JP2006266964A - Strain control type super-high cycle fatigue testing method and fatigue testing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a strain control type super-high cycle fatigue testing method and a fatigue testing apparatus therefor, which can carry out a super-high cycle fatigue test for a short period of time. <P>SOLUTION: The testing method comprises; a first step of setting at least a control velocity f and the total strain range Δεt across referring points of a test piece 1; a second step of obtaining a target control range ΔLt of a noncontact type displacement gauge 20 corresponding to the set value of the total strain range across the referring points of the test piece; and a third step of executing a fatigue test under a strain control of the noncontact type displacement gauge while repeatedly applying loads on the test piece at the control velocity f such that the amplitude of a measurement value of the noncontact type displacement gauge is kept constant within the target control range ΔLt. In the second step, a measurement value of the noncontact type displacement gauge and a measurement value of a contact type strain gauge 10 are acquired while repeatedly applying the loads on the test piece at a strain velocity at which the contact type strain gauge can follow motions, and a correlation between both gauges is obtained from those measurement values, and the target control range ΔLt is obtained based on the correlation. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fatigue test method and fatigue test apparatus by strain control suitable for use in evaluating fatigue damage in an ultra-high cycle region.

In a high cycle fatigue test of steel materials, a horizontal portion appears in the SN curve and a fatigue limit is generally recognized. For this reason, the design of equipment structures and the like used semipermanently is designed based on the fatigue limit. However, according to research examples of fatigue tests using load control in recent years, in high-strength steels and the like, a horizontal part does not appear in the SN curve even in a long life region of 10 ⁷ to 10 ⁸ cycles or more, and the fatigue limit is recognized. No phenomenon or two-stage bending phenomenon of SN curve has been reported. Many of these phenomena tend to be recognized as the strength of steel materials increases (improves quality).

On the other hand, in fast breeder reactor (FBR) equipment, piping, etc., repeated thermal stress is applied to the wetted structure due to irregular temperature fluctuation (thermal striping) due to fluid mixing at the confluence of coolants with different temperatures. It is known to occur. Thermal stress repeated during plant operation requires evaluation up to about 10 ⁹ cycles, and in order to experimentally grasp fatigue damage in such an ultra-high cycle range, it can be performed in a short time. Ultra high cycle fatigue tests with strain control are becoming necessary.

However, with existing strain control fatigue test technology, it takes a very long time of 30 years or more at a standard strain rate (0.1 Hz to 1 Hz) to acquire one point of fatigue damage data in the 10 ⁹ cycle region. I need.
In general, in the strain control method, as shown in FIG. 5A, a fatigue test is performed by applying a repeated load (tensile force, compressive force) to the test piece 1 while keeping the strain rate of the test piece constant. Is performed (see FIG. 6). In the fatigue test by this strain control method, for example, a pressing type strain gauge 10 as shown in FIG. 7 is used to measure the strain between the test marks of the test piece 1 (the parallel part of the test piece). There is a limit to the control speed of the fatigue testing machine, for example, as shown in FIG. 5 (b), the strain rate is approximately 3 Hz or more, slippage occurs in the strain gauge 10, and the problems of followability and durability become obvious. It cannot be used at all above 10 Hz. For this reason, in the conventional fatigue tester (low cycle fatigue test apparatus) as shown in FIG. 7, the fatigue test must be performed at the standard strain rate. Even though fatigue evaluation data could be acquired, fatigue evaluation data in an ultrahigh cycle region of 10 ⁷ to 10 ⁸ cycles or more could not be acquired.

  In recent years, fatigue testing machines up to 1 kHz have been developed, but all are based on a load control system (that is, a control system with a constant load applied to the test piece). In this case, as shown in FIGS. 8A and 8B, even if the load is constant, the amount of strain varies depending on the number of repetitions. It could not be applied to the evaluation of fatigue damage.

The present invention has been made in view of such circumstances, and can perform an ultra-high cycle fatigue test of 10 ⁷ to 10 ⁹ cycles or more by strain control in a shorter time, and moreover, a conventional low cycle fatigue test. An object of the present invention is to provide a strain control type ultra-high cycle fatigue test method and a fatigue test apparatus capable of performing an evaluation equivalent to the fatigue life data.

