METHOD FOR IMPROVING FATIGUE STRENGTH

Abstract

Provided is a novel method for improving fatigue strength, which is applicable to any small portion that is covered with another member. A method for improving fatigue strength according to the present invention includes: disposing an aid (3, 15) to be opposed to an processing object (2A, 2B, 6) on which a fatigue strength improving process is performed, the aid being configured to assist the improving process; and generating sparks between the aid (3, 15) and the processing object (2A, 2B, 6).

Claims

1. A method for improving fatigue strength, comprising: disposing an aid to be opposed to a processing object on which a fatigue strength improving process is performed, the aid being configured to assist the improving process; and generating sparks between the processing object and the aid.

2. The method for improving fatigue strength according to claim 1, wherein welding due to the generation of the sparks is caused on an opposed surface of the processing object, the opposed surface being opposed to the aid, thereby repairing a crack generated on the opposed surface.

3. The method for improving fatigue strength according to claim 2, wherein the opposed surface is formed of a circumferential surface facing one of a groove and a through-hole formed in the processing object.

4. The method for improving fatigue strength according to claim 1, wherein the aid is attached to the processing object before the improving process for generating the sparks is performed.

5. The method for improving fatigue strength according to claim 1, wherein the aid is opposed to the processing object when the improving process for generating the sparks is performed.

6. The method for improving fatigue strength according to claim 1, wherein a first splice plate and a second splice plate constitute the processing object, the first splice plate and the second splice plate being fastened together, a fastener that fastens the first splice plate and the second splice plate constitutes the aid, the processing object and the aid constitute a fastener joint, and the sparks are generated between the fastener and one or both of the first splice plate and the second splice plate.

7. The method for improving fatigue strength according to claim 1, wherein the processing object is made of a metallic material.

8. The method for improving fatigue strength according to claim 6, wherein the processing object is made of a metallic material.

9. The method for improving fatigue strength according to claim 1, wherein the aid is made of a metallic material.

10. The method for improving fatigue strength according to claim 6, wherein the aid is made of a metallic material.

11. The method for improving fatigue strength according to claim 7, wherein the aid is made of a metallic material, and the processing object includes a non-conductive covering layer which is formed on a surface of the processing object.

12. The method for improving fatigue strength according to claim 8, wherein the aid is made of a metallic material, and the processing object includes a non-conductive covering layer which is formed on a surface of the processing object.

13. The method for improving fatigue strength according to claim 9, wherein the aid includes a non-conductive covering layer which is formed on a surface of the aid.

14. The method for improving fatigue strength according to claim 10, wherein the aid includes a non-conductive covering layer which is formed on a surface of the aid.

15. The method for improving fatigue strength according to claim 2, wherein the aid is attached to the processing object before the improving process for generating the sparks is performed.

16. The method for improving fatigue strength according to claim 3, wherein the aid is attached to the processing object before the improving process for generating the sparks is performed.

17. The method for improving fatigue strength according to claim 2, wherein the aid is opposed to the processing object when the improving process for generating the sparks is performed.

18. The method for improving fatigue strength according to claim 3, wherein the aid is opposed to the processing object when the improving process for generating the sparks is performed.

19. The method for improving fatigue strength according to claim 2, wherein a first splice plate and a second splice plate constitute the processing object, the first splice plate and the second splice plate being fastened together, a fastener that fastens the first splice plate and the second splice plate constitutes the aid, the processing object and the aid constitute a fastener joint, and the sparks are generated between the fastener and one or both of the first splice plate and the second splice plate.

20. The method for improving fatigue strength according to claim 3, wherein a first splice plate and a second splice plate constitute the processing object, the first splice plate and the second splice plate being fastened together, a fastener that fastens the first splice plate and the second splice plate constitutes the aid, the processing object and the aid constitute a fastener joint, and the sparks are generated between the fastener and one or both of the first splice plate and the second splice plate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1A is a plan view showing a sample on which a method for improving fatigue strength according to the present invention is carrier out; FIG. 1B is a sectional view taken along a line Ib-Ib shown in FIG. 1A; and FIG. 1C is a partially enlarged view of FIG. 1B;

[0020] FIGS. 2A and 2B are graphs each showing results of tests for demonstrating the method for improving fatigue strength according to the present invention;

[0021] FIG. 3 is a table showing static strength evaluation results according to Example and Comparative Example of the present invention;

[0022] FIGS. 4A and 4B are diagrams each showing another mode for implementing the method for improving fatigue strength according to the present invention;

[0023] FIGS. 5A to 5C are diagrams each showing still another mode for implementing the method for improving fatigue strength according to the present invention; and

[0024] FIGS. 6A to 6C are diagrams each showing still one more mode for implementing the method for improving fatigue strength according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] A method for improving fatigue strength according to the present invention will be described below based on embodiments with reference to the accompanying drawings.

