JP2010216930A - Test specimen preparing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a test specimen preparing method which enables conducting evaluation of strength to be performed with high accuracy. <P>SOLUTION: In a method for preparing a test specimen used in a strength test, at first, an artificial flow is formed to a matrix (step S2b), and a predetermined load is applied to the test specimen, to which the artificial flaw is formed, to form an actual crack from the artificial flaw which is the starting point (step S3b). Next, the region having the artificial flaw formed thereto is removed (step S4b), and hardening is subsequently performed (step S5b). The hardening, for example, is high-frequency hardening. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing a specimen used for a strength test method, and more particularly to a method for producing a specimen having scratches.

  In the manufacturing process, when testing the strength of a part that is cracked for various reasons, the limit stress and the number of breaks are obtained by applying a load statically or dynamically to a specimen without scratches. The allowable crack depth that is acceptable as a product is obtained by performing a test by applying a similar load to a certain specimen.

  FIG. 16 is a flowchart showing the flow of a conventional test method. First, an intact specimen and a specimen having scratches of various depths are created (S301). First, a strength test is performed on an intact specimen (S302), and a critical stress is determined (S303). Next, a strength test is performed on the specimen having an artificial flaw of depth d1 (S304). As a result of the strength test, when the fracture occurs (S305), it is determined that the allowable crack depth is 0 (S306).

  If not broken (S305), a strength test is performed on the specimen having an artificial flaw of d2 deeper than the depth d1 (S307). As a result of the strength test, when the fracture occurs (S308), the allowable crack depth is determined as d1 (S309). As a result of the strength test, when the fracture does not occur (S308), the strength test is performed in the same manner while increasing the depth. In this way, the strength test is performed to determine the allowable crack depth (S310).

  A specimen having a flaw used in such a strength test is created by applying an artificial flaw by various methods such as electric discharge machining and lathe machining (for example, Patent Document 1). The reason why the specimen having an artificial wound is used is that an artificial wound having a different depth can be relatively easily applied.

Registered Utility Model No. 3041280

  When the present inventors analyzed the artificial wound of the test body used in the strength test, as shown in the sectional view of FIG. 17, the artificial wound (FIG. 17A) actually occurs in the manufacturing process. It has been found that the opening width is large and the shape of the tip is greatly different compared to cracks such as burn cracks (FIG. 17B). Specifically, the opening width of actual cracks (hereinafter, actual cracks) is about 10 μm, whereas the opening width of artificial flaws is about 0.1 mm even if it is narrow. The tip of the actual crack is sharp, whereas the tip of the artificial wound is rounded. For this reason, the stress concentration degree in the vicinity of the tip, which is the most important in the strength evaluation of parts, is greatly different between an artificial flaw and an actual crack.

As shown in FIG. 18A, when a micro elliptical hole exists on an infinite plate subjected to uniform tensile stress, the stress at the long axis end A can be expressed as the following equation.
σ _y = σ ₀ {1 + 2 (a / b)} = σ ₀ {1 + 2 (a / r ₀ ) ^1/2 }

  As shown in FIG. 18B, since the actual crack is defined as b≈0 or r≈0, the stress at the crack tip is infinite, that is, a stress singular field where the stress cannot be specified. Therefore, the strength evaluation using the specimen having an artificial wound is greatly different from the strength evaluation for an actual crack. In particular, the test specimen having an artificial flaw is evaluated with higher strength than the test specimen having an actual crack, and the allowable crack depth is deepened, which increases the danger in actual use of the part.

  The present invention has been made to solve such a problem, and an object of the present invention is to provide a method for producing a test specimen capable of performing strength evaluation with high accuracy.

  The method for forming a test body according to the present invention is a method for forming a test body used in a strength test, and includes an artificial wound forming step for forming an artificial wound on a base material, and a test body on which the artificial wound is formed. An actual crack forming step for applying a predetermined load to form an actual crack starting from the artificial wound, and an actual work shape processing step for removing the area where the artificial wound is formed after the actual crack forming step, A quenching step for quenching is provided after the actual workpiece shape machining step.

