A method for judging the characteristics of the force-strain curve that can be used to determine the cyclic plastic zone

Abstract

A method for judging the characteristics of the force-strain curve that can be used to determine the cyclic plastic zone. By using the characteristics of the stress-strain hysteresis loop in the cyclic plastic zone, the characteristics of the force-strain curve are judged. The force-strain curve is abstracted and simplified into two straight lines, and the characteristics of the force-strain curve are judged according to the relationship and position of the two straight lines, so as to separate the elastic zone, the monotonic plastic zone, and the cyclic plastic zone. The present invention can be used for the detection of the cyclic plastic zone.

Description

Technical Field

[0001] The present invention belongs to the field of metal fatigue tests, and provides a method for judging the force-strain curve characteristics of a cyclic plastic zone, which is applicable to the separation of the cyclic plastic zone, monotonic plastic zone, and elastic zone at the crack tip during the fatigue test of Q&P steel. Background Art

[0002] There is plastic deformation at the fatigue crack tip. The area where plastic deformation occurs is called the plastic zone, and other areas are called the elastic zone. The plastic zone can be further divided into a cyclic plastic zone and a monotonic plastic zone. The cyclic plastic zone is wrapped by the monotonic plastic zone. The cyclic plastic zone contains rich information and is of great significance for studying crack propagation characteristics. However, the cyclic plastic zone is very small, and existing detection methods all rely on expensive equipment and are cumbersome to operate. The present invention uses the characteristics of the stress-strain hysteresis loop in the cyclic plastic zone to judge the force-strain curve characteristics, thereby separating the cyclic plastic zone. Summary of the Invention

[0003] In order to overcome the deficiencies of the prior art, the present invention provides a method for judging the force-strain curve characteristics of a cyclic plastic zone, which can effectively judge the area to which it belongs according to the force-strain curve characteristics.

[0004] The technical solution adopted by the present invention to solve its technical problems is:

[0005] A method for judging the force-strain curve characteristics of a cyclic plastic zone, the method comprising the following steps:

[0006] Step 1: Count the proportion of data points with strain less than the yield point threshold. If the proportion is greater than the preset value, it is determined to be the elastic zone; otherwise, proceed to the next step;

[0007] Step 2: Group the collected force-strain data into a loading group and an unloading group;

[0008] Step 3: Subtract the strain data of the unloading group from the strain data of the loading group pairwise, count the number of point pairs cnt with a difference less than 0, and count the average difference avrDiff;

[0009] Step 4: Perform linear fitting on the unloading group and the loading group respectively to obtain the slopes k1, k2, intercepts b1, b2, and confidence levels r1, r2;

[0010] Step 5: Judge whether cnt is 0, whether avrDiff is greater than 0.001, and whether r1 and r2 are >0.98. If all the above conditions are met, it is determined to be the cyclic plastic zone; otherwise, proceed to the next step;

[0011] Step 6: Determine whether (b1 + b2) / 2 is greater than 0.002. If it is satisfied, it is determined as the monotonic plastic region; otherwise, proceed to the next step and determine it as the elastic region.

[0012] Further, in the said Step 1, the preset value is 80%, and a strain of 0.002 is used as the yield point. If the number of data points less than 0.002 in the collected data accounts for the majority, it is considered that plastic deformation has not occurred at this position, and it is determined as the elastic region.

[0013] Further, in the said Step 3, subtract the strain data of the unloading group and the loading group pairwise. Taking the unloading group as the reference, traverse each pair of data in the unloading group, find the data point pairs with equal force values in the loading group, subtract the strain value of the loading group from the strain value of the unloading group. After the traversal, obtain the number of point pairs cnt with a difference less than 0 and the average difference avrDiff.

[0014] Still further, in the said Step 4, perform linear fitting on the unloading group and the loading group respectively, and the RANSAC linear fitting algorithm is used. In the collected original data, the data of the loading group and the unloading group are connected end to end, and only the middle section shows good linearity, and there are noise points in the data. Using the RANSAC linear fitting algorithm can ensure that only the middle section data is selected for fitting and the noise points that have a greater impact on the fitting result are removed, so that the linear fitting result meets the actual situation.

[0015] Even further, in the said Step 5, judge the number of point pairs cnt with a difference less than 0, the average difference avrDiff, the slopes k1, k2, the intercepts b1, b2, and the confidence levels r1, r2. Judge whether cnt is 0, whether avrDiff is greater than 0.001 to distinguish whether the straight line fitted by the unloading group is above the straight line fitted by the loading group and is higher by a certain distance, and judge whether r1, r2 are >0.98 to distinguish whether the data has sufficient linearity. Only when the linearity is sufficient can it be determined as the cyclic plastic region.

[0016] The beneficial effects of the present invention are mainly manifested in: According to the different characteristics of the stress-strain hysteresis loops in different regions (elastic region, monotonic plastic region, cyclic plastic region), analyze the force-strain curve, and abstract and simplify it into two straight lines. According to the relationship and position of the two straight lines, judge the characteristics of the force-strain curve, so as to separate the elastic region, the monotonic plastic region, and the cyclic plastic region. Description of the Drawings

[0017] Figure 1 It is a flow chart of the method for judging the characteristics of the force-strain curve that can be used to judge the cyclic plastic region. Detailed Embodiments

[0018] The present invention will be further described below with reference to the drawings.

