JP6418636B2 - Creep characteristic value acquisition method - Google Patents

Description

  The present invention relates to a creep characteristic value acquiring method used in a creep constitutive equation taking strain rate dependency into consideration.

In order to predict residual stress and deformation that occur when a metal processed at high temperature, such as cast products, reaches room temperature, conventionally, a constitutive equation that expresses the mechanical behavior of the metal in mathematical expressions has been used in analysis programs. .
At that time, in order to predict residual stress and deformation with high accuracy, it has been clarified that a constitutive equation that can take into consideration the strain rate dependency occurring at high temperature should be used. Although strain rate dependency can be taken into consideration by acquiring creep characteristics, conventionally, it is necessary to carry out multiple tensile tests or creep tests at any temperature for acquisition, so test time, test, or test There is a problem that the cost, the number of test pieces, and the like are required.

On the other hand, in recent years, a stress relaxation method has been proposed as a means for rapidly acquiring creep characteristics with various alloy types.
For example, a report of Tanimura et al. Of Yokohama National University (Non-Patent Document 1), a non-patent document 2, and an example of Patent Document 1 can be given. In these proposals, although the effectiveness to the creep property acquisition by the stress relaxation method was shown, there is a possibility that the value originally different from the creep property acquired by the tensile test or the creep test may be acquired.
The problems of the conventional findings related to the invention will be described below.

JP 2008-209262 A

Toshinobu Tanimura, Toshio Tatsuhiro, Tadahiro Tsujitani, Masaki Shiratori: Electronics Packaging Society Journal, Vol. 10, No. 1, 2007, Pages 52-61 “Elasto-plastic / creep / viscoplastic physical properties of solder using stress relaxation method” Acquisition efficiency " Kenichi Oguchi, Atsuko Shibata, Mitsuhiko Kimura: J. JF, Vol. 84, No. 10, 2012, Pages 569-576 “Efficient Evaluation of Elastic-Plastic-Creep Properties of Stainless Steel Cast Steel and Thermal Deformation FEM Analysis” Katsunori Abe, Hideo Yoshinaga, Shotaro Moroshi: Journal of the Metals Society of Japan, Vol. 4, Issue 4, 1976, 393-399 “Determination of internal stress and frictional stress by stress relaxation test”

Here, when the frictional stress can not be ignored for the deformation stress,

となる。 Also, if the frictional stress can be ignored for the deformation stress,

It becomes.

  Non-Patent Document 1 mentioned above reports that, in a solder alloy, by using a stress relaxation method, it is possible to reduce the number of tests required to obtain creep characteristics as compared with a conventional tensile test. However, comparison of creep strain rates before and after stress relaxation is not performed, and there is a possibility that the conditions for establishing the stress relaxation method are not satisfied.

  In Non-Patent Document 2 and Patent Document 1, it is possible to reduce the number of tests required to obtain creep characteristics by using a stress relaxation method in stainless steel cast steel as in Tanimura et al. (Non-Patent Document 1). Although we report it, we do not compare creep strain rates before and after stress relaxation, so it may not meet the conditions for stress relaxation method.

As described above, it has been proposed to use the stress relaxation method with various alloy types in order to reduce the time and cost required for acquiring the creep characteristics and the number of test pieces, but the main objective was to elucidate the deformation mechanism of the metal. Studies have shown that stress relaxation may not hold.
Therefore, the applicable range of the stress relaxation method as a substitute for creep property acquisition methods such as tensile tests is currently limited.

  In addition, in acquisition of creep characteristics using the current stress relaxation method, it is also assumed that creep strain rates before and after stress relaxation match, without considering whether the stress relaxation method is established or not. And has been implemented. Therefore, it may be possible to obtain a value different from the creep property obtained by the tensile test.

  Furthermore, since knowledge on a method for acquiring creep characteristics as an alternative to conventional tests in the case where values different from creep characteristics obtained in tensile tests are obtained does not exist either, there is a new creep characteristic for that case. Acquisition method is required.

  The present invention has been developed in view of the above-mentioned present conditions, and is a method of acquiring creep characteristic values: n value and A value used in creep constitutive equation considering strain rate dependency, and the current stress relaxation method is established. Even in the case of not being used, creep characteristics can be obtained with close accuracy by tensile test and creep test as compared with the stress relaxation method currently used, so that the various problems described above can be solved. .

  In addition, in the present invention, acquisition of creep characteristics is performed without impairing time, cost, reduction in the number of test pieces, etc., which are advantages of the stress relaxation method currently used compared to tensile tests and creep tests. Can.

  By using the present invention, it is processed at high temperature, which has the accuracy equivalent to the conventional creep property acquisition method such as tensile test and the reduction effect of time, cost and the number of test specimens that the current stress relaxation method has. It is possible to obtain a creep characteristic value for constructing a constitutive equation that simulates more accurately the residual stress and deformation generated when the metal reaches the normal temperature.

It is a schematic diagram of a stress relaxation test. Note that (a) shows the strain-time relationship, and (b) shows the stress-time relationship. It is a conceptual diagram of the relationship between the creep strain rate and stress which are obtained by the stress relaxation test when the stress relaxation method does not hold. It is the graph which showed the relationship between creep strain rate and stress in the result of a tension test and a stress relaxation test.

Hereinafter, the present invention will be specifically described.
The stress relaxation test, as shown in FIG. 1, comprises stages of tension and strain holding, and measures the strain rate immediately before stress relaxation from the time of tension. Furthermore, the creep strain rate during stress relaxation including immediately after stress relaxation is determined from the strain holding portion by stress relaxation rate and Young's modulus.

