JP2008209262A - Quick evaluation method of elasticity, plasticity, and creep characteristic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a quick evaluation method of elasticity, plasticity, and a creep characteristic capable of executing evaluation of elasticity, plasticity, and creep characteristic wherein several kinds of tensile tests under strain rate and several kinds of creep test under retention stress are required to be executable hitherto, only by one kind of a step wave load test that comprises an instantaneous load part and a strain retention part. <P>SOLUTION: The step wave load test wherein an instantaneous load and strain retention are repeated is executed, and a stress-elastic/plastic strain curve is acquired from a stress-strain curve, corresponding to the instantaneous load part; and a material constant relative to an elastic/plastic characteristic is derived from the stress-elastic/plastic strain curve, and a relation between the stress and the creep strain rate is acquired from a stress mitigation curve. Then, the ratio between transient creep strain rate and steady creep strain rate is acquired from a repeated stress mitigation curve, and the material constants of a steady creep rule and a transient creep rule are derived, from the relation between the stress and the creep strain rate or the ratio between the transient creep strain rate and the steady creep strain rate. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a rapid evaluation method for elastic / plastic / creep characteristics, which makes it possible to perform an elastic / plastic / creep characteristic evaluation by only one type of staircase load test composed of an instantaneous load section and a strain holding section.

Conventionally, in order to reduce the calculation cost, the opportunity for executing the elastic / plastic / creep finite element analysis is increasing also in the field of equipment design, which is based solely on the elastic / plastic finite element analysis. This is because a higher level of safety is required for a device that is predicted to have a site where creep deformation occurs.
Another reason is that the advancement of computational technology has made it possible to perform elastic / plastic / creep finite element analysis in a shorter time than before.
In order to perform elastic / plastic / creep finite element analysis, it is necessary to investigate the elastic / plastic / creep characteristics of the material to be analyzed and to determine the material constants that reflect the characteristics.
These material constants need to be determined from tensile tests with several strain rates and creep tests with several holding stresses.
In addition, when considering the temperature dependence of deformation characteristics, these tests must be performed at multiple temperatures, and many more tests are required before conducting an elasto-plastic-creep finite element analysis. Implementation is essential.
In particular, the creep test often takes a long time, so performing many such tests requires enormous amounts of time to determine the material constants and perform the elastic / plastic / creep finite element analysis. means.
In other words, even if it becomes possible to execute elastic / plastic / creep analysis at high speed due to the development of calculation technology, if the creep test time until analysis is not shortened, elastic / plastic / creep analysis is executed. The hurdles to remain remain high.
Based on the above, it is hoped that a rapid evaluation method for elastic, plastic, and creep properties that can quickly evaluate the elastic, plastic, and creep properties of materials from very few experiments and accurately derive material constants that reflect these properties is hoped for. It is rare.

In addition, as a publicly known technique, in an electronic device such as an IC package in which an IC (integrated circuit) chip is mounted on a printed circuit board, an electronic device that can easily and accurately evaluate reliability by obtaining the number of life cycles easily and accurately. A reliability evaluation method and a reliability evaluation apparatus thereof are known (see Patent Document 1).
This known technology performs an accelerated test that is exposed to a periodic temperature condition for a specific electronic device, that is, a temperature cycle test, and a universal relationship between the number of life cycles and strain amplitude for all electrical devices, that is, a life strain relationship. Obtain the equation, perform thermal stress simulation on the analysis model of any electronic device, calculate the strain amplitude, and then substitute the strain amplitude for the analysis model of any electronic device into the lifetime strain relational expression, The life cycle number of an analysis model of an arbitrary electronic device is obtained.

JP 2000-46905 A

  In the present invention, an elastic, plastic, and creep property evaluation that has conventionally had to be carried out with a tensile test under several strain rates and a creep test with several holding stresses was performed from an instantaneous load section and a strain holding section. An object of the present invention is to provide a rapid evaluation method for elastic, plastic, and creep characteristics that can be executed only by one type of staircase load test.