  The strain-controlled ultra-high cycle fatigue test method according to the present invention includes a contact-type strain gauge capable of measuring strain between test specimens and a non-contact displacement of a measurement point when a load is applied to the test specimen. Is a test method for performing a fatigue test on a test piece by strain control using a non-contact displacement meter that can be measured by the test method, and the test condition is at least the total strain range between the test specimens (test specimen standard) (The value obtained by dividing the total amount of deformation by the distance between the gauge points multiplied by 100) and the first step of setting the control speed, and the non-corresponding to the set value of the total strain range between the gauge points of the test specimen A second step of determining a target control range of the contact displacement meter, and a load repeatedly applied to the test piece so that the amplitude of the measured value of the non-contact displacement meter is constant within the target control range at the control speed. , Fatigue tests under strain control by non-contact displacement meter In the second step, the measured value of the contact strain gauge when a load is repeatedly applied to the specimen at a strain rate that can be followed by the contact strain gauge, and the non-contact displacement gauge The measurement value is obtained, the correlation between the two is obtained from the measurement value, and based on the correlation, the target of the non-contact displacement meter corresponding to the set value of the total strain range between the gauge points of the test piece is obtained. The control range is obtained.

  In the strain control type ultra-high cycle fatigue test method, the first step to the third step are sequentially performed, and then the stress fluctuation rate reaches a predetermined value or the number of repetitions reaches a predetermined number of times. It is preferable to repeat the two steps and the third step. In the second step, the contact-type strain gauge is arranged at a position in contact with the test piece so that the strain between the test points of the test piece can be measured. In the third step, the position is separated from the test piece. It is desirable to arrange a contact-type strain gauge in

  In addition, the strain control type ultra-high cycle fatigue testing apparatus according to the present invention measures a load portion for applying a tensile force and a compressive force to a test piece mounted between upper and lower chucks, and a strain of the test piece. And a control unit that controls each unit based on the measurement result of the measurement unit, and performs a fatigue test of the test piece by strain control. Consists of a combination of a contact-type strain gauge that can measure strain between gauge points and a non-contact-type displacement meter that can measure the displacement of the measurement point when a load is applied to the test piece in a non-contact manner. The measured value of the contact-type strain gauge when a load is repeatedly applied to the specimen at a strain rate that can be followed by the contact-type strain gauge and the measured value of the non-contact-type displacement gauge before or during the test. And the correlation between the two from the measured values Based on the correlation, the target control range of the non-contact displacement meter corresponding to the set value of the total strain range between the test specimens of the test piece is determined, and then the measurement of the non-contact displacement meter is performed. The control is such that a load is repeatedly applied to the test piece so that the amplitude of the value is constant within the target control range.

  According to the present invention, the measured value of the contact-type strain gauge when a load is repeatedly applied to the test piece at a strain rate that can be followed by the contact-type strain gauge before or during the test, and the non-contact displacement gauge And obtain the correlation between them, and based on the correlation, the target of the non-contact displacement meter corresponding to the set value of the total strain range between the test specimen points After setting the control range, repeatedly apply a load to the specimen so that the amplitude of the measured value of the non-contact displacement meter is constant within the target control range, and fatigue under strain control by the non-contact displacement meter. Since the test was performed, it was possible to solve technical problems related to the followability and durability of the contact-type strain gauge, and compared with the conventional low cycle fatigue test device (Fig. 7), the control speed of the test device. (Set value of strain rate) can be increased To become.

Therefore, a fatigue test can be performed in a short time, and fatigue evaluation data in an ultrahigh cycle region of 10 ⁷ to 10 ⁹ cycles or more can be acquired. As a result, it is possible to reflect the evaluation data on the longer life side in the design and the like, and the reliability of various structural materials can be improved. In addition, labor costs for maintenance can be reduced by shortening the test time.

  In addition, the target control range of the non-contact displacement meter is reset every time the stress fluctuation rate reaches a predetermined value or the number of repetitions reaches a predetermined number of times, so that the setting error of the target control range is reduced. It is possible to perform the same evaluation as the fatigue life data of the conventional low cycle fatigue test.

FIG. 1 shows an embodiment of a strain-controlled ultra-high cycle fatigue testing apparatus according to the present invention, and reference numeral 1 in the drawing is a test piece.
This test apparatus measures the strain of the test piece 1 and a load portion 5 (see FIG. 6) for applying a tensile force and a compressive force to the test piece 1 mounted between the upper and lower chucks 2a and 2b. And a control unit (not shown) that controls each unit based on the measurement result of the measurement unit 6.

