JP2528908B2 - Fatigue measurement method for metallic materials - Google Patents

Description

The present invention relates to a method for measuring the degree of fatigue of a metal material used in a pressure vessel or the like.

[Prior Art] As a method of measuring the degree of fatigue of a metal material, a method of measuring the half-value width of a sample to be measured by X-ray diffraction and measuring the fatigue degree from the change in the half-value width is often used. There is.

That is, when the crystal surface of the metal material is irradiated with X-rays,
X-rays are scattered by the atoms constituting the crystal, and this scattered X
The line causes a diffraction phenomenon that reinforces each other in a specific direction, and this takes advantage of the fact that this correlates with the fatigue level of a metal material. The X-ray half widths of samples with different fatigue levels are statistically obtained in advance. The fatigue level is measured by associating with the full width at half maximum of the sample to be measured.

Here, the degree of fatigue indicates the degree of fatigue, that is, the degree of fatigue or damage, and is synonymous with the degree of fatigue damage or CD (Cumulative Damage). The fatigue level is 0% when the material is not damaged at all and 100% when cracks occur.
It is represented by a value from 0% to 100%. The value is measured by subjecting the material to repeated deformation under predetermined conditions, the number of repetitions is Ni, and the number of repetitions when cracks start to occur in the material is Nf, Ni / Nf
Is expressed as a percentage.

[Problems to be Solved by the Invention] However, in the above measuring method, as shown in FIG. 9, the change in half width and the change in fatigue level are not in a linear proportional relationship, and the fatigue level is 100%. Even when the test sample of No. 2 was measured, the difference in half-width was extremely large between the cracked portion and the portion distant from this, and it was difficult to accurately measure the fatigue level.

Note that there is a method of measuring the fatigue level by changing the hardness of the sample or the microstructure, but all of them lack accuracy.

Therefore, an object of the present invention is to solve the above problems and to provide a measuring method capable of accurately measuring the fatigue level of a metal material.

[Means for Solving the Problems] In order to achieve the above object, the present invention takes a photograph of a plurality of different regions of a sample to be measured by selected-area electron diffraction, and shifts the azimuth deviation angle from these. The average value of is calculated, and this average value is compared with the average value of the deviation angle of the azimuth that is obtained in advance for the test samples of 0% to 100% fatigue level to measure the fatigue level of the measured sample. To do.

[Operation] First, the degree of fatigue of a metal material can be generally evaluated by the deviation with respect to the initial position (fatigue degree 0%) of the crystal plane constituting the metal, and this deviation is proportional to the degree of fatigue.

In conventional X-ray diffraction, the measurement area is large, and
Although it was impossible to measure the diffraction, by using the selected area diffraction method, the diffraction image could be seen in the cell of the crystal unit, and the present invention was completed thereby.

The present invention, by using the selected area electron beam diffraction, it is possible to individually measure the diffraction of crystals in a very narrow region of the measurement sample, and statistically average the deviation angles of the directions of this diffraction image to obtain an average value. Then, this average value can be taken as a value representing the degree of fatigue. That is, the average value of the deviation angle is linearly proportional to the increase of the fatigue degree or the damage degree, and the average value of the deviation angle of the azimuth for each step of the test sample given in advance from 0% to 100% of the fatigue degree. If the average value of the deviation angle with respect to this fatigue degree is compared with the average value of the deviation angle of the azimuth of the diffraction image obtained by the selected-area electron diffraction of the measured sample, the fatigue degree of the measured sample is Can be measured.

Normally, in X-ray diffraction, when the fatigue level is 100%, the change in the half-value width between the vicinity of the crack and the portion further away from the crack is large, but in the present invention, the difference between the average values of the deviation angles is small,
Therefore, the fatigue level can be accurately measured without being influenced by the sampling location of the sample.

EXAMPLES Examples of the present invention will be described in detail below with reference to the accompanying drawings.

When measuring the fatigue level of the sample to be measured, the azimuth shift of the diffraction image by the selected area electron beam diffraction from the test sample to which the fatigue level of 0% to 100% was given in advance for each level of fatigue level Find the average value of the corners.

