JPH1123420A - Method for evaluating life of bolt - Google Patents

Abstract

PROBLEM TO BE SOLVED: To evaluate a life with high reliability by forming a test piece by retaining several threads of a bolt under test and boring a center portion, and executing a general purpose creep or low cycle fatigue test via a jig to be engaged with the threads. SOLUTION: Several threads of a base end of a bolt under test are retained at an engaging part of a bolt under test with female threads, and cut by retaining a base end side. Thereafter, the treads are bored in an axial hole of a predetermined diameter to form coaxial threaded hole to a test piece 7. The base end side of the piece 7 is engaged with a drug rod 9a of a tester, and a thread side is engaged with a jig 8 having female threads engaged with a drug bar 9b. It is applied to a general purpose creep or low cycle fatigue tester. Thus, the piece 7 is formed, several threads are loaded, and testing load is shared at several threads to a load shared at the several threads in a practical use state. Thus, a stress distribution from the threads to roots of the thread becomes similar to a real bolt to obtain a testing condition near using stress condition of the real bolt, and hence its life evaluation has high reliability.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bolt life evaluation method for evaluating the aging of a screw bottom of a high-temperature bolt or the like by a destruction technique.

[0002]

2. Description of the Related Art High temperature bolts are known to suffer damage such as creep and low cycle fatigue during long-term use, and are one of the parts that are regularly replaced. In order to accurately grasp the timing of this replacement and optimize the replacement interval, check the bolts during normal opening inspections, perform trend management, and remove the representative bolts during precision inspection. In addition, it is also practiced to grasp the remaining life of the bolt used in the device by performing a cutting survey. When the representative bolt is extracted and cut and inspected, a test piece for creep or fatigue test is processed from the test bolt, and a creep or low cycle fatigue test is performed on the test piece.

In general, creep or low-cycle fatigue damage of bolts occurs from the bottom of the screw, often leading to cracks and fractures. In addition, from the viewpoint of strength evaluation, the screw bottom generates high local stress due to stress concentration. It is known.

FIGS. 3 to 6 show examples in which a standard creep test piece (smooth test piece 2 and notch test piece 3) is sampled from a test bolt 1. FIG. On the other hand, it is conceivable to perform a test on the entire bolt, but this has not been realized due to a problem in the capability of the testing machine and difficulty in performing the test. For this reason, a method of cutting out a bending test piece 5 partially including a screw bottom from the thread portion of the test bolt 1 as shown in FIGS. 7 to 9 and testing it with a bending creep tester 6 has also been proposed and put into practical use. ing.

[0005]

The above prior art has the following problems and it is difficult to accurately estimate the remaining life.

(1) A standard creep test piece is collected,
The creep or fatigue test method does not allow testing at the severest part of the bolt life.

(2) In a method in which a bending test piece is cut out and a bending creep test is performed, the acting force of the test load is different from the actual bolt screw state.

(3) In the method of testing the entire test bolt, the load or the like becomes too large, and a general-purpose creep tester or fatigue tester cannot be used. (For example, for a bolt with a diameter of about 100 mm, a load of 50 to 100 tons is required)

[0009]

The present invention employs the following means to solve the above-mentioned problems.

That is, a test piece in which a few threads are left and a center portion is cut out from a test bolt for creep or low cycle fatigue test is produced, and a general-purpose creep test machine is passed through a jig which fits the thread. Alternatively, the life is evaluated using a general-purpose low cycle fatigue tester.

[0011] In the above, a cylindrical test piece having a few threads left from the test bolt and a hollow central portion is produced. Then, the life is evaluated by using a general-purpose creep tester and / or a general-purpose low cycle fatigue tester through a jig that fits into the thread of the test piece. At this time, the test load is a load that is shared by several threads in the actual use state, that is, a small load F, since the threads have several threads.
The diameter of the hollow portion is set so that the remaining cross section S has a stress in an actual use state when the load F is small.

As described above, a general-purpose creep tester and a low cycle fatigue tester can be used, and the test is performed in a state similar to the actual use state. Then, the life evaluation is performed. Therefore, highly reliable life evaluation can be performed.

[0013]

1 and 2 show an embodiment of the present invention.
This will be described below.

From the test bolt 1, the threaded portion at the base end of the meshing portion a with the female thread is processed to leave several threads (two threads in the figure), and the tip side is cut perpendicularly to the axis. Also, cut vertically, leaving the base end small. Then, the thread portion is cut out with a coaxial hole having a predetermined diameter D. Further, a coaxial screw hole b is formed on the base end side. This becomes the test piece 7.

The test piece 7 has a draw bar 9a of the test machine at the base end side.
It is glared. Further, the thread side is engaged with a jig 8 having a female screw for engaging with the pull rod 9b. Then, it is applied to a general-purpose creep tester and a low cycle fatigue tester.

By forming the test piece 7 as described above, the test load is set to a load which is shared by several threads in the actual use state since the threads have several threads. For example, if the thread of the engagement portion a has 30 threads and the thread of the test piece 7 has two threads, a small load F of about 1/15 of the actual load may be used. The diameter D of the hollow portion is set so that the remaining cross section S has a stress in an actual use state at a small load F.

In this way, the stress distribution and the stress from the screw thread to the screw bottom are the same as those of the actual bolt, and it is possible to obtain a test condition close to the working stress condition of the actual bolt. Therefore, a general-purpose creep tester or a low cycle fatigue tester can be used, and the test is performed in a state similar to an actual use state. Since the life evaluation is performed, highly reliable life evaluation can be performed.

[0018]

As described above, according to the present invention, a test piece having a few threads left from the test bolt and a central portion cut out is prepared and tested using a general-purpose creep tester or fatigue tester. Therefore, by appropriately selecting the test load, it is possible to realize a condition in which the stress distribution at the screw bottom of the test piece is close to that of the bolt of the actual machine.

For this reason, an accurate creep or low cycle fatigue test is performed, and an accurate evaluation of the remaining life becomes possible.

[Brief description of the drawings]

FIG. 1 is a sectional view of an embodiment of the present invention.

FIG. 2 is a cross-sectional view of the test piece of one embodiment.

FIG. 3 is a side view for explaining an example of the related art.

FIG. 4 is an explanatory front view of the example of the conventional art.

FIG. 5 is a view of a smooth creep test piece of the conventional example.

FIG. 6 is a view of an example of an annular notched creep test piece of the related art.

FIG. 7 is an explanatory side view of another example of the conventional art.

FIG. 8 is an explanatory front view of another example of the conventional art.

FIG. 9 is an explanatory diagram of another example of the related art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Test bolt 2 Smooth test piece 3 Notch test piece 5 Test piece 6 Bending creep tester 7 Test piece 8 Jig 9a, 9b Pull rod

Claims (1)

[Claims] 1. A creep or low cycle fatigue test bolt is formed with a test piece having several threads left and a central portion cut out from a test bolt, and a general-purpose creep test machine is passed through a jig mating with the thread. Alternatively, a method for evaluating the life of a bolt, wherein the life is evaluated using a general-purpose low-cycle fatigue tester.