  As the load unit 5, for example, a well-known load device such as an electrohydraulic servo control type or an electric servo motor control type load device can be used. The measuring unit 6 includes a contact-type strain gauge 10 capable of measuring strain between test marks 1 (test piece parallel parts), and displacement (or two points) of the measurement point when a load is applied to the test piece 1. Change in the distance between the measurement points) can be configured by a combination of the non-contact displacement meter 20 capable of measuring in a non-contact manner. In the present embodiment, a pressing-type strain gauge is used as the contact-type strain gauge 10. This pressing strain gauge includes a pair of parallel quartz rods (or ceramic rods) 11a and 11b having sharp tip portions, and the tips of the quartz rods 11a and 11b are respectively pressed against a test point. The amount of strain between the gauge points of the test piece 1 is measured by measuring the amount of change in the distance between the quartz rods 11 a and 11 b with the strain gauge 13 while being fixed to the piece 1. This pressing-type strain gauge is mounted on the slide moving table 12 and is configured to be able to advance and retract in the direction of approaching and separating from the test piece 1. In the present embodiment, as the non-contact displacement meter 20, a laser displacement meter that uses a laser beam to measure the distance to the reflecting surface by a triangulation method is used. In this laser displacement meter, in order to measure the change in the distance between the upper and lower chucks 2a, 2b, that is, the amount of expansion / contraction of the entire test piece 1, the main body 21 having a light source and a light receiving element is provided on one chuck 2a and the reflecting surface 22a is provided. Each target 22 is attached to the other chuck 2b. The distance between the main body 21 and the target 22 is set within a measurement range of the laser displacement meter so that they do not come into contact with each other when a load is applied to the test piece 1.

  As the control unit, a known computer having a CPU (Central Processing Unit), a RAM (Random Access Memory), a display unit, an input unit, a storage unit, a communication unit, and the like can be used. The storage unit of this control unit is a storage area for storing various processing programs executed by the CPU (including a processing program for performing a strain control type ultra-high cycle fatigue test described later), control data, and the like. In addition, there are provided a storage area for storing measurement data fetched from a pressing-type strain gauge and a laser displacement gauge, a storage area for storing test condition setting data input from the input unit, and the like.

  Next, an embodiment of a strain control type ultra-high cycle fatigue test method using the fatigue test apparatus will be described. This fatigue test method includes a first step of setting test conditions, a second step of executing a calibration process described later, and a third step of performing a fatigue test under strain control by the non-contact displacement meter 20. Have. In the present embodiment, after the first process to the third process are executed in order, the second process and the third process are repeatedly performed each time a calibration process execution condition (described later) is satisfied. This series of processing is continued until the test piece 1 is broken.

  First, in the first step, as test conditions, a set value of the total strain range between the test points of the test piece 1 (hereinafter referred to as a target strain range Δεt) and a set value of a strain rate (hereinafter referred to as a control speed f). .), The calibration process execution condition is input to the input unit of the control unit and stored in the storage unit. As a condition for executing the calibration process, a condition based on the stress fluctuation rate or the number of repetitions (for example, the calibration process is executed when the stress fluctuation rate reaches a predetermined value or the number of repetitions reaches a predetermined number of times). Condition) can be set.

  Next, in the second step, calibration processing is performed. This calibration process is performed before the test is started or during the test (when the execution condition of the calibration process is satisfied). The amplitude value of the non-contact displacement meter 20 corresponding to the target strain range Δεt of the contact strain gauge 10 (hereinafter referred to as the calibration process) , Referred to as a target control range ΔLt).

  In this second step, first, the control unit controls the drive unit of the contact strain gauge 10 to press the tips of the quartz rods 11 a and 11 b of the contact strain gauge 10 against the test point 1. Then, the control unit controls the load unit 5 so that the measured value of the contact strain gauge 10 is a target strain range Δεt (at a strain rate (for example, 0.1 Hz) that the contact strain gauge 10 can follow. For example, the process of repeatedly applying a load to the test piece 1 is executed while gradually increasing the amplitude of the load until it reaches or almost reaches 0.3%). During this process, the control unit takes in the measurement value of the contact strain gauge 10 and the measurement value of the non-contact displacement meter 20 and sequentially stores them in the storage unit. When the target strain range Δεt is reached or almost reached, the operation of the load section 5 is stopped and the contact strain gauge 10 is detached from the test piece 1.

  Thereafter, the control unit obtains a correlation between the measured values of the contact strain gauge 10 and the non-contact displacement meter 20 taken into the storage unit, and based on the correlation, the target strain of the contact strain gauge 10 is obtained. The target control range ΔLt of the non-contact displacement meter 20 corresponding to the range Δεt is obtained, and processing for storing this in the storage unit is performed. FIG. 2 is a graph showing the transition of the measured value of the contact strain gauge 10 and the measured value of the non-contact displacement meter 20 incorporated in the control unit. It can be seen that the measured values of the contact displacement meter 20 are in a correlated relationship. The correlation between the two can be expressed by the following equation using the coefficient a.