  The rapid evaluation method for elastic / plastic / creep characteristics of the present invention performs a step wave load test that repeats instantaneous load and strain retention, and calculates a stress-elasto-plastic strain curve from the stress-strain curve corresponding to the instantaneous load part. A first step of acquiring, a second step of deriving a material constant relating to elastic / plastic properties from the stress-elastic-plastic strain curve, a third step of acquiring a relationship between stress and creep strain rate from the stress relaxation curve, From the fourth step of obtaining the ratio of the transition creep strain rate to the steady creep strain rate from the cyclic stress relaxation curve, and the information obtained in the third step and the fourth step, the material constants of the steady creep law and the transition creep law are obtained. And a fifth step to be derived.

  In the first step, only the elasto-plastic deformation part is extracted from all the instantaneous loads of the staircase wave load, and these are connected to obtain the stress-elastic-plastic strain curve necessary for evaluating the elasto-plastic characteristics. To do.

  In the second step, the Young's modulus and the plastic tangent coefficient of the material constant representing the elastic / plastic characteristics are obtained from the stress-elastic-plastic strain relationship.

  In the third step, the relationship between stress and creep strain rate, which is indispensable for evaluating creep characteristics, is obtained from the relationship between stress and stress rate obtained from the stress relaxation curve.

  In the fourth step, a stress relaxation curve is applied to the third step, the relationship between stress and creep strain rate is obtained, and the third step is applied to a plurality of stress relaxation curves constituting the repeated stress relaxation curve. The relationship between the stress of each stress relaxation curve and the creep strain rate is obtained, the steady creep strain rate is calculated during the creep strain rate of the obtained plurality of stress relaxation curves, and the creep strain rate and the steady creep strain rate are calculated. The transition creep strain rate is calculated from the difference, and the ratio between the transition creep strain rate and the steady creep strain rate is calculated using the calculated steady creep strain rate and transition creep strain rate.

  The rapid evaluation method of elastic / plastic / creep characteristics of the present invention can quickly evaluate the elastic / plastic / creep characteristics of a material from a very small number of experiments and accurately derive a material constant reflecting the characteristics.

One embodiment of the rapid evaluation method for elastic / plastic / creep characteristics of the present invention will be described below with reference to the drawings.
FIG. 1 is a conceptual model diagram of a staircase wave load test of the rapid evaluation method for elastic / plastic / creep characteristics of the present invention.
In FIG. 1, the IS part of the staircase load test corresponds to the instantaneous load part, and the MS part corresponds to the strain holding part.
In the IS part, since the strain increment Δε _is is given instantaneously, the influence of creep is eliminated in the corresponding stress-strain relationship.
Stress relaxation occurs in the MS part.
In this step wave load test, when the strain ε _end is reached, the strain ε _end is held for Δt _end and a stress relaxation curve is acquired over a longer time than the MS portion.
Table 1 shows examples of staircase wave load test conditions, ε _end = 4.4 × 10 ⁻² , Δt _end = 600 sec.

FIG. 2 is a stress-strain curve obtained in a step wave load test.
The increase / decrease in stress at the initial portion indicated by a circle in FIG. 2 is caused by repeated increase in stress due to instantaneous load and stress relaxation due to strain retention as shown in the repeated stress relaxation curve in FIG.
Further, the decrease in stress at the terminal end indicated by a circle is caused by stress relaxation as shown in the stress relaxation curve of FIG. 4 generated by holding the strain during Δt _end with strain ε _end .

In the present invention, the elastic-plastic / creep characteristics are evaluated by applying the stress-strain curve of FIG. 2 and the stress relaxation curves of FIGS. 3 and 4 to the following five techniques.
(1) A technique for obtaining a stress-elasto-plastic strain curve from a stress-strain curve corresponding to an instantaneous load portion.
(2) A technique for deriving material constants relating to elastic / plastic properties from the stress-elastic-plastic strain curve.
(3) Technology for obtaining the relationship between stress and creep strain rate from a stress relaxation curve.
(4) A technique for obtaining a ratio between a transition creep strain rate and a steady creep strain rate from a cyclic stress relaxation curve.
(5) A technique for deriving the material constants of the steady creep law and the transition creep law from the information obtained in (3) and (4) above.
The details of the above five techniques are as follows.

