JPH03152441A - Testing method for thermal fatigue - Google Patents

Abstract

PURPOSE:To enable execution of an accurate test entirely over a wide range of a test temperature from a normal temperature area to a high temperature area by computing a thermal expansion coefficient of a test piece at each temperature on based on a measured strain and by determining a control target value therefrom. CONSTITUTION:First, a control of a tester is executed so that a load on a test piece 1, i.e. a confinement ratio therefore, be zero. Heating of the test piece 1 by a heating coil 5 is started and the temperature of the test piece 1 is varied up and down within the range of a test temperature, while it is monitored by a temperature sensor 9. In cycles of heating and cooling, a control circuit 12 measures a strain of the test piece 1 through a displacement detector 10. Based on the measured strain, subsequently, the control circuit 12 determines of a thermal expansion coefficient of the test piece 1 at each temperature by computation and determines a control target value in a thermal fatigue test by using the determined thermal expansion coefficient. According to this method, an accurate test can be executed entirely over a wide range of the test temperature from a normal temperature area to a high temperature area.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a thermal fatigue testing method, and in particular, a thermal fatigue test is performed by heating and cooling a test piece in a predetermined temperature pattern while keeping a constant restraint rate. This relates to a thermal fatigue test method.

[Prior Art] Generally, in high-temperature equipment, the metal materials used undergo repeated thermal expansion and contraction due to temperature fluctuations during startup and shutdown. When such thermal expansion strain is restrained, thermal stress is generated as shown in FIG. Fatigue due to repeated thermal stress is called thermal fatigue. Total constrained strain range Δε1
is given by Δε,=ηαΔT. Here, η is the constraint ratio, α is the coefficient of thermal expansion,
ΔT is the temperature fluctuation range. In the thermal fatigue test, Δε is determined by the restraint rate under the conditions of the upper and lower temperature limits, and Δ
The number of repetitions Nt until failure for ε is determined.

By the way, from the above formula, the constraint ratio η is expressed as η=ΔεL/αΔT, and is defined by the ratio of thermal expansion to the entire strain range, and is 1 when the constraint is fixed, and −l when the displacement is O1 times the free expansion. . Note that the temperature fluctuation range ΔT is the difference (T−Tm) between the measured temperature T and the fluctuation center temperature Tm.

Now, with the restraint rate η constant, Δ2. When performing a thermal fatigue test using Δ11 as the control target value, conventionally the control target value Δ11 is determined by the following formula.

Δf,=αΔT+Δε.

αΔT−ηαΔT αΔT(1−η) α(T−Tm)(1−η) ・・・・・・(1) At this time, separately measure the coefficient of thermal expansion α in the above formula (1) in advance. , during operation, substitute this into the above equation (1) to obtain the control target value Δ
1. was looking for.

[Problem to be solved by the invention]

However, when the thermal expansion coefficient α is tested over a wide range from normal temperature to high temperature range, it changes as α(T) with temperature as a parameter, so there is a problem that the test cannot be performed accurately.

Therefore, in view of the above-mentioned conventional problems, it is an object of the present invention to provide a thermal fatigue testing method that allows the test to be performed accurately over the entire test temperature range, even when testing is performed in a wide range from room temperature to high temperature range. It is said that

[Means for Solving the Problems] In order to solve the above problems, the thermal fatigue test method according to the present invention is based on the following: the restraint rate, the temperature of a test piece that is heated and cooled in a predetermined temperature pattern, and the heat of the test piece. In a thermal fatigue test method in which a target value for strain control is determined based on the expansion coefficient and a thermal fatigue test is performed using the target value, the temperature of the test piece is adjusted before the start of the thermal fatigue test without restraining the test piece. is varied between the lower limit temperature and upper limit temperature of the temperature pattern, the strain occurring in the test piece at each temperature is measured, and the thermal expansion coefficient of the test piece at each temperature is calculated based on the measured strain. The target value in the thermal fatigue test is determined by using the coefficient of thermal expansion determined by the calculation.

[For production]

In the above method, the control target value for thermal fatigue testing is determined by taking into account the coefficient of thermal expansion at each temperature. Even if the coefficient of thermal expansion is changed as a parameter, a target value can be determined that takes into account the change in the coefficient of thermal expansion.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing an example of a thermal fatigue testing machine for carrying out the thermal fatigue testing method according to the present invention.
is a test piece, one end of which is connected to the piston 2a of the hydraulic cylinder 2 via a chuck 3a, and the other end connected to a load detector 4 via a chuck 3b. The hydraulic cylinder 2 is operated to move the piston 2a to the test piece 1 connected as described above, as indicated by the arrow A in the figure.

By moving in direction B, tensile and compressive loads are applied to the test piece 1.

Further, a high frequency coil 5 serving as a heating means is arranged around the test piece l so as to surround it.

This high frequency coil 5 is provided with a temperature regulator 5a that controls the high frequency current and adjusts the heating temperature. As shown in FIG. 2, a nozzle 6 is arranged around the high-frequency coil 5 to blow cooling air onto the high-frequency coil 5 and the test piece 1. A servo valve 8 that continuously adjusts the flow rate is provided in the flow path 7 that sends cooling air to the nozzle 6.
is provided.

Furthermore, a temperature sensor 9 composed of a thermocouple is attached to the test piece l, and chucks 3a and 3b are attached to the test piece l.
A displacement detector IO is arranged to detect strain in the test piece 1 by measuring the distance between the test pieces 1 and 1.

The outputs of the load detector 4, temperature sensor 9, and displacement detector 10 are amplified by amplifiers lla to 11c, respectively, and input to a control circuit 12 constituted by a microcomputer (CPU) that operates according to a predetermined program. be done.

