel surface that are sandwiched with the fatigue crack parallel to the fatigue crack, and then the area directly above the fatigue crack is further peened.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2018-51608

[0007] Patent Document 2: Japanese Patent Application Publication No. 2018-51609

[0008] Patent Document 3: Japanese Patent Application Publication No. 2000-225567

[0009] Patent Document 4: Japanese Patent No. 4441641 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] The technology disclosed in Patent Document 1 has the following problems: since the bending process is divided into two steps, two types of molds are required, resulting in high manufacturing costs. The technology disclosed in Patent Document 2 has the following problems: its application is limited to the flanged portion. The technology disclosed in Patent Document 3, due to the use of shot peening, requires processing in a sealed container to prevent the scattering of projection material. The technology disclosed in Patent Document 4 has the following problems: although it is a method of hammering a flat plate or weld toe using an air-type hammer peening device, when applied to a component having a bending section formed by bending a metal sheet, it strikes the entire inner bending side of the bending section, a dangerous area for cracking, resulting in low productivity; the component deforms as a whole due to the impact load of the impact pin used in the hammering; furthermore, when the radius of the impact pin's tip is greater than the radius of curvature of the bending section, the bending angle of the bending section increases.

[0012] The present invention was made to solve the above-mentioned problems, and its object is to provide a method for suppressing the fatigue cracking progression of a bent metal sheet without increasing the manufacturing cost of bending the sheet metal, applicable to the bent section after bending, without reducing productivity, without changing the bending angle of the whole component or the bent section, and suppressing the progression of fatigue cracking in the bent section, as well as an automotive component having a bent section formed by bending a sheet metal and suppressing the progression of fatigue cracking at the bent section.

[0013] Methods for solving problems

[0014] The fatigue cracking progression suppression method for a bent portion of a metal sheet of the present invention suppresses the progression of fatigue cracking in a bent portion formed by bending a metal sheet by applying plastic strain along the valleyline direction of the bent portion, at least within a range from the bending start point to the bending end point, in a direction orthogonal to the valleyline direction.

[0015] In addition, the compressive residual stress is generated by forming an indentation band as a series of impact marks on the inner side of the bending part by using a needle peening treatment of the impact pin. The impact pin is a component whose radius of curvature in the cross section orthogonal to the valley line direction at its front end is less than the radius of curvature of the bending part.

[0016] Furthermore, the automotive component of the present invention is an automotive component having a bending section formed by bending a metal sheet and suppressing the progression of fatigue cracking at the bending section. The bending section has an indentation band at least within the bending inner side of the bending section from the bending start point to the bending end point in a direction orthogonal to the valley line direction, spaced apart by a gap of more than the thickness of the metal sheet. The indentation band is a series of impact marks formed by hammering with a needle-shaped part using a hammering pin.

[0017] The effects of the invention

[0018] In this invention, the propagation of cracks in the valley direction generated on the inner side of the bending section formed by bending a metal sheet can be suppressed, thereby effectively improving the fatigue life of automotive parts having the bending section. Attached Figure Description

[0019] Figure 1 The figures illustrate the fatigue cracking progression suppression method of the bending section of the metal sheet according to Embodiment 1 of the present invention and the automobile part according to Embodiment 2 of the present invention ((a) is a top view from the direction facing the bending section, (b) is a cross-sectional view, and (c) is an explanatory diagram of the cracked cross section and compressive residual stress generated inside the bending section).

[0020] Figure 2The figures (a) illustrate the progression of fatigue cracking at a bending section and the concept for suppressing the progression of fatigue cracking, as a means of obtaining the present invention. (b) shows the cracking progression at a bending section in a typical (general) metal sheet.

[0021] Figure 3 The diagram illustrates the stress distribution at the bending section during the process of obtaining the present invention ((a) is immediately after bending, and (b) is after springback).