  A knife edge attaching step for attaching a pair of knife edges to face each other in the vicinity of the artificial wound formed by the artificial wound forming step after the artificial wound forming step and before the actual crack forming step. May be further provided.

  In the actual crack forming step, the surface on which the artificial wound is formed has both sides of the artificial wound as fulcrums, and a predetermined position relative to the position corresponding to the artificial wound on the opposite surface opposite to the surface on which the artificial wound is formed. An actual crack may be formed by applying a load.

  Here, it is preferable that the quenching performed in the quenching step is induction quenching.

  ADVANTAGE OF THE INVENTION According to this invention, the preparation method of the test body which can perform intensity | strength evaluation with high precision can be provided.

It is a block diagram which shows the structure for implementing an intensity | strength test method. It is a figure for demonstrating the difference of the evaluation in an artificial wound, and the evaluation in an actual crack. It is a flowchart which shows the flow of an intensity | strength test method. It is a figure which shows the preparation method of the test body concerning this invention. It is a figure which shows the preparation method of the test body concerning this invention. It is a graph for demonstrating the method of calculating an allowable crack depth in a strength test method. It is a flowchart of the process which estimates the crack depth in a strength test method. It is a figure for demonstrating the process which estimates the crack depth in a strength test method. It is a figure for demonstrating the process which estimates the crack depth in a strength test method. It is a figure for demonstrating the process which estimates the crack depth in a strength test method. It is a figure for demonstrating the process which estimates the crack depth in a strength test method. It is a figure for demonstrating the process which estimates the crack depth in a strength test method. It is a figure for demonstrating the process which estimates the crack depth in a strength test method. It is a figure for demonstrating the method of calculating an allowable crack depth, when judging from other than a crack in a strength test method. It is a figure for demonstrating the process which estimates the crack depth in a strength test method. It is a flowchart which shows the flow of the conventional strength test method. It is sectional drawing which shows the artificial wound and the actual crack provided in the test body. It is a figure for demonstrating distribution of the stress with respect to the artificial flaw provided in the test body and the actual crack.

  First, a strength test method according to an embodiment of the present invention will be described with reference to the block diagram of FIG. The strength test method is characterized in that a strength test is performed on the specimen 1 having an actual crack. Specifically, a strength test of the test body 1 is executed using the strength tester 2, and experimental data obtained as a result are recorded in the data logger 3. Here, the strength tester 2 is, for example, a tensile tester, a bending tester, or a torsion tester, and has a control device.

  The strength test method according to the present embodiment basically differs only in whether an artificial flaw or an actual crack is imparted to the test body. Etc. can be used as they are.

  The strength evaluation with an artificial flaw and the strength evaluation with an actual crack will be described with reference to FIG. As shown in FIG. 2 (a), when a predetermined stress is applied to a test body having artificial flaws of three types of depths d1, d2, and d3 (d1 <d2 <d3), the depth d3 Assuming that only the specimen has progressed, the allowable crack depth is determined as d2. On the other hand, since the specimens having actual cracks having the depths d1, d2, and d3 (d1 <d2 <d3) are more likely to develop than the specimens having artificial flaws, for example, FIG. As shown in (b), cracks develop not only for test specimens having actual cracks of depth d3 but also for test specimens having actual cracks of depth d2. In this case, the allowable crack depth is d1, which is a result different from the case of the specimen having an artificial flaw.

Next, the strength test method according to the present embodiment will be described using the flowchart shown in FIG. First, an intact specimen and a specimen having actual cracks with various depths are prepared (S101). A method for producing a specimen having a real crack will be described in detail later.
First, a strength test is performed on an intact specimen (S102), and a critical stress is determined (S103). Next, a strength test is performed on the specimen having an actual crack with a depth d1 (S104). As a result of the strength test, when the fracture occurs (S105), it is determined that the allowable crack depth is 0 (S106).