  Coefficient a = Contact strain gauge measurement (%) / Non-contact displacement gauge measurement (μm)

  The target control range ΔLt of the non-contact displacement meter 20 can be obtained by substituting the coefficient a and the target strain range Δεt of the contact strain meter 10 into the following equation.

Target control range ΔLt (μm) of non-contact displacement meter
= Target strain range of contact strain gauge Δεt (%) / Coefficient a

  Subsequently, in the third step, a fatigue test is performed under strain control by the non-contact displacement meter 20. Specifically, as shown in FIG. 2, the control unit controls the load unit 5 and gradually increases the amplitude of the load applied to the test piece 1 at the control speed f (for example, 100 Hz) set in the first step. A steady fatigue test is performed by applying a repeated load to the test piece 1 so that the amplitude of the measured value of the non-contact displacement meter 20 becomes constant in the target control range ΔLt (for example, 40 μm). I do.

Thereafter, the control unit monitors whether or not the execution condition of the calibration process set in the first step is satisfied, and detects that the execution condition is satisfied (for example, the number of repetitions reaches a predetermined number). As shown in FIG. 2, the load amplitude applied to the test piece 1 is gradually decreased, and the load portion 5 is controlled so that the amplitude of the load eventually becomes zero. After that, the process proceeds to the second process described above and executes the calibration process described above to correct (reset) the target control range ΔLt, and then proceeds to the third process to resume the steady fatigue test. To do. That is, as the number of repetitions increases, the coefficient a (correlation between the measured values of the contact strain gauge 10 and the non-contact displacement gauge 20) changes. Therefore, in this embodiment, a fatigue test is appropriately performed. The target control range ΔLt is corrected as needed by interrupting and performing calibration processing. The execution condition of the calibration process is preferably set so that the fluctuation of the target control range ΔLt that occurs before and after the process is sufficiently small, thereby reducing the setting error of the target control range ΔLt. be able to.
In this way, the fatigue test is continued while correcting the target control range ΔLt as needed. When the test piece 1 is damaged, the test is terminated.

  As described above, according to the present embodiment, the contact-type strain gauge 10 when the load is repeatedly applied to the test piece 1 at a strain rate that the contact-type strain gauge 10 can follow before or during the test starts. The measurement value and the measurement value of the non-contact displacement meter 20 are acquired, and a correlation (coefficient a) between them is obtained from the measurement value. Based on the correlation, all the points between the test points 1 are obtained. After setting the target control range ΔLt of the non-contact displacement meter 20 corresponding to the set value of the strain range (target strain range Δεt), the amplitude of the measurement value of the non-contact displacement meter 20 is constant within the target control range ΔLt. Since the fatigue test is performed under the strain control by the non-contact displacement gauge 20 by applying a repeated load to the test piece 1, there are technical problems regarding the followability and durability of the contact strain gauge. Which can be resolved The control speed f of the apparatus can be increased to about 100 Hz.

Therefore, the test time can be shortened to 1/50 to 1/100 of the conventional value, and fatigue evaluation data in an ultrahigh cycle region of 10 ⁷ to 10 ⁹ cycles or more, which has been considered impossible in the past, can be acquired. As a result, it is possible to reflect the evaluation data on the longer life side in the design and the like, and it is possible to improve the reliability of materials of various structures. In addition, labor costs for maintenance can be reduced by shortening the test time.

In the present embodiment, the calibration process is executed before the start of the test, and the strain control by the non-contact displacement meter 20 is performed from the start of the test. However, the present invention is not limited to this, for example, In the initial stage where the value of the coefficient a is unstable (for example, until the number of repetitions reaches 2000), strain control by the contact strain gauge 10 is performed at a relatively low control speed (for example, 0.1 to 1 Hz). Then, after the value of the coefficient a is stabilized, calibration processing may be executed to perform strain control by the non-contact displacement meter 20 at a high control speed (for example, 100 Hz).
In the present embodiment, the contact-type strain gauge 10 is exemplified as a contact-type strain gauge using the strain gauge 13 as a sensor portion, and the non-contact displacement gauge 20 is exemplified as a laser displacement gauge. As the contact strain gauge 10, for example, a pressing-type strain gauge that employs a differential transformer type, a capacitance type, an optical displacement gauge as a sensor unit, and a non-contact displacement gauge 20 as an eddy current displacement gauge, etc. Can also be used.