[0022] Figure 4 The figure shows the results obtained in the process of studying the stress intensity factor at the bending section during the process of obtaining the present invention ((a) is a schematic diagram of the cracked section, (b) is the stress intensity factor in the depth direction and transverse direction at the cracked section, and (c) is a figure showing the correspondence between the transverse stress intensity factor K(0) at the crack tip and the cracked section).

[0023] Figure 5 The figure shows a fatigue test piece used in a fatigue test in an embodiment of the invention ((a) is a front view, (b) is a cross-sectional view).

[0024] Figure 6 This is a diagram illustrating a fatigue testing method in an embodiment of the present invention. Detailed Implementation

[0025] <How the Invention Was Obtained>

[0026] Fatigue cracks that occur on the surface of a metal plate are generally like... Figure 2 As shown in (a), the semi-elliptical cracked cross section expands and progresses from the location where the crack 5 originates in both the transverse direction (along the surface of the metal plate 1) and the depth direction (the thickness direction of the metal plate 1). Later, the cracked cross section reaches the reverse side of the metal plate 1 and reaches fatigue fracture.

[0027] Therefore, as Figure 2 As shown in (b), the inventors observed in detail the progression of fatigue cracks generated on the inner side of the bending section 3 formed by bending the metal sheet 1. As a result, the following was determined: the cracks 5 generated on the inner side of the bending section 3 initially progressed both in the depth direction (thickness direction of the metal sheet 1) and laterally (valve direction on the inner side of the bend) of the bending section 3, but... Figure 2 As shown in (b), the propagation of crack 5 in the thickness direction stops when it reaches a depth of about 1 / 4 of the thickness t. After that, crack 5 only propagates in the transverse direction.

[0028] In investigating the reasons, the inventors focused on Figure 3 The stress distribution at the bending section 3 is shown. After bending the metal plate 1, the bending section 3, as shown... Figure 3 As shown in (a), the region closer to the neutral axis on the inside of the bend is under compressive stress, while the region closer to the neutral axis on the outside of the bend is under tensile stress. However, after the bending moment of the bent section 3 is unloaded and springs back, as... Figure 3 As shown in (b), the surface layer on the inside of the bend is reversed to tensile stress, while compressive stress remains at a position approximately 1 / 4 of the plate thickness t from the surface on the inside of the bend at the bending processing section 3.

[0029] Therefore, it has been determined that the reason why the fatigue cracks generated on the inner side of the bending section 3 stop at a depth of about 1 / 4 of the plate thickness from the inner surface of the bending section is the residual compressive stress that remains inside the bending section 3 after springback.

[0030] Therefore, based on the above-mentioned insight that the progression of cracks in the thickness direction of the plate is stopped due to the compressive residual stress remaining inside the bending section, the inventor conceived of whether it is possible to prevent or suppress the cracking progression of the crack 5 at the bending section 3 in the valley direction (lateral direction) by generating compressive residual stress by applying plastic strain at predetermined intervals along the valley direction (lateral direction).

[0031] This invention is based on the above concept and has been developed through various studies. Specifically, it has the following structure.

[0032] [Implementation Method 1]

[0033] <Methods for suppressing fatigue cracking progression in bent sections of metal sheets>

[0034] The fatigue cracking progression suppression method for the bending section of a metal sheet according to Embodiment 1 of the present invention suppresses the progression of fatigue cracks generated at the bending section 3 formed by bending the metal sheet 1, such as... Figure 1 As shown, plastic strain is applied along the valley line direction on the inner side of the bend at the bending processing section 3, at least within the range from the bending start point 3a to the bending end point 3b, in a direction orthogonal to the valley line direction, at least on the inner side of the bend of the bending processing section 3, thereby generating compressive residual stress.

[0035] Furthermore, in this embodiment 1, as Figure 1As shown in (b), by using the needle-shaped hammering process of the striking pin 11, an indentation band 9 as a series of striking marks 7 is formed on the inner side of the bending section 3 from the bending start point 3a to the bending end point 3b (see reference). Figure 1 (a)), thereby generating compressive residual stress. At this time, the interval d in the valley direction of the indentation band 9 is greater than or equal to the plate thickness t of the metal plate 1. Moreover, the radius of curvature r in the cross section orthogonal to the valley direction of the front end 13 of the striking pin 11 is less than or equal to the radius of curvature R of the inner side of the bending section 3.