  When not ruptured (S105), a strength test is performed on a specimen having an actual crack of d2 deeper than the depth d1 (S107). As a result of the strength test, when the fracture occurs (S108), the allowable crack depth is determined as d1 (S109). As a result of the strength test, when the fracture does not occur (S108), the strength test is performed in the same manner while increasing the depth. In this way, the strength test is performed to determine the allowable crack depth (S110).

Next, a method for creating a test specimen according to a comparative example and a method for creating a test specimen according to the present embodiment will be described using the flowchart shown in FIG.
When giving a crack with respect to an induction hardening material, inventors created the test body according to the preparation method of the comparative example shown to Fig.4 (a). First, a material to be a base material was prepared (step S1a). In this example, it is a solid round bar shape with a diameter of 30 mm. This material was quenched after performing the actual workpiece shape processing (S step 2a) (step S3a). Furthermore, after performing the notch process (step S4a) which produces an artificial wound, the real crack was provided (step S5a). In the preparation method of the comparative example, brittle fracture occurred in the test specimen at the stage of imparting actual cracks.

  Therefore, in the present embodiment, a test specimen was created by the creation method shown in FIG. First, a material to be a base material was prepared (step S1b). In this example, it is a solid round bar shape with a diameter of 30 mm. Next, notch processing was performed on this material to create an artificial wound (step S2b). The notch processing was performed within a range that is deleted in the actual workpiece shape processing to be performed later. In this example, it is within 4 mm from the surface.

  An actual crack was given after the notch processing (step S3b). The method for imparting actual cracks will be described in detail later. Next, actual workpiece shape processing (step S4b) was performed. In the actual work shape processing, the outer periphery of the material was cut and removed so as to have a diameter of 26 mm.

  Finally, quenching was performed (step S5b). Quenching is induction hardening. According to such a preparation method, brittle fracture did not occur in the test body, and a highly evaluated test body could be obtained even in the evaluation using a torsion tester or the like.

  Then, the preparation method of the test body which has a real crack is demonstrated using FIG. First, as shown to Fig.5 (a), the artificial wound 11 used as the starting point of an actual crack is provided with respect to the test body 1. FIG. Examples of the method for applying the artificial wound 11 include lathe machining and electric discharge machining. The test body 1 may have various shapes such as a columnar member used in a propeller shaft, instead of a plate-like member as shown in the drawing. Moreover, the test body 1 is carbon steel, such as S45C, for example.

  Next, as shown in FIG.5 (b), the fatigue crack 12 is provided by 3-point bending repetition. In a state where both sides of the artificial wound 11 are supported from below on the surface to which the artificial wound 11 has been applied, a position corresponding to the position of the artificial wound 11 on the surface opposite to the surface to which the artificial wound 11 has been applied (that is, human A load is repeatedly applied several times to a position where the line extending the depth direction of the work wound 11 and the opposite side surface are in contact with each other or in the vicinity thereof. At this time, the magnitude of the load may be constant or may be increased little by little. The occurrence of the fatigue crack 12 may be detected by appropriately checking the side surface, but may be detected by observing the opening width of the artificial wound 11. This method will be described in detail later.

  Next, as shown in FIG. 5C, the surface of the test body 1 on the side where the artificial wound 11 is formed is polished and shaved, and the portion where the artificial wound 11 is formed, ie, the depth of the artificial wound 11 Only scrape parallel to the original surface. By this processing, the thickness of the test body 1 is changed from t1 to t2, and as shown in FIG. 5C, the test body 1 in which only the fatigue crack 11 is formed on one surface is created.

  Then, the calculation method of the allowable crack depth by a tensile durability test is demonstrated using FIG. First, a constant stress is applied to an intact product over a plurality of times, and the number of times of durability until breakage is recorded. In addition, the number of times of endurance until a fracture occurs by applying different stresses is recorded. Similarly, for the test specimen having the actual crack depth d1, the number of times of durability is recorded by changing the stress. Further, the same test is performed on the test specimen having the actual crack depth d2 (d1 <d2). FIG. 6 is a graph plotting the durability test results for each specimen when the horizontal axis is the number of times of durability and the vertical axis is the stress.