In this example, a test round bar was made from high-tensile steel SA508 class 3 for pressure vessels, and 0%, 25%, 50%, and 75% were prepared for each.
Then, the test pieces are scraped off from these round bars, and the deviation angle of the diffraction image by selected area electron beam diffraction is measured for these test pieces. In this selected area electron beam diffraction, an area consisting of several cells of the image of each test piece magnified by a transmission electron microscope is arbitrarily selected by a selected area diaphragm, and an electron diffraction image in that area is taken as a photographic negative film. In this case, the diaphragm is moved to record the diffraction images at five positions on the same film. In addition, such an operation is performed 15 to 20 times in different areas to obtain 15 to 20 photographs each of which is composed of five diffraction images.

And from these pictures, the deviation angle of azimuth (maximum one)
Each θ is measured to obtain 15 to 20 deviation angles θ from one test piece, and the deviation angles θ are plotted on a normal probability distribution paper to obtain an average value μ of the deviation angles θ.

1 to 6 are 0%, 25%, 50%, 75%,
21 pieces of deviation angle θ data are taken for the test piece with 100% fatigue degree and the sample near the crack of the test piece with 100% fatigue degree, and the deviation angle θ and probability (= {n / (N + 1)} × 100
(%) N is the number of data, and n is an integer from 1 to 21. ) Is shown on the normal probability distribution paper, and from these graphs, the value of the deviation angle θ corresponding to the probability of 50% is obtained as the average value μ of the deviation angles.

The standard deviation value σ of the deviation angle θ is obtained from the graph. That is, in the graph of FIG.
The distance in the horizontal axis direction between points a and b on the approximate straight line corresponding to mean and μ−σ is represented as a standard deviation value σ.

The average value μ of the deviation angle θ with respect to the fatigue degree 0 to 100% thus obtained is shown in a graph as shown in FIG. 7. As shown in the figure, the average value μ of the deviation angle is approximately equal to the fatigue degree. There is a linear proportional relationship, and when the fatigue level is 100%, the difference between the above average values in the vicinity of the crack and in the portion further away from this is small.

Therefore, the average value of the deviation angle is obtained from the sample to be measured in the same manner as described above, and this average value is compared with a graph showing the relationship between the average value of the deviation angle and the fatigue degree obtained in advance as shown in FIG. Then, the fatigue level of the sample to be measured can be measured, and the life of the metal material can be determined.

Note that the relationship between the fatigue level and the standard deviation σ value of the deviation angle is as shown in FIG. 8, and as the fatigue level increases, the σ value also increases, but these values are approximately proportional, and therefore FIG. Accurate fatigue measurement can be performed based on the graph.

[Effects of the Invention] In summary, according to the present invention, the following excellent effects are exhibited.

The average value of the azimuth deviation angle of the diffraction image obtained by the selected area electron beam diffraction is in a linear proportional relationship with the fatigue degree, and at the fatigue degree of 100%, the average value of the above-mentioned average value in the vicinity of the crack and the portion apart from this Since the difference is small, the fatigue level of the sample to be measured can be accurately measured without being influenced by the sampling location.

[Brief description of drawings]

FIG. 1 is a graph in which the deviation angle is plotted on a normal probability distribution paper in order to obtain the average deviation angle of the test piece having a fatigue level of 0% in the present invention, and FIG. 2 is the deviation angle of the test piece having a fatigue level of 25%. Fig. 3 shows the average deviation angle of the test pieces with 50% fatigue, and Fig. 4 shows the average deviation angle of the test pieces with 75% fatigue. Graph to put out,
FIG. 5 is a graph for obtaining the average value of the deviation angle at 100% fatigue level, FIG. 6 is a graph for obtaining the average value of the deviation angle near the crack in the test piece having 100% fatigue level, and FIG. Fig. 8 is a graph showing the relationship between the fatigue level and the average deviation angle, Fig. 8 is a graph showing the relationship between the fatigue level and the standard deviation of the deviation angle, and Fig. 9 is the fatigue level and the X-ray half width in the conventional measurement method. It is a graph which shows the relationship with.

Continuation of front page (56) References METALLURGICAL TRA NSACTIONS A Vol. 21 A, JULY 1990, pages 1989-1996 METALLURGCAL TRANSACTIONS A Vol. 24A, OCTOBER 1993, pages 2209-2216.

Claims (1)

(57) [Claims] 1. Diffraction images of a plurality of different regions are photographed from a sample to be measured by selected area electron beam diffraction, an average value of azimuth deviation angles measured from these is determined, and from this average value and 0% fatigue level. A method for measuring the degree of fatigue of a metallic material, which comprises measuring the degree of fatigue of a sample to be measured by comparing it with an average value of azimuth deviation angles obtained in advance for samples at each stage up to 100%.