Next, the effect of the present invention will be clarified by examples.
When the fatigue test was carried out by the strain controlled ultra high cycle fatigue test method according to the present invention, the results shown in the graphs of FIGS. 3 and 4 were obtained. FIG. 3 shows that the target strain range Δεt is 0.3% or less, the temperature condition is 600 ° C., the ultra-high cycle fatigue test method (10 to 60 Hz) according to the present invention, and the low cycle fatigue test defined by the conventional JIS. The method (0.1 Hz) shows the stress behavior when a 316FR steel fatigue test is performed. In the figure, “HMH3D4 (B7)” and “HB8A01 (B8)” are test piece numbers (heat numbers). FIG. 4 also shows SUS304 steel and 316FR steel obtained by the ultra-high cycle fatigue test method according to the present invention and the low cycle fatigue test method defined by the conventional JIS under the temperature condition of 550 ° C. or 600 ° C. The fatigue properties of are shown.

  According to these graphs, in each cycle region, almost the same evaluation data was obtained between the ultra-high cycle fatigue test method according to the present invention and the low cycle fatigue test method defined in the conventional JIS, and there was significant difference between the two. It can be seen that there is no significant difference. In other words, according to the ultra-high cycle fatigue test method according to the present invention, in the high cycle region that cannot be dealt with by the conventional low cycle fatigue test method defined in JIS, it is equivalent to the fatigue life data of the low cycle fatigue test method. It can be seen that the fatigue evaluation data can be obtained.

It is a principal part block diagram which shows one Embodiment of the strain control type | mold super high cycle fatigue test apparatus which concerns on this invention. It is a graph which shows transition of the measured value of a contact type strain gauge, and the measured value of a non-contact type displacement meter. It is a graph which shows the experimental verification result of the tension | pulling / compression stress fluctuation | variation by this invention. It is a graph which shows the experimental verification result of the fatigue characteristic by this invention. It is a figure for demonstrating the conventional distortion control method. It is a schematic block diagram which shows an example of the conventional fatigue test apparatus (low cycle fatigue test apparatus). It is a principal part enlarged view of the fatigue test apparatus of FIG. It is a figure for demonstrating the strain behavior (example of stainless steel) in a load control method.

Explanation of symbols

1 Test piece 10 Contact strain gauge 20 Non-contact displacement gauge

Claims (4)

Using a contact-type strain gauge that can measure the strain between the test specimen points and a non-contact-type displacement gauge that can measure the displacement of the measurement point when a load is applied to the test piece in a non-contact manner. A test method for performing a fatigue test on a test piece under control,
As a test condition, the first step of setting at least the total strain range and the control speed between the test point of the test piece, and the target of the non-contact displacement meter corresponding to the set value of the total strain range between the test point of the test piece A second step of obtaining a control range; and a non-contact displacement meter by applying a load repeatedly to the test piece so that the amplitude of the measured value of the non-contact displacement meter is constant within the target control range at the control speed. A third step of performing a fatigue test under strain control by
In the second step, the measurement value of the contact strain gauge when a load is repeatedly applied to the test piece at a strain rate that can be followed by the contact strain gauge, and the measurement value of the non-contact displacement gauge are obtained, The correlation between the two is obtained from the measured values, and based on the correlation, the target control range of the non-contact displacement meter corresponding to the set value of the total strain range between the test specimen points is obtained. A strain-controlled ultra-high cycle fatigue test method characterized by the above.   After the first step to the third step are sequentially executed, the second step and the third step are repeatedly performed every time the stress fluctuation rate reaches a predetermined value or the repetition number reaches a predetermined number of times. The strain controlled ultra high cycle fatigue test method according to claim 1.   In the second step, the contact-type strain gauge is arranged at a position in contact with the test piece so that the strain between the test points of the test piece can be measured, while in the third process, the contact is made at a position separated from the test piece. 2. The strain control type ultra high cycle fatigue test method according to claim 1, wherein a strain gauge is arranged. A load part for applying tensile force and compressive force to the test piece mounted between the upper and lower chucks, a measurement part for measuring the strain of the test piece, and the measurement result of each part based on the measurement result of this measurement part And a control device that performs control, and performs a fatigue test of the test piece by strain control,
The measurement unit includes a contact strain meter capable of measuring the strain between the gauge points of the test piece, a non-contact displacement meter capable of measuring the displacement of the measurement point when a load is applied to the test piece in a non-contact manner, and Composed of
The above control unit measures the measured value of the contact-type strain gauge when a load is repeatedly applied to the test piece at a strain rate that can be followed by the contact-type strain gauge before or during the test. The measurement value is obtained, the correlation between the two is obtained from the measurement value, and based on the correlation, the target of the non-contact displacement meter corresponding to the set value of the total strain range between the gauge points of the test piece is obtained. A strain characterized in that a control range is obtained, and then control is performed so that a load is repeatedly applied to the specimen so that the amplitude of the measured value of the non-contact displacement meter is constant within the target control range. Controlled ultra-high cycle fatigue testing equipment.

Images