[0036] based on Figure 4 The results of the study on the stress propagation coefficient at the tip of the crack generated on the inside of the bending section 3 of the metal plate 1, as shown, illustrate the reason for suppressing the propagation of the crack in the valley direction of the crack generated on the inside of the bending section 3 using the fatigue crack propagation suppression method of the bending section of the metal plate of Embodiment 1.

[0037] Figure 4 (a) is a schematic diagram showing the shape of the crack surface. For example... Figure 4 As shown in (a), the cracks that are generated on the inner side of the bending process section 3 of the metal plate 1 are assumed to be semi-elliptical in both the depth direction and the transverse direction, and the cracks expand and progress.

[0038] Figure 4 (b) is the result obtained by using the Newman-Raju formula shown below (public literature: JC Newman Jr. and IS Raju: Eng. Fract. Mech., Vol. 15, No.1-2(1981), p.185) to calculate the stress amplification factor K at the crack tip in each direction of progression in the transverse (valley direction on the inside of the bend) and depth (thickness direction).

[0039] [Mathematical Expression 1]

[0040]

[0041] In the above formula, σ t It is tensile stress, σ b This refers to bending stress, where 'a' is the depth length of the crack (in the thickness direction), 'c' is the transverse length of the crack (the distance from the crack initiation point to the transverse crack tip), 'b' is the width of metal plate 1, and 't' is the thickness of the bending section 3 (metal plate 1). The angle H (a,c,t) is formed by the straight line L connecting the location of the crack and the tip of the crack with the surface of the bending part 3. Q(a,c) and F(a,c,b,t) ) is a function provided by each variable (refer to the above-mentioned public literature).

[0042] exist Figure 4 In (b), K(0) is in the transverse direction (the direction of the valley line on the inner side of the curve), i.e. The stress amplification factor, K(90), which propagates in the direction of 0°, is in the depth direction (thickness direction). The stress amplification factor propagating in the 90° direction is assumed in the calculation of K(0) and K(90) to be a plate thickness t (=2.8mm), a plate width of 50mm, a bending load of 100MPa, and a crack depth a to be 1 / 4 of the plate thickness t (2.8mm), i.e., 0.7mm. Furthermore, Figure 4 (b) shows the relationship between the crack section ratio c / a and K(0) and K(90), where the crack section ratio c / a is an indicator of the shape of the semi-elliptical crack section. It should be noted that the effect of the 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 in the above formula.

[0043] from Figure 4 As shown in the graph (b), in the region where the crack cross-sectional area ratio c / a is |c / a|≤2 (hereinafter referred to as "Region P"), that is, when the shape of the crack cross-section is close to a semi-circle, the lateral stress amplification factor K(0) is approximately constant and relatively high, and the driving force for the lateral propagation (progression) of the crack is relatively large. On the other hand, it can be seen that in the region where the crack cross-sectional area ratio |c / a|>2 (hereinafter referred to as "Region Q"), as the shape of the crack cross-section becomes a semi-ellipse that expands laterally (the absolute value of c / a increases), the stress amplification factor K(0) decreases, and the driving force for the lateral propagation of the crack also decreases. That is, the magnitude of the driving force for the lateral propagation of the crack varies with the crack cross-sectional area ratio |c / a|=2 as the boundary.

[0044] Based on the above research results, this paper explains the idea of ​​suppressing the propagation of cracks generated on the inner side of the bending section 3 in the valley direction (lateral direction). When it is assumed that the crack propagation in the depth direction stops at approximately 1 / 4 of the plate thickness t, the length 'a' of the crack cross-section in the depth direction is expressed as a≈t / 4. Therefore, when the crack cross-section ratio c / a=2, the relationship 2c≈t holds. Furthermore, when it is assumed that the crack propagates symmetrically in the lateral direction, 2c is the lateral width of the crack cross-section. Therefore, it can be concluded that when the crack cross-section ratio c / a=2, the lateral width of the crack cross-section (=2c) is approximately the same as the plate thickness t of the metal plate 1.