[0026] A method for improving fatigue strength according to this embodiment is applied to a fastener joint 1 having a structure in which a pair of a first splice plate 2A and a second splice plate 2B is connected by a plurality of fasteners 3, which are aligned, as shown in FIGS. 1A to 1C. In the method for improving fatigue strength according to this embodiment, a high current is applied to a node between the first splice plate 2A and the second splice plate 2B, thereby improving the fatigue strength of each of the first splice plate 2A and the second splice plate 2B.

[0027] The first splice plate 2A and the second splice plate 2B that constitute the fastener joint 1 are made of iron and iron-base alloys, aluminum and aluminum alloys, and other metallic materials. The first splice plate 2A and the second splice plate 2B each include a plurality of fastening holes 5 through which fasteners 3 made of a metallic material penetrate and inserted. The fasteners 3 are fastened in such a manner that the fasteners are inserted into the fastening holes 5, respectively, in the state where the positions of the fastening holes 5 in the first splice plate 2A match the positions of the fastening holes 5 in the second splice plate 2B, respectively.

[0028] When the fastener joint 1 is actually used as a structural element, a circumferential surface 7 which surrounds the fastening holes 5 in the first splice plate 2A and the second splice plate 2B repeatedly receives a stress through the fasteners 3, and a concentration of stress occurs on the circumferential surface 7. In addition, since the fastening holes 5 are formed by cutting, a small crack that cannot be visually confirmed is generated on the cutting processing surface during cutting and the crack remains on the circumferential surface 7. This small crack may cause a fatigue fracture.

[0029] In this embodiment, this small crack C is repaired and eliminated by welding, or the degree of the crack is reduced. In this embodiment, in order to weld a base material, a high-energy current is applied to a node between the first splice plate 2A and the second splice plate 2B by using a power supply 4 as shown in FIG. 1A. With this structure, as shown in FIG. 1C, sparks S are generated between each fastener 3 and the first and second splice plates 2A and 2B. Arc heat generated due to the sparks melts the surface of the circumferential surface 7, and the melted components flow into the small crack C and are then solidified, so that the small crack C is filled with the components. A series of operations described above is herein referred to as welding. The sparks S are more likely to be generated on a discontinuous surface. For this reason, the sparks are preferentially generated on a portion where the small crack C is formed, rather than on a continuous surface of the circumferential surface 7 on which the small crack C is not formed. Therefore, the repair of the small crack C by generating the sparks S is convenient for this embodiment.

[0030] Next, the results of tests for demonstrating the method for improving fatigue strength according to this embodiment by application of a high current will be described.

[0031] Experiments were conducted in such a manner that the fastener joint 1 shown in FIG. 1 was prepared and a high current was applied, and then a fatigue strength test was conducted by repeatedly applying a tensile load and the behavior of fracture stress was measured. The applied current was 210 C (Coulomb) at maximum. An example in which the high current was applied is referred to as an Example. For comparison, a fatigue strength test was conducted in a similar manner to that of the Example, except that a high current was not applied. This example is referred to as Comparative Example. Note that the same material was used for the fastener joint 1 according to Example and the fastener joint 1 according to Comparative Example, and the fastener joints were prepared under the same processing conditions.

[0032] Further, a tensile strength test of applying a static load was conducted on the fastener joint 1 according to Example and the fastener joint 1 according to Comparative Example.

[0033] FIG. 2A shows the results of the fatigue strength test. FIG. 3 shows the results of the strength test of applying a static load. Note that in FIGS. 2A and 2B, the horizontal axis represents the number (cycle) of loads that are repeatedly applied until a rapture occurs, and the vertical axis represents a maximum load.

[0034] As shown in FIG. 2A, it was confirmed that the fatigue strength of the fastener joint 1 according to Example in which a high current was applied was improved as compared with the fastener joint 1 according to Comparative Example in which a high current was not applied. In particular, as the applied load (maximum load) decreases, the effect of improvement in the fatigue strength becomes larger, and the number of cycles to fracture is more than ten times higher than that in Comparative Example. On the other hand, as a result of measuring a breaking load as a static load (n=3) as shown in FIG. 3, there is no significant difference between Example and Comparative Example.

[0035] As described above, it was confirmed that the application of a high current enables an improvement in the fatigue strength without any adverse effect on the static strength.

[0036] Advantageous effects obtained by improving the fatigue strength according to this embodiment will be described.

[0037] In general, an improvement in the fatigue strength cannot be expected only by increasing the dimensions. For example, in the case of the fastener joint 1, when the thickness of each of the first splice plate 2A and the second splice plate 2B is increased, a stress applied to the joint is reduced, but an offset of the load increases. Therefore, an improvement in the fatigue strength cannot be expected.