The control circuit 12 converts analog signals input manually via amplifiers lla to tic into digital signals and captures the digital signals.
The captured signal is processed to control the high frequency current flowing through the high frequency coil 5 in order to raise and lower the temperature of the test piece 1 to a predetermined temperature, and to output a control signal to control the opening degree of the servo valve 8. .

The thermal fatigue testing method according to the present invention, which is carried out using the thermal fatigue testing machine configured as described above, will be described below with reference to the timing chart shown in FIG.

First, the test machine is controlled so that the load applied to the test piece 1 is zero, that is, the restraint rate is 0. At time t1, the heating coil 5 starts heating the test piece l, and the temperature sensor 9 performs the test. While monitoring the temperature of piece 1, it is raised from the lower limit temperature T of the test temperature range to the upper limit temperature T2 at an appropriate rate of increase. Test piece 1
When the temperature reaches the upper limit temperature T2 at time t2, that temperature is maintained for a predetermined period of time until time t, then heating by the heating coil 5 is stopped, and if necessary, cooling air is supplied to the test piece 1 from the nozzle 6. By spraying, the temperature is lowered from the upper limit temperature T2 to the lower limit temperature T at an appropriate rate of decline. When the temperature of the test piece 1 reaches the lower limit temperature T1 at time L4, after holding that temperature for a certain period of time, heating of the test piece 1 by the heating coil 5 is started again at the time point, and the heating of the test piece 1 is continued until time L6. The temperature of the test piece 1 is raised to the thermal fatigue test starting temperature T0.

During the above-described heating/cooling cycle of the test piece l between times 1+ and t4, the control circuit 12 generates an analog signal representative of the strain on the test piece l obtained at the output of the displacement detector 10 and amplified by the amplifier 11C. An analog signal representing the temperature of the test piece I obtained from the output of the temperature sensor 9 and amplified by the amplifier Ilb is converted into a digital signal at an appropriate sampling frequency, and these signals are respectively taken in. Of the digital signals taken in, the signals for the periods from time tl to time t3 and from time t3 to L4 are respectively stored in the internal memory 12a as strain data and temperature data. Of course, the control circuit 12 servo-controls the hydraulic cylinder 2 based on the load signal from the load detector 4 in order to perform load control with zero load in the above cycle.

Thereafter, the control circuit 12 performs calculations based on the strain and temperature data stored in the memory 12a to determine the thermal expansion coefficient α at each temperature, which is then used as the control target value Δ! for the later thermal fatigue test. To use as data for calculating ,
It is made into a table and stored in a predetermined area of the memory 12a.

FIG. 4 is a graph showing changes in the coefficient of thermal expansion α(T) with respect to the temperature stored in the memory 12a.

After determining the thermal expansion coefficient α with respect to temperature as described above, the thermal fatigue test is started at time t7. In this thermal fatigue test, the test piece 1 was placed at a lower limit temperature T+
The control circuit 12 controls the heating coil 5 and the servo valve 8 in order to perform heating and cooling in a predetermined pattern between the temperature T2 and the upper limit temperature T2. At this time, the control circuit 12 controls the constraint rate η
In order to control the strain by fixing η1 to a predetermined constant value, the temperature sensor 9 measures the actual temperature T of the test piece l, reads out the coefficient of thermal expansion α(T) for this temperature from the memory 12a, and sets the control target. The value ΔN is expressed by the above formula (
The following formula is a modification of 1), Δ11−α(T)(T−T, )(
1−η,), and the calculated control target value Δ
! 0 is used to servo control the hydraulic cylinder 2.

Note that the above-described measurement routine for the coefficient of thermal expansion α(T) may be periodically executed at any interval during the thermal fatigue test. In this way, the coefficient of thermal expansion α (
It becomes possible to conduct a thermal fatigue test that also takes T) into consideration, and it becomes possible to conduct an even more accurate test.

〔effect〕

As explained above, according to the present invention, the control target value during the thermal fatigue test is determined by taking into account the coefficient of thermal expansion at each temperature. Even if the coefficient of thermal expansion changes with temperature as a parameter, a target value that takes this change in coefficient of thermal expansion into account is now required. The effect is that the test can be performed accurately.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing an example of a thermal fatigue testing machine used to carry out the thermal fatigue testing method according to the present invention, Fig. 2 is a diagram showing details of a part of Fig. 1, and Fig. 3 is a diagram showing the present invention. Figure 4 is a diagram showing data obtained in the process of carrying out the method of the present invention, Figure 5 shows the relationship between strain, restraint, and thermal stress caused by thermal expansion. FIG. l...Test piece, 2...Hydraulic cylinder, 2a.
...Piston, 4...Load detector, 5...Heating coil, 6...Nozzle, 9...Temperature sensor, 10...
Displacement detector, 12...control circuit, 12a...memory.

Claims (1)

[Claims] A target value for strain control is determined from the restraint ratio, the temperature of the test piece heated and cooled in a predetermined temperature pattern, and the coefficient of thermal expansion of the test piece, and the target value is used to control thermal fatigue. In the thermal fatigue test method for conducting the test, before the start of the thermal fatigue test, the temperature of the test piece is changed between the lower limit temperature and the upper limit temperature of the temperature pattern without restraining the test piece, and at this time, each temperature Measure the strain that occurs in the test piece at , calculate the thermal expansion coefficient of the test piece at each temperature based on the measured strain, and use the calculated thermal expansion coefficient to determine the target in the thermal fatigue test. A thermal fatigue test method characterized by determining a value.