[0045] That is, when the purpose is to prevent or suppress fatigue failure caused by the progress of cracks in the valley line direction (transverse direction) generated on the inner side of the bending section 3, since the progress of cracks in the depth direction stops at 1 / 4 of the plate thickness t, and the cracks further progress only in the transverse direction, during the period when the width of the crack cross section in the transverse direction (=2c) is less than or equal to the plate thickness t, the stress expansion coefficient K(0) in the transverse direction is greater than the stress expansion coefficient K(90) in the depth direction, and the driving force for crack propagation in the transverse direction is larger, it can be considered that it is difficult to stop crack propagation in this region.

[0046] However, in the region (region Q) where the crack grows to a certain extent and extends to a width (=2c) in the transverse direction of the crack cross section that is greater than the plate thickness t, the stress expansion factor K(0) is less than the stress expansion factor K(90) in the depth direction. In addition, it gradually decreases as the absolute value of the crack cross section ratio c / a increases. Therefore, it can be considered that the driving force for crack propagation in this region is also small, making it easy to stop crack propagation.

[0047] Therefore, it can be considered that, compared with stopping the propagation of cracks in region P where the stress expansion factor K(0) is higher in the transverse direction compared to the depth direction, if the growth of cracks is allowed up to region Q where the stress expansion factor K(0) is lower in the transverse direction, then the compressive residual stress can be stopped or suppressed at a lower level than that in region P.

[0048] exist Figure 4 (c) shows a graph representing the relationship between the transverse stress expansion factor K(0) at the crack tip and the crack section ratio c / a, schematically illustrating the relationship as follows: Figure 1 The diagram shows a cracked cross section where compressive residual stress is imparted by forming an indentation band 9 in the cracked region Q through hammering treatment using a needle-shaped part with a striking pin 11. Figure 4 In the example shown in (c), make Figure 1 The spacing d of the indentation band 9 shown is greater than the plate thickness t (≈ the width 2c of the crack cross section in the transverse direction when the crack cross section is more than c / a=2). The stress amplification factor K(0) at the location where the compressive residual stress is imparted by the hammering of the needle is 10 to 12% lower than the stress amplification factor K(0) in region P (-2≤c / a≤2).

[0049] In this way, by forming an indentation band 9 along the valley line direction of the bending processing section 3 at intervals d above the plate thickness t, compressive residual stress is imparted, thereby suppressing the progression of cracks in the valley line direction.

[0050] It should be noted that, in this embodiment 1, as described above, by using the hammering process of the needle-shaped part of the striking pin, an indentation band 9 as a series of striking marks 7 is formed on the inner side of the bending section 3 to impart plastic strain, thereby generating compressive residual stress in the bending section 3. Preferably, the radius of curvature r in the cross section orthogonal to the valley line direction of the front end 13 of the striking pin 11 is less than or equal to the radius of curvature R of the bending section 3.

[0051] This is because when the radius of curvature r of the front end 13 of the striking pin 11 is greater than the radius of curvature R of the bending part 3, the front end 13 of the striking pin 11 cannot directly strike the bending part 3, and the compressive residual stress cannot be properly generated in the bending part 3.

[0052] It should be noted that in a part of the inner side of the bending section 3, buckling deformation sometimes occurs during bending, and the radius of curvature locally becomes extremely small. Except for the case where the radius of curvature of the bending section 3 changes over a large area due to buckling, the radius of curvature R of the bending section 3 can be set to a macroscopic radius of curvature.

