JPH04208837A - Method for evaluating fatigue of machine parts - Google Patents

Abstract

PURPOSE:To accurately detect a fatigue accumulated in machine parts so as to prevent the occurrence of an accident caused by the fatigue by measuring the fatigue accumulated in the machine parts and electrical potential of the parts. CONSTITUTION:After a fatigue test piece 7 with a notch is prepared, platinum wires of current input terminals l and 2 are spot-welded to two points in the axial direction and platinum wires 3 and 4 and 5 and 6 for measuring the potential differences of a damaged and non-damaged sections are spot-welded between the two points. At the time of fatigue tests, an alternating current is made to flow between the terminals 1 and 2 and the ratio ED/EO of the potential difference ED between the terminals 3 and 4 to the potential difference EO between the terminals 5 and 6 is calculated after measuring the potential differences ED and EO. When the value is compared with a real degree of fatigue obtained from Formula I, it becomes clear that the value monotonously increases. Therefore, when the ratio ED/EO is appropriately standardized, the ratio well coincides with the real degree of fatigue.

Description

[Detailed description of the invention]

[00011

FIELD OF THE INVENTION The present invention relates to a non-destructive testing method applied to in-service testing of mechanical parts subjected to repeated strain. [0002] [Prior Art] As a method for detecting fatigue damage in mechanical parts due to repeated strain, there is a method of detecting fatigue cracks generated as a result of fatigue damage using non-destructive defect inspection methods such as ultrasonic flaw detection and penetrant flaw detection. It was used a lot. [0003]

[Problems to be Solved by the Invention] However, with the above-mentioned method that focuses on defects, it is not possible to detect defects that are below the lower limit of defect detection, and in addition, in order to detect defects, it is necessary to remove the oxidized layer on the surface of the mechanical component using a grinder or the like. It needed to be removed. Fatigue damage caused by repeated strain progresses through the generation and propagation of surface cracks, but polishing the surface removes minute surface cracks that occur during the fatigue damage accumulation process, making it possible to underestimate the extent of damage. In addition to this, there was a possibility that early damage could not be evaluated. [0004]Furthermore, since fatigue damage in mechanical parts is often located at discontinuous parts such as weld toes, the amount of surface layer removed by grinders etc. increases at such locations, which causes the above-mentioned problems. was being encouraged. [0005]

[Means for Solving the Problems] In view of the above circumstances, the present invention has the following features. First, the present inventors focused on the electric potential method in place of the above-mentioned method, which requires grinding the surface of a mechanical component that has suffered fatigue damage. In other words, it has been found that when a constant current is applied to a location of the mechanical component where the accumulation of fatigue damage is a problem, the potential difference increases due to surface cracks that occur as fatigue damage accumulates. but,
In the DC four-terminal method, which has been put into practical use as an electrical resistance method,
For shape discontinuities in actual mechanical parts where accumulation of fatigue damage is a problem, measurement values vary due to deviations in measurement positions, so electrode terminals are permanently installed on the mechanical parts. In addition, alternating current was measured in order to suppress the effects of error in measured potential due to thermoelectromotive force and noise. Furthermore, taking advantage of the fact that fatigue damage accumulates locally, the potential difference between the damaged and undamaged parts between the current input terminals is measured, and fatigue damage is evaluated by the ratio of the potential difference in the damaged part to the potential difference in the undamaged part. I decided to do so. [0006]

[Operation] According to the method of the present invention, measurement can be performed without polishing the surface of mechanical parts by utilizing the change in potential difference accompanying the accumulation of fatigue damage. It becomes possible to detect minute surface cracks without the effects of removal. Furthermore, unlike the replica method, it does not require many steps and can be easily evaluated by simply measuring the potential difference. Furthermore, it is now possible to perform phase detection using alternating current as the input current, and a current input terminal and potential difference measurement terminal are permanently installed.
Furthermore, we decided to measure the potential difference between the damaged and non-damaged parts of the same current input terminal, and evaluate the damage based on the ratio. Therefore, it was determined that the difference in measurement position, temperature, and noise that affect the error in electrical potential measurement values were determined. , the influence of thermoelectromotive force can be suppressed, and measurement accuracy can be improved. [0007]