図６の丸印で示す曲線は、この処理を図２の応力−ひずみ曲線に適用して取得した応力−弾塑性ひずみ曲線に相当する。

A curve indicated by a circle in FIG. 6 corresponds to a stress-elastic-plastic strain curve obtained by applying this process to the stress-strain curve of FIG.

Technique (2): The Young's modulus and plastic tangent coefficient of material constants representing elastic / plastic properties are obtained from the stress-elasto-plastic strain relationship.
In particular, if the plastic tangent coefficient is acquired as a function of stress or strain, an accurate deformation simulation is possible.
In this technology, the stress-elastic / plastic strain curve obtained in FIG. 6 is expressed by the Ramberg-Osgood law, which represents the elastic-plastic strain as the sum of elastic strain and plastic strain, as shown in FIG. Approximate.
The Ramberg-Osgood law is given in the form of number 1.

また、右辺第１、２項目は、それぞれ弾性ひずみと塑性ひずみに相当する。
したがって、応力と塑性ひずみの関係を表す塑性接線係数Hは、右辺第２項目を微分すれば、数２のように応力の関数として得られる。

The first and second items on the right side correspond to elastic strain and plastic strain, respectively.
Therefore, the plastic tangent coefficient H representing the relationship between the stress and the plastic strain can be obtained as a function of the stress as shown in Equation 2 by differentiating the second item on the right side.

The approximation process by the Ramberg-Osgood rule is performed in the following order.
(i) The Young's modulus E is determined by linearly approximating the relationship between stress and strain in a low stress region where the stress and strain are in a linear relationship.
(ii) The reference plastic strain is set appropriately (ε ₀ = 5.0 × 10 ^{−4 in} the approximation of FIG. 7), and the corresponding reference stress D is determined.
(iii) The hardening index m is calculated using the values of E and D determined in (i) and (ii) and the stress and strain values at arbitrary points on the stress-elastic / plastic strain curve.
The Young's modulus and plastic tangent coefficient can be obtained through the approximation process based on the above Ramberg-Osgood rule.

Technique (3): In stress relaxation by strain retention, the amount of increase in creep strain is balanced with the amount of decrease in elastic strain.
In this technology, paying attention to this, the “relationship between stress and creep strain rate” which is indispensable for evaluating the creep characteristics is obtained from the “relationship between stress and stress rate” obtained from the stress relaxation curve.
The “relationship between stress and stress rate” is obtained by calculating the stress rate from the slope of the tangent as shown in FIG. 8 at a plurality of points on the stress relaxation curve.
The calculated stress rate is applied to the relationship between the stress rate of Equation 3 and the creep strain rate.

応力緩和曲線上の複数の応力点で取得した応力速度を数３に適用すれば、「応力とクリープひずみ速度の関係」が得られる。

If the stress rates acquired at a plurality of stress points on the stress relaxation curve are applied to Equation 3, the “relationship between stress and creep strain rate” can be obtained.

  The transition creep strain rate is given by a type proportional to the steady creep strain rate, as shown in Equation 5.

The proportionality coefficient C ₁ in Equation 5 is expressed by Equation 6 as a function that becomes zero when the transition creep strain develops.

The technique (4) is for obtaining the “ratio between the transition creep strain rate and the steady creep strain rate” necessary for formulating the equations (5) and (6).
The “ratio between the transition creep strain rate and the steady creep strain rate” is obtained by the following procedure.