[0053] Furthermore, regarding the issues in the technology described in the aforementioned Patent Document 4, such as the deformation of the bending section 3 in the metal plate 1 due to the impact load of the striking pin, and the expansion of the bending angle of the bending section 3 when the radius of curvature of the striking pin 11 is greater than the radius of curvature of the bending section 3, in this embodiment 1, by striking with the striking pin 11 having a radius of curvature r smaller than the radius of curvature of the inner bending side of the bending section 3, it is possible to suppress the deformation of the metal plate 1, the change in the bending angle of the bending section 3, and at the same time, appropriately introduce compressive residual stress into the bending section 3.

[0054] Furthermore, the range of the indentation band 9 formed on the inner side of the bending section 3 in a direction orthogonal to the valley line direction can be any part or all of the range from the bending start point to the bending end point on the inner side of the bending section. Here, the bending start point and the bending end point refer to the bending R termination point on the inner side of the bending section 3.

[0055] At this time, if the location of cracks is obvious in the width direction of the bending section 3 due to cracks, wrinkles, or stress conditions on the metal plate, it is not necessary to process along the valley line direction on the inside of the bend throughout the entire range of the bending section 3. Plastic strain is applied by forming an indentation band 9 only at the periphery of the location where the cracks occur, thereby generating compressive residual stress.

[0056] The upper limit of the interval d of the indentation band 9 in the valley direction on the inner side of the bending section 3 is not specifically specified. It can be determined based on the required lifespan of the component made from the bent metal sheet, the allowable crack length as a component, the lower limit of the crack length that can be found in periodic inspections, and the allowable crack length.

[0057] It should be noted that the present invention can simultaneously perform hammering of needle-shaped parts using multiple striking pins. If combined with automated construction methods such as those utilizing robotic arms, productivity can be further improved. Furthermore, in the case of parts with numerous bending sections, such as automotive components, applying this treatment to the entire area of ​​the bending section would reduce productivity. Therefore, it can be limited to locations with smaller radii of curvature in the bending section, or to locations where fatigue failure on the inner side of the bend is anticipated through prior fatigue testing or stress analysis.

[0058] Furthermore, since this invention generates compressive residual stress in the bending section that can suppress the propagation of cracks towards the valley line of the bending section, the method for generating compressive residual stress is not limited to needle-shaped hammering; the same treatment can be performed using methods such as laser peening, shot peening, and cold spraying. In the case of shot peening, by shielding the area outside the desired indentation location, unwanted plastic deformation is not generated, thus achieving the same effect. These methods generate compressive residual stress by applying impact to the material to impart plastic strain. Laser peening utilizes the shock wave from laser irradiation, while shot peening and cold spraying provide impact to the bending section by projecting material, thereby imparting plastic strain.

[0059] It should be noted that when the bending process of a metal sheet is not purely bending, but involves axial tension and compression, the position in the thickness direction where compressive residual stress is introduced will shift. However, in this case, cracking will still stop at the location where compressive residual stress exists. Therefore, if the location of compressive residual stress is not significantly different from 1 / 4 of the sheet thickness t, construction can proceed under the condition that the relationship between the depth 'a' of the compressive residual stress and the transverse length 'c' of the crack cross-section satisfies 2c > 4a.

[0060] If the location of the compressive residual stress introduced due to springback deviates significantly from 1 / 4 of the plate thickness, the above method is used to recalculate, and the interval d for implementing the needle-shaped hammering treatment is determined based on the value of c / a reduced by K(0).

[0061] In summary, the present invention enables an effective improvement in the fatigue life of a component having a bent section formed from a bent sheet metal. Furthermore, according to the present invention, since hammering of the entire inner surface of the bent section is unnecessary, increased productivity is also expected, as is the improvement in fatigue life resulting from compressive residual stress and work hardening caused by locally providing plastic strain on the inner surface of the bend.