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings and the like. First, a notched fatigue test piece shown in FIG. 1 was prepared. Platinum wires] and 2 were spot welded to the test piece as terminals for current input at two points in the axial direction of the test piece sandwiching the gauge points, and a Platinum wires 3.4 and 5.6 for measuring the potential difference in the damaged and non-damaged parts were spot welded to the region including the notch and the region not including the notch, respectively. First, a fatigue test was conducted on the test piece 7 at room temperature, and the number of repeated fractures Nf was determined. Next, an AC potential difference measuring device shown in FIG. 2 was prototyped, and a separately prepared test piece 8 was used. The test was interrupted at every INf, and the damaged part (platinum wire 5.6) and the undamaged part (platinum wire 3.6) were examined.
4) potential differences, Eo and Eo, respectively, were measured. [0008] Eo, Eo, and E D/E
o and fatigue damage degree (number of interruption cycles/number of repeated ruptures N
The relationship with f is determined and shown in FIG. While the measured values of Eo and ED have large dispersion, Eo /Eo increases monotonically with the degree of fatigue damage, and is a highly accurate non-destructive fatigue damage measurement compared to measuring the absolute value of the fatigue damage area. It was considered to be an evaluation method. Therefore, in order to verify the effectiveness of the application to actual machine parts, a fatigue test was conducted on a test piece simulating the header nozzle section 9 of a thermal power boiler shown in FIG. Fatigue damage evaluation was conducted using this method. A test piece is shown in FIG. 4, and the fatigue test was conducted by applying constant tensile and compressive strain to the nozzle stub 10 in the direction 11 of the arrow shown in the figure. The above test was carried out before the test and at 25
Potential difference measurements were performed on the test piece with interruptions after 00, 5,000, and 7,500 repetitions. In order to show the details of the potential difference measurement position, an enlarged view of the vicinity of the part A in FIG. 4 is shown in FIG. In FIG. 5, numerals 1 to 6 indicate platinum wire electrodes as in FIG. Also,
The measured value of the potential difference EO, ED between the undamaged part and the damaged part, E
D/EO, fatigue damage evaluation results by the method of the present invention evaluated based on Fig. 3, and the number of fracture cycles of the test piece (9
360 times) using Formula 1.
**[0009]

[Equation 1] Number of interruption repeats (0,2500,5000,7500
times) True degree of fatigue damage = Number of a1 interruptions of the test piece (9360 times) [00101 Table 1 shows the degree of fatigue damage at each number of interruptions.
※※[001
1]

[Table 1] [0012] Since the ED/EO before the test in Figure 3 is different from the ED /EO before the test in this test, the ED /EO at each interruption was divided by the Eo /Eo before the test. Damage was evaluated using the normalized value. The degree of fatigue damage evaluated by the method of the present invention was in good agreement with the true degree of fatigue damage, and it was confirmed that the degree of fatigue damage could be evaluated accurately by the method of the present invention. [0013,1] [Effects of the Invention] As described above, according to the method of the present invention, fatigue damage accumulated in mechanical parts in service can be detected with high accuracy simply by measuring the electrical potential. It has become possible to prevent accidents due to the accumulation of fatigue damage of mechanical parts in service, and it has also become possible to extend the period of stable operation of the mechanical parts, thereby promoting effective utilization of equipment.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a notched fatigue test piece according to a first embodiment of the present invention.

FIG. 2 is a block diagram of a potential difference measuring device according to an embodiment of the present invention.
★★

FIG. 3 is a schematic diagram showing potential difference measurement results according to an example of the present invention.

FIG. 4 is a schematic diagram of an actual model fatigue test piece according to an example of the present invention.

FIG. 5 is an enlarged schematic diagram of the vicinity of the part in FIG. 4A showing the electrode attachment state of the actual model fatigue test piece according to the example of the present invention.

[Explanation of symbols]

1 Platinum wire for current input terminal 2 Platinum wire for current input terminal 3 Platinum wire for potential difference measurement terminal in damaged part 4 Platinum wire for potential difference measurement terminal in damaged part 5 Platinum wire for potential difference measurement terminal in undamaged part 6 Platinum wire for potential difference measurement terminal in undamaged part Platinum wire for potential difference measurement terminal 7 Fatigue test piece with notch 8 Fatigue test piece with notch 9 Real model fatigue test piece 10 simulating a boiler header header for thermal power generation Nozzle head 11 Strain loading direction of the real model fatigue test piece

[Figure 2] (72) Inventor Toshiyuki Imazato Nagasaki Heavy Artillery No. 1-1 Uramachi Mitsubishi Heavy Industries Stock % Formula %

Claims (1)

[Claims] Claims: 1. A method for evaluating fatigue damage of a mechanical component, comprising: passing an alternating current through a region including a damaged portion and a region including an undamaged portion of the mechanical component, and measuring a potential difference between the damaged portion and the undamaged portion; Using the relationship created in advance between the ratio of the potential difference of the damaged part to the potential difference of the undamaged part and the degree of fatigue damage (number of strain repetitions from the start of operation to the time of investigation/number of repetitions of strain from the start of operation to failure) 1. A method for evaluating fatigue damage of a mechanical component, characterized in that the degree of fatigue damage of the mechanical component is evaluated from a measured value of a ratio of a potential difference of a damaged part to a potential difference of an undamaged part of the mechanical component.