Method for calculating stress-strain diagrams

Abstract

To calculate a stress distortion diagram with respect to a material whose yielding point is not observed.SOLUTION: A method for calculating a stress distortion diagram from a result of tensile test with respect to a sample includes the steps of: identifying a reduction start point when a reduction of a change rate of a load with respect to a displacement starts in the tensile test based on load and displacement data obtained in the tensile test; calculating the stress distortion diagram when the displacement is larger than that at the reduction start point based on the data after the reduction start point; and calculating the stress distortion diagram when the displacement is smaller than that at the reduction start point based on a Young's modulus of the sample.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The technology disclosed in this specification relates to a method for calculating a stress-strain diagram.

[0002] Patent Document 1 discloses a method for calculating a stress-strain diagram from the results of a tensile test. In this calculation method, the yield point in the unloading process is calculated, and the stress-strain diagram is calculated based on the yield point.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] With the technology of Patent Document 1, it was not possible to calculate a stress-strain diagram for a material in which a clear yield point was not observed. In this specification, a technology capable of calculating a stress-strain diagram for a material in which a yield point is not observed is proposed.

Means for Solving the Problems

[0005] This specification proposes a method for calculating a stress-strain diagram from the results of a tensile test on a sample. This calculation method includes a step of specifying a decrease start point at which the rate of change of the load with respect to the displacement in the tensile test starts to decrease from the load and displacement data obtained by the tensile test, a step of calculating a stress-strain diagram when the displacement is greater than the decrease start point from the data after the decrease start point, and a step of calculating a stress-strain diagram when the displacement is smaller than the decrease start point from the Young's modulus of the sample.

[0006] This technique identifies the point at which the rate of change of load relative to displacement begins to decrease, based on tensile test data. This point is considered the point where the atomic structure transitions and the material begins plastic deformation. Next, the stress-strain diagram is calculated from data after the point at which the decrease begins, specifically for the period when the displacement is greater than the point at which the decrease begins (i.e., the plastic region). Furthermore, this method calculates the stress-strain diagram for the period when the displacement is smaller than the point at which the decrease begins (i.e., the elastic region), based on the Young's modulus of the sample. Therefore, this method allows for the calculation of stress-strain diagrams from the elastic region to the plastic region. This method also allows for the calculation of stress-strain diagrams even when the yield point is not observed. [Brief explanation of the drawing]

[0007] [Figure 1] A graph showing the changes in load and displacement over time in a tensile test. [Figure 2] A graph showing the change over time in the rate of change of load displacement during a tensile test. [Figure 3] A graph showing the calculated stress-strain diagram. [Modes for carrying out the invention]

[0008] Figure 1 shows the results of measuring the load P (i.e., the force applied to the sample in the tensile direction) and displacement L (i.e., the stroke in the tensile direction) when a tensile test was performed on a sample. The upper graph of Figure 1 is the load P, and the lower graph of Figure 1 is the change in displacement L. Note that the dashed line graph in Figure 1 represents the data measured in the tensile test. As shown in Figure 1, immediately after the start of the tensile test, a small elongation occurs in the test equipment body and the clamps holding the sample, so the rise of the load P is delayed. In addition, the data obtained from the tensile test contains small noise due to errors in the test equipment and coil. In the stress-strain diagram calculation method of the example, first, a graph is calculated in which the noise is removed by smoothing the data obtained from the tensile test using a moving average or the like. Each graph shown as a solid line in Figure 1 represents the smoothed graph of the change in load P and displacement L. As shown in Figure 1, in the tensile test, the change in displacement L is maintained at approximately constant. That is, in the tensile test, displacement occurs at approximately constant speed.

[0009] Next, based on the smoothed load P and displacement L data, a graph showing the change over time of the rate of change dP / dL of load P with respect to displacement L is calculated, as shown in Figure 2. In this embodiment, as shown in Figure 1, the amount of change in displacement L is approximately constant, so the rate of change dP / dL is approximately equal to the rate of change of load P with respect to time (i.e., the slope of the load P graph in Figure 1). Also, Figure 2 shows a graph created using unsmoothed data (dashed line) and a graph created using smoothed data (solid line). As shown in Figure 2, by using smoothed data, it is possible to create a graph of the change over time of the rate of change dP / dL with noise removed. Next, based on the graph of the rate of change dP / dL calculated using the smoothed data, the point at which the rate of change dP / dL began to decrease in the tensile test (hereinafter referred to as the decrease initiation point X) is identified. The decrease initiation point X is the point at which the rate of change dP / dL reaches its maximum value or local maximum value. The decrease initiation point X can be considered as the point at which the sample begins plastic deformation in the tensile test. Once the starting point X of the decrease is identified, the time t1 at that point is determined. Note that depending on the smoothing method, the features of the original graph (the graph containing noise) may disappear, and the starting point X may be identified in an inappropriate location. Therefore, as shown in Figure 2, it is preferable to compare the graphs before and after smoothing to confirm that the starting point X is appropriate.

[0010] Next, based on the load P and displacement L data from time t1 onward (see Figure 1), the stress-strain diagram G1 is calculated when the displacement L (i.e., strain) is greater than the decrease starting point X, as shown in Figure 3. This method allows for the accurate calculation of the stress-strain diagram in the plastic region.

[0011] Next, based on the Young's modulus of the sample, the stress-strain diagram G2 is calculated when the displacement L is smaller than the starting point X of the decrease. The Young's modulus may be a known value or a separately measured value. The stress-strain diagram G2 is calculated as a straight line based on the Young's modulus. Furthermore, the stress-strain diagram G2 is created to be continuous with the stress-strain diagram G1 at the starting point X of the decrease. This method allows for the accurate calculation of the stress-strain diagram in the elastic region. Additionally, by connecting the stress-strain diagram G2 to the stress-strain diagram G1 in this way, it is possible to calculate the stress-strain diagram covering the range from the elastic region to the plastic region.

[0012] As described above, according to the calculation method of the embodiment, it is possible to calculate a stress-strain diagram spanning from the elastic region to the plastic region, regardless of whether or not a yield point is observed in the tensile test.

[0013] Although embodiments have been described in detail above, these are merely illustrative and do not limit the scope of the claims. The technologies described in the claims include various modifications and changes to the specific examples illustrated above. The technical elements described in this specification or drawings exhibit technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technologies illustrated in this specification or drawings achieve multiple objectives simultaneously, and achieving even one of these objectives constitutes technical usefulness.

Claims

[Claim 1] A method for calculating a stress-strain diagram from the results of a tensile test on a sample, A step of identifying the starting point of the decrease in the rate of change of the load with respect to the displacement in the tensile test, based on the load and displacement data obtained from the tensile test, A step of calculating a stress-strain diagram when the displacement is greater than the reduction starting point, using data from after the reduction starting point; A step of calculating a stress-strain diagram when the displacement is smaller than the starting point of the decrease, based on the Young's modulus of the sample. A method of having.

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