[0062] Furthermore, according to the present invention, even if no cracks originally occurred on the inner side of the bending section, the smoothing of microscopic irregularities such as wrinkles can reduce microscopic stress concentration. Moreover, by using a striking pin with a small radius of curvature at the front end, the impact load required to deform the metal sheet and form an indentation band can be reduced. Thus, by suppressing reaction forces, stable construction can be achieved, suppressing not only the macroscopic deformation of the entire component that is a concern when applying the method described in Patent Document 4 to a component with a bending section of a metal sheet, but also enabling its application in production lines using robotic arms or the like.

[0063] Furthermore, in this invention, the possibility that the striking pin will not contact the inner surface of the bent part and thus fail to strike sufficiently can be eliminated. Moreover, when this invention is applied to the manufacture of automotive parts, it can be applied to the flanged part of the method described in Patent Document 2 without the need for an additional stamping die for bending as described in Patent Document 1, and it is also unnecessary to use a scattering prevention container required for shot peening as described in Patent Document 3.

[0064] [Implementation Method 2]

[0065] <Automotive Components>

[0066] like Figure 1 As shown, the automotive component of Embodiment 2 of the present invention has a bending section 3 formed by bending a metal sheet 1, and the component suppresses the progression of fatigue cracking at the bending section. The bending section 3 has an indentation band at least within the bending inner side of the bending section 3, from the bending start point to the bending end point, in a direction orthogonal to the valley line direction, at intervals greater than the thickness of the metal sheet. The indentation band is a series of impact marks formed by hammering with a needle-shaped part using a hammering pin.

[0067] In the automotive component of this embodiment 2, similar to the fatigue cracking progression suppression method for the bending section of the metal sheet in embodiment 1 of the present invention, compressive residual stress is generated by using a needle-shaped hammering process to create an indentation band that is separated by a gap of more than the sheet thickness along the valley line direction of the inner side of the bending section. Since the progression of cracks in the valley line direction is suppressed by such compressive residual stress, the fatigue life is improved.

[0068] Furthermore, in the automotive component of this embodiment 2, the plastic strain applied to the bending processing section is generated by needle-shaped hammering. However, the plastic strain applied to the bending processing section of the automotive component of the present invention is not limited to being generated by needle-shaped hammering, but can also be generated by laser hammering, shot peening, cold spraying, or other processes.

[0069] Example

[0070] Since experiments were conducted to confirm the effectiveness of the present invention, the following description will be provided.

[0071] In this embodiment, Figure 5 The fatigue test piece 21 shown was used as the test object for fatigue testing, and the fatigue strength was evaluated.

[0072] The test material was a 200mm x 50mm piece cut from a hot-rolled steel sheet with a thickness of 2.9mm and a yield strength of 850MPa. A press brake was used to bend the sheet to produce a fatigue test piece 21. The fatigue test piece 21 had a bending section 23 with an inner bending radius R = 2mm and a thickness t = 2.8mm, and two sheet sections 25a and 25b extending from both ends of the bending section 23. Each of the sheet sections 25a and 25b had a diameter... Holes 27a and 27b, each 8.5mm in diameter.

[0073] Next, as Figure 1 As shown in (a) and (b), the bending section 23 of the fatigue test piece 21 is subjected to needle-shaped hammering treatment, forming indentation bands 29 at predetermined intervals d along the valley line direction on the inner side of the bend. Here, the radius of curvature r of the front end of the striking pin 11 used in the needle-shaped hammering is set to be 0.2 mm smaller than the radius of curvature R = 2 mm in the cross section in the valley line direction of the bending section 23.

[0074] Next, for fatigue test piece 21, which underwent needle-shaped hammering treatment in the bending section 23, in Figure 6 Fatigue tests were conducted under the boundary conditions shown. Figure 6 As shown, in the fatigue test, the hole 27a of one of the fatigue test pieces 21, portion 25a ( Figure 6 The plate 25b is fixed as a fixed point, and the hole 27b of the other plate 25b is used as a load input point to apply a cyclic load (load 2.3kN, repetition frequency 10Hz).