(i) the strain epsilon _{end The} by holding the strain between Delta] t _{end The} in stress relaxation curve obtained in (see FIGS. 1 and 2), since the creep strain is fully developed, creep deformation occurs only in the steady creep.
Therefore, the stress relaxation curve obtained here is applied to the technique (3) to obtain the “relationship between stress and creep strain rate”.
Then, this relationship is plotted as shown in FIG. 9, and a steady creep law is formulated from the approximate curve of this relationship.
In FIG. 9, the Norton law of Formula 7 is used as the steady creep law.

(iv) Using the steady creep strain rate and the transition creep strain rate calculated in (iii), calculate the “ratio between the transition creep strain rate and the steady creep strain rate”.

Technique (5): The material constant of the steady creep rule is determined by the procedure (i) of technique (4).
The transition creep strain law plots the relationship between the “transition creep strain rate and steady creep strain rate” obtained by the technique (4) and the transition creep strain at the time when the ratio is obtained as shown in FIG. The formulation is formulated by determining the value of the constant in Equation 6 from the approximate curve.

It is a conceptual model figure of the staircase wave load test of the rapid evaluation method of the elastic / plastic / creep characteristics of the present invention. It is a model drawing of the stress-strain curve obtained by a staircase load test. It is a model drawing of a repetitive stress relaxation curve. It is a model drawing of a stress relaxation curve. It is a schematic diagram of the stress-strain curve which repeats increase / decrease in stress by a staircase wave load. It is a schematic diagram of a stress-elastic / plastic strain curve. It is an approximate schematic diagram of a stress-elastic / plastic strain curve. It is a schematic diagram of the stress rate during stress relaxation. It is a schematic diagram of the relationship between the creep strain rate and stress acquired from the stress relaxation curve. It is a schematic diagram of a repetitive stress relaxation curve. It is a schematic diagram of the relationship between the ratio of transition creep strain rate and steady creep strain rate, and transition creep strain.

Claims (5)

  From the stress-elasto-plastic strain curve, a first step of performing a staircase load test that repeats instantaneous load and strain retention, and obtaining a stress-elasto-plastic strain curve from the stress-strain curve corresponding to the instantaneous load portion, and The second step of deriving material constants for elastic and plastic properties, the third step of obtaining the relationship between stress and creep strain rate from the stress relaxation curve, and the transient creep strain rate and steady state creep strain rate from the cyclic stress relaxation curve A fourth step of obtaining the ratio of the above, and a fifth step of deriving the material constants of the steady creep law and the transition creep law from the information obtained in the third step and the fourth step. Rapid evaluation method for plastic and creep properties.   In the first step, only the elasto-plastic deformation part is extracted from all the instantaneous loads of the staircase wave load, and these are connected to obtain the stress-elastic-plastic strain curve necessary for evaluating the elasto-plastic characteristics. The method for quickly evaluating elastic / plastic / creep characteristics according to claim 1.   The rapid evaluation of elastic / plastic / creep characteristics according to claim 1, wherein in the second step, the Young's modulus and the plastic tangent coefficient of the material constant representing the elastic / plastic characteristics are obtained from a stress-elastic-plastic strain relationship. Method.   The relationship between stress and creep strain rate, which are indispensable for evaluating creep characteristics, is obtained from the relationship between stress and stress rate obtained from the stress relaxation curve in the third step. Rapid evaluation method for elastic, plastic and creep properties.   In the fourth step, a stress relaxation curve is applied to the third step, the relationship between stress and creep strain rate is obtained, and the third step is applied to a plurality of stress relaxation curves constituting the repeated stress relaxation curve. The relationship between the stress of each stress relaxation curve and the creep strain rate is obtained, the steady creep strain rate is calculated during the creep strain rate of the obtained plurality of stress relaxation curves, and the creep strain rate and the steady creep strain rate are calculated. The transition creep strain rate is calculated from the difference, and the ratio between the transition creep strain rate and the steady creep strain rate is calculated using the calculated steady creep strain rate and the transition creep strain rate. Rapid evaluation method for elastic, plastic and creep properties.

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