Stress distortion diagram calculation method

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 during 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] JP 2003-194686 A Summary of the Invention [Problem to be solved by the invention]

[0004] The technique of Patent Document 1 was unable to calculate a stress-strain diagram for a material in which a clear yield point was not observed. This specification proposes a technique that makes it possible to calculate a stress-strain diagram for a material in which a yield point was not observed. [Means for solving the problem]

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

[0006] In this technique, a decrease start point where the rate of change of load with respect to displacement starts to decrease is identified from the data of the tensile test. The decrease start point is the point where the atomic structure transitions, and can be considered as the point where the material starts to undergo plastic deformation. Next, a stress-strain curve when the displacement is larger than the decrease start point (i.e., the plastic region) is calculated from the data after the decrease start point. This calculation method also calculates a stress-strain curve when the displacement is smaller than the decrease start point (i.e., the elastic region) from the Young's modulus of the sample. Therefore, this calculation method can calculate a stress-strain curve from the elastic region to the plastic region. This calculation method can calculate a stress-strain curve even when a yield point is not observed. [Brief description of the drawings]

[0007] [Figure 1] Graph showing changes in load and displacement over time in a tensile test. [Diagram 2] 1 is a graph showing the change over time in the rate of change of load relative to displacement in a tensile test. [Diagram 3] Graph showing calculated stress-strain curve. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0009] Next, based on the data of the load P and the displacement L after smoothing, a graph showing the change over time of the rate of change dP / dL of the load P with respect to the displacement L is calculated as shown in FIG. 2. In this embodiment, since the change amount of the displacement L is substantially constant as shown in FIG. 1, the rate of change dP / dL is substantially equal to the rate of change of the load P with respect to time (i.e., the slope of the graph of the load P in FIG. 1). Also, FIG. 2 shows a graph (dashed line) created using the data that has not been smoothed, and a graph (solid line) created using the smoothed data. As shown in FIG. 2, by using the smoothed data, a graph of the change over time of the rate of change dP / dL can be created in a state in which noise has been removed. Next, based on the graph of the rate of change dP / dL calculated using the smoothed data, a point at which the rate of change dP / dL starts to decrease in the tensile test (hereinafter, referred to as the decrease starting point X) is specified. The decrease starting point X is the point at which the rate of change dP / dL becomes a maximum value or a local maximum value. The decrease starting point X can be regarded as the point at which the sample starts to undergo plastic deformation in the tensile test. Once the decrease start point X is identified, the time t1 at the decrease start point X is identified. Depending on the smoothing method, the characteristics of the original graph (graph including noise) may disappear, and the decrease start point X may be identified at an inappropriate position. Therefore, as shown in FIG. 2, it is preferable to compare the graphs before and after smoothing to confirm that the decrease start point X is appropriate.

[0010] Next, based on the data of the load P and the displacement L after time t1 (see Fig. 1), a stress-strain curve G1 is calculated when the displacement L (i.e., the strain) is larger than the decrease start point X, as shown in Fig. 3. This method makes it possible to accurately calculate the stress-strain curve in the plastic region.

[0011] Next, a stress-strain curve G2 when the displacement L is smaller than the decrease start point X is calculated based on the Young's modulus of the sample. The Young's modulus may be a known value or a value measured separately. The stress-strain curve G2 is calculated as a straight line based on the Young's modulus. The stress-strain curve G2 is also created so as to be continuous with the stress-strain curve G1 at the decrease start point X. This method allows the stress-strain curve in the elastic region to be calculated accurately. Furthermore, by connecting the stress-strain curve G2 to the stress-strain curve G1 in this way, the stress-strain curve in the range from the elastic region to the plastic region can be calculated.

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

[0013] Although the embodiments have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes to the specific examples exemplified above. The technical elements described in this specification or drawings exhibit technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in this specification or drawings achieves multiple objectives simultaneously, and achieving one of the objectives itself has technical utility.

Claims

[Claim 1] A method for calculating a stress-strain diagram from the results of a tensile test on a sample, comprising the steps of: identifying a decrease start point at which a rate of change of the load with respect to the displacement starts to decrease in the tensile test from load and displacement data obtained by the tensile test; calculating a stress-strain curve when the displacement is larger than that at the decrease starting point from the data after the decrease starting point; Calculating a stress-strain curve when the displacement is smaller than the decrease starting point from the Young's modulus of the sample; The method according to claim 1,

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