[0075] Then, a penetrant inspection test is performed on the bending section 23 for 10,000 cycles per load input to measure the progression of cracks generated in the bending section 23. Fatigue test pieces with cracks that have passed through two or more indentation bands 29 are deemed unqualified due to crack progression, and the fatigue test is terminated. Furthermore, the fatigue limit is defined as 500,000 cycles of cyclic loading; fatigue test pieces 21 that reach the fatigue limit are deemed qualified, and the fatigue test is terminated.

[0076] In this embodiment, fatigue tests were conducted on fatigue test pieces 21 with varying intervals d of the indentation bands 29. Fatigue test pieces 21 with intervals d of 3 mm, 4 mm, and 5 mm or more (the thickness of the metal plate 1 within the scope of this invention) were designated as Invention Example 1, Invention Example 2, and Invention Example 3. Furthermore, as a comparison, a fatigue test piece with an interval d of 2 mm (outside the scope of this invention) was designated as Comparative Example 1. Table 1 shows the intervals d of the indentation bands 29 formed on the fatigue test piece 21 and the fatigue test results.

[0077] [Table 1]

[0078] Comparative Example 1 was deemed unqualified because the cracking progressed through more than two indentation bands 29 at 200,000 cycles. In contrast, Examples 1 to 3 of the Invention were deemed qualified because no cracking progress through more than two indentation bands 29 was observed and the fatigue limit was reached at 500,000 cycles.

[0079] According to the present invention, it is shown that the progression of fatigue cracking on the inner side of the bent portion formed by bending a metal sheet can be suppressed and fatigue life can be improved.

[0080] Industrial availability

[0081] According to the present invention, a method for suppressing the fatigue cracking progression of a bent portion of a metal sheet is provided, which can be applied to a bent portion after bending without increasing the manufacturing cost of the bending process of the metal sheet, without reducing productivity, without changing the bending angle of the component as a whole or the bent portion, and suppressing the progression of fatigue cracking in the bent portion; and an automotive component having a bent portion formed by bending a metal sheet and suppressing the progression of fatigue cracking at the bent portion.

[0082] Explanation of reference numerals in the attached figures

[0083] 1 Metal plate

[0084] 3 Bending Processing Section

[0085] 3a Bending start point

[0086] 3b End of bend

[0087] 5. Cracks

[0088] 7. Marks of attack

[0089] 9. Indentation tape

[0090] 11. Crack down on sales

[0091] 13. Front end

[0092] 21 Fatigue test pieces

[0093] 23 Bending Processing Department

[0094] 25a and 25b sections

[0095] Holes 27a and 27b

[0096] 29 Indentation tape

Claims

1. A method for suppressing the progression of fatigue cracks in a bent section of a metal sheet, applied to the bent section of a metal sheet after bending and used to suppress the progression of fatigue cracks in the bent section, wherein, Plastic strain is applied at least within the range from the start point to the end point of the bend on the inner side of the bend in a direction orthogonal to the valley line direction of the bending section, at least along the valley line direction of the bending section, at a distance greater than the thickness of the metal plate, thereby generating compressive residual stress.

2. The method for suppressing fatigue cracking progression in the bending section of a metal plate according to claim 1, wherein, The compressive residual stress is generated by hammering the inner side of the bent section with a needle-shaped hammer using a striking pin, forming an indentation band as a series of impact marks. As the striking pin, a component whose radius of curvature in the cross section of its front end orthogonal to the valley line direction is less than or equal to the radius of curvature of the bending processing section.

3. An automotive component having a bending section formed by bending a sheet metal and suppressing the progression of fatigue cracking at the bending section, wherein, Along the valley line direction of the bending section, there is an indentation band at least on the inner side of the bending section from the starting point to the ending point of the bending, with a gap of more than the thickness of the metal plate. The indentation band is a series of impact marks formed by hammering with a needle-shaped part of an impact pin. The impact pin is a component whose radius of curvature in the cross section of its front end orthogonal to the valley line direction is less than or equal to the radius of curvature of the bending section.

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