[0019] Reference Figure 1 , a method for judging the characteristics of the force-strain curve of the cyclic plastic zone, comprising the following steps:

[0020] Step 1: Count the proportion of data points with a strain less than 0.002. If the proportion is greater than 80%, it is determined as the elastic zone; otherwise, proceed to the next step;

[0021] In the above Step 1, the preset value is 80%, and a strain of 0.002 is used as the yield point. If the number of points less than 0.002 in the collected data accounts for the majority, it is considered that plastic deformation has not occurred at this position, and it is determined as the elastic zone.

[0022] Step 2: Group the collected force-strain data into a loading group and an unloading group;

[0023] Step 3: Subtract the strain data of the unloading group and the loading group pairwise, count the number of point pairs cnt with a difference less than 0, and count the average difference avrDiff;

[0024] In the above Step 3, subtract the strain data of the unloading group and the loading group pairwise. Taking the unloading group as the reference, traverse each pair of data in the unloading group, find the data point pairs with equal force values in the loading group, subtract the strain value of the loading group from the strain value of the unloading group. After the traversal, obtain the number of point pairs cnt with a difference less than 0 and the average difference avrDiff.

[0025] Step 4: Perform linear fitting on the unloading group and the loading group respectively to obtain the slopes k1, k2, intercepts b1, b2, and confidence levels r1, r2;

[0026] In the above Step 4, perform linear fitting on the unloading group and the loading group respectively, and the RANSAC linear fitting algorithm is used. In the collected original data, the data of the loading group and the unloading group are connected end to end, and only the middle section shows good linearity, and there are noise points in the data. Using the RANSAC linear fitting algorithm can ensure that only the middle section data is selected for fitting and the noise points that have a greater impact on the fitting result are removed, so that the linear fitting result meets the actual situation.

[0027] Step 5: Judge whether cnt is 0, whether avrDiff is greater than 0.001, and whether r1 and r2 are > 0.98. If all the above conditions are met, it is determined as the cyclic plastic zone; otherwise, proceed to the next step;

[0028] In step 5, judge the number of point pairs cnt with a difference less than 0, the average difference avrDiff, the slopes k1, k2, the intercepts b1, b2, and the confidence levels r1, r2. Judge whether cnt is 0, and whether avrDiff is greater than 0.001 to distinguish whether the line fitted by the unloading group is above the line fitted by the loading group and is higher by a certain distance. Judge whether r1 and r2 are > 0.98 to distinguish whether the data has sufficient linearity. Only when the linearity is sufficient can it be determined as the cyclic plastic region.

[0029] Step 6: Judge whether (b1 + b2) / 2 is greater than 0.002. If it is satisfied, it is determined as the monotonic plastic region; otherwise, go to the next step to be determined as the elastic region.

[0030] The content described in the embodiments of this specification is only an enumeration of the implementation forms of the inventive concept and is only for illustrative purposes. The protection scope of the present invention should not be regarded as limited to the specific forms stated in this embodiment. The protection scope of the present invention also extends to equivalent technical means that can be conceived by those of ordinary skill in the art according to the inventive concept of the present invention.

Claims

1. A method for judging the characteristics of the force-strain curve of a cyclic plastic zone, characterized in that, The method includes the following steps: Step 1: Count the proportion of data points with strain less than the yield point threshold. If the proportion is greater than the preset value, it is determined as the elastic region; otherwise, proceed to the next step; Step 2: Group the collected force-strain data into a loading group and an unloading group; Step 3: Subtract the strain data of the unloading group and the loading group pairwise. Taking the unloading group as the reference, traverse each pair of data in the unloading group, find the data point pairs with equal force values in the loading group, subtract the strain value of the loading group from the strain value of the unloading group. After traversing, obtain the number of point pairs cnt with a difference less than 0 and the average difference avrDiff; Step 4: Perform linear fitting on the unloading group and the loading group respectively to obtain the slopes k1, k2, intercepts b1, b2, and confidence levels r1, r2; Step 5: Judge whether cnt is 0, whether avrDiff is greater than 0.001, and whether r1, r2 are > 0.

98. If all the above conditions are met, it is determined as the cyclic plastic region; otherwise, proceed to the next step; Step 6: Judge whether (b1 + b2) / 2 is greater than 0.

002. If it is satisfied, it is determined as the monotonic plastic region; otherwise, proceed to the next step to be determined as the elastic region.

2. The method for judging the characteristics of the force-strain curve of a cyclic plastic zone according to claim 1, characterized in that, In the said Step 1, the preset value is 80%, and a strain of 0.002 is used as the yield point.

3. The method for judging the characteristics of the force-strain curve of a cyclic plastic zone according to claim 1 or 2, characterized in that, In the said Step 4, when performing linear fitting on the unloading group and the loading group respectively, the RANSAC linear fitting algorithm is used.

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