  As shown in FIG. 6, the test piece of the intact product and the actual crack depth d1 has the same result, whereas the actual crack depth d2 is obtained when the same stress is applied. The number of endurance has decreased. Therefore, the allowable crack depth in this example is determined as d1.

  Subsequently, a method for estimating the crack depth by measuring the opening width (opening displacement) of the actual crack in the test body will be described. As shown in the flowchart of FIG. 7, first, compliance is derived (S201). As shown in FIG. 8, in a state where a load is applied from the opposite side of the actual crack with both sides of the actual crack as fulcrums, the compliance can be defined by the opening displacement amount / the load change amount.

  Here, the opening displacement amount is measured with a strain gauge in the clip gauge 4 by providing an edge at the opening of the actual crack 12 and hooking the clip gauge 4 as shown in FIG. can do. Here, when the edge of the test body 1 cannot be processed, holes 131 and 132 are formed on both sides of the actual crack 12 of the test body 1 as shown in FIG. The knife edges 51 and 52 may be inserted and fixed respectively. Further, when the drilling cannot be performed on the test body 1, the knife edges 61 and 62 may be fixed with an instantaneous adhesive or the like.

  Compliance is determined for specimens with multiple types of actual crack depths. In the example shown in FIG. 10, compliance is obtained for test specimens having actual crack depths d1, d2, and d3. Next, as shown in FIG. 11, a calibration curve of actual crack depth and compliance is created (S202). Then, while applying a load as shown in FIG. 12 to the specimen for which the actual crack depth is estimated, the opening displacement is measured to calculate the compliance, and the calculated compliance is shown in FIG. The actual crack depth is estimated with reference to the calibration curve (S203).

  In this way, if the method for estimating the actual crack depth is used, as shown in FIG. 14, even if the fracture is caused from other than the actual crack, the progress of the crack can be observed, and the progress The depth d1 that is not present can be determined as the allowable crack depth.

  In the case where the opening displacement is measured by the clip gauge 4 in the cylindrical test body (round bar), the curved shape 71 along the outer shape of the test body is formed below the edge 72 as shown in FIG. It is preferable to use a knife edge 7 on the surface.

  In the above-described example, the tensile durability test is taken up. However, the present invention is not limited to this, and the strength test can be applied to a bending test, a torsion test, an impact test, and the like.

DESCRIPTION OF SYMBOLS 1 Test body 2 Strength tester 3 Data logger 4 Clip gauge 51, 52, 61, 62, 7 Knife edge 11 Artificial wound 12 Actual crack

Claims (4)

A method for forming a specimen used in a strength test,
An artificial wound forming step for forming an artificial wound on the base material;
Applying a predetermined load to the test body on which the artificial wound is formed, an actual crack forming step for forming an actual crack starting from the artificial wound, and
After the actual crack forming step, an actual workpiece shape processing step for removing the region where the artificial flaw is formed,
A test body forming method comprising a quenching step of quenching after the actual workpiece shape processing step.   A knife edge attaching step for attaching a pair of knife edges to face each other in the vicinity of the artificial wound formed by the artificial wound forming step after the artificial wound forming step and before the actual crack forming step. The method for forming a specimen according to claim 1, comprising:   In the actual crack forming step, the surface on which the artificial wound is formed has both sides of the artificial wound as fulcrums, and a predetermined position relative to the position corresponding to the artificial wound on the opposite surface opposite to the surface on which the artificial wound is formed. 3. The method for forming a test specimen according to claim 1, wherein an actual crack is formed by applying a load.   The method for forming a specimen according to any one of claims 1 to 3, wherein the quenching performed in the quenching step is induction quenching.

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