[0038] On the other hand, according this embodiment, the fasteners 3, which are aids to assist a fatigue strength improving process, are disposed to be opposed to the first splice plate 2A and the second splice plate 2B which are processing objects on which the improving process is performed, thereby generating sparks between the fasteners 3 and the first and second splice plates 2A, 2B. Therefore, this embodiment is effective for improving the fatigue strength without changing the design.

[0039] Though materials having a high fatigue strength can be used to improve the fatigue strength, this method leads to an increase in cost and requires a design change when existing structural elements are used. Also in this regard, according to this embodiment, material costs are not increased and there is no need to change the design.

[0040] In the method of applying a compressive residual stress by shot peening, for example, which is known as a method for improving fatigue strength, a large workload is required for shot peening. In addition, when the method is applied after the use of the object, it is necessary to remove a covering member from a portion to which a compressive residual stress is applied if the portion is covered with the covering member. Therefore, if there are many covering members, for example, the method is not practical. Further, at a small portion, such as the circumferential surface 7 of this embodiment, it is difficult to apply shot peening.

[0041] In this regard, according to this embodiment, the fatigue strength can be improved only by generating sparks to repair the small crack C by welding. In addition, even at a small portion, such as the circumferential surface 7 of the fastening hole 5, and even when the portion is covered with the fasteners 3, the fatigue strength can be improved only by generating sparks. Therefore, this embodiment can be applied with a small workload required for improving the fatigue strength, without any effort after use.

[0042] The present invention has been described above based on preferred embodiments, but can be modified as appropriate by, for example, choosing some of the components described in the above embodiments, without departing from the scope of the invention.

[0043] For example, in the present invention, a non-conductive layer can be formed on the surface of a region in which sparks are to be generated. In the above embodiments, for example, coating is applied to the first splice plate 2A and the second splice plate 2B and a non-conductive covering layer 9 is formed on the surfaces thereof as shown in FIG. 4A, thereby facilitating the generation of sparks between the circumferential surface 7 and the fasteners 3. As shown in FIG. 4B, the covering layer 9 may be formed by applying coating only to the fasteners 3. Alternatively, although not shown, the non-conductive covering layer 9 may be formed by applying coating to the fasteners 3 and both the first and second splice plates 2A and 2B.

[0044] FIG. 2B shows the results of measuring the fatigue strength in a similar manner to Example except that coating is applied to the first splice plate 2A and the second splice plate 2B of the above Example as shown in FIG. 4A and a non-conductive layer is formed on the surfaces thereof. As shown in FIG. 2B, it was confirmed that the fatigue strength can be further improved in a region with a small load by forming the non-conductive layer. This seems to be because sparks were more easily generated and repair of the small crack C was promoted.

[0045] While the above embodiments illustrate, by way of example, the fastener joint 1 to which the fasteners 3 are attached, the object to which the present invention is applied is not limited to this, and any object can be used as long as sparks can be generated. For example, as shown in FIG. 5A, the present invention can be applied to a through-hole 11 which is formed in a processing object 6 and is not provided with members corresponding to the fastener 3. In this mode, when the diameter of the through-hole 11 is small, as shown in FIG. 5B, the sparks S are generated between different portions of a circumferential surface 13, even if other members are not applied. When the diameter of the through-hole 11 is large, as shown in FIG. 5C, an aid 15 which corresponds to each fastener 3 described above and assists the generation of the sparks S may be inserted into the through-hole 11, and then a high current may be applied.

[0046] The through-holes including the fastening holes 5 are objects suitable for obtaining the advantageous effects of the present invention. However, the present invention can be applied to any structure.

[0047] For example, when a notch 17, which is a V-shaped groove, is formed at an end face of the processing object 6 as shown in FIG. 6A, the method for improving fatigue strength according to the present invention can be applied to the notch 17. In this case, as shown in FIG. 6B, the shape of the aid 15 for generating the sparks S between the aid and the processing object 6 is similar to the shape of the notch 17. The aid 15 is opposed to the notch 17, and a high current is applied between the processing object 6 and the aid 15. Accordingly, the sparks are generated between the aid 15 and the circumferential surface 7 that defines the periphery of the notch 17, which is a groove, and the small crack C generated on the circumferential surface 7 is repaired by welding.

[0048] For example, according to the present invention, as shown in FIG. 6C, the aid 15 is opposed to a flat surface 19 of the processing object 6, and a high current is applied between the processing object 6 and the aid 15, thereby making it possible to generate sparks between the aid 15 and the flat surface 19 of the processing object 6.

[0049] As described above, according to the present invention, the aid may be attached to the processing object before the improving process for generating the sparks S is performed, or the aid may be opposed to the processing object when the improving process for generating the sparks S is performed.

[0050] Furthermore, any current may be applied in the present invention, as long as the process for improving the fatigue strength by generating sparks is generated. Conditions suitable for the object for improving the fatigue strength can be confirmed by a test.