Here, Non-Patent Document 4 of RWRohde et al. Of Sandia National Laboratory, assumes that the deformation mechanism is the same during stress relaxation in the stress relaxation test and in the creep test.
In Non-Patent Document 4, n value among creep characteristic values is widely used as a judgment standard of the deformation mechanism of metal. Therefore, as long as the deformation mechanism is the same during stress relaxation and during the creep test, it is indicated that the n values obtained in the conventional tests such as the stress relaxation method and the creep test and the tensile test are the same.

R. W. Rohde, J. C. Swearengen: ASTM, 1979, Pages 21-35 "Metal Deformation Modeling-Stress Relaxation of Aluminum"

FIG. 2 is a view showing the relationship between the creep strain rate and the stress obtained from the current stress relaxation test.
The relationship between strain rate and steady state stress can be acquired at one point when the object to be measured is tensioned, but with a straight line of slope n obtained from the stress relaxation curve due to the mismatch of creep strain rate immediately before and after stress relaxation There is a gap.

Hereinafter, the procedure of the creep characteristic acquisition method of the present invention will be described.
First, a metal (alloy) which is a target object is cast as a test piece and processed into a tensile test piece shape. This shape is not particularly limited, but JIS Z 2201 No. 4 test piece is preferable.

Since the test temperature assumes implementation in a steady state creep condition that can be expressed by the stress relaxation method and a steady state creep law, the Norton Law, the melting point is generally Tm considering the temperature at which the steady state creep becomes dominant. Then, it may be selected between 0.4Tm and Tm. For example, if it is JIS ADC12 which is an aluminum die-cast alloy, it is the range of about 300 degreeC to 450 degreeC.
The strain rate at which the stress relaxation test is performed depends on the range of strain rates in the creep characteristics for acquisition, and the strain rate above that range may be selected.

  The starting strain amount of stress relaxation may be selected in the range of the strain amount reaching the steady state stress state. For example, if it is said JIS ADC12, 4.0% is in the range.

Next, the procedure for acquiring the creep characteristic value: n value and A value from the above test will be described.
First, the n value can be determined by the method of obtaining the n value of the current stress relaxation method from Non-Patent Document 4 mentioned above. Also in the present invention, the n value depends on the deformation mechanism of the metal.

And in this invention, the creep characteristic in the arbitrary temperature of a target object can be calculated | required from the following formula (2) using the said creep characteristic value: n value and A value.

  It will be shown below as an example that the present invention can obtain creep characteristics closer to the conventional test with less number of tests than the conventional test than the creep characteristics using the stress relaxation method.

First, creep characteristics were acquired by a tensile test and a stress relaxation test, which are conventional tests, targeting JIS ADC12 which is a typical aluminum die-cast alloy.
The test piece was manufactured by casting JIS ADC 12 with a copper rod shape and processing it into a tensile test piece shape.
In the tensile test, the test temperature was set to 300 ° C. and 450 ° C. at which steady-state creep becomes dominant in JIS ADC12.
Since strain rate for the purpose of acquisition of creep characteristics in a range from 1.0 × 10 ^-2 of ^{1.0 × 10 -5 / s, 1.0} × 10 -2 /s,1.0×10 -4 / s and 1.0 × 10 ^-5 / Set to s.
In the stress relaxation test, at the same test temperature, the strain rate is set to 1.0 × 10 ^-3 / s from the ease of test implementation, and after the amount of applied strain reaches 4.0% in the steady state stress, the strain is maintained and the test The stress of the piece was relaxed.

  The strain rates immediately before and after stress relaxation in this stress relaxation test are shown in Table 1.

In Table 1 to JIS ADC 12, the strain rates before and after stress relaxation do not match under the test conditions described above, so it can be judged that the conventional stress relaxation method does not hold.
Actually, looking at FIG. 3 which is the result of acquiring the relationship between the creep strain rate and the stress of each test, it is clear that the gap is a straight line representing the relationship between the creep strain rate and the stress obtained by the tensile test and the stress relaxation test. It is happening.
Therefore, as can be seen from Table 2 in which the A value and the n value are compared, the n value agrees within about a 10% error, but the A value is an about 60 to 90% error.

  Next, Table 3 shows creep characteristics obtained from the above equation (1) using the creep strain rate immediately before the stress relaxation obtained in the above test and the n value.

  From the table, it can be seen that, according to the present invention, it is possible to obtain a value close to the creep characteristics obtained in the tensile test, rather than using the conventional stress relaxation method.

In fact, the steady stress obtained by the high temperature tensile test carried out in [0035] and its stress according to the Norton law shown in the following equation (2) constructed using the conventional stress relaxation method and the creep characteristics obtained in the present invention Comparisons of calculated values are shown in Tables 4 and 5.

表４および５から、現行の応力緩和法による計算値よりも、本発明によって構築された構成式を用いた計算値の方が、より精度よく引張試験で得られる定常応力を予測できていることが分かる。

From Tables 4 and 5, it is possible to predict the steady-state stress obtained in the tensile test more accurately by the calculated value using the constitutive equation constructed according to the present invention than the calculated value by the current stress relaxation method I understand.

  Further, in the present embodiment, in the test method conventionally conducted, although the number of test pieces required is four and the test time is about 2000 seconds to obtain the creep characteristics at an arbitrary temperature, The number of test pieces was only one for creep property acquisition and the test time was about 1000 seconds, and it was confirmed that it was possible to reduce the time required for creep property acquisition, cost, the number of test pieces, etc. doing.

  By using the present invention, it is possible to more simply and accurately predict thermal stress, residual stress, deformation, etc. generated when performing casting and hot plastic working, so that test time, test cost, It becomes possible to solve the problem concerning the number of test pieces etc.

Claims (1)

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